The immune system is a collection of cells, tissues, and organs in the body that defend against attacks by “foreign” invaders. Once the invader is recognized, the immune response is triggered. T cells become activated after recognizing major histocompatibility complex (MHC) molecules on antigen-presenting cells (APCs) with the T cell receptor, and co-signaling molecules. In the human body, the immune response needs to be finely tuned, because an immune response that is too strong may lead to the body attacking its own healthy tissue and cells, while a weak immune response fails to protect the body from invaders. The immune system has its own immunoregulators, referred to as immune checkpoints. The immune checkpoint molecules have both positive and negative modulatory effects on T cell activation.

Immune checkpoint therapies, by targeting regulatory pathways in T cells to enhance antitumor immunity, are the focus of research on the treatment of several forms of cancer. Due to the dynamic nature of the immune response and multifaceted regulation of immune signaling pathways, immunity can overshoot its targets and attack healthy tissues or organs after release from checkpoint arrest. The way forward for immune checkpoint therapy lies in better understanding of the interaction between the immune system and tumor cells.


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Key immune checkpoint targets for detection using antibodies

Summaries of key immune checkpoint targets

T cell activation is generally self-limited, as activated T cells express receptors such as PD-1 (also known as PDCD-1) that mediate inhibitory signals from the antigen-presenting cells (APC). PD-1 can bind to two different but related ligands, PD-L1 and PD-L2. Upon binding to either of these ligands, signals generated by PD-1 decrease TCR-mediated proliferation and cytokine production. Increased PDL-1 expression is associated with many murine and human cancers and can be further up-regulated upon IFN-gamma stimulation. Thus, PD-L1 might play an important role in immune evasion by tumors.

Flow cytometry analysis of PD-1 and B7-H1
Flow cytometry analysis of PD-1 and B7-H1. C57BL/6 splenocytes were stimulated with ConA and then stained with APC anti–mouse CD3e (Cat. No. A18605) and 0.5 µg PE anti–mouse CD279 (PD-1) (Cat. No. A18700) (upper right panel) or 0.5 µg PE Armenian hamster IgG isotype control (upper left panel). Staining of C57BL/6 splenocytes with APC anti–mouse CD3e and PE rat IgG2a kappa isotype control (bottom left panel) or PE anti–mouse CD274 (B7-H1) (Cat. No. A14764) (bottom right panel). Cells in the lymphocyte gate were used for analysis.

DR3/TNFRSF25 is a member of the TNF receptor superfamily. This receptor is expressed preferentially in tissues enriched in lymphocytes, and it may play a role in regulating lymphocyte homeostasis. The sole known ligand for DR3 is the TNF superfamily member TL1A (TNF like cytokine 1A, TNFSF15). The signal transduction of this receptor is mediated by various death domain–containing adaptor proteins. Knockout studies in mice have suggested the role of DR3/TNFRSF25 in the removal of self-reactive T cells in the thymus. Studies have also suggested a role for TL1A and DR3 in various human diseases and animal disease models, including rheumatoid arthritis (RA), collagen-induced arthritis (CIA), and inflammatory bowel disease (IBD).

Immunofluorescence analysis of DR3/TNFRSF25

Immunofluorescence analysis of DR3/TNFRSF25. The experiment was performed on fixed and permeabilized Ramos cells for detection of endogenous DR3/TNFRSF25 using ABfinity Anti-DR3/TNFRSF25 Recombinant Rabbit Monoclonal Antibody (Cat. No. 702277, 2 µg/mL) and labeled with Goat Anti–Rabbit IgG (H+L) Superclonal Secondary Antibody, Alexa Fluor 488 conjugate (Cat. No. A27034, 1:2,000). Panel a shows representative cells that were stained for detection and localization of DR3/TNFRSF25 protein (green); panel b is stained for nuclei (blue) using SlowFade Gold Antifade Mountant with DAPI (Cat. No. S36938); panel c represents cytoskeletal F-actin staining using Alexa Fluor 555 Rhodamine Phalloidin (Cat. No. R415, 1:300); panel d is a composite image of panels a, b, and c clearly demonstrating localization of DR3/TNFRSF25 in the membrane. The images were captured at 60x magnification.


T cell immunoglobin mucin-3 (TIM-3) is expressed on the surface of human myeloid leukemia stem cells in many types of human acute myeloid leukemia (AML), but not on hematopoietic stem cells (HSCs). TIM-3 is overexpressed in Hodgkin's disease tissue and might participate in the interaction between the Hodgkin and Reed-Sternberg (H&RS) cells with their surrounding cells. Galectins are a family of beta-galactoside–binding proteins implicated in modulating cell–cell and cell–matrix interactions. Galectin-9 (Gal-9) is the ligand of TIM-3; together these proteins constitute an autocrine loop critical for leukemic stem cell (LSC) self-renewal and development of human AML and thus might play a role in the pathogenesis of this disease and/or its associated immunodeficiency. 

