Induction Mechanism of Human Blood CD8⁺T Cell Proliferation and Cytotoxicity by Natural Killer Cell Stimulatory Factor (Interleukin‐12)

Natural killer cell stimulatory factor (NKSF/IL-12) has been found to induce cytotoxic activity of human blood T cells. In the present study, the effect of NKSF on induction of cytotoxic CD8+ T cells in the presence or absence of monocytes was examined. Highly purified lymphocytes (> 99%) and monocytes (> 90%) were isolated by centrifugal elutriation from peripheral blood of normal donors. Then, CD8+ cells were isolated with antibody-bound magnetic beads from purified lymphocytes. The cytotoxicity of CD8+ cells was measured by 51Cr release assay for 4 h. NKSF enhanced the proliferative response of CD8+ cells stimulated with suboptimal concentrations of interleukin-2 (IL-2), but rather inhibited their proliferative and cytotoxic responses on stimulation with an optimal concentration of IL-2. NKSF stimulated CD8+ cells to produce interferon gamma (IFN gamma) irrespective of the presence of added IL-2, and this effect was augmented by co-cultivation with monocytes. Blood monocytes upregulated induction of cytotoxic CD8+ cells stimulated with NKSF alone, and this effect was abolished by addition of antibody against IFN gamma, but not of antibody against tumor necrosis factor alpha. Induction of NKSF-inducible cytotoxic CD8+ cells was inhibited by addition of transforming growth factor beta, but not of IL-4. These observations suggest that in situ induction of NKSF-stimulated cytotoxic CD8+ cells may be regulated by complex cytokine networks, depending on the participation of monocytes.