Immune Checkpoint Inhibitors: Targets, Mechanisms & Research Reagents

Immune checkpoint therapy has transformed cancer treatment. By blocking inhibitory receptors on T cells or their ligands on tumor cells, checkpoint inhibitors unleash anti-tumor immune responses that can produce durable remissions across multiple cancer types. Since the approval of ipilimumab (anti-CTLA-4) in 2011, the field has expanded to include eight approved checkpoint targets and dozens more in clinical development.

This guide provides a research-focused overview of the major immune checkpoint targets, their mechanisms of action, approved and investigational therapeutics, and the research reagents available for studying them in the laboratory. For a broader view of all oncology research areas, see our Cancer Research Reagents hub.

The Checkpoint Landscape

Research Applications

Target expression profiling. Use anti-checkpoint antibodies for IHC on tumor tissue (PD-L1 scoring), flow cytometry on TILs (exhaustion phenotyping), and IF for co-localization studies. Checkpoint expression patterns are closely linked to the broader tumor microenvironment composition.

Functional blocking assays. Neutralizing antibodies and research biosimilars enable cell-based checkpoint blockade assays, T cell re-activation studies, and mixed lymphocyte reactions (MLRs).

PK/ADA bioanalytical assays. Research biosimilar versions of approved checkpoint inhibitors serve as calibration standards and positive controls in pharmacokinetic and anti-drug antibody assays during drug development.

Combination therapy screening. Recombinant checkpoint proteins and matched antibodies enable in vitro screening of dual checkpoint blockade combinations (e.g., PD-1 + LAG-3, PD-1 + TIGIT) before advancing to animal models.

Frequently Asked Questions

Q: Which checkpoint antibodies work for IHC on FFPE tissue?

PD-L1, PD-1, and LAG-3 antibodies are routinely used on FFPE tissue after heat-induced epitope retrieval. CTLA-4 and TIGIT IHC on FFPE requires careful antibody clone selection and optimization. Check the antibody datasheet for validated IHC-P (paraffin) data before purchasing. For diagnostic panel design guidance, see our IHC Panel Design for Cancer Diagnosis guide.

Q: Can I use research biosimilar pembrolizumab in a cell-based checkpoint blockade assay?

Yes. Research biosimilar pembrolizumab binds PD-1 with the same specificity as the clinical drug and can block PD-1/PD-L1 interaction in cell-based reporter assays and MLRs. It is widely used as a positive control and reference standard in checkpoint blockade research. For more on how research biosimilars differ from originator drugs, see our Research Biosimilar Antibodies guide.

Q: How do I build a T cell exhaustion flow cytometry panel?

A comprehensive exhaustion panel should include: PD-1, LAG-3, TIM-3, and TIGIT on CD8+ T cells (gated as CD3+CD8+). Add CD39 (ENTPD1) as a marker of antigen-experienced exhausted T cells, and TCF1 (intracellular) to distinguish progenitor exhausted from terminally exhausted subsets. Use fluorochrome-conjugated monoclonal antibodies with minimal spectral overlap. For step-by-step staining protocols and gating strategies, see our Flow Cytometry Staining Guide and TIL Analysis Guide.

References

1. Pardoll DM. The blockade of immune checkpoints in cancer immunotherapy. Nat Rev Cancer. 2012;12(4):252-264. doi: 10.1038/nrc3239

2. Topalian SL, Drake CG, Pardoll DM. Immune checkpoint blockade: a common denominator approach to cancer therapy. Cancer Cell. 2015;27(4):450-461. doi: 10.1016/j.ccell.2015.03.001

3. Wei SC, Duffy CR, Allison JP. Fundamental mechanisms of immune checkpoint blockade therapy. Cancer Discov. 2018;8(9):1069-1086. doi: 10.1158/2159-8290.CD-18-0367

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