Ординатура / Офтальмология / Английские материалы / Retinal Degenerations biology, diagnostics, and therapeutics_Tombran-Tink, Barnstable_2007

Fig. 9. Thickness of the outer nuclear layer of the retina of an eye that had received a transplantation of gene-transduced IPE cells. Rats were exposed to 3000 lux light 1 d after the transplantation of gene-transduced IPE cells into the subretinal space. Light microphotographs of rat retina taken 700 m from the optic disc in the transplanted hemisphere of the eye.

half of the eye (Fig. 9). In the AAV2-BDNF-IPE cells, the rats were placed under constant light on days 1 and 90 after transplantation and examined.

The expression of the BDNF gene in the subretinal space was higher in AAV-BDNF- IPE transplantation than transplantation of IPE only. A statistically significant photoreceptor protection was observed on days 1 and 90 in eyes receiving the AAV2-BDNF- IPE transplant, in both the superior transplant site and the inferior hemispheres which did not receive the transplant (60).

bFGF-IPE and BDNF-IPE cells expressed higher levels of the mRNA and proteins of each NTfactor than nontransfected IPE cells (53,63). A significant increase in the protection of photoreceptor cells against NMDA neurotoxicity was observed in the

neuroretinal cells cultured with BDNF-transfected IPE cells than in those cultured with nontransfected IPE cells (p = 0.0029) or with nontreated cells (p = 0.0010) (60). bFGFIPE cells could protect photoreceptor outer segments (53). These results suggest IPE cells with transfection of the BDNF genes may be a useful tool for delivering these factors to the subretinal space and have protective effects against the mechanisms leading to the degeneration of photoreceptor cells.

OUR THERAPEUTIC STRATEGY FOR RETINAL DYSTROPHIES AND AMD

Gene therapy is now being used to treat a broad variety of diseases and many approaches for delivering the targeted genes to the appropriate sites have been attempted. Recombinant viral vectors have been the most extensively used, and the vectors can transport a gene to a cell to replace the defective gene, to suppress the expression of a mutant gene, or to deliver a protective gene to delay degeneration. The AAVs are members of the Parvoviridae family, and the AAV2 is one of the vectors most extensively studied and developed for clinical use (64,65). AAV2 transduction in animal models has progressed from rodents to nonhuman primates and is now being used in humans in phase I safety trials (66,67).

We are continuing our basic and clinical studies of autologous IPE cells proceeding logically from our observations described previously. Our results to date have demonstrated that IPE cells transduced with recombinant AAV2-mediated genes transplanted into the subretinal space may be the most promising treatment for protecting or slowing apoptotic photoreceptor cell death caused by retinal degeneration or AMD.

ACKNOWLEDGMENTS

I wish to thank many of my collaborators in the Department of Ophthalmology, Tohoku University School of Medicine since 1986 to the present. I wish to acknowledge especially Drs. S.-I. Ishiguro, T. Abe, K. Yamaguchi, Y. Wada, H. Tomita, and M. Yoshida for their hard work and cooperation in performing and analyzing the basic and clinical studies for many years. I also thank Dr. Duco Hamasaki for helpful advice. The main part of this chapter was presented as the special lecture at the 108th Japanese Ophthalmology Congress in April 17, 2004.

REFERENCES

1.Hewitt AT, Adler R. The retinal pigment epithelium and interphotoreceptor matrix: structure and specialized functions. In: Ryan S, ed. Retina. (St. Louis, MO: Mosby-Year Book, Inc., 1994;58–71.

2.Marshall J. The aging retina: physiology or pathology. Eye 1987;1:282–295.

3.Chang GQ, Hao Y, Wong F. Apoptosis: final common pathway of photoreceptor death in rd, rds, and rhodopsin mutant mice. Neuron 1993;11:595–605.

4.Thompson DA, Gal A. Vitamin A metabolism in the retinal pigment epithelium: genes, mutations, and diseases. Prog. Retinal Eye Res 2003;22:683–703.

