Expression of interleukin-1 alpha, interleukin-1 beta, and an interleukin-1 receptor antagonist in human retinal pigment epithelial cells

ABCB5+ Limbal Epithelial Stem Cells Inhibit Developmental but Promote Inflammatory (Lymph) Angiogenesis While Preventing Corneal Inflammation

The limbus, the vascularized junction between the cornea and conjunctiva, is thought to function as a barrier against corneal neovascularization. However, the exact mechanisms regulating this remain unknown. In this study, the limbal epithelial stem cell (LESC) marker ABCB5 was used to investigate the role of LESCs in corneal neovascularization. In an ABCB5KO model, a mild but significant increase of limbal lymphatic and blood vascular network complexity was observed in developing mice (4 weeks) but not in adult mice. Conversely, when using a cornea suture model, the WT animals exhibited a mild but significant increase in the number of lymphatic vessel sprouts compared to the ABCB5KO, suggesting a contextual anti-lymphangiogenic effect of ABCB5 on the limbal vasculature during development, but a pro-lymphangiogenic effect under inflammatory challenge in adulthood. In addition, conditioned media from ABCB5-positive cultured human limbal epithelial cells (ABCB5+) stimulated human blood and lymphatic endothelial cell proliferation and migration. Finally, a proteomic analysis demonstrated ABCB5+ cells have a pro(lymph)angiogenic as well as an anti-inflammatory profile. These data suggest a novel dual, context-dependent role of ABCB5+ LESCs, inhibiting developmental but promoting inflammatory (lymph)angiogenesis in adulthood and exerting anti-inflammatory effects. These findings are of high clinical relevance in relation to LESC therapy against blindness.

FoxP3 expression by retinal pigment epithelial cells: transcription factor with potential relevance for the pathology of age-related macular degeneration

Background

Forkhead-Box-Protein P3 (FoxP3) is a transcription factor and marker of regulatory T cells, converting naive T cells into Tregs that can downregulate the effector function of other T cells. We previously detected the expression of FoxP3 in retinal pigment epithelial (RPE) cells, forming the outer blood–retina barrier of the immune privileged eye.

Methods

We investigated the expression, subcellular localization, and phosphorylation of FoxP3 in RPE cells in vivo and in vitro after treatment with various stressors including age, retinal laser burn, autoimmune inflammation, exposure to cigarette smoke, in addition of IL-1β and mechanical cell monolayer destruction. Eye tissue from humans, mouse models of retinal degeneration and rats, and ARPE-19, a human RPE cell line for in vitro experiments, underwent immunohistochemical, immunofluorescence staining, and PCR or immunoblot analysis to determine the intracellular localization and phosphorylation of FoxP3. Cytokine expression of stressed cultured RPE cells was investigated by multiplex bead analysis. Depletion of the FoxP3 gene was performed with CRISPR/Cas9 editing.

Results

RPE in vivo displayed increased nuclear FoxP3-expression with increases in age and inflammation, long-term exposure of mice to cigarette smoke, or after laser burn injury. The human RPE cell line ARPE-19 constitutively expressed nuclear FoxP3 under non-confluent culture conditions, representing a regulatory phenotype under chronic stress. Confluently grown cells expressed cytosolic FoxP3 that was translocated to the nucleus after treatment with IL-1β to imitate activated macrophages or after mechanical destruction of the monolayer. Moreover, with depletion of FoxP3, but not of a control gene, by CRISPR/Cas9 gene editing decreased stress resistance of RPE cells.

Conclusion

Our data suggest that FoxP3 is upregulated by age and under cellular stress and might be important for RPE function.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02620-w.

Complement-mediated release of fibroblast growth factor 2 from human RPE cells

PURPOSE

Complement activation is associated with choroidal neovascularization (CNV) in age-related macular degeneration (AMD). Fibroblast growth factor 2 (FGF2) and membrane attack complex (MAC) are present in eyes of patients with CNV. Herein, we investigated the effect of complement activation on FGF2 release in human retinal pigment epithelial (RPE) cells.

METHODS

Cultured human RPE cells were primed with an anti-RPE antibody and then treated with C1q-depleted human serum in the presence or absence of Tec kinases inhibitor (LFM-A13). 38 cytokines/chemokines levels were measured by Luminex technology. Secretion of FGF2 and interleukin (IL)-6 was assessed by ELISA. Tec protein was measured by Western blot. mRNA expression of FGF2, chemokine (C-X-C motif) ligand 1 (CXCL-1), and family members of Tec kinases was evaluated by qPCR. Cell viability and MAC deposition were determined by WST-1 assay and flow cytometry, respectively.

RESULTS

Complement activation caused increased FGF2 and IL-6 release. FGF2 was released when C6-depleted human serum was reconstituted with C6. Anti-C5 antibody significantly attenuated complement-mediated FGF2 release, but not IL-6. FGF2 mRNA levels were not affected, while CXCL-1 mRNA levels were increased by complement activation. FGF2-containing extracellular vesicles were detected in response to complement challenge. Tec mRNA and protein were expressed in RPE cells. In the presence of LFM-A13, secretion of FGF2, but not IL-6, and MAC deposition were significantly decreased and cell viability was significantly increased in complement-treated cells when compared to controls.

CONCLUSIONS

Complement plays an important role to release FGF2 from RPE cells. Tec kinase is involved in MAC formation and complement-mediated FGF2 release. This information suggests a role for complement activation to mediate neovascularization in conditions such as AMD, and may elucidate potential therapeutic targets.

