Arthritis is developed in Borrelia-primed and -infected mice deficient of interleukin-17.

Interleukin-17 (IL-17) has been shown to participate in the development of Lyme arthritis in experimental mice. For example, neutralization of IL-17 with antibodies inhibits induction of arthritis in Borrelia-primed and -infected C57BL/6 wild-type mice. We hypothesized that mice lacking IL-17 would fail to develop Borrelia-induced arthritis. IL-17-deficient and wild-type C57BL/6 mice were primed with heat-inactivated Borrelia and then infected with viable spirochetes 3 weeks later. No swelling or major histopathological changes of the hind paws were detected in IL-17-deficient or wild-type mice that were primed with Borrelia or infected with viable spirochetes. By contrast, IL-17-deficient and wild-type mice that were primed and subsequently infected with heterologous Borrelia developed severe swelling and histopathological changes of the hind paws. In addition, Borrelia-primed and -infected IL-17-deficient mice exhibited elevated gamma-interferon (IFN-γ) levels in sera and increased frequencies of IFN-γ-expressing lymphocytes in popliteal lymph nodes compared to Borrelia-primed and -infected wild-type mice. These results demonstrate that IL-17 is not required for development of severe pathology in response to infection with Borrelia burgdorferi, but may contribute to disease through an interaction with IFN-γ.

IL-17A deficiency promotes periosteal bone formation in a model of inflammatory arthritis.

BACKGROUND: Interleukin-17A (IL-17A) plays a pathogenic role in several rheumatic diseases including spondyloarthritis and, paradoxically, has been described to both promote and protect from bone formation. We therefore examined the effects of IL-17A on osteoblast differentiation in vitro and on periosteal bone formation in an in vivo model of inflammatory arthritis. METHODS: K/BxN serum transfer arthritis was induced in IL-17A-deficient and wild-type mice. Clinical and histologic inflammation was assessed and periosteal bone formation was quantitated. Murine calvarial osteoblasts were differentiated in the continuous presence of IL-17A with or without blockade of secreted frizzled related protein (sFRP)1 and effects on differentiation were determined by qRT-PCR and mineralization assays. The impact of IL-17A on expression of Wnt signaling pathway antagonists was also assessed by qRT-PCR. Finally, regulation of Dickkopf (DKK)1 expression in murine synovial fibroblasts was evaluated after treatment with IL-17A, TNF, or IL-17A plus TNF. RESULTS: IL-17A-deficient mice develop significantly more periosteal bone than wild-type mice at peak inflammation, despite comparable severity of inflammation and bone erosion. IL-17A inhibits calvarial osteoblast differentiation in vitro, inducing mRNA expression of the Wnt antagonist sFRP1 in osteoblasts, and suppressing sFRP3 expression, both potentially contributing to inhibition of osteoblast differentiation. Furthermore, a blocking antibody to sFRP1 reduced the inhibitory effect of IL-17A on differentiation. Although treatment with IL-17A suppresses DKK1 mRNA expression in osteoblasts, IL-17A plus TNF synergistically upregulate DKK1 mRNA expression in synovial fibroblasts. CONCLUSIONS: IL-17A may limit the extent of bone formation at inflamed periosteal sites in spondyloarthritis. IL-17A inhibits calvarial osteoblast differentiation, in part by regulating expression of Wnt signaling pathway components. These results demonstrate that additional studies focusing on the role of IL-17A in bone formation in spondyloarthritis are indicated.

IL-10-producing regulatory B10 cells ameliorate collagen-induced arthritis via suppressing Th17 cell generation.

