Activation of human natural killer cells by Plasmodium falciparum
The purpose of work described in this thesis was to (i) determine the contribution of innate immune responses to the early pro-inflammatory cytokine response to Plasmodium falciparum, (ii) describe the kinetics and cellular sources ofIFN-y production by human PBMC in response to activation by intact, infected erythrocytes (iRBC) or freeze-thawed schizont lysate (PfSL) and (iii) determine the activation requirements for innate immune cells responding to P. falciparum. Infected erythrocytes induce a more rapid and intense IFN-y response from malaria naive PBMC than does PfSL, correlating with rapid iRBC activation of CD3-CD56+ natural killer (NK) cells to produce IFN-y. There is marked heterogeneity between donors in the magnitude of the NK-IFN-y response not correlating with mitogen or cytokine-induced NK activation or prior malaria exposure. The NK-IFN-y response is highly IL-I2 dependent, partly IL-I8 dependent and highly dependent on direct contact between the NK cell and the parasitized erythrocyte. Exogenous rIL-I2 or rIL-I8 did not augment NK-IFN-y responses indicating that IL-I2 and IL-18 production is not the limiting factor explaining differences in NK cell reactivity between live and dead parasites or between donors. The possibility that donor heterogeneity is due to genetic variation in killer immunoglobulin- like receptors (KIR) and/or differential expression of C-type lectin receptors was also investigated. A significant up-regulation ofCD94 and NKG2A was observed in IFN-y+ NK cells of responding donors, suggesting that the inhibitory CD94:NKG2A heterodimer may serve a regulatory function on P. falciparum activated NK cells. Collectively, these data indicate that NK cells may represent an important early source oflFN-y, a cytokine implicated in induction of various anti-parasitic effector mechanisms. The heterogeneity of this early IFN-y response between donors suggests variation in their ability to mount a rapid pro-inflammatory cytokine response to malaria that may, in turn, influence their innate susceptibility to malaria infection, malaria-related morbidity or death from malaria.