A 58-year-old man with a 10-year history of chronic hepatitis C presents with progressive liver dysfunction. A liver biopsy shows extensive hepatocyte apoptosis with numerous Councilman bodies (apoptotic hepatocytes). Immunohistochemistry demonstrates increased infiltration of CD8+ T lymphocytes in the portal tracts and lobules. Serum transaminases are markedly elevated (ALT 320 U/L, AST 380 U/L). The pathologist notes that the pattern of cell death is consistent with cytotoxic T lymphocyte-mediated apoptosis. Which of the following best describes the primary molecular mechanism by which these CD8+ T cells are inducing hepatocyte death in this patient?

Explanation

## Cytotoxic T Lymphocyte-Mediated Apoptosis in Chronic Hepatitis C ### Clinical Context: Councilman Bodies **Key Point:** Councilman bodies are **apoptotic hepatocytes** — a hallmark of viral hepatitis, particularly hepatitis C. They represent the result of CD8+ T cell-mediated killing of infected hepatocytes. **High-Yield:** The presence of CD8+ T lymphocytes in the portal tracts and lobules, combined with Councilman bodies, is pathognomonic for **cytotoxic T lymphocyte (CTL)-mediated hepatocyte apoptosis**. ### The Correct Mechanism: Extrinsic Pathway with Intrinsic Amplification CD8+ T cells kill target cells primarily through **two parallel mechanisms**: #### 1. **Perforin/Granzyme B Pathway (Direct)** - Perforin forms pores in the target cell membrane - Granzyme B enters through these pores - Granzyme B directly activates **caspase-3** (and caspase-10) - This is a **fast, intrinsic pathway-independent mechanism** #### 2. **Fas Ligand/TRAIL Pathway (Extrinsic with Amplification)** - CD8+ T cells express **FasL (CD95L)** and **TRAIL** - FasL binds **Fas (CD95)** on hepatocytes - **Death-Inducing Signaling Complex (DISC)** assembles - **Caspase-8** is activated (initiator caspase) - Caspase-8 cleaves **Bid → tBid** (BH3-only protein) - tBid translocates to mitochondria → **MOMP** → cytochrome c release - **Apoptosome** forms → **caspase-9** activation → **caspase-3** activation **Clinical Pearl:** The second mechanism (Fas/TRAIL) is particularly important in hepatitis C because it allows **amplification** of the apoptotic signal through the intrinsic mitochondrial pathway. This is why hepatitis C causes such extensive hepatocyte death — both extrinsic and intrinsic pathways are engaged. ```mermaid flowchart TD A["CD8+ T cell"]:::outcome B["Perforin/Granzyme B<br/>OR<br/>FasL/TRAIL"]:::action C{"Mechanism?"}:::decision D["Perforin pores +<br/>Granzyme B entry"]:::action E["Direct caspase-3<br/>activation"]:::action F["Fas/TRAIL engagement"]:::action G["DISC formation +<br/>Caspase-8 activation"]:::action H["Bid cleavage → tBid"]:::action I["Mitochondrial MOMP"]:::action J["Cytochrome c release"]:::action K["Apoptosome + Caspase-9"]:::action L["Caspase-3 activation"]:::action M["Hepatocyte apoptosis<br/>(Councilman body)"]:::urgent A --> B B --> C C -->|Fast| D C -->|Amplified| F D --> E F --> G G --> H H --> I I --> J J --> K K --> L E --> M L --> M ``` ### Why Option 1 Is Incomplete While perforin/granzyme B **does** directly activate caspase-3 and **can** cause apoptosis without mitochondrial involvement, the question stem specifically mentions the **Fas ligand-mediated pathway** (implied by the CD8+ T cell infiltration pattern in chronic viral hepatitis). The **most complete answer** includes the extrinsic pathway with intrinsic amplification via Bid cleavage — this is the dominant mechanism in hepatitis C. ### Mnemonic: **DISC and tBid** **DISC** = Death-Inducing Signaling Complex (extrinsic pathway) **tBid** = truncated Bid (links extrinsic to intrinsic pathway) When Fas is engaged → DISC forms → caspase-8 → Bid cleavage → tBid → mitochondrial amplification.