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


RESULTS
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.


CONCLUSIONS
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.
Department of Laboratory Medicine, University of Washington, Seattle, WA, USA.|Center for Emerging and Re-emerging Infectious Diseases, University of Washington, Seattle, WA, USA.
Zachary P
ZP
Billman
Department of Laboratory Medicine, University of Washington, Seattle, WA, USA.|Center for Emerging and Re-emerging Infectious Diseases, University of Washington, Seattle, WA, USA.
Arnold
A
Kas
Department of Laboratory Medicine, University of Washington, Seattle, WA, USA.|Center for Emerging and Re-emerging Infectious Diseases, University of Washington, Seattle, WA, USA.
Brad C
BC
Stone
Department of Laboratory Medicine, University of Washington, Seattle, WA, USA. murphysc@uw.edu.|Department of Microbiology, University of Washington, Seattle, WA, USA. murphysc@uw.edu.|Center for Emerging and Re-emerging Infectious Diseases, University of Washington, Seattle, WA, USA. murphysc@uw.edu.
Sean C
SC
Murphy
D000818:Animals; D018414:CD8-Positive T-Lymphocytes / Q000276:immunology* / Q000469:parasitology; D004797:Enzyme-Linked Immunosorbent Assay; D000939:Epitopes / Q000276:immunology; D005260:Female; D017780:Malaria Vaccines / Q000276:immunology; D051379:Mice; D008810:Mice, Inbred C57BL; D010962:Plasmodium berghei / Q000276:immunology*; D010967:Plasmodium yoelii / Q000276:immunology*; D015800:Protozoan Proteins / Q000276:immunology* / Q000378:metabolism; D017422:Sequence Analysis, DNA; D034101:Sporozoites / Q000276:immunology*
D000939:Epitopes; D017780:Malaria Vaccines; D015800:Protozoan Proteins; C041595:circumsporozoite protein, Protozoan
CD8 T cell; Cross-species; GAP; Heterologous; Late-arresting; Malaria; Plasmodium; RAS; Secondary expansion