Stimulation natural cytotoxic activity in mixed cell cultures and by culture supernatants containing interferon.

We have previously demonstrated that lymphocyte preparations cultured for 5 days with a number of different mitomycin-C-treated lymphoid cell lines showed increased non-specific (NK-like) cytotoxic activity. We have now examined the activity induced in T- and B-cell-enriched responder populations, prepared by sheep red blood cell (SRBC) rosette separation, and found that increased cytotoxic activity was present mainly in the T-cell-enriched fraction. To investigate whether soluble factors were involved in the mediation of enhanced cytotoxicity, supernatants from mixed cell cultures were used to pretreat freshly isolated effector cell populations before measurement of cytotoxicity against K562 cells. Effector cells pretreated with supernatants from mixed cell cultures incubated for 2–6 days showed increased cytotoxic activity, the greatest activity being obtained with supernatants from 4 and 5 day co-cultures. Increased activity was observed when effector cells were pretreated for 2, 4 and 18 hr with supernatants from mixed cell cultures, regardless of the nature of the stimulator cell (T- and B-cell-derived lines, K562 and normal allogeneic lymphocytes). Active supernatants were produced by T-cell-enriched, but not B-cell-enriched responder populations. Supernatants from mixed cell cultures were also examined for the presence of interferon (IFN) and in general, those mediating large increases in cytotoxic activity also contained significant anti-viral activity. The IFN detected was not neutralized by antiserum to IFN-β but was partly neutralized by antiserum to IFN-α. pH 2 treatment also removed part of the activity while a combination of pH 2 and anti-α completely neutralized the activity suggesting the presence of IFN-γ and IFN-α. The candidacy of IFNs as mediators of enhanced cytotoxicity in this system is discussed.