Experimental melanin-protein induced uveitis (EMIU) is the sole type of uveitis evoked by a diversity of ocular melanin preparations and melanin-derived soluble polypeptides.

Experimental melanin-protein induced uveitis (EMIU) is a CD4 T cell-mediated disease involving the choroid and iris, but sparing the retina. The present study was designed to solubilize uveitogenic antigen from melanin granules without enzymatic digestion, and to investigate some of its elements by comparison with different purified melanin preparations. Many melanin surface-derived polypeptides with molecular weights ranging from 1 to > 100 kDa were obtained by extractions of the prepurified granules with hot lithium dodecyl sulfate (LDS). The mixture was electrophoretically separated into seven subfractions, each containing many components and capable of evoking the typical features of EMIU after footpad immunization of Lewis rats. The five low-molecular-weight fractions between M, 1 kDa and 30 kDa exhibited most pathogenicity which was evenly distributed among the fractions. Highly uveitogenic material remained in the melanin preparations even after multiple exhaustive extractions with LDS, and represented about 70% of the detectable protein. The uveal pathogen (UP-X) thus proved to be antigenically stable, and the major part of the pathogenic material was strongly bound to the granule surface layer. Concentrated urea solution was also capable of extracting many uveitogenic melanin polypeptides, but in a different composition than LDS did, and less effectively. Human choroidal melanin provided an LDS-soluble fraction with low pathogenicity. A single intraperitoneal injection of bovine melanin polypeptides together with pertussis toxin, but without footpad immunization in Freund's complete adjuvant, evoked EMIU as well. In all experiments, no uveitis except EMIU was observed, indicating that only one type of uveitogenic epitope was present in a wide variety of carrier molecules. An explanation for this phenomenon is discussed.