5.Wada Y, Nakazawa T, Abe T, Fuse N, Tamai M. Clinical variability of patients associated with gene mutations of visual cycle protein, arrestin, RPE65 and RDH5. Invest Ophthalmol Vis Sci 2000;41(4):S617.

6.Li L, Turner JE. Optimal conditions for long-term photoreceptor cell rescue in RCS rats: The necessity for healthy RPE transplants. Exp Eye Res 1991;52:669–679.

7.Yamaguchi K, Gaur VP, Turner JE. Retinal pigment epithelial cell transplantation into aging retina: A possible approach to delay age-related cell death. Jpn J Ophthalmol 1993;37: 16–27.

8.Rezai KA, Kohen L, Wiedemann P, Heimann K. Iris pigment epithelium transplantation. Graefes Arch Clin Exp Ophthalmol 1997;235:558–562.

9.Heriot WJ, Machemer R. Pigment epithelial repair. Graefe’s Arch Clin Exp Ophthalmol 1992;230:91–100.

10.Wallow IH. Repair of the pigment epithelial barrier following photocoagulation. Arch Ophthalmol 1984;102:126–135.

11.Bird A, Marshall J. Retinal pigment epithelial detachments in the elderly. Trans Ophthalmol Soc UK 1986;105:674–682.

12.Abe T, Tomita H, Kano T, et al. Autologous iris pigment epithelial cell transplantation in monkey subretinal region Curr Eye Res 2000;20:268–275.

13.Kimizuka Y, Yamada T, Tamai M. Quantitative study on regenerated retinal pigment epithelium and the effects of growth factor. Curr Eye Res 1997;16:1081–1087.

14.Valentino TL, Kaplan HJ, Del Priore LV, Fang SR, Berger A, Silverman MS. Retinal pigment epithelial repopulation in monkeys after submacular surgery. Arch Ophthalmol, 1995;113:932–938.

15.Sheedlo H, Li L, Turner J. Functional and structural characteristics of photoreceptor cells rescued in RPE-cell grafted retinas of RCS dystrophic rats. Exp Eye Res 1989;48:841–854.

16.Grossniklaus HE, Hutchinson AK, Capone A, Woolfson J, Lambert HM. Clinicopathologic features of surgically excised choroidal neovascular membranes. Ophthalmology 1994; 101:1099–1111.

17.Seregard S, Algvere PV, Berglin L. Immunohistochemical characterization of surgically removed subfoveal fibrovascular membranes. Graefe’s Arch Clin Exp Ophthalmol 1994;232:325–329.

18.Abe T, Yoshida M, Kano T, Tamai M. Visual function after removal of suberetinal neovascular membranes in patients with age-related macular degeneration. Graefe’s Arch Clin Exp Ophthalmol 2001;239:927–936.

19.Gass JDM. Biomicroscopic and histopathologic considerations regarding the feasibility of surgical excision of subfoveal neovascular membranes. Am J Ophthalmol 1994;118:285–298.

20.Algvere PV, Berglin L, Gouras P, Sheng Y. Transplantation of fetal retinal pigment epithelium in age-related macular degeneration with subfoveal neovascularization. Graefe’s Arch Clin Exp Ophthalmol 1994;232:707–716.

21.Algvere PV, Berglin L, Gouras P, Sheng Y, Kopp ED. Transplantation of RPE in age-related macular degeneration: observations in disciform lesions and dry RPE atrophy. Graefe’s Arch Clin Exp Ophthalmol 1997;235:149–158.

22.Weisz JM, Humayun MS, De Juan Jr., E, et al. Allogenic fetal retinal pigment epithelial cell transplant in a patient with geographic atrophy. Retina 1999;19:540–545.

23.Algvere PV, Gouras P, Dafgard KE, et al. Long-term outcome of RPE allografts in nonimmunosuppressed patients with AMD. Eur J Ophthalmol 1997;9:217–230.