Expression and regulation of alarmin cytokine IL-1α in human retinal pigment epithelial cells

Human retinal pigment epithelial (hRPE) cells play important immune-regulatory roles in a variety of retinal pathologic processes, including the production of inflammatory cytokines that are essential mediators of the innate immune response within the ocular microenvironment. The pro-inflammatory "alarmin" cytokine IL-1α has been implicated in both infectious and non-infectious retinal diseases, but its regulation in the retina is poorly understood. The purpose of this study was to elucidate the expression and regulation of IL-1α within hRPE cells. To do this, IL-1α mRNA and protein in hRPE cells was assessed by RT-PCR, qPCR, ELISA, Western blot, and immunofluorescence following treatment with a variety of stimuli and inhibitors. ER stress, LPS, IL-1β, and TLR2 activation all significantly increased intracellular IL-1α protein. Increasing intracellular calcium synergized both LPS- and Pam3CSK4-induced IL-1α protein production. Accordingly, blocking calcium signaling and calpain activity strongly suppressed IL-1α protein expression. Significant but more moderate inhibition occurred following blockage of TLR4, caspase-4, or caspase-1. Neutralizing antibodies to IL-1β and TLR2 partially eliminated LPS- and TLR2 ligand Pam3CSK4-stimulated IL-1α protein production. IFN-β induced caspase-4 expression and activation, and also potentiated LPS-induced IL-1α expression, but IFN-β alone had no effect on IL-1α protein production. Interestingly, all inhibitors targeting the PI3K/Akt pathway, with the exception of Ly294002, strongly increased IL-1α protein expression. This study improves understanding of the complex mechanisms regulating IL-1α protein expression in hRPE cells by demonstrating that TLR4 and TLR2 stimulation and exposure to IL-1β, ER stress and intracellular calcium all induce hRPE cells to produce intracellular IL-1α, which is negatively regulated by the PI3K/Akt pathway. Additionally, the non-canonical inflammasome pathway was shown to be involved in LPS-induced hRPE IL-1α expression through caspase-4 signaling.

Distinct CD40L receptors mediate inflammasome activation and secretion of IL-1β and MCP-1 in cultured human retinal pigment epithelial cells

CD40L signaling occurs in several diseases with inflammatory components, including ocular and retinal diseases. However, it has never been evaluated as a pathogenic mechanism in age-related macular degeneration (AMD) or as an inducer of inflammasome formation in any cell type. mRNA and protein levels of CD40, IL-1β, NALP1, NALP3, caspase-1, and caspase-5 were determined by RT-PCR, qPCR, and Western blot. CD40L receptor (CD40, α5β1, and CD11b) expression was determined by Western and immunofluorescent staining. IL-1β, IL-18, and MCP-1 secretions were determined by ELISA. NALP1 and NALP3 inflammasome formation were determined by Co-IP. Experiments were conducted on primary human retinal pigment epithelial (hRPE) cells from four different donors. Human umbilical vein endothelial (HUVEC) and monocytic leukemia (THP-1) cells demonstrated the general applicability of our findings. In hRPE cells, CD40L-induced NALP1 and NALP3 inflammasome activation, cleavage of caspase-1 and caspase-5, and IL-1β and IL-18 secretion. Interestingly, neutralizing CD11b and α5β1 antibodies, but not CD40, reduced CD40L-induced IL-1β secretion in hRPE cells. Similarly, CD40L treatment also induced HUVEC and THP-1 cells to secret IL-1β through CD11b and α5β1. Additionally, the CD40L-induced IL-1β secretion acted in an autocrine/paracrine manner to feed back and induce hRPE cells to secrete MCP-1. This study is the first to show that CD40L induces inflammasome activation in any cell type, including hRPE cells, and that this induction is through CD11b and α5β1 cell-surface receptors. These mechanisms likely play an important role in many retinal and non-retinal diseases and provide compelling drug targets that may help reduce pro-inflammatory processes.

Scaffolds for retinal pigment epithelial cell transplantation in age-related macular degeneration

In several retinal degenerative diseases, including age-related macular degeneration, the retinal pigment epithelium, a highly functionalized cell monolayer, becomes dysfunctional. These retinal diseases are marked by early retinal pigment epithelium dysfunction reducing its ability to maintain a healthy retina, hence making the retinal pigment epithelium an attractive target for treatment. Cell therapies, including bolus cell injections, have been investigated with mixed results. Since bolus cell injection does not promote the proper monolayer architecture, scaffolds seeded with retinal pigment epithelium cells and then implanted have been increasingly investigated. Such cell-seeded scaffolds address both the dysfunction of the retinal pigment epithelium cells and age-related retinal changes that inhibit the efficacy of cell-only therapies. Currently, several groups are investigating retinal therapies using seeded cells from a number of cell sources on a variety of scaffolds, such as degradable, non-degradable, natural, and artificial substrates. This review describes the variety of scaffolds that have been developed for the implantation of retinal pigment epithelium cells.

The pro-inflammatory role of high-mobility group box 1 protein (HMGB-1) in photoreceptors and retinal explants exposed to elevated pressure

To determine the role of high-mobility group box 1 protein (HMGB-1) in cellular and tissue models of elevated pressure-induced neurodegeneration, regeneration, and inflammation. Mouse retinal photoreceptor-derived cells (661W) and retinal explants were incubated either under elevated pressure or in the presence of recombinant HMGB-1 (rHMGB-1) to investigate the mechanisms of response of photoreceptors. Immunohistochemistry, western blotting, and the quantitative real-time PCR were used to examine the expression levels of immunological factors (eg, HMGB-1, receptor for advanced glycation end products (RAGE)), Toll-like receptors 2 and 4 (TLR-2, TLR-4), apoptosis-related factors (eg, B-cell lymphoma 2 (Bcl-2), Bcl-2-associated death promoter (Bad)) as well as cytokine expression (eg, tumor necrosis factor alpha (TNF-α), interleukin (IL)-4, IL-6, and vascular endothelial growth factor (VEGF)). The data revealed increased the expression of HMGB-1 and its receptors RAGE, TLR-2, and TLR-4, and TNF-α as well as pro-apoptotic factors (eg, Bad) as well as apoptosis in 661W cells exposed to elevated pressure. Co-cultivation of 661W cells with rHMGB-1 increased the expression levels of pro-apoptotic Bad and cleaved Caspase-3 resulting in apoptosis. Cytokine array studies revealed an increased release of TNF-α, IL-4, IL-6, and VEGF after incubation of 661W cells with rHMGB-1. Upregulation of HMGB-1, TLR-2, and RAGE as well as anti-apoptotic Bcl-2 expression levels was found in the retinal explants exposed to rHMGB-1 or elevated pressure. The results suggest that HMGB-1 promotes an inflammatory response and mediates apoptosis in the pathology of photoreceptors and retinal homeostasis. HMGB-1 may have a key role in ongoing damage of retinal cells under conditions of elevated intraocular pressure.

