Secondary progressive multiple sclerosis (SPMS) is a chronic form of disease that occurs after relapsing-remitting MS, with a progressive disease course, and its pathogenesis remains unclear. CX3C chemokine receptor 1 (CX3CR1) is a G protein-coupled receptor that may be a useful marker of Eomes+ Th cells; the antigen has been shown to be expressed by cytotoxic Th cells and required for late-onset disease.