Transplantation of photoreceptors into the degenerative retina: Current state and future perspectives

The mammalian retina displays no intrinsic regenerative capacities, therefore retinal degenerative diseases such as age-related macular degeneration (AMD) or retinitis pigmentosa (RP) result in a permanent loss of the light-sensing photoreceptor cells. The degeneration of photoreceptors leads to vision impairment and, in later stages, complete blindness. Several therapeutic strategies have been developed to slow down or prevent further retinal degeneration, however a definitive cure i.e. replacement of the lost photoreceptors, has not yet been established. Cell-based treatment approaches, by means of photoreceptor transplantation, have been studied in pre-clinical animal models over the last three decades. The introduction of pluripotent stem cell-derived retinal organoids represents, in principle, an unlimited source for the generation of transplantable human photoreceptors. However, safety, immunological and reproducibility-related issues regarding the use of such cells still need to be solved. Moreover, the recent finding of cytoplasmic material transfer between donor and host photoreceptors demands reinterpretation of several former transplantation studies. At the same time, material transfer between healthy donor and dysfunctional patient photoreceptors also offers a potential alternative strategy for therapeutic intervention. In this review we discuss the history and current state of photoreceptor transplantation, the techniques used to assess rescue of visual function, the prerequisites for effective transplantation as well as the main roadblocks, including safety and immune response to the graft, that need to be overcome for successful clinical translation of photoreceptor transplantation approaches.

Association of serum levels of anti-myeloperoxidase antibody with retinal photoreceptor ellipsoid zone disruption in diabetic retinopathy

AIM

To study the association of serum levels of anti-myeloperoxidase (MPO) antibody with retinal photoreceptor ellipsoid zone (EZ) disruption in diabetic retinopathy.

METHODS

Consecutive patients with type 2 DM [diabetes mellitus with no retinopathy (NODR; n=20); non-proliferative diabetic retinopathy (NPDR; n=18); proliferative diabetic retinopathy (PDR; n=16)] and healthy controls (n=20) between the ages of 40 and 65years were included. Disruption of EZ was graded by spectral domain optical coherence tomography as no disruption of EZ and disrupted EZ. The serum levels of anti-MPO antibody was analyzed using standard protocol. Association between the variables was evaluated using multiple regression analysis.

RESULTS

A significant difference was found between the serum levels of anti-MPO antibody in various study groups (p<0.001). A positive association was found between EZ disruption and levels of anti-MPO antibody [adjusted odd's ratio (AOR)=1.079, CI 1.010-1.124, p=0.04]. A significant positive correlation was found between logMAR visual acuity and grade of disruption (AOR=1.008, CI 1.006-5.688, p=0.04).

CONCLUSIONS

An increased serum anti-MPO antibody levels is associated with retinal photoreceptor EZ disruption and decreased visual acuity in diabetic retinopathy.

Retinal photoreceptor expresses toll-like receptors (TLRs) and elicits innate responses following TLR ligand and bacterial challenge

Toll-like receptors (TLRs) play an important role in host defense against microbial pathogens. Our previous studies have shown that TLRs are expressed on various retinal cells (Microglia and Müller glia) and orchestrate retinal innate responses in bacterial endophthalmitis. In this study, we used a well-characterized mouse cone photoreceptor cell line (661W); and demonstrated that these cells express all known TLRs. Although the stimulation of 661W cells with TLR ligands (Pam3Cys, PolyI:C, LPS, Flagellin, Poly DT, and ODN) did not alter TLR expression, downstream TLR-signaling pathways (NF-κB, p38, and ERK) are activated. Moreover, TLR-activated 661W cells secreted significant amounts of inflammatory mediators (IL-6, IL-1β, MIP-2, and KC) in their culture supernatant, as assessed by ELISA. A similar trend was observed in 661W cells challenged with live bacteria (Staphylococcus aureus). Interestingly, the neutralization of TLR2, a major receptor for S. aureus recognition, did not significantly attenuate bacterial-induced inflammatory mediators, suggesting the existence of TLR2-independent mechanisms in photoreceptor cells. Together, these results indicate that photoreceptors constitutively express functional TLRs and possess the ability to initiate innate responses following pathogen challenge, implicating their role in retinal innate immunity.

