ENTPD1 and myeloid sarcoma: Surprisingly, CD39 expression on CD4+ T cells and increased proportions of this cell population can also be induced by polysaccharide A (PSA), a molecule derived from Bacteroides fragilis, a commensal microorganism that inhabits the gut (54–56), which highlights the important role of the gut microbiota in the pathophysiology of MS (57, 58), especially in the context of ADO signaling.