Many variant proteins encoded by Plasmodium-specific multigene families are exported into red blood cells (RBC). P. falciparum-specific variant proteins encoded by the var, stevor and rifin multigene families are exported onto the surface of infected red blood cells (iRBC) and mediate interactions between iRBC and host cells resulting in tissue sequestration and rosetting. However, the precise function of most other Plasmodium multigene families encoding exported proteins is unknown. To understand the role of RBC-exported proteins of rodent malaria parasites (RMP) we analysed the expression and cellular location by fluorescent-tagging of members of the pir, fam-a and fam-b multigene families. Furthermore, we performed phylogenetic analyses of the fam-a and fam-b multigene families, which indicate that both families have a history of functional differentiation unique to RMP. We demonstrate for all three families that expression of family members in iRBC is not mutually exclusive. Most tagged proteins were transported into the iRBC cytoplasm but not onto the iRBC plasma membrane, indicating that they are unlikely to play a direct role in iRBC-host cell interactions. Unexpectedly, most family members are also expressed during the liver stage, where they are transported into the parasitophorous vacuole. This suggests that these protein families promote parasite development in both the liver and blood, either by supporting parasite development within hepatocytes and erythrocytes and/or by manipulating the host immune response. Indeed, in the case of Fam-A, which have a steroidogenic acute regulatory-related lipid transfer (START) domain, we found that several family members can transfer phosphatidylcholine in vitro. These observations indicate that these proteins may transport (host) phosphatidylcholine for membrane synthesis. This is the first demonstration of a biological function of any exported variant protein family of rodent malaria parasites.

bModel antigens are frequently introduced into pathogens to study determinants that influence T-cell responses to infections. To address whether an antigen's subcellular location influences the nature and magnitude of antigen-specific T-cell responses, we generated Plasmodium berghei parasites expressing the model antigen ovalbumin (OVA) either in the parasite cytoplasm or on the parasitophorous vacuole membrane (PVM). For cytosolic expression, OVA alone or conjugated to mCherry was expressed from a strong constitutive promoter (OVA hsp70 or OVA::mCherry hsp70 ); for PVM expression, OVA was fused to HEP17/EXP1 (OVA::Hep17 hep17 ). Unexpectedly, OVA expression in OVA hsp70 parasites was very low, but when OVA was fused to mCherry (OVA::mCherry hsp70 ), it was highly expressed. OVA expression in OVA::Hep17 hep17 parasites was strong but significantly less than that in OVA::mCherry hsp70 parasites. These transgenic parasites were used to examine the effects of antigen subcellular location and expression level on the development of T-cell responses during blood-stage infections. While all OVA-expressing parasites induced activation and proliferation of OVA-specific CD8؉ T cells (OT-I) and CD4 ؉ T cells (OT-II), the level of activation varied: OVA::Hep17 hep17 parasites induced significantly stronger splenic and intracerebral OT-I and OT-II responses than those of OVA::mCherry hsp70 parasites, but OVA::mCherry hsp70 parasites promoted stronger OT-I and OT-II responses than those of OVA hsp70 parasites. Despite lower OVA expression levels, OVA::Hep17 hep17 parasites induced stronger T-cell responses than those of OVA::mCherry hsp70 parasites. These results indicate that unconjugated cytosolic OVA is not stably expressed in Plasmodium parasites and, importantly, that its cellular location and expression level influence both the induction and magnitude of parasite-specific T-cell responses. These parasites represent useful tools for studying the development and function of antigenspecific T-cell responses during malaria infection.T cells play a central role in the immune response to malaria and can help to control blood-stage infections (1, 2). For example, in human and rodent malaria infections, effector CD4 ϩ T cells promote antiparasitic antibody production and regulate macrophage-based antiparasitic effector responses (1, 2). However, it is also clear that proinflammatory T-cell responses, if not regulated appropriately or if present in the wrong environment, can contribute to the development of immunopathology during malaria infection (3, 4). Thus, understanding how malarial proteins are recognized by the immune system to initiate adaptive T-cell responses and identifying the antigen-specific T-cell responses involved in protection and pathology during infection have significant importance for vaccine development and for identification of predictive immunological biomarkers for severe malarial disease.Difficulties in identifying endogenous T-cell epitopes within blood-stage malaria parasites have hampered the inves...

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