Protective gene expression changes elicited by an inherited defect in photoreceptor structure

Inherited defects in retinal photoreceptor structure impair visual transduction, disrupt relationship with the retinal pigment epithelium (RPE), and compromise cell viability. A variety of progressive retinal degenerative diseases can result, and knowledge of disease etiology remains incomplete. To investigate pathogenic mechanisms in such instances, we have characterized rod photoreceptor and retinal gene expression changes in response to a defined insult to photoreceptor structure, using the retinal degeneration slow (rds) mouse model. Global gene expression profiling was performed on flow-sorted rds and wild-type rod photoreceptors immediately prior and subsequent to times at which OSs are normally elaborated. Dysregulated genes were identified via microarray hybridization, and selected candidates were validated using quantitative PCR analyses. Both the array and qPCR data revealed that gene expression changes were generally modest and dispersed amongst a variety of known functional networks. Although genes showing major (>5-fold) differential expression were identified in a few instances, nearly all displayed transient temporal profiles, returning to WT levels by postnatal day (P) 21. These observations suggest that major defects in photoreceptor cell structure may induce early homeostatic responses, which function in a protective manner to promote cell viability. We identified a single key gene, Egr1, that was dysregulated in a sustained fashion in rds rod photoreceptors and retina. Egr1 upregulation was associated with microglial activation and migration into the outer retina at times subsequent to the major peak of photoreceptor cell death. Interestingly, this response was accompanied by neurotrophic factor upregulation. We hypothesize that activation of Egr1 and neurotrophic factors may represent a protective immune mechanism which contributes to the characteristically slow retinal degeneration of the rds mouse model.

The alternative pathway is required, but not alone sufficient, for retinal pathology in mouse laser-induced choroidal neovascularization

Human genetic studies have demonstrated that polymorphisms in different complement proteins can increase the risk for developing AMD. There are three pathways of complement activation, classical (CP), alternative (AP), and lectin (LP), which all activate a final common pathway. Proteins encoded by the AMD risk genes participate in the AP (CFB), CP/LP (C2), or in the AP and final common pathway (C3). Here we tested which pathway is essential in mouse laser-induced CNV. CNV was analyzed using single complement pathway knockouts (i.e., eliminating one complement pathway at a time), followed by a double knockout in which only the AP is present, and the CP and LP are disabled, using molecular, histological and electrophysiological outcomes. First, single-gene knockouts were analyzed and compared to wild type mice; C1q(-/-) (no CP), MBL(-/-) (no LP), and CFB(-/-) (no AP). Six days after the laser-induced lesion, mice without a functional AP had reduced CNV progression (P<0.001) and preserved ERG amplitudes, whereas those without a functional CP or LP were indistinguishable from the wild type controls (P>0.3). Second, AP-only mice (C1q(-/-)MBL(-/-)) were as protected from developing CNV as the CFB(-/-) mice. The degree of pathology in each strain correlated with protein levels of the angiogenic and anti-angiogenic protein VEGF and PEDF, respectively, as well as levels of terminal pathway activation product C5a, and C9. The analysis of complement activation pathways in mouse laser-induced CNV allows for the following conclusions. Comparing the single pathway knockouts with those having only a functional AP showed: (1) that AP activation is necessary, but not alone sufficient for injury; and (2) that initial complement activation proceeds via both the LP and CP. Thus, these data indicate an important role for the AP in the generation of complement-dependent injury in the RPE and choroid via amplification of CP- and LP-initiated complement activation. Improving our understanding of the local regulation of this pathway in the eye is essential for developing improved treatment approaches for AMD.

[Application of MHC-Ig/peptide polymer in detecting antigen-specific cytotoxic T lymphocytes in mice with experimental autoimmune uveitis]

OBJECTIVE

To explore the application of MHC-Ig/peptide polymer technique for detecting antigen-specific cytotoxic T lymphocytes (CTLs) in mice with experimental autoimmune uveitis (EAU).

