Purification and partial characterization of a tumor-specific blocking factor from sera of mice with growing chemically induced sarcomas.

The purification of a blocking factor from the sera of tumor-bearing mice is described. Whole serum with blocking activity-the ability to inhibit specific cell-mediated anti-tumor immunity in microcytotoxicity tests-was fractionated on immunoadsorbent columns containing Sepharose-bound syngeneic normal mouse immunoglobulins and immunoglobulins from tumor-immune donors. The blocking serum was derived from mice which had carried a transplanted methylocholanthrene-induced sarcoma for 21 to 28 days. Elution of the immunoadsorbents recovered the blocking activity in a single fraction. This fraction was blocking activity in a single fraction. This fraction was radiolabeled and analyzed by SDS gel electrophoresis and Sephadex G-200 column chromatography. The active component of the blocking serum was shown to be a polypeptide of m.w. 56,000. Specificity testing implied that the factor was likely to be either tumor antigen or an antigen-specific suppressor molecule.