Complement anaphylatoxin receptors C3aR and C5aR are required in the pathogenesis of experimental autoimmune uveitis

A CD6-targeted antibody-drug conjugate as a potential therapy for T cell-mediated disorders

The selective targeting of pathogenic T cells is a holy grail in the development of new therapeutics for T cell-mediated disorders, including many autoimmune diseases and graft versus host disease. We describe the development of a CD6-targeted antibody-drug conjugate (CD6-ADC) by conjugating an inactive form of monomethyl auristatin E (MMAE), a potent mitotic toxin, onto a mAb against CD6, an established T cell surface marker. Even though CD6 is present on all T cells, only the activated (pathogenic) T cells vigorously divide and thus are susceptible to the antimitotic MMAE-mediated killing via the CD6-ADC. We found CD6-ADC selectively killed activated proliferating human T cells and antigen-specific mouse T cells in vitro. Furthermore, in vivo, whereas the CD6-ADC had no significant detrimental effect on normal T cells in naive CD6-humanized mice, the same dose of CD6-ADC, but not the controls, efficiently treated 2 preclinical models of autoimmune uveitis and a model of graft versus host disease. These results provide evidence suggesting that CD6-ADC could be further developed as a potential therapeutic agent for the selective elimination of pathogenic T cells and treatment of many T cell-mediated disorders.

Therapeutic targeting of the complement system in ocular disease

The complement system is involved in the pathogenesis of several ocular diseases, providing a rationale for the investigation of complement-targeting therapeutics for these conditions. Dry age-related macular degeneration, as characterised by geographic atrophy (GA), is currently the most active area of research for complement-targeting therapeutics, with a complement C3 inhibitor approved in the United States earlier this year marking the first approved therapy for GA. This review discusses the role of complement in ocular disease, provides an overview of the complement-targeting agents currently under development for ocular conditions, and reflects on the lessons that can be learned from the preclinical investigations and clinical trials conducted in this field to date.

The role of C5a receptors in autoimmunity

The complement system is an essential component of the innate immune response and plays a vital role in host defense and inflammation. Dysregulation of the complement system, particularly involving the anaphylatoxin C5a and its receptors (C5aR1 and C5aR2), has been linked to several autoimmune diseases, indicating the potential for targeted therapies. C5aR1 and C5aR2 are seven-transmembrane receptors with distinct signaling mechanisms that play both partially overlapping and opposing roles in immunity. Both receptors are expressed on a broad spectrum of immune and non-immune cells and are involved in cellular functions and physiological processes during homeostasis and inflammation. Dysregulated C5a-mediated inflammation contributes to autoimmune diseases such as rheumatoid arthritis, systemic lupus erythematosus, multiple sclerosis, epidermolysis bullosa acquisita, antiphospholipid syndrome, and others. Therefore, targeting C5a or its receptors may yield therapeutic innovations in these autoimmune diseases by reducing the recruitment and activation of immune cells that lead to tissue inflammation and injury, thereby exacerbating the autoimmune response. Clinical trials focused on the inhibition of C5 cleavage or the C5a/C5aR1-axis using small molecules or monoclonal antibodies hold promise for bringing novel treatments for autoimmune diseases into practice. However, given the heterogeneous nature of (systemic) autoimmune diseases, there are still several challenges, such as patient selection, optimal dosing, and treatment duration, that require further investigation and development to realize the full therapeutic potential of C5a receptor inhibition, ideally in the context of a personalized medicine approach. Here, we aim to provide a brief overview of the current knowledge on the function of C5a receptors, the involvement of C5a receptors in autoimmune disorders, the molecular mechanisms underlying C5a receptor-mediated autoimmunity, and the potential for targeted therapies to modulate their activity.

Differential expression of C5aR1 and C5aR2 in innate and adaptive immune cells located in early skin lesions of bullous pemphigoid patients

