IL21 and relapsing-remitting multiple sclerosis: One study demonstrated that simvastatin, with good CNS penetration, inhibited Th17 cell differentiation and lowered IL-17A, IL-17F, IL-21, and IL-22 secretion in in vitro-differentiated naive CD4(+) T cells from RRMS patients by blocking IFN regulatory factor 4 (IRF4) expression, which was identified as a key transcription factor for human Th17 cell differentiation [223].