METHODS

B10RIII mice were immunized with an interphotoreceptor retinal-binding protein (IRBP) synthetic peptide (IRBP161-180). The in vivo primed T cells were separated and stained with MHC-Ig polymer combined with a panel of truncated peptides derived from IRBP161-180. The level of IRBP-specific CTLs cells was determined by FACS analysis. The CD8+ T cells were isolated from the primed T cells and stimulated with complex polymers containing MHC-Ig and various IRBP-derived peptides. The proliferation of CD8+ T cells was measured by H thymidine incorporation. The production of interferon- (IFN-) in the cell suspensions was measured by ELISA.

RESULTS

The IRBP-specific CTLs were detected by MHC-Ig/peptide polymers. The MHC-Ig/IRBP168-177 peptide polymer dtected 12.3% specific CTLs, showing greater ability in stimulating proliferation of CTLs and production of IFN--than the other MHC-Ig/ peptide polymers (P < 0.01). The truncated 10-mer peptide, IRBP168-177, was the major antigenic epitope for the IRBP-specific CTLs. The MHC-Ig/IRBP168-177 peptide polymer detected the highest level(4.9% +/- 1.1%) of specific CTLs from peripheral blood mononuclear cells (PBMCs) at the acute stage of EAU.

CONCLUSION

The MHC-Ig polymer technique is an effective instrument for detecting antigen-specific CTLs, with good sensitivity and specificity in EAU studies.

Local Administration of an Adeno-associated Viral Vector Expressing IL-10 Reduces Monocyte Infiltration and Subsequent Photoreceptor Damage during Experimental Autoimmune Uveitis

Autoimmune posterior uveitis is a chronic, potentially blinding inflammatory disease of the eye. It is commonly treated with immunosuppressive drugs that have adverse long-term effects. Advances in gene transfer techniques have enabled long-term, stable transduction of retinal cells following subretinal injection with adeno-associated viral (AAV) vectors. Here we report for the first time that subretinal injection of rAAV-2 encoding murine IL-10 into the retina of C57BL/6 mice significantly decreases the median experimental autoimmune uveitis (EAU) disease severity. This protection is shown to be due to a decrease in the number and activation status of infiltrating monocytes during EAU, as determined by costimulatory molecule expression and nitrotyrosine detection. No differences within splenocyte proliferative responses or serum antibody levels were detected, emphasizing the potential of gene therapy strategies in ameliorating autoimmune responses in local microenvironments without unwanted systemic effects.

Chronic recurrent autoimmune uveitis with progressive photoreceptor damage induced in rats by transfer of IRBP-specific T cells

Recurrent uveitis is a common cause of vision blindness. Using a rat model of chronic recurrent uveitis, we examined the relationship between clinical expression, pathological changes, and the heterogeneity of the disease. Chronic recurrent uveitis was induced by adoptive transfer of interphotoreceptor retinoid-binding protein (IRBP)-specific T cells in a total of more than 60 Lewis rats. In about 75% of cases recurrent uveitis was pathologically a chronic and progressive disease. The major pathological changes included the gradual loss of photoreceptor cells. However, disease progression did not always parallel the severity of ocular inflammation and clinical recurrent disease, with about a quarter showing no pathological damage in the eye.

Otx2Homeobox Gene Induces Photoreceptor-Specific Phenotypes in Cells Derived from Adult Iris and Ciliary Tissue

PURPOSE

It remains unclear which gene induction effectively generates photoreceptor-specific phenotypes from nonretinal tissues. The purpose of this study was to determine whether Crx and Otx2--homeobox genes related to photoreceptor development--can induce the generation of these phenotypes in cells derived from adult ciliary and iris tissue and in mesencephalon-derived neural stem cells.

METHODS

Crx and Otx2 were transferred into adult rat ciliary- and embryonic mesencephalon-derived neurospheres and adult rat iris-derived cells with the aid of a recombinant retrovirus. The presence of photoreceptor-specific phenotypes was confirmed by immunocytochemistry and Western blot analysis.

