Background Chronic gut inflammation occurring in AS patients has been linked to active axial inflammation and the gut has been proposed as the main site of IL-23 production in AS patients. IL-23 has been demonstrated to be essential in murine enthesitis by acting on a unique subset of entheseal resident T cells that share some immunological features with a subset of IL-23-responsive gut derived innate lymphoid cells (type III ILCs).
Objectives Aim of the study was to better characterize the immunologic origin and the behavior of ILCs in the gut, synovial fluid and bone marrow of AS patients.
Methods Consecutive ileal gut biopsies were obtained from 20 HLA-B27+ AS patients with axial active disease (BASDAI mean 6.7±2.2) and 15 healthy subjects. Paired samples of peripheral blood and synovial fluid (form knee joints) were obtained from 10 of the AS and 10 osteoarthritis patients. Bone marrow (BM) biopsies (N=5) from inflamed sacroiliac joints (SI) as demonstrated by MRI and from subjects undergoing BM biopsy for suspected monoclona gammopathy (N=5) were also obtained. IL-23R+CD3+CD4–CD8–CD56+T-bet+NKp44+ (type III ILCs) cells were determined and characterized for IL-17 and IL-22 production by flow cytometry and confocal microscopy on isolated lamina propria (LPMC), peripheral blood (PBMC), BM and synovial mononuclear cells (SMC). a4b7 integrin and its counter-receptor MADCAM-1 were evaluated in ileal and BM samples. Quantitative gene expression analysis by rt-PCR of IL-7 and IL-15 (involved in ILCs differentiation) and of IL-17, IL-22 and IL-23p19 was also performed. IL-17, IL-22, IL-23p19, IL-7 and IL-15 protein expression was also analyzed by immunohistochemistry (IHC). Tissue distribution of lymphoid tissue inducer cells (LTi) was also assessed by confocal microscopy.
Results CD3–/CD3+CD4–CD8–RORc–Tbet+CD56+NKp44+ (type III ILCs) cells expressing the IL-23R were significantly expanded in the gut, synovial fluid, and bone marrow of AS patients compared to controls and produced high levels of IL-17 and IL-22. Type III ILCs isolated from the peripheral blood, synovial fluid and BM of AS displayed a significant over-expression of a4b7 suggesting an intestinal origin for these cells. MADcAM1 increased co-expression was demonstrated in both BM and ileal samples. Strong expression of both IL-17 and IL-22 was confirmed by IHC in AS BM. IL-7 was significantly increased in AS gut compared to control gut, especially in the context of Paneth cells (PC) and surrounded by aggregates of c-kit/IL-7R+ cells (LTi) that have been demonstrated to be precursors of gut type III ILC.
Conclusions In this study we demonstrated that gut-derived IL-23R-expressing type III ILCs are expanded in the synovial fluid and inflamed BM of AS patients and produced IL-17 and IL-22, both cytokines suggested as key players in the pathogenesis of AS. The increased expression of IL-7 in AS gut and the presence of clusters of LTi cells close to Paneth cells suggest an important role of innate intestinal immunity in the differentiation of type III ILCs. The increased intestinal and BM expression of MADCAM-1 occurring in AS also suggests the presence of an active homing axis between the gut and the inflamed SI joints.
Disclosure of Interest None declared