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    • PD is expressed on activated CD and CD T cells

    2018-10-23

    PD-1 is expressed on activated CD4+ and CD8+ T cells, as well as on B cells, myeloid cells (monocytes and some dendritic cells) and NK T cells, suggesting multiple roles in immune regulation (Keir et al., 2008). Persistent expression of PD-1 on CD8+ T cells leads to exhaustion, associated with decreased effector functions, including diminished secretion of IL-2, IFN-γ and TNF-α, and the inability to produce cytolytic molecules such as perforin (Hofmeyer et al., 2011). In addition, PD-1 is constitutively expressed on CD4+Foxp3+ regulatory T cells (Tregs), regulating the development, maintenance and functional response of induced Treg cells, and facilitating de novo conversion of naïve CD4+ T cells to Foxp3+ Tregs (Francisco et al., 2010, 2009). Thus, PD-1 makes broad contributions to T cell-mediated immunity by reducing effector T cell signaling and by enhancing immunosuppressive Treg function, which impacts the establishment and maintenance of immunological tolerance. Currently three monoclonal niclosamide Supplier targeting PD-1 are under Phase I/III clinical trials for the treatment of various solid tumors, and two of them, pembrolizumab and nivolumab were granted FDA approval in 2014 for the treatment of metastatic melanoma (Sharma and Allison, 2015a), and subsequently for the treatment of advanced non-small cell lung cancer (NSCLC). PD-1 recognizes two ligands, PD-L1 (B7-H1 or CD274) and PD-L2 (B7-DC, CD273), which belong to the B7 family and share 34% identity. PD-L1 mRNA is ubiquitously expressed by immune cells, as well as non-hematopoietic cells, and cell surface PD-L1 is upregulated upon activation. Cytokines such as IFN-γ and TNF-α induce PD-L1 expression on T and B cells, endothelial and epithelial cells (Freeman et al., 2000; Ishida et al., 2002). PD-L1 overexpression on tumor cells, and PD-1 on infiltrating lymphocytes have been recognized as important immune evasion mechanisms. Currently four different monoclonal PD-L1 blocking antibodies are in Phase I/II clinical trials (Ohaegbulam et al., 2015). PD-L2 exhibits ~3-fold higher affinity to PD-1 than PD-L1, but its expression is primarily restricted to antigen presenting cells such as dendritic cells, macrophages, B1 B cells and mast cells (Keir et al., 2008). PD-L2 expression is upregulated by cytokines such as IL-4 and IFN-γ (Latchman et al., 2001; Tseng et al., 2001; Zhong and Rothstein, 2011). Due to the broader expression of PD-1 and its ligands, compared to other costimulatory receptor-ligand pairs (such as CTLA4/B7, which are restricted to T cells/APCs), PD-1 signaling regulates immune responses at multiple levels, including, but not limited to, effector responses at the level of peripheral cells and tissues. PD-1 and its ligands are single-pass type I transmembrane proteins, similar to other members of the CD28/B7 family (Chattopadhyay et al., 2009). PD-1 consists of an extracellular immunoglobulin variable (IgV) domain, a transmembrane segment and a cytoplasmic tail harboring two tyrosine-based signaling motifs. The ectodomains of the PD-Ligands are composed of membrane-distal IgV and membrane-proximal immunoglobulin constant (IgC) domains, followed by transmembrane and cytoplasmic segments. The human and mouse PD-1 genes share 60% and 70% identity at the amino acid and nucleotide levels, respectively (Finger et al., 1997). Binding of the PD-Ligands to PD-1 in the context of antigen receptor signaling induces phosphorylation of the two signaling tyrosines within the cytoplasmic tail of PD-1, one of which is part of an Immunoreceptor Tyrosine-based Inhibitory Motif (ITIM), and the other an Immunoreceptor Tyrosin-based Switch Motif (ITSM). Src homology 2-containing tyrosine phosphatase (SHP-2) is recruited to the phosphorylated ITSM motif, which dephosphorylates signaling molecules such as TCR-associated CD3ζ and ZAP70, resulting in inhibition of the downstream PI3K/Akt signaling pathway, and disruption of glucose metabolism and IL-2 production in T cells (Keir et al., 2005; Okazaki et al., 2001; Sharpe et al., 2007).