Adaptive and Innate Immunity

The immune system has two functional divisions: the innate immune system and the adaptive immune system. The innate immune system acts nonspecifically as the body’s first line of defense against pathogens. However, this type of response, if perpetuated, would result in unwanted nonspecific damage to the host. Therefore a secondary, antigen-specific response develops and leads the attack. This is mediated by T cells and B cells, which are equipped with antigen-specific receptors. The effector cells release mediators and trigger other components of the immune system to eliminate the target. Subpopulations of T and B cells develop and maintain immunological memory, which facilitates a more rapid response in the case of recurrent infection.

The innate immune system consists of the following components:

The adaptive immune response consists of the following components:

Principal Components of the Immune System

Cells of the immune system arise from the pluripotent stem cells in the bone marrow and diverge into the lymphoid or myeloid lineages. The myeloid lineage primarily contains cells with phagocytic functions such as neutrophils, basophils, eosinophils, and macrophages. The lymphoid lineage consists of T cells, B cells, and NK cells.

T-Cell Receptors

The TCR consists of two glycosylated polypeptide chains, alpha (α) and beta (β), of 45,000 and 40,000 dalton molecular weight, respectively. This heterodimer of an α and β chain is linked by disulfide bonds. Amino acid sequences show that each chain consists of variable (V), joining (J), and constant (C) regions closely resembling Igs (Fig. 41.1). There are about 10² TCR-variable genes grouped by homology into a small number of families, compared with 10³ or greater for Igs (see later discussion). The principles governing generation of diversity in the TCR are very similar to those for Ig genes. T cells can only recognize short peptides that are associated with MHC molecules. In contrast, the Ig receptor can recognize peptides, whole proteins, nucleic acids, lipids, and small chemicals.

Fig. 41.1 Molecular and genetic organization of the T-cell receptor (TCR) and immunoglobulin (Ig) molecule. A-B, Structural organization of the TCR and Ig molecule. The TCR is a heterodimer consisting of two chains, a and b; the Ig molecule consists of two heavy and light chains. Both molecules are stabilized by interchain and intrachain disulfide bonds. Variable-region domains are located at the amino terminal, and constant-region domains are located on the carboxy terminal. The antigen-binding site on the Ig molecule is located between the variable-region domains of the heavy and light chains. The variable region of the TCR recognizes foreign peptides in the context of self-MHC (major histocompatibility complex) molecules. The TCR is also associated with the CD3 antigen (consisting of g, d, e, and z-z chains) to form the TCR complex. C, Organization of the gene families of Ig and TCR. The common feature of the four gene pools is that they contain a number of variable (V) gene segments that are separated from the constant (C) region genes by the joining (J) genes. In the case of the TCR b chain and the Ig heavy-chain gene, additional diversity (D) genes are present. During ontogeny, one of the V gene segments is juxtaposed to the J segment through a process of chromosomal rearrangement to form the V(D)J gene. This, along with the constant region genes, is transcribed to form messenger RNA and then protein.

T cells also express a variety of nonpolymorphic antigens on their surfaces. The most abundantly expressed is CD45, comprising 10% of lymphocyte membrane proteins. CD45 exists as a number of isoforms that differ in the molecular weight of their extracellular domains as a result of RNA splicing. These isoforms can be distinguished serologically. The low molecular weight (CD45RO) isoforms define activated, or memory, T-cell populations.

Genetics of the Immune System

Major Histocompatibility and Human Leukocyte Antigens

Major histocompatibility complex gene products or the human leukocyte antigens (HLAs) serve to distinguish self from nonself. In addition, they serve the important function of presenting antigen to the appropriate cells. The MHC class I gene product contains an MHC-encoded α chain, and a smaller non-MHC-encoded β₂-microglobulin chain. The MHC class II gene product consists of two polypeptide chains, α and β, which are noncovalently linked. Both class I and class II proteins are stabilized by intrachain disulfide bonds. Class I antigens are expressed on all nucleated cells, whereas class II antigens are constitutively expressed only on dendritic cells, macrophages, and B cells and are also expressed on a variety of activated cells including T cells, endothelial cells, and astrocytes.

In humans, class I molecules are HLA-A, B, and C, whereas the class II molecules are HLA-DP, DQ, and DR. Several alleles are recognized for each locus; thus the HLA-A locus has at least 20 alleles, and HLA-B has at least 40. The number of alleles for the D region appears to be as extensive as that for HLA-A, HLA-B, and HLA-C. In view of the extensive polymorphisms present, the chances of two unrelated individuals sharing identical HLA antigens are extremely low. The reason for the extensive diversity and evolutionary pressure that lead to this are not fully understood.

Class I antigens regulate the specificity of cytotoxic CD8⁺ T cells, which are responsible for killing cells bearing viral antigens or foreign transplantation antigens (Fig. 41.2). The target cells share class I MHC genes with the cytotoxic cell. Thus the cytotoxic cell that is specific for a particular virus is capable of recognizing the antigenic determinants of the virus only in association with a particular MHC class I gene product. The function of class II MHC gene products appears to be to regulate the specificity of T-helper cells, which in turn regulate DTH and antibody response to foreign antigens. Similarly, an immunized T-cell population will recognize a foreign antigen only if it is presented on the surface of an APC that shares the same class II MHC antigen specificity as the immunized T-cell population. Thus the functional specificity of the T-cell population is restricted by the MHC molecules they recognize. CD8⁺ T cells (cytotoxic) and CD4⁺ T cells (helper) are referred to as MHC class I and MHC class II restricted T cells, respectively (Fig. 41.3).

Fig. 41.2 The phenomenon of major histocompatibility complex (MHC) restriction. For antigen-specific cytolysis of virus-infected targets to occur, T cells should be sensitized to the virus and share the same class I human leukocyte antigen (HLA) with the target cell. In the lower part of the figure, the MHC class I antigen expressed on the CD8⁺ T cell is different from the MHC class 1 antigen expressed on the target cell; therefore lysis does not occur. TCR, T-cell receptor.

Fig. 41.3 Antigenic recognition of cytotoxic and helper T cells. The cytotoxic T cell recognizes viral peptides associated with human leukocyte antigen-A (HLA-A), HLA-B, or HLA-C molecules. The coreceptor for the helper T cell is the CD4 molecule. MHC, Major histocompatibility complex; TCR, T-cell receptor.

The analysis of the three-dimensional structure of the class I and class II molecules has confirmed the notion that these molecules are carriers of immunogenic peptides that are processed by APCs and presented on the cell surface (Fig. 41.4). Both MHC class I and class II molecules share similarities in crystal structure that allow them to accept and retain immunogenic peptides in grooves, or pockets, and present them to T cells.

Fig. 41.4 Schematic diagram of the human leukocyte antigen (HLA) complex in humans, located on chromosome 6. The HLA class I gene (HLA-A, -B, and -C) codes for a single heavy-chain molecule. The β₂-microglobulin is coded by genes on a different chromosome. The HLA class II genes (DR, DP, and DQ) form the αβ heterodimer. The HLA class III genes include those encoding for members of the complement family of proteins. MHC, Major histocompatibility complex.

Initiation of the Immune Response

Accessory Molecules for T-Cell Activation

The interaction of MHC-peptide complex with T cells, although necessary, is insufficient for T-cell activation. Other classes of molecules are involved in T-cell antigen recognition, activation, intracellular signaling, adhesion, and trafficking of T cells to their target organs. The distinction between the functions of these classes of molecules is not absolute, and many may be involved in interactions between other cells of the immune system.

CD3

Molecules whose primary role is signaling include the CD3 molecule. The CD3 molecule is part of the TCR complex. Although the TCR interacts with the MHC-peptide complex on APCs, the signals for the subsequent enactment of T-cell activation and proliferation are delivered by the CD3 antigen. The cytoplasmic tail of the CD3 proteins contains one copy of a sequence motif important for signaling functions, called the immunoreceptor tyrosine-based activation motif (ITAM). Phosphorylation of the ITAM initiates intracellular signaling events. In experimental situations, anti-CD3 antibodies can nonspecifically activate these intracellular signals, producing activated T cells in the absence of antigen.

