The role of adjuvant-induced innate immune activation in shaping vaccine responses

Abstract: Adjuvants are components added to non-live vaccine formulations to enhance the effect of the vaccine by alerting the immune system to initiate a response against the vaccine. Powerful new adjuvants will be critical for the development of next generation vaccines to diseases such as tuberculosis, HIV-1/AIDS, malaria, and therapeutic cancer vaccines. My thesis work has focused on the responses induced by adjuvants targeting different immune-modulatory receptors in the innate immune system. The overall aim of the studies was to better understand the mechanisms by which adjuvants can alter innate immune activation and thereby influence the magnitude, polarization, and longevity of adaptive vaccine responses. In paper I, I investigated an adjuvant combining the toll-like receptor (TLR)3-agonist, Poly IC:LC, and an agonistic monoclonal antibody targeting CD40 (anti-CD40Ab) for the potential to induce T cell responses. We found low T cell responses in the blood, but remarkable frequencies of vaccine-specific T cells restricted to the lung and bronchoalveolar lavage after vaccination. The majority of the vaccine-specific T cells in the lung expressed CD103, representing tissue-resident memory T cells (TRM). However, we found that the anti-CD40Ab was widely disseminated after vaccination to all organs analyzed, and therefore lung-specific adjuvant activation alone could not explain the compartmentalized TRM. We consequently expanded the studies in paper II to compare the intravenous (IV) and subcutaneous (SQ) routes of administration. In contrast to IV, the CD40Ab stayed localized to the skin and the skin draining lymph nodes following SQ administration. While both groups induced equivalent vaccine-specific T cell homing to the lung, IV immunization induced a significantly higher proportion of CD103+ TRM. IV immunization induced an innate profile skewed towards IL-10 production, which strongly correlated with the proportion of TRM. By in vitro studies, we found that blood monocytes were the main producers of IL-10 and could mediate increased CD103 expression on T cells. IL-10 did not directly cause CD103 upregulation, but instead conditioned monocytes to release TGFb which in turn induced the TRM phenotype. In paper III, I compared how adjuvants targeting either TLR4, TLR7/8, or TLR9 induced different innate immune responses to polarize the adaptive vaccine responses. In a large preclinical vaccine study, the TLR-adjuvants were added to polymer-based nanoparticles encapsulating the malaria transmission-blocking vaccine antigen Pfs25, to identify correlates of immunity leading to robust, long-lived, functional Ab titers. All groups induced high Ab titers and transmission reducing activity in mosquitoes at peak responses. However, the adjuvants targeting TLR7/8 or TLR9 induced higher levels of IFNα production and type I IFN associated gene signatures than the adjuvant targeting TLR4. The IFNα signature showed strong correlations with the increased Ab half-life observed in these groups. All adjuvants generated Pfs25-specific CD4 T cell responses when combined with the nanoparticle encapsulated antigen, which correlated with increased IgG Ab avidity. In conclusion, the thesis provides increased understanding of the mechanisms by which adjuvants potentiate and regulate vaccine responses and will hopefully aid in refining future vaccine formulations.