Inflammatory mediators induced by amyloid-beta in the retina and RPE in vivo: implications for inflammasome activation in age-related macular degeneration

PURPOSE

Drusen are hallmarks of age-related macular degeneration (AMD). Amyloid-beta 1-40 (Aβ 1-40), a constituent of drusen, is known to stimulate inflammatory pathways in RPE; however, its effect in vivo is not known. The purpose of this study was to examine the effect of Aβ 1-40 on cytokine expression and inflammasome activation relevant to AMD in an animal model.

METHODS

Wild-type rats received intravitreal injections of Aβ 1-40, and eyes were taken at days 1, 4, 14, and 49 postinjection. The RPE, neuroretina, and vitreous were analyzed for cytokine expression, inflammasome activation, and microglial response via RT-PCR, immunohistochemistry, and suspension array assay. Retinal cell loss was assessed via apoptotic markers and retinal thickness.

RESULTS

Aβ 1-40 stimulated upregulation of IL-6, TNF-α, IL-1β, IL-18, caspase-1, NLRP3, and XAF1 genes in the RPE/choroid and the neuroretina. Increased IL-1β and IL-6 immunoreactivity was found in retinal sections, and elevated levels of IL-1β and IL-18 were found in the vitreous of Aβ-injected eyes. Aβ 1-40 induced a moderate increase in CD11b/c-reactive cells on day 1 postinjection only. No evidence of the proapoptotic XAF1 protein, p53, TUNEL immunoreactivity, or retinal thinning was observed.

CONCLUSIONS

These results confirm earlier in vitro work and support the proinflammatory role of drusen component Aβ 1-40 in the RPE and retina. Inflammasome activation may be responsible for this effect in vivo. This model is useful for understanding cellular triggers of inflammasome activation and proposed early inflammatory events in the outer retina associated with the etiology of AMD.

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.

Lack of TNF-alpha promotes caspase-3-independent apoptosis during murine cytomegalovirus retinitis

PURPOSE

Both caspase-dependent and caspase-independent apoptosis contribute to retinal damage during murine cytomegalovirus (MCMV) retinitis, and TNF-α is among the inducers of apoptosis. The aim of this study was to determine the contribution of TNF-α by studying virus replication and apoptosis in immunosuppressed (IS) TNF-α(-/-) mice.

METHODS

IS TNF-α(-/-) mice or wild-type mice were inoculated with MCMV by the supraciliary route. Injected eyes were examined by plaque assay, electron microscopy, Western blot analysis (caspase-3, caspase-8, caspase-12, Bid, NF-κB, cFlip, XIAP), staining for MCMV early antigen, and TUNEL assay.

RESULTS

Although the titer of MCMV was similar in both groups, significantly more apoptotic cells were observed in the retinas of IS TNF-α(-/-) mice than in those of wild-type mice. The level of active caspase-3 was similar in both groups; however, more activated proteins for genes involved in the mitochondrial pathway (cleaved caspase-8, tBid) and endoplasmic reticulum (ER) stress (cleaved caspase-12) and, though less active, NF-κB subunits and antiapoptotic proteins (XIAP and cFlip) were detected in the TNF-α(-/-) eyes compared with wild-type mice.

CONCLUSIONS

Although TNF-α is an inducer of apoptosis, the results of this study suggest that TNF-α is also antiapoptotic by the following mechanism: TNF-α activation of NF-κB promotes the production of the antiapoptosis genes, c-flip or XIAP, which, in turn, inhibit the activation of caspase-8 and the mitochondrial pathway or the activation of caspase-12 and ER stress.

Microarray analysis identifies changes in inflammatory gene expression in response to amyloid-beta stimulation of cultured human retinal pigment epithelial cells

PURPOSE

Age-related macular degeneration (AMD) is a common cause of irreversible vision loss in the elderly. The hypothesis was that in vitro stimulation of RPE cells with Abeta(1-40), a constituent of drusen, promotes changes in gene expression and cellular pathways associated with the pathogenesis of AMD, including oxidative stress, inflammation, and angiogenesis.

METHODS

Confluent human RPE cells were stimulated with Abeta(1-40), or the reverse peptide Abeta(40-1), and genome wide changes in gene expression were studied with gene microarrays. Selected genes were verified by qRT-PCR and ELISA. Pathway analysis with gene set enrichment analysis (GSEA) and ingenuity revealed top functional pathways in RPE after Abeta(1-40) stimulation.

RESULTS

RPE cells stimulated with Abeta(1-40) (0.3 microM) for 24 hours resulted in 63 upregulated and 22 downregulated previously known genes. The upregulated genes were predominantly in inflammatory and immune response categories, but other categories were also represented, including apoptosis, cell signaling, cell proliferation, and signal transduction. Categories of downregulated genes included immune response, transporters, metabolic functions and transcription factors. ELISA confirmed that secreted levels of IL-8 were two times higher than control levels. GSEA and ingenuity analysis confirmed that the top affected pathways in RPE cells after Abeta(1-40) stimulation were inflammation and immune response related. Surprisingly, few angiogenic pathways were activated at the doses and exposure times studied.

