Stimulated rat T cell-derived inhibitory factor for cellular DNA synthesis (STIF). I. Isolation and characterization

A lymphokine inhibitory for cellular DNA synthesis (termed STIF) was isolated from the culture supernatants of concanavalin A (Con A)-stimulated SD rat spleen cells. STIF inhibited the DNA synthesis of mouse bone marrow cells as well as mouse leukemia cells. STIF has an apparent m.w. of 45,000 to 50,000 and is separable from IL 2, m.w. 20,000 to 25,000, by Sephacryl S-200 gel filtration, but not from immune interferon (IFN) having the same m.w. as STIF. Con A-Sepharose chromatography of the fraction containing STIF and IFN could separate these lymphokines into Con A-unbound and Con A-bound fractions, respectively. Further fractionation of the STIF fraction by DEAE-Sephadex A-50 or Mono Q-FPLC anion exchange chromatography indicated that the STIF fraction contained two components of STIF activity, both showing the same pI value (5.1 to 5.6) on flat-bed isoelectric focusing. STIF was characterized as a sugar-free lymphokine of trypsin-sensitive protein nature.