B-Cell Targeted Therapies
Anti-CD20 antibodies, BAFF inhibitors, BTK inhibitors, and plasma cell-targeting agents -- how B-cell depletion and modulation treat autoimmune disease.
## B-Cell Biology and Drug Targets
B lymphocytes contribute to autoimmune disease through multiple mechanisms: autoantibody production, antigen presentation to T cells, and cytokine secretion (IL-6, TNF, lymphotoxin). B-cell depletion can be therapeutic even in diseases not traditionally considered antibody-mediated, highlighting these non-antibody functions.
Key B-cell surface targets include CD20 (expressed from pre-B to mature B cell, absent on stem cells and plasma cells), CD19, BAFF receptor, and BCMA (B-cell maturation antigen, expressed on plasma cells).
## Anti-CD20 Antibodies
**Rituximab** was the first anti-CD20 monoclonal antibody, originally approved for B-cell lymphoma. It depletes B cells via complement-dependent cytotoxicity (CDC), antibody-dependent cellular cytotoxicity (ADCC), and direct apoptosis. Off-label and approved autoimmune uses include rheumatoid arthritis (with methotrexate), granulomatosis with polyangiitis, and pemphigus vulgaris.
**Ocrelizumab** is a humanized anti-CD20 with enhanced ADCC and reduced CDC, approved for relapsing and primary progressive multiple sclerosis. **Ofatumumab** targets a distinct CD20 epitope and is approved for relapsing MS as monthly subcutaneous self-injection.
**Obinutuzumab** is glycoengineered for enhanced ADCC, used in CLL and follicular lymphoma.
## BAFF/APRIL Pathway Inhibitors
B-cell activating factor (BAFF/BLyS) and APRIL are TNF superfamily members that promote B-cell survival and class switching. **Belimumab** is a monoclonal antibody against soluble BAFF, approved for systemic lupus erythematosus (SLE) and lupus nephritis. It reduces disease flares and corticosteroid requirements. **Atacicept** targets both BAFF and APRIL but failed in MS trials due to unexpected disease worsening.
## BTK Inhibitors in Autoimmunity
Bruton's tyrosine kinase (BTK) is essential for B-cell receptor signaling. Originally developed for B-cell malignancies (ibrutinib, acalabrutinib, zanubrutinib), selective BTK inhibitors are now in late-stage trials for SLE, rheumatoid arthritis, and MS. Their advantage over anti-CD20 is reversible B-cell inhibition without depletion, potentially preserving vaccine responses.
## Practical Considerations
B-cell depletion causes prolonged hypogammaglobulinemia in some patients, increasing sinopulmonary infection risk. Immunoglobulin levels should be monitored, with IgG replacement considered if levels fall below 4 g/L with recurrent infections. Hepatitis B screening is mandatory, as B-cell depletion can trigger fatal reactivation. Vaccine responses are impaired for 6-12 months after rituximab; plan vaccinations before initiating therapy.
## Key Takeaways
- Anti-CD20 antibodies deplete B cells via CDC, ADCC, and apoptosis mechanisms
- B-cell depletion benefits extend beyond antibody-mediated diseases
- BAFF inhibition with belimumab reduces lupus flares with favorable safety
- BTK inhibitors offer reversible B-cell modulation without depletion