Interaction between human interleukin-2-activated natural killer cells and heat-killed germ tube forms of Candida albicans.

Human interleukin-2-activated natural killer (LAK) cells are able to recognize and to bind to both live and heat-killed germ tube forms of Candida albicans, establishing a wide and intimate contact as revealed by electron microscopic observations. Following the interaction, LAK cells are activated: an increased expression of some cytokine mRNA (in particular, TNF-alpha, GM-CSF, and IFN-gamma) has been revealed by RT-PCR and perforin secretion has been suggested by immunofluorescence microscopy. Nonetheless, neither morphological damage or growth inhibition of fungal target cells have been detected. Instead, evident signs of cell damage could be noticed in interacting LAK cells. Moreover, the observation by transmission electron microscopy of LAK cell-germ tube conjugates revealed the presence of apoptotic cells. The analysis of LAK cell cytotoxic activity against DAUDI cells showed that the lymphocytic effector underwent a significant reduction in its lytic capability after the interaction with C. albicans. The results obtained in this in vitro study seem to indicate that in such an interaction LAK cells cannot directly inhibit or kill the fungal pathogen by using their lytic machinery but they secrete those cytokines which have stimulatory effects on phagocytic cells. The ultimate results are the programmed death of LAK cells and the enhancement of the fungicidal activity exerted by competent cells.