Therapy targeting antigen-specific T cells by a peptide-based tolerizing vaccine against autoimmune arthritis.

A longstanding goal has been to find an antigen-specific preventive therapy, i.e., a vaccine, for autoimmune diseases. It has been difficult to find safe ways to steer the targeting of natural regulatory antigen. Here, we show that the administration of exogenous mouse major histocompatibility complex class II protein bounding a unique galactosylated collagen type II (COL2) peptide (Aq-galCOL2) directly interacts with the antigen-specific TCR through a positively charged tag. This leads to expanding a VISTA-positive nonconventional regulatory T cells, resulting in a potent dominant suppressive effect and protection against arthritis in mice. The therapeutic effect is dominant and tissue specific as the suppression can be transferred with regulatory T cells, which downregulate various autoimmune arthritis models including antibody-induced arthritis. Thus, the tolerogenic approach described here may be a promising dominant antigen-specific therapy for rheumatoid arthritis, and in principle, for autoimmune diseases in general.

Fcgr2b and Fcgr3 are the major genetic factors for cartilage antibody-induced arthritis, overriding the effect of Hc encoding complement C5.

Like rheumatoid arthritis (RA) in humans, collagen-induced arthritis (CIA) in mice is associated with not only MHC class II genetic polymorphism but also, to some extent, with other loci including genes encoding Fc gamma receptors (FCGRs) and complement C5. In this study, we used a cartilage antibody-induced arthritis (CAIA) model in which arthritis develops within a 12-h timeframe, to determine the relative importance of FCGRs and C5 (Hc). In CAIA, inhibiting or deleting FCGR3 substantially hindered arthritis development, underscoring the crucial role of this receptor. Blocking FCGR3 also reduced the levels of FCGR4, and vice versa. When employing an IgG1 arthritogenic cocktail that exclusively interacts with FCGR2B and FCGR3, joint inflammation was promptly initiated in Fcgr2b-- mice but not in Fcgr3-- mice, suggesting that FCGR3 is sufficient for CAIA development. Regarding complement activation, Fcgr2b++.Hc** mice with C5 mutated were fully resistant to CAIA, whereas Fcgr2b--.Hc** mice developed arthritis rapidly. We conclude that FCGR3 is essential and sufficient for CAIA development, particularly when induced by IgG1 antibodies. The human ortholog of mouse FCGR3, FCGR2A, may be associated with RA pathogenesis. FCGR2B deficiency allows for rapid arthritis progression and overrides the resistance conferred by C5 deficiency.

Stereoselective Synthesis of the Gal-α-(1→3)-Gal-ß-(1→3)-GlcNAc Trisaccharide: a new Ligand for DCAR and Mincle C-Type Lectin Receptors.

In this work, we have discovered that the Gal-α-(1→3)-Gal-ß-(1→3)-GlcNAc trisaccharide, a fragment of the B antigen Type-1, is a new ligand of two C-type lectin receptors (CLRs) i. e. DCAR and Mincle which are key players in different types of autoimmune diseases. Accordingly, we report here on a straightforward methodology to access pure Gal-α-(1→3)-Gal-ß-(1→3)-GlcNAc trisaccharide. A spacer with a terminal primary amine group was included at the reducing end of the GlcNAc residue thus ensuring the further functionalization of the trisaccharide Gal-α-(1→3)-Gal-ß-(1→3)-GlcNAc.

Pathogenic antibody response to glucose-6-phosphate isomerase targets a modified epitope uniquely exposed on joint cartilage.

OBJECTIVES: To identify the arthritogenic B cell epitopes of glucose-6-phosphate isomerase (GPI) and their association with rheumatoid arthritis (RA). METHODS: IgG response towards a library of GPI peptides in patients with early RA, pre-symptomatic individuals and population controls, as well as in mice, were tested by bead-based multiplex immunoassays and ELISA. Monoclonal IgG were generated, and the binding specificity and affinity were determined by ELISA, gel size exclusion chromatography, surface plasma resonance and X-ray crystallography. Arthritogenicity was investigated by passive transfer experiments. Antigen-specific B cells were identified by peptide tetramer staining. RESULTS: Peptide GPI293-307 was the dominant B cell epitope in K/BxN and GPI-immunised mice. We could detect B cells and low levels of IgM antibodies binding the GPI293-307 epitopes, and high affinity anti-GPI293-307 IgG antibodies already 7 days after GPI immunisation, immediately before arthritis onset. Transfer of anti-GPI293-307 IgG antibodies induced arthritis in mice. Moreover, anti-GPI293-307 IgG antibodies were more frequent in individuals prior to RA onset (19%) than in controls (7.5%). GPI293-307-specific antibodies were associated with radiographic joint damage. Crystal structures of the Fab-peptide complex revealed that this epitope is not exposed in native GPI but requires conformational change of the protein in inflamed joint for effective recognition by anti-GPI293-307 antibodies. CONCLUSIONS: We have identified the major pathogenic B cell epitope of the RA-associated autoantigen GPI, at position 293-307, exposed only on structurally modified GPI on the cartilage surface. B cells to this neo-epitope escape tolerance and could potentially play a role in the pathogenesis of RA.

