Exploring protective and pathogenic immune responses in the non-human primate model of tuberculosis

Abstract

To this day, tuberculosis remains a complex global health problem, the elimination of which is hindered by knowledge gaps related to protective immunity and diagnosis. In an effort to address these knowledge gaps, this thesis utilized the non-human primate model of tuberculosis to investigate protective and pathogenic immune responses after vaccination and Mycobaterium tuberculosis (Mtb) infection, as well as the dynamics of a potential biomarker of active tuberculosis (TB) disease. First, by applying a dose-escalation study design, we showed that rhesus and cynomolgus macaques are not dissimilar in their susceptibility to infection. However, also at low challenge doses, cynomolgus macaques remained more resistant to the development of TB pathology than rhesus macaques. We used this differential disease susceptibility to characterize early and local immune responses that may underlie tuberculosis disease development and resistance. An early, local proinflammatory innate immune response was observed in cynomolgus macaques, while rhesus macaques presented with increased expression of markers associated with immune suppression. We next built upon of the observation that administration of 1 CFU of Mtb resulted in infection in a subset of exposed rhesus macaques. Intradermal or pulmonary BCG vaccinated rhesus macaques were subjected eight times to this limiting infectious dose to investigate whether we could assess vaccine-mediated prevention of infection, in addition to (partial) prevention of disease. We observed that pulmonary BCG vaccination was superior to intradermal BCG in preventing both infection and disease. This enhanced protection correlated with the induction of local, polyfunctional, IL17A producing T-cells and IL-10 production. Subsequently, we evaluated whether these correlates of protection would also be elicited by mucosal administration of MTBVAC, a novel, live attenuated, Mtb-derived TB vaccine. Like BCG, MTBVAC was found to induce local IL10 production and IL17A producing T-cells. These antigen-specific IL17A-producing T-cells were profiled more in depth, and we showed that cytokine-producing T-cells induced by vaccination display a distinct tissue-residency phenotype and express more mucosal homing markers. Cytokine producing T-cells induced by Mtb infection lacked this phenotype. Antibodies induced by both mucosal vaccination with BCG and MTBVAC were able to bind live Mtb and enhance pathogen uptake by phagocytes. Finally, we utilized biosamples of the studies described in this thesis as well as prior studies performed to assess the association of complement component C1q with TB disease severity. As observed in patients, C1q levels in serum were increased in animals with more TB pathology. In macaques, C1q levels correlated with the amount of pathology assessed post-mortem or by PET-CT imaging. Elevated C1q levels were detected from 6 weeks post-infection onward. An increase in C1q levels in BAL fluid and C1q production by BAL cells could be observed in Mtb infected, but also pulmonary BCG vaccinated animals. Taken together, the NHP studies described in this thesis have added to the model and contributed to our knowledge of protective TB immunity, thereby furthering the efforts towards elimination of TB.