Genetically Attenuated Parasite Vaccines Induce Contact-Dependent CD8+ T Cell Killing of Plasmodium yoelii Liver Stage-Infected Hepatocytes

Abstract

The production of IFN-γ by CD8+ T cells is an important hallmark of protective immunity induced by irradiation-attenuated sporozoites against malaria. Here, we demonstrate that protracted sterile protection conferred by a Plasmodium yoelii genetically attenuated parasite (PyGAP) vaccine was completely dependent on CD8+ T lymphocytes but only partially dependent on IFN-γ. We used live cell imaging to document that CD8+ CTL from PyGAP-immunized mice directly killed hepatocyte infected with a liver stage parasite. Immunization studies with perforin and IFN-γ knockout mice also indicated that the protection was largely dependent on perforin-mediated effector mechanisms rather than on IFN-γ. This was further supported by our observation that both liver and spleen CD8+ T cells from PyGAP-immunized mice induced massive apoptosis of liver stage-infected hepatocytes in vitro without the release of detectable IFN-γ and TNF-α. Conversely, CD8+ T cells isolated from naive mice that had survived wild-type P. yoelii sporozoite infection targeted mainly sporozoite-traversed and uninfected hepatocytes, revealing an immune evasion strategy that might be used by wild-type parasites to subvert host immune responses during natural infection. However, CTLs from wild-type sporozoite-challenged mice could recognize and kill infected hepatocytes that were pulsed with circumsporozoite protein. Additionally, protection in PyGAP-immunized mice directly correlated with the magnitude of effector memory CD8+ T cells. Our findings implicate CTLs as key immune effectors in a highly protective PyGAP vaccine for malaria and emphasize the critical need to define surrogate markers for correlates of protection, apart from IFN-γ.