Jeong-Hyeon Sohn

Role of Complement and Complement Membrane Attack Complex in Laser-Induced Choroidal Neovascularization

Choroidal neovascularization (CNV), or choroidal angiogenesis, is the hallmark of age-related macular degeneration and a leading cause of visual loss after age 55. The pathogenesis of new choroidal vessel formation is poorly understood. Although inflammation has been implicated in the development of CNV, the role of complement in CNV has not been explored experimentally. A reliable way to produce CNV in animals is to rupture Bruch’s membrane with laser photocoagulation. A murine model of laser-induced CNV in C57BL/6 mice revealed the deposition of C3 and membrane attack complex (MAC) in the neovascular complex. CNV was inhibited by complement depletion using cobra venom factor and did not develop in C3−/− mice. Anti-murine C6 Abs in C57BL/6 mice inhibited MAC formation and also resulted in the inhibition of CNV. Vascular endothelial growth factor, TGF-β2, and β-fibroblast growth factor were elevated in C57BL/6 mice after laser-induced CNV; complement depletion resulted in a marked reduction in the level of these angiogenic factors. Thus, activation of complement, specifically the formation of MAC, is essential for the development of laser- induced choroidal angiogenesis in mice. It is possible that a similar mechanism may be involved in the pathophysiology of other angiogenesis essential diseases.

Tolerance is dependent on complement C3 fragment iC3b binding to antigen-presenting cells.

Systemic tolerance can be induced by the introduction of antigen into an immune-privileged site. Here we investigated the role of complement in the induction of tolerance after intraocular injection. We found that the development of antigen-specific tolerance is dependent on a complement activation product. The ligation of the complement C3 activation product iC3b to complement receptor type 3 (the iC3b receptor) on antigen-presenting cells resulted in the sequential production of transforming growth factor-β2 and interleukin-10, which is essential for the induction of tolerance. These observations may extend to the development of both neonatal tolerance and other forms of acquired tolerance.

Complement Activation via Alternative Pathway Is Critical in the Development of Laser-Induced Choroidal Neovascularization: Role of Factor B and Factor H

The objective of this study was to explore the role of classical, lectin, and alternative pathways of complement activation in laser-induced choroidal neovascularization (CNV). The classical and alternative pathways were blocked in C57BL/6 mice by small interfering RNAs (siRNA) directed against C1q and factor B, respectively. C4−/− mice developed CNV similar to their wild-type controls and inhibition of C1q by siRNA had no effect on the development of CNV. In contrast, CNV was significantly inhibited (p < 0.001) in C5−/− mice and C57BL/6 mice treated with factor B siRNA. Inhibition of the alternative pathway by factor B siRNA resulted in decreased levels of membrane attack complex and angiogenic factors–vascular endothelial growth factor and TGF-β2. Furthermore, factor B was up-regulated in complement sufficient C57BL/6 mice at day 1 postlaser and remained elevated at day 7. Significantly reduced levels of factor H were observed at day 3 in these animals. In conclusion, our results demonstrate that activation of the factor B-dependent alternative pathway, but not the classical or lectin pathways, was essential for the development of CNV in mouse model of laser-induced CNV. Thus, specific blockade of the alternative pathway may represent a therapeutically relevant strategy for the inhibition of CNV.

Immunotherapy for choroidal neovascularization in a laser-induced mouse model simulating exudative (wet) macular degeneration

Age-related macular degeneration (AMD) is the leading cause of blindness after age 55 in the industrialized world. Severe loss of central vision frequently occurs with the exudative (wet) form of AMD, as a result of the formation of a pathological choroidal neovasculature (CNV) that damages the macular region of the retina. We tested the effect of an immunotherapy procedure, which had been shown to destroy the pathological neovasculature in solid tumors, on the formation of laser-induced CNV in a mouse model simulating exudative AMD in humans. The procedure involves administering an Icon molecule that binds with high affinity and specificity to tissue factor (TF), resulting in the activation of a potent cytolytic immune response against cells expressing TF. The Icon binds selectively to TF on the vascular endothelium of a CNV in the mouse and pig models and also on the CNV of patients with exudative AMD. Here we show that the Icon dramatically reduces the frequency of CNV formation in the mouse model. After laser treatment to induce CNV formation, the mice were injected either with an adenoviral vector encoding the Icon, resulting in synthesis of the Icon by vector-infected mouse cells, or with the Icon protein. The route of injection was i.v. or intraocular. The efficacy of the Icon in preventing formation of laser-induced CNV depends on binding selectively to the CNV. Because the Icon binds selectively to the CNV in exudative AMD as well as to laser-induced CNV, the Icon might also be efficacious for treating patients with exudative AMD.

