Experimental transplantation of retinal and iris pigment epithelial cells into the subretinal space

Abstract

A dysfunction of the retinal pigment epithelium (RPE) is the main cause for the development of age-related macular degeneration (ARMD) and visual loss in elderly people. For about 10 years experimental and clinical attempts to transplant RPE cells have been performed. The aims of this study were to elucidate the long term results of RPE allografts, to develop an atraumatic transplantation technique, and to explore the cellular response to RPE allografts, melanin granules, and autologous IPE cells. Surgery was performed on rabbits with a follow-up period of up to six months. After a pars plan vitrectomy, a subretinal bleb was created into which a suspension of RPE/IPE donor cells or melanin granules was injected. Pigmented RPE donor cells or preparations of melanin granules were implanted subretinally in albino rabbits. Pigmented rabbits were used in IPE transplantation. The eyes were monitored with ophthalmoscopy, fundus photography, light microscopy, electron microscopy and immunohistochemistry. Transplantation of suspensions of fresh pigmented RPE cells to the subretinal space in rabbits is feasible and induces virtually no complications when an atraumatic surgical procedure is used. The allograft forms a monolayer in conjunction with the native RPE and persists almost intact up to three months. At six months after transplantation, there was a cellular response exhibiting multilayers of cells, such as RPE and macrophages. Damage to adjacent photoreceptors in combination with melanin granules in the subretinal space indicates graft failure. No infiltration of lymphocytes was seen. Whether the cellular response was due to immunological or non-immunological mechanisms could not be determined from this experiment. Cyclosporine (CsA) could not prevent disintegration of the RPE transplant and graft failure. CsA was not capable of promoting graft survival as compared to the controls. The transplant seems to be disrupted either by immunological mechanisms that are not inhibited by CsA, or by non-immunologic events. Implantation of melanin granules to the subretinal space of albino rabbits induces a considerable phagocytic cellular response involving the host’s RPE, macrophages and glial cells. The migration of pigment-laden cells into the neural retina was frequently associated with focal photoreceptor damage. The cellular response was identical to that ensuing RPE cell transplantation. These findings support the concept that non-immunological events have a considerable influence on the outcome. In order to evaluate the impact of non-immunological mechanisms, a technique of transplanting fresh autologous IPE cells to the subretinal space of the same eye was developed. Grafted IPE cells were seen to survive for six months. There was a remodeling of the compound cellular layers in the subretinal space over time where grafted IPE cells joined the native RPE cells. The cellular response that developed exhibited macrophages, but no lymphocytes, and was in this respect similar to that observed following RPE transplantation. In RPE allografts, the photoreceptors appeared normal on light microscopy at three months, but at six months, the photoreceptors overlying the transplants generally exhibited pathological changes. In autologous IPE grafts, on the other hand, the photoreceptors displayed normal outer segment length and outer nuclear layer on top of grafted IPE cells. Focally, multilayers of both grafted IPE and RPE cells, together with macrophages, induce damage to adjacent photoreceptors as observed at 6 months. Cellular multilayers in the subretinal space, irrespective of genesis, are likely to have adverse effects on photoreceptors. The experiments using autologous IPE grafts show that non-immunogenic mechanisms have a decisive impact on the outcome of the transplant in the subretinal space. This thesis is based on the following original publications Crafoord S, Algvere PV, Seregard S, Dafgård Kopp EME (1999): Long-term outcome of RPE allografts to the subretinal space of rabbits. Acta Ophthalmol Scand 77: 247–254. Crafoord S, Algvere PV, Dafgård-Kopp E, Seregard S (2000): Cyclosporine treatment of RPE allografts in the rabbit subretinal space. Acta Ophthalmol Scand 78: 122–129. Crafoord S, Dafgård-Kopp E, Seregard S, Algvere PV (2000): Cellular migration into neural retina following implantation of melanin granules to the subretinal space. Graefe’s Arch Clin Exp Ophthalmol (2000) 238: 682–689. Crafoord S, Geng L, Seregard S, Algvere PV (2000): Transplantation of autologous iris pigment epithelial cells to the subretinal space: I. Morphological features (manuscript). V. Crafoord S, Geng L, Algvere PV (2000): Transplantation of autologous iris pigment epithelial cells to the subretinal space: II. Photoreceptor survival and consequences of cellular multilayers (manuscript). Author Sven Crafoord From: Division of Ophthalmology Department of Neuroscience and Locomotion Faculty of Health Sciences, E-581 85 Linköping Department of Ophthalmology Örebro Medical Centre Hospital SE-701 85 Örebro Department of Ophthalmology Karolinska Institutet St Erik’s Eye Hospital SE-112 82 Stockholm Sweden Accepted by: Faculty of Health Sciences Linköping University Sweden, 2000 ISBN 91-7219-586-X ISSN 0345-0082