Interleukin-12 (IL-12) alone or in synergistic combination with IL-2 for in vitro activation of human bone marrow: Differential effects at different time points

IL-12 augments many effector functions associated with T and NK cells including induction of cytotoxic activity and secretion of various cytokines. In addition, synergistic responses with IL-2 in most of the functions attributed to that cytokine make IL-12 an attractive candidate to have a potential role as an immunotherapeutic agent. We have evaluated the effect of rhIL-12 either alone or in combination with IL-2 on the generation of cytotoxic effecters from normal human bone marrow. IL-12 induced cytotoxic effecters against NK sensitive targets within 24 h of stimulation and this activity was maintained in cultures with IL-12 supplementation for up to 2 weeks tested. The decline in NK activity seen in control cultures over a period of 2 weeks was thus not observed in IL-12 stimulated cultures. In addition, a small increase was also observed in cytotoxic activity against NK resistant Daudi cell targets. IL-12, when combined with a low to intermediate dose of IL-2, led to enhancement in IL-2 induced cytotoxicity against both NK sensitive and resistant targets. However an increase in cytotoxicity against NK resistant targets was evident only after 1 week of stimulation. Cellular yield and number of hematopoietic precursors (LTC-IC, CFU-GM, BFU-e) in IL-12 stimulated cultures was comparable to or higher than control cultures at all the three time points (days 1, 7, 14) tested. In contrast, cultures stimulated with a combination of IL-12/IL-2 showed an increased number of hematopoietic precursors at day 1 followed by a marked decrease in the number of these precursors at day 7 and day 14. The several-fold higher levels of inhibitory cytokines such as IFN-γ and TNF-β detected in these combination cultures may be responsible for this biphasic behavior of hematopoietic precursors in combination cultures. Thus, IL-12 has differential effects alone or in combination with IL-2 on lymphoid and myeloid cells.