An in vivo investigation of the roles of CD4+ T cell help in the generation of CD8+ T cell responses

CD8+ T cells are vital effectors of adaptive immunity, playing key roles in protection from viral and intracellular bacterial pathogens, tumor surveillance, and transplant rejection. CD4+ T cells are now appreciated to play critical roles in nearly all effective CD8+ T cell responses (help). Help can drive primary expansion of CD8+ T cells, deliver key signals for the generation of effective memory cells, and support secondary expansion upon antigen re-encounter. Although advances have been made in the more than 25 years since the original description of help, precise molecular mechanisms are still elusive. Identifying CD4 + T cell-derived proteins critical for delivery of helper signals is the goal of this thesis.;Immunization with peptide-pulsed dendritic cells results in the primary expansion of endogenous antigen specific CD4+ and CD8 + T cell responses that persist into a lasting memory population. Our lab has developed a non-inflammatory, non-infectious model where CD4 +T cell help is critically required at every stage including primary expansion of CD8+ T cells. CD4+ and CD8 + T cell responses in this model are primed exclusively by the immunizing DC. This isolation of the antigen presenting cell from the responding endogenous T cell pool has allowed our lab to identify crucial DC proteins involved in help. The goal of my thesis project was to identify novel roles for CD4+ T cells in the generation of CD8+ T cell responses. To isolate the helper T pool from endogenous responding T cells in recipients, OT-II TCR transgenic CD4+ T cells were adoptively transferred into mice that were later immunized with DC pulsed with pOVAII, the cognate antigen for OT-II cells, and a class I epitope. The CD8+ T cell responses generated in this new system are critically dependant on the presence of the OT-II cells for primary expansion. Furthermore, help from adoptively transferred T cells appears functionally equivalent to help from endogenous T cells in that it is effected by naive cells that need to recognize antigen in vivo presented on the same APC as the CD8+ T cell epitope. This important advancement in our model system allows for genetic separation of the helper T cell pool and facilitates analysis of CD4+ T cell functions in help for CD8+ T cell responses in the absence of infection and inflammation.;The critical role of CD40/CD154 interactions in help for CD8+ T cell responses has been well described; however, many open questions with regard to mechanism remain. Use of CD154-deficient OT-II cells as helpers in our experimental model has definitively shown that CD8+ T cell responses to non-infectious, cell associated antigens are dependent on CD4+ T cell expression of CD154 and that expression by any other cell type is not required in these types of responses. The consequences of CD40 ligation for CD4+ T cell responses are also investigated.;Our model has also revealed a novel role for CD4+ T cell-derived IL-2 in help. When IL-24 OT-II cells are adoptively transferred, we find that they are incapable of providing help for CD8+ T cell responses to DC immunization. However, IL-2-/- OT-II cells are able to up-regulate CD154 in vitro, and home to and expand in lymph nodes in vivo. CD4+ T cells must also express IL-2 in addition to CD154 to be effective helpers in our model system. This result definitively shows that there are additional functions of CD4 + T cell help for CD8+ T cell responses beyond just ligation of CD40 on DC.;Our system has revealed critical roles for CD4+ T cell-derived CD154 and IL-2 in the generation of CD8+ T cell responses to cell associated antigens in the presence of minimal inflammation. We hypothesize that these molecules will have important effects on the quality of the CD8 + T cell responses generated and the propagation of lasting memory CD8+ T cells even in the context of active infection. Understanding the signals necessary for optimal memory CD8+ T cell responses will greatly enhance vaccination strategies.