PDCD1 and cryohydrocytosis: Though PD-1 was demonstrated to be expressed on Tregs and negatively regulated CD4+CD25+Foxp3+ Tregs function by preventing STAT-5 phosphorylation in CHC patients [23], it was still far from well understanding the molecular mechanisms of PD-1/PD-L1 pathway involving in inhibition of CD4+ T cell function, including Foxp3- effector T cells, Foxp3- memory T cells and Tregs.