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A haplotype with tightly linked Fc gamma receptor (FcγR) genes is known as a major locus controlling immune responses and autoimmune diseases, including arthritis. Here, we split a congenic fragment derived from the NOD mouse (Cia9) to study its effect on immune response and arthritis in mice. We found that arthritis susceptibility was indeed controlled by the FcγR gene cluster and a recombination between the FcγR2b and FcγR3 loci gave us the opportunity to separately study their impact. We identified the NOD‐derived FcγR2b and FcγR3 alleles as disease‐promoting for arthritis development without impact on antibody secretion. We further found that macrophage‐mediated phagocytosis was directly correlated to FcγR3 expression in the congenic mice. In conclusion, we positioned FcγR2b and FcγR3 alleles as disease regulatory and showed that their genetic polymorphisms independently and additively control innate immune cell activation and arthritis., The closely linked polymorphic Fcgr2b and Fcgr3 NOD alleles promote the development of arthritis and enhance the inflammatory response capacity.

Ankylosing spondylitis (AS) is associated with autoantibody production to class II MHC‐associated invariant chain peptide, CD74/CLIP. In this study, we considered that anti‐CD74/CLIP autoantibodies present in sera from AS might recognize CD74 degradation products that accumulate upon deficiency of the enzyme signal peptide peptidase‐like 2A (SPPL2a). We analyzed monocytes from healthy controls (n = 42), psoriatic arthritis (n = 25), rheumatoid arthritis (n = 16), and AS patients (n = 15) for SPPL2a enzyme activity and complemented the experiments using SPPL2a‐sufficient and ‐deficient THP‐1 cells. We found defects in SPPL2a function and CD74 processing in a subset of AS patients, which culminated in CD74 and HLA class II display at the cell surface. These findings were verified in SPPL2a‐deficient THP‐1 cells, which showed expedited expression of MHC class II, total CD74 and CD74 N‐terminal degradation products at the plasma membrane upon receipt of an inflammatory trigger. Furthermore, we observed that IgG anti‐CD74/CLIP autoantibodies recognize CD74 N‐terminal degradation products that accumulate upon SPPL2a defect. In conclusion, reduced activity of SPPL2a protease in monocytes from AS predisposes to endosomal accumulation of CD74 and CD74 N‐terminal fragments, which, upon IFN‐γ‐exposure, is deposited at the plasma membrane and can be recognized by anti‐CD74/CLIP autoantibodies., Ankylosing spondylitis (AS) patient sera contain anti‐CD74 autoantibodies. Signal peptide peptidase‐like 2a (SPPL2a) is involved in CD74 processing. These results show that a subset of AS patients have impaired SPPL2a enzyme function. SPPL2a dysfunction leads to CD74 fragment accumulation on the plasma membrane, which can be recognized by anti‐CD74 autoantibodies.

IL-23 plays an important role in the development of arthritis and the IL-23 receptor (IL-23R) is expressed on different types of T cells. However, it is not fully clear which IL-23R+ T cells are critical in driving T cell-mediated synovitis. We demonstrate, using knock-in IL-23R-GFP reporter (IL-23RGFP/+ ) mice, that CD4+ CCR6+ T cells and γδ T cells, but not CD8+ T cells, express the IL-23R(GFP). During early arthritis, IL-23R(GFP)+ CD4+ CCR6+ T cells, but not IL-23R(GFP)+ γδ T cells, were present in the inflamed joints. IL-23RGFP/+ mice were bred as homozygotes to obtain IL-23RGFP/GFP (IL-23R deficient/IL-23R-/- ) mice, which express GFP under the IL-23R promotor. Arthritis progression and joint damage were significantly milder in IL-23R-/- mice, which revealed less IL-17A+ cells in their lymphoid tissues. Surprisingly, IL-23R-/- mice had increased numbers of IL-23R(GFP)+ CD4+ CCR6+ and CCR7+ CD4+ CCR6+ T cells in their spleen compared to WT, and IL-23 suppressed CCR7 expression in vitro. However, IL-23R(GFP)+ CD4+ CCR6+ T cells were present in the synovium of IL-23R-/- mice at day 4. Finally, adoptive transfer experiments revealed that CD4+ CCR6+ T cells and not γδ T cells drive arthritis progression. These data suggest that IL-23R-dependent T cell-mediated synovitis is dependent on CD4+ CCR6+ T cells and not on γδ T cells.

