Effect of metalloprotease inhibitors on corneal allograft survival

The FasLane to ocular pathology-metalloproteinase cleavage of membrane-bound FasL determines FasL function

Fas ligand (FasL) is best known for its ability to induce cell death in a wide range of Fas-expressing targets and to limit inflammation in immunoprivileged sites such as the eye. In addition, the ability of FasL to induce a much more extensive list of outcomes is being increasingly explored and accepted. These outcomes include the induction of proinflammatory cytokine production, T cell activation, and cell motility. However, the distinct and opposing functions of membrane-associated FasL (mFasL) and the C-terminal soluble FasL fragment (sFasL) released by metalloproteinase cleavage is less well documented and understood. Both mFasL and sFasL can form trimers that engage the trimeric Fas receptor, but only mFasL can form a multimeric complex in lipid rafts to trigger apoptosis and inflammation. By contrast, a number of reports have now documented the anti-apoptotic and anti-inflammatory activity of sFasL, pointing to a critical regulatory function of the soluble molecule. The immunomodulatory activity of FasL is particularly evident in ocular pathology where elimination of the metalloproteinase cleavage site and the ensuing increased expression of mFasL can severely exacerbate the extent of inflammation and cell death. By contrast, both homeostatic and increased expression of sFasL can limit inflammation and cell death. The mechanism(s) responsible for the protective activity of sFasL are discussed but remain controversial. Nevertheless, it will be important to consider therapeutic applications of sFasL for the treatment of ocular diseases such as glaucoma.

Immune privilege in corneal transplantation

Corneal transplantation is the most successful solid organ transplantation performed in humans. The extraordinary success of orthotopic corneal allografts, in both humans and experimental animals, is related to the phenomenon of "immune privilege". Inflammation is self-regulated to preserve ocular functions because the eye has immune privilege. At present, three major mechanisms are considered to provide immune privilege in corneal transplantation: 1) anatomical, cellular, and molecular barriers in the cornea; 2) tolerance related to anterior chamber-associated immune deviation and regulatory T cells; and 3) an immunosuppressive intraocular microenvironment. This review describes the mechanisms of immune privilege that have been elucidated from animal models of ocular inflammation, especially those involving corneal transplantation, and its relevance for the clinic. An update on molecular, cellular, and neural interactions in local and systemic immune regulation is provided. Therapeutic strategies for restoring immune privilege are also discussed.

Therapeutic Use of Soluble Fas Ligand Ameliorates Acute and Recurrent Herpetic Stromal Keratitis in Mice

PURPOSE

The present study was designed to test the therapeutic value of soluble FasL (sFasL) in an acute model of herpetic stromal keratitis (HSK) and, more importantly, a recurrent model of HSK using BALB/c, BALB-lpr, and National Institutes of Health (NIH) mice.

METHODS

Mice were infected either acutely with the KOS strain of herpes simplex virus 1 (HSV-1) or latently with the McKrae strain of HSV-1. Acutely infected mice as well as ultraviolet-B (UV-B) reactivated mice (recurrent infection) were treated with sFasL, or soluble TNF-related apoptosis inducing ligand (sTRAIL), or BSA daily or 3 times/wk by using either a combination of subconjunctival injection and topical ointment, or with topical ointment alone. These mice then were evaluated for corneal opacity and neovascularization for 6 weeks.

RESULTS

Following acute and recurrent HSV-1 infection, wild-type BALB/c mice treated with sFasL displayed significantly reduced incidence of corneal opacity and neovascularization compared to the control animals. However, BALB-lpr mice, which are deficient in Fas+ inflammatory cells, displayed no such differences in ocular disease, as expected. Latently infected NIH mice treated with sFasL displayed similar results. Flow cytometric analysis revealed that the corneal inflammatory infiltrate in those treated with sFasL was significantly less than in sTRAIL- or BSA-treated mice. Furthermore, corneas from sFasL-treated mice displayed relatively more cells undergoing apoptosis.

CONCLUSIONS

This study provides evidence that sFasL treatment has potential therapeutic benefit in reducing inflammatory infiltrate and neovascularization in primary and recurrent forms of HSK, and that it does so by augmenting the restriction of Fas+ inflammatory cells mediated by membrane FasL.

