Inclusion of Non-target antigen in vaccination favors generation of OVA specific CD4 Memory T cells

Abstract

T cells are important players of naturally acquired protective immune responses; the ultimate goal of any successful vaccination is inducing heterogeneous pool of long-lived T cells. Often we notice that immune response generated by vaccines fail to achieve the desired goal particularly T cell responses. It is well established that long-lived T cells could be generated in hosts infected by pathogens naturally. Out of several Ags derived from the pathogen, host mounts response to few Ags called immunogen (s) (Target antigens). As vaccination is perceived to mimic infection, adjuvant and Target Ags are inoculated together with the hope to elicit memory response as natural infection. It is very likely that immune response to Target antigen is influenced by the presence of other Ags derived from pathogen, called Non-Target Ags that are excluded in vaccine formulations. The Non-Target Ags could be non-immunogen, poor immunogen or immunogen. Furthermore, we often experience that individuals during particular season of the year get exposed to multiple infections, or while evoking immune response to one, they encounter another pathogen. In this thesis, we wanted to understand how inclusion of Non-Target Ags with Target Ags in vaccine formulations influences generation of qualitatively better effector and memory response. We studied whether Ags of the secondary infection influence the development of memory T cells to Ag(s) encountered from the first infection. Also, we studied how memory T cell response is influenced by Non-Target Ags through modulation of APC function. In this study we mimic natural infection by inclusion of Non-Target Ags with vaccine candidate, OVA, in the inoculum containing TLR ligands to suffice the minimal condition of pathogen to provoke immune response and understand the role of Non-Target Ags in inducing CD4+ T cell response and formation of memory. We immunized C57BL/6 mice with OVA in the presence/absence of immunogenic or poorly immunogenic Non-Target Ag, HEL (Hen egg lysozyme) or MBP (Myelin Basic Protein), respectively, and measured CD4 T cell responses to OVA. Memory T cells were characterized based on phenotypic markers (CD44, CD62L) and their response to Target antigen was measured by cell proliferation and intracellular IFN-, IL-2 and TNF- production upon ex vivo OVA challenge. We found that inclusion of Non-Target Ags HEL or MBP enhances the OVA specific CD4 T cell responses. Interestingly, poorly immunogenic MBP was found to strongly favor the generation of OVA specific memory CD4 T cells. Further, the OVA specific CD4 T cells generated in the presence of MBP were also found to be multiple cytokine producing in nature. We also observed that challenge of host with MBP favors establishment of Memory. We also tested the ability of Non-Target Ags in modulating APC function. We found that CD11c MHCII+ DCs showed up regulation of CD40, CD86 on inclusion of MBP, which might facilitate synapse formation, activation of T cells. In Summary, our results demonstrate that Non-Target Ags depending on their nature of immunogenicity might influence the generation of memory. Exploitation of the unique role of antigens would help develop novel strategies to generate long-lived memory. Having better insights on these issues will improve our understanding of why natural infection is better alternative of inducing protective immunity and will also pave the path for developing new adjuvants.