PD-1

Programmed Death 1, or PD-1, is a Type I membrane protein of 268 amino acids. PD-1 is a member of the extended CD28/CTLA-4 family of T cell regulators. The protein's structure includes an extracellular IgV domain followed by a transmembrane region and an intracellular tail. The intracellular tail contains two phosphorylation sites located in an immunoreceptor tyrosine-based inhibitory motif and an immunoreceptor tyrosine-based switch motif, which suggests that PD-1 negatively regulates TCR signals. This is consistent with binding of SHP-1 and SHP-2 phosphatases to the cytoplasmic tail of PD-1 upon ligand binding. PD-1 is expressed on the surface of activated T cells, B cells, and macrophages, suggesting that compared to CTLA-4, PD-1 more broadly negatively regulates immune responses.

Ligands
PD-1 has two ligands, PD-L1 and PD-L2, which are members of the B7 family. PD-L1 protein is upregulated on macrophages and dendritic cells (DC) in response to LPS and GM-CSF treatment, and on T cells and B cells upon TCR and B cell receptor signaling, whereas in resting mice, PD-L1 mRNA can be detected in the heart, lung, thymus, spleen, and kidney. PD-L1 is expressed on almost all murine tumor cell lines, including PA1 myeloma, P815 mastocytoma, and B16 melanoma upon treatment with IFN-γ. PD-L2 expression is more restricted and is expressed mainly by DCs and a few tumor lines.

Function
Many studies indicate that PD-1 and its ligands negatively regulate immune responses. First, PD-1 knockout mice develop lupus-like glomerulonephritis and dilated cardiomyopathy on the C57BL/6 and BALB/c backgrounds, respectively. In vitro, treatment of anti-CD3 stimulated T cells with PD-L1-Ig results in reduced T cell proliferation and IFN-γ secretion. Reduced T cell proliferation correlated with attenuated IL-2 secretion, which can be rescued by addition of cross-linking anti-CD28 antibodies or exogenous IL-2. Together, these data suggest that PD-1 negatively regulates T cell responses. Experiments using PD-L1 transfected DCs and PD-1 expressing transgenic (Tg) CD4 and CD8+ T cells suggest that CD8+ T cells are more susceptible to inhibition by PD-L1, although this could be dependent on the strength of TCR signaling. Consistent with a role in negatively regulating CD8+ T cell responses, using an LCMV model of chronic infection, Rafi Ahmed’s group showed that the PD-1-PD-L1 interaction inhibits activation, expansion and acquisition of effector functions of virus specific CD8+ T cells, which can be reversed by blocking the PD-1-PD-L1 interaction.

As CTLA-4 negatively regulates anti-tumor immune responses, the closely related molecule PD-1 has been independently explored as a target for immunotherapy. The 2C TCR recognizes the peptide SIYRYYGL in the context of H 2kb. 2C CD8 T cells incubated with IFN-γ treated B16 targets expressing SIYRYYGL peptide poorly lyse their targets and secrete low levels of IL-2. However, PD-1 knockout 2C T cells have heightened cytolytic capacity and IL-2 secretion, suggesting that PD-1 negatively regulates anti-tumor CD8 T cell responses. Similarly, P815 mastocytoma, which does not express PD-L1 unless treated with IFN-γ, can be transduced to express PD-L1, resulting in inhibition of in vitro CD8-mediated cytotoxicity and enhanced in vivo tumor growth. In vitro cytotoxicity and in vivo inhibition of growth can be restored by anti-PD-L1 antibodies or by genetic ablation of PD-1 ). Together, these data suggest that expression of PD-L1 on tumor cells inhibits anti-tumor activity through engagement of PD-1 on effector T cells. Expression of PD-L1 on tumors is correlated with reduced survival in esophageal, pancreatic and other types of cancers, highlighti      pathway as a target for immunotherapy.  Said et al. showed that triggering PD-1, expressed on monocytes and up-regulated upon monocytes activation, by its ligand PD-L1 induces IL-10 production which inhibits CD4 T-cell function.