Induction of Memory T Cells by Mimicking Natural Course of Infection

Abstract

nducing robust and sustained T cell responses is necessary for therapeutic intervention of most infectious diseases. Unfortunately, many current subunit vaccine regimens fail to induce substantial cellular immunity. Potent cellular responses are an absolute necessity when it comes to achieving a state of complete protection and durable immunological memory in host systems. Vaccine invention and development has its roots lying under the concept of natural infection induced memory. Thus, mimicking infection can represent a good road map for guiding the development of novel subunit vaccine regimens. It has been noted that often responses to vaccination differ from those induced by infectious challenge, naturally. The failure of vaccines to a greater extent could be correlated to an incomplete understanding of the signals and cell types that operate at different stages of the immune response to influence the quantity and quality of developing memory T cells. Immunological rules guiding subunit vaccine elicited T cell responses would not be very different from those guiding infection induced T cell responses. Hence, understanding infection in its natural settings could help in designing better antigen-adjuvant systems and strategies, for generating multifunctional memory responses. In our study we wished to obtain insights on the effects of fragmenting doses of antigen and adjuvant on memory T cell generation in a typical natural viral infection setting. We thus designed two objectives each focusing on initial/acute phase and contraction/resolution phase of infection. As part of first objective, we intended to mimic infection by administering TLR ligands as adjuvants and OVA as model antigen; following the course of infection. We immunized groups of C57BL/6 mice with combinations of TLR agonists (R848 + Poly I: C to mimic viral infection) along with Ag OVA by creating conditions analogous to graded inflammation and Ag dose. By adopting the modified approach, we found enhanced memory CD4+ T cell response as compared to the vaccine regimen. We have evaluated the OVA specific memory CD4 T cells by phenotypic characterization (CD44 & CD62L) as well as measuring the effector function in terms of intracellular cytokine (IFN-γ, IL-2 & TNF-α). We have also looked at the generation of antigen specific memory T cells using MHC class II OVA tetramer. To validate our findings, we also tested our alternative vaccination strategy using antigen PAD4 (Protective Antigen Domain 4) from Bacillus anthracis along with TLR Ligands. The results show generation of multifunctional memory CD4+ T cells in groups of mice vaccinated with our new strategy. For the second objective, we designed a strategy in which groups of mice were subjected to a standard dose of antigen and TLR ligands (OVA, HEL and Poly I: C + R848) as part of initial priming phase. Ten days later, they were administered with very low dose of antigen. The idea behind designing this strategy was to understand the effect of low dose antigen during the contraction phase. An acute infection generally subsides by seven to ten days and this is the window when memory T cells are believed to be generated. We measured CD4 T cell response to OVA. CD44 and CD62L were used for phenotypic characterisation and response to ex-vivo OVA stimulation was measured in terms of intracellular cytokine production (IFN-γ, IL-2 and TNF-α). The results obtained indicate that qualitatively better memory type CD4 T cells are generated in groups following the modified strategy. Overall we could conclude that by modifying the vaccination strategies in a way which could closely mimic infection, better memory T cell responses could be generated. Alternative strategies focusing on each phase of infection could lead to the desired/tailor-made responses particular to the type of infection being targeted. The present study thus has important implications for developing novel antigen-adjuvant delivery systems.