Filarial parasites induce NK cell activation, type 1 and type 2 cytokine secretion, and subsequent apoptotic cell death.

NK cells are an important source of early cytokine production in a variety of intracellular viral, bacterial, and protozoan infections; however, the role of NK cells in extracellular parasitic infections such as filarial infections is not well-defined. To investigate the role of NK cells in filarial infections, we have used an in vitro model system of culturing live infective-stage larvae (L3) or live microfilariae (Mf) of Brugia malayi, a causative agent of human lymphatic filariasis, with PBMC of normal individuals. We found that NK cells undergo early cell activation and produce IFN-gamma and TNF-alpha within 24 h after stimulation with both live L3 and Mf. Interestingly, NK cells also express IL-4 and IL-5 at this time point in response to live Mf but not L3. This is accompanied by significant alterations in NK cell expression of costimulatory molecules and natural cytotoxicity receptors. This activation is dependent on the presence of monocytes in the culture, IL-12, and direct contact with live parasites. The early activation event is subsequently followed by apoptosis of NK cells involving a caspase-dependent mechanism in response to live L3 but not live Mf. Thus, the NK cell-parasite interaction is complex, with filarial parasites inducing NK cell activation and cytokine secretion and finally NK cell apoptosis, which may provide an additional mechanism of down-regulating the host immune response.