CONCLUSIONS

Abeta(1-40) promotes RPE gene expression changes in pathways associated with immune response, inflammation, and cytokine and interferon signaling pathways. Results may relate to in vivo mechanisms associated with the pathogenesis of AMD.

B cells from periodontal disease patients express surface Toll-like receptor 4

Chronic systemic inflammation links periodontal disease (PD) to increased incidence of cardiovascular disease. Activation of TLRs, particularly TLR4, promotes chronic inflammation in PD by stimulating myeloid cells. B cells from healthy individuals are generally refractory to TLR4 agonists as a result of low surface TLR4 expression. Unexpectedly, a significantly increased percentage of gingival and peripheral blood B cells from patients with PD expressed surface TLR4. Surface expression correlated with an active TLR4 promoter that mimicked the TLR4 promoter in neutrophils. B cells from PD patients were surface myeloid differentiation protein 2-positive and also packaged the enhancer of a proinflammatory cytokine, IL-1 beta, into an active structure, demonstrating that these cells harbor key characteristics of proinflammatory cell types. Furthermore, B cells lacked activating signatures of a natural IL-1 beta inhibitor, IL-1 receptor antagonist. Surprisingly, despite multiple signatures of proinflammatory cells, freshly isolated B cells from PD patients had decreased expression of TLR pathway genes compared with B cells from healthy individuals. Decreases in inflammatory gene expression were even more dramatic in B cells stimulated with a TLR4 ligand from a periodontal pathogen, Porphyromonas gingivalis LPS 1690. In contrast, B cell TLR4 was not activated by the prototypic TLR4 ligand Escherichia coli LPS. These findings raise the unexpected possibility that TLR4 engagement modulates B cell activation in PD patients.

Pro-inflammatory cytokines increase reactive oxygen species through mitochondria and NADPH oxidase in cultured RPE cells

Reactive oxygen species (ROS) generated during inflammation are believed to play critical roles in various ocular diseases. However, the underlying mechanisms remain poorly understood. We investigated if pro-inflammatory cytokines, tumor necrosis factor (TNF)-alpha, interleukin-1 beta (IL-1 beta), and interferon-gamma (IFN-gamma), induce ROS in human retinal pigment epithelial (RPE) cells. TNF-alpha, IL-1 beta and IFN-gamma increased both intracellular and extracellular ROS production in a time- and dose-dependent manner. Thenoyltrifluoroacetone (TTFA), an inhibitor of mitochondrial respiratory chain, blocked TNF-alpha- and IFN-gamma-, but not IL-1 beta-induced ROS, whereas other two mitochondrial respiratory chain inhibitors, rotenone and antimycin A, had no effect. NADPH oxidase inhibitor (diphenylene iodinium) abolished the ROS production induced by IL-1 beta or IFN-gamma, but not by TNF-alpha, whereas 6-aminonicotinamide (6AN), an inhibitor of the hexose monophosphate shunt (HMS), had no significant effects on the ROS induced by all three cytokines. ROS scavengers, pyrrolidinedithiocarbamate (PDTC) and N-acetyl-cysteine (NAC), reduced the levels of ROS induced by TNF-alpha, IL-1 beta and IFN-gamma (P<0.05). Collectively, these results demonstrate that TNF-alpha, IL-1 beta and IFN-gamma increase mitochondrial- and NADPH oxidase-generated ROS in human RPE cells.

Major histocompatibility complex molecules on parenchymal cells of the target organ protect against autoimmune disease

Parenchymal cells of the autoimmune organ may only express major histocompatibility complex (MHC) molecules during the disease process. In this paper, we hypothesize that the appearance of MHC molecules on parenchymal cells may augment the activation of invading autoreactive T cells and either exacerbate or suppress local inflammation. It is speculated that like many biological responses this is a two-edge sword - namely, the expression of modest levels of MHC molecules may inhibit the activation of invading T cells, whereas overexpression of these molecules may promote activation of autoimmune T cells, enhancing the inflammatory cascade, thus leading to tissue damage.

The role of tumour necrosis factor (TNF-alpha) in experimental autoimmune uveoretinitis (EAU)

The pleiotropic cytokine tumour necrosis factor-alpha (TNF-alpha) is released from cells that include macrophages and T-cells during inflammatory responses, orchestrating the initiation of further leucocytic infiltration via adhesion molecule upregulation, dendritic cell maturation and survival, macrophage activation and driving Th1 T-cells responses within tissues. Exposure to TNF also plays a role in maintaining tissue homeostasis, particularly relating to resident cell responses of both microglia and retinal pigment epithelium. Depending on the balance between duration and dose of TNF exposure, an environment where full expression of inflammatory and autoimmune responses within tissues may occur. In experimental autoimmune uveoretinitis (EAU), increased tissue concentrations of TNF facilitate the on-going T-cell effector responses and macrophage activation. These are responsible for targeted and bystander tissue damage and can be suppressed by anti-TNF therapies, in particular, those directed at the p55 TNF receptor. The ability to suppress disease experimentally has led to the successful translation of anti-TNF therapy for treatment of uveitis in cohort studies and phase I/II trials where, additionally, altered peripheral blood CD4(+) T-cell profiles can be demonstrated following each treatment.

Retinal pigment epithelium-immune system interactions: cytokine production and cytokine-induced changes

Vision is dependent on proper function of several intraocular structures. Immune responses to eliminate invading pathogens from the eye may threat vision by causing damage to these structures. Therefore, immunological defence of the eye should be carefully balanced between efficacy and maintenance of functional integrity. The eye is equipped with several regulatory mechanisms to prevent certain immune and inflammatory responses and is, therefore, regarded as an immune privileged site. The retinal pigment epithelium (RPE) contributes to the immune privileged status of the eye as part of the blood-eye barrier and by the secretion of immunosuppressive factors inside the eye. RPE cells, however, may also play an important role in the development of immune and inflammatory responses in the posterior part of the eye. During the last decade it has become clear that RPE cells are highly sensitive to a variety of inflammatory cytokines. Under inflammatory conditions, RPE cells produce a myriad of cytokines that may activate the resident ocular cells or attract and activate leukocytes. Cytokine stimulation of RPE cells causes profound effects, including nitric oxide secretion, cell surface expression of MHC class II and adhesion molecules and abrogation of barrier function. This article provides a comprehensive review of the literature concerning RPE cells and cytokines.