RESULTS

More than 90% of the Crx- and Otx2-transfected ciliary- and iris-derived cells exhibited rod opsin immunoreactivity, whereas few of the similarly transfected mesencephalon-derived neural stem cells expressed rod opsin. At least two additional key components of the phototransduction cascade, recoverin and Gdeltat1, were expressed by Crx- and Otx2-transfected iris-derived cells.

CONCLUSIONS

Crx and Otx2 effectively induced the generation of photoreceptor-specific phenotypes from ciliary- and iris-derived cells. That both Crx and Otx2 induced phenotype generation in cells derived from iris or ciliary tissue may suggest an approach to photoreceptor cell preparation for retinal transplantation.

Immune responses to retinal self-antigens in CD25(+)CD4(+) regulatory T-cell-depleted mice

PURPOSE

Prior work has shown that autoimmune uveoretinitis develops spontaneously in CD25(+)CD4(+) regulatory T-cell-depleted mice (Tr-depleted mice). In this study, the generation of autoantibodies and autoreactive T-cells specific to retinal antigens was examined in Tr-depleted mice with uveoretinitis, and the pathogenic and immunogenic abilities of the autoreactive T cells were evaluated.

METHODS

Tr-depletion was achieved in (C57BL/6 x A/J) F1 (B6A) mice by thymectomy on day 3 of life followed by intraperitoneal injection of an anti-CD25 mAb. At 6 months of age, autoantibodies to the retina were evaluated by indirect immunofluorescence, and total IgG2a levels in sera were assessed by ELISA. The pathogenic abilities of the splenic T cells were examined by adoptive transfer to syngeneic nu/nu mice, and the proliferation responses and the secretion of granulocyte-macrophage-colony-stimulating factor (GM-CSF), IFN-gamma, and IL-10 on stimulation by retinal self-antigens was also evaluated.

RESULTS

Autoantibodies to the retinal photoreceptor cell layer were detected in Tr-depleted mice, and the titers correlated well with the grades of inflammatory lesions. The splenic CD4(+) T cells of Tr-depleted mice induced uveoretinitis in the recipients by adoptive transfer and exhibited proliferative responses and secretion of IFN-gamma, but not of IL-10, by in vitro stimulation with S-Ag and interphotoreceptor retinoid-binding protein (IRBP). Moreover, the total IgG2a level in serum was markedly and significantly augmented in Tr-depleted mice.

CONCLUSIONS

The results suggest that in Tr-depleted mice in which uveoretinitis develops, S-Ag and IRBP-specific T cells are spontaneously sensitized and shifted to a Th1-phenotype. These sensitized T cells may account for the development of autoimmune uveoretinitis in Tr-depleted mice.

Stable rAAV-mediated transduction of rod and cone photoreceptors in the canine retina

Recombinant adeno-associated virus (rAAV) vectors are attractive candidates for the treatment of inherited and acquired retinal disease. Although rAAV vectors are well characterized in rodent models, a prerequisite to their clinical application in human patients is the thorough evaluation of their efficacy and safety in intermediate animal models. In this study, we describe rAAV-2-mediated expression of GFP reporter gene in retinal cells following local vector delivery in dogs. Subretinal delivery of rAAV.CMV.GFP was performed unilaterally in eight normal dogs from 6 weeks of age. The area of retinal transduction was maximized by the optimization of surgical techniques for subretinal vector delivery by pars-plana vitrectomy and the use of fine-gauge subretinal cannulae to create multiple retinotomies. rAAV-2 vectors mediated efficient stable reporter gene expression in photoreceptors and retinal pigment epithelial cells. We found efficient transduction of cone photoreceptors in addition to rods in both the canine retina and after subretinal vector delivery in another intermediate animal model, the feline retina. GFP expression in dogs was confined to the area of the retinal bleb and was sustained in cells at this site for at least 18 months. Electroretinography demonstrated a modest reduction in global rod-mediated retinal function following subretinal delivery of rAAV.CMV.GFP. Three of the eight animals developed delayed-onset intraocular inflammation, in two cases associated with a serum antibody response to GFP protein. We conclude that rAAV-2 vectors mediate efficient sustained transgene expression in rod and cone photoreceptors following subretinal delivery in this intermediate animal model. The possibility of adverse effects including intraocular immune responses and reduced retinal function requires further investigation prior to clinical applications in patients.