Hyper‐activated or deviated immune responses can result in immunopathological diseases. Paradoxically, immunodeficiency represents a frequent cause of such immune‐mediated pathologies. Immunopathological manifestations are commonly treated by immunosuppression, but in situations in which immunodeficiency is the basis of disease development, enhancing immunity may represent an alternative treatment option. Here, we tested this counterintuitive concept in a preclinical model using infection of mice with lymphocytic choriomeningitis virus (LCMV). Firstly, we demonstrate that infection of B‐cell‐deficient (B−/−) but not of wild‐type (WT) mice with the LCMV strain Docile induced a rapid and fatal CD8+ T‐cell‐mediated immunopathological disease. Similar to WT mice, LCMV‐infected B−/− mice generated a potent, functional LCMV‐specific CD8+ T‐cell response but exhibited prolonged viral antigen presentation and increased vascular leakage in liver and lungs. Secondly, we were able to prevent this virus‐induced immunopathology in B−/− mice by active or passive T‐cell immunizations or by treatment with LCMV‐specific virus neutralizing or non‐neutralizing monoclonal antibodies (mAb). Thus, boosting antiviral immunity did not aggravate immunopathology in this model, but prevented it by decreasing the formation of target structures for damage‐causing CD8+ T cells.

Human CD117+CRTH2neg innate lymphoid cells (ILC) comprise multipotent precursors (ILCp), which are able to differentiate into subtypes in response to different signals received in peripheral tissues. NKp46+ ILCp have been reported to associate with ILC3 whereas KLRG1+ILCp with ILC2, although the latter can also generate other ILC subsets, thus, maintaining a substantial plasticity. We here showed that CD62L is expressed by ILCp exclusively within KLRG1+ population and its expression marks a loss of their broad differentiation potential. Analysis of cytokine production and relevant markers demonstrated that CD62L+ILCp mainly differentiate into ILC2 whereas CD62Lneg counterpart can also differentiate into other ILC subsets depending on the signals they receive. Remarkably, in peripheral blood of psoriatic patients, where ILC3 are usually enriched, CD62L+ILC were drastically reduced, whereas CD62LnegILC2 upregulated both RORγt and NKp46, thus, suggesting an ongoing conversion to ILC3. Therefore, CD62L now emerges as a potential marker to identify a skewing toward type 2 among ILCp., CD62L expression identifies a population of ILCp more biased toward type 2 profile. Circulating ILCp expressing CD62L are significantly reduced in psoriatic patients resulting in a skewing toward ILC3.

In MS, B cells survive peripheral tolerance checkpoints to mediate local inflammation, but the underlying molecular mechanisms are relatively underexplored. In mice, the MIF pathway controls B‐cell development and the induction of EAE. Here, we found that MIF and MIF receptor CD74 are downregulated, while MIF receptor CXCR4 is upregulated in B cells from early onset MS patients. B cells were identified as the main immune subset in blood expressing MIF. Blocking of MIF and CD74 signaling in B cells triggered CXCR4 expression, and vice versa, with separate effects on their proinflammatory activity, proliferation, and sensitivity to Fas‐mediated apoptosis. This study reveals a new reciprocal negative regulation loop between CD74 and CXCR4 in human B cells. The disturbance of this loop during MS onset provides further insights into how pathogenic B cells survive peripheral tolerance checkpoints to mediate disease activity in MS.

Primary Sjögren's syndrome (pSS) is a complex systemic autoimmune disease with heterogeneous disease manifestations. Genetic predisposition, hormonal and environmental factors are all thought to contribute to disease etiology and pathogenesis. A better understanding of the disease pathogenesis is required in order to establish new targeted therapies. We analysed MAPK/ERK and JAK/STAT signalling networks in peripheral blood mononuclear cells (PBMCs) upon stimulation with interferon alpha 2b (IFN‐α2b) by flow cytometry to define potentially dysfunctional intracellular signalling pathways involved in disease pathogenesis. Cells derived from pSS patients displayed small but significant increases in basal phosphorylation levels of numerous signalling proteins compared to cells from healthy donors. The phosphorylation profiles following stimulation with IFNα2b differed significantly between pSS patients and healthy donors, especially regarding STAT1 Y701. PCA further grouped patients according to clinical characteristics. Type I IFN induced gene expression was found to negatively correlate with the IFN‐α2b induced phosphorylation of STAT3 S727 in T cells and positively with pSTAT1 Y701 in B cells. Increases in pSTAT1 Y701 were associated with the presence of autoantibodies. Our results indicate involvement of both STAT3 S727 and STAT1 Y701 pathways in pSS patients. Therapies targeting these pathways might therefore be beneficial for certain subgroups of patients.

