PD1 signal transduction pathways in T cells

Abstract

The use of immune checkpoint inhibitors for the treatment of cancer is revolutionizing oncology. Amongst these therapeutic agents, antibodies that block PD-L1/PD1 interactions between cancer cells and T cells are demonstrating high efficacies and low toxicities. Despite all the recent advances, very little is yet known on the molecular intracellular signaling pathways regulated by either PD-L1 or PD1. Here we review the current knowledge on PD1-dependent intracellular signaling pathways, and the consequences of disrupting PD1 signal transduction.

Keywords: B7-H1; PD-L1; PD1; PDL1; cancer.

Figure 1. Antigen presentation and T cell activation through the T cell receptor signalosome

(A) T cells receive from antigen presenting cells (APCs, left of the picture) three signals. APCs present antigenic peptides complexed to MHC molecules (pMHC, as indicated) to T cells through binding to their TCR as shown. This interaction triggers intracellular signaling events (signal 1) as indicated within the T cell on the right. T cells simultaneously receive additional “positive” and “negative” signals through ligand-receptor interactions within the immunological synapse. On top is shown the co-stimulatory interaction driven by CD80-CD28, and below two inhibitory interactions between PD-L1/PD1 and CD80-CTLA4. The integration of these signals delivers a second signal regulating the extent of T cell activation. A third signal is also provided by cytokines (top). (B) Simplified scheme of the TCR signalosome including the αβ TCR chains associated to CD3 molecules as indicated. In this example, only CD3ζ dimers are shown. Signal 1 is initiated through αβ TCR and CD4/CD8 molecules. This signal depends on LCK activation that phosphorylates CD3 and CD28 cytoplasmic domains. ZAP70 then binds to CD3ζ and phosphorylates LAT and p38 as shown. Phosphorylated LAT recruits other enzymes and adaptor molecules as shown which will trigger calcium-dependent and MAPK-dependent pathways. Signal 2 depends on PI3K associated to CD28 when associated to CD80 on the surface of the APC. PI3K generates PIP₃ leading to proliferation and survival by regulating AKT-mTOR pathways. CD28 engagement prevents apoptosis and acts synergistically with CD3-dependent signals. In green, adaptor molecules. In red, kinases and phospholipase C.

Figure 2. PD1-dependent inhibitory mechanisms

(A) Direct inhibitory mechanisms over the TCR signalosome are shown. The figure represents PD1-dependent proximal inhibitory mechanisms, which depend on the recruitment of SHP1 and SHP2 phosphatases as shown. These phosphatases inhibit ZAP70 and PI3K activities (blue arrows). Downstream intracellular pathways are also terminated, as exemplified in the figure with ERK and PKCθ. (B) Indirect inhibitory mechanisms over TCR signaling and T cell proliferation are shown through regulation of CK2 expression and activities. On the left, the PI3K-dependent signaling pathway activating CDK2 and inhibiting SMAD3 is shown. Briefly, PIP3 activates AKT leading to production of the ubiquitin ligase SCF that degrades the CDK2 inhibitor Kip1. Activated CDK2 triggers cell cycle progression and inactivates SMAD3 by phosphorylation. These pathways are negatively regulated by the PTEN phosphatase that degrades PIP₃. During TCR activation CK2 phosphorylates PTEN with a concomitant decrease in its activities. When PD1 is engaged CK2 expression and activities decrease resulting in active PTEN that eliminates PIP₃ shutting off AKT activation. (C) Regulation of TCR surface expression by PD1. PD1 engagement promotes expression of E3 ubiquitin ligases of the CBL family, as shown. As indicated, other ligases may be up-regulated as well. These ubiquitin ligases ubiquitylate TCR chains and PI3K, leading to the removal of TCRs from the T cell surface, possibly by endocytosis. Thus, T cells cannot respond to antigenic stimulation. (D) Metabolic control by PD1. Engaged PD1 alters T cell metabolism from glycolysis to β-oxidation by inhibition of ERK and PI3K-AKT activities. PD1-stimulated T cells would then metabolically resemble long-lived memory T cells.

Figure 3. Immunotherapies and targeted therapies display distinct patterns of response

The graph displays a model of responses for immunotherapies and targeted therapies such as kinase inhibitors. The percentage of survival is plotted over time after the initial treatment. While targeted therapies exhibit fast anti-tumor effects, tumors acquire resistance to treatments. This is exemplified as a sudden drop of survival as indicated within the dotted square on the right. In contrast, immunotherapies show a significant group of patients that are intrinsically resistant to therapy, evidenced as an initial sudden drop in survival as indicated within the dotted square on the left. Then, long-term survivors are found with durable responses. So far, there are not effective biomarkers discriminating patients that could benefit from immunotherapies from those with intrinsic resistance.