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OP01. Card9 is a crucial adapter protein for gastro-intestinal immune response

H. Sokol1, K. Conway2, C.‑W. Png2, Z. Mei2, Z. Cao2, B. Morin2, C. Li2, A. Gardet2, H. Shi2, R. Xavier2

1Saint-Antoine Hospital, Paris, France; 2Massachusetts General Hospital and Harvard Medical School, United States

Background: Polymorphisms in Card9 (Caspase Recruitment Domain 9) gene are associated with Crohn's disease (CD) and ulcerative colitis (UC). Card9 is an adapter protein expressed in myeloid cells playing a central role for the integration of signals downstream of many Pattern Recognition Receptors such as Dectin‑1, Mincle, Nod2, Rig‑I and some TLRs. It has been shown that Card9 was required to mount an appropriate Th17 imune response toward fungal infection in human. However, its role in the gastrointestinal tract and particularly in colitis setting is unknown. The aim of this study was to identify the role of Card9 in gastro-intestinal homeostasis and inflammation.

Methods: We used the DSS-induced and the Citrobacter rodentium infection colitis models in wild type (WT) and Card9 KO (KO) mice. DSS was given for 7 days followed or not by a recovery phase of 5 days, before sacrifice. We assessed colitis severity (weight loss, histology), expression of cytokines in colon, mesenteric lymph nodes (MLN) and spleen (rtPCR and/or ELISA) and cellular subtypes levels in colon and small intestine lamina propria, MLN and spleen (flow cytometry).

Results: DSS-induced colitis was more severe in Card9 KO mice and have notably a recovery defect. KO mice exhibited a down regulation of IL‑6, IFNγ and Th17 cytokines (IL‑17A, IL‑17F, IL‑21, IL‑22) in the colon and in MLN. IL‑17A and IL‑22 expression were particularly defective in recovery phase and were associated with a decrease expression of antimicrobial peptides (RegIIIg and DEFB1) in the colon. IL‑17A and IL‑22 are required for the protection against C. rodentium infection. As expected, KO mice exhibited a more severe colitis. Bacterial load in feces and spleen as well as mortality were higher in KO compared to WT mice. KO mice exhibited a decreased expression of IL‑6, IL‑17A, IL‑22 and RegIIIg in colon and lower IL‑17A and IFNγ secretion by MLN cells. The analysis of cells subpopulations in gastro-intestinal immune compartments showed a decrease proportion of Th1, Th17 and innate lymphoid cells (ILCs) in KO mice. Finally, some of these abnormalities were also found at baseline (without colitis) in KO mice.

Conclusions: Card9 is required for the development of an appropriate intestinal immune response in steady state and in colitis setting. Card9 is notably involved in “Th17” and IL‑22 responses (involving Th17 cells but also ILCs) which are required to eradicate some intestinal pathogens and to recover after intestinal inflammation.