Learning Objectives
Understand the molecular signaling of Immune Checkpoints and how tumors exploit these pathways to cause T-cell exhaustion. Identify the specific roles of PD-1/PD-L1 and CTLA-4, and master the clinical pharmacology of the monoclonal antibodies used to inhibit them.
1. The PD-1 / PD-L1 Pathway
The interaction between PD-1 (Programmed Cell Death Protein 1) on T-cells and its ligands (PD-L1/2) on tumor cells acts as an “off switch” for the immune system. Tumors upregulate these ligands to induce T-cell dysfunction or exhaustion.
| Checkpoint Component | Location | Mechanism of Action |
|---|---|---|
| PD-1 | T-cells | Receives inhibitory signal from tumor cells. |
| PD-L1 / PD-L2 | Tumor Cells / Microenvironment | Sends an inhibitory signal to T-cells to evade attack. |
Activity:
2. The CTLA-4 Pathway
CTLA-4 is a checkpoint that functions during the initial stages of T-cell activation. It outcompetes the stimulatory molecule CD28 for binding to B7 on Antigen-Presenting Cells (APCs), effectively preventing the necessary “second signal” for T-cell activation.
| Molecule | Interaction | Outcome on T-cell |
|---|---|---|
| CD28 + B7 | Costimulatory (Signal 2) | Activation and proliferation. |
| CTLA-4 + B7 | Inhibitory Competition | Inhibition of activation. |
Activity:
3. Therapeutic Checkpoint Inhibitors
Monoclonal antibodies are used to block these inhibitory interactions, allowing the patient’s own immune system to recognize and destroy the cancer.
| Target | Drug Names |
|---|---|
| Anti-PD-1 | Cemiplimab, Nivolumab, Pembrolizumab |
| Anti-PD-L1 | Atezolizumab, Durvalumab, Avelumab |
| Anti-CTLA-4 | Ipilimumab |
Activity
High-Yield Mnemonics & Tips:
- CTLA-4 = Early: Think of CTLA-4 as the checkpoint that acts “early” in the lymph nodes during T-cell priming.
- PD-1 = Peripheral: Think of PD-1 as acting in the “periphery” at the actual tumor site.
- Antibody Suffixes: Most of these end in “-mab” (monoclonal antibody). Ipilimumab is the classic anti-CTLA-4 drug often tested for its association with immune-related adverse effects (colitis, dermatitis).