Acquired Immunity Against Virus Infections

The importance of acquired immunity against viruses is demonstrated by fatal or persistent infections of viruses in severe combined immunodeficient (SCID) mice, which lack functional T and B cells, under conditions where normal immunocompetent mice would clear the infection without apparent disease or mortality. Most of our present understanding of antiviral immune mechanisms comes from experiments performed with inbred laboratory mice, because different components of the immune system can be easily manipulated in this animal model. This chapter describes the dynamics of T- and B-cell responses elicited by virus infections and illustrates the functions and importance of these cells with examples from well-studied viral models. Immunodominant epitopes are highly immunogenic and therefore elicit T-cell responses that are easily detectable, whereas T cells specific for weakly immunogenic, subdominant epitopes are often difficult to detect. Studies with poliovirus have indicated that virus-specific CD8 T cells can recognize antigen presenting cells (APC) class I MHC-presented peptides derived from exogenous proteins. FcγRIII, which are expressed on natural killer (NK) cells in addition to monocytes and macrophages, bind mainly IgG2a, IgG2b, and IgG1 antibodies, and have the potential to mediate antibody-dependent cell-mediated cytotoxicity (ADCC) of virus-infected cells.