PDE4D and relapsing-remitting multiple sclerosis: Of note, in contrast to other CD4 memory cells, the top marker genes for cluster T09 included numerous genes, which have previously been proven to be encephalitogenic in MS animal models, such as MAF, PDE4D, ABCB1, CCR1, and LGALS1.24, 25, 26, 27, 28 Together, cluster T09 consists of unique highly differentiated CD4+ T cells that possess traits of both Th17 and Tfh cells and express a rich repertoire of migratory and encephalitogenic factors, which make it plausible that they contribute to both the pathogenesis of RRMS and progressive MS (Figure 7G).