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IL-33 is a cytokine of the IL-1 family, which signals through the ST2 receptor. Previous studies emphasized a role for IL-33 in shaping innate and adaptive immune responses. IL-33 was also reported to modulate myelopoiesis and myeloid cell activity. In this article, we describe IL-33-overexpressing CMV/IL33 and LysM/IL33 mice, which display an inflammatory phenotype associated with growth retardation and paw swelling. The phenotype of CMV/IL33 mice is dependent on activation of the ST2 receptor and is characterized by extensive neutrophil infiltration into different organs, including the paws. Local or systemic levels of proinflammatory mediators such as IL-1β, Cxcl-1, G-CSF, and IL-6 are increased. CMV/IL-33 mice also suffer from anemia, thrombocytosis, and a marked dysregulation of myelopoiesis, leading to an important increase in myeloid cell production or accumulation in bone marrow (BM), spleen, and peripheral blood. Consistently, recombinant IL-33 induced proliferation of myeloid lineage cells in BM-derived granulocyte cultures, whereas IL-33 knockout mice exhibited minor deficiencies in spleen and BM myeloid cell populations. Our observations reveal a neutrophil-dominated inflammatory phenotype in IL-33-overexpressing CMV/IL33 and LysM/IL33 mice, and highlight important regulatory effects of IL-33 on myelopoiesis in vitro and in vivo, where excessive IL-33 signaling can translate into the occurrence of a myeloproliferative disorder., (Copyright © 2015 by The American Association of Immunologists, Inc.)

Previous work suggested implication of the interleukin (IL)-1 family cytokine IL-33, signaling through its receptor ST2, in the pathogenesis of human and mouse arthritis. In this study, we directly investigated the role of endogenous IL-33 in antigen-induced arthritis (AIA) and collagen-induced arthritis (CIA) using IL-33 KO mice. AIA was induced by injection of methylated bovine serum albumin (mBSA) into knee joints of previously immunized mice. CIA was induced by immunization with bovine type II collagen. Disease severity was evaluated by clinical and histological scoring and cellular immune responses were assessed in cultured draining lymph node cells. Our results indicate that the development of AIA or CIA, as assessed by clinical or histological evaluation, is not impaired in IL-33 deficient mice. We did not observe any consistent modifications in humoral or cellular immune responses in IL-33 KO mice, although IL-33 deficiency enhanced antigen-specific IFN-γ production, proliferation or IgG2a titers in some experiments, suggesting that endogenous IL-33 may contribute to shaping the adaptive immune response. In conclusion, our data suggest that IL-33 plays a modifying rather than a pivotal role in disease development in two models of immune-mediated arthritis., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Introduction: Interleukin (IL)-33 is a cytokine of the IL-1 family, which signals through the ST2 receptor. Previous work suggested implication of the IL-33/ST2 axis in the pathogenesis of human and mouse arthritis. Here, we directly investigated the role of endogenous IL-33 in K/BxN serum transfer-induced arthritis by using IL-33 knockout (KO) mice., Methods: Arthritis was induced by injection of complete K/BxN serum or purified IgG. Disease severity was monitored by clinical and histological scoring., Results: K/BxN serum transfer induced pronounced arthritis with similar incidence and severity in IL-33 KO and wild-type (WT) mice. In contrast, disease development was significantly reduced in ST2 KO mice. IL-33 expression in synovial tissue was comparable in arthritic WT and ST2 KO mice, and absent in IL-33 KO mice. Transfer of purified arthritogenic IgG instead of complete K/BxN serum also resulted in similar arthritis severity in IL-33 KO and WT mice, excluding a contribution of IL-33 contained in the serum of donor mice to explain this result. We investigated additional potential confounding factors, including purity of genetic background, but the mechanisms underlying reduced arthritis in ST2 KO mice remained unclear., Conclusions: The data obtained with IL-33 KO mice indicate that endogenous IL-33 is not required for the development of joint inflammation in K/BxN serum transfer-induced arthritis. On the contrary, arthritis severity was reduced in ST2 KO mice. This observation might relate to IL-33 independent effects of ST2, and/or reveal the existence of confounding variables affecting the severity of joint inflammation in these KO strains.