A single nucleotide polymorphism within the PTPN22 gene is a strong genetic risk factor predisposing to the development of multiple autoimmune diseases. PTPN22 regulates Syk and Src family kinases downstream of immuno-receptors. Fungal β-glucan receptor dectin-1 signals via Syk, and dectin-1 stimulation induces arthritis in mouse models. We investigated whether PTPN22 regulates dectin-1 dependent immune responses. Bone marrow derived dendritic cells (BMDCs) generated from C57BL/6 wild type (WT) and Ptpn22−/− mutant mice, were pulsed with OVA323-339 and the dectin-1 agonist curdlan and co-cultured in vitro with OT-II T-cells or adoptively transferred into OT-II mice, and T-cell responses were determined by immunoassay. Dectin-1 activated Ptpn22−/− BMDCs enhanced T-cell secretion of IL-17 in vitro and in vivo in an IL-1β dependent manner. Immunoblotting revealed that compared to WT, dectin-1 activated Ptpn22−/− BMDCs displayed enhanced Syk and Erk phosphorylation. Dectin-1 activation of BMDCs expressing Ptpn22R619W (the mouse orthologue of human PTPN22R620W) also resulted in increased IL-1β secretion and T-cell dependent IL-17 responses, indicating that in the context of dectin-1 Ptpn22R619W operates as a loss-of-function variant. These findings highlight PTPN22 as a novel regulator of dectin-1 signals, providing a link between genetically conferred perturbations of innate receptor signaling and the risk of autoimmune disease.

An association between T‐cell lymphopenia and autoimmunity has long been proposed, but it remains to be elucidated whether T‐cell lymphopenia affects B‐cell responses to autoantigens. Human neonatal thymectomy (Tx) results in a decrease in T‐cell numbers and we used this model to study the development of autoreactivity. Two cohorts of neonatally thymectomized individuals were examined, a cohort of young (1–5 years post‐Tx, n = 10–27) and older children (>10 years, n = 26), and compared to healthy age‐matched controls. T‐cell and B‐cell subsets were assessed and autoantibody profiling performed. Early post‐Tx, a decrease in T‐cell numbers (2.75 × 109/L vs. 0.71 × 109/L) and an increased proportion of memory T cells (19.72 vs. 57.43%) were observed. The presence of autoantibodies was correlated with an increased proportion of memory T cells in thymectomized children. No differences were seen in percentages of different B‐cell subsets between the groups. The autoantigen microarray showed a skewed autoantibody response after Tx. In the cohort of older individuals, autoantibodies were present in 62% of the thymectomized children, while they were found in only 33% of the healthy controls. Overall, our data suggest that neonatal Tx skews the autoantibody profile. Preferential expansion and preservation of Treg (regulatory T) cell stability and function, may contribute to preventing autoimmune disease development after Tx.

The TNF family cytokines B‐cell activating factor (BAFF) and a proliferation‐inducing ligand (APRIL) support plasma cell survival. It is known that inhibitors of BAFF only (BAFFR‐Fc) or BAFF and APRIL (TACI‐Fc) administered early enough in an NZB/NZW F1 mouse model of systemic lupus erythematosus (SLE) ameliorate clinical outcomes, pointing to a pathogenic role of BAFF. In the present study, TACI‐Fc administrated at a later stage of disease, after onset of autoimmunity, decreased the number of bone marrow plasma cells and slowed down further formation of autoantibodies. TACI‐Fc prevented renal damage during a 12‐week treatment period regardless of autoantibody levels, while BAFFR‐Fc did not despite a similar BAFF‐blocking activity in vivo. TACI‐Fc also decreased established plasma cells in a T‐dependent hapten/carrier immunization system better than single inhibitors of BAFF or APRIL, and sometimes better than combined single inhibitors with at least equivalent BAFF and APRIL inhibitory activities. These results indicate that TACI‐Fc can prevent symptoms of renal damage in a mouse model of SLE when BAFFR‐Fc cannot, and point to a plasticity of plasma cells for survival factors. Targeting plasma cells with TACI‐Fc might be beneficial to prevent autoantibody‐mediated damages in SLE.