24.Peyman GA, Blinder KJ, Paris CL, Alturki W, Nelson Jr, NC, Billson FA. A technique for retinal pigment epithelium transplantation for age-related macular degeneration secondary to extensive subfoveal scarring. Ophthalmic Surgery 1991;22:102–108.

25.Binder S, Krebs I, Hilgers R-D, et al. Outcome of transplantation of autologous retinal pigment epithelium in age-related macular degeneration: a prospective trial. Invest Ophthalmol Vis Sci 2004;45:4151–4160.

26.Streilein JW. Anterior chamber associated immune deviation: the privilege of immunity in the eye. Surv Ophthalmol 1990;35:67–73.

27.Zhang X, Bok D. Transplantation of retinal pigment epithelial cells and immune response in the subretinal space. Invest Ophthalmol Vis Sci 1998;39:1021–1027.

28.Abe T, Tomita H, Ohashi T, et al. Characterization of Iris Pigment Epithelial Cell for Auto Cell Transplantation. Cell Transplant 1999;8:501–510.

29.Durlu YK, Tamai M. Transplantation of retinal pigment epithelium using viable cryopreserved cells. Cell Transplant 1997;6:149–162.

30.Abe T, Sato M, Tamai M. Dedifferentiation of the retinal pigment epithelium compared to the proliferative membranes of proliferative vitreoretinopaty. Curr Eye Res 1998;17: 1103–1109.

31.Tamai M. Retinal pigment epithelial cell transplantation: Perspective. J Jpn Ophthalmol Soc 1996;100:982–1006.

32.Kociok N, Heppekausen H, Schraermeyer U, et al. The mRNA expression of cytokines and their receptors in cutured iris pigment epithelial cells: a comparison with retinal pigment epithelial cells. Exp Eye Res 1998;67:237–250.

33.Rezai KA, Lappas A, Farrokh-Siar L, Kohen L, Wiedeman P, Heimann K. Iris pigment epithelial cells of long evans rats demonstrate phagocytic activity. Exp Eye Res 1997;65: 23–29.

34.Sakuragi M, Tomita H, Abe T, Tamai M. Changes of phagocytic capacity in basic fibroblast growth factor-transfected iris pigment epithelial cells in rats. Curr Eye Res 2001;23:185–191.

35.Sugano E, Tomita H, Abe T, Yamashita A, Tamai M. Comparative study of cathepsins D and S in rat IPE and RPE cells. Exp Eye Res 2003;77:203–209.

36.Li L, Turner JE. Optimal conditions for long-term photoreceptor cell rescue in RCS rats: The necessity for healthy RPE transplants. Exp Eye Res 1991;52:669–679.

37.Sheedlo HJ, Li L, Turner JE. Photoreceptor cell rescue at early and late RPE-cell transplantation periods during retinal disease in RCS dystrophic rats. J Neural Transplant Plast 1991;2:55–63.

38.Little CW, Castillo B, DiLoreto DA, et al. Transplantation of human fetal retinal pigment epithelium rescues photoreceptor cells from degeneration in the Royal College of Surgeons rat retina. Invest Ophthalmol Vis Sci 1996;37:204–211.

39.Bhatt NS, Newsome DA, Fenech T, et al. Experimental transplantation of human retinal pigment epitheliual cells on collagen substrates. Am J Ophthalmol 1994;117:214–221.

40.Akaishi K, Ishiguro S-I, Durlu YK, Tamai M. Quantitative analysis of major histocompatibility complex class II-positive cells in posterior segment of Royal College of Surgeons rat eyes. Jpn J Ophthalmol 1998;42:357–362.

41.Fealy MJ, Most D, Huie P, et al. Association of down-regulation of cytokine activity with rat hind limb allograft survival. Transplantation 1995;59:1475–1480.

42.Abe T, Takeda Y, Yamada K, et al. Cytokine gene expression after subretinal transplantation. Tohoku J Exp Med 1999;189:179–189.

43.Abe T, Yoshida M, Tomita H, et al. Functional analysis after auto iris pigment epithelial cell transplantation in patients with age-related macular degeneration. Tohoku J Exp Med 1999;189:295–305.

