MAIT cell activation augments adenovirus vector vaccine immunogenicity

Mucosal-associated invariant T (MAIT) cells are innate sensors of viruses, which can augment early immune responses and contribute to protection from lethal infection. Thus, we reasoned MAIT cells may have an adjuvating role in the immunogenicity of replication-incompetent adenovirus vectors, which are novel vaccine platforms for pandemic pathogens such as Ebola virus and SARS-CoV-2. In both mice and human volunteers, immunization with ChAdOx1 (Chimpanzee Adenovirus Ox1) robustly activated MAIT cells. Activation required transduction of plasmacytoid dendritic cells and monocytes to produce IFN- and IL-18, respectively. IFN--induced monocyte-derived TNF was identified as a novel intermediate in this activation pathway, and activation required combinatorial signaling of all three cytokines both in vitro and in vivo. Strikingly, vaccine-induced activation of MAIT cells positively correlated with vaccine-induced T cell responses in human volunteers. Supporting a causal relationship, MAIT cell-deficient mice displayed impaired CD8+ T cell responses to multiple vaccine-encoded antigens. These findings define a novel role for MAIT cells in the immunogenicity of adenovirus vector vaccines, with potential implications for vaccine design. Four experiments were performed: 1) Human PBMCs were stimulated in vitro for 24 hours using a ChAdOx1 vector, or left untreated (n=4 of each condition) (AD119). 2) C57BL/6J mice were immunized intramuscularly with a ChAdOx1 vector, or left unimmunized. After 24 hours, draining inguinal lymph nodes were harvested (n=5 of each condition) (AD122). 3) Human volunteers were immunized intramuscularly with a ChAdOx1 vector. PBMCs were collected one day prior to and one day following immunization (n=14) (AD142_AD143). 4) C57Bl/6J, Il18rap-/-, Ifnar-/-, and Tnfrsf1a-/-Tnfrsf1b-/- mice were immunized intramuscularly with a ChAdOx1 vector. After 24 hours, draining inguinal lymph nodes from these mice, or unimmunized C57BL/6J control mice were collected (n=4 of each condition) (AD148). For all four experiments, mucosal-associated invariant T (MAIT) cells were isolated by FACS, RNA was extracted by phenol/chloroform, and cDNA libraries were generated using the Smart-Seq2 protocol. The comparisons performed were: Experiments 1-3: MAIT cells from the ChAdOx1-stimulated condition versus unstimulated/naïve/pre-vaccine to identify vaccine-induced genes. Following this, the differentially expressed genes between each of these conditions were compared. Experiment 4: MAIT cells from vaccinated and unvaccinated C57BL6/J were compared to identify differentially expressed genes in response to vaccination. Then wild-type mice and each of the knockouts were compared to determine which of the vaccine-induced genes were regulated by the particular cytokine pathway perturbed in each knockout mouse.