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Zorluk: ZorCellular Injury and Adaptation

Following a recent medical mission in Southeast Asia where she sustained an accidental needlestick injury, a 24-year-old woman develops progressive fatigue, right upper quadrant abdominal pain, and dark urine. Physical examination confirms marked scleral icterus and a tender, palpable liver edge. Laboratory studies reveal severely elevated aspartate aminotransferase (AST) and alanine aminotransferase (ALT), alongside positive serologies for Hepatitis B surface antigen (HBsAg) and IgM anti-HBc. A liver biopsy is obtained to evaluate the extent of injury, demonstrating scattered shrunken, intensely eosinophilic cells with fragmented nuclei, known as Councilman bodies. The targeted elimination of these infected hepatocytes is primarily driven by cytotoxic T lymphocytes.

Arrange the following subcellular events in the correct chronological sequence that leads to the specific histological findings described in this patient's liver biopsy.

  1. 1Binding of the T-lymphocyte Fas ligand to the hepatocyte CD95 surface receptor
  2. 2Oligomerization of receptor intracellular death domains
  3. 3Recruitment of the adapter protein Fas-associated death domain (FADD)
  4. 4Auto-catalytic cleavage and activation of procaspase-8
  5. 5Activation of executioner caspases resulting in DNA fragmentation

Cevap

The correct sequence is: Binding of Fas ligand to CD95 -> Oligomerization of death domains -> Recruitment of FADD -> Cleavage of procaspase-8 -> Activation of executioner caspases.
The clinical vignette describes a patient with acute Hepatitis B viral infection. The liver biopsy reveals Councilman bodies, which represent apoptotic hepatocytes. In viral hepatitis, infected cells are cleared by CD8+ cytotoxic T lymphocytes. One of the primary mechanisms by which CD8+ T cells induce apoptosis in target cells is the extrinsic pathway via Fas ligand (FasL). The correct chronological sequence of this cascade begins with the T cell's FasL binding to the Fas receptor (CD95) on the hepatocyte. This interaction induces trimerization of the receptor's intracellular death domains. The clustered death domains act as a docking site for the adapter protein FADD (Fas-associated death domain). FADD then recruits procaspase-8, leading to its proximity-induced auto-catalytic cleavage into active caspase-8. Finally, initiator caspase-8 cleaves and activates executioner caspases (such as caspase-3), which activate endonucleases and proteases that dismantle the cell, resulting in the characteristic shrunken, eosinophilic appearance of the Councilman body.

Adım Adım Çözüm

1
Diagnose the clinical condition and mechanism of injury.
The patient has acute Hepatitis B with liver biopsy showing Councilman bodies, which are apoptotic hepatocytes killed by CD8+ cytotoxic T cells.
Identifying the mechanism (apoptosis via T-cells) is required to determine which cellular cascade is active.
2
Identify the specific apoptotic pathway utilized by cytotoxic T cells.
Cytotoxic T cells induce apoptosis in target cells primarily through the extrinsic Fas/FasL death receptor pathway or the perforin/granzyme pathway. The items listed correspond to the Fas/FasL cascade.
Distinguishing between the intrinsic (mitochondrial) and extrinsic (receptor-mediated) pathways is necessary to sequence the molecular events.
3
Sequence the molecular events of the extrinsic apoptosis pathway.
FasL binds to Fas (CD95) -> Receptors trimerize -> FADD is recruited -> Procaspase-8 auto-cleaves -> Executioner caspases (caspase-3) activate endonucleases.
This is the exact biochemical signaling cascade that translates extracellular immune recognition into programmed cell death.

Anahtar Kavram

The extrinsic (death receptor) pathway of apoptosis is initiated by FasL binding to Fas (CD95), leading to FADD recruitment, caspase-8 activation, and subsequent executioner caspase-mediated cell death (e.g., Councilman bodies in viral hepatitis).
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Konular

Cellular Injury and Adaptation

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