Vitamin D3 receptor polymorphisms regulate T cells and T cell-dependent inflammatory diseases.

It has proven difficult to identify the underlying genes in complex autoimmune diseases. Here, we use forward genetics to identify polymorphisms in the vitamin D receptor gene (Vdr) promoter, controlling Vdr expression and T cell activation. We isolated these polymorphisms in a congenic mouse line, allowing us to study the immunomodulatory properties of VDR in a physiological context. Congenic mice overexpressed VDR selectively in T cells, and thus did not suffer from calcemic effects. VDR overexpression resulted in an enhanced antigen-specific T cell response and more severe autoimmune phenotypes. In contrast, vitamin D3-deficiency inhibited T cell responses and protected mice from developing autoimmune arthritis. Our observations are likely translatable to humans, as Vdr is overexpressed in rheumatic joints. Genetic control of VDR availability codetermines the proinflammatory behavior of T cells, suggesting that increased presence of VDR at the site of inflammation might limit the antiinflammatory properties of its ligand.

Key interactions in the trimolecular complex consisting of the rheumatoid arthritis-associated DRB1*04:01 molecule, the major glycosylated collagen II peptide and the T-cell receptor.

OBJECTIVES: Rheumatoid arthritis (RA) is an autoimmune disease strongly associated with the major histocompatibility complex (MHC) class II allele DRB1*04:01, which encodes a protein that binds self-peptides for presentation to T cells. This study characterises the autoantigen-presenting function of DRB1*04:01 (HLA-DRA*01:01/HLA-DRB1*04:01) at a molecular level for prototypic T-cell determinants, focusing on a post-translationally modified collagen type II (Col2)-derived peptide. METHODS: The crystal structures of DRB1*04:01 molecules in complex with the peptides HSP70289-306, citrullinated CILP982-996 and galactosylated Col2259-273 were determined on cocrystallisation. T cells specific for Col2259-273 were investigated in peripheral blood mononuclear cells from patients with DRB1*04:01-positive RA by cytofluorometric detection of the activation marker CD154 on peptide stimulation and binding of fluorescent DRB1*0401/Col2259-273 tetramer complexes. The cDNAs encoding the T-cell receptor (TCR) α-chains and ß-chains were cloned from single-cell sorted tetramer-positive T cells and transferred via a lentiviral vector into TCR-deficient Jurkat 76 cells. RESULTS: The crystal structures identified peptide binding to DRB1*04:01 and potential side chain exposure to T cells. The main TCR recognition sites in Col2259-273 were lysine residues that can be galactosylated. RA T-cell responses to DRB1*04:01-presented Col2259-273 were dependent on peptide galactosylation at lysine 264. Dynamic molecular modelling of a functionally characterised Col2259-273-specific TCR complexed with DRB1*04:01/Col2259-273 provided evidence for differential allosteric T-cell recognition of glycosylated lysine 264. CONCLUSIONS: The MHC-peptide-TCR interactions elucidated in our study provide new molecular insights into recognition of a post-translationally modified RA T-cell determinant with a known dominant role in arthritogenic and tolerogenic responses in murine Col2-induced arthritis.

Identification and analyses of inhibitors targeting apolipoprotein(a) kringle domains KIV-7, KIV-10, and KV provide insight into kringle domain function.