CD4 and CD8

CD4 or CD8 antigens are expressed on mature T cells and serve an accessory role in signaling and antigen recognition. CD4 binds to a nonpolymorphic site on the MHC class II β chain, and CD8 binds to the α3 domain of the MHC class I molecule. Signals for cell division that are delivered to the nucleus are mediated by second messengers. When the receptor binds its ligand, it causes the activation of protein kinases. These kinases add phosphate groups to other proteins that ultimately signal the cell to divide. CD4, CD8, and CD3 on T cells and CD19 on B cells are examples of receptors that are linked to kinases. CD4 is the cell surface receptor for human immunodeficiency virus (HIV-1), and the fact that certain non-T cells such as microglia and macrophages can express low levels of CD4 may explain the propensity of the virus for the central nervous system (CNS).

Costimulatory Molecules

Costimulatory molecules serve as a “second signal” to facilitate T-cell activation. Costimulatory pathways that are critical for T-cell activation include the B7-CD28 and CD40-CD154 pathways. Members of the integrin families including vascular cell adhesion molecule 1 (VCAM-1), intercellular adhesion molecule (ICAM-1), and leukocyte function antigen 3 (LFA-3) can provide costimulatory signals, but they also play critical roles in T-cell adhesion, facilitate interaction with the APCs, mediate adhesion to nonhematopoietic cells such as endothelial cells, and guide cell traffic (Fig. 41.5).

Fig. 41.5 Antigen-driven activation of helper T cells. Proliferation of T cells requires the delivery of a number of concordant signals. Along with stimulation through the T-cell receptor–CD3 complex, the presence of appropriate costimulatory signals via CD28 antigen, adhesion molecules, leukocyte function antigen 1 (LFA1) and CD2, and the coreceptor molecule CD4 are essential for T-cell activation and proliferation. The membrane events of antigen recognition lead to activation of second messengers. The second messengers signal the nucleus and cell to divide and secrete cytokines. Interleukin 2 (IL-2) acts as an autocrine growth stimulator, thereby amplifying the response. ICAM, Intercellular adhesion molecule; IFN-γ, interferon γ; MHC, major histocompatibility complex; TCR, T-cell receptor; TNF, tumor necrosis factor.

The B7-CD28 interaction is one of the most extensively studied costimulatory systems. The B7 molecules are expressed on antigen-presenting cells, and their expression is induced in activated cells. There are two forms of B7, B7-1 (CD80) and B7-2 (CD86), that share some homology but have different expression kinetics. The B7 molecules interact with their ligand, CD28, which is constitutively expressed on most T cells. Binding of the CD28 molecule mediates intracytoplasmic signals that increase expression of the growth factor, IL-2, and enhance expression of the anti-apoptotic molecule, Bcl-x_L. An alternate ligand for B7 is CTLA-4, which is homologous to CD28 in structure, but in contrast to CD28, CTLA-4 functions to inhibit T-cell activation. Costimulatory molecules may deliver either a stimulatory (positive) or inhibitory (negative) signal for T-cell activation (Brunet et al., 1987). Examples of molecules delivering a positive costimulatory signal for T-cell activation include the B7-CD28, CD40-CD154 pathways. Examples of molecular pathways delivering a negative signal for T-cell activation include B7-CTLA4 and PD1-PD ligand (Khoury and Sayegh, 2004). The delicate balance between positive and negative regulatory signals can determine the outcome of a specific immune response.

Cell Migration

Molecules primarily involved in cell migration into tissues include chemokines, integrins, selectins, and matrix metalloproteinases (MMPs). Chemokines constitute a large family of chemoattractant peptides that regulate the vast spectrum of leukocyte migration events through interactions with chemokine receptors. The integrin family includes VCAM-1, ICAM-1, LFA-3, CD45, and CD2 and mediates adhesion to endothelial cells and guiding cell traffic. L-selectins facilitate the rolling of leucocytes along the surface of endothelial cells and function as a homing receptor to target peripheral lymphoid organs. The MMPs are a family of proteinases secreted by inflammatory cells; MMPs digest specific components of the extracellular matrix, thereby facilitating lymphocyte entry through basement membranes including the blood-brain barrier (BBB).

Regulation of the Immune Response

Termination of an Immune Response

The primary goal of the immune response is to protect the organism from infectious agents and generate memory T- and B-cell responses that provide accelerated and high-avidity secondary responses on reencountering antigens. It is desirable to terminate these responses once an antigen has been cleared. In parallel, the immune system must constantly function to prevent autoimmune activation and maintain self-tolerance. A number of systems operate to prevent uncontrolled responses. Here we discuss termination of individual components of the immune response. Following is a discussion of the mechanisms that maintain self-tolerance, many of which are also involved in immune-response termination.

Central Tolerance

Bone marrow stem cells migrate to the thymus, thereby becoming thymocytes, or T cells. In this location, T-cell VDJ germline genetic elements recombine to create α and β chains, which in turn form the TCR. Thymocytes then undergo a process of education that involves positive and negative selection. Positive selection of thymocytes occurs in the thymus cortex when the cells are in the double negative stage, CD4⁻ CD8⁻. The cortex contains dendritic and epithelial cells that present MHC antigens to the developing thymocytes. T cells with receptor having no affinity to MHC will fail to receive signals needed for maturation and will die in situ. Those with low affinity toward MHC survive and become single-positive thymocytes depending on their affinity toward MHC I (CD8⁺) or MHC II (CD4⁺). In the thymus medulla, thymocytes that display a high affinity toward self-antigen are deleted by apoptosis, a process called negative selection. Most T-cell education occurs in the thymus; however, extrathymic sites may exist.

Peripheral Tolerance

Self-reactive lymphocytes may escape central tolerance; therefore peripheral mechanisms exist to maintain self-tolerance. This is termed peripheral tolerance. Peripheral tolerance is maintained through clonal anergy or clonal deletion. It is not clear to what extent each of these mechanisms functions in maintaining human self-tolerance; however, extensive research has been done to elucidate the mechanisms through which anergy and deletion work. In addition, self-tolerance may be maintained despite the presence of antigen-responsive lymphocytes. It is postulated that this is due to the presence of suppressor T cells or other factors that may interfere with a successful lymphocyte response.

Anergy Due to Failure of T-cell Activation

In normal circumstances, an APC presents antigen as a peptide + MHC complex (signal one). In the absence of signal one, the T cell dies because of neglect. If signal one is presented in the absence of costimulatory signals (signal two), the T cell becomes anergic. An example of this situation occurs when an antigen is presented by nonprofessional APCs that lack the appropriate costimulatory molecules (Fig. 41.6). However, when a T cell is activated, it up-regulates the expression of an alternate costimulatory molecule, CTLA-4. CTLA4 engagement by CD80 and CD86 on the surface of APCs sends a negative signal to the T cell, inhibiting cell growth and proliferation. Animals deficient for CTLA-4 expression on their T lymphocytes have an uncontrolled lymphoproliferative phenotype with autoreactivity (Waterhouse et al., 1995).

Fig. 41.6 A two-signal model of T-cell activation. Activation of the T-cell receptor (TCR) by an antigen major histocompatibility complex (MHC) provides signal 1, which is sufficient to induce the T cell to enter the cell cycle and begin blast transformation, which is characterized by an increase in cell size. Signal 2, the costimulatory signal, can be provided to the T cell through interaction of CD28 with molecules of the B7 family found on the surface of bone marrow–derived antigen-presenting cells (APCs). A, In this instance, TCR signals are complemented, enabling the T cell to proliferate, produce cytokines, and develop mature effector functions. B, In the absence of a second signal, T-cell activation is abortive, and the cell becomes anergic. Signal 2 might not be delivered if the APC does not express a costimulatory ligand on its surface, perhaps because a nonprofessional APC, such as an epithelial cell, is presenting antigen. IL, Interleukin.

