Natural killer (NK) cell activating factor released from murine thymocytes stimulated with an anti-tumor streptococcal preparation, OK-432.

Natural killer (NK) activity of mouse splenocytes was significantly augmented when the splenocytes were incubated for 3 to 4 hr with culture supernatants of mouse thymocytes stimulated by OK-432, an antitumor preparation from the Streptococcus pyogenes SU-strain. Antiviral activity was also detected in the culture supernatants, but IL 2 activity was not. When the culture supernatants of thymocytes stimulated by OK-432 were fractionated on a column of Blue Sepharose CL-6B, NK enhancing activity and antiviral activity were observed in partly overlapping fractions that bound to the column. However, the antiviral activity in the Blue Sepharose-bound fraction was neutralized completely by treatment with anti-IFN (alpha, beta) antiserum, whereas significant NK cell enhancing activity was still observed after treatment with anti-IFN (alpha, beta) antiserum. When the Blue Sepharose-bound fraction was subjected to gel filtration, the NK cell enhancing activity was detected in the 25,000 to 35,000 and 40,000 to 67,000 m.w. regions, but antiviral activity was observed in the over 67,000 m.w. region. These results indicate that a new kind of lymphokine, called natural killer cell activating factor (NKAF), distinct from IFN and IL 2, was found. The NKAF was found to have the following properties: its pI value is between pH 5.5 and 6.5, it binds to concanavalin A- and lentil agglutinin-Sepharose, and it is stable with pH 2-24 hr treatment. In addition, NKAF-producing cells were peanut agglutinin (PNA)-thymocytes when thymocytes were fractionated by the agglutination-sedimentation method with the use of PNA.