More than they seem: Unveiling diverse roles for hepatic γδ T cells in the immune response to live-attenuated malaria vaccines

Abstract

Malaria is a devastating global disease caused by Plasmodium parasites that are transmitted through mosquitos. Within mammalian hosts, parasites first invade and replicate within the liver as clinically silent liver stages before egressing to the blood, where infection of red blood cells results in all clinical symptoms, disease-associated mortality, and transmission back to mosquitos. Whole parasite vaccines, composed of live-attenuated Plasmodium parasites that arrest during the liver stage of infection, induce a robust hepatic memory CD8+ T cell response that confers sterile protection against malaria in malaria-naïve adults. However, these vaccines are less efficacious in children and individuals with previous malaria exposure; thus, identifying cell types and signaling pathways that can augment the critical memory CD8+ T cell response in the liver may enable targeted improvements of whole parasite vaccines to enhance efficacy. In humans, among the peripheral immune correlates of protection that have emerged during whole parasite vaccine clinical trials are a population of innate-like T cells called γδ T cells. To better understand the mechanisms by which γδ T cells regulate whole parasite vaccine efficacy, we comprehensively characterized the murine γδ T cell response to the most efficacious whole parasite vaccines ever tested: late-arresting replication-competent genetically attenuated parasites (LARC GAPs). Using high-parameter single-cell mass cytometry, we find that IFNγ-producing γδ T cell subsets are induced early in the liver after LARC GAP vaccination. IFNγ signaling is critical for liver-stage parasite elimination and may regulate immunization-induced CD8+ T cell responses, leading us to hypothesize that these IFNγ-producing γδ T cells play roles in both the innate and adaptive immune responses to Plasmodium liver stages. At innate timepoints, we demonstrate that LARC GAP-induced γδ T cells act in concert with innate-like ɑβ T cells to contribute to the restriction of liver-stage Plasmodium parasites. We also find that a specific subset of IFNγ-producing γδ T cells, called γδ NKT cells, regulates the protective CD8+ T cell response to LARC GAP immunization. We extend these findings to humans utilizing single-cell RNA-sequencing and flow cytometric analyses of peripheral blood samples from a whole parasite vaccine clinical trial, revealing for the first time that tissue-associated γδ T cells are uniquely responsive to whole parasite immunization and correlate with vaccine-induced protection. Together, our findings demonstrate that hepatic γδ T cells are multi-functional players in the response to whole parasite vaccines that, if targeted by novel adjuvant approaches, may be able to enhance vaccine efficacy in difficult-to-protect populations by supporting the development and effector activity of protective hepatic anti-Plasmodium memory CD8+ T cells.