Identification and partial purification of a human natural killer cell proliferation-inducing factor

Culture supernatants of Concanavalin A activated human peripheral blood mononuclear cells were found to contain a factor which induced proliferative response in normal peripheral blood mononuclear cells. This proliferation-inducing factor specifically induced and sustained proliferation of purified human NK cells but not of T or B cells. Although interleukin 2 (IL12) also has proliferation-inducing effects on NK cells, the partially purified proliferationinducing factor preparations contained no measurable IL2 contamination. Moreover, neutralizing anti-IL2 antibodies did not block the growth effect of proliferation-inducing factor on purified human NK cells. Other cytokines which were tested, including IL4, IL6, IL7, IL12, TNF and IFN, were all found to be inactive in the proliferation-inducing factor assay. While proliferation-inducing factor by itself had no effect on T-cell proliferation, IL2-induced proliferation of T cells was significantly enhanced in the presence of proliferation-inducing factor, as was IL2-induced NK-cell proliferation. NK cells could be maintained in culture for at least a month in the presence of proliferation-inducing factor alone, but the cells lost their cytolytic activity after 3–4 weeks in culture. Addition of IL2, to NK cells which had been cultured in the presence of proliferation-inducing factor, restored their cytotoxicity. Proliferation-inducing factor activity was partially purified on an anion exchange HPLC column. The molecular weight of proliferation-inducing factor appeared to be about 10 kDa, based on its elution profile on a sizing HPLC column. Our results indicate that proliferation-inducing factor is a novel NK-cell proliferationinducing factor.