Autoantibody-induced apoptosis as a possible mechanism of autoimmune retinopathy

Circulating antibodies specific to retinal proteins have been associated with retinal dysfunction in patients with retinopathy. Anti-recoverin antibodies found in patients with cancer-associated retinopathy (CAR) represent a unique model to study the relationship between retinal degeneration and autoimmunity. A body of evidence from in vitro and in vivo studies indicates that anti-recoverin autoantibodies are cytotoxic to retinal cells and induce apoptotic death of retinal photoreceptor cells, which leads to the degeneration of the photoreceptor cell layer. Similar to anti-recoverin autoantibodies, antibodies with other retinal specificities induce their target retinal cell death by activating a caspase 3-dependent apoptotic pathway. Thus, autoantibody-induced apoptosis may be a common pathway that leads to retinal death and blindness.

Progressive subretinal fibrosis and blindness associated with multifocal granulomatous chorioretinitis: A variant of sympathetic ophthalmia

OBJECTIVE

To report a case of bilateral progressive subretinal fibrosis and blindness with multifocal granulomatous chorioretinitis occurring after intraocular surgery. We propose that this is a variant of sympathetic ophthalmia.

DESIGN

Clinicopathologic case report.

METHODS

The left enucleated globe was examined by histopathologic methods. The patient's sera were subjected to immunohistochemical studies against retinal antigens, and collagen 2 types in areas of fibrosis were identified. Polymerase chain reaction was used to test for herpes virus DNA in microdissected, formalin-fixed, paraffin-embedded tissue.

RESULTS

The enucleated globe demonstrated histopathologic features similar to an entity previously described as progressive subretinal fibrosis with multifocal granulomatous chorioretinitis. The patient's sera demonstrated antibodies directed against retinal photoreceptors and pigment epithelium. Polymerase chain reaction for herpes virus was negative. Immunohistochemical studies demonstrated types III, IV, V, and VI collagen in areas of fibrosis.

CONCLUSIONS

The clinical history along with the histopathologic and immunohistochemical findings suggest that progressive subretinal fibrosis with multifocal granulomatous chorioretinitis may represent a variant of sympathetic ophthalmia and that retinal autoimmunity may play a role in its pathogenesis.

Immunopathology of recurrent uveitis in spontaneously diseased horses

Equine recurrent uveitis (ERU) is the most serious eye disease in horses worldwide. Despite the fact that ERU is generally considered to be immune mediated, a detailed description of the histopathology of the posterior part of ERU eyes is lacking. Here, we examined sections of paraffin-embedded eyes using histological and immunhistological methods. Twenty seven eyes of 20 horses with ERU and 30 eyes of 15 healthy control horses were included in this study. We could consistently demonstrate an involvement of the retina and the choroid in all examined eyes of horses with spontaneous ERU. In eyes with minimal histopathological changes, the infiltrates consisted almost exclusively of T-cells. Histopathological changes start with the destruction of the photoreceptor outer segments, which often leads to focal retinal detachment. In more severely affected eyes, there is additional disintegration of the ganglion cell layer and the inner nuclear layer. In almost all examined eyes, lymphoid follicle formation could be demonstrated. Typical localizations of these follicles were the iris stroma and the choroid underneath the transition zone of the retina without photoreceptor cells to the region containing photoreceptor cells. These follicles consist of a T-cell rich periphery with a small center of CD3-negative lymphocytes. In cases with extreme histopathological changes, the retinal architecture is widely disintegrated with massive infiltration of the retina, the choroid, and the ciliary body by several types of inflammatory cells. Necrotic remnants of the retina are end-stage findings and there is only a minor inflammatory infiltration left. This study provides clear evidence that the retina is involved in all stages of ERU. Inflammation is mainly driven by T-cells as T-cells were demonstrated in mild stages of the disease and are also the predominating cell type in all other stages of ERU.

