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Difficulty: HardImmunologic and Inflammatory Processes

An oncologist requests a consultation in the emergency department for a 67-year-old woman complaining of frequent, voluminous, non-bloody diarrhea. She was diagnosed with advanced non-small cell lung cancer four months ago and recently completed her third cycle of cemiplimab therapy. She reports up to 10 bowel movements daily, accompanied by diffuse abdominal cramping, but denies fever, sick contacts, or recent antibiotic use. Vital signs are stable. Physical examination reveals mild lower abdominal tenderness without guarding or rebound. Laboratory studies show mild hypokalemia and a normal white blood cell count. Stool testing for Clostridioides difficile toxin and routine enteric pathogens is negative. Flexible sigmoidoscopy demonstrates diffuse mucosal erythema, loss of vascular pattern, and ulceration. Biopsy reveals dense cytotoxic T-lymphocytic infiltrates in the lamina propria with crypt abscesses. The pathogenesis of this patient's current gastrointestinal condition is most directly linked to pharmacological interference with which of the following immunologic mechanisms?

  1. Maintenance of peripheral self-toleranceAnswer
  2. B
    Elimination of autoreactive T cells in the thymic medulla
  3. C
    Fas-mediated apoptosis of autoreactive lymphocytes
  4. D
    Generation of FOXP3-positive regulatory T cells
  5. E
    Intestinal mucosal protection via secretory IgA

Answer

Maintenance of peripheral self-tolerance
The correct answer accurately identifies the role of the PD-1/PD-L1 pathway. Cemiplimab is an anti-PD-1 monoclonal antibody used in cancer immunotherapy. Under normal physiological conditions, PD-1 on T cells binds to PD-L1 expressed on peripheral tissues, delivering an inhibitory signal that prevents unchecked T-cell activation and autoimmune tissue damage (a mechanism known as peripheral tolerance). By blocking this interaction, cemiplimab enhances the anti-tumor T-cell response but simultaneously disrupts peripheral tolerance, leading to immune-related adverse events (irAEs) such as the severe lymphocytic colitis seen in this patient.

Step-by-Step Solution

1
Analyze the patient's presentation and medication history.
The patient developed severe, non-infectious colitis featuring cytotoxic T-lymphocyte infiltration shortly after receiving cemiplimab for lung cancer.
Establishing the temporal relationship between the antineoplastic drug and the acute colonic inflammation is essential for diagnosis.
2
Identify the pharmacological class and target of the medication.
Cemiplimab is an immune checkpoint inhibitor, specifically a monoclonal antibody directed against Programmed Cell Death Protein 1 (PD-1).
Understanding the drug's target is necessary to deduce the underlying immunopathologic mechanism of the adverse event.
3
Link the drug's target to foundational immunologic principles.
The PD-1 receptor is expressed on mature T cells; its interaction with PD-L1 on tissue cells normally induces T-cell exhaustion, serving as a critical checkpoint for peripheral tolerance.
Immune-related adverse events (irAEs) like colitis occur because blocking this checkpoint removes the "brakes" on the immune system, allowing autoreactive T cells to attack healthy peripheral tissues.

Key Concept

Immune checkpoint inhibitors (e.g., anti-PD-1 agents like cemiplimab, pembrolizumab, nivolumab) cause immune-related adverse events, such as severe colitis, by blocking the PD-1/PD-L1 pathway, which normally maintains peripheral self-tolerance by inhibiting autoreactive T-cell activation in host tissues.
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Topics

Immunologic and Inflammatory Processes

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