Impaired Fas-Fas Ligand Interactions Result in Greater Recurrent Herpetic Stromal Keratitis in Mice

Herpes simplex virus-1 (HSV-1) infection of the cornea leads to a potentially blinding condition termed herpetic stromal keratitis (HSK). Clinical studies have indicated that disease is primarily associated with recurrent HSK following reactivation of a latent viral infection of the trigeminal ganglia. One of the key factors that limit inflammation of the cornea is the expression of Fas ligand (FasL). We demonstrate that infection of the cornea with HSV-1 results in increased functional expression of FasL and that mice expressing mutations in Fas (lpr) and FasL (gld) display increased recurrent HSK following reactivation compared to wild-type mice. Furthermore, both gld and lpr mice took longer to clear their corneas of infectious virus and the reactivation rate for these strains was significantly greater than that seen with wild-type mice. Collectively, these findings indicate that the interaction of Fas with FasL in the cornea restricts the development of recurrent HSK.

Murine corneal transplantation: a model to study the most common form of solid organ transplantation

Corneal transplantation is the most common form of organ transplantation in the United States with between 45,000 and 55,000 procedures performed each year. While several animal models exist for this procedure and mice are the species that is most commonly used. The reasons for using mice are the relative cost of using this species, the existence of many genetically defined strains that allow for the study of immune responses, and the existence of an extensive array of reagents that can be used to further define responses in this species. This model has been used to define factors in the cornea that are responsible for the relative immune privilege status of this tissue that enables corneal allografts to survive acute rejection in the absence of immunosuppressive therapy. It has also been used to define those factors that are most important in rejection of such allografts. Consequently, much of what we know concerning mechanisms of both corneal allograft acceptance and rejection are due to studies using a murine model of corneal transplantation. In addition to describing a model for acute corneal allograft rejection, we also present for the first time a model of late-term corneal allograft rejection.

Doxycycline inhibits inflammation-induced lymphangiogenesis in mouse cornea by multiple mechanisms

Lymphangiogenesis is significantly involved in the pathogenesis of diseases, including graft rejection, cancer metastasis and various inflammatory conditions. The inhibition of lymphangiogenesis has become a new therapeutic target for the treatment of these diseases. Here, we explored the anti-lymphangiogenic effects of doxycycline in inflammation-induced lymphangiogenesis (ILA) in the cornea and the underlying mechanisms. In the present study, mice with ILA of the cornea were treated with topical doxycycline (0.1%) or vehicle control. Lymphangiogenesis was quantified using corneal immunostaining of lymphatic vessel endothelial hyaluronan receptor-1 (LYVE-1). Human dermal lymphatic endothelial cells (HDLECs) and a murine macrophage cell line (RAW264.7) were used to further explore the underlying mechanisms of doxycycline-mediated anti-lymphangiogenesis in vitro. Our results showed that doxycycline treatment dramatically inhibited ILA in the mouse cornea (p<0.001), with a significant decrease in vascular endothelial growth factor (VEGF)-C/VEGF receptor 3 signalling, macrophage infiltration and inflammatory cytokine expression. Doxycycline also significantly inhibited VEGF-C-induced HDLEC proliferation in vitro by modulating the PI3K/Akt/endothelial nitric oxide (NO) synthase (eNOS) pathway and significantly suppressed interleukin-1β (IL-1β), TNF-α and VEGF-C production in the RAW264.7 cell line by modulating the PI3K/Akt/nuclear factor-kappaB (NF-κB) pathway. Additionally, doxycycline treatment dramatically reduced the phosphorylation of NF-κBp65, Akt and eNOS in ILA and significantly inhibited matrix metalloproteinases (MMPs) activity in vitro and in ILA. In conclusion, doxycycline inhibited ILA, possibly through suppression of VEGF-C signalling, macrophage function and MMPs activity. This observation suggests that doxycycline is a potential therapeutic agent for lymphangiogenesis-related diseases.

Age-dependent changes in FasL (CD95L) modulate macrophage function in a model of age-related macular degeneration

PURPOSE

We examined the effect of aging on Fas ligand (FasL) function in a mouse model of choroidal neovascularization (CNV).

