Rabbit Pigmented Ciliary Epithelium Produces Interleukin-6 in Response to Inflammatory Cytokines

Abstract

Interleukin-6 is a multifunctional cytokine that is found in high concentrations in intraocular fluids during the uveitic response. Although monocytic cells are a major source of interleukin-6, resident intraocular cells may also contribute to its accumulation in intraocular fluids during uveitis. The purpose of this study was to determine whether interleukin-6 is produced by pigmented ciliary epithelial cells and whether agents known to stimulate interleukin-6 production, such as interleukin-1β, tumor necrosis factor-α, bacterial endotoxin, and stimulators of the adenylyl cyclase/adenosine 3′,5′-cyclic monophosphate system, increase interleukin-6 production by these cells. Primary and first-passage cultures of nontransformed rabbit pigmented ciliary epithelial cells were incubated with the test agents for varying periods of time in serum-free medium and interleukin-6 levels in the cell-conditioned medium were measured by bioassay.Little, if any interleukin-6 was released from pigmented ciliary epithelial cells incubated for up to 18 hr in serum-free medium. Interleukin-1βstimulated interleukin-6 release in a time- and concentration-dependent manner. Tumor necrosis factor-α, although ineffective alone, increased interleukin-1β-induced interleukin-6 release in a concentration-dependent manner when co-incubated with interleukin-1βfor 18 hr. However, tumor necrosis factor-αdid not enhance interleukin-1β-induced interleukin-6 release if co-incubated with interleukin-1βfor a shorter time (6 hr). A 6 hr exposure to bacterial endotoxin did not stimulate interleukin-6 release from pigmented ciliary epithelial cells. Co-incubation of pigmented ciliary epithelial cells with interleukin-1βand agents that stimulate the adenyl cyclase/adenosine 3′,5′-cyclic monophosphate system through cell surface G-protein transduced receptors, i.e. isoproterenol, vasoactive intestinal peptide or prostaglandin E2, significantly enhanced the ability of interleukin-1βto stimulate interleukin-6 release. However, neither the adenyl cyclase activator, forskolin or the adenosine 3′,5′-cyclic monophosphate-mimetic, dibutyryl 3′,5′-cyclic monophosphate enhanced interleukin-1β-induced release of interleukin-6.These results indicate that the pigmented ciliary epithelium is one potential source of interleukin-6 and may contribute to the elevation in intraocular fluid interleukin-6 levels observed during various intraocular inflammatory episodes. Although agents that activate the adenyl cyclase/adenosine 3′,5′-cyclic monophosphate system through cell surface G-protein transduced receptors increased interleukin-1β-induced release of interleukin-6, the ineffectiveness of forskolin and dibutryl 3′,5′-cyclic monophosphate suggest that simply increasing intracellular 3′,5′-cyclic monophosphate is not sufficient to augment interleukin-1β-induced release of interleukin-6. The significance of interleukin-6 in the intraocular inflammatory response is discussed in terms of its proposed role in an endogenous antiinflammatory system acting through induction of interleukin-1 receptor antagonist, soluble tumor necrosis factor receptor, acute-phase proteins and corticosteroids.