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 degrees 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-gamma, transforming growth factor-beta or alpha-fetoprotein.