Purpose: Classic studies have demonstrated that corneal epithelial cell density in culture can alter the balance of stimulatory and inhibitory cytokines controlling the elaboration of collagenolytic activity by co-cultured stromal cells. The current study attempts to bring the understanding of this mechanism to a molecular level.
Methods: A rabbit primary corneal cell culture model was used.
Results: Using molecular probes that bind to and neutralize specific cytokines, a major stimulator for stromal cell collagenase synthesis released by corneal epithelial cells into culture medium was identified as interleukin-1 alpha (IL-1 alpha), and a secondary stimulator was characterized as a heparin-binding cytokine. An inverse relationship between net collagenase stimulatory activity and epithelial cell plating density was demonstrated. In contrast, the release of inhibitory activity for IL-1-stimulated collagenase synthesis was not subject to the cell density effect. Direct measurement of IL-1 alpha protein levels revealed that this cytokine was released much more efficiently on a per cell basis when cells were plated at low density than when they were plated at high density. The effect was not caused by greater cell lysis at low cell density and was mediated only partially by changes at the IL-1 alpha synthesis level.
Conclusions: These data provide evidence that epithelial cells release stimulatory cytokines for collagenase expression more efficiently when they have limited contact with their neighbors and that this has important consequences for the overall paracrine cytokine balance controlling collagenase synthesis. Alteration of the paracrine cytokine balance by changes in cell contact may be an important means for regulating epithelial-stromal interactions involved in corneal development and repair.