Costimulatory signals are required for optimal proliferation of human natural killer cells

CD56dim NK cells, which comprise approximately 90% of human peripheral blood NK cells, respond to IL-2 with cytokine production, up-regulation of functionally relevant surface molecules, and augmented cytolytic activity. Nevertheless, CD56dim NK cells proliferate poorly in response to IL-2 alone. We found that other NK cell mitogens, including IL-4, IL-7, and IL-12, also induced little proliferation of CD56dim NK cells. Indeed, IL-2 stimulated at least 10-fold more NK cell proliferation than did IL-4, IL-7, or IL-12. In contrast, leukocyte-conditioned medium (LCM) induced two- to threefold greater proliferation of CD56dim NK cells than did optimal concentrations of IL-2. Although the calcium ionophore ionomycin did not stimulate proliferation by itself, it markedly augmented LCM-induced proliferation of CD56dim NK cells. Proliferation in response to either LCM alone or LCM together with ionomycin was almost completely abrogated by anti-IL-2R antibodies. Thus, IL-2 appears to be necessary but not sufficient for optimal proliferation of CD56dim NK cells. LCM-induced proliferation of ionomycin-activated CD56dim NK cells was inhibited 24% by anti-IL-1 heteroantisera and 57% by anti-TNF antisera; a combination of both antisera inhibited proliferation by 73%. Furthermore, although rIL-1 and TNF did not induce proliferation by themselves, both cytokines could augment IL-2-induced proliferation of resting or ionomycin-activated NK cells. Hence IL-1 and TNF do not appear to be primary NK cell mitogens, but rather accessory factors that can enhance IL-2-dependent NK cell proliferation. Stimulation through CD2 or CD16 Ag did not enhance LCM-induced NK cell proliferation. However, stimulation with NK-sensitive K562 cells strongly augmented CD56dim NK cell proliferation to LCM or to IL-2, IL-1, and TNF in combination. NK-resistant Daudi cells did not promote the proliferation of highly purified NK cells. Thus, NK cell proliferation may be enhanced by triggering through putative receptors for natural killing, and ionomycin may mimic such triggering. Although IL-2 by itself can induce NK cell proliferation, most NK cells resemble T and B lymphocytes in that they require multiple signals for optimal proliferation.