Increased plasma concentrations of lipoprotein(a) (Lp(a)) are associated with an increased risk for cardiovascular disease. Lp(a) is composed of apolipoprotein(a) (apo(a)) covalently bound to apolipoprotein B of low-density lipoprotein (LDL). Many of apo(a)'s potential pathological properties, such as inhibition of plasmin generation, have been attributed to its main structural domains, the kringles, and have been proposed to be mediated by their lysine-binding sites. However, available small-molecule inhibitors, such as lysine analogs, bind unselectively to kringle domains and are therefore unsuitable for functional characterization of specific kringle domains. Here, we discovered small molecules that specifically bind to the apo(a) kringle domains KIV-7, KIV-10, and KV. Chemical synthesis yielded compound AZ-05, which bound to KIV-10 with a Kd of 0.8 µm and exhibited more than 100-fold selectivity for KIV-10, compared with the other kringle domains tested, including plasminogen kringle 1. To better understand and further improve ligand selectivity, we determined the crystal structures of KIV-7, KIV-10, and KV in complex with small-molecule ligands at 1.6-2.1 Å resolutions. Furthermore, we used these small molecules as chemical probes to characterize the roles of the different apo(a) kringle domains in in vitro assays. These assays revealed the assembly of Lp(a) from apo(a) and LDL, as well as potential pathophysiological mechanisms of Lp(a), including (i) binding to fibrin, (ii) stimulation of smooth-muscle cell proliferation, and (iii) stimulation of LDL uptake into differentiated monocytes. Our results indicate that a small-molecule inhibitor targeting the lysine-binding site of KIV-10 can combat the pathophysiological effects of Lp(a).

Structure and biophysical characterization of the human full-length neurturin-GFRa2 complex: A role for heparan sulfate in signaling.

Neurturin (NRTN) provides trophic support to neurons and is considered a therapeutic agent for neurodegenerative diseases, such as Parkinson's disease. It binds to its co-receptor GFRa2, and the resulting NRTN-GFRa2 complex activates the transmembrane receptors rearranged during transfection (RET) or the neural cell adhesion molecule (NCAM). We report the crystal structure of NRTN, alone and in complex with GFRa2. This is the first crystal structure of a GFRa with all three domains and shows that domain 1 does not interact directly with NRTN, but it may support an interaction with RET and/or NCAM, via a highly conserved surface. In addition, biophysical results show that the relative concentration of GFRa2 on cell surfaces can affect the functional affinity of NRTN through avidity effects. We have identified a heparan sulfate-binding site on NRTN and a putative binding site in GFRa2, suggesting that heparan sulfate has a role in the assembly of the signaling complex. We further show that mutant NRTN with reduced affinity for heparan sulfate may provide a route forward for delivery of NRTN with increased exposure in preclinical in vivo models and ultimately to Parkinson's patients.

Cutting Edge: Processing of Oxidized Peptides in Macrophages Regulates T Cell Activation and Development of Autoimmune Arthritis.

APCs are known to produce NADPH oxidase (NOX) 2-derived reactive oxygen species; however, whether and how NOX2-mediated oxidation affects redox-sensitive immunogenic peptides remains elusive. In this study, we investigated a major immunogenic peptide in glucose-6-phosphate isomerase (G6PI), a potential autoantigen in rheumatoid arthritis, which can form internal disulfide bonds. Ag presentation assays showed that presentation of this G6PI peptide was more efficient in NOX2-deficient (Ncf1m1J/m1J mutant) mice, compared with wild-type controls. IFN-γ-inducible lysosomal thiol reductase (GILT), which facilitates disulfide bond-containing Ag processing, was found to be upregulated in macrophages from Ncf1 mutant mice. Ncf1 mutant mice exhibited more severe G6PI peptide-induced arthritis, which was accompanied by the increased GILT expression in macrophages and enhanced Ag-specific T cell responses. Our results show that NOX2-dependent processing of the redox-sensitive autoantigens by APCs modify T cell activity and development of autoimmune arthritis.

Regulatory T cells control epitope spreading in autoimmune arthritis independent of cytotoxic T-lymphocyte antigen-4.

CD4+  Foxp3+ regulatory T (Treg) cells can control both cellular and humoral immune responses; however, when and how Treg cells play a predominant role in regulating autoimmune disease remains elusive. To deplete Treg cells in vivo at given time-points, we used a mouse strain, susceptible to glucose-6-phosphate isomerase peptide-induced arthritis (GIA), in which the deletion of Treg cells can be controlled by diphtheria toxin treatment. By depleting Treg cells in the GIA mouse model, we found that a temporary lack of Treg cells at both priming and onset exaggerated disease development. Ablation of Treg cells led to the expansion of antigen-specific CD4+ T cells including granulocyte-macrophage colony-stimulating factor, interferon-γ and interleukin-17-producing T cells, and promoted both T-cell and B-cell epitope spreading, which perpetuated arthritis. Interestingly, specific depletion of cytotoxic T-lymphocyte antigen-4 (CTLA-4) on Treg cells only, was sufficient to protect mice from GIA, due to the expansion of CTLA-4- Treg cells expressing alternative suppressive molecules. Collectively, our findings suggest that Treg cells, independently of CTLA-4, act as the key driving force in controlling autoimmune arthritis development.

