Transforming growth factor-beta2 (TGF-beta2) is elevated in the aqueous humor of patients with primary open-angle glaucoma (POAG), and high levels of TGF-beta2 are thought to contribute to the pathogenesis of POAG. Most TGF-beta2 in the eye is present in a latent, inactive form and the mechanisms of its in vivo activation are unclear. Since thrombospondin-1 (TSP-1) is one of the most potent in vivo activating molecules of TGF-betas, we investigated the localization and expression of TSP-1 in the aqueous humor outflow pathways. TSP-1 immunohistochemistry was performed in the eyes of human donors (8 normal and 17 with glaucoma). In addition, the eyes of Tsp-1(-/-)-deficient mice and normal Tsp-1(+/+) mice were investigated. TSP-1 mRNA expression was assessed by reverse transcription-polymerase chain reaction and Northern blotting of RNA from fresh trabecular meshwork (TM), and human and mouse TM cells in vitro. In addition, Northern and Western blot analyses of TM cells after incubation with TGF-beta and dexamethasone were performed. In most of the eyes, TSP-1 immunolabeling was predominantly observed in extracellular areas of the juxtacanalicular (cribriform) part of the TM. Some focal staining was observed in the corneoscleral and uveal parts of the TM. In the eyes of six glaucoma patients (including one with steroid-induced glaucoma), TSP-1 immunoreactivity was considerably more intense and all regions of the TM were positively labeled. In double labeling experiments, staining for TSP-1 did not overlap with that of fibronectin or type VI collagen. mRNA for TSP-1 was detected in both fresh and cultured TM cells. Incubation of TM cells with TGF-beta1 and dexamethasone caused a marked increase in TSP-1 expression. TSP-1 in the TM might act as a potent local endogenous activator of TGF-betas in the aqueous humor and mediate any local effects of TGF-beta and/or dexamethasone on the outflow of aqueous humor.