Immunochemical studies of epiretinal membranes using APAAP complexes: evidence for macrophage involvement in traumatic proliferative vitreoretinopathy

Mononuclear Phagocytes in Proliferative Vitreoretinopathy (PVR). A Specific Role of Microglial Cells in Non-Traumatic Disease?

Mononuclear phagocytes have been a focus of attention in the cellular biology of proliferative vitreoretinopathy (PVR) for more than ten years. The pattern of phagocyte participation in periretinal traction membrane formation in PVR depends on the etiology, i.e. trauma, rhegmatogenous retinal detachment, previous therapy, i.e. multiple surgical interventions, and the clinical stage of the disease. We have recently identified microglial cells as a distinct cellular population, in membranes from patients with non-traumatic PVR. Current evidence of mononuclear phagocyte function in PVR suggests a role for resident phagocytes of the vitreous and retina in PVR subsequent to rhegmatogenous detachment, and a role for blood-derived monocytes in post-traumatic PVR. The cellular biology of PVR may be much more heterogeneous than previously assumed.

Conjunctival Fibrosis after Application of Mitomycin C: An Animal Experimental Study

Mitomycin C (MMC) is widely used as an antimetabolite to minimize scarring of filtering blebs. Its effectiveness has been demonstrated in several clinical studies. The drug's mechanism of action is attributed to its antiproliferative effect on the subconjunctival cells, reducing the wound healing response. We placed sponges soaked with various concentrations of MMC, from 0.05 to 1.0 mg/ml, under the conjunctiva of 18 rabbits. Four weeks later, there was marked concentration-dependent scarring of the substantia proppria of the conjunctiva. Macrophages were more numerous in treated eyes, as determined by immunohistochemistry. The reason for MMC-related fibrosis in this animal model is not clear but may be related to the substances toxic effects. Whether it is only present in rabbits remains to be clarified.

Thrombospondin: A New Attachment Protein in Preretinal Traction Membranes

Thrombospondin (TSP), an adhesive integrin-binding protein of plasma and platelets, was detected in preretinal traction membranes from patients with idiopathic (8/8) and traumatic (7/8) proliferative vitreoretinopathy (PVR) and proliferative diabetic retinopathy (PDR) (6/8). TSP immunoreactivity was compared to the pattern of von Willebrand factor, plasma transglutaminase (blood coagulation factor XIII), fibronectin, and mononuclear phagocytes, using double-label immunoflurorescence microscopy. TSP was partially colocalised with the endothelial cell marker, von Willebrand factor, in PDR. The codistribution of catalytic factor XIII and two cross-linking substrates, fibronectin and TSP, suggests a functional role of the enzyme in the extracellular matrix build-up in PVR and PDR. No significant TSP synthesis by mononuclear phagocytes was observed. Western blotting indicated a plasmin-mediated intravitreal breakdown of presumably plasmatic TSP in PVR and PDR.

MCP-1-activated monocytes induce apoptosis in human retinal pigment epithelium

PURPOSE

The inflammatory response in age-related macular degeneration (AMD) is characterized by mononuclear leukocyte infiltration of the outer blood-retina barrier formed by the retinal pigment epithelium (RPE). A key mechanistic element in AMD progression is RPE dysfunction and apoptotic cell loss. The purpose of this study was to evaluate whether monocyte chemoattractant protein (MCP)-1-activated monocytes induce human RPE apoptosis and whether Ca(2+) and reactive oxygen species (ROS) are involved in this process.

METHODS

A cell-based fluorometric assay was used to measure intracellular Ca(2+) concentrations ([Ca(2+)](i)) in RPE cells loaded with fluorescent Ca(2+) indicator. Intracellular RPE ROS levels were measured by using the 5- and 6-chloromethyl-2',7'-dichlorodihydrofluorescence diacetate acetyl ester (CM-H(2)DCFDA) assay. RPE apoptosis was evaluated by activated caspase-3, Hoechst staining, and apoptosis ELISA.

