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