Antigen-specific purification of blocking factors from sera of mice with chemically induced tumors.

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Abstract
Serum from mice with growing tumors can prevent (block) the destruction of tumor cells by immune lymphocytes as measured in a microcytotoxicity assay. Factors responsible for this blocking activity were purified by binding to immune adsorbents prepared from antibodies obtained by immunizing BALB/c mice to the homologous tumors. Two transplantable BALB/c sarcoma lines [MCA 1321 and MCA 1408] with individually different tumor-specific transplantation antigens were studied in parallel. The original tumor-specific blocking activity was recovered by elution of the immune adsorbents; i.e., eluates blocked the reduction of surviving tumor cell targets by immune lymphocytes only if the tumor specificity of targets and lymphocytes corresponded to the serum specificity, and immune adsorbent columns prepared from tumor-immune sera recognized the purified blocking fractions in a tumor-specific fashion, indicating that a portion of the humoral response in the immune mice was directed against a factor that was individually distinct for each tumor. Absorption of eluates with the homologous tumor cells removed their blocking activity, indicating that the blocking factors have antigen-binding properties. Blocking activity in the purified fractions resided in molecules presumptively identified as glycoproteins by affinity chromatography on concanavalin A-Sepharose.