Human MHC Class II and Invariant Chain Knock-in Mice Mimic Rheumatoid Arthritis with Allele Restriction in Immune Response and Arthritis Association.

Transgenic mice expressing human major histocompatibility complex class II (MHCII) risk alleles are widely used in autoimmune disease research, but limitations arise due to non-physiologic expression. To address this, physiologically relevant mouse models are established via knock-in technology to explore the role of MHCII in diseases like rheumatoid arthritis. The gene sequences encoding the ectodomains are replaced with the human DRB1*04:01 and 04:02 alleles, DRA, and CD74 (invariant chain) in C57BL/6N mice. The collagen type II (Col2a1) gene is modified to mimic human COL2. Importantly, DRB1*04:01 knock-in mice display physiologic expression of human MHCII also on thymic epithelial cells, in contrast to DRB1*04:01 transgenic mice. Humanization of the invariant chain enhances MHCII expression on thymic epithelial cells, increases mature B cell numbers in spleen, and improves antigen presentation. To validate its functionality, the collagen-induced arthritis (CIA) model is used, where DRB1*04:01 expression led to a higher susceptibility to arthritis, as compared with mice expressing DRB1*04:02. In addition, the humanized T cell epitope on COL2 allows autoreactive T cell-mediated arthritis development. In conclusion, the humanized knock-in mouse faithfully expresses MHCII, confirming the DRB1*04:01 alleles role in rheumatoid arthritis and being also useful for studying MHCII-associated diseases.

A subset of type-II collagen-binding antibodies prevents experimental arthritis by inhibiting FCGR3 signaling in neutrophils.

Rheumatoid arthritis (RA) involves several classes of pathogenic autoantibodies, some of which react with type-II collagen (COL2) in articular cartilage. We previously described a subset of COL2 antibodies targeting the F4 epitope (ERGLKGHRGFT) that could be regulatory. Here, using phage display, we developed recombinant antibodies against this epitope and examined the underlying mechanism of action. One of these antibodies, R69-4, protected against cartilage antibody- and collagen-induced arthritis in mice, but not autoimmune disease models independent of arthritogenic autoantibodies. R69-4 was further shown to cross-react with a large range of proteins within the inflamed synovial fluid, such as the complement protein C1q. Complexed R69-4 inhibited neutrophil FCGR3 signaling, thereby impairing downstream IL-1ß secretion and neutrophil self-orchestrated recruitment. Likewise, human isotypes of R69-4 protected against arthritis with comparable efficiency. We conclude that R69-4 abrogates autoantibody-mediated arthritis mainly by hindering FCGR3 signaling, highlighting its potential clinical utility in acute RA.

Autoantibodies to Disease-Related Proteins in Joints as Novel Biomarkers for the Diagnosis of Rheumatoid Arthritis.

OBJECTIVE: This study was undertaken to develop and characterize a multiplex immunoassay for detection of autoantibodies against peptides derived from proteins known to play a role in development of arthritis and that are also expressed in joints. METHODS: We selected peptides from the human counterpart of proteins expressed in the joints, based on mouse models that showed these to be targeted by pathogenic or regulatory antibodies in vivo. Using bead-based flow immunoassays measuring IgG antibodies, we selected triple helical or cyclic peptides, containing the epitopes, to avoid collinear reactivity. We characterized the analytical performance of the immunoassay and then validated it in 3 independent rheumatoid arthritis (RA) cohorts (n = 2,110), Swedish age- and sex-matched healthy controls, and patients with osteoarthritis (OA), patients with psoriatic arthritis (PsA), and patients with systemic lupus erythematosus (SLE). RESULTS: Screening assays showed 5 peptide antigens that discriminated RA patients from healthy controls with 99% specificity (95% confidence interval [CI] 98-100%). In our validation studies, we reproduced the discriminatory capacity of the autoantibodies in 2 other RA cohorts, showing that the autoantibodies had high discriminatory capacity for RA versus OA, PsA, and SLE. The novel biomarkers identified 22.5% (95% CI 19-26%) of early RA patients seronegative for anti-cyclic citrullinated peptide and rheumatoid factor. The usefulness of the biomarkers in identifying seronegative RA patients was confirmed in validation studies using 2 independent cohorts of RA patients and cohorts of patients with OA, PsA, and SLE. CONCLUSION: A multiplex immunoassay with peptides from disease-related proteins in joints was found to be useful for detection of specific autoantibodies in RA serum. Of note, this immunoassay had high discriminatory capacity for early seronegative RA.

