Differential expression of the complement regulatory proteins in the human eye

The presence of complement activation products in the human eye during infection or inflammation has been well described. During complement activation the host must be protected from attack against self tissue; this is achieved by three membrane-bound complement regulatory proteins: membrane cofactor protein (MCP, CD46), decay accelerating factor (DAF, CD55), and membrane attack complex inhibiting protein (CD59). This study was undertaken to analyze the expression of these proteins in the normal human eye. Tissues were sectioned by cryostat and both polyclonal and monoclonal antibodies to MCP, DAF, and CD59 were used. Control stains were performed with nonrelevant antibodies of the same immunoglobulin subclass and normal rabbit serum as well as by omission of the primary and secondary antibodies. All three proteins were found to be differentially expressed in the human eye. With anti-MCP, strong staining of the corneal epithelium and weak staining of the corneal keratocytes in stroma and photoreceptor cells was observed. Staining with anti-DAF was very strong in the corneal epithelium and the ciliary body and moderate in the corneal stroma (keratocytes) and iris. In contrast, anti-CD59 stained very strongly in the corneal epithelium, corneal stroma (keratocytes), iris, choroid, and all layers of the retina, and moderately in the ciliary body. Identification of MCP, DAF, and CD59 in the human eye provides evidence that a regulatory system exists to protect these cells from destruction by complement-activating events. It remains to be determined if other more specialized functions exist for these proteins, especially in the case of CD59 because of its extensive expression in the retina.