Bullous pemphigoid (BP), the by far most frequent autoimmune subepidermal blistering disorder (AIBD), is characterized by the deposition of autoantibodies against BP180 (type XVII collagen; Col17) and BP230 as well as complement components at the dermal-epidermal junction (DEJ). The mechanisms of complement activation in BP patients, including the generation of C5a and regulation of its two cognate C5aRs, i.e., C5aR1 and C5aR2, are incompletely understood. In this study, transcriptome analysis of perilesional and non-lesional skin biopsies of BP patients compared to site-, age-, and sex-matched controls showed an upregulated expression of C5AR1, C5AR2, CR1, and C3AR1 and other complement-associated genes in perilesional BP skin. Of note, increased expressions of C5AR2 and C3AR1 were also observed in non-lesional BP skin. Subsequently, double immunofluorescence (IF) staining revealed T cells and macrophages as the dominant cellular sources of C5aR1 in early lesions of BP patients, while C5aR2 mainly expressed on mast cells and eosinophils. In addition, systemic levels of various complement factors and associated molecules were measured in BP patients and controls. Significantly higher plasma levels of C3a, CD55, and mannose-binding lectin-pathway activity were found in BP patients compared to controls. Finally, the functional relevance of C5aR1 and C5aR2 in BP was explored by two in vitro assays. Specific inhibition of C5aR1, resulted in significantly reduced migration of human neutrophils toward the chemoattractant C5a, whereas stimulation of C5aR2 showed no effect. In contrast, the selective targeting of C5aR1 and/or C5aR2 had no effect on the release of reactive oxygen species (ROS) from Col17-anti-Col17 IgG immune complex-stimulated human leukocytes. Collectively, this study delineates a complex landscape of activated complement receptors, complement factors, and related molecules in early BP skin lesions. Our results corroborate findings in mouse models of pemphigoid diseases that the C5a/C5aR1 axis is pivotal for attracting inflammatory cells to the skin and substantiate our understanding of the C5a/C5aR1 axis in human BP. The broad expression of C5aRs on multiple cell types critical for BP pathogenesis call for clinical studies targeting this axis in BP and other complement-mediated AIBDs.

CDCP1 regulates retinal pigmented epithelial barrier integrity for the development of experimental autoimmune uveitis

Cub domain-containing protein 1 (CDCP1) is a protein that is highly expressed on the surface of many cancer cells. However, its distribution in normal tissues and its potential roles in nontumor cells are poorly understood. We found that CDCP1 is present on both human and mouse retinal pigment epithelial (RPE) cells. CDCP1-KO mice developed attenuated retinal inflammation in a passive model of autoimmune uveitis, with disrupted tight junctions and infiltrating T cells detected in RPE flat mounts from WT but not CDCP1-KO mice during EAU development. Mechanistically, we discovered that CDCP1 on RPE cells was upregulated by IFN-γ in vitro and after EAU induction in vivo. CD6 stimulation induced increased RPE barrier permeability of WT but not CDCP1-knockdown (CDCP1-KD) RPE cells, and activated T cells migrated through WT RPE monolayers more efficiently than the CDCP1-KD RPE monolayers. In addition, CD6 stimulation of WT but not the CDCP1-KD RPE cells induced massive stress fiber formation and focal adhesion disruption to reduce cell barrier tight junctions. These data suggest that CDCP1 on RPE cells interacts with CD6 on T cells to induce RPE cytoskeleton remodeling and focal adhesion disruption, which open up the tight junctions to facilitate T cell infiltration for the development of uveitis.

Recent developments of neuroprotective agents for degenerative retinal disorders

Retinal degeneration is a debilitating ocular complication characterized by the progressive loss of photoreceptors and other retinal neurons, which are caused by a group of retinal diseases affecting various age groups, and increasingly prevalent in the elderly. Age-related macular degeneration, diabetic retinopathy and glaucoma are among the most common complex degenerative retinal disorders, posing significant public health problems worldwide largely due to the aging society and the lack of effective therapeutics. Whilst pathoetiologies vary, if left untreated, loss of retinal neurons can result in an acquired degeneration and ultimately severe visual impairment. Irrespective of underlined etiology, loss of neurons and supporting cells including retinal pigment epithelium, microvascular endothelium, and glia, converges as the common endpoint of retinal degeneration and therefore discovery or repurposing of therapies to protect retinal neurons directly or indirectly are under intensive investigation. This review overviews recent developments of potential neuroprotectants including neuropeptides, exosomes, mitochondrial-derived peptides, complement inhibitors, senolytics, autophagy enhancers and antioxidants either still experimentally or in clinical trials. Effective treatments that possess direct or indirect neuroprotective properties would significantly lift the burden of visual handicap.

Cytokine and Complement Response in the Glaucomatous βB1-CTGF Mouse Model

Glaucoma is a complex neurodegenerative disease leading to a loss of retinal ganglion cells (RGCs) and optic nerve axons. An activation of the complement system seems to contribute to cell loss in this disease. Hence, we investigated a possible initiation of the complement system and the cytokine response in the βB1-CTGF glaucoma model. In these mice, intraocular pressure is elevated, which is the main glaucoma risk factor in patients, and RGC loss occurs at 15 weeks of age. Therefore, quantitative real-time PCR and immunohistological experiments were performed in 5-, 10-, and 15-week-old βB1-CTGF animals and their corresponding wildtypes (WT) to analyze the expression of several complement system factors. We could show that mRNA levels of the terminal complement pathway components C3 and C5 (Hc) were upregulated at 10 weeks. In accordance, more C3+ and membrane attack complex+ cells were observed in transgenic retinae. Further, the C5a receptor anaphylatoxin receptor (C5ar) and the complement component C5a receptor 1 (C5ar1; CD88) mRNA levels were upregulated in 10- and 15-week-old βB1-CTGF mice. Interestingly, all three activation routes of the complement system were elevated in βB1-CTGF mice at some age. Especially C1q, as a marker of the classical pathway, was significantly increased at all investigated ages. Furthermore, mRNA expression levels of interferon-γ (Infg) were upregulated at 5 weeks, while Cxcl1 and Cxcl2 mRNA levels were upregulated at 10 and 15 weeks. The mRNA levels of the chemokines Cxcl10 were increased at all ages in βB1-CTGF mice. These results lead to the assumption that in these transgenic mice, a complement activation mainly through the classical pathway as well as a cytokine response plays a major role in cell death.