                                         Galectin-9

TIM3/Galectin-9
Immunohistochemical analysis of galectin-9. The target was detected in perfusion-fixed and frozen sections of mouse colon using Rat Anti–Mouse Galectin-9 Monoclonal Antibody (Cat. No. MA5-24369) at 25 µg/mL overnight at 4°C. The tissue was stained using an anti-rat IgG secondary antibody, HRP conjugate, with DAB substrate (brown), and counterstained with hematoxylin (blue). Specific staining was localized to plasma membranes and cytoplasm of epithelial cells.

Immunocytochemical analysis of galectin-9. The target was detected in immersion-fixed mouse splenocytes using Goat Anti–Mouse Galectin-9 Antigen Affinity-Purified Polyclonal Antibody (Cat. No. PA5-47503) at 15 µg/mL for 3 hr at room temperature. Cells were stained using a fluorophore-conjugated anti–goat IgG secondary antibody (red) and counterstained with DAPI (blue). Specific staining was localized to cytoplasm.

Flow cytometry analysis of galectin-9. Mouse thymocytes were stained with Goat Anti–Mouse Galectin-9 Short Isoform Affinity-Purified Polyclonal Antibody (Cat. No. PA5-47503) (orange histogram) or control antibody (open histogram), followed by an APC-conjugated anti–rat IgG secondary antibody. To facilitate intracellular staining, cells were fixed with paraformaldehyde and permeabilized with saponin.


CTLA4 is a member of the immunoglobulin superfamily and encodes a protein that transmits an inhibitory signal to T cells. Mutations in this gene have been associated with insulin-dependent diabetes mellitus, Graves’ disease, Hashimoto thyroiditis, celiac disease, systemic lupus erythematosus, thyroid-associated orbitopathy, and other autoimmune diseases. CD80 (B7-1) and CD86 (B7-2) are ligands of T cell critical costimulatory molecule CD28 and of an inhibitory receptor CTLA-4 (CD152). Both B7 molecules are expressed on professional antigen-presenting cells and are essential for T cell activation. Differences in CD80 and CD86 competency have not been fully elucidated yet; results from existing studies conflict about their respective roles in initiating or sustaining the T cell immune response.

CTLA4 CD80

Immunocytochemical analysis of CTLA-4. The target was detected in immersion-fixed human peripheral blood mononuclear cells treated with PMA and calcium ionomycin using Goat Anti–Human CTLA-4 Antigen Affinity-Purified Polyclonal Antibody (Cat. No. PA5-47547) at 15 µg/mL for 3 hr at room temperature. Cells were stained using a fluorophore-conjugated anti–goat IgG secondary antibody (red) and counterstained with DAPI (blue). Specific staining was localized to cell surfaces.


TIGIT, a recently identified co-inhibitory receptor, is expressed on the surface of a variety of lymphoid cells, especially on Foxp3+ regulatory T cells. The expression of TIGIT is markedly enriched on tumor-infiltrating T cells. The poliovirus receptor (PVR, CD155), a member of the Ig superfamily with 3 Ig domains in the arrangement V-C-C, is the high-affinity receptor for TIGIT. Binding with its receptor CD155, TIGIT causes increased secretion of IL-10 and decreased secretion of IL-12B and suppresses T cell activation by promoting the generation of mature immunoregulatory dendritic cells.

TIGIT

Immunofluorescence analysis of CD155 (green) showing staining in HeLa cell membranes. Formalin-fixed cells were permeabilized with 0.1% Triton X-100 in TBS for 5–10 min and blocked with 3% BSA-PBS for 30 min at room temperature. Cells were probed with a CD155 monoclonal antibody (Cat. No. MA5-13493) in 3% BSA-PBS at a dilution of 1:20 and incubated overnight at 4°C in a humidified chamber. Cells were washed with PBST and incubated with a DyLight conjugated secondary antibody in PBS at room temperature in the dark. F-actin was stained with phalloidin (red) and nuclei were stained with Hoechst or DAPI (blue). Image was taken at 60x magnification.

LAG3 (CD223) is a cell surface molecule expressed on activated T cells, NK cells, B cells, and plasmacytoid dendritic cells. Binding with its major ligand, Class II MHC, LAG3 plays a role in modulating dendritic cell function. Based on research studies, it is also extensively co-expressed with PD-1 on tumor-infiltrating CD4+ and CD8+ T cells in several transplantable tumors.

