Monocytes are known to engage in reciprocal crosstalk with NK cells but their influence on NK-cell-associated antibody-dependent cellular cytotoxicity (ADCC) is not well understood. We demonstrate that in humans FcγRIII (CD16)-dependent ADCC by NK cells is considerably enhanced by monocytes, and that this effect is regulated by FcγRII (CD32) crosslinking in healthy individuals. It is known that during HIV-1 infection, NK cells are known to express low levels of CD16 and exhibit reduced ADCC. We show that immune regulation of CD16-mediated NK-cell cytotoxicity by monocytes through CD32 engagement is substantially disturbed in chronic progressive HIV-1 infection. Expression of activating isoform of CD32 represented a compensatory mechanism for reduced expression of CD16 on NK cells during HIV-1 infection. As a result, the regulation of NK-cell-associated ADCC by monocytes is skewed and eventually constitutes a novel factor that contributes to HIV-1-associated immune deficiency, dysregulation and pathogenesis. Our data therefore provide evidence, for the first time, that in humans monocytes act as a rheostat for FcγRIII-mediated NK-cell functions maintaining a well-balanced immune response.