44.Abe T, Yoshida M, Tomita H, et al. Auto iris pigment epithelial cell transplantation in patients with age-related macular degeneration: short term results. Tohoku J exp Med 2000;191:7–20.

45.Tamai M. Progress in pathogenesis and therapeutic research in retinitis pigmentosa and age-related macular degeneration. J Jpn Ophthalmol Soc 2004;108:750–769.

46.Chang GQ, Hao Y, Wong F. Apoptosis: final common pathway of photoreceptor death in rd, rds, and rhodopsin mutant mice. Neuron 1993;11:595–605.

47.Wong P. Apoptosis, retinitis pigmentosa, and degeneration. Biochem Cell Biol 1994;72:489–498.

48.Ranganathan R. Cell biology: a matter of life or death. Science 2003;299:1677–1679.

49.LaVail MM, Yasumura D, Matthes MT, et al. Protection of mouse photoreceptors by survival factors in retinal degenerations. Invest Ophthalmol Vis Sci 1998;39:592–602.

50.Okoye G, Zimmer J, Sung J, et al. Increased expression of brain-derived neurotrophic factor preserves retinal function and slows cell death from rhodopsin mutation or oxidative damage. J Neurosci 2003;23:4164–4172.

51.Lawrence JM, Keegan DJ, Muir EM, et al. Transplantation of Schwann cell line clones secreting GDNF or BDNF into the retinas of dystrophic Royal College of Surgeons rat. Invest Ophthalmol Vis Sci 2004;45:267–274.

52.Unoki K, LaVail MM. Protection of the rat retina from ischemic injury by brain-derived neurotrophic factor, ciliary neurotrophic factor, and basic fibroblast growth factor. Invest Ophthalmol Vis Sci 1994;35:907–915.

53.Tamai M, Takeda Y, Yamada K, et al. bFGF transfected iris PE may rescue photoreceptor cell degeneration in RCS rat. In: LaVail MM, Anderson RE, Hollyfield JG, eds. Retinal Degeneration. New York: Plenum Press, 1997;323–328.

54.Itaya H, Gullapalli V, Sugino IK, Tamai M, Zarbin M. Iris pigment epithelium attachment to aged submacular human Bruch’s membrane. Invest Ophthalmol Vis Sci 2004;45: 4520–4528.

55.Bibel M, Barde YA. Neurotrophins: key regulators of cell fate and cell shape in the vertebrate nervous system. Genes Dev 2000;14:2919–2937.

56.Leibrock J, Lottspeich F, Hohn A, et al. Molecular cloning and expression of bain-derived neurotrophic factor. Nature 1989;341:149–152.

57.von Bartheld CS. Neurotrophins in the developing and regenerating visual system. Histol Histopathol 1998;13:437–459.

58.Liang FQ, Aleman TS, Dejneka NS, et al. Long-term protection of retinal structure but not function using RAAV.CNTF in animal models of retinitis pigmentosa. Mol Ther 2001;4:461–472.

59.Saigo Y, Abe T, Hojo M, Tomita H, Sugano E, Tamai M. Transplantation of Transduced Retinal Pigment Epithelium in Rats. Invest Ophthalmol Vis Sci 2004;45:1996–2004.

60.Hojo M, Abe T, Sugano E, et al. Photoreceptor Protection by Iris Pigment Epithelial Transplantation Transduced with AAV-Mediated Brain-Derived Neurotrophic Factor Gene. Invest Ophthalmol Vis Sci 2004;45:3721–3726.

61.Bennett J, Maguire AM, Cideciyan AV, et al. Stable transgene expression in rod photoreceptors after recombinant adeno-associated virus-mediated gene transfer to monkey retina. Proc Natl Acad Sci USA 1999;96:9920–9925.