Hemophagocytic lymphohistiocytosis (HLH) is a life‐threatening inflammatory syndrome characterized by hyperactivation of lymphocytes and histiocytes. T cells play a key role in HLH pathogenesis, but their differentiation pattern is not well characterized in patients with active HLH. We compared T‐cell activation patterns between patients with familial HLH (1°HLH), 2°HLH without apparent infectious trigger (2°HLH) and 2°HLH induced by a viral infection (2°V‐HLH). Polyclonal CD8+ T cells are highly activated in 1°HLH and 2°V‐HLH, but less in 2°HLH as assessed by HLA‐DR expression and marker combination with CD45RA, CCR7, CD127, PD‐1 and CD57. Absence of increased HLA‐DR expression on T cells excluded active 1° HLH with high sensitivity and specificity. A high proportion of polyclonal CD127−CD4+ T cells expressing HLA‐DR, CD57, and perforin is a signature of infants with 1°HLH, much less prominent in virus‐associated 2°HLH. The similar pattern and extent of CD8+ T‐cell activation compared to 2° V‐HLH is compatible with a viral trigger of 1°HLH. However, in most 1°HLH patients no triggering infection was documented and the unique activation of cytotoxic CD4+ T cells indicates that the overall T‐cell response in 1°HLH is different. This may reflect different pathways of pathogenesis of these two HLH variants., The illustrated phenotype is characteristic for patients presenting with different subtypes of HLH in the first year of life and demonstrates the pathophysiological principle. In older patients, most markers of T cell differentiation (apart from HLA‐DR) are very heterogenous also in healthy individuals, such that these phenotypic markers are of limited diagnostic value in patients beyond infancy.

The pro‐inflammatory cytokine IL‐17A has been implicated in the immunopathology of inflammatory arthritis. IL‐17F bears 50% homology to IL‐17A and has recently been suggested to play a role in inflammation. We investigated the induction and cytokine profile of IL‐17F+ CD4+ T cells, and how IL‐17F may contribute to inflammation. Upon culture of healthy donor CD4+ T cells with IL‐1β, IL‐23, anti‐CD3, and anti‐CD28 mAb, both IL‐17A and IL‐17F‐expressing cells were detected. In comparison to IL‐17A+IL‐17F− CD4+ T cells, IL‐17F+IL‐17A− and IL‐17A+IL‐17F+ CD4+ T cells contained lower proportions of IL‐10‐expressing and GM‐CSF‐expressing cells and higher proportions of IFN‐γ‐expressing cells. Titration of anti‐CD28 mAb revealed that strong co‐stimulation increased IL‐17F+IL‐17A− and IL‐17A+IL‐17F+ CD4+ T cell frequencies, whereas IL‐17A+IL‐17F− CD4+ T cell frequencies decreased. This was partly mediated via an IL‐2‐dependent mechanism. Addition of IL‐17A, IL‐17F, and TNF‐α to synovial fibroblasts from patients with inflammatory arthritis resulted in significant production of IL‐6 and IL‐8, which was reduced to a larger extent by combined blockade of IL‐17A and IL‐17F than blockade of IL‐17A alone. Our data indicate that IL‐17A and IL‐17F are differentially regulated upon T cell co‐stimulation, and that dual blockade of IL‐17A and IL‐17F reduces inflammation more effectively than IL‐17A blockade alone., High‐strength T cell stimulation enhances the frequency of IL‐17F+ CD4+ T cells in part via an IL‐2 dependent manner. Results demonstrate IL‐17A+ and IL‐17F+ CD4+ T cell populations display different cytokine profiles. Combined blockade of IL‐17A and IL‐17F is more effective at reducing inflammation than blockade of IL‐17A alone.

The balance between proinflammatory and regulatory CD4(+) T cells is tightly controlled in lymphoid organs. In autoimmune diseases this balance is altered in the periphery and target tissue of patients. However, not much is known about the balance initiated in lymphoid organs during the development of disease. Since systemic autoimmunity is present years before the clinical manifestations of rheumatoid arthritis (RA), it is possible to study the immunoregulatory balance during the earliest (preclinical) phases of disease. Here, we report for the first time the frequency and phenotype of proinflammatory and regulatory CD4(+) T cells in lymph node biopsies obtained from autoantibody positive individuals at risk for developing RA, patients with established disease and healthy controls. The frequency of proinflammatory LN Th1 cells was increased in RA patients compared with HCs, while the frequency of regulatory T cells was lower in LN biopsies of RA-risk individuals. Upon in vitro stimulation LN CD4(+) T cells produced lower levels of proinflammatory cytokines, IFN-γ and IL-17A, in both RA-risk individuals and early RA patients. This study shows that already during the earliest phases of systemic autoimmunity the immunoregulatory balance between proinflammatory and regulatory CD4(+) T cells is altered in LN tissue