Apoptosis

Apoptosis is the process in which a cell undergoes programmed cell death. As opposed to necrosis, when interruption of the supply of nutrients triggers cell death, apoptosis may be triggered by various signals including withdrawal of growth factors, cytokines, exposure to corticosteroids, and repeated exposure to antigens. Mediators of apoptosis include the Bcl family of genes, which are mostly antiapoptotic, and the Fas family of genes, which are proapoptotic. Activated T cells also express Fas ligand (CD95L or FasL) and Fas (CD95); ligation of Fas and FasL induces apoptosis of the T cells.

Repeated stimulation with an antigen may also induce apoptosis via the Fas/FasL pathway, a process termed activation-induced cell death (AICD). Therefore an autoreactive T lymphocyte may encounter large doses of self-antigen in the periphery and consequently may be deleted by AICD. Mice lacking Fas or FasL develop a lupus-like syndrome (Zhou et al., 1996), and mutations in the Fas gene were associated with an autoimmune disease with lymphoproliferation in humans (Drappa et al., 1996).

IL-2 is the prototypical growth factor, inducing clonal expansion of antigen-stimulated lymphocytes; paradoxically, disruption of the IL-2 gene leads to accumulation of activated lymphocytes and autoimmune syndromes (Sadlack et al., 1993). This is because IL-2 induces the transcription and surface expression of Fas ligand (FasL). Interactions of Fas with FasL lead to cell death (Fig. 41.7). Therefore IL-2 plays a dual role in T-cell regulation, reflecting a possible role for cytokine concentration and timing of exposure. Other cytokines that mediate apoptosis and cell death are TNF-α and IFN-γ. Complete absence of either of these cytokines results in deficient T-cell apoptosis, inability to terminate the immune response, and uncontrolled autoimmune disease.

Fig. 41.7 Activation of the T cell leads to coexpression of the death receptor Fas (CD95) and its ligand (FasL), resulting in death of the cell and neighboring cells. APC, Antigen-presenting cell; MHC, major histocompatibility complex; TCR, T-cell receptor.

Immune Privilege in the Central Nervous System

Immunological reactions in the CNS differ from those in the rest of the body because of its unique architecture, cellular composition, and molecular expression. The CNS has been termed an immunologically privileged site because of the relative improved survival of allografts within this region. Indeed, the same factors that play a role in immunological tolerance in the CNS play a role in immune-mediated diseases involving the CNS, infections of the CNS, tumor survival, and therapies.

Important factors relevant to immunological responses in the CNS are: (1) absence of lymphatic drainage, limiting the immunological circulation; (2) the blood-brain barrier (BBB), which limits the passage of immune cells and factors; (3) the low level of expression of MHC factors, particularly MHC II in the resident cells of the CNS; (4) low levels of potent APCs, such as dendritic or Langerhans cells; and (5) the presence of immunosuppressive factors such as TGF-β (Wilbanks and Streilein, 1992) and CD200 (Webb and Barclay, 1984).

Because of the lack of a lymphatic system, antigens drain along perivascular spaces. Monocyte-derived CNS resident cells, termed microglia, play an important role in immune surveillance in these areas. The BBB is composed of tight junctions between endothelial cells and a layer of astrocytic foot processes that prevent entry of inflammatory cells and other factors into the CNS. Entry of inflammatory cells across the BBB is facilitated by upregulation of adhesion molecules ICAM-1 and VCAM-1 on endothelial cells. T cells must be activated before crossing the BBB. Entry is facilitated by expression of receptors for adhesion molecules, including α4-integrin.

The CNS houses cells that are capable of antigen presentation under certain conditions in vitro, but to what extent this occurs in vivo remains under debate. In the CNS, endogenous expression of MHC class I and class II on APCs such as microglia is low, and in oligodendrocytes and astrocytes, it is almost undetectable. Neurons express MHC class I only when damaged and in the presence of IFN-γ (Neumann et al., 1995). Expression of MHC antigens on both microglia and astrocytes are enhanced by the presence of cytokines, TNF-α, and IFN-γ. Under certain conditions, microglial cells may play a role as APCs in the nervous system (Perry, 1994). More recently, populations of perivascular dendritic cells capable of antigen presentation have been identified in rodents (Greter et al., 2005), with analogous populations demonstrated in humans; however, their role in human disease is unclear.

Immune privilege in the CNS is also influenced by the constitutive expression of a number of immunoregulatory factors, some of which are common to immune privilege in the anterior chamber of the eye. Anterior chamber immune privilege is due in part to expression of TGF-β in the aqueous of the eye. In the CNS, TGF-β is produced by astrocytes and microglia and may play a role in down-regulating immune responses locally. Neurons are also capable of producing TGF-β, which in animal models has been shown to facilitate the differentiation of regulatory T cells (Liu et al., 2006). Increased expression of Fas ligand in the CNS compared with the peripheral nervous system (PNS) may increase apoptosis of T cells, thereby down-regulating the immune response (Moalem et al., 1999). Some CNS tumors express large amounts of TGF-β, which may play a role in protecting them from immune surveillance. CNS tumors may also express Fas or Fas ligand, facilitating protection from immune surveillance. Some populations of neurons express a cell surface marker named CD200. CD200 is a nonsignaling molecule but serves to inhibit activation of cells including microglia and macrophages that express the CD200 receptor (CD200R) (Hoek et al., 2000; Wright et al., 2000). CD200 has been shown to down-regulate inflammatory responses in models of MS (Liu et al., 2010) and uveitis (Banerjee and Dick, 2004; Broderick et al., 2002). Fractalkine (CXCL1) is a chemokine that is constitutively expressed on some populations of neurons. Interaction with its receptor, CX3CR1, present on microglia and NK cells, serves to down-regulate microglial-mediated neurotoxicity both in vitro and in animal models of Parkinson disease and ALS (Cardona et al., 2006). In the animal model of MS, absence of fractalkine or its receptor resulted in a reduction of NK cells in the CNS and exacerbation of disease, supporting the view that NK cells play an inhibitory role in CNS inflammation (Huang et al., 2006).

Neuroglial Cells and the Immune Response

Neuroglial cells including microglia and astrocytes participate in immune responses within the CNS, and there is increasing evidence that these cells play a central role in initiating and propagating immune-mediated diseases of the CNS.

Microglia are derived from bone marrow cells during ontogeny (Hickey and Kimura, 1988) and reside within the CNS as three principal types of cells: perivascular microglia, parenchymal microglia, and Kolmer cells, which reside in the choroid plexus. Microglia have mitotic potential and can differentiate from bone marrow–derived cells to perivascular microglia and parenchymal microglia. Compared to macrophages, microglia are relatively radioresistant. Microglia may exist either in a resting (ramified) form or activated or phagocytic forms within the CNS. Activated microglia express higher levels of MHC class II and produce higher levels of proinflammatory cytokines including TNF-α, IL-6, and IL-1, as well as nitric oxide and glutamate. Microglia express chemokine receptors and various pattern recognition receptors (PRRs) including Toll-like receptors. PRRs recognize pathogen-associated molecular patterns (PAMPs) expressed by a variety of microbes, and interaction results in microglial activation. The primary functions of microglia are immune surveillance for foreign antigens and phagocytic scavengers of cellular debris. Microglia, particularly perivascular microglia, may also participate in antigen presentation within the CNS under certain conditions. Microglia play a role in regulating the programmed elimination of neural cells during brain development and, in some cases, enhance neuronal survival by producing neurotrophic and antiinflammatory cytokines. Microglia may also play a role in neuroregeneration and repair. However, there is overwhelming evidence that microglia play a deleterious role in several neurodegenerative diseases: MS, ALS, Parkinson disease and HIV-associated dementia. Their role in Alzheimer disease (AD) is less clear. Overactivation of microglia, possibly by microbes or other environmental factors through PRRs, may result in chronic proinflammatory milieu in the CNS, leading to progressive neurodegeneration. Strategies to down-regulate such responses are under investigation (Block et al., 2007).

