In October 2025, the FDA approved an anti-CD20 monoclonal antibody for adults with active lupus nephritis receiving standard therapy. In the phase 3 REGENCY trial, 46.4% of patients receiving B-cell depletion plus standard therapy achieved a complete renal response at week 76, compared with 33.1% receiving standard therapy alone. The treatment effect was accompanied by reductions in anti-dsDNA antibodies and improvement in complement levels.[1]
That serologic response matters. As B-cell-directed treatment becomes deeper and more precise—from anti-CD20 antibodies to CD19-directed CAR-T cells and approaches that target plasma-cell populations through B cell maturation antigen (BCMA)—it is tempting to assume that autoantibody testing becomes less relevant. The emerging evidence suggests the opposite. These therapies are changing what autoantibodies can tell us: not only about diagnosis, but about the cellular compartments sustaining autoimmunity, the importance of a pretreatment baseline and the durability of an immune reset.
What deep B-cell depletion is telling us
The most informative evidence comes from therapies capable of producing profound B-cell depletion. In the phase 1/2 CASTLE basket trial of CD19-directed CAR T-cell therapy in treatment-refractory systemic lupus erythematosus, systemic sclerosis and idiopathic inflammatory myopathy, different autoantibody specificities followed markedly different trajectories.[2]
Antibodies against dsDNA, histones, nucleosomes and Smith antigen were among those frequently eliminated. Anti-Scl-70 levels fell substantially but remained detectable in some patients. By contrast, anti-Ro52, anti-Ro60 and anti-La antibodies showed moderate decreases without conversion to seronegativity. Investigators suggested that this persistence reflected, at least in part, production by CD19-negative plasma cells that escape CD19-directed therapy.[2]
That distinction is clinically and biologically important. A peripheral B-cell count can show that a therapy engaged its target, but it cannot fully describe what happened to the autoreactive cells responsible for a particular antibody specificity. Serial autoantibody measurements may therefore provide an indirect functional readout of how deeply treatment has affected different components of the B-cell and plasma-cell lineage.
Newer data reinforce the point. In 2026, investigators reported outcomes with CD19×CD3 (blinatumomab) and BCMA×CD3 T-cell engagers (teclistamab) in patients with treatment-refractory autoimmune connective tissue diseases. Both approaches reduced autoantibody titers, including in patients who had previously received anti-CD20 therapy.[3] These findings add to growing evidence that autoantibody kinetics can help reveal meaningful differences in the antibody-producing compartments reached by treatment.
Lastly, although not directly targeting B-cells, neonatal IgG receptor blockers have become a promising avenue in the treatment of autoimmune diseases. Such antibodies also alter autoantibody levels and have shown promising results in diseases such as RA, myositis, and Sjögren’s disease.
From diagnostic marker to treatment-context biomarker
The strongest evidence for using serology to inform treatment selection still comes from established B-cell-depleting therapy. In rheumatoid arthritis, a pooled analysis of 16 registries involving 27,583 patients found that seropositivity was associated with greater effectiveness of non-TNF biologic therapies, with the largest difference observed for rituximab.[4] Earlier work also found that higher anti-CCP titers were associated with a greater likelihood of response to rituximab.[5]
Those data do not yet mean that an autoantibody profile can select the right patient for CAR T-cell or BCMA-directed therapy. That remains a hypothesis to test prospectively. But the underlying question is becoming increasingly relevant: if different therapies reach different stages of the autoreactive B-cell lineage, could antibody specificity, concentration and cellular source eventually help identify which therapeutic strategy is most likely to succeed?
Trials should begin answering that question now, before treatment rather than after it.
The importance of the pretreatment baseline
Deep depletion can erase or remodel portions of a patient’s serologic signature. That makes comprehensive pretreatment characterization increasingly valuable.
Patients with refractory autoimmune disease often arrive at advanced therapies with years of fragmented laboratory testing performed across different laboratories, platforms, and time points. A structured baseline profile provides a clearer record of the autoantibody repertoire before intervention and creates a reference against which subsequent kinetics can be interpreted.
This is not simply documentation. Without a reliable baseline, it becomes difficult to distinguish an antibody that disappeared with therapy from one that was never present, or to determine whether its re-emergence accompanies B-cell repopulation or clinical recurrence. As immune-reset strategies advance, baseline serology should become part of the translational framework used to understand what treatment changed.
Seronegativity is not the same as cure
The field should be equally careful not to overinterpret the disappearance of autoantibodies. Clinical remission can occur even when disease-associated antibodies remain detectable.
The CASTLE investigators found persistent anti-Ro antibodies in some patients despite sustained drug-free clinical responses.[2]
These observations are an important warning against turning seronegativity into a surrogate endpoint for cure. Some autoantibodies may contribute directly to pathogenesis; others may primarily mark an autoreactive immune compartment. B-cells also contribute to disease through mechanisms beyond antibody production, including antigen presentation and cytokine signaling.
At the same time, sustained absence—or re-emergence—of selected autoantibodies after immune reset may ultimately prove prognostically useful. We do not yet know. Answering that question will require prospective studies that pair standardized serial serology with clinical disease activity, immune-cell reconstitution, and long-term outcomes.
Precise and accurate detection is the next translational challenge
That work depends on test method accuracy and precision. Autoantibody assays differ in antigen source, platform, calibration, analytical range, and reporting units. Those differences become particularly important when the question shifts from whether an antibody is present to whether its concentration has fallen, disappeared or returned over time.
The next generation of B-cell-directed trials should therefore build in harmonized longitudinal autoantibody assessment, shared reference materials and consistent reporting wherever possible.
As we learn to reset the autoreactive immune system with greater precision, we should resist the assumption that autoantibodies become less important. Instead, their role is changing.
The more precisely we learn to remove B-cells and reduce IgG levels, the more it matters what pathological function such antibodies carry.
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