Serum-derived immunosuppressive substances. II. An evaluation of various sources for an endogenous regulator of lymphocyte activation.

Four published methods (ion-exchange chromatography, heat, molecular sieving and alcohol fractionation) for the preparation of fractions of normal sera and tissues which possess immunosuppressive activity are compared. Human and bovine serum and human, bovine and ovine tissues including spleen, placenta and thymus were investigated as sources of substances which were immunosuppressive when added to mixed lymphocyte cultures and when injected into mice 24 h before antigenic challenge. Cohn fraction IV of human serum purified by ion-exchange chromatography at pH 5 was the most convenient preparation examined. The problems of establishing the nature of or the identity between substances being studied in different laboratories are discussed and the recommendation is made that a standard assay for activity be adopted.