HISTORICAL DEVELOPMENT OF RPE TREATMENT

Peyman and colleagues8 were the first to transplant RPE in 2 patients suffering from terminal AMD in 1991. In 1994, Algvere et al.9 treated 5 patients with neovascular AMD by using a patch of cultured human fetal RPE to compensate for the lesion after subretinal choroidal neovasculariztion (CNV) removal. At 3 months macular edema and rejection were observed; after 12 months mild visual loss was registered. Four dry AMD cases received very small circular patches of human fetal RPE outside the fovea, attempting to reach overgrowth of the foveal area. No visual improvement along with a milder rejection was observed, and this was explained by the more intact blood–retinal barrier in dry AMD. When different amounts of cell suspensions of fetal human RPE were used to cover larger RPE defects, rejection seemed related to higher numbers of cells.

IMMUNE REACTION AND RPE

TRANSPLANTATION

Like the anterior chamber, the subretinal space was shown to possess qualities of an immune-privileged site (anterior chamber-associated immune deviation), and initial reports on RPE allografts in the Royal College of Surgeons (RCS) rat suggested that there was no rejection in eyes with up to 1-year follow-up.10 The former seems contingent on an intact RPE monolayer and the observed tolerance is therefore unexpected, since blood–retinal barrier breakdown occurs relatively early in this animal. A subsequent study then demonstrated evidence of chronic rejection mediated through major histocompatibility complex (MHC) II expression.11 Evidence for MHC II-mediated rejection had already been suggested in an earlier study, where interferon-γ (IFN-γ)-activated cultured RPE transplants were acutely rejected. IFN-γ stimulation of RPE induces MHC II expression in the RPE and thereby renders these as antigen-presenting cells for T cells. Alternative pathways for T-cell activation in the scenario of insufficient MHC molecule expression have also been described. It was further demonstrated that the immunologic response was related to the amount of transplanted cells and increases over time, perhaps related to the amounts of MHC. Taken together, these mechanisms may in part explain why fetal allografts transplanted into patients with dry AMD enjoyed a more prolonged survival than cases with wet AMD.12

Strategies have subsequently been devised to improve RPE graft survival. Cyclosporine given to rabbits receiving RPE allografts, was not able to prevent RPE allograft destruction in the subretinal space. Similarly disappointing results were obtained even using a triple immune suppression for fetal pig RPE xenografts into albino rabbits.

Conceivably, from the above discussion the foundation for the use of syngeneic (autologous) cells has been laid. Their advantages and limitations are further discussed.

AUTOLOGOUS TREATMENT

IRIS PIGMENT EPITHELIUM

Harvesting autologous RPE is challenging, so the first startegy to eliminate rejection was the use of iris pigment epithelium (IPE) cells. At that time several investigators had demonstrated the ability of IPE to phagocytose outer segments. IPE and RPE share a common embryogenesis, yet develop early distinct molecular markers. IPE cells have been mainly used by groups in Japan and Germany.13,14

In a clinical study of 20 eyes with exudative macular degeneration IPE transplantation showed stabilization of vision or improvement along with a low CNV recurrance rate which remained stable up to 3 years.15

RETINAL PIGMENT EPITHELIUM

Suspension

The delivery of a RPE cell suspension subretinally (Figure 50.1) can be improved as follows:

1.A gentle localized retinal detachment to avoid RPE damage is achieved by a Ca2+-, Mg2+-free solution.

2.A new instrument was developed to improve cell harvesting (Binder–Parel–Lee cannula).3

3.After membrane removal the subretinal area is cleaned of blood and debris, and rebleeding is avoided with a small perfluorocarbon bubble.

4.The subfoveal delivery of cells is also done under perfluorocarbon and in a soft eye to avoid reflux.

5.A small air bubble is initially aspirated in the tuberculin syringe to ensure that all harvestered RPE cells are transplanted and no residuals remain in the cannula.

In a pilot study consisting of 14 eyes where we applied the former technique, a 2-line or more improvement was achieved in 57%; 21% regained useful reading vision between Jaeger (Jg) 1 and 4. There was neither intraoperative complication nor recurrent CNV after a mean of 17 months of observation in this series.16 However, this study was performed at a time when PDT was not available and lesions were smaller and mainly classic. A prospective trial was conducted then, in which transplantation after membrane excision was compared with a control group in which membrane excision alone was performed. Although the results after 12 months (n = 54) showed improvement of 2 or more lines in 52.5% in the group with RPE transplants (n = 40), in the statistical analysis there was a trend in favor of the transplanted group, but no significant difference between both groups. Reading acuity and multifocal electroretinogram however showed significantly better results in the transplanted group.17 These results showed that membrane excision combined with simultaneous transplantation of an autologous suspension of RPE cells was superior to membrane excision alone. Furthermore, the CNV recurrence rate was very low – 4.4% after 12 months, 13.3% after 24 months, and 15.4% after 36 months. As a major complication retina detachments occurred in 8.6% but could be managed with a second surgery. A different suspension technique was used by van Meurs et al. in 8 eyes: they harvested the cells inferiorly and added 2 µg poly-l lysine subretinally to improve cell adhesion. Meurs et al. reported proliferative vitreopathy (PVR) in 3/8 eyes and no visual gain.18

RPE-BM Choroid Sheet

Transplanted intact RPE sheets come as a polarized monolayer (Figure 50.2). In human eyes the technique has improved over time and surgical trauma has decreased. The first autologous transplant series taken from ajacent areas of the CNV lesion was performed by Aylward’s group.19 The group of Jan van Meurs reported about 6 cases where the transplant was excised from the midperiphery of the retina and translocated under the fovea after subretinal membrane removal. They reported fixation on the transplant in 4 of the 6 cases. Experimental studies on pig eyes from the same group showed vascularization of such grafts. With larger numbers now treated, stabilization of vision or mean gain of 1 line has been reported by the same group.20

Joussen et al. also reported on 45 cases after patch transplantation: 3 of them were dry AMD, 16 had PED, and the rest had neovascular AMD.21 Only 8.8% had a 15-letter improvement, PVR rate was 37.7%, and revision surgery was needed in 48.8%. Also MacLaren performed a similar procedure in a small series, showing very limited visual improvement and a high PVR rate of more than 40%.22

Bindewald et al. suggested in a small case series a modified technique,23 whereby choroidal vessels of the RPE/choroid sheets is ablated with a 308-nm excimer laser. The rationale was to reduce diffusion distance from the choroidal wound bed, the graft, and outer retinal layers, as well as untoward cellular reactions that would limit subsequent visual recovery. In vitro studies have demonstrated safety of the

Surgery and Pharmacotherapy • 5 section