Th17.1 lymphocytes: emerging players in the orchestra of immune-mediated inflammatory diseases.

It is now well established that Th17 lymphocytes associate with myriad immune-mediated inflammatory diseases. Over the past one and a half decades, a subset of Th17 lymphocytes viz. Th17.1 lymphocytes has been identified in pre-clinical and clinical models of inflammatory rheumatic diseases. These lymphocytes secrete IL-17A (signature cytokine of Th17 lymphocytes) as well as IFN-γ (the signature cytokine of Th1 lymphocytes). They express the chemokine markers for Th1 (CXCR3) as well as Th17 (CCR6) lymphocytes. Th17.1 lymphocytes also express the drug efflux protein p-glycoprotein, which associates with resistance to corticosteroids and other immunosuppressive drugs. This narrative review overviews the evidence regarding Th17.1 lymphocytes in different inflammatory rheumatic diseases. It is now recognized that Th17.1 lymphocytes are increased in the synovial fluid of affected joints in rheumatoid arthritis (RA) and associate with poor treatment response to abatacept. Th17.1 lymphocytes from synovial fluid of RA are less responsive to immunosuppression than those from the peripheral blood. In sarcoidosis, Th17.1 lymphocytes are concentrated in mediastinal lymph nodes and alveolar lining. Such Th17.1 lymphocytes in sarcoidosis are the predominant source of IFN-γ in the sarcoid lung. Th17.1 lymphocytes are elevated in lupus and Takayasu arteritis and associate with disease activity. Future studies should evaluate isolated Th17.1 lymphocytes from peripheral blood or sites of pathology such as synovial fluid and assess their modulation with immunosuppressive therapy in vitro. The analysis of gene expression signature of isolated Th17.1 lymphocytes might enable the identification of newer therapeutic strategies specifically targeting these cell populations in inflammatory rheumatic diseases. Key Points • Th17.1 lymphocytes are a subset of Th17 lymphocytes secreting both IFN-γ and IL-17 • Th17.1 lymphocytes drive neutrophilic inflammation, granuloma formation, and corticosteroid resistance • Th17.1 lymphocytes are elevated in rheumatoid arthritis and sarcoidosis at sites of inflammation • Increased circulating Th17.1 lymphocytes have been identified in lupus and Takayasu arteritis and associate with active disease.