Astrocytes play multiple roles in the CNS, including their role in the glia limitans at the BBB and physical support of neuronal and axonal structures, as well as provision of growth factors. Astrocytes secrete cytokines including TGF-β and are also influenced by IL-1 and interferons to divide and express proteins such as costimulatory molecules and Toll-like receptors on their surfaces. There is increasing evidence against the role of astrocytes in antigen presentation within the CNS. Astrocytes play a critical role in converting glutamate to glutamine, a less toxic substance, so impairment of astrocyte function may result in increased glutamate-mediated neurotoxicity. Astrocytes also produce chemokines including stromal-derived factor-1 (SDF-1), which plays a significant role in HIV-associated dementia.

Cells of the CNS not only respond to inflammatory stimuli but are also capable of producing cytokines and other inflammatory factors, often directly under the influence of lymphocytes. These observations led to the conclusion that the brain is not an immunologically sequestered organ but that it interacts, produces immunologically active factors, and is closely involved with the systemic immune response.

Genetic Factors

Genetic makeup plays a role in susceptibility to autoimmune diseases. In particular, an association between certain MHC haplotypes and disease has been noted. MS is linked to the HLA-DR2 allele, and the relative risk of having this allele in the Northern European population is 3.8. MG has been linked to HLA-DR3. However, the presence of the allele does not guarantee disease. In general, the relative risk of developing disease among individuals who carry the antigen may be calculated by the following formula:

Association of a particular HLA haplotype with autoimmune disease may be due to the ability of a particular MHC molecule to bind and present autoantigen to the T cell, as in MS where the MHC class II allele, DRB1*1501, has been shown to be effective in presenting myelin basic protein (MBP peptide) to T-cell clones isolated from MS patients (Wucherpfennig et al., 1995; Wucherpfennig et al., 1997). Conversely, if an MHC molecule does not bind a particular self-antigen in the thymus, the developing T cell will not recognize that antigen as self and will escape negative selection. Therefore certain MHC haplotypes have an association with disease, whereas others protect against disease. Disease linkage tends to be with class II genes of the MHC rather than class I, suggesting a key role for T-cell autoimmunity.

Association of a particular HLA-haplotype with disease may be due to its linkage to another locus or disease susceptibility gene. Linkage disequilibrium refers to the increased chance of inheriting two alleles together because they are genetically linked, as opposed to inheriting them together as separate random events.

Sex is one of the most important genetic determinants associated with autoimmune disease. Many autoimmune diseases are more frequent in females; systemic lupus erythematosus (SLE) is 10 times more common in women, and MS twice as common. Evidence from animal models has shown that females are more resistant to infections and reject foreign skin grafts sooner than their male counterparts. This is especially true during periods of high estrogen availability. Estrogen levels decrease after ovulation or during pregnancy, and this is associated with a progesterone surge. The lowering of estrogen ensures immunological tolerance toward the sperm and subsequently toward the fetus. Therefore estrogen’s effects on the immune system may predispose women toward autoimmune diseases. This is reflected in experimental disease models of autoimmunity. Only female (NZB × NZW)F1 mice develop the SLE-like disease, and this is abrogated by androgen treatment. Similarly, in experimental autoimmune encephalomyelitis (EAE), an experimental model for MS, female SJL mice are more susceptible to disease induction and are protected with testosterone (Dalal et al., 1997). Preliminary studies testing the effectiveness of a testosterone gel in males with MS have shown encouraging results but require additional validation. Initial studies investigating estriol effects in women with MS have shown a potent effect on reduction of new lesion formation, evident on gadolinium-enhanced magnetic resonance imaging (MRI) (Sicotte et al., 2002).

Environmental Factors

Environmental factors may play a role in the pathogenesis of autoimmune diseases. Molecular mimicry is one of the mechanisms implicated. In this situation, an environmental antigen resembling a self-antigen elicits an immune response to both itself and the self-antigen. The environmental antigen involved in molecular mimicry may be a superantigen. Superantigens have the property of stimulating all T cells that express a given TCR variable gene family, regardless of their exact specificity, because of direct TCR-superantigen interaction. They are usually of bacterial or viral origin and bind as intact molecules to MHC.

In many cases of molecular mimicry, the environmental antigen is a pathogen, and autoimmune disease follows the pathogen-caused disease. The classic example of this is streptococcal-induced endocarditis. Neurological diseases caused by this mechanism include streptococcal-induced chorea, Gd1b axonal neuropathy, Semple rabies vaccine-induced encephalomyelitis, and the anti-Hu paraneoplastic syndrome. Several studies have demonstrated that both adult (Ascherio and Munger, 2007) and pediatric (Alotaibi et al., 2004; Banwell et al., 2007; Lunemann et al., 2008; Pohl et al., 2006) MS patients more frequently demonstrate evidence of a remote infection with Epstein-Barr virus (EBV) than controls, implicating a role for this virus in disease pathogenesis. Interestingly, epitopes of EBV resemble myelin basic protein (MBP), supporting a role for molecular mimicry in disease pathogenesis (Lang et al., 2002). Despite these associations, however, it is clear that the majority of persons are infected with EBV without autoimmune sequelae. Recent studies have integrated risk factors in the pathogenesis of MS and found that the relative risk of MS among DR15-positive women with elevated (>1 : 320) anti-EBNA-1 titers was ninefold higher than that of DR15-negative women with low (<1 : 80) anti-EBNA-1 titers (De Jager et al., 2008).

It is possible that once an inflammatory reaction proceeds, the tissue injury may expose other self-antigens that were previously unrecognized by the immune system. For unknown reasons, peripheral tolerance mechanisms may fail, and an autoimmune reaction ensues. The spreading of the autoimmune reaction from one antigen to another is termed determinant spreading or epitope spreading. Epitope spreading may play a role in perpetuating immune-mediated reactions and therefore in causing chronic diseases. Therapies that inhibit molecular mimicry or epitope spreading may be useful in preventing autoimmune diseases.

Multiple Sclerosis

Multiple sclerosis is a heterogeneous disease and is characterized by neurological deficits disseminated in time and space. It is a major cause of disability in the adult population in North America. Women are predominantly affected, in a ratio of 2 : 1. The disease is characterized by a varying array of neurological deficits. There are four main disease types, classified on the basis of the clinical disease course: relapsing-remitting (RR), secondary progressive (SP), primary progressive (PP), and progressive relapsing (PR). RR disease affects 65% of patients and is characterized by onset of neurological deficits that remit over a period of weeks to months. After 15 years, most RR patients go on to have an SP form of disease in which neurological deficits become fixed and accumulate. PP patients accumulate permanent neurological deficits from the onset of disease, whereas patients with PR disease have a combination of progressive and stepwise deficits. Disease onset generally occurs in the early 20s for RR disease and in the mid-30s for PP disease, although childhood-onset MS is becoming increasingly recognized.

MS is a complex polygenic disease. Monozygotic twins carry a concordance rate of 27%, whereas dizygotic twins of the same sex display a 2.3% concordance rate. The incidence for first-degree relatives of MS patients is 2% to 5%, whereas the incidence for the general population is under 0.1%. Genetic linkage studies have been performed, and several regions of interest have been found, but the most robust association remains with the HLA region. There is an increased incidence of MS in patients with the HLA-DR2 (DR1501) haplotype (Haines et al., 1998; Sawcer and Goodfellow, 1998; Stewart et al., 1981). More recently, a large genome-wide study identified single nucleotide polymorphism of IL-2R and IL-7R alleles as risk alleles for MS (Hafler et al., 2007). MS remains most prevalent among people of Northern European descent. There is a lower prevalence in other populations, such as Arabic and Mediterranean people, but among those with disease, there is a higher incidence of other disease-associated haplotypes such as DR4 and DR6.

