NK cells are important in combating herpes virus infections

The NK response to murine cytomegalovirus (MCMV) is especially well-characterized (Fig. 13.w1) and has given important insights into virus-NK interactions.

Fig. 13.w1 The NK cell response to MCMV in C57BL/6 mice

In uninfected mice, host cells express ligands (e.g. MHC class I) that engage inhibitory receptors on NK cells and prevent NK activation. When mice are infected with murine cytomegalovirus (MCMV), production of innate cytokines such as IL-12, IL-15, and IL-18 is upregulated, which drives nonspecific NK cell activation and proliferation. MHC class I expression is actively reduced on MCMV-infected cells and viral proteins including m157 are expressed on the cell surface. NK cells expressing the activating receptor Ly49H⁽⁺⁾ are activated (emboldened cell surface) on contact with infected cells, as few of their inhibitory receptors are engaged but they receive activating signals, most importantly as a consequence of Ly49H binding to m157. This results in triggering of NK effector functions (leading to lysis of the infected cell) and also drives enhanced activation and proliferation of the Ly49H-expressing NK population. After the infection has been cleared, Ly49H-expressing NK cells retain an enhanced ability to combat virus replication if reinfection occurs.

In the early stages of MCMV infection NK cells respond to locally-produced innate cytokines by undergoing non-specific activation and proliferation. In C57BL/6 mice, specific recognition of MCMV-infected cells is mediated by NK cells expressing the activating NK receptor Ly49H, which interacts with the MCMV protein m157 on infected cells. This results in a clonal expansion of m157-specific NK cells, which play a critical role in controlling virus replication. Mouse strains which lack Ly49H or other activating receptors that can specifically recognize MCMV are highly susceptible to infection with this virus. Notably, passage of MCMV in C57BL/6 mice leads to selection of viruses bearing mutations in the m157 gene, the replication of which is not well-controlled in these mice.

If the m157 protein of MCMV targets infected cells for recognition by NK cells, why does the virus express this protein? Although m157 enables mouse strains expressing the activating NK receptor Ly49H to recognize and destroy MCMV-infected cells, certain other inbred mouse strains express an inhibitory receptor, Ly49I, which also binds to m157 and protects the virus by inhibiting NK cell responses in these animals.

The m157-driven activation and expansion of Ly49H-expressing NK cells in C57BL/6 mice bears similarities to clonal expansion of antigen-specific T cells. Following virus clearance, the frequency of Ly49H-expressing NK cells declines – but the remaining population of cells retains the capacity to mediate a more efficient response on secondary exposure to MCMV, reminiscent of the heightened responsiveness of memory T cells. These recent findings of antigen specificity and memory (defining features of adaptive responses) in NK populations suggest that there can be overlap between features of the innate and adaptive response during virus infections – nonetheless the rapid response to naive NK cells to infection and multiple ‘generic’ mechanisms for NK activation demonstrate that their principal role is as an innate effector subset.

Antibodies can neutralize the infectivity of viruses

Antibodies provide a major barrier to virus spread between cells and tissues and are particularly important in restricting virus spread in the bloodstream. IgA production becomes focused at mucosal surfaces where it serves to prevent reinfection.

An important mechanism of IgA-mediated neutralization occurs intracellularly as IgA passes from the luminal to the apical surface of the cell. During this transcytosis vesicles containing IgA interact with those containing virus, leading to neutralization.

Antibodies may be generated against any viral protein in the infected cell.

Defense against free virus particles involves neutralization of infectivity, which can occur in various ways (Fig. 13.4). Such mechanisms are likely to operate in vivo because injection of neutralizing monoclonal antibodies is highly effective at inhibiting virus replication. The presence of circulating virus-neutralizing antibodies is an important factor in the prevention of reinfection. Passively-administered monoclonal antibodies have been used therapeutically to inhibit respiratory syncytial virus and influenza virus infections.

