Implications/Analysis of Family History

Richard’s clinical history combined with a number of healthy sisters and evidence for the death of an uncle from bacterial pneumonia is highly suggestive of an X-linked congenital defect in immune functioning.

Implications/Analysis of Clinical History

A clinical history that includes chronic sinusitis and recurrent diarrhea caused by protozoa or bacteria, as well as hospitalization with bacterial pneumonia in a male infant suggests an X-linked immunodeficiency disorder. Defects in B cells, phagocytes, or complement could lead to some of these symptoms. However, sinopulmonary infections are highly suggestive of a deficiency in IgA. Therefore, laboratory investigations should focus initially on total serum immunoglobulin levels and levels of various antibody subclasses.

Implications/Analysis of Laboratory Investigation

Richard’s initial blood work consisting of a complete blood cell count and differential was normal so this is not a problem in T cell or B cell development (see Case 1). Serum immunoglobulin measures (see Case 3) indicated little to no IgG, IgA, or IgE, but IgM was present in higher concentrations than the normal reference value, suggesting that there is a problem in isotype switching. The normal numbers of B cells and the high concentrations of IgM rule out X-linked (Bruton’s) agammaglobulinemia (XLA) as a possible diagnosis.

B cell activation leading to isotype switching requires T cell–derived cytokines as well as cognate interaction with activated T cells (Fig. 4-1A). As such, a deficiency or defect in T cells could account for the observed levels of IgG, IgA, and IgE. However, we are told that T cell function is normal. In light of normal T cell function, the molecular interactions between B and T cells should be investigated.

FIGURE 4-1 B cell activation in response to T cell–independent antigen. A, B cell activation to T cell–independent antigens requires cognate interaction with T cells, as well as T cell cytokines. This leads to isotype switching, differentiation to plasma cells, or memory cells. A required signal for isotype switching is CD40 (on B cells) with CD154 (on T cells). B, T cells do not express CD154, and so isotype switching does not occur (hyper IgM syndrome).

(A, Modified with permission from Gorczynski R, Stanley J: Clinical Immunology. Austin, Landes, 1999.)

Additional Laboratory Tests

Flow cytometric analysis of cell surface markers on T and B cells revealed normal expression of CD40 on B cells but no detectable expression of CD154 (CD40 ligand) on activated T cells.

Role of CD28 and CD152

CD80 and CD86 (formerly known as B7-1 and B7-2) are expressed on antigen-presenting cells, including dendritic cells, macrophages, and B cells (see Fig. 4-2A). They are both ligands for CD28, which is constitutively expressed on T cells, and CD152 (formerly CTLA-4), which is induced after T cell activation. It has long been recognized that T cell activation requires at least two signals, the first being delivered via the T cell antigen receptor and the second via CD28, whose interaction with CD80/86 triggers a signal transduction cascade that results in the stabilization of mRNA for IL-2, a growth factor for T cells. CD152 is induced on activated T cells and probably plays a more important role in downregulating the T cell response after binding to the same ligands CD80 and CD86. Because CD80 and CD86 have a higher affinity for CD152, the inhibitory signal plays the dominant role once CD152 is expressed on the cell surface.

FIGURE 4-2 Interaction of T cells and B cells. A, Interaction of CD80/86 (B cells) with CD28 leads to T cell activation, whereas interaction with CD152 downregulates activated T cells. B, ICOS-B7h interaction upregulates CD154, which is a ligand for CD40. Activated cells express the inducible co-stimulator (ICOS), which is a ligand for B7h on B cells. This interaction leads to the upregulation of CD154 (CD40L) on T cells, which is a ligand for CD40 on B cells.

Inducible Co-stimulator and CD154 Expression

Studies with knockout mice for the ICOS (inducible co-stimulator) gene also show profound defects in isotype switching after B cell activation. ICOS is expressed only on the surface of activated T cells and has been shown to upregulate CD154 on T cells following ICOS-B7h interaction (Fig. 4-2B). B7h is constitutively expressed on naive B cells and is a newly discovered member of the B7 (CD80/86) family. This defect in isotype switching is reversed when anti-CD40 antibodies are used to stimulate CD40 directly (instead of binding CD154). Cytokine secretion is also affected in the ICOS knockout mice. T cells from such animals show increased production of IL-2 and interferon gamma (IFNγ) but a decrease in IL-4 and IL-10 after immune stimulation. IL-4 is required for isotype switching to IgE. Note: The reader should beware of some potential confusion caused by redundant nomenclature. B7h (the ligand for ICOS) has also been named B7RP-1, GL-50, ICOSL, and LICOS. The inducible co-stimulator (ICOS) is also known as activation-inducible lymphocyte immunomodulator (AILIM).

Autosomal Recessive Forms of Hyper IgM

Although hyper IgM type I is the most common form of this disorder, autosomal recessive forms have been identified. The autosomal recessive forms of this disease, hyper IgM types II and III, are caused by mutations in the AID (activation-induced cytidine deaminase) gene and CD40, respectively. Both class switch recombination and somatic hypermutation (somatic mutation) require the action of the AID enzyme, which deaminates cytidine to uridine. More recently, a recessive mutation in the gene encoding uracil-DNA-glycosylase (UNG), an enzyme in the base-excision repair pathway for removal of uracils occurring in DNA, has been identified as also causing hyper IgM syndrome.

It is becoming increasingly clear that mutations can occur in several genes that encode proteins acting at different stages of class switching and that all such mutations can produce similar clinical manifestations (phenotype), although the genotypes are distinct. One model that has been put forth by Durandy and Honjo (see Further Reading) is that AID deaminates cytosine in the immunoglobulin switch region (DNA), thereby forming uracil. However, the uracil, which should only be present in RNA, must be removed and this is mediated by UNG. As such, mutations in either AID or UNG enzymes would lead to the same clinical picture.