PDCD1 and Sepsis: Immune checkpoint receptors including programmed death-1 (PD-1), programmed death ligand-1 (PD-L1), cytotoxic T lymphocyte antigen-4 (CTLA-4) and B and T lymphocyte attenuator (BTLA) have been shown to be increased on immune cells during sepsis and hypothesized to be one of the major contributors causing sepsis induced immune cell dysfunction [17].