Elution of "blocking factors" from human tumors, capable of abrogating tumor-cell destruction by specifically immune lymphocytes

Sera from patients with growing neoplasms have been previously shown capable of specifically blocking the cytotoxic effect of lymphocytes immune to tumor-associated antigens (TAA) of the respective patients' neoplasms. The present investigation demonstrates that a similar blocking activity can be eluted at pH 3.1 from human tumor tissues, obtained at surgery (seminomas and osteogenic sarcomas) and from tumor cells growing in ascites and pleural effusions (carcinomas of endometrium, breast and ovary). A high (MW above 100,000) and a low (MW below 100,000) molecular weight fraction can be separated from the eluates by ultrafiltration. Neither of these fractions could block lymphocyte-mediated cytotoxicity in vitro when tested by itself, while a 1:1 mixture of them could. Blocking was also obtained when the tumor cells were first exposed to the high and then to the low molecular weight fractions, but not when the sequence was reversed. The observations obtained are analogous to previous findings in animal tumor systems and provide evidence that tumors growing in human patients are coated by “blocking factors”.