METHODS

Young and aged mice were laser treated to induce CNV. Bone marrow chimeras were performed between young and aged mice. FasL protein expression was examined in the eye and soluble FasL (sFasL) was measured in the blood. Young and aged mice were treated with a matrix metalloprotease (MMP) inhibitor and systemic sFasL was neutralized by antibody treatment. Macrophages from young and aged mice were tested for sFasL-mediated cytokine production and migration.

RESULTS

The elevated CNV response observed with aging was dependent on bone marrow-derived cells. FasL expression in the eye was increased with age, but decreased following laser treatment. Aged mice had higher levels of sFasL in the blood compared to young mice. Systemic treatment with an MMP inhibitor decreased bloodborne sFasL, and reduced CNV in young and aged mice. Systemic neutralization of sFasL reduced CNV only in aged mice. sFasL increased cytokine production in aged macrophages and proangiogenic M2 macrophages. Aged M2 macrophages had elevated Fas (CD95) expression and displayed increased migration in response to sFasL compared to M1 macrophages derived from young animals.

CONCLUSIONS

Age modulates FasL function where increased MMP cleavage leads to a loss of function in the eye. The released form of FasL (sFasL) preferentially induces the migration of proangiogenic M2 macrophages into the laser lesions and increases proangiogenic cytokines promoting CNV. FasL may be a viable target for therapeutic intervention in aged-related neovascular disease.

Allograft survival enhancement using doxycycline in alkali-burned mouse corneas

PURPOSE

To explore the inhibitory effects of doxycycline on allograft rejection in alkali-burned cornea beds.

METHODS

The corneas of BALB/c mice were injured using a 1 mol/l NaOH solution. Following the injury, the corneas from C57BL/6 mice were transplanted into the eyes of BALB/c mice after being randomized into three groups: allogeneic corneal transplantation (group A), topical use of doxycycline after allogeneic corneal transplantation (group B) and syngeneic corneal transplantation (group C). Corneal angiogenesis was examined using whole-mount immunofluorescence, and corneal inflammation was evaluated using inflammation index scoring. The immune rejection of the grafts was examined using a slit lamp. In addition, the expression of vascular endothelial growth factor A and interleukin-1β in the transplanted corneas was examined using a real-time polymerase chain reaction, immunohistochemistry and an enzyme-linked immunosorbent assay.

RESULTS

The outgrowth of the corneal blood vessels in the group A mice was faster than that in the group B and group C mice. The inflammation index levels were highest in the group A mice, intermediate in the group B mice and lowest in the group C mice. Vascular endothelial growth factor and the interleukin-1β protein and mRNA levels decreased dramatically in the group B mice compared with the group A mice (all p-values < 0.01). In addition, the mean survival time in the group B mice (27.00 ± 2.00 days) was significantly longer than that in the group A mice (11.67 ± 1.51 days; p < 0.05).

CONCLUSIONS

Doxycycline may have had a significant role in preventing corneal angiogenesis and inflammation in alkali-burned corneal beds, which resulted in higher allograft survival rates.

Mice with mutations in Fas and Fas ligand demonstrate increased herpetic stromal keratitis following corneal infection with HSV-1

HSV-1 infection of the cornea leads to a potentially blinding immunoinflammatory lesion of the cornea, termed herpetic stromal keratitis. It has also been shown that one of the factors limiting inflammation of the cornea is the presence of Fas ligand (FasL) on corneal epithelium and endothelium. In this study, the role played by FasL expression in the cornea following acute infection with HSV-1 was determined. Both BALB/c and C57BL/6 (B6) mice with HSV-1 infection were compared with their lpr and gld counterparts. Results indicated that mice bearing mutations in the Fas Ag (lpr) displayed the most severe disease, whereas the FasL-defective gld mouse displayed an intermediate phenotype. It was further demonstrated that increased disease was due to lack of Fas expression on bone marrow-derived cells. Of interest, although virus persisted slightly longer in the corneas of mice bearing lpr and gld mutations, the persistence of infectious virus in the trigeminal ganglia was the same for all strains infected. Further, B6 mice bearing lpr and gld mutations were also more resistant to virus-induced mortality than were wild-type B6 mice. Thus, neither disease nor mortality correlated with viral replication in these mice. Collectively, the findings indicate that the presence of FasL on the cornea restricts the entry of Fas(+) bone marrow-derived inflammatory cells and thus reduces the severity of HSK.