Although genetic factors play an important role in pathogenesis, migration and other studies have demonstrated that environment also plays a critical role. Epidemiological studies have shown that residence in certain geographical areas and migration to these areas before the age of 15 increases the incidence of MS. In addition, there is a diminishing north-to-south gradient in MS prevalence in the Northern Hemisphere, with an opposite trend in the Southern Hemisphere. This led to the hypothesis and demonstration of an inverse association between sunlight exposure and MS (van der Mei et al., 2003).

An extension of this hypothesis has led to exploration of the role of vitamin D in MS, since vitamin D is metabolized in the skin by ultraviolet (UV) irradiation. A prospective study in army recruits found that 25-hydroxyvitamin D levels in the highest quintile (above 99.1 nmol/L) were associated with a lower risk of MS (odds ratio [OR], 0.38) (Munger et al., 2006). Treatment of animal models of MS with vitamin D ameliorates disease, and several studies have shown that the active form of vitamin D, calcitriol, can down-regulate proinflammatory dendritic cells (DC) and reduce T_H1 lymphocyte responses while promoting antiinflammatory T_H2 lymphocyte responses (Adorini et al., 2004; Griffin et al., 2001; Penna and Adorini, 2000; Penna et al., 2007). Studies exploring the therapeutic effects of vitamin D in MS are underway (Burton et al., 2010).

Pathologically, MS is characterized by inflammatory infiltrates in the CNS white matter, with resultant demyelination and axonal transections (Trapp et al., 1998) producing sclerotic plaques. Inflammation is generally perivenular, and lesions typically occur in the periventricular subcortical white matter, corpus callosum, optic nerve, brainstem, cerebellum, and spinal cord. Cortical lesions have also been described. The pathology of MS lesions has been classified into four distinct subtypes with the following predominant features: (1) cellular infiltration, (2) antibody deposition, (3) oligodendrocyte apoptosis, and (4) oligodendrocyte death without apoptosis (Lucchinetti et al., 2000). Observations to date show a single subtype of lesion in each patient, raising the possibility of distinct MS disease pathogenetic types. Activated microglia have been demonstrated in non-lesion, or otherwise normal-appearing white matter (NAWM) and may play a role in axonal damage and disease progression.

It is clear that the immune system plays a central role in mediating CNS damage in MS. Oligoclonal bands are commonly observed in the CNS of MS patients; however, the target of these antibodies has yet to be elucidated. Cell-mediated immunity, primarily involving T-helper cells, is believed to play an important role in initiating the disease, and immunological studies have substantiated the presence of activated T cells in MS. Most of the therapies for MS also target T cells. No clear autoantigen has been described in MS, and it therefore remains an immune-mediated disease rather than an autoimmune disease. Reactivity to various myelin antigens, including myelin basic protein (MBP), proteolipid protein (PLP), and myelin oligodendrocyte protein (MOG) has been investigated. MBP-reactive T cells are present in normal individuals; however, MS patients have a higher frequency of activated MBP-reactive T cells in the peripheral blood and the cerebrospinal fluid (CSF) (Zhang et al., 1994).

The immunopathogenesis of MS is thought to involve activation of myelin-specific T cells via molecular mimicry or by a superantigen presumably in the periphery. These cells are believed to predominantly be of the T_H1 phenotype, as evidenced by increased production of IL-12 (the major inducer of T_H1 cytokines) by APCs in the peripheral blood of MS patients with active disease, and the observation that a clinical trial of IFN-γ worsened MS. These cells then cross the BBB and get reactivated in the CNS when they are presented with their cognate antigen. Perivascular dendritic-like cells have been shown to play a role in T-cell reactivation in animal models of MS (Greter et al., 2005), but their role in the human disease is unclear. Adhesion molecules and their ligands are expressed on T cells and endothelial cells to facilitate passage through the BBB. VCAM-1 and its ligand, VLA-4, are up-regulated in T cells during chronic disease, thus perpetuating inflammation, and VLA-4 antibody therapy (natalizumab) has recently been shown to be effective in reducing MS relapses and lesion formation.

Invasion of activated T cells into the brain and reactivation within the CNS initiates a cascade of cytokines. IL-2, IFN-γ, and TNF-α activate macrophages, which in turn elicit nitric oxide and TNF-α. In experimental models, myelin damage is mediated by nitric oxide lipid peroxidation, direct TNF-α damage, and complement-induced pore formation. As mentioned previously, T_H2 cytokines can induce B-cell activation and antibody production that further damage myelin. Each of these steps in the pathogenesis may be targeted for therapeutic intervention.

There is mounting evidence indicating that T cells play a key role in the relapsing-remitting phase of MS. However, recent work has demonstrated that disease progression in MS may be due to distinct mechanisms (Weiner, 2009). Epitope spreading may occur within the CNS, and in animal models can be facilitated by microglia, macrophages, and dendritic cells (McMahon et al., 2005). Activated microglia are found in progressive forms of the disease and have been associated with axonal damage and demyelination (Kutzelnigg et al., 2005). B-cell follicles have been demonstrated at autopsy in patients with chronic MS (Serafini et al., 2004) and may play a role in facilitating an autonomous inflammatory response within the CNS. Cortical demyelination is also associated with progressive forms of MS (Kutzelnigg et al., 2005).

Neuromyelitis optica (NMO), or Devic disease, is a rare subtype of MS characterized by clinical episodes of optic neuritis and transverse myelitis and demonstration of contiguous lesions in the spinal cord (Wingerchuk et al., 2006). The presence of serum antibodies targeting the aquaporin-4 water channel present on the surface of the glia limitans at the BBB has been shown to be a sensitive and specific marker of NMO (Lennon et al., 2004). Injection of aquaporin-4 antibodies into animal models of disease have demonstrated enhanced complement deposition around blood vessels, loss of aquaporin-4, and astrocyte and myelin damage (Kutzelnigg et al., 2005). This study, as well as others, indicate increasing recognition of the role of glial pathology in MS.

A number of immune-mediated autoimmune disorders of the nervous system are available for study in laboratory animals. Besides allowing for the analysis of the immunoregulatory network, they have been critical in designing immunotherapies. Two major animal models exist that mimic the clinical manifestations of MS. These are experimental autoimmune encephalomyelitis (EAE) and Theiler murine encephalomyelitis virus-induced disease (TMEV-IDD). Although animal models are not identical to the human disease, each model may represent certain pathogenic aspects of MS.

EAE is a T cell–mediated autoimmune demyelinating disease of the CNS. The disease can be induced in a number of experimental laboratory animals, including primates, by subcutaneous injection of whole-brain homogenate or of a purified preparation of MBP, PLP, or MOG emulsified in adjuvant. By altering the immunization protocols and animal strains, a relapsing-remitting or a chronic form of the disease may be induced. Various EAE models have demonstrated that mice with a T_H1 cytokine profile are more susceptible to EAE than wild-type mice, whereas animals with a T_H2 (IL-4, IL-10) cytokine profile are generally resistant to disease development (Chitnis et al., 2001). Transfer of T_H17-producing cells induces a more severe form of EAE than transfer of T_H1 cells (Jager et al., 2009); the role of these cells in MS is starting to be elucidated.

TMEV-IDD is induced by injecting TMEV picornavirus into the cerebral hemisphere. The virus infects neurons and glial cells. In some strains of mice, the host is unable to clear the virus, resulting in encephalitis and death; in other cases, the virus is cleared completely, and the host is resistant to demyelination. In TMEV-IDD-susceptible strains of mice, the virus is partially cleared, saving the host from death but inciting an immune response that results in demyelination. Thus the damage induced by the virus is due to a failure of the host to mount a fully protective response, which predisposes the pathogen to persistence, resulting in immunopathology. TMEV-IDD is a T cell–mediated disease; although the exact immunological mechanisms inducing demyelination remain unclear, damage may be a result of epitope spreading (Miller et al., 1997). Studies in both EAE and TMEV have contributed vastly to our understanding of MS, and these models offer a system for testing new therapeutics.

Details of available therapies in MS are discussed in Chapter 60. Here, we discuss the immunological mechanisms of currently used medications, as well as experimental therapeutic strategies.

