PDCD1 and viral infectious disease: Recent data on the ability of DCs to express B7-H1 suggested that activation of PD-1/PD-L1 (B7-H1) signaling pathway upon DCs and T-lymphocyte interaction could mediate DC-cytotoxic activity against activated T-lymphocytes and increased expression of B7-H1 on DCs in tumor growth [6] and virus infections, and in such a way result in T cell deficiency [16, 17].