RESULTS

MCP-1-activated human monocytes increased [Ca(2+)](i), ROS levels, and apoptosis in RPE cells, all of which were inhibited by 8-bromo-cyclic adenosine diphosphoribosyl ribose (8-Br-cADPR), an antagonist of cADPR. Although the ROS scavengers pyrrolidinedithiocarbamate (PDTC) and N-acetylcysteine (NAC) significantly inhibited ROS production and apoptosis induced by activated monocytes, they did not affect induced Ca(2+) levels. The induced Ca(2+) levels and apoptosis in RPE cells were inhibited by an antibody against cluster of differentiation antigen 14 (CD14), an adhesion molecule expressed by these cells.

CONCLUSIONS

These results indicate that CD14, Ca(2+), and ROS are involved in activated monocyte-induced RPE apoptosis and that cADPR contributes to these changes. Understanding the complex interactions among CD14, cADPR, Ca(2+), and ROS may provide new insights and treatments of retinal diseases, including AMD.

Differential cytokine expression of human retinal pigment epithelial cells in response to stimulation by C5a

Human retinal pigment epithelial (RPE) cells form part of the blood-retina barrier where they potentially can regulate leucocyte function. RPE cells are known to secrete several cytokines in response to stimulation by other cytokines. Anaphylatoxin C5a, a potent inflammatory mediator produced during complement activation, binds to G-protein coupled C5a receptors (C5aR) on monocytes/macrophages and releases various cytokines from the cells. We previously reported that the human RPE cell line ARPE-19 possesses C5aR and expresses IL-8 mRNA in response to C5a stimulation. In this study, we used a primary human RPE cell line (RPE43) and found that C5a induces increased expression of IL-1beta, IL-6, MCP-1 and GM-CSF mRNAs as well as IL-8 mRNA. ARPE-19 cells showed similar increases in the same cytokines. Interestingly, the kinetics of expression of the various cytokines differed. These results provide further evidence that C5a stimulation of RPE cells may play a role in regulating leucocyte function during ocular inflammation in which there is complement activation.

Apoptosis in proliferative vitreoretinal disorders: possible involvement of TGF-beta-induced RPE cell apoptosis

The targeted induction of apoptosis is a novel therapeutic approach to control the unlimited growth of proliferating cells. Since massive proliferation of cells at the vitreoretinal interface is a key feature of proliferative vitreoretinal disorders, we sought to identify apoptosis in epiretinal membranes from patients with proliferative vitreoretinopathy (PVR), proliferative diabetic retinopathy (PDR) and macular pucker. Further, we evaluated the possible induction of apoptosis of retinal pigment epithelial (RPE) cells by transforming growth factor-beta(TGF-beta). Apoptotic cells were identified by in situ DNA end labeling and acridine orange staining on paraffin-embedded tissue sections from epiretinal membranes of patients with all vitreoretinal disorders examined. Labeled nuclei or condensed chromatin were scattered throughout the membranes or occurred in clusters. Most apoptotic cells were RPE-derived, as assessed by cytokeratin immunochemistry. No apoptotic glial cells were detected. In PVR, proliferative activity, as confirmed by Ki-67 immunochemistry, was associated with short history and rapid disease progression. Apoptotic nuclei were observed more frequently in long-standing PVR or slow progression towards traction retinal detachment. TGF-beta was detected in all control vitreous samples by bioassay at concentrations below 20 ng ml-1. TGF-beta levels increased up to 20-fold in pathological vitreous. Marked heterogeneity was observed in all patient groups. The degree of TGF-beta activation was significantly higher in PVR than in PDR. Proapoptotic effects of TGF-beta were demonstrated in cultured human RPE cells by electron microscopy, in situ DNA end labeling, comet assay and a photometric enzyme immunoassay for histone-associated DNA fragments. Apoptosis appears to be a key regulatory mechanism of growth control of specific cell populations in proliferative vitreoretinal disorders. Administration of proapoptotic growth factors such as TGF-beta may provide a novel approach to inhibit cellular proliferation at the vitreoretinal interface.

