Immune checkpoints are regulators of the immune system. These pathways are crucial for self-tolerance, which prevents the immune system from attacking cells indiscriminately. However, some cancers can protect themselves from attack by stimulating immune checkpoint targets. Inhibitory checkpoint molecules are targets for cancer immunotherapy due to their potential for use in multiple types of cancers. Currently approved checkpoint inhibitors block CTLA4, PD-1 and PD-L1. For the related basic science discoveries, James P. Allison and Tasuku Honjo won the Tang Prize in Biopharmaceutical Science and the Nobel Prize in Physiology or Medicine in 2018.
Stimulatory checkpoint molecules Four stimulatory checkpoint molecules are members of the tumor necrosis factor (TNF) receptor superfamily—CD27, CD40, OX40, GITR and CD137. Another two stimulatory checkpoint molecules belong to the B7-CD28 superfamily—CD28 itself and ICOS.
CD27: This molecule supports antigen-specific expansion of naïve T cells and is vital for the generation of T cell memory. CD27 is also a memory marker of B cells. CD27's activity is governed by the transient availability of its ligand, CD70, on lymphocytes and dendritic cells. CD27 costimulation is known to suppress Th17 effector cell function. The American biotech company Celldex Therapeutics is working on CDX-1127, an agonistic anti-CD27 monoclonal antibody which in animal models has been shown to be effective in the context of T cell receptor stimulation. CD28: This molecule is constitutively expressed on almost all human CD4+ T cells and on around half of all CD8 T cells. Binding with its two ligands are CD80 and CD86, expressed on dendritic cells, prompts T cell expansion. CD28 was the target of the TGN1412 'superagonist' which caused severe inflammatory reactions in the first-in-man study in London in March 2006. CD40: This molecule, found on a variety of immune system cells including antigen presenting cells has CD40L, otherwise known as CD154 and transiently expressed on the surface of activated CD4+ T cells, as its ligand. CD40 signaling is known to 'license' dendritic cells to mature and thereby trigger T-cell activation and differentiation. A now-defunct Seattle-based biotechnology company called VLST in-licensed an anti-CD40 agonist monoclonal antibody from Pfizer in 2012. The Swiss pharmaceutical company Roche acquired this project when VLST was shut down in 2013. CD122: This molecule, which is the Interleukin-2 receptor beta sub-unit, is known to increase proliferation of CD8+ effector T cells. The American biotechnology company Nektar Therapeutics is working on NKTR-214, a CD122-biased immune-stimulatory cytokine Phase I results announced in Nov 2016. CD137: When this molecule, also called 4-1BB, is bound by CD137 ligand, the result is T-cell proliferation. CD137-mediated signaling is also known to protect T cells, and in particular, CD8+ T cells from activation-induced cell death. The German biotech company Pieris Pharmaceuticals has developed an engineered lipocalin that is bi-specific for CD137 and HER2. OX40: This molecule, also called CD134, has OX40L, or CD252, as its ligand. Like CD27, OX40 promotes the expansion of effector and memory T cells, however it is also noted for its ability to suppress the differentiation and activity of T-regulatory cells, and also for its regulation of cytokine production. OX40's value as a drug target primarily lies in the fact that, being transiently expressed after T-cell receptor engagement, it is only upregulated on the most recently antigen-activated T cells within inflammatory lesions. Anti-OX40 monoclonal antibodies have been shown to have clinical utility in advanced cancer. The pharma company AstraZeneca has three drugs in development targeting OX40: MEDI0562 is a humanised OX40 agonist; MEDI6469, murine OX4 agonist; and MEDI6383, an OX40 agonist GITR: short for Glucocorticoid-Induced TNFR family Related gene, prompts T cell expansion, including Treg expansion. The ligand for GITR is mainly expressed on antigen presenting cells. Antibodies to GITR have been shown to promote an anti-tumor response through loss of Treg lineage stability. The biotech company TG Therapeutics is working on anti-GITR antibodies ICOS: This molecule, short for Inducible T-cell costimulator, and also called CD278, is expressed on activated T cells. Its ligand is ICOSL, expressed mainly on B cells and dendritic cells. The molecule seems to be important in T cell effector function. The American biotechnology company Jounce Therapeutics is developing an ICOS agonist.
Inhibitory checkpoint molecules
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