CX3CR1 and rheumatoid arthritis: RA FLS also significantly express higher amounts of CX3CL1 (fractalkine), and the expression of its sole receptor, CX3CR1, is upregulated in CD4+ and CD8+ T cells of patients with RA, suggesting the involvement of the CX3CL1/CX3CR1 axis in the pathogenesis of RA (44).