Analysis of the secretion pattern of monocyte chemotactic protein-1 (MCP-1) and transforming growth factor-beta 2 (TGF-beta2) by human retinal pigment epithelial cells

Retinal pigment epithelial (RPE) cells, situated between the neurosensory retina and the vascularized choroid, form part of the blood-eye barrier and are important for homeostasis of the outer retina. These cells are able to produce a variety of cytokines which may play a role in the maintenance of the immunosuppressive milieu inside the eye and in intraocular inflammatory responses. In the present study, we investigated whether RPE cells secreted the anti-inflammatory cytokine TGF-beta2 and the proinflammatory cytokine MCP-1 in a polarized manner. Monolayers of human donor RPE cells were cultured on transwell filters. Secretion of TGF-beta2 and MCP-1 at either the apical or basal side of the RPE cell monolayers, that were not treated or stimulated with IL-1beta (200 U/ml), was analysed by ELISA. All three cell lines examined had a different TGF-beta2 secretion pattern. In two of the three donor RPE cell lines tested, TGF-beta2 secretion was polarized, but not in the same direction. TGF-beta2 secretion was not up-regulated by stimulation with IL-1beta. In contrast, IL-1beta strongly induced MCP-1 secretion preferentially into the basal compartment of all RPE monolayers tested. These data indicate that human RPE cells are able to secrete TGF-beta2 and MCP-1 in a polarized fashion. Our results suggest that MCP-1 can be secreted by RPE cells in the direction of choroidal vessels during inflammatory responses in the posterior part of the eye, which may limit damage to the neurosensory retina.

Expression of multiple forms of IL-1 receptor antagonist (IL-1ra) by human retinal pigment epithelial cells: identification of a new IL-1ra exon

The eye is considered an immunologically privileged organ and is separated from the rest of the body by blood-ocular barriers. Part of the blood-retina barrier consists of the retinal pigment epithelium (RPE). In addition to the physical barrier which the monolayer of RPE cells forms, these cells contribute to ocular immune privilege by producing anti-inflammatory molecules that down-regulate potential damaging immune reactions. In this study the mRNA expression of IL-1 receptor antagonist (IL-1ra) by RPE cells was studied in 15 donor-derived cell lines. Expression of both the intracellular and secreted IL-1ra was detected in unstimulated and IL-1beta- or phorbol 12-myristate 13-acetate-exposed RPE. Analysis of IL-1ra protein in RPE cell lysates and cell culture supernatants indicated that these cells produce mainly intracellular IL-1ra. No correlation between IL-1ra expression levels and the IL-1ra gene polymorphism could be detected. In addition to the two known intracellular IL-1ra variants (intracellular IL-1ra type I and type II) evidence is provided for the expression of a hitherto unknown splice variant of the IL-1ra mRNA by RPE cells. Expression was not confined to RPE cells and could also be detected in cultured human fibroblasts and macrophages. This variant, which we have tentatively named intracellular IL-1ra type III, encodes a C-terminally truncated protein of only 27 amino acids.

Caspase 1-independent IL-1beta release and inflammation induced by the apoptosis inducer Fas ligand

Fas ligand is a well-characterized apoptosis inducer. Here we demonstrate that Fas ligand induces the processing and secretion of interleukin-1beta (IL-1beta) in peritoneal exudate cells. This IL-1beta secretion is independent of IL-1beta converting enzyme (caspase 1), yet it is inhibited by caspase inhibitors, indicating that a caspase(s) in addition to IL-1beta converting enzyme can process IL-1beta. Inoculation of tumor cells expressing Fas ligand into wild-type mice induces a massive neutrophil infiltration that is, in contrast, suppressed in IL-1alpha/beta knockout mice. These results demonstrate a newly discovered role for Fas ligand in inflammation, and challenge the dogma that apoptosis does not induce inflammation.

The mRNA expression of cytokines and their receptors in cultured iris pigment epithelial cells: a comparison with retinal pigment epithelial cells