62.Yoshioka Y, et al. (in preparation)

63.Kano T, Abe T, Tomita H, Sakata T, Ishiguro S-I, Tamai M. Protective effect against ischemia and light damage of iris pigment epithelial cells transfedted with the BDNF gene. Invest Ophthalmol Vis Sci 2002;43:3744–3753.

64.Rabinowitz JE, Samulski J. Adeno-associated virus expression systems for gene transfer. Curr Opin Biotechnol 1998;9:470–475.

65.Monahan PE, Samulski RJ. AAV vectors: Is clinical success on the horizon? Gene Ther 2000;7:24–30.

66.Kay MA, Manno CS, Ragni MV, et al. Evidence for gene transfer and expression of factor IX in haemophilia B patients treated with an AAV vector. Nat Genet 2000;24:257–261.

67.Rabinowitz JE, Rolling F, Li C, et al. Cross-packaging of a single adeno-associated virus (AAV) type 2 vector genome into multiple aav serotypes enables transduction with broad specificity. J Virol 2002;76:791–801.

In such diseases, photoreceptors and/or RPE become dysfunctional or degenerate and need to be replaced whereas the neural retina that connects to the brain can still remain functional (3–6) (reviewed in ref. 7). If the diseased cells can be replaced and the new cells can make appropriate connections with the functional part of the host retina, a degenerated retina might be repaired and vision restored.

Vitamin supplements with zinc (8) and gene therapy to introduce trophic factors (9) or to correct mutated genes (10,11) may be helpful in the early stages of a disease, but once photoreceptors are lost, they must be replaced to restore vision (reviewed in refs. 12,13). With the exception of microchip implantation (reviewed in refs. 14,15), there are presently no realistic alternative techniques to retinal transplantation for the treatment of end-stage retinal diseases. In many retinal diseases, both photoreceptors and RPE are affected (7) and need to be replaced. To meet this need, our group has developed a procedure to transplant sheets of fetal RPE together with its neuroblastic retina (16–18).

There is now evidence that transplanted fetal neurosensory retina can re-establish connections with the residual neural network (19). In addition, the transplanted tissue might exert a positive rescue effect on the recipient’s retina, as has been shown in animal experiments in vitro and in vivo (20). The first FDA-approved clinical trials in retinal transplantation have shown very promising results (21).

Retinal Remodeling

In pigmented Royal College of Surgeons (RCS) rat retina up to the age of 515 d, no ultrastructural abnormalities in synaptic counts and in ganglion cell characteristics were found (22). However, subsequent studies showed that ganglion cells change their properties about 3 mo after photoreceptor loss in the RCS rat, owing to the abnormal in growth of blood vessels from the choroid (23–25). Similar changes occur in the rd mouse retina (26–28).

Remodeling of the inner retina is a major secondary effect of outer retinal degenerations (29,30). It is thought that this process occurs as a result of denervation of the inner retinal neurons, and subsequent attempts by these neurons to find new synaptic input. This process involves cell death; rewiring, i.e., the formation of new circuits to replace lost innervation; and cell migration (30).

Properties and Use of Fetal Donor Tissue

There are many reasons why all of our studies have used fetal donor cells and not adult donor cells. Fetal cells have a high capacity to sprout processes and to produce trophic substances that will aid host and transplant cells to establish contacts. They can multiply, so that the transplant can grow to cover a larger area, and transplants of retinal aggregates to nude rats can grow larger the younger the donor age (31). Fetal retinal cells can also overcome the trauma of transplantation much easier than adult cells because they do not depend as heavily on oxygen (32). Further, fetal retinal tissue is likely less immunogenic than adult tissue because it contains less microglia than older tissue (33,34). Research has shown no rejection if the tissue is transplanted to the central nervous system (CNS) or the eye of the same species (see the Retinal Transplant Immunology section).

In January of 1993, President Clinton overturned the ban on federally funded fetal tissue research. Public Law 103-43 (also known as the National Institutes of Health [NIH] Revitalization Act of 1993) explicitly made funding of fetal tissue transplantation research legal when certain conditions are met. Strict ethical guidelines must be followed so as not to give any incentive for abortions. Donors must remain anonymous and informed consent procedures must be in place to explain the research purpose and potential risks. Our research has used private, nongovernmental sources, and we have adhered to all NIH guidelines regardless of our funding source.