CD47 Deficiency Ameliorates Ocular Autoimmune Inflammation

Autoimmune uveitis is a sight-threatening ocular inflammatory condition in which the retina and uveal tissues become a target of autoreactive immune cells. The CD47 is a ubiquitously expressed transmembrane protein which plays multiple roles in fundamental cellular functions including phagocytosis, proliferation, and adhesion. Signal regulatory protein alpha (SIRPα), one of the CD47 ligands, is predominantly expressed in myeloid lineage cells such as dendritic cells (DCs) or macrophages, and CD47-SIRPα signaling pathway is implicated in the development of autoimmune diseases. Our current study demonstrates how CD47 depletion is effective in the prevention of experimental autoimmune uveitis (EAU), an animal model of human autoimmune uveitis, in animals deficient of CD47 (CD47-/- ). Systemic suppression of SIRPα+ DCs in animals deficient in CD47 resulted in the inability of autoreactive CD4+ T cells to develop, which is crucial to induction of EAU. Of interest, retinal microglia, the resident immune cell of the retina, express SIRPα, however these cells were not operative in EAU suppression in response to CD47 depletion. These results identify CD47 as a significant regulator in the development of SIRPα+ DCs that is vital to disease induction in EAU.

The innate immune system in diabetic retinopathy

The prevalence of diabetes has been rising steadily in the past half-century, along with the burden of its associated complications, including diabetic retinopathy (DR). DR is currently the most common cause of vision loss in working-age adults in the United States. Historically, DR has been diagnosed and classified clinically based on what is visible by fundoscopy; that is vasculature alterations. However, recent technological advances have confirmed pathology of the neuroretina prior to any detectable vascular changes. These, coupled with molecular studies, and the positive impact of anti-inflammatory therapeutics in DR patients have highlighted the central involvement of the innate immune system. Reminiscent of the systemic impact of diabetes, immune dysregulation has become increasingly identified as a key element of the pathophysiology of DR by interfering with normal homeostatic systems. This review uses the growing body of literature across various model systems to demonstrate the clear involvement of all three pillars of the immune system: immune-competent cells, mediators, and the complement system. It also demonstrates how the relative contribution of each of these requires more extensive analysis, including in human tissues over the continuum of disease progression. Finally, although this review demonstrates how the complex interactions of the immune system pose many more questions than answers, the intimately connected nature of the three pillars of the immune system may also point to possible new targets to reverse or even halt reverse retinopathy.

Immunological Insights in Equine Recurrent Uveitis

Horses worldwide suffer from equine recurrent uveitis (ERU), an organ-specific, immune-mediated disease with painful, remitting-relapsing inflammatory attacks alternating with periods of quiescence, which ultimately leads to blindness. In course of disease, both eyes can eventually be affected and since blind horses pose a threat to themselves and their surroundings, these animals have to be killed. Therefore, this disease is highly relevant for veterinary medicine. Additionally, ERU shows strong clinical and pathological resemblance to autoimmune uveitis in man. The exact cause for the onset of ERU is unclear to date. T cells are believed to be the main effector cells in this disease, as they overcome the blood retinal barrier to invade the eye, an organ physiologically devoid of peripheral immune cells. These cells cause severe intraocular inflammation, especially in their primary target, the retina. With every inflammatory episode, retinal degeneration increases until eyesight is completely lost. In ERU, T cells show an activated phenotype, with enhanced deformability and migration ability, which is reflected in the composition of their proteome and downstream interaction pathways even in quiescent stage of disease. Besides the dysregulation of adaptive immune cells, emerging evidence suggests that cells of the innate immune system may also directly contribute to ERU pathogenesis. As investigations in both the target organ and the periphery have rapidly evolved in recent years, giving new insights on pathogenesis-associated processes on cellular and molecular level, this review summarizes latest developments in ERU research.

