6. Project Summary Malaria infection starts when mosquitoes inject sporozoites into the skin. The parasites enter the bloodstream, and after reaching the liver, they develop into exoerythrocytic forms (EEFs) inside hepatocytes. The EEFs mature and then divide rapidly to form thousands of merozoites that re-enter the blood and infect erythrocytes causing the disease we recognize as malaria. Sterile immunity against malaria infection has been achieved by vaccination with irradiated sporozoites (IrSp) or with Genetically Attenuated Sporozoites (GAS) obtained by targeting their uis3, or uis4 or P52/36 genes. Using CStransgenic mice that are unable to make antibodies, we have shown that circumsporozoite (CS) antigen alone accounts for close to 90% of the protection elicited by immunization with IrSp of a rodent malaria parasite, Plasmodium yoelii. However, we have found that hyper-immunizing these mice with IrSp could induce a very strong protective anti-malaria immunity, which is mainly mediated by CD8+ T cells. This finding underscores the presence of sub-dominant [unreadable]minor[unreadable] protective antigens in IrSp. Therefore, in this proposal, we seek 1) To identify CD8+ T cell responses against some of the non-CS [unreadable]minor[unreadable] protective antigens generated by immunization with IrSp, and 2) To compare the mechanisms of T cell-mediated protection induced by GAS and IrSp. The identification of novel malaria antigen(s) should facilitate the development of preerythrocytic malaria vaccines. In addition, the systematic comparison of the role of non- CS-specific CD8+ T cells induced by GAS versus by IrSp should contribute to our understanding of the nature of sporozoite-based vaccines against malaria.