Technical Challenges With Fetal Sheet Transplantation

Although fetal tissue has many advantages, one major weakness is that it is very fragile and presents several challenges during transplantation. It is of outmost importance not to damage the donor tissue or the host. The first challenge is to precisely dissect an intact monolayer RPE sheet attached to the neural retinal sheet. To perform this delicate task, our group uses custom-designed ultra-precision forceps. The second challenge is to implant the fragile donor sheets into the subretinal space without damage, and we have developed a proprietary instrument to perform this step. The implantation instrument is a hand-held tool (stainless-steel hand piece) with a flat, flexible disposable nozzle tube (many different sizes, according to the purpose) that fits over a stationary mandrel (not a movable plunger). This innovative device provides the surgeon with precise manual control and allows gentle placement of the transplant into the target area of the subretinal space with minimal trauma to the donor tissue and host eye. When the nozzle tip containing the tissue is on the target, the surgeon holds his hand completely still and releases a spring that retracts the nozzle. The surgeon has complete control over the speed of retraction of the nozzle tip and in this way exposes and “places” the tissue on target. The nozzle size is chosen according to the donor tissue so that very little fluid is delivered. All other methods (35,36) push or inject the tissue into a large subretinal bleb so that trauma is exerted on the host and donor tissue and the increased pressure can easily push out the donor tissue through the retinotomy site. In our earlier research (16,37,38), we used a matrix coating to protect the donor tissue. However, our custom-made implantation instrument so exceeded expectations that the procedure could be performed without use of the matrix coating. The implantation tool provides the surgeon with the precise control required for very gentle delivery of the fragile fetal graft.

Retinal Transplant Immunology

The subretinal space is regarded as an immunological privileged site (39) so that there is a reduced probability of rejection of allografts of fetal tissue. The neural retina is non-immunogenic but the RPE and the microglial cells in the donor retina are immunogenic (40,41). Dissociated RPE cells seem to initiate an immune response after transplantation to the subretinal space (42,43). However, allografted sheets of RPE are not rejected when transplanted to the kidney capsule and thus are immunologically privileged (41). Postnatal retinal tissue however was rejected. Despite the potential for rejection based on the microglia in the donor retina, our hypothesis is that rejection will

probably not happen because of the immunological privileged site of the subretinal space. So far, our hypothesis has been confirmed in our results with patients (21,44).

Most of the microglial cells are associated with blood vessels and migrate postnatally into the rat retina (33) and from 16-wk gestation into the human retina (34). The number of immunogenic microglial cells in fetal rat retina is much lower than in postnatal retina (33). Therefore, it is likely that fetal retina is less immunogenic than postnatal retina because fetal retina still lacks inner retinal vessels. However, no group has yet tested this hypothesis. In our model, we have seen stable transplants in rats 6 to 10 mo after surgery. This indicates that allogeneic retinal sheet transplants can be tolerated in the subretinal space of rats with retinal degeneration.

Use of Stem Cells in Our Model

Stem cell transplantation is considered to have a great potential. Stem cells are cells early in development that have the capacity to differentiate into different cell types and different tissues. Progenitor cells are still multipotential but restricted to a specific tissue. For example, fetal retina contains progenitor cells that have the capacity to differentiate into various retinal cell types, but not other cells. Our laboratory has worked with retinal progenitor cells derived from rat E17 retina in vitro or in vivo for more than 5 yr.

