Detection of Exostosin 1 in Lupus Nephritis: Prevalence, Clinico-Pathologic and Renal Outcome Correlations

Background/Objectives: Lupus nephritis (LN) is one of the most severe complications of systemic lupus erythematosus (SLE); it is associated with increased morbidity and mortality, underscoring the need for new diagnostic markers and therapeutic strategies. In this context, the exostosin 1 (EXT1)/exostosin 2 (EXT2) heterodimer has emerged as a novel antigen in membranous nephropathy associated with SLE. This study evaluated EXT1 prevalence in renal biopsies from patients with lupus membranous nephropathy (LMN) and compared clinical, laboratory, and histopathological characteristics on diagnosis and renal outcomes. Methods: This retrospective study included 97 LMN patients whose renal biopsy underwent immunohistochemistry (IHC) for EXT1. EXT1-positive and EXT1-negative groups were compared using descriptive analyses and repeated measures models. Results: EXT1 positivity was observed in 35% of the cohort, and is more frequent in pure LMN (40%) than in cases with a proliferative component (32%). Regarding SLE diagnostic criteria, EXT1-positive patients showed a higher frequency of antiphospholipid antibodies, although data were available for only a subset of patients. This group also exhibited lower serum creatinine levels, but without statistical significance. EXT1-negative patients more frequently received cyclophosphamide as induction therapy (57.6% vs. 34.5%; p = 0.041). No differences in clinical outcomes were observed during follow-up. Conclusions: EXT1 prevalence was consistent with the literature, reinforcing the epidemiological reproducibility of this marker. EXT1-positive and EXT1-negative groups did not differ regarding clinical presentation, disease progression, and renal outcomes, heightening the need for prospective studies to further elucidate the diagnostic and prognostic role of EXT1 in LMN.