A 38-year-old man with chronic lymphocytic leukaemia (CLL) is treated with rituximab, a monoclonal antibody against CD20 on B lymphocytes. Within 48 hours, a marked reduction in circulating B cells is observed. Flow cytometry shows increased annexin V positivity in the remaining CLL cells. Which pathway of apoptosis is primarily responsible for rituximab-induced cell death in this patient?

Explanation

## Rituximab-Induced Apoptosis: The Extrinsic Pathway ### Mechanism of Rituximab Action Rituximab is a chimeric monoclonal antibody (IgG1) that binds to **CD20**, a B-cell surface antigen. CD20 clustering by rituximab initiates three mechanisms of B-cell death: 1. **Extrinsic apoptosis** (primary in CLL) 2. Complement-dependent cytotoxicity (CDC) 3. Antibody-dependent cellular cytotoxicity (ADCC) ### The Extrinsic Pathway: Death Receptor Signalling **Key Point:** CD20 clustering recruits adaptor proteins including **FADD** (Fas-Associated protein with Death Domain) and **caspase-8** into a signalling complex called the **DISC** (Death-Inducing Signalling Complex). Sequential steps: 1. **CD20 clustering** by rituximab → conformational change 2. **FADD recruitment** → brings pro-caspase-8 molecules into proximity 3. **Caspase-8 activation** → initiator caspase of the extrinsic pathway 4. **Caspase-8 cleaves caspase-3** (executioner caspase) → apoptosis 5. **Caspase-8 can also cleave Bid** → amplification via the intrinsic pathway (crosstalk) ### Why This Is Extrinsic, Not Intrinsic | Feature | Rituximab-Induced Apoptosis | |---------|-----------------------------| | **Trigger** | Antibody-mediated CD20 clustering (external signal) | | **Initiator Caspase** | Caspase-8 (not caspase-9) | | **Mitochondrial Involvement** | Optional (via Bid cleavage); not required | | **p53 Dependence** | No — works in p53-mutant CLL | | **DISC Formation** | Yes — FADD + caspase-8 | | **Annexin V Positivity** | Yes — phosphatidylserine exposure on outer membrane | **High-Yield:** Rituximab is effective in CLL even when p53 is mutated (del(17p)), because it uses the extrinsic pathway, which is p53-independent. This is a major advantage over chemotherapy in high-risk CLL. **Clinical Pearl:** Annexin V binds to phosphatidylserine (PS), which is externalized on apoptotic cells. Its presence confirms apoptosis is occurring, not just cell lysis. ### Intrinsic vs. Extrinsic Pathways: Key Distinction ```mermaid flowchart TD A[Cell Stress/Signal]:::outcome --> B{Type of Trigger?}:::decision B -->|DNA Damage, Hypoxia, Growth Factor Withdrawal| C[Intrinsic Pathway]:::outcome B -->|Death Receptor Clustering| D[Extrinsic Pathway]:::outcome C --> E[p53 activation]:::action C --> F[Bcl-2 family upregulation]:::action F --> G[Mitochondrial MOMP]:::action G --> H[Cytochrome c release]:::action H --> I[Apoptosome + Caspase-9]:::action D --> J[DISC formation]:::action J --> K[Caspase-8 activation]:::action K --> L[Caspase-3 activation]:::action I --> M[Apoptosis]:::outcome L --> M ``` **Mnemonic:** **DISC** = Death-Inducing Signalling Complex (extrinsic pathway hallmark). **MOMP** = Mitochondrial Outer Membrane Permeabilization (intrinsic pathway hallmark).