Antigen-presenting autoreactive B cells activate regulatory T cells and suppress autoimmune arthritis in mice.

B cells undergo several rounds of selection to eliminate potentially pathogenic autoreactive clones, but in contrast to T cells, evidence of positive selection of autoreactive B cells remains moot. Using unique tetramers, we traced natural autoreactive B cells (C1-B) specific for a defined triple-helical epitope on collagen type-II (COL2), constituting a sizeable fraction of the physiological B cell repertoire in mice, rats, and humans. Adoptive transfer of C1-B suppressed arthritis independently of IL10, separating them from IL10-secreting regulatory B cells. Single-cell sequencing revealed an antigen processing and presentation signature, including induced expression of CD72 and CCR7 as surface markers. C1-B presented COL2 to T cells and induced the expansion of regulatory T cells in a contact-dependent manner. CD72 blockade impeded this effect suggesting a new downstream suppressor mechanism that regulates antigen-specific T cell tolerization. Thus, our results indicate that autoreactive antigen-specific naïve B cells tolerize infiltrating T cells against self-antigens to impede the development of tissue-specific autoimmune inflammation.

A subset of antibodies targeting citrullinated proteins confers protection from rheumatoid arthritis.

Although elevated levels of anti-citrullinated protein antibodies (ACPAs) are a hallmark of rheumatoid arthritis (RA), the in vivo functions of these antibodies remain unclear. Here, we have expressed monoclonal ACPAs derived from patients with RA, and analyzed their functions in mice, as well as their specificities. None of the ACPAs showed arthritogenicity nor induced pain-associated behavior in mice. However, one of the antibodies, clone E4, protected mice from antibody-induced arthritis. E4 showed a binding pattern restricted to skin, macrophages and dendritic cells in lymphoid tissue, and cartilage derived from mouse and human arthritic joints. Proteomic analysis confirmed that E4 strongly binds to macrophages and certain RA synovial fluid proteins such as α-enolase. The protective effect of E4 was epitope-specific and dependent on the interaction between E4-citrullinated α-enolase immune complexes with FCGR2B on macrophages, resulting in increased IL-10 secretion and reduced osteoclastogenesis. These findings suggest that a subset of ACPAs have therapeutic potential in RA.

Glucomannan and beta-glucan degradation by Mytilus edulis Cel45A: Crystal structure and activity comparison with GH45 subfamily A, B and C.

The enzymatic hydrolysis of barley beta-glucan, konjac glucomannan and carboxymethyl cellulose by a ß-1,4-D-endoglucanase MeCel45A from blue mussel, Mytilus edulis, which belongs to subfamily B of glycoside hydrolase family 45 (GH45), was compared with GH45 members of subfamilies A (Humicola insolens HiCel45A), B (Trichoderma reesei TrCel45A) and C (Phanerochaete chrysosporium PcCel45A). Furthermore, the crystal structure of MeCel45A is reported. Initial rates and hydrolysis yields were determined by reducing sugar assays and product formation was characterized using NMR spectroscopy. The subfamily B and C enzymes exhibited mannanase activity, whereas the subfamily A member was uniquely able to produce monomeric glucose. All enzymes were confirmed to be inverting glycoside hydrolases. MeCel45A appears to be cold adapted by evolution, as it maintained 70% activity on cellohexaose at 4 °C relative to 30 °C, compared to 35% for TrCel45A. Both enzymes produced cellobiose and cellotetraose from cellohexaose, but TrCel45A additionally produced cellotriose.

Rheumatoid arthritis sera antibodies to citrullinated collagen type II bind to joint cartilage.

