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The recently cloned cytotoxic lymphocyte maturation factor [CLMF] also called NK cell stimulatory factor [NKSF] or interleukin-12 [IL-12] has been described as a growth factor for mature lymphoid cells. The present study investigated whether purified recombinant human IL-12 could stimulate CFU colony growth. Source of progenitor cells were peripheral blood cells depleted of adherent, CD2- and CD56-positive cells. RhIL-12 was investigated either alone or in combination with rhIL-3, rhIL-6 and rhGM-CSF. RhIL-12 alone did not support colony formation of myeloid or erythroid progenitors. RhIL-12 in combination with rhIL-3 increased the numbers of BFU-E and CFU-GM. No synergism or additive effect was seen with the combination of rhIL-12 and rhGM-CSF or rhIL-12 and rhIL-6. An additive increase in the number of granulocytic colonies was observed when rhIL-3, rhIL-6 and rhGM-CSF were used together with rhIL-12. Our result therefore suggest that, in addition to being a potent lymphopoietic stimulator, IL-12 acts synergistically with IL-3 in enhancing the sensitivity of hemopoietic progenitors to IL-3.

To gain additional informations on the role played by Natural Killer cells (NK) in the differentiation of human hematopoietic precursors, we have studied the effect of NK-cell depletion on the in vitro proliferation of hematopoietic cells. NK cells were depleted from blood mononuclear cells by FACS using anti CD3 and anti CD56 monoclonal antibodies. Depletion of NK cells suppressed CFU-GM up to 69% (P < 0,01), while no significant effect on either BFU-E and CFU-Mix growth was observed. To define the threshold of CD56+ cells required to support CFU-GM formation, NK cells were added to NK-depleted cells in a titrated fashion. Enhancement of CFU-GM colony growth was observed at NK/NK - depleted cells ratio of 0.15/1. A dose dependent suppression of CFU growth was observed at ratios ranging from 0.25/1 to 0.5/1. Addition of neutralizing antibodies against IL3 and GM-CSF abrogate the stimulating effect of NK cells. Our results suggest that cells with LGL morphology and NK markers play an important role in differentiation of myeloid precursors and exert a moderate influence on erythroid progenitors. The modulatory effect on hematopietic progenitors depend on the number of NK cells present in the mixed culture. © 1994 Wiley-Liss, Inc.