Leukotriene B4 and Its Receptor in Experimental Autoimmune Uveitis and in Human Retinal Tissues: Clinical Severity and LTB4 Dependence of Retinal Th17 Cells

Nomacopan, a drug originally derived from tick saliva, has dual functions of sequestering leukotriene B₄ (LTB₄) and inhibiting complement component 5 (C5) activation. Nomacopan has been shown to provide therapeutic benefit in experimental autoimmune uveitis (EAU). Longer acting forms of nomacopan were more efficacious in mouse EAU models, and the long-acting variant that inhibited only LTB₄ was at least as effective as the long-acting variant that inhibited both C5 and LTB₄, preventing structural damage to the retina and a significantly reducing effector T helper 17 cells and inflammatory macrophages. Increased levels of LTB₄ and C5a (produced upon C5 activation) were detected during disease progression. Activated retinal lymphocytes were shown to express LTB₄ receptors (R) in vitro and in inflamed draining lymph nodes. Levels of LTB₄R-expressing active/inflammatory retinal macrophages were also increased. Within the draining lymph node CD4⁺ T-cell population, 30% expressed LTB₄R⁺ following activation in vitro, whereas retinal infiltrating cells expressed LTB₄R and C5aR. Validation of expression of those receptors in human uveitis and healthy tissues suggests that infiltrating cells could be targeted by inhibitors of the LTB₄-LTB4 receptor 1 (BLT1) pathway as a novel therapeutic approach. This study provides novel data on intraocular LTB₄ and C5a in EAU, their associated receptor expression by retinal infiltrating cells in mouse and human tissues, and in attenuating EAU via the dual inhibitor nomacopan.

Quantitative proteomic analysis of rat retina with experimental autoimmune uveitis based on tandem mass tag (TMT) peptide labeling coupled with LC-MS/MS

Uveitis is a recurrent, inflammatory eye disease that occurs in the retina, iris, ciliary body and choroid. Currently, the detailed mechanism is still unclear. Proteomics can offer a powerful set of tools for the direct high-throughput study and a key contribution to the understanding of protein functions. This approach can also allow us to compare the protein profiling of the cells in healthy and diseased states that can be used to identify proteins associated with disease development and progression. In the present study, we first established an autoimmune uveitis (EAU) rat model. On day 12 after immunization, we isolated the rat retinas from both normal and EAU animals to collect total proteins. Using tandem mass tag (TMT) peptide labeling coupled with LC-MS/MS quantitative proteomics technique, we identified the differentially expressed proteins in EAU rat retinas, performed bioinformatics analyses, validated the expression of the COX1, NADH1, C3, and C9 proteins, and determined the adenosine triphosphate (ATP) levels. The results indicated that there were 190 upregulated and 103 downregulated proteins in EAU rat retinas. Bioinformatics analysis revealed the differentially expressed proteins were mainly involved in acute inflammatory response, visual perception and eye photoreceptor cell differentiation that were mainly related to complement and coagulation cascades, phagosome, PI3K-Akt signaling, and metabolic pathways. In conclusion, based on the TMT-based quantitative proteomics technique, the differentially expressed proteins in EAU rat retinas were mainly associated with complement and coagulation cascades and metabolic pathways. Our findings will facilitate the understanding of the pathogenesis of uveitis and will be useful for subsequent studies.

Retinal endothelial cell phenotypic modifications during experimental autoimmune uveitis: a transcriptomic approach

BACKGROUND

Blood-retinal barrier cells are known to exhibit a massive phenotypic change during experimental autoimmune uveitis (EAU) development. In an attempt to investigate the mechanisms of blood-retinal barrier (BRB) breakdown at a global level, we studied the gene regulation of total retinal cells and retinal endothelial cells during non-infectious uveitis.

METHODS

Retinal endothelial cells were isolated by flow cytometry either in Tie2-GFP mice (CD31+ CD45- GFP+ cells), or in wild type C57BL/6 mice (CD31+ CD45- endoglin+ cells). EAU was induced in C57BL/6 mice by adoptive transfer of IRBP1-20-specific T cells. Total retinal cells and retinal endothelial cells from naïve and EAU mice were sorted and their gene expression compared by RNA-Seq. Protein expression of selected genes was validated by immunofluorescence on retinal wholemounts and cryosections and by flow cytometry.

RESULTS

Retinal endothelial cell sorting in wild type C57BL/6 mice was validated by comparative transcriptome analysis with retinal endothelial cells sorted from Tie2-GFP mice, which express GFP under the control of the endothelial-specific receptor tyrosine kinase promoter Tie2. RNA-Seq analysis of total retinal cells mainly brought to light upregulation of genes involved in antigen presentation and T cell activation during EAU. Specific transcriptome analysis of retinal endothelial cells allowed us to identify 82 genes modulated in retinal endothelial cells during EAU development. Protein expression of 5 of those genes (serpina3n, lcn2, ackr1, lrg1 and lamc3) was validated at the level of inner BRB cells.

