Mechanism of Action of Rituximab
Rituximab is a chimeric murine-human monoclonal antibody that targets the CD20 antigen expressed on the surface of pre-B and mature B-lymphocytes, mediating B-cell lysis through complement-dependent cytotoxicity (CDC) and antibody-dependent cell-mediated cytotoxicity (ADCC). 1
Primary Mechanism
Rituximab functions by specifically binding to the CD20 protein, which is present on B cells from the pre-B stage through mature B cells (but not on plasma cells or stem cells). Upon binding to CD20, rituximab triggers B-cell destruction through two main immune-mediated pathways 1:
- Complement-Dependent Cytotoxicity (CDC): The antibody-antigen complex activates the complement cascade, leading to direct cell lysis
- Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC): Natural killer cells and other immune effector cells recognize the Fc portion of rituximab bound to B cells and destroy them
Additional Mechanisms
Beyond these classical immune mechanisms, rituximab also induces direct cell death through intracellular signaling pathways 2, 3. This includes:
- Lipid raft modifications on the cell membrane
- Activation of apoptotic pathways through caspase activation
- Effects on intracellular kinases and transcription factors
Clinical Pharmacodynamics
The functional consequence of rituximab's mechanism is rapid and sustained B-cell depletion 1:
- Circulating CD19-positive B cells deplete within 2-3 weeks of first infusion
- Depletion persists for 6-9 months in most patients
- B-cell recovery typically begins around 6 months, with median levels returning to normal by 12 months post-treatment
- A small proportion (~4%) may experience prolonged depletion lasting more than 3 years
Disease-Specific Relevance
In rheumatoid arthritis, B cells contribute to pathogenesis through multiple mechanisms including production of rheumatoid factor and autoantibodies, antigen presentation, T-cell activation, and proinflammatory cytokine production—all of which are disrupted by rituximab-mediated B-cell depletion 1.
In chronic graft-versus-host disease (cGVHD), B cells are implicated in pathogenesis, making rituximab's anti-CD20 mechanism particularly effective for steroid-refractory cases, especially those with skin involvement 4.
Important Caveats
- Rituximab does not target plasma cells (which lack CD20 expression), limiting its effect on established antibody production
- Serum immunoglobulin levels (particularly IgM) decrease during treatment, with potential infectious complications 1
- The relative contribution of CDC versus ADCC versus direct signaling to clinical efficacy varies by disease context and is not fully defined 5, 6