Suppression of Complement Regulatory Proteins (CRPs) Exacerbates Experimental Autoimmune Anterior Uveitis (EAAU)

This study was undertaken to explore the role of complement regulatory proteins (CRPs) in experimental autoimmune anterior uveitis (EAAU). We observed that the levels of CRPs, Crry and CD59, in the eyes of Lewis rats increased during EAAU and remained elevated when the disease resolved. The in vivo role of these CRPs in EAAU was explored using neutralizing mAbs, antisense oligodeoxynucleotides (AS-ODNs), and small interfering RNAs against rat Crry and CD59. Suppression of Crry in vivo at days 9, 14, or 19 by neutralizing mAb or AS-ODNs resulted in the early onset of disease, the exacerbation of intraocular inflammation, and delayed resolution. Suppression of CD59 was only effective when the Abs and ODNs were given before the onset of disease. The most profound effect on the disease was observed when a mixture of Crry and CD59 mAbs or AS-ODNs was administered. A similar effect was observed with a combination of Crry and CD59 small interfering RNA. There was no permanent histologic damage to ocular tissue after the inflammation cleared in these animals. Increased complement activation as determined by increased deposition of C3, C3 activation fragments, and membrane attack complex was observed in the eyes of Lewis rats when the function and/or expression of Crry and CD59 was suppressed. Thus, our results suggest that various ocular tissues up-regulate the expression of Crry and CD59 to avoid self-injury during autoimmune uveitis and that these CRPs play an active role in the resolution of EAAU by down-regulating complement activation in vivo.

Type I Collagen Is the Autoantigen in Experimental Autoimmune Anterior Uveitis

This study was undertaken to identify and characterize the Ag responsible for the induction of experimental autoimmune anterior uveitis (EAAU). Melanin-associated Ag isolated from bovine iris and ciliary body was digested with the proteolytic enzyme V8 protease to solubilize the proteins and the pathogenic protein was purified to homogeneity. Lewis rats were sensitized to various fractions and investigated for the development of anterior uveitis and an immune response to the purified Ag. The uveitogenic Ag had a mass of 22 kDa (SDS-PAGE) and an isoelectric point of 6.75. The N-terminal amino acid sequence of this protein demonstrated 100% homology with the bovine type I collagen α-2 chain starting from amino acid 385 and will be referred to as CI-α2 (22 kDa). Animals immunized with bovine CI-α2 (22 kDa) developed both cellular and humoral immunity to the Ag. They developed anterior uveitis only if the CI-α2 chain underwent proteolysis and if the bound carbohydrates were intact. EAAU induced by CI-α2 (22 kDa) can be adoptively transferred to naive syngenic rats by primed CD4+ T cells. EAAU could not be induced by the adoptive transfer of sera obtained from animals immunized with CI-α2 (22 kDa). The α-1 and α-2 chains (intact or proteolytically cleaved) of type I collagen from calfskin were not pathogenic. Although human anterior uveitis has been historically characterized as a collagen disease, this is first time collagen has been directly identified as the target autoantigen in uveitis.

Complement, innate immunity and ocular disease

The complement system is a major component of innate immunity. During an inflammatory reaction, the eye is potentially threatened by homologous complement attack, and unregulated complement activation could lead to tissue damage and vision loss. The complement system is continuously activated at low levels in the normal eye, and intraocular complement-regulatory proteins (CRPs) tightly regulate this spontaneous complement activation so that there is elimination of potential pathogens without the induction of destructive intraocular inflammation. The presence of a complement activation product (iC3b) during the early phase of antigen and antigen-presenting cell contact is essential for the induction of systemic tolerance to antigen injected into the anterior chamber of the eye and the establishment of ocular immune privilege. The complement system and complement-regulatory proteins control intraocular inflammation in autoimmune anterior uveitis and may play an important role in the development of age-related macular degeneration. Thus, in the eye, complement functions as a double-edged sword - on one hand it provides innate immunity against pathogens while simultaneously instructing the adaptive immune response to develop tolerance to such pathogens to avoid inadvertent tissue damage in a critical organ.

Anti-Inflammatory Effects of Specific Cyclooxygenase 2,5-Lipoxygenase, and Inducible Nitric Oxide Synthase Inhibitors on Experimental Autoimmune Anterior Uveitis (EAAU)

Purpose: Inflammation, in general, causes the release of a variety of inflammatory mediators that in turn induce cyclooxygenase (COX) 2, nitric oxide synthase (iNOS) and 5-lipoxygense (LP) synthesis, producing large amounts of inflammatory prostaglandins (PG), nitric oxide (NO), and leukotriene (LT) B4. Therefore, inhibition of these enzymes may abrogate intraocular inflammation in experimental autoimmune anterior uveitis (EAAU). Methods: Lewis rats were immunized with melanin-associated antigen (MAA) isolated from bovine iris and ciliary body. These animals were divided into three groups. The first group of rats received subcutaneous injection of COX 2 inhibitor CS 236 at different time points. The second and third groups of animals received subcutaneous aminoguanidine (AG), an iNOS inhibitor, and nordihydroguaiaretic acid (NDGA), a 5-LP inhibitor, respectively. Control animals received vehicle. Rat eyes were examined daily by slit-lamp biomicroscopy from Day 7 to 30 post injection for uveitis. Animals were also sacrificed at various time points for histologic analysis. Results: Control animals developed severe EAAU in both eyes. The disease started in these animals on Day 12 post immunization and lasted for ten days. Interestingly, CS 236, a potent COX 2 inhibitor, completely abrogated EAAU when the animals were treated daily from the Day 0 to 14 or Day 0 to 20 after MAA injection. Furthermore, daily CS 236 treatment after the onset of EAAU (Day 14–20) significantly reduced the severity (both clinical and histologic) of EAAU and shortened the duration of disease. iNOS inhibitor (AG) and 5-LP inhibitor (NDGA) partially attenuated EAAU. Conclusions: Our results show that EAAU was partially attenuated by AG and NDGA. Interestingly, CS 236, a potent COX 2 inhibitor, completely inhibited EAAU in male Lewis rats most likely by inhibiting the initial phase and onset of the disease.