CONCLUSION

Those data not only confirm the involvement of known pathogenic molecules but further provide a list of new candidate genes and pathways possibly implicated in inner BRB breakdown during non-infectious posterior uveitis.

CD55 Is Essential for CD103+ Dendritic Cell Tolerogenic Responses that Protect against Autoimmunity

Recent studies traced inflammatory bowel disease in some patients to deficiency of CD55 [decay-accelerating factor (DAF)], but the mechanism underlying the linkage remained unclear. Herein, we studied the importance of DAF in enabling processes that program tolerance in the gut and the eye, two immune-privileged sites where immunosuppressive responses are continuously elicited. Unlike oral feeding or ocular injection of ovalbumin in wild-type (WT) mice, which induced dominant immune tolerance, identical treatment of DAF^-/- mice or DAF^-/- to WT bone marrow chimeras did not. While 10% to 30% of mesenteric and submandibular lymph node CD4⁺ cells became robust T-regulatory cells (Tregs) in WT forkhead box P3 (Foxp3)-green fluorescent protein mice, few in either site became Tregs with little suppressor activity in DAF^-/- Foxp3-green fluorescent protein mice. Phenotyping of CD103⁺ dendritic cells (DCs) from the ovalbumin-fed DAF^-/- mice showed impaired expression of inducer of costimulation (ICOS) ligand, programmed death receptor 1-ligand 1 (PD1-L1), CxxxC chemokine receptor 1 (Cx3CR1), CCR7, and CCR9. Analyses of elicited DAF^-/- Foxp3⁺ Tregs showed reduced expression of interferon regulatory factor 8 (IRF-8)/aldehyde dehydrogenase 1 family member A2 (Aldh1a2) and glycoprotein A repetitions predominant/latency-associated protein associated with Treg transforming growth factor-β production and presentation, as well as integrin β6/integrin β8 associated with Treg and CD103⁺ DC transforming growth factor-β release. Thus, DAF is required for the properties of CD103⁺ DCs and their naïve CD4⁺ cell partners that together program tolerance.

Retinal microglia initiate neuroinflammation in ocular autoimmunity

Autoimmune uveitis is a sight-threatening ocular inflammatory condition in which the retina and uveal tissues become a target of autoreactive immune cells. While microglia have been studied extensively in autoimmune uveitis, their exact function remains uncertain. The objective of the current study was to determine whether resident microglia are necessary and sufficient to initiate and amplify retinal inflammation in autoimmune uveitis. In this study, we clearly demonstrate that microglia are essential for initiating infiltration of immune cells utilizing a murine model of experimental autoimmune uveoretinitis (EAU) and the recently identified microglia-specific marker P2ry12. Initiating disease is the primary function of microglia in EAU, since eliminating microglia during the later stages of EAU had little effect, indicating that the function of circulating leukocytes is to amplify and sustain destructive inflammation once microglia have triggered disease. In the absence of microglia, uveitis does not develop, since leukocytes cannot gain entry through the blood-retinal barrier, illustrating that microglia play a critical role in regulating infiltration of inflammatory cells into the retina.

Immunity against the Obligate Intracellular Bacterial Pathogen Rickettsia australis Requires a Functional Complement System

The complement system has a well-defined role in deterring blood-borne infections. However, complement is not entirely efficacious, as several bacterial pathogens, including some obligate intracellular pathogens, have evolved mechanisms for resistance. It is presumed that obligate intracellular bacteria evade complement attack by residing within a host cell; however, recent studies have challenged this presumption. Here, we demonstrate that the complement system is activated during infection with the obligate intracellular bacterium Rickettsia australis and that genetic ablation of complement increases susceptibility to infection. Interaction of Rickettsia australis with serum-borne complement leads to activation of the complement cascade, producing three effector mechanisms that could negatively influence R. australis. The C9-dependent membrane attack complex can lead to deposition of a bacteriolytic membrane pore on the bacteria, but this system does not contribute to control of rickettsial infection. Similarly, complement receptor (CR1/2)-dependent opsonophagocytosis may lead to engulfment and killing of the bacteria, but this system is also dispensable for immunity. Nevertheless, intact complement is essential for naturally acquired and antibody-mediated immunity to Rickettsia infection. Comparison of infection in mice lacking the central complement protein C3 with infection in their wild-type counterparts demonstrated decreases in gamma interferon (IFN-γ) production, IgG secretion, and spleen hyperplasia in animals lacking complement. The correlation between loss of secondary immune functions and loss of complement indicates that the proinflammatory signaling components of the complement system, and not membrane attack complex or opsonophagocytosis, contribute to the immune response to this pathogen.