Cytokines in proliferative diabetic retinopathy and proliferative vitreoretinopathy

We determined whether interleukin-8, monocyte chemotactic protein-1, and macrophage-colony stimulating factor are present in the vitreous of patients with proliferative diabetic retinopathy (PDR) or proliferative vitreoretinopathy (PVR). The levels of these cytokines were measured by specific enzyme-linked immunoassays in vitreous from 30 patients with PDR, 13 patients with PVR, and 26 control individuals, including 10 cadaver eyes and 16 patients with idiopathic macular holes, idiopathic macular puckers, vitreous hemorrhages, or uncomplicated retinal detachments. Detectable levels of interleukin-8 were found in 90% of vitreous samples of patients with PDR, 85% with PVR, and 58% of control samples. IL-8 was significantly increased in PDR (mean +/- SEM; 25.0 +/- 5.3 ng/ml; p = 0.01), but not in PVR (11.9 +/- 3.9 ng/ml; p = 0.50) compared to control human vitreous (8.5 +/- 2.5 2.5 ng/ml). MCP-1 was detected in 90% of vitreous samples of patients with PDR, 92% with PVR, and 81% of control samples. MCP-1 was significantly increased in PDR (6.2 +/- 0.9 ng/ml, p = 0.001) and PVR (7.7 +/- 2.5 ng/ml, p = 0.001) over the levels in control vitreous (1.2 +/- 0.2 ng/ml). M-CSF was detected in 94% of vitreous samples of patients with PDR, 88% with PVR, and 92% from control vitreous. M-CSF was significantly elevated in PDR (32.3 +/- 8.3 ng/ml, p = 0.03), but not in PVR (23.6 +/- 12.8 ng/ml, p = 0.4) compared to control (10.7 +/- 3.5 ng/ml). Our results suggest that IL-8, MCP-1, and M-CSF participate in the pathogenesis of PDR and PVR.

Onset and recurrence of proliferative vitreoretinopathy in various vitreoretinal disease

AIMS

This study aimed to evaluate both the mean time intervals between retinal disease and the development of proliferative vitreoretinopathy (PVR) and the time intervals at which recurrent PVR develops following various kinds of vitreoretinal disease and surgery.

METHODS

One hundred and thirty six consecutive cases of PVR that were seen and operated on between 1991 and 1994 were evaluated retrospectively. Intervals between onset of disease and PVR or recurrent PVR were noted. Conditions leading to PVR, surgical procedures, and the final anatomical and functional results were evaluated.

RESULTS

Before PVR was noted for the first time, an average of 1.06 vitreoretinal surgical procedures were performed (range 0 to 3). A mean of 1.99 surgeries were performed to control PVR or to reach an end stage where further surgery would have been unreasonable (range 0 to 5). The average (median) time interval between the onset of the retinal disease and PVR was 2.0 months (range 0.5 to 45 months). The median time intervals between surgery and recurrence for the second, third, or fourth times was also 2.0 months (range 0.5 to 34 months).

CONCLUSIONS

PVR starts earliest at 2 weeks after an event and subsequently quietens down within a maximum of 45 months after treatment (median 2 months). PVR may recur more than once but the time course of the disease is likely to be the same for each recurrence.

Immunohistochemistry of anterior proliferative vitreoretinopathy. Report of 11 cases

Anterior proliferative vitreoretinopathy is characterized by epiretinal proliferation that extends anteriorly over the vitreous base, and may, in addition to the cells usually contributing to proliferative vitreoretinopathy, also contain cells of ocular structures located in that area. We examined 11 complete globes with aPVR that were enucleated after previous severe trauma or perforating injuries (n = 8) and complicated retinal detachment (n = 3) by a panel of immunohistochemical markers. We found presence of RPE, glial cells, macrophages and fibrocytes, as consistently reported in PVR membranes. In addition, T-cell lymphocytes were present in 6 of the cases, and cells expressing the common leucocyte antigen on 8 cases. Cells staining positive for the intracytoplasmic contractile filament alpha-smooth muscle actin were present in 5 cases and cells staining for desmin in one case. Collagen type IV was part of most of the membranes, and vessels with leakage of plasma factors were present in more than half of the cases.

