Background/Purpose: Immune-cell mediated activation of joint-lining fibroblast-like synoviocytes (FLS) play a key role in joint inflammation and destruction during Rheumatoid arthritis (RA). Thus, identifying factors that promote aggressive and joint-destructive FLS behavior is viewed as a high priority in the development of novel anti-RA therapies. Innate Lymphoid Cells (ILC) are a family of tissue resident innate immune cells that are the innate counterparts to T cells. A major function of ILCs is to interact with local tissue stromal cells in order maintain tissue homeostasis. Although ILCs have been identified in the synovium of RA patients, their role in disease pathogenesis and their potential interaction with FLS remains largely unknown. Here, we identified a novel pathogenic role for ILC2 in arthritis that is mediated through their production of the fibroblast-activating growth factor amphiregulin (AREG).

Methods: Subsets of synovial ILC were assessed in the active K/BxN model and the K/BxN serum transfer-inducing arthritis (STIA) model. Role of ILCs in arthritis was determined by subjecting Rag2-deficient (Rag2^-/-) and ILC-deficient (Rag2^-/-IL2rg^–) mice to STIA. Role of AREG in the STIA model was evaluated using AREG-KO mice and mice with a conditional deletion of AREG in ILC2 (Rag2^-/- IL5-Cre Areg-floxed). Clinical scoring of arthritis was followed by histological assessment. Effect of ILC-derived cytokines on FLS migration was evaluated in vitro. Mann-Whitney or un-paired T tests were used for statistical differences.

Results: Assessment of ILC subsets revealed that ILC2 is the predominant ILC population in the arthritic synovium of mice with active K/BxN arthritis and STIA (P=0.05 vs. non-arthritic control). ILC-deficient mice (Rag2^-/-IL2rg^–) showed reduced development of STIA (P=0.0072 vs. Rag2^-/- mice), indicating that ILCs promote arthritis development. Assessment of ILC signature cytokines identified the ILC2-derived growth factor AREG to be significantly increased in arthritic joints (P=0.0001 vs. non-arthritic control). A pathogenic role for AREG in arthritis development was supported by a significant alleviation in disease severity in AREG-KO mice subjected to STIA (P=0.05 vs. control littermate mice). We identified ILC2 as the major source of synovial AREG during arthritis and conditional deletion of AREG in ILC2 resulted in reduced arthritis development (P=0.0044 vs. WT littermate controls). Mechanistically, we found that AREG promotes activation and migration of FLS (P=0.0001 vs control).

Conclusion: We find that ILC2 is the predominant ILC subset in arthritic joints of mice with K/BxN arthritis. Furthermore, ILC2-derived AREG enhances development of arthritis in mice and promotes FLS activation and migration. We propose that the AREG signaling pathway may provide a novel therapeutic option for the treatment of RA.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/role-of-group-2-innate-lymphoid-cells-in-the-pathogenesis-of-rheumatoid-arthritis/