Complement-Mediated Activation of the NLRP3 Inflammasome and Its Inhibition by AAV-Mediated Delivery of CD59 in a Model of Uveitis

Uveitis is an inflammatory disorder of the eye responsible for approximately 10%-15% of blindness in the US. In this study, we examined the role of the complement membrane attack complex (MAC) and the NLRP3 inflammasome in the pathogenesis of experimental autoimmune uveitis (EAU) in normal and C9^-/- mice that are incapable of assembling the MAC. We discovered that the MAC and the NLRP3 inflammasome and associated production of IL-1β are elevated in EAU mice and that MAC may be involved in regulation of Th1 and Th17 cell differentiation. In contrast, MAC and the NLRP3 inflammasome were not elevated in C9^-/- mice. However, EAU-associated pathophysiology including retinal structure and function were not rescued in C9^-/- mice. Unexpectedly, AAV-mediated delivery of sCD59, an inhibitor of C9 incorporation into the MAC, successfully attenuated activation of the NLRP3 inflammasome and EAU pathology as well as MAC. Our studies provide an improved understanding of the role of the MAC and the NLRP3 inflammasome in EAU as well as suggest a novel approach for the treatment of uveitis.

Targeting CD6 for the treatment of experimental autoimmune uveitis

OBJECTIVE

CD6 is emerging as a new target for treating many pathological conditions in which T cells are integrally involved, but even the latest data from studies of CD6 gene engineered mice were still contradictory. To address this issue, we studied experimental autoimmune uveitis (EAU), a model of autoimmune uveitis, in wild-type (WT) and CD6 knockout (KO) mice.

METHODS

After EAU induction in WT and CD6 KO mice, we evaluated ocular inflammation and compared retinal antigen-specific T-cell responses using scanning laser ophthalmoscopy, spectral-domain optical coherence tomography, histopathology, and T cell recall assays. Uveitogenic T cells from WT and CD6 KO mice were adoptively transferred into WT naïve mice to confirm the impact of CD6 on T cells. In addition, we immunized CD6 KO mice with recombinant CD6 protein to develop mouse anti-mouse CD6 monoclonal antibodies (mAbs) in which functional antibodies exhibiting cross-reactivity with human CD6 were screened and identified for treatment studies.

RESULTS

In CD6 KO mice with EAU, we found significantly decreased retinal inflammation and reduced autoreactive T-cell responses, and confirmed the impaired uveitogenic capacity of T cells from these mice in an adoptive transfer experiment. Notably, one of these cross-reactive mAbs significantly ameliorated retinal inflammation in EAU induced by the adoptive transfer of uveitogenic T cells.

CONCLUSIONS

Together, these data strongly suggest that CD6 plays a previously unknown, but pivotal role in autoimmune uveitis, and may be a promising new treatment target for this blinding disease. In addition, the newly developed mouse anti-mouse/human CD6 mAbs could be valuable tools for testing CD6-targeted therapies in other mouse models of human diseases.

Increased Complement 3a Receptor is Associated with Behcet's disease and Vogt-Koyanagi-Harada disease

Behcet's disease (BD) and Vogt-Koyanagi-Harada disease (VKH) are systemic and recurrent autoimmune diseases associated with abnormal T cell immune response. Complement 3a receptor (C3aR) and complement 5a receptor (C5aR) have been reported to be involved in T cell mediated autoimmune disease. This study aimed to investigate the role of C3aR and C5aR in these two diseases. The C3aR expression in PBMCs was increased in patients with active BD (aBD) and active VKH (aVKH). No statistical difference was found concerning the expression of C5aR in PBMCs between patients with aBD or aVKH and normal controls. After the intraocular inflammation in BD and VKH patients was controlled, the C3aR expression returned back to normal levels. The serum from patients with aBD and aVKH significantly induced C3aR expression by PBMCs. C3a induced IL-6, IL-1β and TNF-α secretion, while inhibited the production of IL-10 by monocytes. Activation of C3aR in CD4+T cells could upregulate IL-17 production and inhibit IL-10 production, but had no detectable influence on IFN-γ production. Our data indicates that increased C3aR expression may lead to over activation of the Th17 cell response and may therefore contribute to the pathogenesis of BD and VKH disease.