Granuloma annulare: an immunohistochemical study

The monocyte/histiocytic response in granuloma annulare has not been extensively studied. We studied the immunohistochemical staining pattern in granuloma annulare by using a panel of markers for "histiocytic" cells including Ham 56, KP1, factor XIIIa, Mac 387, vimentin, and lysozyme. The infiltrate failed to stain with the histiocytic markers, except with antibodies against vimentin and lysozyme. Commonly used histiocytic markers for infectious, immunogenic, and foreign body granulomas fail to stain the infiltrate in granuloma annulare. We hypothesize that a locally derived, immunologically distinct population of histiocytes produces the reaction pattern of granuloma annulare.

Effect of leukopenia on experimental post-traumatic retinal detachment

Macrophages are invariably present in the intraocular membranes of patients with traumatic proliferative vitreoretinopathy (PVR). There are two sources from which these macrophages could be recruited: adjacent tissues and the systemic circulation. In the study described herein, the role of circulating white blood cells and monocytes in experimental, traumatic PVR was studied. The circulating white blood cells of 20 rabbits were depleted by intravenous injection of strontium-89. Posterior perforating eye injury with subsequent intravitreal injection of autologous whole blood or autologous activated macrophages was then performed on these leukopenic animals. The experiments demonstrated that severe bone marrow depression reduced significantly the incidence of retinal detachments in eyes receiving whole blood, and reduced the severity of retinal detachments in eyes injected with activated macrophages. An association between the degree of leukopenia, monocytopenia, and protection from retinal detachment was demonstrated. These results support the hypothesis that macrophage infiltration is an important component of intraocular cellular proliferation, but does not exclude the role of other types of white blood cells in the pathogenesis of PVR.

Macrophages in proliferative vitreoretinopathy and proliferative diabetic retinopathy: differentiation of subpopulations

Macrophages have long been known to play a major role in the pathogenesis of proliferative vitreoretinal disorders. Using the monoclonal antibodies EBM11 (pan macrophage), 27E10 (early inflammatory stage marker), and RM3/1 (healing phase marker), different subpopulations of macrophages were differentiated in surgically removed membranes from patients with macular pucker (n = 6), proliferative vitreoretinopathy (PVR) following rhegmatogenous retinal detachment (n = 11), traumatic PVR (n = 19), and proliferative diabetic retinopathy (PDR) (n = 11). Macrophages were predominantly found in traumatic PVR and PDR. Some healing phase (RM3/1) macrophages were detected in all disease entities. Inflammatory stage macrophages (positive staining for 27E10) could not be detected in PVR following rhegmatogenous retinal detachment and idiopathic macular pucker. In traumatic PVR inflammatory stage macrophages were associated with a short history of disease whereas in PDR all types of macrophages could be detected regardless of clinical history and duration of the disease.

Origin of fibronectin in epiretinal membranes of proliferative vitreoretinopathy and proliferative diabetic retinopathy

Fibronectins, high molecular multifunctional glycoproteins of the extracellular matrix and plasma, have been a popular area of research in the pathogenesis of proliferative disorders of the retina. Several immunohistochemical studies have revealed that fibronectin is a major constituent of epiretinal membranes and that the cell types involved in proliferative intraocular disorders may synthesise it. However, owing to the fact that plasma and cellular fibronectin are similar in their overall structure, the origin of fibronectin in epiretinal membranes has not yet been clearly defined. In this study, we used two monoclonal antibodies: FN-3, which recognises an extra domain present in the cellular but not plasma form of fibronectin; and FN-4, which reacts with an antigenic site on both plasma and cellular fibronectin. In 37 epiretinal membranes obtained from eyes with proliferative vitreoretinopathy and proliferative diabetic retinopathy, we demonstrated the presence of cellular fibronectin, thus indicating local production. The significantly stronger and positive immunostain with FN-4 in the same specimens suggests the colocalisation of plasma fibronectin, that may be derived from the breakdown of the blood-retinal barrier and trapped in membranes during their formation. In pathological vitreous we demonstrated both types of fibronectin by western blot analysis.

Retinal microglia: a new cell in idiopathic proliferative vitreoretinopathy?

