SummaryInterleukin (IL)-36a, IL-36b and IL-36g are expressed highly in skin and are involved in the pathogenesis of psoriasis, while the antagonists IL-36Ra or IL-38, another potential IL-36 inhibitor, limit uncontrolled inflammation. The expression and role of IL-36 cytokines in rheumatoid arthritis (RA) and Crohn's disease (CD) is currently debated. Here, we observed that during imiquimod-induced mouse skin inflammation and in human psoriasis, expression of IL-36a, g and IL-36Ra, but not IL-36b and IL-38 mRNA, was induced and correlated with IL-1b and T helper type 17 (Th17) cytokines (IL-17A, IL-22, IL-23, CCL20). In mice with collageninduced arthritis and in the synovium of patients with RA, IL-36a, b, g, IL36Ra and IL-38 were all elevated and correlated with IL-1b, CCL3, CCL4 and macrophage colony-stimulating factor (M-CSF), but not with Th17 cytokines. In the colon of mice with dextran sulphate sodium-induced colitis and in patients with CD, only IL-36a, g and IL-38 were induced at relatively low levels and correlated with IL-1b and IL-17A. We suggest that only a minor subgroup of patients with RA (17-29%) or CD (25%) had an elevated IL-36 agonists/antagonists ratio, versus 93% of patients with psoriasis. By immunohistochemistry, IL-36 cytokines were produced by various cell types in skin, synovium and colonic mucosa such as keratinocytes, CD68 1 macrophages, dendritic/Langerhans cells and CD79a 1 plasma cells. In primary cultures of monocytes or inflammatory macrophages (M1), IL-36b and IL-36Ra were produced constitutively, but IL-36a, g and IL-38 were produced after lipopolysaccharide stimulation. These distinct expression profiles may help to explain why only subgroups of RA and CD patients have a potentially elevated IL-36 agonists/ antagonists ratio.

Interleukin-34 (IL-34) was recently characterized as the M-CSF "twin" cytokine, regulating the proliferation/differentiation/survival of myeloid cells. The implication of M-CSF in oncology was initially suspected by the reduced metastatic dissemination in knock-out mice, due to angiogenesis impairment. Based on this observation, our work studied the involvement of IL-34 in the pathogenesis of osteosarcoma. The in vivo effects of IL-34 were assessed on tissue vasculature and macrophage infiltration in a murine preclinical model based on a paratibial inoculation of human osteosarcoma cells overexpressing or not IL-34 or M-CSF. In vitro investigations using endothelial cell precursors and mature HUVEC cells were performed to analyse the involvement of IL-34 in angiogenesis and myeloid cell adhesion. The data revealed that IL-34 overexpression was associated with the progression of osteosarcoma (tumor growth, lung metastases) and an increase of neo-angiogenesis. In vitro analyses demonstrated that IL-34 stimulated endothelial cell proliferation and vascular cord formation. Pre-treatment of endothelial cells by chondroitinases/heparinases reduced the formation of vascular tubes and abolished the associated cell signalling. In addition, IL-34 increased the in vivo recruitment of M2 tumor-associated macrophages into the tumor tissue. IL-34 increased in vitro monocyte/CD34 1 cell adhesion to activated HUVEC monolayers under physiological shear stress conditions. This work also demonstrates that IL-34 is expressed by osteosarcoma cells, is regulated by TNF-a, IL-1b, and contributes to osteosarcoma growth by increasing the neo-angiogenesis and the recruitment of M2 macrophages. By promoting new vessel formation and extravasation of immune cells, IL-34 may play a key role in tumor development and inflammatory diseases.Angiogenesis and vasculogenesis are central events during embryogenesis and growth.

This work for the first time identifies IL-34 expression in the synovial tissue of patients with arthritis. This cytokine, as a downstream effector of TNFα and IL-1β, may contribute to inflammation and bone erosions in RA.