Belimumab in Systemic Lupus Erythematosus: From B-Cell Biology to Disease Modification.

Systemic lupus erythematosus (SLE) is a heterogeneous autoimmune disease in which B-cell dysregulation plays a central pathogenic role beyond autoantibody production. Advances in B-cell biology have led to the development of targeted therapies, including inhibition of the B-cell activating factor (BAFF) pathway. Belimumab, a monoclonal antibody that neutralizes soluble BAFF, modulates B-cell survival signals upstream, promoting progressive immunologic remodeling rather than rapid depletion. This review integrates current knowledge on BAFF-dependent B-cell biology with mechanistic, pharmacokinetic, and clinical data to provide a comprehensive framework for understanding belimumab's effects in SLE and lupus nephritis (LN). Belimumab preferentially reduces transitional and naïve B cells, while memory B cells show a relative transient increase followed by a gradual return to baseline levels, reflecting redistribution rather than expansion, and long-lived plasma cells are largely unaffected. These effects result in progressive remodeling of B-cell compartment dynamics and contribute to broader modulation of adaptive immune amplification pathways. Pharmacokinetic data support a threshold-based model of BAFF neutralization, with exposure influenced by disease-related factors such as proteinuria in LN. Clinical response is primarily determined by baseline disease biology, with greater benefit observed in patients with serologically active disease and less established organ involvement. Across clinical trials and real-world studies, belimumab reduces disease activity and flares, enables glucocorticoid tapering, and slows organ damage accrual. In LN, it improves renal outcomes and reduces the risk of kidney-related events. Collectively, these findings support belimumab as a disease-modifying therapy in SLE. Further research is needed to refine patient selection and optimize treatment sequencing and combination strategies.