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Difficulty: HardInfectious Disease Mechanisms

A 75-year-old man is transferred from a skilled nursing facility to the intensive care unit for hypotension, fever, and severe abdominal pain. He recently completed a 10-day course of clindamycin for aspiration pneumonia. Vital signs show a temperature of 38.9°C (102.0°F), pulse of 122/min, and blood pressure of 86/50 mm Hg. Physical examination reveals a distended abdomen with diffuse tenderness and guarding. Laboratory studies show a leukocyte count of 32,000/mm³ and serum creatinine of 2.4 mg/dL. A non-contrast CT scan of the abdomen demonstrates severe, diffuse colonic wall thickening with mucosal edema (thumbprinting). Stool polymerase chain reaction is positive for Clostridioides difficile toxin genes. Arrange the following steps in the correct chronological sequence detailing the cellular pathogenesis of the primary toxin (Toxin B) responsible for this patient's mucosal necrosis.

  1. 1Binding of the holotoxin to surface receptors (e.g., Frizzled-1, CSPG4) on the apical membrane of colonic epithelial cells.
  2. 2Internalization of the toxin-receptor complex into the host cell via receptor-mediated endocytosis.
  3. 3Acidification of the endosomal lumen, inducing a conformational change that inserts the translocation domain into the endosomal membrane.
  4. 4Binding of cytosolic inositol hexakisphosphate (InsP6) to the autoprotease domain, triggering self-cleavage.
  5. 5Release of the active glucosyltransferase domain into the host cell cytosol.
  6. 6Covalent transfer of a glucose moiety to host Rho family GTPases, causing actin cytoskeleton depolymerization and cell rounding.

Answer

The correct sequence begins with receptor binding, followed by endocytosis, endosomal acidification/insertion, InsP6-mediated autoproteolysis, cytosolic release of the active domain, and finally, glycosylation of host Rho GTPases.
The pathogenesis of Clostridioides difficile Toxin B (TcdB) follows an intricate intracellular delivery pathway. It begins with the heavy chain binding to specific host receptors (e.g., Frizzled-1, CSPG4). The toxin is then internalized via receptor-mediated endocytosis. Inside the early endosome, a decrease in pH triggers a conformational change that causes the hydrophobic translocation domain to insert into the endosomal membrane. This exposes the toxin's autoprotease domain to the host cell's cytosol. Cytosolic inositol hexakisphosphate (InsP6) binds to this domain, serving as an allosteric activator that induces autoproteolytic cleavage. This self-cleavage liberates the toxic 'A' subunit (the glucosyltransferase domain) into the cytoplasm. Once free, this domain irreversibly inactivates host Rho family small GTPases (Rho, Rac, and Cdc42) by transferring a glucose moiety to a conserved threonine residue. This inactivation halts actin polymerization, leading to tight junction disruption, profound fluid secretion, and apoptosis, which clinically manifests as pseudomembranous colitis and toxic megacolon.

Step-by-Step Solution

1
Identify the initial contact mechanism.
Binding of the holotoxin to surface receptors on colonic epithelial cells.
Large bacterial exotoxins must first identify and attach to specific host membrane receptors (like Frizzled-1) to initiate entry.
2
Determine the cellular uptake process.
Internalization of the toxin-receptor complex via receptor-mediated endocytosis.
Host cells actively engulf the receptor-toxin complex, sequestering it within an intracellular vesicle (endosome).
3
Analyze the mechanism of endosomal escape.
Acidification of the endosomal lumen inducing a conformational change and membrane insertion.
As endosomal ATPases pump protons into the vesicle, the acidic environment forces the toxin's translocation domain to unfold and embed into the lipid bilayer, creating a pore.
4
Identify the host factor required for toxin activation.
Binding of cytosolic inositol hexakisphosphate (InsP6) triggering self-cleavage.
Clostridioides difficile toxins uniquely rely on the host's cytosolic InsP6 to activate an internal cysteine protease domain, initiating autoproteolysis.
5
Determine the outcome of autoproteolysis.
Release of the active glucosyltransferase domain into the cytosol.
Cleavage separates the catalytic domain from the delivery machinery, allowing it to diffuse freely within the target cell cytoplasm.
6
Identify the ultimate pathogenic molecular target.
Glycosylation of host Rho family GTPases causing actin depolymerization.
The active domain covalently adds glucose to Rho, Rac, and Cdc42, paralyzing actin dynamics. This destroys the cell's structural integrity and tight junctions, forming the pseudomembranes seen clinically.

Key Concept

The intracellular pathogenesis of Clostridioides difficile Toxin B relies on endosomal acidification for membrane insertion, host InsP6 for autoproteolytic release, and glycosylation of Rho GTPases for cytoskeleton destruction.
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Topics

Infectious Disease Mechanisms

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