Defining rules of CD8⁺ T cell expansion against pre-erythrocytic Plasmodium antigens in sporozoite-immunized mice

Whole Plasmodium sporozoites serve as both experimental tools and potentially as deployable vaccines in the fight against malaria infection. Live sporozoites infect hepatocytes and induce a diverse repertoire of CD8⁺ T cell responses, some of which are capable of killing Plasmodium-infected hepatocytes. Previous studies in Plasmodium yoelii-immunized BALB/c mice showed that some CD8⁺ T cell responses expanded with repeated parasite exposure, whereas other responses did not.

Here, similar outcomes were observed using known Plasmodium berghei epitopes in C57BL/6 mice. With the exception of the response to PbTRAP, IFNγ-producing T cell responses to most studied antigens, such as PbGAP50, failed to re-expand in mice immunized with two doses of irradiated P. berghei sporozoites. In an effort to boost secondary CD8⁺ T cell responses, heterologous cross-species immunizations were performed. Alignment of P. yoelii 17XNL and P. berghei ANKA proteins revealed that >60 % of the amino acids in syntenic orthologous proteins are continuously homologous in fragments ≥8-amino acids long, suggesting that cross-species immunization could potentially trigger responses to a large number of common Class I epitopes. Heterologous immunization resulted in a larger liver burden than homologous immunization. Amongst seven tested antigen-specific responses, only CSP- and TRAP-specific CD8⁺ T cell responses were expanded by secondary homologous sporozoite immunization and only those to the L3 ribosomal protein and S20 could be re-expanded by heterologous immunization. In general, heterologous late-arresting, genetically attenuated sporozoites were better at secondarily expanding L3-specific responses than were irradiated sporozoites. GAP50 and several other antigens shared between P. berghei and P. yoelii induced a large number of IFNγ-positive T cells during primary immunization, yet these responses could not be re-expanded by either homologous or heterologous secondary immunization.

These studies highlight how responses to different sporozoite antigens can markedly differ in recall following repeated sporozoite vaccinations. Cross-species immunization broadens the secondary response to sporozoites and may represent a novel strategy for candidate antigen discovery.