Review of ocular immune privilege in the year 2010: modifying the immune privilege of the eye

The original evidence for the existence of immunologically privileged sites in the body was based on the prolonged survival of genetically disparate transplanted tissue in the anterior chamber of the eye. The failure of the immune system to elicit an immune response in this and other such sites constitutes the hallmark of the immune privilege status. The remarkably successful field of corneal transplantation in clinical practice is undoubtedly associated with corneal immune privilege. Several investigations have addressed the regulatory mechanisms governing this phenomenon, which involves a complex interplay between multiple molecular and cellular pathways. Furthermore, the use of various transgenic mouse models has facilitated the identification of critical pathways, which upon disruption can modify the immune privileged status of the eye. Understanding these pathways not only reveals the mechanisms underlying various ocular inflammatory disease conditions, but also has clinical implications for the transplantation field and for the treatment of autoimmunity.

Targeting immune privilege to prevent pathogenic neovascularization

PURPOSE. Current studies suggest that the immune system plays a critical role in blinding eye disorders. The eye is an immune-privileged site, and FasL expression is a major part of that mechanism because Fas/FasL interactions regulate inflammation and neovascularization, preventing damage to delicate ocular structures. These studies were undertaken to test the idea that modulating immune privilege might be an effective therapeutic approach to pathogenic angiogenesis in the eye. METHODS. C57BL/6 mice or FasL-defective B6-gld mice were laser treated to induce choroidal neovascularization (CNV). Mice were injected with cytotoxic FasL in the vitreous cavity or were treated with oral doxycycline in the drinking water. They were evaluated for CNV 7 days later. In some experiments eye tissue was harvested and evaluated for FasL expression, macrophage influx by immunohistochemistry, and release of sFasL. RESULTS. Injection of cytotoxic FasL successfully prevented neovascularization in a mouse model of CNV. Oral doxycycline increased functional FasL in the eye and substantially inhibited neovascularization. Doxycycline treatment increased FasL expression on the RPE cells and reduced circulating and tissue-associated sFasL. Treatment was ineffective in B6-gld mice, demonstrating that CNV inhibition was mediated by FasL. CONCLUSIONS. Targeting immune privilege using cytotoxic molecules or by increasing expression of the proapoptotic protein FasL may be a viable approach to treating neovascular eye disease.

Feasibility study of lamellar keratoplasty in a murine model

PURPOSE

In contrast to penetrating keratoplasty (PK), the donor cornea in lamellar keratoplasty (LK) remains separated from the host aqueous humor. There is debate about relative merits of each approach, but experimental comparisons have never been performed in animal models. Therefore, the authors developed a murine LK model.

METHODS

For allogeneic PK and LK surgeries, corneas of C57BL/6 mice were transplanted to BALB/c mice, assessed by slit lamp, and scored for opacity, edema, and neovascularization up to 46 d post-transplantation. Additional PK or LK surgeries were performed for histological assessment.

RESULTS

Graft rejection rate was less in LK vs. PK (69.2 vs. 100%), as was neovascularization (84.6 vs. 100%). In LK, inflammatory cells infiltrated primarily the button; in PK, heavier infiltration was observed throughout the cornea.

CONCLUSIONS

This study demonstrates the feasibility of LK in mice and presents data suggesting that the inflammatory response in LK differs from that in PK.

Graft failure IV. Immunologic mechanisms of corneal transplant rejection

Corneal transplantation is the oldest and the most common form of solid tissue transplantation in humans. Immunologic graft rejection is one of the main causes of short and long-term graft failure. Rejection involves donor tissue recognition and destruction by allo-specific immune cells of the recipient. This review outlines (1) the immunobiology of transplantation, with reference to ocular immune privilege, (2) factors that confer "high-risk" status to a graft and (3) the pathophysiologic mechanisms of corneal transplant rejection.

