== The presence of memory B cells was assessed by a modification of a previously described adoptive transfer system (31). produced IFN- uponin vitrostimulation. A key role for T cells was confirmed by the fact that TCR/mice also generated strong neutralizing IgG responses to VSV, whereas TCR//mice produced very low titers. The neutralizing IgG responses of TCR/mice were accompanied by the development of memory B cells, but not by antigen-specific germinal center (GC) formation. Thus, during viral contamination of T cell-deficient mice, T cells may provide the signals that are required for isotype switching. Vesicular stomatitis virus (VSV) contamination of immunocompetent mice induces a rapid neutralizing IgM response, which occurs independently of T cell help, followed by production of neutralizing IgG antibodies that are strictly dependent on T cell help (1). Although VSV also induces cytotoxic T lymphocytes (2,3), the neutralizing IgG response seems to be crucial for recovery from primary infections (4). The generation of high-affinity antibodies and class switching from IgM to IgG is usually the result of cognate interactions between antigen-specific B cells and CD4+ T cells, the latter secreting cytokines that regulate Ig isotype switching (57). These T-dependent B cell responses are accompanied by the formation of specialized microenvironments, germinal centers (GC), within which somatic mutation and affinity maturation occur (8,9). However, evidence is growing that there may be alternative, albeit less efficient, mechanisms for the generation of IgG antibody responses. For example, mice that have been rendered T cell-deficient by targeted disruption of the TCR gene (TCR/) exhibit elevated serum IgG levels b-AP15 (NSC 687852) and enhanced numbers of GC in comparison to TCR+/littermates (1012). Furthermore, athymic nude mice, although generally having low levels of IgG antibodies and few GC, can produce VSV-specific IgG antibodies (13) and can clear intracerebral sindbis virus contamination (14). Finally, in the case of polyoma virus contamination of T cell-deficient mice, it was recently shown that B cells alone were capable of mounting a protective T cell-independent antiviral antibody response, including the production of IgG antibodies (15). In this study, we have examined the anti-viral antibody responses of T cell-deficient mice infected with VSV and have characterized the factors involved in the generation of T-independent anti-viral IgG responses. == MATERIALS AND METHODS == == Mice. == C57BL/6 (H-2b) mice were from the b-AP15 (NSC 687852) Institut fr Labortierkunde (Zrich, Switzerland). Mice with a mutant T cell receptor gene (TCR/) (16) were obtained from M. J. Owen, (Imperial Cancer Research Fund, London), and TCR/mice (17) were obtained from U. Steinhoff, (Max-Planck-Institut fr Infektionsbiologie, Berlin). Mice with nonfunctional TCR and genes (TCR//) (17) were obtained from The Jackson Laboratory. All T cell-deficient strains had been backcrossed to C57BL/6 (H-2b) mice. == Viruses, Immunization, Measurement of VSV-Specific Antibody Responses, and Immunohistology. == VSV Indiana (VSV-IND, MuddSummers isolate) was used in this research and was propagated and UV-inactivated (UV-VSV) as referred to previously (18,19). Recombinant vaccinia infections expressing the VSV-IND surface area Rabbit Polyclonal to OR4A16 glycoprotein (Vacc-IND-G) or lymphocytic choriomeningitis disease nucleoprotein (Vacc-LCMV-NP) or murine interferon (Vacc-IFN-) had been prepared as referred to previously (20,21). Mice had been immunized i.v. with 2 106pfu of infectious VSV-IND, UV-VSV, or Vacc-IND-G. VSV-neutralizing IgM and IgG antibody titers had been assayed as referred to (22,23). Mean titers from sets of three mice are demonstrated, and intragroup variants had been 2 titer measures. The isotype distribution of VSV-specific IgG was dependant on ELISA on VSV-coated plates (24). Immunohistological evaluation of freezing spleen areas was performed as referred to previously (25). == Reagents forin VivoDepletions. == To deplete T cell or organic killer (NK) cell populationsin vivo, mice i were injected.p. with b-AP15 (NSC 687852) 0.5 mg anti-Thy1.2 mAb (26) or with polyclonal rabbit anti-AsialoGM1 antiserum (Waco) on times 3 and 1 before immunization. Antibody remedies had been repeated on times 5 and 11 after immunization. IFN- was depleted by daily.