Immune checkpoint pathways

T-Cell Activation - The Two-Signal Handshake

• Signal 1 (Activation): T-cell receptor (TCR) on a naïve T-cell binds to an antigen presented by the Major Histocompatibility Complex (MHC) on an antigen-presenting cell (APC).

  • CD4+ helper T-cells recognize MHC-II.
  • CD8+ cytotoxic T-cells recognize MHC-I.

• Signal 2 (Co-stimulation/Survival): A crucial safety check to prevent autoimmunity.

  • CD28 protein on the T-cell binds to the B7 protein (CD80/CD86) on the APC.

⭐ Without Signal 2, T-cells become anergic (unresponsive) or undergo apoptosis. This is a key mechanism of peripheral tolerance.

Inhibitory Pathways - Pumping the Brakes

• CTLA-4 Pathway (Cytotoxic T-Lymphocyte-Associated Protein 4):

  • Location: Primarily acts in the lymph node during T-cell activation.
  • Mechanism: CTLA-4 on T-cells has a higher affinity for B7 ligands (CD80/86) on antigen-presenting cells (APCs) than the co-stimulatory receptor CD28.
  • Outcome: This competitive binding prevents T-cell co-stimulation, effectively ↓ T-cell activation.

• PD-1 Pathway (Programmed Cell Death Protein 1):

  • Location: Functions later in peripheral tissues and the tumor microenvironment.
  • Mechanism: PD-1 on activated T-cells binds to its ligands, PD-L1 and PD-L2. Tumor cells often upregulate PD-L1 to protect themselves.
  • Outcome: Induces T-cell anergy or exhaustion, ↓ cytokine release, and suppresses cytotoxic T-cell activity.

⭐ Exam Favorite: CTLA-4 blockade acts earlier in the immune response (priming phase in lymph nodes), while PD-1/PD-L1 blockade acts later (effector phase in peripheral tissues).

Checkpoint Inhibitors - Releasing the Hounds

• Mechanism: Monoclonal antibodies that block inhibitory signals on T-cells, unleashing an anti-tumor immune response.
  • CTLA-4 Blockade: Prevents CD28 co-stimulation negation. Primarily acts in lymph nodes.
    • Drug: Ipilimumab
  • PD-1/PD-L1 Blockade: Prevents T-cell "exhaustion" signal. Primarily acts in peripheral tissues.
    • PD-1 Drugs: Nivolumab, Pembrolizumab
    • PD-L1 Drugs: Atezolizumab, Durvalumab, Avelumab

• Clinical Use: Melanoma, Non-Small Cell Lung Cancer (NSCLC), Renal Cell Carcinoma (RCC), Bladder Cancer.

• Adverse Effects (Immune-Related Adverse Events - irAEs):

  • Common: Dermatitis, colitis (diarrhea), hepatitis, endocrinopathies (e.g., hypophysitis, thyroiditis), pneumonitis.
  • 📌 "Ipi, Nivo, Pembro... itis!" - most side effects are inflammatory.

⭐ High-Yield: Management of immune-related adverse events (irAEs) often involves high-dose corticosteroids (e.g., prednisone) and temporary or permanent discontinuation of the checkpoint inhibitor.

High‑Yield Points - ⚡ Biggest Takeaways

• CTLA-4 and PD-1 are crucial inhibitory pathways that downregulate T-cell activation, preventing autoimmunity.
• CTLA-4 outcompetes the co-stimulatory molecule CD28 for its ligand, B7, on antigen-presenting cells.
• PD-1 on activated T-cells binds to PD-L1 on tumor cells, leading to T-cell exhaustion.
• Cancers exploit this by upregulating PD-L1 to hide from the immune system.
• Checkpoint inhibitors block these interactions, “releasing the brakes” on anti-tumor immunity.
• Their main toxicity is immune-related adverse events (irAEs) like colitis, dermatitis, and endocrinopathies.