Despite numerous studies of the role of mononuclear phagocytes in proliferative vitreoretinopathy, the origin of these cells has remained obscure. Notably, retinal microglial cells have consistently been neglected. Applying double label immunohistology with a set of new cell markers to 37 preretinal traction membranes, we have identified a distinct population of proliferating cells presumably of microglial origin. The identification of microglia relies on positive labels for LN-1, Ricinus communis agglutinin-1, vimentin, HLA-DR, and nucleoside diphosphatase, and negative labels for Leu-M1, Leu-M3, EBM-11, von Willebrand factor, CD22, cytokeratin, and glial fibrillary acidic protein. Microglia are much more prevalent in idiopathic than in traumatic proliferative vitreoretinopathy and insignificant in proliferative diabetic retinopathy. HLA-DR expression was not restricted to pigment epithelium as previously reported but also observed in microglia, macrophages, endothelial and glial cells. The detection of retinal microglial cell proliferation suggests a pathogenetic role of these cells and questions current concepts of the cellular biology of proliferative vitreoretinopathy.

Vitronectin and proliferative intraocular disorders. I. A colocalisation study of the serum spreading factor, vitronectin, and fibronectin in traction membranes from patients with proliferative vitreoretinopathy

The presence of a scaffold for cellular spreading and proliferation is a precondition for the development of traction membranes in proliferative vitreoretinopathy (PVR). This study shows the presence of the serum spreading factor, vitronectin, in the extracellular matrix of periretinal membranes removed during vitreoretinal surgery. By means of a double label immunofluorescence protocol, a partial colocalisation of vitronectin with fibronectin at the magnification of light microscopy can be detected. Fibronectin is a high-molecular glycoprotein with multiple biological functions including the mediation of cell attachment and migration. Both proteins share a special cell recognition site which could be a target for experimental pharmacological approaches to PVR. Preliminary studies of vitreous aspirates using electrophoresis and Western blotting indicate that vitronectin may play a more important role in post-traumatic PVR as compared to PVR following rhegmatogenous retinal detachment.

Vitronectin and proliferative intraocular disorders. II. Expression of cell surface receptors for fibronectin and vitronectin in periretinal membranes

Several cell types participate in the formation of vitreoretinal traction membranes in proliferative intraocular disorders. The communication between these cells involves hormones, growth factors, and the interaction with extracellular matrix molecules. We have previously demonstrated a partial colocalisation of two potent mediators of cell attachment, fibronectin and vitronectin, in periretinal membranes from patients with proliferative vitreoretinopathy (PVR). We found a similar pattern of vitronectin and fibronectin deposition in proliferative diabetic retinopathy (PDR) (n = 6). Now we show the expression of the corresponding cell surface receptors, integrins, for fibronectin and vitronectin by proliferating cells in 22 periretinal membranes, including traumatic (n = 8) and idiopathic (n = 8) PVR as well as PDR membranes (n = 6). Integrins are membrane receptors for extracellular matrix macromolecules which are involved in such basic biological phenomena as embryogenesis and metastasis. Future studies on the pathogenesis of vitreoretinal proliferation will have to focus on the initiation, maintenance, and regulation of this intercellular communication network involving attachment proteins and integrins.

Immunoglobulin G, complement factor C3 and lymphocytes in proliferative intraocular disorders

This study examines a possible immunological contribution to the development of proliferative intraocular disorders (PID) with traction retinal detachment. We analysed 24 periretinal membranes and 35 vitreous aspirates from patients with idiopathic proliferative vitreoretinopathy (PVR), traumatic PVR, and proliferative diabetic retinopathy (PDR). Lymphocytes and complement factor C3 deposits could not be detected in any of the membrane specimens. IgG was present in all but one of the PVR membranes but in less than half of the PDR specimens and there to a lesser extent. The IgG immunoreactivity was not collocalized with macrophages but instead located to the extracellular matrix. The intravitreal levels of IgG (ELISA) and protein were elevated in PID but the range of these biochemical changes was so wide that there were no significant differences of the IgG levels between the single types of PID. Using electrophoresis and Western blotting, C3 was detected in normal and pathologic vitreous but smaller C3 fragments indicative of C3 breakdown were only found in PID.

