OBJECTIVE The intraocular microenvironment is an immune-privileged site where immunogenic inflammation is suppressed. Suppression of immunogenic inflammation has been associated with immunosuppressive factors found in aqueous humor produced by ocular tissues. To further understand the mechanisms suppressing immunogenic inflammation in the eye, we have examined the production of lymphokines by primed T-cells activated in the presence of aqueous humor. METHODS Enriched in vivo primed T-cells were T-cell receptor-stimulated in the presence of fresh aqueous humor. The culture supernatant was assayed for IFN-gamma and IL-4 by sandwich ELISA. TGF-beta production by T-cells stimulated in the presence of aqueous humor was assayed by a TGF-beta bioassay of the culture supernatant and by quantitative RT-PCR for TGF-beta 1 mRNA expression. Aqueous humor-treated T-cells were assayed for their capacity to suppress IFN-gamma production by stimulated, primed T-cells. RESULTS Aqueous humor-enhanced proliferation but irreversibly suppressed production of both IFN-gamma and IL-4 by in vitro-activated, in vivo-primed T-cells. Aqueous humor induced in vivo primed T-cells to produce TGF-beta in vitro, and these TGF-beta-producing T-cells suppressed IFN-gamma production by other T-cells activated in co-cultures. CONCLUSIONS Aqueous humor alters the functional program of TCR-ligand-activated, primed T-cells, converting the cells to TGF-beta-producing regulatory cells.