Fig. 13.4 Antiviral effects of antibody

Mechanisms by which antibody acts to neutralize virus or kill virally infected cells.

Complement is involved in the neutralization of some free viruses

Complement can also damage the virion envelope, a process known as virolysis, and some viruses can directly activate the classical and alternative complement pathways. However, complement is not considered to be a major factor in the defense against viruses because individuals with complement deficiencies are not predisposed to severe viral infection.

This should be contrasted with those herpesviruses and poxviruses that carry viral homologues of complement regulatory proteins (CD46, CD55) that regulate complement activation. Presumably these viruses are susceptible to control by complement-dependent mechanisms.

CD8⁺ T cells target virus-infected cells

The principal T cell surveillance system operating against viruses is highly efficient and selective. CD8⁺ T cells identify virus-infected cells by recognizing MHC class I molecules presenting virus-derived peptides on the cell surface, and are triggered to mediate effector functions that clear the infection.

CD8⁺ T cells:

‘Curative’ mechanisms are particularly important when infection is very widespread and it would be neither feasible for CD8⁺ T cells to interact with and kill every infected cell, nor desirable for so many host cells to be destroyed.

Virtually all cells in the body express MHC class I molecules, making this an important mechanism for identifying and eliminating or curing virus-infected cells.

Because of the central role played by MHC class I in targeting CD8⁺ T cells to infected cells, some viruses have evolved elaborate strategies to disrupt MHC class I expression, thereby interfering with T cell recognition and favoring virus persistence (see below).

Almost any viral protein can be processed in the cytoplasm to generate peptides that are transported to the endoplasmic reticulum where they interact with MHC class I molecules.

The importance of T cells in in vivo control of virus infections has been identified using various techniques.

In animals:

In humans:

The ability of knockout mice that lack particular lymphocyte populations to mediate control of some virus infections illustrates the redundancy that can occur in the immune system. For example, in the absence of CD8⁺ T cells, CD4⁺ T cells, antibodies or other mechanisms, individuals are sometimes able to compensate and still bring the infection under control.

CD4⁺ T cells are a major effector cell population in the response to some virus infections

CD4⁺ T cells have also been identified as a major effector cell population in the immune response to some virus infections. A good example is in HSV-1 infection of epithelial surfaces. Here, CD4⁺ T cells participate in a delayed-type hypersensitivity response (see Chapter 26) that results in accelerated clearance of virus. They produce cytokines such as IFNγ and TNFα, which mediate direct antiviral effects and also help to activate macrophages at the site of infection. Macrophages play an important role in inhibiting virus infection, probably through the generation and action of nitric oxide (Fig. 13.5).

Fig. 13.5 Experiment demonstrating that CD4⁺ T cells, macrophages, and IFNγ all have a protective role in cutaneous infections with HSV-1

CD4⁺ T cells were obtained from mice infected with HSV-1 8 days previously. The cells were transferred to syngeneic mice infected with HSV-1 in the skin. These mice were treated with anti-CR3 (to block macrophage migration to the site of infection), or anti-IFNγ (to block the activation of macrophages), or were untreated. An additional control group was infected, but did not receive CD4⁺ T cells. The amount of infectious virus remaining after 5 days was then determined. The results demonstrate that the protective effects of CD4⁺ T cells are mediated by macrophages and IFNγ.

In measles virus and Epstein–Barr virus (EBV) infections, CD4⁺ CTLs are generated that recognize and kill MHC class II-positive cells infected with the virus using the cytolytic mechanisms also employed by CD8⁺ CTLs. This suggests that measles virus and EBV peptides are generated by normal pathways of antigen presentation (i.e. following phagocytosis and degradation, see Chapter 8). However, other pathways have been implicated in which some measles proteins/peptides enter class II vesicles from the cytosol.

A summary of antiviral defense mechanisms is illustrated in Figure 13.6.