Objective: Interleukin-33 (IL-33; or, IL-1F11) was recently identified as the ligand of the IL-1 family receptor T1/ST2. The aim of this study was to examine IL-33 production in human and mouse joints and to investigate the role of IL-33 and T1/ST2 in experimental arthritis., Methods: IL-33 expression was examined in human synovial tissue, rheumatoid arthritis (RA) synovial fibroblasts, and arthritic mouse joints. Mice with collagen-induced arthritis (CIA) were treated with blocking anti-ST2 antibody or control antibody beginning at the onset of disease. Arthritis severity was assessed by clinical and histologic scoring. Draining lymph node (LN) cell responses were examined ex vivo, and joint messenger RNA (mRNA) was used for expression profiling., Results: IL-33 was highly expressed in human RA synovium. In cultured synovial fibroblasts, IL-33 expression was strongly induced by IL-1beta and/or tumor necrosis factor alpha. Furthermore, IL-33 mRNA was detected in the joints of mice with CIA and increased during the early phase of the disease. Administration of a blocking anti-ST2 antibody at the onset of disease attenuated the severity of CIA and reduced joint destruction. Anti-ST2 antibody treatment was associated with a marked decrease in interferon-gamma production as well as with a more limited reduction in IL-17 production by ex vivo-stimulated draining LN cells. Finally, RANKL mRNA levels in the joint were reduced by anti-ST2 treatment., Conclusion: IL-33 is produced locally in inflamed joints, and neutralization of IL-33 signaling has a therapeutic effect on the course of arthritis. These observations suggest that locally produced IL-33 may contribute to the pathogenesis of joint inflammation and destruction.

Objective: To determine the role and expression of the cytokine/receptor pair interleukin-21 (IL-21)/IL-21 receptor (IL-21R) in rheumatoid arthritis (RA)., Methods: The expression of IL-21R and IL-21 was analyzed by TaqMan real-time polymerase chain reaction (PCR) and in situ hybridization of synovial biopsy samples from patients with RA and osteoarthritis (OA). Double labeling by immunohistochemistry after in situ hybridization was performed with anti-CD68 antibodies. The expression of IL-21R at the protein level was confirmed by Western blotting. Stimulation experiments were performed with recombinant IL-1beta, tumor necrosis factor alpha (TNFalpha), platelet-derived growth factor (PDGF), and transforming growth factor beta (TGFbeta). The role of IL-21R in cartilage destruction was analyzed in the SCID mouse coimplantation model of RA., Results: IL-21R was found in total RNA extracts and in synovial biopsy samples from RA patients, whereas no expression or only minimal expression was seen in samples from OA patients. Double labeling indicated that both synovial macrophages and synovial fibroblasts expressed IL-21R. Western blotting with anti-IL-21R antibodies confirmed the expression of IL-21R protein in RA synovial fibroblasts (RASFs). Of note, IL-21 was not detectable by real-time PCR and in situ hybridization in the same samples in vivo as in vitro. The level of expression of IL-21R messenger RNA (mRNA) was not altered by stimulation with IL-1beta, TNFalpha, PDGF, or TGFbeta. Interestingly, in the SCID mouse coimplantation model, RASFs did not maintain their expression of IL-21R at sites of invasion into the cartilage. Similarly, IL-21R mRNA was not expressed at sites of invasion into cartilage and bone in RA synovium., Conclusion: Our data demonstrate that IL-21R is expressed in RA synovium by RASFs and synovial macrophages. IL-21R is associated with the activated phenotype of RASFs independently of the major proinflammatory cytokines IL-1beta and TNFalpha, but correlates negatively with the destruction of articular cartilage and bone.