Roxithromycin inhibits tumor necrosis factor-alpha-induced matrix metalloproteinase-1 expression through regulating mitogen-activated protein kinase phosphorylation and Ets-1 expression

BACKGROUND AND OBJECTIVE

In periodontitis, matrix metalloproteinases (MMPs) are upregulated in response to locally released inflammatory cytokines, resulting in pathologic processes. Roxithromycin is a 14-membered ring macrolide antibiotic with broad-spectrum antibacterial effects against oral pathogens and immunomodulatory effects. Recently, we reported that roxithromycin inhibits tumor necrosis factor (TNF)-alpha-induced vascular endothelial growth factor expression in human periodontal ligament (HPDL) cell cultures. In the present study, we examined the effect of roxithromycin on TNF-alpha-induced MMP-1 production by HPDL cells.

MATERIAL AND METHODS

Cultured cells were incubated with 1% fetal bovine serum for 24 h, followed by treatment with 10 ng/ml TNF-alpha, 10 microM roxithromycin, and mitogen-activated protein kinase inhibitor at various concentrations. Culture supernatants and sediments were collected at different time-points and used for enzyme-linked immunosorbent assays, and northern and western blot analyses.

RESULTS

In HPDL cell cultures, roxithromycin strongly inhibited TNF-alpha-induced MMP-1 mRNA expression and production. The inhibition of MMP-1 gene expression by roxithromycin was dependent on de novo protein synthesis and was regulated at the transcriptional level. Roxithromycin significantly inhibited TNF-alpha-induced c-Jun N-terminal kinase activation (JNP) and marginally inhibited extracellular signal-regulated kinase (ERK) 1/2 activation, but not p38 mitogen-activated protein kinase activation. Furthermore, roxithromycin reduced the induction of Ets-1, one of the critical factors in MMP-1 transcription.

CONCLUSION

Roxithromycin inhibits TNF-alpha-mediated MMP-1 induction through the downregulation of ERK1/2 and JNK activation and the subsequent reduction of Ets-1, suggesting that roxithromycin may have therapeutic use in periodontitis and other chronic inflammatory conditions involving MMP-1 induction.

The role of Fas ligand as an effector molecule in corneal graft rejection

Previous studies have shown that the expression of Fas ligand (FasL; CD95L) by donor corneas is critical to their survival when placed on allogeneic recipients. Since there have been reports that the cornea expresses Fas, we tested the idea that FasL on lymphoid cells could be an effector molecule during rejection episodes. When FasL defective BALB/c-gld mice were engrafted with allogeneic corneas, significantly more of these corneas were accepted than by normal BALB/c mice. However, this was not due to impaired FasL-mediated effector function in these mice as the allogeneic corneas did not express detectable Fas by Western blot or RT-PCR analysis. Furthermore, donor corneas without Fas were given no survival advantage, but were rejected similar to wild-type donor allogeneic corneas. Examination of the T cell compartment in gld mice revealed that these cells express higher levels of Fas and are more susceptible to Fas-mediated death than wild-type cells. These results indicate that FasL is not an effector molecule in corneal graft rejection and that gld mice show reduced graft rejection due to greater susceptibility of their T cells to Fas-mediated apoptosis.

Alpha-1--antitrypsin in aqueous humour from patients with corneal allograft rejection

PURPOSE

To investigate the presence and concentration of alpha1-antitrypsin in aqueous humour at the time of corneal rejection and to compare results obtained from patients with reversible and irreversible rejection.

METHODS

Samples of aqueous humour were obtained from 17 patients with acute corneal endothelial allograft rejection. The presence of alpha1-antitrypsin in aqueous humour was confirmed by immunoblotting and measured employing a sandwich ELISA. Total protein concentrations in aqueous humour were measured using Bradford's method. The outcome of corneal rejection episodes was determined 1 month after diagnosing corneal rejection and described as reversible or irreversible rejection.

RESULTS

alpha1-antitrypsin was detected in aqueous humour. Patients with reversible rejection had significantly higher alpha1-antitrypsin concentration than patients with irreversible rejection (p = 0.044). There was no significant difference in total protein concentrations (p = 0.745), and no correlation was found between alpha1-antitrypsin and total protein concentrations (p = 0.368).

CONCLUSIONS

alpha1-antitrypsin in aqueous humour seems to signal a favourable outcome of corneal rejection. The possible mechanism is discussed.