β-Interferon (IFN-β) therapy for MS is one of the most important advances in the treatment of this disease. It is available in three different forms: subcutaneous IFN-β-1b (Betaseron), subcutaneous IFN-β-1a (Rebif), or intramuscular IFN-β-1a (Avonex). Interferons have many properties, including suppressing proliferation of viruses and T cells. The mechanisms of IFN-β action in MS has been attributed to several different mechanisms. Interferon-associated increased production of IL-10 by macrophages down-regulates the number of T_H1 cells. IFN-β has also been shown to decrease production of IL-12 by dendritic cells, potential CNS APCs, further inhibiting T_H1 cell formation. In addition, IFN-β modulates adhesion molecule expression, primarily by facilitating the conversion of cell-associated VCAM-1 into soluble VCAM-1. These drugs also down-regulate costimulatory molecule expression, thus decreasing T-cell activation and migration to the CNS (Yong, 2002).

Glatiramer acetate (GA), also known as copolymer-1 or Copaxone, is another class of drug used in the treatment of MS. In contrast to IFN-β, GA is a synthetic molecule that was originally designed to resemble MBP. It is composed of random repetitive sequences of the amino acids glutamic acid, lysine, alanine, and tyrosine (G-L-A-T). Its mechanism of action is unclear; however, it is thought to bind with high affinity to the MHC groove, leading to the generation of GA-specific T cells. Several studies have suggested that GA-specific T cells display a T_H2 bias (Duda et al., 2000), and animal models have demonstrated that these cells can migrate to the CNS and ameliorate EAE disease through a local down-regulation of the immune responses (Aharoni et al., 1997). It has also been demonstrated that GA-specific T cells protect from optic nerve crush injuries, possibly mediated by the production of brain-derived neurotrophic factor (BDNF) (Kipnis et al., 2000).

Altered peptide ligands (APL) resembling MBP have been used in phase 1 trials for MS with little success. Two concurrent trials were initiated using the same compound, CGP77116. One showed an increased number of lesions on MRI in some patients after the initiation of treatment (Bielekova et al., 2000). In the other trial, 9% of patients developed allergic-type reactions associated with a T_H2 deviation (Kappos et al., 2000). Both trials were stopped because of safety concerns.

Several new therapies for the treatment of MS are monoclonal antibodies targeting specific receptors on immune cell populations. Natalizumab, an α4β1-integrin antibody (Tysabri), is approved for the treatment of relapsing forms of MS and effectively blocks T- and B-cell migration across the BBB. However, blockade of CNS immune surveillance has produced profound adverse effects, as evidenced by the development of progressive multifocal leukoencephalopathy (PML) with a frequency of approximately 1 : 1000. The experience with Tysabri has highlighted the importance of balancing potential adverse effects such as infection and tumor with therapeutic efficacy.

Campath-1H targets the CD52 receptor present on lymphocytes and monocytes. Phase 2 studies have shown potent effects in MS, with depletion of peripheral lymphocytes. However, a quarter of treated patients develop autoimmune thyroid disease, and rarely immune thrombocytopenic purpura, suggesting that Campath enhances immune dysregulation in a subset of patients (Coles et al., 2008; Jones et al., 2009). Daclizumab blocks the IL-2R α chain (CD25) present in the high-affinity IL-2 receptor on T cells, thus inhibiting T-cell replication and making more IL-2 available to the low-affinity CD25 receptor present on NK cells, which induces a regulatory NK cell population (Bielekova et al., 2006).

Rituximab, an antibody that primarily targets activated B cells, has recently been shown to reduce disease activity in relapsing-remitting (Hauser et al., 2008) and a subset of primary progressive MS patients (Hawker et al., 2009). Interestingly, open-label studies have suggested that rituximab treatment is effective in patients with neuromyelitis optica, which is increasingly thought of as an antibody-mediated disease. Another therapy currently under investigation is CTLA4Ig, which blocks B7-CD28 costimulatory signals on T cells and may induce T-cell anergy in vivo.

Many therapeutic strategies that have been used in the past nonspecifically target components of the immune response. Nonspecific strategies include cyclophosphamide, mitoxantrone, and cladribine, which depress bone marrow production of cells, including T cells. Cyclophosphamide may also function by inducing a cytokine switch, with a decrease in IL-12 and an increase in IL-4, IL-5, and TGF-β (Comabella et al., 1998). A novel strategy to suppress immune responses in MS is a small molecule, fingolimod, which targets the sphingosine-1-phosphate receptor, which is necessary for lymphocyte egress from lymph nodes (Comi et al., 2010).

Acute Disseminated Encephalomyelitis

Acute disseminated encephalomyelitis (ADEM) is a monophasic demyelinating disease associated with vaccination or a systemic viral infection; it can affect both adults and children. ADEM was originally described in association with rabies and smallpox vaccines, both of which were prepared with neural tissues, suggesting a parallel with EAE, the animal model of MS. These vaccines have since been modified, using non-neural human diploid cell lines. ADEM has not been associated with any vaccines that are currently administered in the United States. The parainfectious variant of ADEM has been associated with measles infection, rubella, mumps, and several other viruses. Viral or bacterial epitopes resembling myelin antigens have the capacity to activate myelin-reactive T-cell clones through molecular mimicry (Wucherpfennig, Hafler, and Strominger, 1995) and can thereby elicit a CNS-specific autoimmune response. Thus, it has been suggested that microbial infections elicit a cross-reactive antimyelin response through molecular mimicry, resulting in ADEM. Myelin peptides have been shown to resemble several viral sequences, and in some cases, cross-reactive T-cell responses have been demonstrated. MBP is the prototypical inducer of EAE, but other myelin proteins like MOG and PLP have also been extensively studied. Examples of cross-reactive T cells with MBP antigens include HHV-6 (Tejada-Simon et al., 2003), coronavirus (Talbot et al., 1996), influenza virus hemagglutinin (Markovic-Plese et al., 2005), and EBV (Lang et al., 2002). PLP shares common sequences with Haemophilus influenzae (Olson et al., 2001). Semliki Forest virus (SFV) peptides mimic MOG (Mokhtarian et al., 1999).

The lesions in ADEM resemble those of MS. The CNS white matter contains perivascular inflammatory infiltrates as well as demyelination. The most likely mechanism by which this disease occurs is molecular mimicry. Experimental evidence has shown that T cells isolated from patients with ADEM are 10 times more likely to react with MBP than controls, likening this disease to EAE in animal models (Pohl-Koppe et al., 1998). We have recently found that 30% of patients with ADEM demonstrate serum antibodies to MOG, which were absent in MS patients (O’Connor K et al., 2007). Because of the monophasic nature of ADEM, it appears that the immunological response occurs acutely, but in contrast to MS, further amplification of inflammation within the CNS is suppressed.

MRI demonstrates multifocal white matter lesions involving the cerebrum, brainstem, cerebellum, and spinal cord, which may or may not enhance with gadolinium. Lesions generally resolve over time. CSF is characterized by normal pressure, moderately elevated cell count (5–100/µL), moderately elevated protein (40–100 mg/dL), and normal glucose. The presence of red blood cells may indicate a diagnosis of hemorrhagic leukoencephalitis. Oligoclonal bands may very rarely be present, and these cases should be followed for the development of MS.

Acute episodes of ADEM should be treated with intravenous corticosteroids. The usual dose is 1 g/day of methylprednisolone for 5 days in adults. Refractory cases have been treated with plasmapheresis or cyclophosphamide. Cases that are suspicious for MS should be followed with MRI.

Immune-Mediated Neuropathies

The immune-mediated neuropathies are a large and heterogeneous group of diseases. We shall focus on acute inflammatory demyelinating polyneuropathy (AIDP) and chronic inflammatory demyelinating polyneuropathy (CIDP), which may be defined by the time to peak disability; in the former, 4 weeks, and in the latter, 2 months. Although AIDP and CIDP share many characteristics, the question of whether one is a continuum of the other is still under debate. AIDP or Guillain-Barré syndrome (GBS) usually presents with symmetrical ascending weakness and may be associated with autonomic dysfunction and respiratory depression. Sensory systems may be involved and may present with paresthesias or numbness. Demyelination and axonal damage may be involved to varying degrees. If the patient’s symptoms continue to progress beyond 4 weeks, the illness is termed CIDP.