OBJECTIVE: To investigate the occurrence and frequency of anti-citrullinated protein antibodies (ACPA) to cyclic citrullinated type II collagen (COL2) epitope with a capacity to bind joint cartilage. METHODS: Luminex immunoassay was used to analyze serum antibody reactivity to 10 COL2-citrullinated peptides (ACC10) and corresponding arginine peptide controls in rheumatoid arthritis (RA), osteoarthritis (OA), and healthy individuals' cohorts. Top ten "promiscuous" sera (cross-reactive with all ACC10) and top ten "private" sera (restrictedly reactive with one ACC10 peptide) from RA and OA cohorts were selected. Enzyme-linked immunosorbent assay (ELISA) was used to detect response to native COL2. Sera were analyzed with naive and arthritic joints from DBA/1J mice by immunohistochemistry, using monoclonal ACPAs and COL2 reactive antibodies with human Fc as comparison. Staining specificity was confirmed with C1 (a major antibody epitope on COL2) mutated mice and competitive blocking with epitope-specific antibodies. RESULTS: All patient sera bound ACC10 compared with control peptides but very few (3/40) bound native triple-helical COL2. Most sera (27/40) specifically bound to arthritic cartilage, whereas only one private RA serum bound to healthy cartilage. Despite very low titers, private sera from both RA and OA showed an epitope-specific response, documented by lack of binding to cartilage from C1-mutated mice and blocking binding to wild-type cartilage with a competitive monoclonal antibody. As a comparison, monoclonal ACPAs visualized typical promiscuous, or private reactivity to joint cartilage and other tissues. CONCLUSION: ACPA from RA and OA sera, reactive with citrullinated non-triple-helical COL2 peptides, can bind specifically to arthritic cartilage.

Surface Ig variable domain glycosylation affects autoantigen binding and acts as threshold for human autoreactive B cell activation.

The hallmark autoantibodies in rheumatoid arthritis are characterized by variable domain glycans (VDGs). Their abundant occurrence results from the selective introduction of N-linked glycosylation sites during somatic hypermutation, and their presence is predictive for disease development. However, the functional consequences of VDGs on autoreactive B cells remain elusive. Combining crystallography, glycobiology, and functional B cell assays allowed us to dissect key characteristics of VDGs on human B cell biology. Crystal structures showed that VDGs are positioned in the vicinity of the antigen-binding pocket, and dynamic modeling combined with binding assays elucidated their impact on binding. We found that VDG-expressing B cell receptors stay longer on the B cell surface and that VDGs enhance B cell activation. These results provide a rationale on how the acquisition of VDGs might contribute to the breach of tolerance of autoreactive B cells in a major human autoimmune disease.

Cartilage Oligomeric Matrix Protein Induced Arthritis-A New Model for Rheumatoid Arthritis in the C57BL/6 Mouse.

The most commonly used strains in experimental research, including genetically modified strains, are C57BL/6 mice. However, so far, no reliable model for rheumatoid arthritis is available, mainly due to the restriction by the MHC class II haplotype H-2b. Collagen-induced arthritis (CIA) is the most widely used animal model of rheumatoid arthritis, but C57BL/6 strain is resistant to CIA because there is no collagen II peptide associated with H-2b. To establish a rheumatoid arthritis model in C57BL/6 mice, we immunized C57BL/6NJ (B6N) mice with human cartilage oligomeric matrix protein (COMP), which induced severe arthritis with high incidence, accompanied by a strong auto-antibody response. Native COMP was required, as denatured COMP lost its ability to induce arthritis in B6N mice. An immunodominant COMP peptide was identified as the key T cell epitope, with a perfect fit into the Ab class II peptide binding pocket. A critical amino acid in this peptide was found to be phenylalanine at position 95. Recombinant COMP mutated at position 95 (COMP_F95S) lost its ability to induce arthritis or a strong immune response in the B6N mice. In conclusion, A new model for RA has been established using C57BL/6 mice through immunization with COMP, which is dependent on a COMP specific peptide binding Ab, thus in similarity with CIA in Aq expressing strains.

Glycan Activation of Clec4b Induces Reactive Oxygen Species Protecting against Neutrophilia and Arthritis.

Animal models for complex diseases are needed to position and analyze the function of interacting genes. Previous positional cloning identified Ncf1 and Clec4b to be major regulators of arthritis models in rats. Here, we investigate epistasis between Ncf1 and Clec4b, two major regulators of arthritis in rats. We find that Clec4b and Ncf1 exert an additive effect on arthritis given by their joint ability to regulate neutrophils. Both genes are highly expressed in neutrophils, together regulating neutrophil availability and their capacity to generate reactive oxygen species. Using a glycan array, we identify key ligands of Clec4b and demonstrate that Clec4b-specific stimulation triggers neutrophils into oxidative burst. Our observations highlight Clec4b as an important regulator of neutrophils and demonstrate how epistatic interactions affect the susceptibility to, and severity of, autoimmune arthritis.