Surface expression of inhibitory (CTLA-4) and stimulatory (OX40) receptors by CD4(+) regulatory T cell subsets circulating in human malaria.

Several CD4(+) T cell subtypes contribute to immune homeostasis in malaria, but the markers that define the main suppressive T cell subsets induced by this infection remain largely unknown. Here we provide a detailed phenotypic characterization of immunoregulatory CD4(+) T cell populations in uncomplicated human malaria. We found an increased proportion of CD4(+) T cells expressing CTLA-4, OX40, GITR, TNFRII, and CD69 in acute-phase single-species infections with Plasmodium vivax, Plasmodium falciparum, or both. Such an increase was not proportional to parasite density in P. vivax infections, and did not persist after parasite clearance. Significantly, less than 10% of CD4(+) T cells expressing these regulatory molecules had the classical T regulatory (Treg) phenotype (CD4(+)CD25(+)CD127(-)FoxP3(+)). Two major Treg cell subpopulations, which together accounted for 19-23% of all Treg cells circulating in malaria patients, expressed surface receptors with opposing regulatory functions, either CTLA-4 or OX40. OX40(+) Treg cells outnumbered their CTLA-4(+) counterparts (1.8:1) during acute P. vivax infection, while a more balanced ratio (1.3:1) was observed following parasite clearance These data reveal new players in the complex CD4(+) Treg cell network that maintains immune homeostasis in malaria and suggest potential targets for therapeutic interventions to improve parasite-specific effector immune responses.