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Objective Methods Results Conclusion Autoantibodies are a hallmark of lupus nephritis (LN), but their association with LN classes and treatment response are not adequately known. In this study, we quantified circulating autoantibodies in the Accelerating Medicines Partnership LN longitudinal cohort to identify serological biomarkers of LN histologic classification and treatment response and how these biomarkers change over time based on treatment response.Peripheral blood samples were collected from 279 patients with systemic lupus erythematosus undergoing diagnostic kidney biopsy based on proteinuria. Of these, 268 were diagnosed with LN. Thirteen autoantibody specificities were measured by bead‐based assays (Bio‐Rad Bioplex 2200) and anti‐C1q by enzyme‐linked immunosorbent assay at the time of biopsy (baseline) and at 3, 6, and 12 months after biopsy. Clinical response was determined at 12 months.Proliferative LN (International Society of Nephrology/Renal Pathology Society class III/IV±V, n = 160) was associated with higher concentrations of anti‐C1q, anti‐chromatin, anti–double‐stranded DNA (dsDNA), and anti–ribosomal P autoantibodies compared to nonproliferative LN (classes I/II/V/VI, n = 108). Anti‐C1q and‐dsDNA were independently associated with proliferative LN. In proliferative LN, higher baseline anti‐C1q levels predicted complete response (area under the curve [AUC] 0.72; P = 0.002) better than baseline proteinuria (AUC 0.59; P = 0.21). Furthermore, all autoantibody levels except for anti‐La/SSB decreased over 12 months in patients with proliferative, but not membranous, LN with a complete response.Baseline levels of anti‐C1q and anti‐dsDNA may serve as noninvasive biomarkers of proliferative LN, and anti‐C1q may predict complete response at the time of kidney biopsy. In addition, tracking autoantibodies over time may provide further insights into treatment response and pathogenic mechanisms in patients with proliferative LN. [ABSTRACT FROM AUTHOR]