Priming of CD8+ T cell responses following immunization with heat-killed Plasmodium sporozoites

Protective immune responses against malaria are induced by immunization with radiation-attenuated Plasmodium sporozoites. In contrast, non-viable, heat-killed sporozoites do not induce protection, emphasizing the requirement for live parasites to achieve effective immune responses. Using an experimental system with CD8+ T cells from T cell receptor-transgenic mice, we analyzed the primary CD8+ T cell responses elicited by heat-killed inactivated sporozoites. We found that the numbers of specific CD8+ T cells induced were much lower compared to when immunizing with attenuated sporozoites; however, the kinetics of activation and the phenotype of these T cells were similar in both groups. Despite their low frequency after priming, high numbers of specific CD8+ T cells were observed after boosting with a recombinant vaccinia virus. Upon induction of the recall response, the same level of protection was observed when either heat-killed or attenuated sporozoites were used for priming. We propose that live parasites are not critical for the induction of memory T cell populations against the malaria liver stages.