LAG3/MHC II

Flow cytometry analysis of human PBMCs. Cells either untreated (A) or treated (B) with 1 µg/mL PHA for 5 days were stained with Goat Anti–Human LAG-3 Antigen Affinity-Purified Polyclonal Antibody (Cat. No. PA5-47298) followed by APC-conjugated anti–goat IgG secondary antibody and PE-conjugated mouse anti–human CD3ε monoclonal antibody. Quadrant markers were set based on control antibody staining.


View available immune checkpoint antibodies

Target Antibodies Target synonyms
B7-H3/CD276 B7-H3 Antibodies 4Ig-B7-H3; B7 homolog 3; B7-H3; B7H3; B7RP-2; CD276; CD276 antigen; Costimulatory molecule; PSEC0249
B7-H4/VTCN1 B7-H4 Antibodies B7 homolog 4; B7-H4; B7H4; B7S1; B7X; FLJ22418; Immune costimulatory protein B7-H4; PRO1291; Protein B7S1; RP11-229A19.4; T cell costimulatory molecule B7x; V-set domain containing T-cell activation inhibitor 1; VCTN1; vtcn1
B7-H5/VISTA VISTA Antibodies 4632428N05Rik; B7-H5; B7H5; C10orf54; chromosome 10 open reading frame 54; Dies1; GI24; PD-1H; platelet receptor Gi24; PP2135; Protein L259; SISP1; stress induced secreted protein 1; Uncharacterized protein C1orf54 homolog
BTLA/CD272 BTLA Antibodies B and T lymphocyte attenuator; B- and T-lymphocyte attenuator; B- and T-lymphocyte-associated protein; BTLA1; CD272
CD27 CD27 Antibodies CD antigen 27; CD27; CD27 antigen; CD27L receptor; T cell activation antigen CD27; T cell activation antigen S152; T-cell activation antigen CD27; T14; Tumor necrosis factor receptor superfamily member 7; tumor necrosis factor receptor superfamily, member 7
CD28 CD28 Antibodies CD28; CD28 antigen; MGC138290; T-cell-specific surface glycoprotein CD28; Tp44
CD30 CD30 Antibodies CD30; CD30 antigen; CD30L receptor; cytokine receptor CD30; Ki-1 antigen; Lymphocyte activation antigen CD30; Tumor necrosis factor receptor superfamily member 8; tumor necrosis factor receptor superfamily, member 8
CD30L/CD153 CD153 Antibodies CD153; CD30 ligand; CD30-L; Cd30l; CD30LG; Tumor necrosis factor ligand superfamily member 8
CD40 CD40 Antibodies B-cell surface antigen CD40; Bp50; CD40; CD40L receptor; CDW40; MGC9013; T-cell differentiation antigen; TNFRSF5; Tumor necrosis factor receptor superfamily member 5; tumor necrosis factor receptor superfamily, member 5
CD40L/CD154 CD154 Antibodies CD154; CD40 antigen ligand; CD40 ligand; CD40-L; Cd40l; gp39; HIGM1; IGM; IMD3; Ly-62; Ly62; RP23-153G22.3; T-B cell-activating molecule; T-BAM; T-cell antigen Gp39; TNF-related activation protein; Tnfsf5; TRAP; tumor necrosis factor (ligand) superfamily member 5; Tumor necrosis factor ligand superfamily member 5
CD70 CD70 Antibodies CD27 ligand; CD27-L; CD27L; CD27LG; CD70; CD70 antigen; Ki-24 antigen; member 7; surface antigen CD70; TNFSF7; tumor necrosis factor (ligand) superfamily, member 7; tumor necrosis factor ligand superfamily; Tumor necrosis factor ligand superfamily member 7
CD80 CD80 Antibodies Activation B7-1 antigen; B-lymphocyte activation antigen B7; B7; B7-1; B7.1; BB1; Cd28l; CD28LG; CD28LG1; CD80; CD80 antigen (CD28 antigen ligand 1, B7-1 antigen); costimulatory factor CD80; costimulatory molecule variant IgV-CD80; CTLA-4 counter-receptor B7.1; LAB7; Ly-53; Ly53; MIC17; T-lymphocyte activation antigen CD80; TS/A-1
CD86 CD86 Antibodies Activation B7-2 antigen; B-lymphocyte activation antigen B7-2; B-lymphocyte antigen B7-2; B7-2; B7-2 antigen; B7.2; B70; BU63; CD28 antigen ligand 2; CD28LG2; CD86; CD86 antigen (CD28 antigen ligand 2, B7-2 antigen); CTLA-4 counter-receptor B7.2; FUN-1; LAB72; MGC34413; T-lymphocyte activation antigen CD86
