Cross-talk between T cells and NK cells generates rapid effector responses to Plasmodium falciparum-infected erythrocytes

Rapid cell-mediated immune responses, characterized by production of proinflammatory cytokines, such as IFN-γ, can inhibit intraerythrocytic replication of malaria parasites and thereby prevent onset of clinical malaria. In this study, we have characterized the kinetics and cellular sources of the very early IFN-γ response to Plasmodium falciparum-infected RBCs among human PBMCs. We find that NK cells dominate the early (12-18 h) IFN-γ response, that NK cells and T cells contribute equally to the response at 24 h, and that T cells increasingly dominate the response from 48 h onward. We also find that although γδ T cells can produce IFN-γ in response to P. falciparum-infected RBCs, they are greatly outnumbered by αβ T cells, and thus, the majority of the IFN-γ⁺ T cells are αβ T cells and not γδ T cells; γδ T cells are, however, an important source of TNF. We have previously shown that NK cell responses to P. falciparum-infected RBCs require cytokine and contact-dependent signals from myeloid accessory cells. In this study, we demonstrate that NK cell IFN-γ responses to P. falciparum-infected RBCs are also crucially dependent on IL-2 secreted by CD4⁺ T cells in an MHC class II-dependent manner, indicating that the innate response to infection actually relies upon complex interactions between NK cells, T cells, and accessory cells. We conclude that activation of NK cells may be a critical function of IL-2-secreting CD4⁺ T cells and that standard protocols for evaluation of Ag-specific immune responses need to be adapted to include assessment of NK cell activation as well as T cell-derived IL-2.