Plasmodium berghei Radiation-Attenuated Sporozoite-Immunized Mice Require Infectious Sporozoite Challenge to Maintain Protective Immunity

Abstract

Plasmodium radiation-attenuated sporozoites (RAS) confer sterile protection in mammalian hosts. The duration of protection is affected by the dose of RAS, the route of immunization, and the timing of primary challenge (PC). Giving PC shortly after the last Plasmodium berghei (Pb) RAS immunization of C75BL/6 mice led to the long-term sterile protection, whereas delaying PC beyond 6 months resulted in parasitemia. The mechanisms responsible for the divergent outcome remain unknown. Because liver effector/memory CD8⁺T cells are associated with lasting protection, herein we asked if any functions of CD8⁺T cells would be diminished or lost by delaying PC. Using the Pb protection model, we characterized functional attributes and phenotypes of liver and spleen CD8⁺T cells following early and delayed PC. Compared with CD8⁺T cells before the challenge, liver KLRG-1^intCD107⁺ and IFN-γ⁺IL-2⁺CD8⁺T cells increased after early but decreased following delayed PC. Memory CD8⁺T cells exhibited higher expression of Bcl-2 at early rather than delayed PC. Finally, splenic and liver-draining lymph node CD8⁺T cells expressed significantly higher CXCR6 and the respective ligands but only following early PC. Collectively, our results show that enhanced proliferation, migration, and elevated effector functions of CD8⁺T cells are associated with the longevity of sterile protection in the Pb RAS murine model.