IL17A and arthritic joint disease: While IL-17 exhibits a protective role in host defense against pathogenic infections in the epithelium and mucosal barrier [40,42,43], elevated levels of IL-17 have been detected in the synovium and synovial fluid of RA patients [44–49], and have been shown to exacerbate inflammation and bone damage in arthritis models such as RA [44,48–50].