Blood coagulation factor XIII contributes to the development of traction retinal detachment

The blood coagulation factor XIII catalyzes the crosslinking of fibrin monomers at the end of the coagulation cascade. Additional functions are the enzymatic coupling of fibrinectin to itself, fibrin, and collagen. We located the two subunits of factor XIII in 20 surgically obtained periretinal membranes, using double label immunofluorescence microscopy. Both subunits of factor XIII could be detected in all specimens. The positive staining in all specimens examined prompted us to determine the source of factor XIII. The abundant fibroblastic cells did not contain factor XIII. Macrophages, half of which stained for the alpha-subunit of factor XIII could not account for the presence of factor XIII because these cells were not present in all specimens, and did not stain for the beta-subunit. Factor XIII is probably derived from the exudation of plasma and platelets through disrupted blood-ocular barriers. This is confirmed by the detection of both subunits in vitreous aspirates from patients with proliferative intraocular disorders (n = 15) by Western blotting.

Proliferative vitreoretinopathy--is it anything more than wound healing at the wrong place?

Proliferative vitreoretinopathy (PVR) is a reactive process of the ocular tissue after perforating trauma, retinal detachment, and surgical manipulations. Although several studies, most of them experimental, have focused on the detection of specific etiologic factors in the development of PVR, there is compelling evidence that PVR is nothing more than a physiologic tissue repair process with undesirable consequences for the retina. Important features of PVR involving the role of platelets, mononuclear phagocytes, and fibroblasts parallel the chain of events observed in tissue repair elsewhere in the body. Numerous experimental models for PVR, originally designed to find specific stimuli for the generation of intraocular traction membrane formation, have shown that the process of PVR is the common pathway of the eye's reaction to vitreoretinal trauma of any kind. Accordingly, vitreoretinal surgeons could learn a lot from the work of other disciplines, e.g. surgery and dermatology, on wound healing, and the factors known to modify wound healing elsewhere in the body should be taken into consideration. The well-established impairment of tissue repair processes caused by medical treatment with corticosteroids and cytotoxic agents suggests a combined medical approach to PVR as an adjunct to surgical treatment, using refined methods of application and dosage. Steroids and cytotoxic drugs will influence the course of PVR by suppressing macrophage recruitment and the initial inflammatory reaction as well as the proliferative phase of wound healing with traction retinal detachment, respectively.

The pathogenesis of vitreoretinal proliferation and traction: a working hypothesis

Traction retinal detachment due to proliferative vitreoretinopathy (PVR) is a serious complication of ocular trauma, retinal detachment, and previous vitreoretinal surgery. The cause is the active proliferation of fibroblasts, glial cells, and retinal pigment epithelial cells in the periretinal spaces, leading to the formation of contractile cellular membranes. The generation of growth and mitosis stimulation for these cells has remained obscure. We postulate that invading macrophages and local microglia secrete growth factors, notably PDGF (platelet-derived growth factor), which in turn mediates the mitogenic effects of transferrin (TF), a protein present in huge amounts in native vitreous, in plasma and in intraocular proliferative tissue.

Transferrin and transferrin receptor expression in intraocular proliferative disease. APAAP-immunolabeling of retinal membranes and ELISA for vitreal transferrin

Transferrin (TF) is the major transport protein involved in human iron metabolism. The expression of the cell-surface TF receptor is associated with cellular proliferation, the dominant feature of proliferative vitreoretinal disorders with traction retinal detachment. A total of 14 retinal membranes from patients with different clinical diagnoses contained immunoreactive TF. Expression of the cell-surface TF receptor was confirmed by a monoclonal anti-human TF-receptor antibody label in 11 of the 14 specimens. We developed a noncompetitive enzyme-linked immunosorbent assay (ELISA) for TF and found it to be a significant component of vitreal protein, with a level of 65.7 +/- 33.9 mg/l. Vitreal TF as a major iron acceptor probably has a protective function, but its interaction with macrophages and its growth-promoting activity may subsequently stimulate the proliferation of fibroblasts and retinal pigment epithelial cells.