CD4 T Cell-Mediated Lysis and Polyclonal Activation of B Cells During Lymphocytic Choriomeningitis Virus Infection: A Dissertation

Abstract

CD4 T cells and B cells are cells associated with the adaptive immune system. The adaptive immune system is designed to mount a rapid antigen-specific response to pathogens by way of clonal expansions of T and B cells bearing discrete antigen-specific receptors. During viral infection, interactions between CD4 T cells and B cells occur in a dynamic process, where B cells that bind to the virus internalize and degrade virus particles. The B cells then present viral antigens to virus-specific CD4 T cells that activate the B cells and cause them to proliferate and differentiate into virus-specific antibody-secreting cells. Yet, non-specific hypergammaglobulinemia and the production of self-reactive antibodies occur during many viral infections, and studies have suggested that viral antigen-presenting B cells may become polyclonally activated by CD4 T cells in vivo in the absence of viral engagement of the B cell receptor. This presumed polyclonal B cell activation associated with virus infection is of great medical interest because it may be involved in the initiation of autoimmunity or contribute to the long-term maintenance of B cell memory. In order to directly examine the interactions that occur between T cells and B cells, I asked what would happen to a polyclonal population of B cells that are presenting viral antigens, if they were transferred into virus-infected hosts. I performed these studies in mice using the well-characterized lymphocytic choriomeningitis virus (LCMV) model of infection. I found that the transferred population of antigen-presenting B cells had two fates. Some antigen-expressing B cells were killed in vivo by CD4 T cells in the first day after transfer into LCMV-infected hosts. However, B cells that survived the cytotoxicity underwent a dynamic polyclonal activation manifested by proliferation, changes in phenotype, and antibody production. The specific elimination of antigen-presenting B cells following adoptive transfer into LCMV-infected hosts is the first evidence that MHC class II-restricted killing can occur in vivo during viral infection. This killing was specific, because only cells expressing specific viral peptides were eliminated, and they were only eliminated in LCMV-infected mice. In addition to peptide specificity, killing was restricted to MHC class II high cells that expressed the B cell markers B220 and CD19. Mice depleted of CD4 T cells prior to adoptive transfer did not eliminate virus-specific targets, suggesting that CD4 T cells are required for this killing. I found that CD4 T cell-dependent cytotoxicity cannot be solely explained by one mechanism, but Fas-FasL interactions and perforin are mechanisms used to induce lysis. Polyclonal B cell activation, hypothesized to be the cause of virus-induced hypergammaglobulinemia, has never been formally described in vivo. Based on previous studies of virus-induced hypergammaglobulinemia, which showed that CD4 T cells were required and that hypergammaglobulinemia was more likely to occur when virus grows to high titer in vivo, it was proposed that the B cells responsible for hypergammaglobulinemia may be expressing viral antigens to virus-specific CD4 T cells in vivo. CD4 T cells would then activate the B cells. However, because the antibodies produced during hypergammaglobulinemia are predominantly not virus-specific, nonvirus-specific B cells must be presenting viral antigens in vivo. In my studies, the adoptively transferred B cells that survived the MHC class II-restricted cytotoxicity became polyclonally activated in LCMV-infected mice. Most of the surviving naïve B cells presenting class II MHC peptides underwent an extensive differentiation process involving both proliferation and secretion of antibodies. Both events required CD4 cells and CD40/CD40L interactions to occur but B cell division did not require MyD88-dependent signaling, type I interferon signaling, or interferon γ signaling within B cells. No division or activation of B cells was detected at all in virus-infected hosts in the absence of cognate CD4 T cells and class II antigen. B cells taken from immunologically tolerant donor LCMV carrier mice with high LCMV antigen load became activated following adoptive transfer into LCMV-infected hosts, suggesting that B cells can present sufficient antigen for this process during a viral infection. A transgenic population of B cells presenting viral antigens was also stimulated to undergo polyclonal activation in LCMV-infected mice. Due to the high proportion of B cells stimulated by virus infection and the fact that transgenic B cells can be activated in this manner, I conclude that virus-induced polyclonal B cell activation is independent of B cell receptor specificity. This approach, therefore, formally demonstrates and quantifies a virus-induced polyclonal proliferation and differentiation of B cells which can occur in a B cell receptor-independent manner. By examining the fate of antigen-presenting B cells following adoptive transfer into LCMV-infected mice, I have been able to observe dynamic interactions between virus-specific CD4 T cells and B cells during viral infection. Adoptive transfer of antigen-presenting B cells results in CD4 T cell-mediated killing and polyclonal activation of B cells during LCMV infection. Studies showing requirements for CD4 T cells or MHC class II to control viral infections must now take MHC class II-restricted cytotoxicity into account. Polyclonal B cell activation after viral infection has the potential to enhance the maintenance of B cell memory or lead to the onset of autoimmune disease.