The biological logic behind B-cell depletion in lupus is compelling. At the heart of the disease lies a breakdown of self-tolerance, with polyclonal activation of B cells that differentiate into plasmablasts and plasma cells churning out autoantibodies. These antibodies bind self-antigens to form immune complexes, which deposit in the kidneys, joints, and skin, activating the complement cascade and driving progressive organ damage. Lupus B cells are also quantitatively and qualitatively abnormal: they overproduce pro-inflammatory cytokines such as tumor necrosis factor, interleukin-6, and interferon-gamma, present autoantigens to T cells in a self-perpetuating activation loop, and are further stimulated when their toll-like receptors engage nucleic acid-protein complexes, the most prominent autoantigens in the disease. Striking at this cellular linchpin, the authors argue, is a rational therapeutic strategy.

Rituximab itself is a chimeric monoclonal antibody, part murine variable region and part human constant region, assembled as an immunoglobulin G1 molecule directed against CD20, a surface marker found on pre-B and mature B cells. Its principal killing mechanism is complement-dependent cytotoxicity, which classifies it as a type I anti-CD20 antibody. Recent structural work has added molecular texture to that picture: researchers solved the structure of CD20 bound to rituximab, revealing a compact double-barrel dimer gripped by two antibody antigen-binding fragments. That architecture suggests rituximab cross-links CD20 into circular assemblies within lipid rafts, a clustering event thought to supercharge complement activation at the cell surface.

The clinical record, however, is more complicated than the biology. Two pivotal randomized, placebo-controlled trials, EXPLORER in extrarenal lupus and LUNAR in lupus nephritis, both missed their primary endpoints, casting a long shadow over the drug. The review’s authors dissect why those trials may have failed on design rather than biology. EXPLORER lumped together heterogeneous disease manifestations, making uniform improvement hard to demonstrate, and both trials layered rituximab on top of intensive background immunosuppression, including high-dose corticosteroids and, in LUNAR, 3 grams per day of mycophenolate mofetil, regimens robust enough to mask any added benefit. The endpoints were also stringent and arguably ill-suited to capturing partial responses, and both trials judged efficacy at 52 weeks, an early timepoint in a disease where clinical responses often take longer to mature. Both trials did confirm that rituximab reliably depletes peripheral B cells, even when that depletion did not translate into measured clinical improvement.

Real-world experience tells a very different story. Observational studies and registries, encompassing cohorts from 18 to 261 patients, have consistently reported response rates between roughly 60 and 85 percent when rituximab is added to standard care in refractory disease. Follow-up periods in these studies range from six months to five years, with only one study extending beyond a decade, a small real-world analysis that documented disease activity improvements persisting at ten years or more. The evidence base is admittedly messy: endpoints vary from clinician judgment to BILAG and SELENA-SLEDAI activity scores, dosing schemes differ across and even within studies, and most analyses lack control groups. Yet the consistency of benefit across so many heterogeneous settings has made rituximab a mainstay for patients whose disease has shrugged off conventional therapy, and scientific society guidelines now sanction its use in refractory SLE.

Nowhere is that benefit clearer than in lupus nephritis, the kidney complication that afflicts roughly 60 percent of patients and can progress to end-stage kidney disease. A Bayesian meta-analysis found that ESKD risk in lupus nephritis fell from the 1970s to the mid-1990s, plateaued, and then paradoxically rose again in the late 2000s despite escalating immunosuppression. Against that trend, the steroid-sparing RITUXILUP protocol stands out: 50 patients received two 1-gram rituximab infusions with limited methylprednisolone and mycophenolate maintenance but no oral steroids, achieving complete and partial renal responses in 52 and 34 percent at 52 weeks. Long-term follow-up of that regimen showed preserved renal function beyond five years. A pooled European analysis of 164 patients with biopsy-proven nephritis found the highest complete response rates in class III and mixed disease, roughly 62 and 50 percent, and the lowest in class IV and pure class V, while nephrotic syndrome and renal failure at treatment initiation predicted worse outcomes. A network meta-analysis ranked rituximab and tacrolimus as the most efficacious induction agents for lupus nephritis.