It has been suggested that human iris pigment epithelial (IPE) cells isolated from iridectomized tissue could be used as autologous cells for transplantation into the subretinal space in diseases with dysfunctional retinal pigment epithelium (RPE). RPE cells synthesize a number of cytokines and their receptors which are important for its proper function. Nearly nothing is known about the capacity of IPE to synthesize cytokines or responding to them. To compare the mRNA expression of 36 cytokines or their receptors in cultured adult IPE cells and RPE cells we used semi-quantitative reverse transcription polymerase chain reactions (RT-PCR). Included in our assay were cytokines with known expression in RPE to get a broad basis for comparing IPE cells: basic fibroblast growth factor (bFGF or FGF-2), and one of its receptor (FGFR-1), epidermal growth factor (EGF), and its receptor EGF-R, transforming growth factor beta(TGFbeta), and its type III receptor TGFbeta-R3, the platelet-derived growth factors and receptors (PDGF A, PDGF B, PDGF-Ralpha, PDGF-Rbeta), tumor necrosis factor alpha(TNFalpha), and two receptors TNF-R1 and TNF-R2, insulin (INS) with receptor INS-R, insulin-like growth factors (IGF1, IGF2), and receptors (IGF1-R, IGF2-R), vascular endothelial growth factor (VEGF), and two receptors (VEGF-R1 or FLT-1 and VEGF-R2 or FLK-1), the receptor for VEGF-C: VEGF-R3 or FLK-4, interleukin 6 (IL6), and its receptor (IL6-R), nerve growth factor (NGF), interleukin 1alpha(IL1alpha), and a receptor (IL1-R). In addition, cytokines or their receptors not known to be expressed in RPE were included to widen our picture of cytokine gene expression in the eye: stem cell factor (SCF), its receptor (SCF-R), low-affinity nerve growth factor receptor p75 (p75(NGF-R), ciliary neutrothropic factor (CNTF), and its receptor (CNTF-R), glycoprotein 130 interleukin 6 transducer (gp130 (IL6-SD), leukemia inhibitory factor (LIF), and its receptor (LIF-R). Semi-quantitative expression data were obtained using series of fivefold dilutions of each cDNA and a fixed number of PCR cycles. The expression of RPE 65, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and beta2-microglobulin (B2MG) was used as a control for cellular origin, RNA quality and PCR conditions. With the exception of insulin and tumor necrosis factor alphaall other cytokines analysed and their receptors were expressed in both IPE and RPE cells, even though the levels varied. No qualitative or quantitative difference were observed in the mRNA expression level of 34 (94%) of the cytokines or receptors between IPE and RPE. In contrast, the mRNA expression level of vascular endothelial growth factor (VEGF) and vascular endothelial growth factor receptor 2 [VEGF-RS (FLK-1)] was lower in IPE than in RPE cells. As an increased expression of VEGF in the RPE in maculae with age-related macular disease could be involved in its pathogenesis, a decreased expression of angiogenic growth factors in IPE cells could possibly be beneficial for the therapy of age-related maculopathy if indeed other tasks of non-functional RPE cells could be performed by IPE cells. The similarity of the mRNA expression pattern in 94% of the cytokines analyzed supports the assumption that IPE cells potentially can perform functions of RPE cells in the appropriate environment.

Interleukin-1 beta enhances interleukin-1 receptor antagonist content in human somatotroph adenoma cell cultures

In addition to the well-known modulation of immune and inflammatory responses, the interleukin-1 (IL-1) system has been shown to be involved in the regulation of anterior pituitary hormone secretion and growth. We previously demonstrated that IL-1 receptor antagonist (IL-1ra) is expressed in human pituitary adenomas cultured in vitro. In the present study, we investigated the regulation of IL-1ra protein by IL-1 beta (1-100 U/mL) in human somatotroph adenomas (n = 9) cultured for 12-48 h. IL-1 beta significantly enhanced the concentration of IL-1ra dose dependently in the somatotroph adenoma cell lysates, whereas IL-1ra concentrations remained unchanged in the culture supernatants. Furthermore, basal IL-1ra concentrations were significantly higher in the cell lysates compared with the corresponding culture supernatants. The regulation of IL-1ra in somatotroph adenoma cells is different from human cultured monocytes, in which IL-1 beta significantly stimulated IL-1ra secretion into the culture supernatants, and no change of intracellular IL-1ra content was observed. Incubation of the somatotroph adenoma cells with 100 U/mL IL-1 beta did not result in a change of GH concentrations in the culture supernatants. Enhancement of intracellular IL-1ra protein by IL-1 beta may represent a mechanism intrinsic to somatotroph adenoma cells to counterregulate the response to IL-1 beta on hormone secretion or cellular growth.

Polarized secretion of IL-6 and IL-8 by human retinal pigment epithelial cells

A number of cell types situated along interfaces of various tissues and organs such as the peritoneum and the intestine have been shown to secrete inflammatory cytokines in a polarized fashion. Retinal pigment epithelial (RPE) cells are positioned at the interface between the vascularized choroid and the avascular retina, forming part of the blood-retina barrier. These cells are potent producers of inflammatory cytokines and are therefore considered to play an important role in the pathogenesis of ocular inflammation. Whether cytokine secretion by these cells also follows a vectorial pattern is not yet known, and was therefore the subject of this study. Monolayers of human RPE cells (primary cultures and the ARPE-19 cell line) cultured on transwell filters were stimulated to produce IL-6 and IL-8 by adding IL-1beta (100 U/ml) to either the upper or the lower compartment. After stimulation, the human RPE cell lines showed polarized secretion of IL-6 and IL-8 towards the basal side, irrespective of the side of stimulation. The ARPE- 19 cell line also secreted IL-6 and IL-8 in a polarized fashion towards the basal side after basal stimulation; polarized secretion was, however, not apparent after apical stimulation. The observation that human RPE cells secrete IL-6 and IL-8 in a polarized fashion towards the choroid may represent a mechanism to prevent damage to the adjacent fragile retinal tissue.

Secretion of proinflammatory cytokines by human conjunctival epithelial cells

The production of cytokines by human conjunctival epithelial cells following stimulation was investigated. Primary cultures of human conjunctival epithelial cells were characterized by morphology and keratin expression. Cultured epithelial cells were treated with varying concentrations of lipopolysaccharide, interleukin (IL)-1 beta, calcium ionophore A23187, or phorbol myristate acetate, and cytokine secretion was determined over specified intervals. Culture supernatants and cell lysates were analyzed by ELISA for IL-1 beta, IL-3, IL-4, IL-5, IL-6, IL-8, IL-11, IL-1 receptor antagonist (IL-1ra), tumor necrosis factor-alpha (TNF-alpha), and granulocyte-macrophage colony stimulating factor (GM-CSF). With the exception of IL-1ra, unstimulated conjunctival epithelial cells produced cytokines at relatively low or undetectable levels. IL-1ra was detected in both culture supernatants and cell lysates under basal conditions. In response to stimuli, conjunctival epithelial cells secreted the proinflammatory cytokines TNF-alpha, IL-6, IL-8, and GM-CSF in a dose- and time-dependent fashion. After stimulation, the intracellular levels of IL-1ra increased in these cells but the supernatant-associated levels remained unchanged. None of the other cytokines evaluated (IL-1 beta, IL-3, IL-4, IL-5, and IL-11) were detected in supernatants or lysates of resting or stimulated cells. These findings suggest that conjunctival epithelial cells may contribute to the pathogenesis of human ocular diseases by production of proinflammatory cytokines. Further evaluation of these cells as targets of therapy is warranted.