Evaluation of expression levels and mechanism of complement activation

The present study aimed to reveal the expression changes of complement system activation and complement activation product C3a receptor during acute myocardial infarction. Blood samples were collected from healthy individuals and from patients with coronary artery stenosis or acute myocardial infarction. The subjects received physical examination in hospital between January and July 2015 (n=5). Cytometric bead array was performed to measure the levels of complement system activation product anaphylatoxin C3a, C4a and C5a. Immunohistochemical investigations were performed in tissues of patients who underwent coronary artery bypass grafting between January and July 2015 to detect the expression of C3a receptor. The results of cytometric bead array showed that the content of complement activation products C3a, C4a and C5a in the plasma of patients with coronary artery stenosis and acute myocardial infarction were significantly higher than those of the control group (P<0.01). The results of immunoblotting suggested that the protein expression of C3a receptor in infarct tissues of patients with acute myocardial infarction was significantly higher than that of normal tissues adjacent to the infarcted area (P<0.05). There is complement system activation in patients with acute myocardial infarction. Additionally, the increase in the expression of complement C3a receptor in tissues of infarct area suggested that C3a-C3a receptor signaling pathway may be involved in the development of myocardial infarction.

Complement Component C4 Regulates the Development of Experimental Autoimmune Uveitis through a T Cell-Intrinsic Mechanism

In addition to its conventional roles in the innate immune system, complement has been found to directly regulate T cells in the adaptive immune system. Complement components, including C3, C5, and factor D, are important in regulating T cell responses. However, whether complement component C4 is involved in regulating T cell responses remains unclear. In this study, we used a T cell-dependent model of autoimmunity, experimental autoimmune uveitis (EAU) to address this issue. We compared disease severity in wild-type (WT) and C4 knockout (KO) mice using indirect ophthalmoscopy, scanning laser ophthalmoscopy, spectral-domain optical coherence tomography, and histopathological analysis. We also explored the underlying mechanism by examining T cell responses in ex vivo antigen-specific recall assays and in in vitro T cell priming assays using bone marrow-derived dendritic cells, splenic dendritic cells, and T cells from WT or C4 KO mice. We found that C4 KO mice develop less severe retinal inflammation than WT mice in EAU and show reduced autoreactive T cell responses and decreased retinal T cell infiltration. We also found that T cells, but not dendritic cells, from C4 KO mice have impaired function. These results demonstrate a previously unknown role of C4 in regulating T cell responses, which affects the development of T cell-mediated autoimmunity, as exemplified by EAU. Our data could shed light on the pathogenesis of autoimmune uveitis in humans.

Fungal-derived cues promote ocular autoimmunity through a Dectin-2/Card9-mediated mechanism

Uveitis (intraocular inflammation) is a leading cause of loss of vision. Although its aetiology is largely speculative, it is thought to arise from complex genetic-environmental interactions that break immune tolerance to generate eye-specific autoreactive T cells. Experimental autoimmune uveitis (EAU), induced by immunization with the ocular antigen, interphotoreceptor retinoid binding protein (IRBP), in combination with mycobacteria-containing complete Freund's adjuvant (CFA), has many clinical and histopathological features of human posterior uveitis. Studies in EAU have focused on defining pathogenic CD4⁺ T cell effector responses, such as those of T helper type 17 (Th17) cells, but the innate receptor pathways precipitating development of autoreactive, eye-specific T cells remain poorly defined. In this study, we found that fungal-derived antigens possess autoimmune uveitis-promoting function akin to CFA in conventional EAU. The capacity of commensal fungi such as Candida albicans or Saccharomyces cerevisae to promote IRBP-triggered EAU was mediated by Card9. Because Card9 is an essential signalling molecule of a subgroup of C-type lectin receptors (CLRs) important in host defence, we evaluated further the proximal Card9-activating CLRs. Using single receptor-deficient mice we identified Dectin-2, but not Mincle or Dectin-1, as a predominant mediator of fungal-promoted uveitis. Conversely, Dectin-2 activation by α-mannan reproduced the uveitic phenotype of EAU sufficiently, in a process mediated by the Card9-coupled signalling axis and interleukin (IL)-17 production. Taken together, this report relates the potential of the Dectin-2/Card9-coupled pathway in ocular autoimmunity. Not only does it contribute to understanding of how innate immune receptors orchestrate T cell-mediated autoimmunity, it also reveals a previously unappreciated ability of fungal-derived signals to promote autoimmunity.

Complement C5a receptor knockout has diminished light-induced microglia/macrophage retinal migration

PURPOSE

The complement system is involved in the pathogenesis of age-related macular degeneration (AMD). Because activated microglia are also associated with AMD, we studied the relationship between complement anaphylatoxin receptors and microglial recruitment.

METHODS

We assessed the effect of anaphylatoxin C3a receptor (C3aR) and C5a receptor (C5aR) knockout (KO) on light damage-induced migration of microglia/macrophages into the mouse outer retina via immunofluorescence and real-time quantitative PCR.

RESULTS

We found that the mRNA levels of C3, C5, C3aR, C5aR, and two activators of the complement alternative pathway, Cfb and Cfd, were all upregulated after light exposure. Retinal Iba1-positive microglia/macrophages express receptors for C3a and C5a. Light damage increased the number of retinal Iba1-positive cells and the mRNA levels of Iba1. Compared with the wild-type (WT) mice, these increases were attenuated in the C5aR KO mice but not in the C3aR KO mice.