IL-10-producing CD1d(hi)CD5(+) B cells, also known as B10 cells, have been shown to possess a regulatory function in the inhibition of immune responses, but whether and how B10 cells suppress the development of autoimmune arthritis remain largely unclear. In this study, we detected significantly decreased numbers of IL-10-producing B cells, but increased IL-17-producing CD4(+) T (Th17) cells in both spleen and draining lymph nodes of mice during the acute stage of collagen-induced arthritis (CIA) when compared with adjuvant-treated control mice. On adoptive transfer of in vitro expanded B10 cells, collagen-immunized mice showed a marked delay of arthritis onset with reduced severity of both clinical symptoms and joint damage, accompanied by a substantial reduction in the number of Th17 cells. To determine whether B10 cells directly inhibit the generation of Th17 cells in culture, naive CD4(+) T cells labeled with carboxyfluorescein succinimidyl ester (CFSE) were co-cultured with B10 cells. These B10 cells suppressed Th17 cell differentiation via the reduction of STAT3 phosphorylation and retinoid-related orphan receptor γt (RORγt) expression. Moreover, Th17 cells showed significantly decreased proliferation when co-cultured with B10 cells. Although adoptive transfer of Th17 cells triggered the development of collagen-induced arthritis in IL-17(-/-)DBA/1J mice, co-transfer of B10 cells with Th17 cells profoundly delayed the onset of arthritis. Thus, our findings suggest a novel regulatory role of B10 cells in arthritic progression via the suppression of Th17 cell generation.

Development of proteoglycan-induced arthritis is independent of IL-17.

IL-17 is the hallmark cytokine for the newly identified subset of Th cells, Th17. Th17 cells are important instigators of inflammation in several models of autoimmune disease; in particular, collagen induced arthritis (CIA) and experimental autoimmune encephalomyelitis (EAE), which were previously characterized as Th1-mediated diseases. Although high levels of IFN-gamma are secreted in CIA and EAE, disease is exacerbated in IFN-gamma- or IFN-gamma receptor-deficient mice due to the ability of IFN-gamma to suppress IL-17 secretion. However, in proteoglycan-induced arthritis (PGIA), severe arthritis is dependent on the production of IFN-gamma. We were therefore interested in determining the role of IL-17 in PGIA. We assessed the progression of arthritis in IL-17-deficient (IL-17-/-) mice and found the onset and severity of arthritis were equivalent in wild-type (WT) and IL-17-/- mice. Despite evidence that IL-17 is involved in neutrophil recruitment, synovial fluid from arthritic joints showed a comparable proportion of Gr1+ neutrophils in WT and IL-17-/- mice. IL-17 is also implicated in bone destruction in autoimmune arthritis, however, histological analysis of the arthritic joints from WT and IL-17-/- mice revealed a similar extent of joint cellularity, cartilage destruction, and bone erosion despite significantly reduced RANKL (receptor activator of NK-kappaB ligand) expression. There were only subtle differences between WT and IL-17-/- mice in proinflammatory cytokine expression, T cell proliferation, and autoantibody production. These data demonstrate that IL-17 is not absolutely required for autoimmune arthritis and that the production of other proinflammatory mediators is sufficient to compensate for the loss of IL-17 in PGIA.

Autoimmune inflammation from the Th17 perspective.

Recent studies demonstrated an IL-17-producer CD4+ T cell subpopulation, termed Th17, distinct from Th1 and Th2. It represents a different pro-inflammatory Th-cell lineage. This notion is supported by gene-targeted mice studies. Mice lacking IL-23 (p19-/-) do not develop experimental autoimmune encephalomyelitis (EAE) or collagen-induced arthritis (CIA), while knockout mice for the Th1 cytokine IL-12 (p35-/-) strongly develop both autoimmune diseases. Disease resistance by IL-23 knockout mice correlates well with the absence of IL-17-producing CD4(+) T lymphocytes in target organs despite normal presence of antigen-specific-IFN-gamma-producing Th1 cells. This finding may thus explain previous contradictory reports showing that anti-IFN-gamma-treated mice, IFN-gamma- or IFNR-deficient mice develop CIA or EAE. TGF-beta, IL-6 and IL-1 are the differentiation factors of Th17 cells. IL-23 is dispensable for this function, but necessary for Th17 expansion and survival. The master regulator that directs the differentiation program of Th17 cells is the orphan nuclear receptor RORgammat. IL-27, a member of the IL-12/IL-23 family, potently inhibits Th17 development. Evidence suggesting rheumatoid arthritis and multiple sclerosis as primarily IL-17 autoimmune inflammatory-mediated diseases is rapidly accumulating. The IL-17/23 axis of inflammation and related molecules may rise as therapeutic targets for treating these and perhaps other autoimmune diseases.