Examkin

Question

Difficulty: Very hardImmunologic and Inflammatory Processes

A 34-year-old woman is admitted to the intensive care unit with a 2-week history of unremitting high-grade fevers, progressive lethargy, and a new-onset generalized tonic-clonic seizure. Her family notes she had a mild upper respiratory tract illness resembling infectious mononucleosis approximately three weeks prior. Temperature is 39.8°C (103.6°F), blood pressure is 90/50 mmHg, and pulse is 125/min. Physical examination reveals massive splenomegaly extending to the umbilicus and diffuse petechiae over the extremities. Neurologic examination shows profound encephalopathy.

Laboratory studies demonstrate:
TestResult
Hemoglobin7.2 g/dL
Platelets24,000/mm³
Leukocytes1,200/mm³ (Absolute neutrophil count: 400/mm³)
Fibrinogen85 mg/dL
Ferritin22,500 ng/mL
Triglycerides420 mg/dL
Soluble CD25Markedly elevated

Bone marrow aspiration reveals prominent macrophages actively engulfing erythrocytes and nucleated hematopoietic cells. Based on the fundamental immunopathogenesis of this patient's condition, the unrestrained activation of these macrophages is most directly driven by the excessive secretion of which of the following mediators from which primary cellular source?

  1. Interferon-gamma from CD8+ cytotoxic T lymphocytes and natural killer cellsAnswer
  2. B
    Interleukin-6 from tissue-resident macrophages
  3. C
    Tumor necrosis factor-alpha from CD4+ helper T cells
  4. D
    Tissue factor from injured vascular endothelial cells
  5. E
    Granulocyte-macrophage colony-stimulating factor from bone marrow stromal cells

Answer

Interferon-gamma from CD8+ cytotoxic T lymphocytes and natural killer cells
The correct answer identifies Interferon-gamma (IFN-gamma) secreted by CD8+ cytotoxic T cells and natural killer (NK) cells as the primary pathogenic driver. This patient's presentation of unremitting fever, massive splenomegaly, severe cytopenias, profound hyperferritinemia, hypofibrinogenemia, hypertriglyceridemia, and elevated soluble CD25 (a marker of intense T-cell activation) is characteristic of Hemophagocytic Lymphohistiocytosis (HLH). In HLH, cytotoxic cells (CD8+ T cells and NK cells) are unable to successfully induce apoptosis in infected antigen-presenting cells. This failure results in a prolonged, ineffective immune synapse and the massive, unregulated release of IFN-gamma. This continuous IFN-gamma exposure directly hyperactivates macrophages, driving them to phagocytose healthy hematopoietic cells (hemophagocytosis) and release large quantities of secondary downstream inflammatory cytokines.

Step-by-Step Solution

1
Identify the clinical syndrome based on the classic pentad of findings.
The patient has Hemophagocytic Lymphohistiocytosis (HLH).
Prolonged fever, severe cytopenias, splenomegaly, extreme hyperferritinemia, hypofibrinogenemia, hypertriglyceridemia, hemophagocytosis, and elevated soluble CD25 are highly specific diagnostic criteria for HLH.
2
Determine the inciting cellular defect.
Cytotoxic T lymphocytes and NK cells fail to properly clear the virally infected cells.
Secondary HLH is often triggered by viral infections (like Epstein-Barr virus) where an acquired or genetic defect in the cytolytic exocytosis pathway prevents effective target cell lysis.
3
Trace the resulting molecular cascade that drives the disease state.
The prolonged, ineffective immune synapse leads to a continuous, massive outpouring of Interferon-gamma (IFN-gamma).
Without successful target cell apoptosis, the normal negative feedback loop is broken, leaving the T cells and NK cells in a hyperactivated, secretory state.
4
Identify the direct effect on the reticuloendothelial system.
The excessive IFN-gamma hyperactivates tissue macrophages.
IFN-gamma is the most potent macrophage-activating cytokine, turning them into highly aggressive phagocytes that consume healthy host cells and secrete the massive downstream cytokine storm responsible for the critical illness.

Key Concept

The immunopathogenesis of Hemophagocytic Lymphohistiocytosis (HLH) is fundamentally driven by a failure of cytotoxic T cells and NK cells to clear target cells, leading to unrestrained secretion of Interferon-gamma (IFN-gamma), which in turn hyperactivates macrophages.
Rate this question

Topics

Immunologic and Inflammatory Processes

More questions from this topic