FOXP3 and acquired polycythemia vera: In particular, IL-6 has been found to correlate with PV disease severity.16 Treg cells have potent immunosuppressive functions through inhibiting T-cell activation, differentiation, and effector functions including T-cell cytokine production and B-cell antibody production.17 The increased Th2 activity and loss of self-tolerance suggests reduced Treg activity, which is supported by the results of this review as levels of FOXP3, a transcription factor specific to Treg cells, is consistently reduced in PV patients (Supplemental Table 5).