Functional assessment of the native retinal pigment epithelium after the surgical excision of subfoveal choroidal neovascular membranes type II: preliminary results

RPE transplantation and its role in retinal disease

Retinal pigment epithelial (RPE) transplantation aims to restore the subretinal anatomy and re-establish the critical interaction between the RPE and the photoreceptor, which is fundamental to sight. The field has developed over the past 20 years with advances coming from a large body of animal work and more recently a considerable number of human trials. Enormous progress has been made with the potential for at least partial restoration of visual function in both animal and human clinical work. Diseases that have been treated with RPE transplantation demonstrating partial reversal of vision loss include primary RPE dystrophies such as the merTK dystrophy in the Royal College of Surgeons (RCS) rat and in humans, photoreceptor dystrophies as well as complex retinal diseases such as atrophic and neovascular age-related macular degeneration (AMD). Unfortunately, in the human trials the visual recovery has been limited at best and full visual recovery has not been demonstrated. Autologous full-thickness transplants have been used most commonly and effectively in human disease but the search for a cell source to replace autologous RPE such as embryonic stem cells, marrow-derived stem cells, umbilical cord-derived cells as well as immortalised cell lines continues. The combination of cell transplantation with other modalities of treatment such as gene transfer remains an exciting future prospect. RPE transplantation has already been shown to be capable of restoring the subretinal anatomy and improving photoreceptor function in a variety of retinal diseases. In the near future, refinements of current techniques are likely to allow RPE transplantation to enter the mainstream of retinal therapy at a time when the treatment of previously blinding retinal diseases is finally becoming a reality.

Transplantation of the RPE in AMD

The retinal pigment epithelium (RPE) maintains retinal function as the metabolic gatekeeper between photoreceptors (PRs) and the choriocapillaries. The RPE and Bruch's membrane (BM) suffer cumulative damage over lifetime, which is thought to induce age-related macular degeneration (AMD) in susceptible individuals. Unlike palliative pharmacologic treatments, replacement of the RPE has a curative potential for AMD. This article reviews mechanisms leading to RPE dysfunction in aging and AMD, laboratory studies on RPE transplantation, and surgical techniques used in AMD patients. Future strategies using ex vivo steps prior to transplantation, BM prosthetics, and stem cell applications are discussed. The functional peculiarity of the macular region, epigenetic phenomena leading to an age-related shift in protein expression, along with the accumulation of lipofuscin may affect the metabolism in the central RPE. Thickening of BM with age decreases its hydraulic conductivity. Drusen are deposits of extracellular material and formed in part by activation of the alternative complement pathway in individuals carrying a mutant allele of complement factor H. AMD likely represents an umbrella term for a disease entity with multifactorial etiology and manifestations. Presently, a slow progressing (dry) non-neovascular atrophic form and a rapidly blinding neovascular (wet) form are discerned. No therapy is currently available for the former, while RPE transplantation and promising (albeit non-causal) anti-angiogenic therapies are available for the latter. The potential of RPE transplantation was demonstrated in animal models. Rejection of allogeneic homologous transplants in patients focused further studies on autologous sources. In vitro studies elucidated cell adhesion and wound healing mechanisms on aged human BM. Currently, autologous RPE, harvested from the midperiphery, is being transplanted as a cell suspension or a patch of RPE and choroid in AMD patients. These techniques have been evaluated from several groups. Autologous RPE transplants may have the disadvantage of carrying the same genetic information that may have led to AMD manifestation. An intermittent culturing step would allow for in vitro therapy of the RPE, its rejuvenation and prosthesis of BM to improve the success RPE transplants. Recent advances in stem cell biology when combined with lessons learned from studies of RPE transplantation are intriguing future therapeutic modalities for AMD patients.

Autologous translocation of the choroid and retinal pigment epithelium in age-related macular degeneration

PURPOSE

To evaluate the autologous translocation of peripheral choroid and retinal pigment epithelium (RPE) in 45 eyes of 43 patients with age-related macular degeneration (AMD).

DESIGN

Prospective nonrandomized study.

METHODS

All patients had visual loss due to AMD (n = 5 classic membranes, n = 14 occult, n = 2 mixed, n = 16 pigment epithelial detachment (PED), n = 5 subretinal hemorrhage, n = 3 geographic atrophy). After extraction of the neovascular complex, an autologous peripheral full-thickness explant of RPE, Bruch membrane, and choroid was translocated from the midperiphery to the macula.

RESULTS

Preoperative distant visual acuity ranged from 20/800 to 20/40. Reading vision ranged from 1.4 logarithm of reading acuity determination (logRAD) to 0.5 logRAD (0.04 to 0.32 Snellen equivalent). Revision surgery was required in 22 eyes as a result of proliferative vitreoretinopathy (PVR), retinal detachment, macular pucker, or vitreous hemorrhage. In eight patients, the patch was renewed. At six months, distant visual acuity ranged from light perception to 20/50 (increase of 15 letters in four eyes). Reading vision ranged from 1.4 to 0.4 logRAD. Visual outcome was unrelated to the type of AMD. Vascularization of the transplant was visible on indocyanine green (ICG) angiography in 40 of 42 eyes. In most patients, autofluorescence of the pigment epithelium was coincident with revascularization of the graft. Fixation on the patch was positively related to visual acuity.

CONCLUSIONS

Autologous translocation of a full-thickness transplant of choroid and RPE usually results in a vascularized and functioning graft. Vascularization was even achieved in patients with geographic atrophy. Fixation stability and microperimetry before the patch translocation may be helpful in selecting patients who will profit from surgery.