Effects of the IL-23–IL-17 pathway on bone in spondyloarthritis

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Abstract

Over the past several years, a pathophysiological role for the IL-23-IL-17 pathway in human disease has been defined. A subset of rheumatic diseases, including psoriatic arthritis (PsA) and ankylosing spondylitis (AS), are now acknowledged to be triggered by dysregulated IL-23-IL-17 pathway activation. Genetic evidence links the IL-23-IL-17 pathway to inflammation in these rheumatic diseases, and mechanistic data from mice support a functional role for IL-23-IL-17 pathway activation in the development of enthesitis and in entheseal bone formation. Furthermore, analysis of human tissue samples, as well as data from clinical trials, also supports a role for activation of the IL-23-IL-17 pathway in these diseases. The unique bone phenotype that occurs in PsA and AS is a surprising coexistence of both systemic bone loss and periosteal and entheseal bone formation and is likely to be the result of the actions of IL-23 and/or IL-17 on bone. However, the effects of these cytokines on bone cells are complex, and controversy remains regarding their exact roles in the specific bone microenvironments relevant to PsA and AS.

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Interleukin-23 rather than interleukin-12 is the critical cytokine for autoimmune inflammation of the brain.

Daniel Cua, Jonathan P Sherlock, Yi. Chen … (2003)

Interleukin-12 (IL-12) is a heterodimeric molecule composed of p35 and p40 subunits. Analyses in vitro have defined IL-12 as an important factor for the differentiation of naive T cells into T-helper type 1 CD4+ lymphocytes secreting interferon-gamma (refs 1, 2). Similarly, numerous studies have concluded that IL-12 is essential for T-cell-dependent immune and inflammatory responses in vivo, primarily through the use of IL-12 p40 gene-targeted mice and neutralizing antibodies against p40. The cytokine IL-23, which comprises the p40 subunit of IL-12 but a different p19 subunit, is produced predominantly by macrophages and dendritic cells, and shows activity on memory T cells. Evidence from studies of IL-23 receptor expression and IL-23 overexpression in transgenic mice suggest, however, that IL-23 may also affect macrophage function directly. Here we show, by using gene-targeted mice lacking only IL-23 and cytokine replacement studies, that the perceived central role for IL-12 in autoimmune inflammation, specifically in the brain, has been misinterpreted and that IL-23, and not IL-12, is the critical factor in this response. In addition, we show that IL-23, unlike IL-12, acts more broadly as an end-stage effector cytokine through direct actions on macrophages.

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Generation of Pathogenic Th17 Cells in the Absence of TGF-β Signaling

Kamran Ghoreschi, Arian Laurence, Xiang-Ping Yang … (2010)

CD4+ T cells that selectively produce interleukin (IL)-17, are critical for host defense and autoimmunity 1–4 . Crucial for T helper17 (Th17) cells in vivo 5,6 , IL-23 has been thought to be incapable of driving initial differentiation. Rather, IL-6 and transforming growth factor (TGF)-β1 have been argued to be the factors responsible for initiating specification 7–10 . Herein, we show that Th17 differentiation can occur in the absence of TGF-β signaling. Neither IL-6 nor IL-23 alone efficiently generated Th17 cells; however, these cytokines in combination with IL-1β effectively induced IL-17 production in naïve precursors, independently of TGF-β. Epigenetic modification of the Il17a/Il17f and Rorc promoters proceeded without TGF-β1, allowing the generation of cells that co-expressed Rorγt and T-bet. T-bet+ Rorγt+ Th17 cells are generated in vivo during experimental allergic encephalomyelitis (EAE), and adoptively transferred Th17 cells generated with IL-23 without TGF-β1 were pathogenic in this disease model. These data suggest an alternative mode for Th17 differentiation. Consistent with genetic data linking IL23R with autoimmunity, our findings re-emphasize the importance of IL-23 and therefore have may have therapeutic implications.