Mechanisms and Functional Consequences of CD4 T Cell Cooperation

CD4 “helper” T cells are central players in the regulation of immune responses. These T cells affect the magnitude and phenotype of immune responses, and indeed, affect whether or not the host responds, at all, to a given antigen. Thus, knowledge of the cellular and molecular requirements for the activation, differentiation, and survival of CD4 T cells is critical for understanding immune responses in general; this understanding may lead to advances in rational vaccine design and effective immunotherapy. Evidence supports a role for CD4 T cells in the activation and acquisition of effector function of other CD4 T cells. In this thesis I set out to develop experimental systems to directly observe, and explore the mechanisms involved in, cooperative interactions between CD4 T cells, “CD4 T cell cooperation”. I developed two major experimental systems wherein cooperative effects in the activation and differentiation of CD4 T cells were observed. Firstly, we show that simultaneous administration, in incomplete Freund’s adjuvant, of multiple peptides, but not single peptides, known to bind host class-two major histocompatability antigens (MHCII) and to stimulate endogenous naive CD4 T cells, results in the cooperative generation of cytokine-producing effector CD4 T cells, in vivo, as assessed by cytokine ELISpot assay. We demonstrate that these cooperative interactions depend on the presence of CD4 T cells with specificity for the administered peptides in the context of host MHCII. In the second experimental system, syngeneic, splenic antigen-presenting cells (APC), loaded in vitro with exogenous peptides, that bind the distinct MHCII molecules, I-A and I-E, mediate cooperation between peptide-specific CD4 T cell populations, in vivo, upon adoptive transfer. Furthermore, we find that, in order for cooperative interactions to occur, these peptides must be simultaneously presented by the same APC. In addition it appears that B cells, and not splenic dendritic cells, efficiently mediate this cooperation. We demonstrate that cooperation between CD4 T cell populations depends on OX40L, as blocking this molecule abrogates cooperative effects. Enhanced primary effector CD4 T cell generation, influenced by cooperation, results in enhanced secondary effector CD4 T cell responses, indicating that cooperation between CD4 T cells plays a role in the establishment of immunological memory. A third experimental system allowed us to address the role of CD4 T cell cooperation in immunological self-tolerance. We find that, in general, activated CD4 cells are susceptible to inactivation by systemically administered peptides. However, under conditions that normally result in inactivation of effector CD4 T cells, the systemic ligation of CD40 or OX40 by agonistic antibodies, but not the administration of isotype-matched control antibodies, rescues cytokine production by these cells. Given the role of these molecules in CD4 T cell cooperation, our findings lead us to propose that cooperation between autoreactive CD4 T cells may, occasionally, result in sustained autoimmune responses. The findings presented in this thesis allow us to make comprehensive models of the role of CD4 T cell cooperation in the activation, differentiation and, the maintenance of self-tolerance in CD4 T cells. Our findings have advanced our understanding sufficiently to allow for further experimentation in models of disease or immunological misregulation