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A ubiquitous clinically silent murine pathogen, polyomavirus has enjoyed long-term co-evolution with the mouse, a highly tractable and genetically and immunologically informative small animal model. Thus, polyomavirus has provided a valuable experimental construct to decipher the host immune mechanisms that come into play to control systemic low-level persistent viral infections. Impaired immunosurveillance for infected cells puts the murine host at risk both to injury resulting from excessive direct virus cytolysis and development of virus-induced tumors. In this review, we present our current understanding of the multifaceted immune response invoked by the mouse to maintain détente with this potentially deleterious persistent natural pathogen, and discuss implications of these studies for therapeutic interventions for human polyomavirus infection.

Polyomavirus (PyV) infection induces protective T cell-independent (TI) IgM and IgG antibody responses in T cell-deficient mice, but these responses are not generated by immunization with viral proteins or virus like particles. We hypothesized that innate signals contribute to the generation of isotype-switched antiviral antibody responses. We studied the role of complement receptor (CR2) engagement in TI and T cell-dependent (TD) antibody responses to PyV using CR2-deficient mice. Antiviral IgG responses were reduced by 80-40% in CR2-/- mice compared to wild type. Adoptive transfer experiments demonstrated the need for CR2 not only in TD, but also in TI IgG responses to PyV. Transfer of CR2-/- B lymphocytes to SCID mice resulted in TI antiviral IgG responses that corresponded to 10% of that seen in wild-type B cell-reconstituted mice. Thus, our studies revealed a profound dependence of TI and TD antiviral antibody responses on CR2-mediated signals in PyV-infected mice, where the viral antigen is abundant and persistent.

Polyomavirus (PyV) infection induces protective T-cell-independent (TI) IgM and IgG responses in T-cell-deficient (TCR beta x delta-/-) mice. In this study, we show that PyV is a TI -2 antigen: B cells with a mutated Bruton's tyrosine kinase (Xid mutants) do not respond to PyV with antibody secretion in the absence of T cells. We also demonstrate that NK-cell-mediated "help" is not absolutely required for the induction of the TI-2 antibodies to PyV; thus for the first time, we provide evidence for protective IgM and IgG responses against a viral infection induced in mice lacking T and NK cells (CD3Etg). Comparison of the antibody responses observed in T- and NK-cell-deficient mice with those of mice lacking only T cells, however, suggests that NK cells may promote isotype switching to IgG2a. This effect is probably mediated by IFN gamma secretion. In support of this idea, studies on the antibody responses of PyV-infected SCID mice that had been reconstituted with IFN gamma R-/- B cells or wild-type B cells demonstrated the IFN gamma dependence of PyV-specific TI IgG2a secretion and provided evidence that IFN gamma acting directly on B cells plays an important role in TI pathways of isotype switching to IgG2a in vivo., (Copyright 2001 Academic Press.)