Characterization of the rat retinal pigment epithelial molecule RET-PE2

Purpose. The RET-PE2 antigen was originally described as a marker of rat retinal pigment epithelium (RPE) (Neill and Barnstable, Exp. Eye Res. 51, 573, 1990). To further characterize this molecule we have studied its expression in a variety of other tissues using the RET-PE2 monoclonal antibody. Methods. RET-PE2 immunoreactivity was demonstrated on cryostat sections by using an indirect immunofluorescence technique. Sections of retina, liver, testis, kidney and brain in the rat were analyzed by confocal laser-scanning microscopy. Reactivity of the antibody was also determined by Western blotting. Results. RET-PE2 labeling was found in the apical, lateral and basal surfaces of the RPE. RET-PE2 labeling was also seen in liver (basolateral membrane of hepatocytes), testis (basal cells and Leydig cells), kidney epithelium of kidney tubules, but not glomeruli) and brain (choroid plexus). The RET-PE2 monoclonal antibody detected bands in crude homogenates of rat retina, liver, testis, kidney and brain that varied between 40 and 55 kDa. Conclusions. Our results show that the distribution of the RET-PE2 molecule is very similar to that of a previously described lymphocyte antigen MRC-OX47. MRC-OX47 is a membrane molecule present at low levels on many lymphocytes but whose expression is markedly increased on activation with mitogens. MRCOX47 is a member of the irnmunoglobulin superfamily and so may serve as a receptor or adhesion molecule. Our results suggest that RET-PE2 may serve as an interface that regulates interactions between the retina and circulating cells or molecules.