Hyalocytes synthesize and secrete inhibitors of retinal pigment epithelial cell proliferation in vitro

Background: Retinal pigment epithelial (RPE) cells that enter the vitreous in pathologic conditions, such as retinal detachment, may proliferate and contribute to the formation of epiretinal membranes. Objective: To study whether hyalocytes, endogenous vitreous cells, play a role in modulating the proliferation of RPE cells. Methods: Cell proliferation was measured by tritiated thymidine incorporation in density-arrested human RPE cells after incubation with media that had been conditioned by cultured bovine hyalocytes. Preliminary characterization of inhibitory activity in hyalocyte- conditioned medium was performed, including blocking experiments with a neutralizing antibody to transforming growth factor-β₂ (TGF-β) and proliferation assays that used MV-1-Lu mink lung epithelial cells. Northern blots were done to assess hyalocyte expression of TGF-β messenger RNA. Results: Hyalocyte-conditioned medium inhibited tritiated thymidine incorporation in RPE cells and MV-1-Lu mink lung epithelial cells in the presence or absence of serum or protease inhibitors. A portion of the inhibitory activity was neutralized by an antibody directed against TGF-β. Northern blots of hyalocyte RNA demonstrated the presence of messenger RNA for TGF-β₂. These data suggest that TGF-β is responsible for a portion of the inhibitory activity secreted by hyalocytes. Additional inhibitory activity is attributable to one or more low-molecular-weight molecules distinct from TGF-β. Conclusion: Hyalocyte-conditioned medium inhibits RPE cell proliferation in vitro through TGF-β and at least one other molecule. Production of these factors by hyalocytes in vivo could provide a deterrent for epiretinal membrane formation that may be perturbed under pathologic conditions.