The photoreceptor cell-specific nuclear receptor is an autoantigen of paraneoplastic retinopathy

OBJECTIVES

To report a novel antibody associated with paraneoplastic retinopathy and to characterize the retinal autoantigen.

METHODS

Immunohistochemistry of rat and human tissues was used to identify antiretinal antibodies. Serologic screening of a bovine retinal cDNA expression library was performed to clone the target antigen.

RESULTS

A 72-year-old woman presented with a 6-month history of progressive visual loss, bilateral central scotomas, light flashes, and night blindness. Visual acuity was 20/40 OD and 20/30 OS. There was generalized loss of retinal pigment and narrow arterioles; discs were normal in appearance. The electroretinogram showed no response. Chest computed tomograph scan demonstrated a right lung mass; biopsy revealed poorly differentiated carcinoma. The patients' serum contained antibodies that immunolabeled nuclei of cells of the outer--and to a lesser extent, the inner--nuclear layer of the adult rat retina. No reactivity was identified with nonretinal adult human or rat tissues. Reactivity was seen in the developing rat embryo. Serologic screening of a bovine retinal library resulted in the isolation of three overlapping clones, encoding a protein highly homologous to the human photoreceptor cell-specific nuclear receptor gene product.

CONCLUSIONS

The target antigen of an antibody associated with paraneoplastic retinopathy is the photoreceptor cellspecific nuclear receptor, a member of a conserved family of nuclear receptors involved in photoreceptor cell development or maintenance.

Immunological problems of transplantation into the subretinal space

The objective of retinal transplantation is to substitute destroyed or degenerated retina to improve visual function. Photoreceptors and retinal pigment epithelium cells of embryonic and nonembryonic origin have been transplanted into the subretinal space in different animal models. Recently, retinal cells have also been used for transplantation in untreated or immunosuppressed patients with retinitis pigmentosa and age-related macular degeneration. Transplants performed in animals such as the RCS rat have maintained retinal function at the transplantation site for long periods of time, although such transplantations in humans have not proved conclusively, to date, whether there is a real effect or not. One reason for this phenomenon seems to be an immune response to transplanted retinal cells at the transplantation site. The detectable rejection process shows that the posterior part of the eye is not absolutely immunologically privileged and that rejection is a serious problem in human retinal transplantation. Many questions concerning transplantation technique, graft treatment and postoperative treatment will be answered through more intensive experiments and clinical trials regarding the immunology. However, rejection of transplanted material is one of the main reasons why retinal transplantation has not yet proved successful.

Antigenic epitopes of the photoreceptor synaptic ribbon

The purpose of this study was twofold: 1) to purify and identify a protein containing an epitope recognized by an anti-synaptic ribbon antibody B16 and 2) to identify and sequence the epitope. B16 recognizes several unrelated proteins in retina immunoblots. Purification and microsequencing of the strongest band (88 kDa) demonstrate 94% identity to aconitase over 111 amino acids. Polyclonal antibodies against aconitase recognize aconitase on Western blots, but not synaptic ribbons in sections. We conclude that although aconitase contains the epitope, aconitase is not the synaptic ribbon protein. The B16 epitope was identified to be 542DTYQHPPKDS551. A synthetic peptide to this sequence absorbs B16 activity in both Western blots and immunohistochemistry studies, whereas partial peptides fail to absorb activity. Additional antibodies against this peptide label synaptic ribbons. When mouse retina were double labeled with B16 and anti-alpha-actinin, B16 was found to label synaptic ribbons in the outer plexiform layer that partially enclosed the alpha-actinin label. We have determined the amino acid sequence of the B16 epitope and found that the B16 labeling colocalizes with alpha-actinin at the photoreceptor synapse.