Suppressor cell-inducing factor: A new lymphokine secreted by a natural suppressor cell line with natural cytotoxic activity. I. Purification to apparent homogeneity and initial characterization of suppressor cell-inducing factor

The lymphokine suppressor cell-inducing factor (SIF), obtained from 15 liters of serum-free culture supernatants of the natural suppressor cell line, M1-A5, has been purified to apparent homogeneity by a combination of gel filtration, ion exchange chromatography, and reverse-phase-HPLC. Purity of SIF was assessed by the migration of the factor as a single band on SDS-PAGE, and the elution from reverse-phase-HPLC column as a single and sharp peak. SIF activity was retained after both procedures. Two protein factors with SIF activity were isolated from M1-A5 culture supernatants. The first protein factor (SIFα) had a M(r) of 43 kDa, and the second protein factor (SIFβ) had a M(r) of 6 kDa. Final purification of SIFα yielded 5 μg protein with specific activity of 4 x 10⁶ U/mg protein. Final purification of SIFβ yielded 40 μg protein with specific activity of 7.5 x 10⁷ U/mg protein. The relationship between SIFα and SIFβ, as well as the relationship with other suppressor factors, will be addressed.