Occurrence of a Soluble Nonspecific Suppressor Factor in the Serum Early after Birth

A nonspecific suppressor factor has been identified in serum of newborn rats and calves. This factor, designated SUF-s, was shown to interfere—across species barriers—with lymphocyte responses in vitro and in vivo. SUF-s interferes in vitro with T- and B-cell proliferation induced by different mitogens and IL-2. Our findings indicate that the activity of SUF-s in vitro, which is of a reversible nature, is directed at an early event in the cascade of T-cell activation. SUF-s does not affect intrinsically regulated proliferation, such as that of tumor cells or established cell lines. In vivo, SUF-s prevents graft-vs-host disease induced by transplantation of allogeneic bone marrow cells in lethally irradiated mice. Using of affinity chromatography, hydrophobic interaction chromatography, and gel filtration, a 15,000-fold purification of the suppressive factor was attained. The moiety engaged in suppression was identified as a 20- to 40-kDa protein. The biological activity is destroyed at temperatures above 70°C, by proteolytic enzyme digestion and under alkaline conditions but was resistant to acidic and reducing conditions. Judged by its biological activity and some of its physical properties. SUF-s is most likely distinct from other described suppressor factors or known cytokines with suppressor activity, such as IL-4, IL-10, interferon-γ, transforming growth factor-β or α-fetoprotein.