AIDP is the most common acute paralytic disease in the Western world, with a mean annual incidence of 1.8 per 100,000 persons. There is an increasing incidence with age. Mortality was generally due to respiratory failure and has now been significantly reduced with the introduction of positive-pressure ventilation. Epidemics have been found, most notably in northern China, where a high incidence has been associated with C. jejuni infections (McKhann et al., 1993).

AIDP or GBS is characterized pathologically by an endoneurial lymphocytic, monocytic, and macrophage infiltrate. Several autoantibodies to myelin glycolipids have been identified, including GM1, GD1a, and GD1b. Antibody-mediated demyelination due to complement fixation has been identified in pathology specimens. In some cases, axonal damage is present and is believed to be a result of bystander damage. Activation of calcium-dependent processes within the nerve, including calpain activation, has been shown in animal models to augment axonal degeneration (O’Hanlon et al., 2003). GBS is primarily an antibody-mediated disease, as evidenced by the fact that many patients improve after treatment with plasmapheresis, and that serum from GBS patients causes demyelination after transfer into experimental animals and peripheral nerve cultures. The Miller-Fisher variant of GBS is characterized by ophthalmoplegia, ataxia, and areflexia and is associated with the presence of GQ1b antibodies in the serum.

The occurrence of AIDP has been linked to many infectious diseases, including C. jejuni, herpesvirus, Mycoplasma pneumonia, and many other bacterial and viral infections, as well as vaccinations. The incidence of infection has been reported to be 90% in the 30 days before occurrence of GBS. C. jejuni is one of the most commonly identifiable agents, and molecular mimicry and host susceptibility play a role in disease pathogenesis. Autoantibodies not present in controls have been identified in the sera of GBS patients associated with C. jejuni, including autoantibodies to the gangliosides GM1, GD1a, GD1b, and GQ1b (Sheikh et al., 1998).

In contrast to AIDP, in CIDP no specific autoantibodies have yet been discovered. The histopathological picture is similar to AIDP; however, most studies identify fewer inflammatory infiltrates. Nerve biopsy reveals mixed demyelination and axonal changes. Onion bulbs may be present, indicating attempts at remyelination. There is little laboratory evidence that this disease is antibody mediated, but paradoxically, patients do improve with plasmapheresis. There is indirect evidence that CIDP is T-cell mediated; however, this area is still under investigation.

Treatment of AIDP involves supportive care and cardiac and respiratory monitoring. Plasmapheresis or intravenous immunoglobulin (IVIG) have been used for acute treatment of AIDP and have been shown to be equally effective in shortening recovery time. Plasmapheresis is a short-term immunotherapy that nonspecifically removes antibodies from the circulation. IVIG is an immunomodulating agent commonly used in the treatment of allergic and autoimmune diseases. It works in part through the presence of Fc fragments that interact with the inhibitory Fc receptor, FcγRIIB, which is also induced on macrophages following IVIG administration (Samuelsson et al., 2001). Additionally, IVIG may displace low-affinity autoantibodies from the nerve. High-dose steroids have not been found to be effective in AIDP.

In contrast to AIDP, CIDP responds well to high-dose oral corticosteroids. Both plasmapheresis and IVIG are also used with success. Immunosuppressants such as cyclosporine A, cyclophosphamide, and azathioprine have had positive outcomes in refractory cases but require further testing in controlled studies. Future therapies for AIDP or CIDP may target complement activation or inhibition of axonal calpain activation.

Autoimmune Myasthenia Gravis

Myasthenia gravis is a disorder of the neuromuscular junction. It is an autoimmune disorder, and 80% to 90% of cases have detectable autoantibodies to the α subunit of the acetylcholine receptor (AChR). MG is characterized by fluctuating weakness and fatigability, primarily in muscles innervated by the cranial nerves, but may occur in skeletal and respiratory muscles. MG has a biphasic age distribution. Most cases occur in women between the ages of 20 to 40 years, the remainder in older patients, with an equal sex distribution. Thymomas occur in 10% to 15% of cases; most are in the older age group. Some 75% of patients will have a thymic abnormality, 85% being thymic hyperplasia. MG is often associated with other autoimmune diseases, thyroid disorders, rheumatoid arthritis (RA), pernicious anemia, and SLE. A similar syndrome, Lambert-Eaton, is associated with antibodies against the presynaptic voltage-gated calcium channel, generally in the setting of small cell cancer of the lung.

Autoimmune MG is caused by the presence of α₁ nicotinic acetylcholine receptor (nAChR) antibodies and is a B cell–mediated disease. Eighty percent to 90% of patients have detectable autoantibodies. These are polyclonal and may be of any IgG subtype. Transfer of serum from myasthenic patients to experimental animals results in neuromuscular blockade. The mechanism by which antibody mediates neurological symptoms is controversial. Possible mechanisms include neuromuscular blockade or damage to the AChR from complement-mediated damage after attachment of the IgG antibody. There is, however, poor correlation between serum antibody titers and disease course and severity.

Although the B cell is the effector cell producing antibodies, experimental evidence has shown that autoreactive T cells are necessary for the disease to occur (Yi and Lefvert, 1994). Removal of the thymus results in improvement of disease in 80% to 90% of myasthenic patients. The role of thymic abnormalities remains unclear, and patients with thymomas have antibodies to additional skeletal muscle proteins such as the ryanodine receptor and titin, as well as the neuromuscular junctional protein, MuSK. Patients may also display symptoms of neuromyotonia. Antibodies directed towards α₃-nAChR are associated with autoimmune autonomic neuropathy.

A large body of research is targeted at understanding the reasons for the failure of T-cell and subsequent failure of B-cell tolerance in MG. Both normal and myasthenic thymus glands contain myoid cells and epithelial cells that express the AChR. T cells expressing the V_β5.1⁺ TCRs are overrepresented, both in the core of germinal centers and in perifollicular areas of hyperplastic thymuses, suggesting a role in the autoimmune response (Truffault et al., 1997). Failure of central or thymic tolerance may play an important role in disease pathogenesis.

Genetic factors play a role in the pathogenesis of autoimmune MG, but monozygotic twins demonstrate less than 50% concordance rate. There is a moderate association of MG with the HLA antigens B8 and DRw3 in young women. The stronger association with HLA-DQw2 remains controversial. There is an unusually high incidence of other autoimmune diseases such as SLE, RA, and thyroid diseases in first-degree relatives of myasthenic patients, suggesting the presence of shared autoimmune genes.

Therapies in MG are targeted toward alleviating symptoms with acetylcholinesterase inhibitors and using strategies to reduce the damage being done by the immune system. Thymectomy is recommended for patients 15 to 65 years old, with 80% to 90% remission rate (Durelli et al., 1991). The thymus plays an important role in T-cell education in the developing human; therefore prepubertal thymectomy is discouraged. A variety of anticholinesterase inhibitors provide temporary symptomatic relief in most patients. Pyridostigmine bromide (Mestinon) and neostigmine bromide (Prostigmin) are the most commonly used agents and must be taken daily.

MG is an antibody-mediated disease and therefore responds to therapies that nonspecifically target antibodies. Both plasmapheresis and treatment with IVIG are used for acute MG exacerbations or in preparation for surgery (Gajdos et al., 1997). Because the autoantigen is known in MG, investigational therapies may target specific molecules such as the B-cell surface Ig or the TCR and deliver immunotoxins.

Immunosuppressives such as cyclosporine, azathioprine, and mycophenolate are used to augment treatment when symptoms are not adequately controlled by the previously mentioned methods, but the decision to use such agents must balance the need and the side effects. Corticosteroids are used at various stages of treatment and have multiple effects on the immune system, including reducing AChR antibody levels.

