The Subcellular Location of Ovalbumin in Plasmodium berghei Blood Stages Influences the Magnitude of T-Cell Responses

Abstract

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...

CD8+ T Cells Induce Fatal Brainstem Pathology during Cerebral Malaria via Luminal Antigen-Specific Engagement of Brain Vasculature

“…To determine the anatomical relationship between myelomonocytic cells and iRBCs during the development of ECM, we infected LysM gfp/+ mice with recombinant PbA expressing mCherry and OVA (PbA-OVA-mCherry) [45]. In symptomatic mice at d6 p.i., we observed fluorescent iRBCs adherent to cerebral vasculature (Fig 2B; S3 Movie), similar to what is found in human CM patients.…”

“…PbA.OVA::mCherry hsp70 (PbA-OVA-mCherry) parasites were kindly provided by C. Janse and S. Khan [45]. PbTg (PbA-OVA-GFP) parasites were kindly provided by W. Heath [38].…”

cGAS-mediated control of blood-stage malaria promotes Plasmodium-specific germinal center responses

“…To measure antigen-specific CD4 + T cell expansion, we utilized a P. yoelii parasite modified to stably express amino acids 51-84 of the LCMV GP under the Hep17 promoter (Supplemental Figure 3A). Expression under the Hep17 promoter localizes peptide expression to the parasitophorous vacuole, and this promoter is active during the erythrocytic stage of infection (29). This LCMV sequence encodes an immunogenic CD4 + T cell epitope presented on I-A b MHC molecules (aa 61-80) (30,31).…”

The use of transgenic parasites in malaria vaccine research

“…Reduced sporozoite production #69 infections [9,10,[56][57][58]. For example, intravital two-photon microscopy of livers of mice infected with P. berghei parasites that express OVA and GFP in their cytoplasm showed that transferred OVA-specific CD8 + T cells recognize and forms clusters around infected hepatocytes, leading to the elimination of the intra-hepatic parasites [56].…”

“…These parasites have been used to visualize and analyze parasite growth and development in vitro and in vivo and have been valuable tools to analyze cellular and molecular features of malaria parasite biology (reviewed in [4][5][6][7]). Transgenic rodent parasites have also been used to provide mechanistic insights into host-parasite interactions that regulate host (immune) responses to infection or those that mediate malarial pathology [8][9][10][11][12][13].…”