The effect of tetracycline fiber therapy on beta-glucuronidase and interleukin-1 beta in crevicular fluid

Treatment with the tetracycline HCL-containing (Actisite infinity) fiber has been shown to improve clinical measures of periodontitis, as well as reduce the number of sites infected with putative periodontal pathogens. In this study, we examined the effect of the tetracycline fiber on biochemical mediators of the host's inflammatory response in gingival crevicular fluid (GCF). The total amount of the lysosomal enzyme beta-glucuronidase (beta G), considered a marker of primary granule release from polymorphonuclear leukocytes and interleukin-1 beta, a cytokine with important proinflammatory effects, were examined in GCF. Patients with localized recurrent periodontitis were followed over a 16 week period. Treated teeth (Tx), teeth adjacent to treated teeth (ADJ) and control teeth (Cx) were studied. Following fiber therapy, the Tx teeth displayed statistically significant reductions in mean probing depth, depth of the deepest site and bleeding on probing over the 16 weeks of the trial. Significant reduction in the depth of the deepest site was also seen for the ADJ teeth over 16 weeks. Total beta G in GCF was reduced for the Tx teeth comparing baseline to 16 weeks, but no significant changes were observed for the ADJ or Cx teeth. Prior to treatment, total beta G for the Tx teeth was 211 +/- 49 U (mean +/- standard error), versus 146 +/- 174 U for the ADJ teeth and 121 +/- 33 U for the Cx teeth. 16 weeks treatment, the mean values for these 3 categories of teeth were comparable (Tx = 95 +/- 20 U, ADJ = 93 +/- 42 U and Cx = 103 +/- 29 U). For the Tx teeth, the maximum reduction in total beta G following therapy occurred at 6 weeks (65%). Total IL-1 beta was significantly reduced for the Tx teeth at 3 and 6 weeks, but rebounded at 16 weeks. In contrast to what was seen for beta G, the maximum reduction in total IL-1 beta for the Tx teeth was observed at 3 weeks (68%). These data suggest that host mediators associated with increased risk for active disease are reduced following tetracycline fiber therapy. Future studies will determine the relative importance of a reduced microbial challenge versus a tetracycline-mediated direct modification of the host response to account for the reduction in the host inflammatory response in GCF following tetracycline fiber therapy.

A redox-based mechanism for induction of interleukin-1 production by nitric oxide in a human colonic epithelial cell line (HT29-Cl.16E)

We evaluated the effects of two NO donors, sodium nitroprusside (SNP) and 3-morpholino-sydnonimine (SIN-1), characterized by alternative redox states, i.e. nitrosonium ion (NO+) and nitric oxide (NO.) respectively, on intracellular interleukin-1 (IL-1) production, by a human colonic epithelial cell line (HT29-Cl.16E). SNP was able to induce intracellular IL-1 alpha production up to 10 h incubation, in a dose-dependent manner. Several experiments provide evidence that the NO+ redox form, and not the free radical NO., is implicated in the IL-1 alpha production: (i) SIN-1, devoid of any NO+ character, led to a very weak IL-1 production as compared with SNP; (ii) the reductive action of a thiol such as cysteine on NO+ led to a dose-dependent increase in NO, concentration, measured as NO2-/NO3- accumulation, and to large decrease in IL-1 production. Dibutyryl cGMP had no effect on IL-1 production, this finding supporting the concept that a cGMP-independent pathway is involved in the intracellular signalling of NO+. Together these results point out that NO, depending on its redox form, is able to modulate IL-1 production in cultured colonic epithelial cells.

SV40-immortalized and primary cultured human retinal pigment epithelial cells share similar patterns of cytokine-receptor expression and cytokine responsiveness

Retinal pigment epithelial (RPE) cells produce and respond to a variety of cytokines; however, molecular and biochemical studies are restricted by the limited access to large numbers of pure cells and the variability associated with different donor sources. Despite success in establishing primary human RPE (HRPE) cell cultures, the inability to sustain consistent proliferation rates and morphology over several passages remains a concern. This problem was approached by using an immortalized line of simian virus (SV)40 transformed fetal HRPE cells (SVRPE). Cytokine production, receptor expression and responsiveness in the SVRPE cell line was analyzed to determine the usefulness of this model for studying HRPE-cytokine interactions. Using reverse transcriptase polymerase chain reaction (RT-PCR), HRPE and SVRPE cells demonstrated an identical pattern of interleukin-1 receptor (IL-1R), IL-2R (alpha sub-unit), IL-6R, interferon (IFN)-gamma R and tumor necrosis factor-alpha (TNF)R p55 expression. No amplification products for TNFR p75 or granulocyte/macrophage colony stimulating factor (GM-CSF)R were demonstrated in either population. IFN-gamma stimulation induced surface human leukocyte antigen (HLA)-DR in both SVRPE and HRPE, while TNF treatment induced surface expression of intercellular adhesion molecule (ICAM)-1 on SVRPE and upregulated ICAM from basal levels on HRPE. Both cell types showed amplification products for interleukin (IL)-1 beta, IL-6 and transforming growth factor (TGF)-beta 1 using RT-PCR. The bioassays demonstrated that both populations of unstimulated cells constitutively secrete very low levels of TGF-beta and no IL-6.(ABSTRACT TRUNCATED AT 250 WORDS)

Monocyte-induced cytokine expression in cultured human retinal pigment epithelial cells