Inflammatory Muscle Diseases

Polymyositis (PM), dermatomyositis (DM), and inclusion body myositis (IBM) are all inflammatory and presumably immune-mediated diseases of the muscle and the surrounding connective tissue. Each has its own unique clinical and immunohistological features. Both PM and DM are more common in females, whereas IBM is more common in males. DM in adults is associated with an increased risk of cancer, and therefore a full cancer screening should be part of patient management.

PM is thought to result from a multitude of causes, including systemic autoimmune connective-tissue disorders and viral and bacterial infections. PM is characterized histopathologically by an endomysial inflammatory infiltrate containing predominantly CD8⁺ T cells. There is relative sparing of blood vessels. In one subtype of PM, T cells with γδ receptors have been identified surrounding non-necrotic muscle fibers (Hohlfeld et al., 1991).

In contrast, DM is characterized by perifascicular atrophy. There is hypoperfusion and subsequent degeneration of the muscle fibers in the periphery of the fascicle, secondary to microvascular damage. Damage to capillaries, resulting in muscle fiber ischemia, is mediated by complement. Immunofluorescence studies have revealed immune-complex deposition within the endothelium, indicating that this is an antibody-mediated disease; therefore the disease differs from PM (Kissel et al., 1986).

As with PM, IBM is mediated by CD8⁺ T cells. However, in contrast to PM, the muscle biopsy in IBM may also demonstrate the presence of characteristic autophagic “rimmed” vacuoles. Amyloid deposits may be demonstrated in the muscle, similar to those seen in AD, suggesting similarities in pathogenesis of these two diseases (Askanas et al., 1992).

Various autoantibodies directed against nuclear and cytoplasmic cell components are found in up to 30% of inflammatory myopathies. Most are nonspecific for connective-tissue disease. Viruses including coxsackie B are implicated in the pathogenesis of disease, and both PM and DM patients may have anti-Jo-1 antibodies to the viral enzyme, histidyl-tRNA synthetase (Mathews and Bernstein 1983). More recently, the presence of B cells, and in particular antibody-secreting plasma cells with V-D-J rearrangements, has been demonstrated in muscle biopsies from both IBM and PM and to a lesser extent in DM (Greenberg et al., 2005).

The mainstay of treatment of PM and DM is corticosteroids. Dosages may vary from 60 to 100 mg/day of prednisone, and duration is determined by clinical outcome. Alternative treatment options for the inflammatory myositis diseases include IVIG, methotrexate, azathioprine, cyclophosphamide, cyclosporine, and in extreme cases, total lymphoid or whole-body irradiation (Mastaglia et al., 1998). Mortality rates vary between 15% and 35% and are generally due to cardiac or respiratory failure. Because there is a higher incidence of malignancy with PM and DM, screening for breast, lung, hematological, ovary, stomach, and colon carcinoma should be performed on patients with these diagnoses. IBM may be more resistant to corticosteroid therapy and is often diagnosed after an assumed PM fails to respond to treatment.

Alzheimer Disease and Amyotrophic Lateral Sclerosis

It may seem strange to include AD and ALS in a chapter on neuroimmunology. These diseases have traditionally been considered neurodegenerative, but recent studies and therapies have suggested a role for the immune system in disease pathogenesis and protection.

β-Amyloid plaques and neurofibrillary tangles consisting of hyperphosphorylated tau protein are the hallmarks of AD. Clearance of amyloid plaques consisting of amyloid-β-fibrils is considered a primary goal of therapy. Amyloid plaques are often surrounded by activated microglia and reactive astrocytes, and are associated with complement activation, leading to the hypothesis that the immune response participates in the clearance of amyloid deposits. Further studies in animal models of AD demonstrated that immunization with amyloid-β peptide resulted in the induction of amyloid-β-specific antibodies, which enhanced the clearance of amyloid plaques (Janus et al., 2000; Morgan et al., 2000). Passive transfer of amyloid-β-specific antibodies yielded similar results. Amyloid plaque clearance is believed to occur through either microglial- and complement-mediated clearance or through direct antibody-amyloid interactions. These studies led to a clinical trial investigating an amyloid-β vaccine administered in conjunction with an adjuvant, which enhanced T_H1 responses. Although cognitive testing results were favorable, 6% of patients developed meningoencephalitis, which is generally believed to be a result of T-cell responses to amyloid-β (Gilman et al., 2005; Orgogozo et al., 2003). Thus, induction of an antibody and microglial-mediated clearance of amyloid in the absence of a prominent T_H1 response is the current goal of therapy. An intriguing study has recently demonstrated that nasal administration of glatiramer acetate (Copaxone), which induces a predominantly T_H2 cellular response, to a murine model of AD resulted in the clearance of amyloid-β plaques in association with activated microglia, but in the absence of antibody formation (Frenkel et al., 2005). Future immunotherapeutic strategies for AD include modified vaccines and strategies to induce activation of microglial cells in the absence of deleterious side effects.

In ALS, a new avenue of investigation has emerged: exploring the role of microglia in disease pathogenesis and, in particular, on disease progression. Pathological analysis and neuroimaging using positron electron tomography (PET) studies have demonstrated activated microglia in areas of severe motor neuron loss. Studies in the animal model of ALS have demonstrated that the presence of the SOD1 mutation in microglia enhanced disease progression (Boillee et al., 2006). Use of minocycline, which acts in part by inhibiting microglial activation, has shown some initial promise in the treatment of ALS. Cyclooxygenase-2 (COX-2) inhibitors have reduced disease severity in animal models of ALS but have been ineffective in humans. Lymphocytes, including T cells, do not appear to play a significant role in this disease, and this is reinforced by the failure of studies using T cell–targeted therapies including total lymphoid irradiation (TLI) or cyclophosphamide. Thus, possible future therapies for ALS include strategies to down-regulate microglial activation.

Paraneoplastic Syndromes

Neurological paraneoplastic disorders are defined as neurological syndromes arising in association with a distant cancer. These are mediated by antibodies produced by the immune system in reaction to a tumor antigen, which cross-react with neural tissue. It is likely that aberrant, primitive, or hamartomatous antigens are expressed by the tumor cells. Enhanced cellular infiltrates are found in tumors associated with paraneoplastic syndrome compared to those not associated with a paraneoplastic syndrome. Therefore one can postulate that the immune system is more active in these situations. Cancers associated with paraneoplastic syndromes are generally associated with a better outcome. Several autoantibodies associated with paraneoplastic syndromes have been identified. The anti-Hu antibody arises in association with small-cell cancer of the lung and cross-reacts with neurons; it is linked to a syndrome of encephalomyelitis and/or sensory neuropathy. Similarly, anti-Yo antibody produces cerebellar degeneration due to cross-reactivity with Purkinje cell cytoplasm and is associated with breast and ovarian cancer. Opsoclonus-myoclonus syndrome is associated with anti-Ri antibody and is found in cases of cancer of the ovary and breast. Cases of paraneoplastic and immune-mediated brainstem and limbic encephalitis have recently been reported to be caused by antibodies targeting LGI1, a neuronal-secreted protein that interacts with presynaptic ADAM23 and postsynaptic ADAM22 (Lai et al., 2010). NMDA receptor encephalitis typically presents with seizures and psychiatric symptoms, but a recent case report describes a patient presenting with optic neuritis and transverse myelitis mimicking neuromyelitis optica (Kruer et al., 2010). Antibodies directed against VGKC are also associated with acquired neuromyotonia, or Isaac syndrome, and Morvan syndrome, characterized by neuromyotonia and insomnia. Lambert-Eaton myasthenic syndrome is caused by antibodies directed against the P/Q type of voltage-gated calcium channels and is generally found in the setting of small-cell cancer of the lung. The same antibodies have also been associated with cases of lung cancer–associated cerebellar ataxia. Stiff person syndrome is associated with antibodies to glutamic acid decarboxylase (GAD), and both autoimmune and paraneoplastic forms, principally associated with breast cancer, have been described.