Fig. 13.6 Effector mechanisms by which adaptive responses combat virus replication

CD8⁺ T cells typically play a dominant role in control of established virus infections, mediating lysis of virus-infected cells and producing antiviral cytokines. CD4⁺ T cells are important effectors in the control of certain virus infections, producing cytokines that mediate antiviral effects and activating macrophages to produce cytolytic and antiviral factors. They also help B cells to make antibodies which are important in controlling free virus, and can also target infected cells by activating complement-mediated lysis or triggering antibody-dependent cell-mediated virus inhibition (ADCVI) by effector cells including NK cells and macrophages.

Viruses avoid recognition by T cells by reducing MHC expression on infected cells

Other viruses try to render the cells they infect ‘less visible’ to host adaptive responses. The critical role played by CD8⁺ T cells in elimination of virus infections is again underlined by the plethora of strategies that viruses have evolved to reduce the level of MHC class I expression on the surface of infected cells.

MHC class I expression can be disrupted by:

Similar mechanisms apply for MHC class II molecules where some herpesviruses block transcription, whereas other mechanisms involve premature targeting of MHC class II for degradation.

Downregulation of MHC class I may disrupt CD8⁺ T cell recognition, but NK cells are more efficient killers in the absence of MHC class I. Human and murine CMV have tried to redress the balance by encoding their own MHC class I homologues, which are expressed on infected cells and can inhibit NK cell activation.

Mutation of viral target antigen allows escape from recognition by antibodies or T cells

Antigenic variation involves a virus acquiring sequence changes (mutations) in sites on proteins that are normally targeted by antibody or T cells so that these sites are no longer recognized. Antigenic variation can promote virus persistence within a given host, e.g. during HIV-1 infection mutations are frequently selected for in and around the epitopes recognized by the initial T cell responses and neutralizing antibody responses, which confer escape from recognition by these responses and enable enhanced virus replication. It can also promote virus persistence at the population level, as exemplified by the antigenic shift and drift seen with influenza virus (Fig. 13.8). Humoral immunity to influenza virus provides protection against re-infection only until a new virus strain emerges, making effective longlasting vaccines difficult to produce.

Fig. 13.8 Examples of strains of influenza A virus that have caused pandemics since 1933

The major surface antigens of influenza virus are the hemagglutinin and neuraminidase. Hemagglutinin is involved in attachment to cells, and antibodies to hemagglutinin are protective. Antibodies to neuraminidase are much less effective. Influenza virus can change its antigenic properties slightly (antigenic drift) or radically (antigenic shift). Alterations in the structure of the hemagglutinin antigen render earlier antibodies ineffective and new virus epidemics therefore break out. The diagram shows strains that have emerged by antigenic shift since 1933. The official influenza antigen nomenclature is based on the type of hemagglutinin (H₁, H₂, etc.) and neuraminidase (N₁, N₂, etc.) expressed on the surface of the virion. Note that, although new strains replace old strains, the internal antigens remain largely unchanged.

Poorly-neutralizing antibodies can enhance viral infectivity

An unusual pathological consequence of some virus interactions, where weakly-neutralizing antibodies are produced, is antibody-dependent enhancement of virus infection (ADE). This involves Fc receptor-mediated uptake of antibody–virus complexes by macrophages and subsequent enhancement of virus infectivity. This is seen in many persistent or viremic virus infections where the common target for replication is the macrophage. An example is dengue virus infection where weakly cross-reactive antibodies induced during prior infections with different dengue virus subtypes can result in ADE with the initiation of:

Antiviral antibodies can form immune complexes that cause tissue damage

Immune complexes may arise in body fluids or on cell surfaces and are most common during persistent or chronic infections (e.g. with hepatitis B virus). Antibody is ineffective (non-neutralizing) in the presence of large amounts of the viral antigen; instead, immune complexes form and are deposited in the kidney or in blood vessels, where they evoke inflammatory responses leading to tissue damage (e.g. glomerulonephritis, see Chapter 25).