Monocytes and retinal pigment epithelial cells are intimately associated in membranes of eyes with proliferative vitreoretinopathy and in certain types of uveitis. The goal of this study was to determine whether monocytes modulate cytokine expression in retinal pigment epithelial cells, and if so, to identify the monocyte products responsible for this effect. Cultured human retinal pigment epithelial cells were exposed to varying concentrations of monocyte-conditioned medium from unstimulated human monocytes for 1-48 hr, or from monocytes prestimulated with lipopolysaccharide. mRNA expression of interleukin-1 beta, interleukin-6, interleukin-8, melanoma growth stimulating activity/gro alpha and gamma, macrophage colony stimulating factor, transforming growth factor-beta 2, basic fibroblast growth factor and activin beta A chain was determined by reverse transcription polymerase chain reaction. Protein secretion of selected cytokines, interleukin-1 beta, interleukin-6, interleukin-8, macrophage colony stimulating factor and transforming growth factor-beta 2 was measured in RPE-conditioned medium by ELISA. Retinal pigment epithelial cells constitutively expressed mRNA for interleukin-6, macrophage colony stimulating factor, transforming growth factor-beta 2, basic fibroblast growth factor and activin beta A chain. Interleukin-1 beta, melanoma growth stimulating activity/gro alpha and gamma and interleukin-8 were not expressed under basal conditions. Stimulated monocyte-conditioned medium markedly induced mRNA of all cytokines except basic fibroblast growth factor and transforming growth factor-beta 2 in a dose- and time-dependent manner. Unstimulated monocyte-conditioned medium was a less potent inducing agent, but still enhanced mRNA expression of interleukin-6, interleukin-8 and melanoma growth stimulating activity/gro alpha. Stimulated monocyte-conditioned medium also induced a time-dependent increase in interleukin-6, Interleukin-8, macrophage colony stimulation factor and transforming growth factor-beta 2, but not interleukin-1 beta protein secretion (p < 0.05 for all time points). Neutralizing antibodies to interleukin-1 beta, or tumour necrosis factor alpha, but not interleukin-1 alpha, significantly reduced cytokine mRNA expression induced by stimulated monocyte-conditioned medium. The combination of all three neutralizing antibodies almost entirely eliminated monocyte-induced mRNA expression and protein production of all cytokines studied. Activated monocytes secrete a heterogeneous mixture of products that together strongly induce expression of multiple cytokines in human retinal pigment epithelial cells. Most if not all of the inducing effect can be accounted for by interleukin-1 beta and tumour necrosis factor alpha. Because cytokines have been implicated in proliferative vitreoretinopathy and uveitis, monocyte-mediated cytokine expression by RPE cells may serve to initiate and perpetuate these diseases.

Inflammation induced changes in adenosine 3',5'-cyclic monophosphate production by ciliary epithelial cell bilayers

Despite extensive evidence implicating the cytokines interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNF alpha) in the intraocular inflammatory response, little is known about their effects on signal transduction in anterior uveal tissue. Since these cytokines have been shown to alter the adenylyl cyclase system in nonocular tissues, we tested the hypothesis that IL-1 beta and TNF alpha affect the anterior uvea by altering production of the intracellular second messenger adenosine 3',5'-cyclic monophosphate (cAMP) in ciliary epithelial bilayers. This was accomplished by measuring the levels of cAMP in bilayers ex vivo, following intraocular inflammation induced by intravitreal injection of IL-1 beta, TNF alpha or bacterial endotoxin, and in vitro, following exposure to IL-1 beta, TNF alpha or bacterial endotoxin. Although cAMP production was enhanced in bilayers from IL-1 beta-, TNF alpha- or endotoxin-inflamed eyes, ex vivo, exposure of normal bilayers to IL-1 beta (15 U ml-1), TNF alpha (20 U ml-1), or a low concentration of endotoxin (0.01 microgram ml-1) for 4 hr, in vitro, had no effect on cAMP production. The inability of IL-1 beta, TNF alpha, or the low concentration of endotoxin to increase cAMP production by bilayers, in vitro, suggests that the enhanced cAMP production observed with inflamed bilayers, ex vivo, was not due to a direct action of these inflammatory agonists on the ciliary epithelial bilayer. Although direct exposure to cytokines or endotoxin did not change cAMP production, treatment with IL-1 beta, TNF alpha, or a higher concentration of endotoxin (1 microgram ml-1) did affect signal transduction mechanisms.(ABSTRACT TRUNCATED AT 250 WORDS)

Survival factors in retinal degenerations

Recent experiments on the retina have examined the effectiveness of various factors (e.g. growth factors, neurotrophins and cytokines) for enhancing survival and reducing injury of retinal neurons, such as photoreceptors and ganglion cells, whose death leads to blindness in degenerative retinal diseases. It has also been shown that retinal injury stimulates intrinsic survival mechanisms that promote survival of these neurons.

Retinal pigment epithelial cells produce interleukin-1 beta and granulocyte-macrophage colony-stimulating factor in response to interleukin-1 alpha

The retinal pigment epithelium (RPE) is clinically involved in diverse ocular inflammatory diseases. Because perturbed RPE cells produce a variety of inflammatory substances, RPE cells may play an integral part in these diseases. Interleukin-1 (IL-1) and granulocyte-macrophage colony-stimulating factor (GM-CSF) are pleiotropic cytokines with the ability to trigger numerous inflammatory responses. This report shows that cultured human RPE cells synthesize interleukin-1 beta (IL-1 beta) and GM-CSF in response to the potentially inflammatory cytokine, IL-1 alpha, but not to E. coli endotoxin. Control RPE cells made little or no mRNA or protein for either IL-1 beta or GM-CSF. Upon stimulation of the cells by IL-1 alpha, both IL-1 beta and GM-CSF mRNAs were readily apparent by 3 hours, persisted for over 24 hours, and were translated into immunologically detectable proteins. GM-CSF protein was secreted into the culture medium, whereas IL-1 beta protein remained cell associated. The IL-1 alpha-induced mRNA and protein production were inhibited by dexamethasone. These observations provide additional evidence that RPE cells are capable of playing a pivotal role during ocular inflammation.