T cells can react to self-cells bearing the syngenic major histocompatibility complex class II molecule Ia. Decreased autoreactive T cell responses are associated with cancer. Tumor growth causes syngeneic macrophages (M phi) to suppress autoreactive T cell proliferation by decreasing M phi Ia expression and increasing M phi production of the suppressor molecule prostaglandin E2 (PGE2). Because M phi produce tumor necrosis factor-alpha (TNF-alpha) during cancer, and TNF-alpha stimulates M phi PGE2 synthesis, we determined if TNF-alpha mediates tumor-induced suppression of autoreactive T cell proliferation stimulated by syngeneic M phi. We showed that tumor growth increases TNF-alpha production because tumor-bearing host (TBH) M phi synthesized more TNF-alpha than normal host (NH) M phi when cultured with lipopolysaccharide. Exogenous TNF-alpha increased NH CD4+ autoreactive T cell proliferation stimulated by syngeneic NH M phi but not by TBH M phi. When endogenous TNF-alpha activity was neutralized by anti-TNF-alpha antibody addition, T cell proliferation decreased when stimulated by NH M phi but increased when stimulated by TBH M phi. Kinetic studies showed that TNF-alpha affected M phi-stimulated T cell proliferation during the first few hours (4h) of the 96 h culture time. Indomethacin-treatment allowed TNF-alpha to increase T cell proliferation stimulated by TBH M phi. A PGE2-specific enzyme-linked immunosorbent assay showed that TBH M phi T cell cultures contained significantly more PGE2 than those containing NH M phi, and that exogenous TNF-alpha increased PGE2 production in TBH M phi cultures more than in NH M phi cultures. Short-term (4h) pretreatment of M phi with TNF-alpha increased T cell proliferation stimulated by NH, but not TBH, M phi. However, long-term (16 h) TNF-alpha pretreatment reversed TBH M phi-mediated suppression, suggesting that early suppressor molecular production inhibits synthesis or activity of TNF-alpha-induced stimulatory monokines. Although TNF-alpha is known to increase T cell proliferation, these results show that the tumor-induced increase in M phi TNF-alpha synthesis suppress autoreactive T cell proliferation, which is mediated by PGE2 production.