The Mechanism and Significance of Deletion of Parasite-specific CD4 ⁺ T Cells in Malaria Infection

It is thought that both helper and effector functions of CD4⁺ T cells contribute to protective immunity to blood stage malaria infection. However, malaria infection does not induce long-term immunity and its mechanisms are not defined. In this study, we show that protective parasite-specific CD4⁺ T cells were depleted after infection with both lethal and nonlethal species of rodent Plasmodium. It is further shown that the depletion is confined to parasite-specific T cells because (a) ovalbumin (OVA)-specific CD4⁺ T cells are not depleted after either malaria infection or direct OVA antigen challenge, and (b) the depletion of parasite-specific T cells during infection does not kill bystander OVA-specific T cells. A significant consequence of the depletion of malaria parasite–specific CD4⁺ T cells is impaired immunity, demonstrated in mice that were less able to control parasitemia after depletion of transferred parasite-specific T cells. Using tumor necrosis factor (TNF)-RI knockout– and Fas-deficient mice, we demonstrate that the depletion of parasite-specific CD4⁺ T cells is not via TNF or Fas pathways. However, in vivo administration of anti–interferon (IFN)-γ antibody blocks depletion, suggesting that IFN-γ is involved in the process. Taken together, these data suggest that long-term immunity to malaria infection may be affected by an IFN-γ–mediated depletion of parasite-specific CD4⁺ T cells during infection. This study provides further insight into the nature of immunity to malaria and may have a significant impact on approaches taken to develop a malaria vaccine.