Retinal progenitor (stem) cells have been isolated from the ciliary margin of the adult retina of different ages (45,46), or from fetal retina (47–51). After transplantation, progenitor cells integrate and migrate into the retina depending on the age, the disease, or the injury status of the recipient retina. Previous studies showed that a limited percentage of progenitor cells can express opsin (47), but most appeared to be limited to a glial lineage after transplantation to an adult host with retinal degeneration (49). However, retinal progenitor cells that have been maintained in defined culture conditions (51) develop to mostly opsin expressing cells after transplantation to recipients with slow and fast retinal degeneration (52). Initially, in our laboratory, we included “stem cells,” progenitor cells derived from younger rat E17 retina, with our fetal sheet transplants with the hypothesis that the progenitor cells could help with the connections of transplant and host. These experiments did not produce the expected results. The experiments with retinal progenitor cells alone continued (48,49), and showed some promise after switching to cells maintained and proliferating in serum-free medium (51,52). It is interesting to compare the results of these “stem” retinal progenitor cells with the results of retinal sheet progenitor cells (rat E19) in our transplant model. The photoreceptors of the sheet transplants show a high degree of differentiation. By example, they can show migration of the phototransduction proteins dependent on the light cycle, indicating that they are able to transfer light into electrical signals (38). In contrast, transplanted retinal progenitor cells have not yet been shown to contain photoreceptor outer segments (OS).

Ongoing studies in our laboratory are investigating various aspects of “stem” retinal progenitor cells to determine if these cells can do what has been shown for retinal sheet transplants: (1) develop to fully functional retinal cells, (2) contain all substances specific for each cell, (3) send out processes to host cells and establish synapses with the host, and (4) establish meaningful communications and restore vision in a host with

retinal degeneration. Although stem/progenitor cells show great promise, there appears to be a long road of research ahead before any possible clinical application.

CLINICAL TRIALS

Clinical trials of retinal transplantation have been motivated by the lack of available treatment to recover or prevent vision loss from RP and other diseases of the outer retina. Although oral vitamin A therapy has been shown to slow the rate of electroretinogram (ERG) loss in RP, it has no effect on vision loss (53). Gene therapy and pharmacological therapy are underway but are still under development, with clinical gene therapy trials to be started soon (54,55). A clinical trial to deliver encapsulated ciliary neurotrophic factor-producing RPE cells to the eye of patients with RP has just begun (no published data available yet) (56). These trials aim at delaying photoreceptor degeneration or correcting gene defects. Finally, development and use of a visual prosthesis is being actively pursued in many centers but the potential of existing devices is not known (14,15,57–59).

With the exception of the visual prosthesis, gene and pharmacological therapies can only help when the retinal degeneration has not progressed too far. Once most of the photoreceptors are lost, replacement of degenerating cells by RPE together with neural retina may be the only means to restore the atrophying neural retina, the pigment epithelium, and part of the choroid.

RPE and IPE Transplantation

The success of RPE transplants in RCS rats (60,61) led to clinical trials in patients with AMD by a team in Sweden in collaboration with Columbia University, NY (42) and in the United States (62). The results were mixed; rejection was observed depending on the status of retinal degeneration, the presence of an intact blood–brain barrier, and immunosuppression (63). In summary, problems with RPE allografts, related to rejection, inflammation and/or changes in the RPE cells after tissue culture, prevented any long-term beneficial effects. Immunosuppressive treatment appeared to prevent graft failure (63).

To avoid rejection, autologous transplants of adult RPE cells (64) and iris pigment epithelial (IPE) cells (65,66) have been performed, mostly to patients with “wet” AMD. Subjective improvements in visual acuity were reported.

Another strategy has been macular translocation to expose the macula to still healthy RPE cells (67). However, several patients with non-exudative AMD developed clear evidence of new geographic atrophy of the RPE in the area of the translocated fovea after macular translocation (68). This means that it is likely an intrinsic defect in the photoreceptors that negatively affects previously healthy RPE, and gives another argument for the need of combined transplants of retina together with its RPE.

Transplants of Neural Retina

The rationale for transplantation of photoreceptors or neural retina is photoreceptors cannot regenerate once they have undergone apoptosis (reviewed in ref. 69). Fourteen patients with RP in India (70), and eight patients with RP and one patient with AMD in the United States received aggregate fetal retinal transplants (71). Ten patients received