CD96/Tactile CD96 Antibodies CD96; CD96 antigenMGC22596; CD96 molecule; Cell surface antigen CD96; DKFZp667E2122; T cell activation, increased late expression; T cell-activated increased late expression protein; T-cell surface protein tactile; TACTILE; TACTILEincreased late expression
CD112/Nectin-2 Nectin 2 Antibodies CD112; Herpes virus entry mediator B; Herpesvirus entry mediator B; herpesvirus entry protein B; MYH2B; MyHC-2B; Nectin-2; poliovirus receptor-like 2; poliovirus receptor-related 2 (herpesvirus entry mediator B); Poliovirus receptor-related protein 2
CD134/OX40 OX40 Antibodies ACT35 antigen; ATC35 antigen; CD134; CD134 antigen; cluster designation 134; lymphoid activation antigene ACT35; OX40 antigen; OX40 cell surface antigen; OX40 homologue; OX40L receptor; TAX transcriptionally-activated glycoprotein 1 receptor; tax-transcriptionally activated glycoprotein 1 receptor; TNFRSF4; Tumor necrosis factor receptor superfamily member 4; tumor necrosis factor receptor superfamily, member 4
CD137/4-1BB CD137 Antibodies 4-1BB ligand receptor; CD137; CD137 antigen; CDw137; homolog of mouse 4-1BB; induced by lymphocyte activation (ILA); interleukin-activated receptor, homolog of mouse Ly63; receptor protein 4-1BB; T cell antigen ILA; T-cell antigen 4-1BB homolog; T-cell antigen ILA; Tumor necrosis factor receptor superfamily member 9; tumor necrosis factor receptor superfamily, member 9
CD137L/4-1BB L 4-1BB Ligand Antibodies 4-1BB ligand; 4-1BB-L; 4-1BBL; CD137 Ligand; homolog of mouse 4-1BB-L; receptor 4-1BB ligand; TNFSF9; Tumor necrosis factor ligand superfamily member 9
CD152/CTLA-4 CTLA-4 Antibodies CD152; CD152 isoform; celiac disease 3; CTLA-4; cytotoxic T lymphocyte associated antigen 4 short spliced form; Cytotoxic T-lymphocyte protein 4; Cytotoxic T-lymphocyte-associated antigen 4; cytotoxic T-lymphocyte-associated protein 4; cytotoxic T-lymphocyte-associated serine esterase-4; insulin-dependent diabetes mellitus 12; ligand and transmembrane spliced cytotoxic T lymphocyte associated antigen 4
CD155/PVR CD155 Antibodies CD155; NECL-5; nectin-like 5; Nectin-like protein 5; Poliovirus receptor; PVR; PVS
CD223/LAG3 CD223 Antibodies CD223; LAG-3; Ly66; Lymphocyte activation gene 3 protein; lymphocyte-activation gene 3; Protein FDC
CD226/DNAM1 CD226 Antibodies CD226; CD226 antigen; DNAM1; platelet and T cell activation antigen 1; Platelet and T-cell activation antigen 1; PTA1; TLiSA1
CD252/OX40L OX40L Antibodies Ath-1; Ath1; atherosclerosis 1; CD252; gp34; OX40 Ligand; OX40L; tax-transcriptionally activated glycoprotein 1 ligand; TNFSF4; Tumor necrosis factor ligand superfamily member 4; TXGP1; Txgp1l
CD258/LIGHT LIGHT Antibodies TNFSF14; LTg; CD258; HVEML; LIGHT; tumor necrosis factor superfamily member 14; tumor necrosis factor ligand superfamily member 14; herpesvirus entry mediator ligand; tumor necrosis factor (ligand) superfamily, member 14; tumor necrosis factor ligand 1D
CD273/PD-L2 CD273 Antibodies B7 dendritic cell molecule; B7-DC; B7DC; bA574F11.2; Btdc; Butyrophilin B7-DC; butyrophilin-like protein; CD273; PD-1 ligand 2; PD-1-ligand 2; PD-L2; PDCD1 ligand 2; PDCD1L2; PDL2; Programmed cell death 1 ligand 2; programmed death ligand 2
CD274/PD-L1 PD-L1 Antibodies B7 homolog 1; B7-H; B7-H1; B7H1; CD274; CD274 antigen; PD-L1; PDCD1 ligand 1; PDCD1L1; PDCD1LG1; PDL1; Programmed cell death 1 ligand 1; programmed death ligand 1
