2G and H). However, in the absence of T cells, addition of exogenous

IL-2 up to 100 IU/mL was unable to rescue IFN-γ production by NK cells (Fig. 2H). Thus, IL-2 contributes to, but find more alone is insufficient for NK IFN-γ production against PfRBC. Possibly, the unique immunological characteristics of PfRBC, i.e. a protozoan pathogen residing within a host cell without MHC class-I molecules, might explain the requirement of further activation signals. One potentially interesting candidate in this regard might be the IL-2 family member IL-21, which is produced by activated T cells 20 and enhances IFN-γ production by NK cells 21. Nonetheless, if T-cell help is required for NK-cell activation, this clearly suggests that it is in fact the immunological memory residing within the T-cell population, rather than intrinsic NK memory, that underlies the observed recall responses by NK cells against PfRBC. Finally, we investigated the

relative contribution of different lymphocyte subpopulations to the total IFN-γ production against PfRBC (Fig. 3A). Depletion of NK and NKT cells from PBMC with anti-CD56 beads prior to stimulation with PfRBC resulted in a reduction of Trichostatin A price IFN-γ production by approximately 60%. Thus, although these two cell types together form only around 20% of IFN-γ-producing cells following exposure (Fig. 1H), their contribution to total cytokine secretion is much greater, presumably in part due to a positive

feedback effect on T cells (Fig. 2A). Once more, however, these anti-CD3 depletion of all T cells resulted in the total abrogation of IFN-γ secretion into the supernatant by remaining PBMC, including NK (Fig. 3A). Thus NK cells are incapable of producing even small amounts of IFN-γ in response to PfRBC in the absence of T cells. In order to understand whether such patterns are representative of naturally acquired immunity to malaria, we performed similar experiments with PBMC from representative samples of three other groups: unexposed Caucasian donors (Fig. 3B), Caucasians exposed by visiting malaria-endemic areas (Fig. 3C) and semi-immune African adults living in an area of intense seasonal transmission (Fig. 3D). Although relative contributions of CD56+ cells varied slightly between individual volunteers, the overall pattern was remarkably similar in all, confirming the generality of our findings. Of particular note, two out of the six malaria-naïve donors responded to PfRBC with considerable IFN-γ production (Fig. 3B), a well-known phenomenon 4, 22–24, yet even in these “innate” responders depletion of CD3+ cells but not CD56+ cells resulted in total abrogation of IFN-γ production. In these donors, “memory” is presumably provided by cross-reacting T cells 22, 23, 25.