Abstract Background: Males are 3 times more at risk to develop muscle-invasive bladder cancer (MIBC), yet females present with higher-grade disease, worse prognosis, and reduced response to available treatments. The biological basis of these sex differences remains elusive. Secretory IgA (SIgA), including SIgA1 and SIgA2, maintains mucosal homeostasis by binding commensal microbes. SIgA is secreted via pIgR-mediated transcytosis and microbe-bound SIgA2 can traffic back to the lamina propria by binding to Dectin-1 and Siglec-5 (reverse transcytosis). This is known in other mucosae but not the bladder. Urinary IgA is elevated in MIBC patients, yet existing studies suggest conflicting roles: IgA is linked to reduced immunosuppression and improved immunotherapy response, yet also to poor survival in cohorts largely dominated by male cases. This leaves sex-specific IgA biology uncharacterized in urinary immunity. Hence, we hypothesize that bladder IgA secretion is distinct in males and females and may influence MIBC outcomes. Methods: Digital Spatial Profiling (GeoMx) was performed on pre-treatment MIBC tumors (n=171; 35F/136M). RNAscope pan-bacterial 16S and multiplex IHC (IgA1, IgA2, ER, AR) were applied to the same tissues. Urinary IgA1/IgA2 were quantified by ELISA (n=20F/20M). Wild-type (n=20F/20M), gonadectomized (n=15F/15M), and sex-reversed FCG mice (FCG, n=5-8 XX-F, XY-F, XY-M and XX-M) received 0.05% N-butyl-N-(4-hydroxybutyl)-nitrosamine (BBN) for 20 weeks. HBLAK cells were cultured on inserts for stratification± β-estradiol then exposed to commercial IgA (2h). Q-omics was used to query TCGA MIBC datasets. Results: Female tumors were enriched for IGHA1, with increased urinary IgA2. In TCGA, high tumoral IGHA2 correlated with improved survival in women only, particularly in tumors with low ESR1 expression. We also found increased CLEC7A (dectin-1) and SIGLEC5 expression in tumors compared to matched normal area. Female mice displayed higher urinary and urothelial IgA, with ovariectomy modulating IgA only after tumor establishment. We found a staining gradient in the urothelium and IgA-positive vesicles, indicating active transport of complexed IgA. No differences in urinary IgA were observed between sex-reversed FCG and WT mice, supporting hormonal rather than autosomal control of IgA. HBLAK pIgR expression was downregulated by β-estradiol. In stratified cultures, IgA added to either side of culture inserts was detected on the opposite side, demonstrating both transcytosis and reverse transcytosis. Long-term exposure to β-estradiol led to significantly less transcytosis, but not reverse-transcytosis. Impact: This is the first investigation of bladder IgA biology in MIBC; we show human and mice tumors are broadly poised for IgA secretion and retro-transport, modulated by female hormones. IgA may represent a novel sex-specific biomarker, its integration in a predictive response model is pending. Citation Format: Yasmine Ilkhani, Sirine Bouabda-Barc, Rachel Chevalier, Orlando Yuan, Jose Joao Mansure, Madhuri Koti, Wassim Kassouf, Éva Michaud. Sex-specific IgA secretion in muscle-invasive bladder cancer: A preliminary study abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 2089.
Ilkhani et al. (Fri,) studied this question.