CD278/ICOS ICOS Antibodies Activation-inducible lymphocyte immunomediatory molecule; AILIM; CD278; CVID1; inducible costimulator; inducible T-cell co-stimulator; Inducible T-cell costimulator
CD279/PD-1 PD-1 Antibodies CD279; hPD-1; hPD-l; mPD-1; PD1; Programmed cell death protein 1; Protein PD-1; SLEB2
CD357/GITR CD357 Antibodies AITR; CD357; GITR-D; Glucocorticoid-induced TNFR-related protein; TNFRSF18; Tumor necrosis factor receptor superfamily member 18
DR3/TNFRSF25 DR3 Antibodies Apo-3; APO3; apoptosis inducing receptor; Apoptosis-inducing receptor AIR; Apoptosis-mediating receptor DR3; Apoptosis-mediating receptor TRAMP; DDR3; death domain receptor 3 soluble form; Death receptor 3; death receptor beta; DR3; LARD; Lymphocyte-associated receptor of death; Protein WSL; Protein WSL-1; TNFRSF12; Tumor necrosis factor receptor superfamily member 25; tumor necrosis factor receptor superfamily, member 12; tumor necrosis factor receptor superfamily, member 12 (translocating chain-association membrane protein); tumor necrosis factor receptor superfamily, member 25; WSL; WSL1
Galectin-9 Galectin 9 Antibodies Ecalectin; Gal-9; galectin 9; Galectin-9; HUAT; lectin, galactoside binding soluble 9; lectin, galactoside-binding, soluble, 9; LGALS9A; Tumor antigen HOM-HD-21; urate transporter/channel protein
GITRL GITRL Antibodies Activation-inducible TNF-related ligand; AITR ligand; AITRL; GITR ligand; GITRL; Glucocorticoid-induced TNF-related ligand; glucocorticoid-induced TNFR-related protein ligand; glucocorticoid-induced-tumor necrosis factor receptor ligand; hGITRL; RP1-15D23.1, AITRL, GITRL, TL6, hGITRL; tumor necrosis factor (ligand) superfamily member 18; tumor necrosis factor (ligand) superfamily, member 18; tumor necrosis factor ligand 2A; Tumor necrosis factor ligand superfamily member 18
HVEM TNFRSF14 Antibodies ATAR; herpes virus entry mediator; HVEA; HVEM; LIGHTR; RP3-395M20.6; tumor necrosis factor receptor superfamily member 14
ICOSL/B7RP1/B7-H2 ICOSLG Antibodies B7 homolog 2; B7 homologue 2; B7-like protein Gl50; B7-related protein 1; B7H2; B7RP1; GL50; ICOS Ligand; ICOS-L; ICOSL; inducible T-cell co-stimulator ligand; KIAA0653; transmembrane protein B7-H2 ICOS ligand
IDO IDO Antibodies 3-dioxygenase; IDO-1; indolamine 2,3 dioxygenase; indole 2,3-dioxygenase; Indoleamine 2,3-dioxygenase 1; Indoleamine-2; Indoleamine-pyrrole 2,3-dioxygenase
TIGIT TIGIT Antibodies T-cell immunoreceptor with Ig and ITIM domains; tigit; V-set and immunoglobulin domain containing 9; V-set and transmembrane domain containing 3; V-set and transmembrane domain-containing protein 3; VSIG9; VSTM3; Washington University cell adhesion molecule; WUCAM
TIM-3 TIM3 Antibodies HAVcr-2; HAVCR2; Hepatitis A virus cellular receptor 2; kidney injury molecule-3; T cell immunoglobulin mucin 3; T cell immunoglobulin mucin-3; T-cell immunoglobulin and mucin domain-containing protein 3; T-cell immunoglobulin mucin family member 3; T-cell immunoglobulin mucin receptor 3; T-cell membrane protein 3; TIM-3; TIMD-3
TL1A TL1A Antibodies bM20K13.3 (tumor necrosis factor (ligand) superfamily, member 15); MGC129934; MGC129935; TL1; TL1A; TNF ligand-related molecule 1; TNF superfamily ligand TL1A; tumor necrosis factor (ligand) superfamily member 15; tumor necrosis factor (ligand) superfamily, member 15; tumor necrosis factor ligand 1B; Tumor necrosis factor ligand superfamily member 15; Vascular endothelial cell growth inhibitor; vascular endothelial growth inhibitor; vascular endothelial growth inhibitor-192A; VEGI; VEGI192A