Supporting this hypothesis, SPMS and PPMS patients were reported to have significantly decreased levels of IFN-γ mRNA compared to RRMS patients (28), whereas SPMS patients exhibited significantly higher levels of serum IL-17F (44), increased frequency of CD4+ROR+ T cells (indicative of a Th17 phenotype) (45), and enhanced IL-17-inducible myeloid factors (11), in comparison to RRMS patients. Here, IL17A is linked to primary progressive multiple sclerosis.