Calf melanin immunomodulates RPE cell attachment to extracellular matrix protein

It is widely accepted that RPE melanin has a protective effect against oxidative damage in RPE cells. It is possible that an additional protective characteristic of melanin is the ability to modulate RPE cell immune response. In this study, in vitro modeling was used to probe the relationship between RPE pigmentation and immune response by monitoring IL-6 expression and secretion in calf melanin pigmented ARPE-19 cells seeded onto glycated extracellular matrix as a stressor.

ARPE-19 cells were left unpigmented or were pigmented with either calf melanin or latex beads, and were then seeded onto RPE-derived extracellular matrix (ECM) or tissue culture-treated plates (no ECM). ECMs were modified by glycation. IL-6 expression was measured using qPCR and IL-6 secretion was determined using an ELISA, both at 30 min and 24 h after seeding. MTT assay was used to quantify cell attachment to glycated matrices 30 min after seeding. In unpigmented ARPE-19 cells, rate of cell attachment to substrate was monitored for 60 min after seeding using a hemacytometer to count unattached cells. Additionally, cell viability was evaluated using the Neutral Red assay 24 h after seeding.

A significant increase in IL-6 expression was observed in calf melanin pigmented cells versus latex bead and unpigmented controls (p < 0.0001) 30 min after seeding onto ECM. Twenty-four hours after seeding, a significant decrease in IL-6 expression was observed in calf melanin pigmented cells (p < 0.0001) versus controls, implicating down-regulation of the cytokine. Additionally, calf melanin pigmented cell populations showed significant increase in attachment compared to unpigmented controls on either no ECM or unmodified ECM.

Pigmentation of RPE cells with calf melanin resulted in significant changes in IL-6 expression regardless of ECM modification, in vitro. These findings suggest that melanin in the RPE may participate in immune response modulation in the retina with particular regard to cell attachment to protein substrates. The results of this study further implicate the role of chemical changes to melanin in regulating inflammation in retinal disease.