The interaction between organ-resident cells from the anterior uvea of the eye and T helper (Th) cells was investigated. Cells from Lewis rat ciliary body processes (CB cells), grown in tissue culture using an explant technique, could be induced to express major histocompatibility complex class I1 (Ia) antigens by incubation with rat interferon-gamma. Ia+ CB cells only poorly functioned as antigen-presenting cells (APC) for a syngeneic, uveitogenic T h cell line specific for the retinal soluble antigen (SAg). Moreover, if added to an Ag-driven lymphocyte proliferation assay in the presence of conventional APC, the rat CB cells had an inhibiting effect on Th proliferation. This inhibitory activity was not species specific, since similar effects were observed with bovine and human ciliary epithelial cells. The suppressive activity of CB cells was composed of a soluble factor, as well as a membrane-associated inhibitor. The soluble activity did not appear to be related to transforming growth factor-beta (TGF-(3), since no reversal of inhibition by a neutralizing antibody toTGF-P was found. Part of the soluble inhibitory activity could be reversed by indomethacin treatment. The membrane-associated component was trypsin sensitive, suggesting a protein molecule. After abrogation of the inhibitory capacity by trypsin treatment and fixation by glutaraldehyde, CB cells effectively presented SAg to Th cells. These data suggest that CB cells are capable of mediating both Ag presentation and inhibition of Th cell proliferation.