Stimulation of natural killer activity in peripheral blood lymphocytes of healthy donors and melanoma patients in vitro: synergism between interleukin (IL)-12 and IL-15 or IL-12 and IL-2.

Interleukin-2 (IL-2) and IL-12 modify the functional status of T- and natural killer (NK) cells by regulating proliferation, cytolytic activity, cytokine induction, and T-cell subset differentiation. These effects are exploited in immunotherapy of cancer patients with IL-2 or IL-12, which, however, is limited by potentially life-threatening side effects. IL-15 shares many of the biological activities of IL-2 and may therefore represent a therapeutic alternative. Here we have compared the ability of these interleukins to stimulate NK activity in peripheral blood lymphocytes (PBLs) isolated from healthy donors (n = 12) as well as from patients (n = 12) suffering from metastatic disease (melanoma). Target (K562) cell lysis was assessed by determining the release of 51Cr and lactate dehydrogenase (LDH) which gave equivalent results. The NK-resistant DAUDI cell line served as control target. Unstimulated NK activity was significantly lower in PBLs purified from melanoma patients. However, cytolytic activity was readily stimulated by preincubation of PBLs (18 h) with cytokines such that the maximum target cell lysis was comparable to that seen in PBL of healthy donors. Similarly, the potency of IL-2 (EC50 = 20.2+/-1.3 and 22.0+/-1.3 u/ml in healthy donors and patients, respectively), IL-12 (EC50 = 11.0+/-1.1 and 4.3+/-1.6 u/ml) and IL-15 (EC50 = 0.3+/-0.1 and 0.2+/-0.1 u/ml) was comparable. Importantly, if the preincubation was carried out with cytokine concentrations in the EC50 range, the effects of two cytokines (tested in all combinations) were additive. A synergism was evident in PBLs obtained both from healthy donors and melanoma patients if concentration-response curves for IL-12 were determined in the presence of increasing concentrations of IL-2 (enhanced efficacy) or IL-15 (enhanced efficacy and potency). Our observations suggest possible alternatives to the monotherapy with IL-2 (or IL-12) in cancer treatment. Provided that the present findings can be extrapolated to the situation in vivo, the combined administration of IL-12 and IL-15 may be as efficacious as the immunotherapy with IL-2; this approach ought to allow for a marked reduction in cytokine dose and thereby improve the therapeutic index.