CONCLUSIONS

C5aR but not C3aR promoted the recruitment of microglia/macrophages. These divergent properties of complement anaphylatoxins in the light damage model provide a rationale for testing the differential effects of these receptors in additional retinal and neurodegeneration models.

Notch signalling suppresses regulatory T-cell function in murine experimental autoimmune uveitis

Autoimmune uveitis is an intraocular inflammatory disorder in developed countries. Understanding the mechanisms underlying the development and modulation of immune reaction in uveitic eyes is critical for designing therapeutic interventions. Here we investigated the role of Notch signalling in regulatory T-cell (Treg cell) function during experimental autoimmune uveitis (EAU). Using the Foxp3-GFP reporter mouse strain, the significance of Notch signalling for the function of infiltrating Treg cells was characterized in an EAU model. We found that infiltrating Treg cells substantially expressed Notch-1, Notch-2, JAG1 and DLL1 in uveitic eyes. Activation of Notch signalling, represented by expression of HES1 and HES5, was enhanced in infiltrating Treg cells. Treatment with JAG1 and DLL1 down-regulated Foxp3 expression and immunosuppressive activity of isolated infiltrating Treg cells in vitro, whereas neutralizing antibodies against JAG1 and DLL1 diminished Notch ligand-mediated negative effects on Treg cells. To investigate the significance of Notch signalling for Treg cell function in vivo, lentivirus-derived Notch short hairpin RNAs were transduced into in vitro expanded Treg cells before adoptive transfer of Treg cells into EAU mice. Transfer of Notch-1-deficient Treg cells remarkably reduced pro-inflammatory cytokine production and inflammatory cell infiltration in uveitic eyes. Taken together, Notch signalling negatively modulates the immunosuppressive function of infiltrating Treg cells in mouse EAU.

Targeting the complement system for the management of retinal inflammatory and degenerative diseases

The retina, an immune privileged tissue, has specialized immune defense mechanisms against noxious insults that may exist in diseases such as age-related macular degeneration (AMD), diabetic retinopathy (DR), uveoretinitis and glaucoma. The defense system consists of retinal innate immune cells (including microglia, perivascular macrophages, and a small population of dendritic cells) and the complement system. Under normal aging conditions, retinal innate immune cells and the complement system undergo a low-grade activation (parainflammation) which is important for retinal homeostasis. In disease states such as AMD and DR, the parainflammatory response is dysregulated and develops into detrimental chronic inflammation. Complement activation in the retina is an important part of chronic inflammation and may contribute to retinal pathology in these disease states. Here, we review the evidence that supports the role of uncontrolled or dysregulated complement activation in various retinal degenerative and angiogenic conditions. We also discuss current strategies that are used to develop complement-based therapies for retinal diseases such as AMD. The potential benefits of complement inhibition in DR, uveoretinitis and glaucoma are also discussed, as well as the need for further research to better understand the mechanisms of complement-mediated retinal damage in these disease states.

Mincle Activation and the Syk/Card9 Signaling Axis Are Central to the Development of Autoimmune Disease of the Eye

Uveitis, which occurs in association with systemic immunological diseases, presents a considerable medical challenge because of incomplete understanding of its pathogenesis. The signals that initiate T cells to target the eye, which may be of infectious or noninfectious origin, are poorly understood. Experimental autoimmune uveoretinitis (EAU) develops in mice immunized with the endogenous retinal protein interphotoreceptor retinoid binding protein in the presence of the adjuvant CFA. EAU manifests as posterior ocular inflammation consisting of vasculitis, granulomas, retinal damage, and invasion of self-reactive T cells, which are key clinical features of human uveitis. Our studies uncover Card9 as a critical genetic determinant for EAU. Card9 was responsible for Th17 polarization and Th17-associated Ag-specific responses, but not Th1-associated responses. Nonetheless, Card9 expression was essential for accumulation of both lineages within the eye. Consistent with its recently identified role as an intracellular signaling mediator for C-type lectin receptors (CLRs), a Card9-dependent transcriptional response in the neuroretina was observed involving genes encoding the CLRs Dectin-1, Dectin-2, and Mincle. Genetic deletion of these individual CLRs revealed an essential role for Mincle. Mincle activation was sufficient to generate the EAU phenotype, and this required activation of both Syk and Card9. In contrast, Dectin-1 contributed minimally and a possible repressive role was shown for Dectin-2. These findings extend our understanding of CLRs in autoimmune uveitis. The newly identified role of Mincle and Syk/Card9-coupled signaling axis in autoimmune uveitis could provide novel targets for treatment of patients with ocular inflammatory disease.