Cellular migration into neural retina following implantation of melanin granules in the subretinal space

Background: In some retinal diseases and following transplantation of retin al pigment epithelium (RPE), melanin granules are liberated to the subretin al space. Our aim was to investigate the cellular response to implanted ext racellular melanin. Methods: After pars plana vitrectomy, 17 albino rabbits received a suspension of melanin granules in the subretinal space. Postope rative examination included ophthalmoscopy, color fundus photography, histo logy using monoclonal antibodies identifying RPE cells (AE1/3), macrophages (RAM II), B-lymphocytes (CD20) and T-lymphocytes (CD45), and electron micr oscopy. The follow-up time was 2 weeks, 4 weeks and 6 months. Results: On f undus photographs, the layer of melanin showed focal attenuation with light er areas at 6 months. Melanin granules were phagocytosed by RPE cells and m acrophages at 2 weeks, as identified by monoclonal antibodies. In areas whe re an abundance of melanin was present, multilayers of macrophages were see n associated with considerable photoreceptor damage. Pigment-laden cells in vaded the neural retina. The cellular infiltration of the retina was focal, and when it involved the outer nuclear layer the photoreceptor damage was severe. Electron microscopy demonstrated the presence of melanosomes intrac ellularly in Muller glia. The process of phagocytosis and removal of melani n granules from the subretinal space was slow and not completed at 6 months . Conclusion: Our experiments show that implantation of melanin granules in the subretinal space of albino rabbits may induce a considerable phagocyti c cellular response featuring the host's RPE, macrophages and glial cells. The migration of pigment-laden cells into the neural retina was associated with focal photoreceptor damage.