IgA nephropathy-associated breast milk B cell alterations

Background: IgA nephropathy (IgAN) is a mucosal immune-associated disease characterized by the overproduction of galactose-deficient IgA1 (Gd-IgA1) and formation of nephritogenic immune complexes. Epstein-Barr virus (EBV), which preferentially infects IgA+ B cells, has been implicated in IgAN pathogenesis, although its role remains unclear. Given mucosa involvement, we characterized breast milk B cells as available representatives of mucosal tissue to better understand the pathogenesis of IgAN. Methods: We performed a phenotypic analysis of B cells (CD19+), plasmablasts (CD38+), and CD138+ cells (representing migrating pre-plasma cells, CPC) in the breast milk of IgAN and healthy mothers (HC). EBV infection of cells was detected by using EBV-encoded RNA (EBER). B cell differentiation, IgA/Gd-IgA1 expression, and homing-related markers were characterized using complementary cytometric and microscopic approaches. Results: Breast milk from mothers with IgAN contains a significantly higher proportion of EBER+ B cells compared with HC. Moreover, the proportion of EBER+ CPCs in the breast milk of IgAN mothers is significantly higher than in HC. B cells present in the breast milk of mothers with IgAN exhibit a higher expression of the mucosal homing receptor CCR9 compared to B cells from HC. IgA+ B cells from healthy mothers exhibit a higher overall frequency of surface Gd-IgA1 expression and lack CD138 marker and thus could be classified as memory B cells or plasmablasts considering their class-switched phenotype. In contrast, breast milk from IgAN mothers was enriched for Gd-IgA1+ CD138+ CPC cells, indicating a shift toward terminally differentiated antibody-producing cells. These findings suggest disease-associated alterations in B cell differentiation and compartmentalization rather than increased mucosal Gd-IgA1 production per se. Conclusion: Despite the limited number of analyzed samples, we detected interesting differences in B cells in the breast milk of IgAN mothers and HC. The analysis of B cell populations in the breast milk of IgAN mothers indicates EBV-associated B cell dysregulation. The enrichment of EBV+ CPC and Gd-IgA1+ CPC in the breast milk of IgAN mothers agrees with a former model proposing aberrant mucosal B cell differentiation and trafficking involvement in IgAN pathogenesis.