The trabecular meshwork (TM)/Schlemm's canal (SC) outflow pathway is the tissue responsible for maintaining normal levels of intraocular pressure. In the present study, we investigate the effects of mechanical stress oil the expression of IL-6 in the TM meshwork,, as well as the effects of this cytokine oil outflow pathway function. Application of cyclic mechanical stress to human TM primary cultures resulted in a statistically significant increase in both secretion and transcription of IL-6, compared to nonstressed controls. Addition of TGF-beta 1, which has been reported to be upregulated in TM cells under mechanical stress, also induced a significant activation of both the transcription and secretion of IL-6. Moreover, anti-TGF-b1 antibodies partially blocked the stretch-induced IL-6 production. Injection of IL-6 into perfused porcine anterior segments resulted in a 30% increase in outflow facility, as well as increased permeability through SC cell monolayers. These results suggest a role for IL-6 in the homeostatic modulation of aqueous humor outflow resistance. (c) 2005 Elsevier Inc. All rights reserved.