Importantly, recent studies have indicated that increasing the number of CD4+ CD25bright FoxP3+ Treg cells, by adoptive transfer or use of the super-agonistic anti-CD28 antibody D665, could suppress antibody production in mouse models of PV, whereas depleting them enhanced the autoantibody production in the same model (49, 50). Here, CD4 is linked to acquired polycythemia vera.