The stakes of achieving renal response are enormous. Ten-year renal survival reaches 95 percent in patients who achieve a complete response, 45 percent with partial response, and just 19 percent with no response; at 15 years, chronic kidney disease-free survival is 95.2, 87.6, and 55.4 percent respectively. An early proteinuria drop below 0.8 grams per day at 12 months is the best predictor of seven-year renal survival, and each renal flare adds to irreversible damage. Intriguingly, a small biopsy study in membranous lupus nephritis showed that immune complex resorption increased significantly after rituximab but not after conventional immunosuppression, with all seven rituximab-treated patients responding compared with 53 percent of conventionally treated controls, hinting at histological benefit beyond proteinuria numbers.

Perhaps the review’s most provocative thread concerns timing. Data from 131 patients showed that those needing only a single rituximab cycle had multi-organ but non-refractory disease and far less prior immunosuppressive exposure, while patients previously treated with azathioprine or methotrexate were significantly more likely to require repeated cycles. A multicenter study of 147 patients found treatment failure in 27 percent at six months, with fewer prior immunosuppressants and higher complement C4 levels protecting against failure. In newly diagnosed patients, early rituximab plus cyclophosphamide produced dramatic BILAG score reductions and slashed cumulative prednisolone doses, with seven-year follow-up showing 2.63 mean flares versus 4 in controls and cumulative steroid exposure of roughly 4,746 milligrams versus 12,554. A retrospective study of newly diagnosed patients found modified lupus low disease activity state in 76 percent of first-line rituximab recipients versus 45.8 percent on conventional first-line therapy. A dedicated clinical trial, FIRST, now underway in 128 patients aged five and older with new moderate-to-severe lupus, will randomize early rituximab against placebo on top of standard care to test whether prompt B-cell depletion can genuinely rewrite the disease trajectory.

Safety data offer reassurance with caveats. Infusion reactions range from 3.5 to 16.4 percent in most cohorts, though some report up to 44 percent; infections occur in 40 to 57 percent of patients, with serious infections requiring hospitalization in 7 to 20 percent, most commonly respiratory and urinary tract infections and herpes zoster. Mortality is low, 0 to nearly 3 percent, mostly from severe infection. A network meta-analysis found rituximab carried the lowest serious adverse event rate among B-cell-targeting biologics, with anifrolumab showing the lowest infection likelihood at 78.7 percent ranking probability followed by rituximab at 72.6. Hypogammaglobulinemia after repeated cycles remains uncommon, affecting 21 percent for IgM and 7 percent for IgG, though rituximab-treated patients face heightened COVID-19 severity and mortality. Low-dose regimens, two 200-milligram infusions or 100 milligrams weekly for four weeks, have shown encouraging response rates in lupus-related thrombocytopenia and lupus enteritis, potentially trimming both toxicity and cost.

The therapeutic landscape around rituximab is shifting rapidly. Obinutuzumab, a glycoengineered type II anti-CD20 antibody that kills B cells through antibody-dependent cellular phagocytosis rather than complement, has been FDA-approved for active lupus nephritis after the REGENCY trial, and small series show it rescuing patients with secondary non-response to rituximab, with median SLEDAI-2K scores falling from 16 to 5 at six months. Ofatumumab and ocrelizumab offer fully human or humanized alternatives with lower immunogenicity, though their phase III lupus programs stumbled on infections and lack of efficacy. Belimumab after rituximab eliminates severe flares and works best, exploratory data suggest, in patients with high baseline serum IgA2 anti-dsDNA antibody concentrations, a potential biomarker for personalized sequencing. And at the frontier, allogeneic CD19-targeted CAR-T cell therapy has induced durable, drug-free remission in refractory lupus, albeit at enormous cost and with risks including cytokine release syndrome and prolonged B-cell aplasia. For now, the review concludes, rituximab’s accessibility, two decades of safety experience, and biosimilar-driven affordability keep it central to lupus care, even as deeper depletion strategies wait in the wings.

Subject of Research: The role of rituximab, an anti-CD20 B-cell-depleting antibody, in therapeutic strategies for systemic lupus erythematosus

Article Title: Positioning rituximab in the therapeutic strategies for SLE

Article References: Positioning rituximab in the therapeutic strategies for SLE. (n.d.). https://doi.org/10.3389/flupu.2026.1877692

Image Credits: AI Generated

DOI: 10.3389/flupu.2026.1877692

Keywords: rituximab, systemic lupus erythematosus, lupus nephritis, B-cell depletion, anti-CD20, belimumab, obinutuzumab, CAR-T therapy, autoimmunity, refractory disease, biologic therapy, rheumatology