Increasing evidence points to an important role for neutrophils in participating in the pathogenesis of the human demyelinating disease MS and the animal model EAE. Therefore, a better understanding of the signals controlling migration of neutrophils as well as evaluating the role of these cells in demyelination is important to define cellular components that contribute to disease in MS patients. In this study, we examined the functional role of the chemokine CXCL1 in contributing to neuroinflammation and demyelination in EAE. Using transgenic mice in which expression of CXCL1 is under the control of a tetracycline‐inducible promoter active within glial fibrillary acidic protein‐positive cells, we have shown that sustained CXCL1 expression within the CNS increased the severity of clinical and histologic disease that was independent of an increase in the frequency of encephalitogenic Th1 and Th17 cells. Rather, disease was associated with enhanced recruitment of CD11b+Ly6G+ neutrophils into the spinal cord. Targeting neutrophils resulted in a reduction in demyelination arguing for a role for these cells in myelin damage. Collectively, these findings emphasize that CXCL1‐mediated attraction of neutrophils into the CNS augments demyelination suggesting that this signaling pathway may offer new targets for therapeutic intervention.

IFN-γ-producing T helper 1 (Th1) cell responses mediate protection against infections but uncontrolled Th1 activity also contributes to a broad range of autoimmune diseases. Autocrine complement activation has recently emerged as key in the induction and contraction of human Th1 immunity: Activation of the complement regulator CD46 and the C3aR expressed by CD4(+) T cells via autocrine generated ligands C3b and C3a, respectively, are critical to IFN-γ production. Further, CD46-mediated signals also induce co-expression of immunosuppressive IL-10 in Th1 cells and transition into a (self)-regulating and contracting phase. In consequence, C3 or CD46-deficient patients suffer from recurrent infections while dysregulation of CD46 signalling contributes to Th1 hyperactivity in rheumatoid arthritis and multiple sclerosis. Here, we report a defect in CD46-regulated Th1 contraction in patients with systemic lupus erythematosus (SLE). We observed that MMP-9-mediated increased shedding of soluble CD46 by Th1 cells was associated with this defect and that inhibition of MMP-9 activity normalized release of soluble CD46 and restored Th1 contraction in patients' T cells. These data may deliver the first mechanistic explanation for the increased serum CD46 levels observed in SLE patients and indicate that targeting CD46-cleaving proteases could be a novel avenue to modulate Th1 responses. This article is protected by copyright. All rights reserved.

Polymorphisms in the MHC class II (MHCII) genes are strongly associated with rheumatoid arthritis, supporting the importance of autoreactive T helper (Th) cells for the development of this disease. Here, we used pristane‐induced arthritis (PIA), induced by the non‐antigenic hydrocarbon pristane, to study the impact of different MHCII alleles on T‐cell activation and differentiation. In MHCII‐congenic rats with disease‐promoting MHCII alleles, pristane primarily induced activation of Th1 cells, whereas activated T cells were Th17 biased in rats with protective MHCII alleles. Neutralization of IFN‐γ during T‐cell activation abrogated the development of disease, suggesting that Th1 immunity is important for disease induction. Neutralization of IL‐17, by contrast, suppressed arthritis only when performed in rats with established disease. Adoptive T‐cell transfers showed that T cells acquired arthritogenic capacity earlier in strains with a prevailing Th1 response. Moreover, upon pristane injection, these strains exhibited more Ag‐primed OX40+ and proliferating T cells of polyclonal origin. These data show that T cells are polarized upon the first encounter with peptide‐MHCII complexes in an allele‐dependent fashion. In PIA, the polyclonal expansion of autoreactive Th1 cells was necessary for the onset of arthritis, while IL‐17 mediated immunity contributed to the progression to chronic disease., Using MHCII‐congenic rats injected with the hydrocarbon pristane, we show that Th‐differentiation is determined by the MHCII‐allele rather than by adjuvant‐induced cytokines. Strains with MHCII‐alleles that bias the immune response toward Th1 (FR61, DA) develop more severe arthritis with an earlier onset than strains with a Th17‐biased response (UR10, HR10).