XCL1 and rheumatoid arthritis: Published research indicated that XC chemokines and their receptors (such as XCL1 and XCR1) and CX3C chemokines and their receptors (e.g., CX3CL1 and CX3CR1) are upregulated in RA patients' mononuclear cells (MNCs) and FLS, respectively [47, 48].