Suppression of mitogen- and tumor-cell-induced lymphocyte stimulation by tumor-associated fetal antigens

Abstract

The ability of tumor-associated fetal antigens (TAFA) to suppress mitogen and tumor-cell-induced blastogenesis was investigated. Three different TAFA (I, III, and IV) were tested in PHA and Con A lymphocyte proliferation assays. Spleen cells from New Zealand black rats (NBR) were fractionated over nylon wool; and nonadherent (NA) and adherent (AD) cells were compared with unfractionated (UF) cell responses. Preincubation of NA cells with TAFA-I was followed by addition of phytohemagglutinin (PHA) elicited suppression in a 3-to 4-day assay. UF cells were unresponsive to TAFA and/or PHA at all concentrations tested. TAFA dose—response titration curves in Con A proliferation assays revealed that all TAFA tested (TAFA I, -III, and -IV from fibrosarcomas; TAFA-I and -III from osteosarcomas) were suppressive. For some TAFA, nanogram quantities produced significant suppression. In mixed leukocyte tumor cell assays (MLTC) both UF and NA normal rat spleen cells were tested for proliferative responses to syngeneic tumor cells. Four distinct TAFA, purified by high-pressure liquid chromatography, suppressed lymphocyte proliferation when added to MLTC cultures in 5-day assays. Specificity experiments demonstrated that trinitrophenol-bovine serum albumin did not produce similar immunosuppression. TAFA did not block recognition of tumor antigen nor produce nonspecific cytotoxicity of the spleen cells. Significant suppression of DNA synthesis was produced by TAFA-1 following cocultivation with spleen and tumor cells for 1, 2, and 3 days, compared to no suppression when spleen and tumor cells were washed free of TAFA-I prior to tumor cell addition at Day 0. Similar experiments using rat embryo fibroblasts (REF) as stimulators demonstrated that pre-REF cocultivation treatment of lymphocytes with TAFA-I significantly reduced subsequent blastogenic responses. This effect was not reversible; however, if TAFA-I was added to responders previously stimulated by REF, a suppressive response was not seen. Experiments were also carried out to determine the reversibility of TAFA-I effects. Cells were treated with TAFA-I from 1 to 5 days, followed by determination of lymphocyte blastogenesis. TAFA-I effects are reversible and antigen presence is required to completely suppress (or inhibit) stimulation by tumor cells.