Questions: Immune Checkpoint Molecules and T Cell Exhaustion

Question 1

CTLA-4 and PD-1 are both checkpoint receptors that inhibit T cells, but they act at different stages. Which best describes the distinction?

• A. CTLA-4 acts only in cancer; PD-1 acts in autoimmune disease — they have non-overlapping disease contexts
• B. CTLA-4 competes with CD28 for B7 ligands on APCs, primarily dampening early T cell priming in lymph nodes; PD-1 binds PD-L1/PD-L2 in peripheral tissues to suppress effector T cells already at the site of antigen
• C. CTLA-4 is expressed on B cells; PD-1 is expressed on T cells — they regulate different cell types
• D. CTLA-4 promotes T cell activation while PD-1 inhibits it; together they provide bidirectional control

Answer: (B)

The spatial and temporal distinction is clinically important. CTLA-4 acts early: it is upregulated on T cells during priming in lymph nodes and outcompetes CD28 for B7 (CD80/CD86) binding with higher affinity, blocking the critical costimulatory signal. PD-1 acts late: it is expressed on chronically activated T cells in peripheral tissues and tumors, where PD-L1 (expressed on many normal cells and tumor cells) delivers an inhibitory signal that suppresses proliferation and cytokine production. This is why CTLA-4 and PD-1 blockade have partially complementary mechanisms and can be combined for greater antitumor effect.

Question 2

A patient receives anti-PD-1 therapy for melanoma and develops severe immune-mediated hepatitis (liver inflammation). Which statement best explains this adverse event?

• A. The anti-PD-1 antibody cross-reacted with a liver antigen due to structural similarity
• B. The hepatitis resulted from tumor antigens released as cancer cells died, triggering systemic inflammation
• C. By blocking PD-1 on T cells throughout the body, the therapy removed suppression from T cells that might otherwise attack liver tissue — a direct consequence of the same mechanism that enables antitumor activity
• D. Anti-PD-1 therapy impairs regulatory T cells specifically in the liver, leaving it uniquely vulnerable

Answer: (C)

Immune-related adverse events are a predictable, mechanistic consequence of checkpoint blockade — not off-target drug effects. PD-1 and its ligands normally suppress self-reactive T cells in peripheral tissues, preventing autoimmunity. When PD-1 is blocked systemically, this suppression is reduced everywhere, including in the liver, colon, skin, and endocrine glands. T cells that were being held in check can now attack normal tissue. This is why checkpoint blockade side effects resemble autoimmune conditions and why the same immunosuppressive agents (steroids, infliximab) used for autoimmunity are used to manage them.

Question 3

T cell exhaustion and T cell anergy are functionally equivalent states — both describe T cells that fail to respond to antigen because they lack adequate activation signals.

• T. True
• F. False

Answer: False

Anergy and exhaustion have distinct mechanisms and origins. Anergy results from TCR engagement (signal 1) without costimulation (signal 2) — the T cell was never fully activated. Exhaustion results from prolonged, chronic antigen stimulation of previously activated T cells — the cell was activated but became progressively dysfunctional over time under sustained exposure. Exhausted cells upregulate multiple inhibitory receptors (PD-1, TIM-3, LAG-3, TIGIT) and lose cytokine production and proliferative capacity in a graduated manner. The distinction matters clinically: some exhausted cells retain partial function and can be reinvigorated by checkpoint blockade, whereas anergic cells typically cannot be rescued by the same approach.

Question 4

Checkpoint blockade immunotherapy (anti-PD-1, anti-CTLA-4) works by directly killing tumor cells through antibody-dependent cytotoxicity.

• T. True
• F. False

Answer: False

Checkpoint inhibitors do not target tumor cells — they target inhibitory receptors on T cells. By blocking PD-1 or CTLA-4, the drugs relieve suppression on tumor-specific T cells that were being held in an exhausted or inhibited state. These reinvigorated T cells then recognize and kill tumor cells through cytotoxic mechanisms (perforin/granzyme, Fas-FasL). The drug's target is the immune cell's brake, not the tumor itself. This distinction explains why checkpoint blockade only works when tumor-infiltrating T cells are present and antigen-specific — 'cold' tumors with few infiltrating T cells often do not respond because there is no T cell response to reinvigorate.

Question 5

Why is 'removing the brakes on the immune system' an incomplete description of how checkpoint blockade works, and what important consequence does it obscure?

Model answer: The metaphor implies a simple on/off switch with no downside to releasing inhibition. But immune checkpoints exist for essential physiological reasons: they prevent autoimmunity and limit immunopathological tissue damage during normal immune responses. Blocking PD-1 or CTLA-4 removes inhibitory signaling not just in tumors but throughout the body, including in tissues where self-reactive T cells are being suppressed. The direct result is immune-related adverse events — colitis, hepatitis, pneumonitis, endocrinopathies — that are mechanistically identical to autoimmune disease and can be life-threatening. These are not side effects in the conventional sense; they are the predictable cost of the same biology that enables antitumor efficacy. Additionally, not all exhausted T cells respond: terminally exhausted cells cannot be reinvigorated by checkpoint blockade alone and may require additional costimulation. The more accurate description is that checkpoint blockade shifts the immune activation/suppression balance, with simultaneous effects on tumor and self-tissue immunity.