Introduction: Inositol-requiring enzyme 1Α (IRE1Α), an endoplasmic reticulum-located transmembrane protein kinase, is the activator of transcription factor X-box-binding protein 1 (XBP1) that is required for plasma cell (PC) differentiation. This study seeks to determine whether IRE1Α plays a role in cardiac allograft rejection by regulating B cell differentiation and DSA production. Methods: B cell specific IRE1Α knockout (KO) mice (on B6 background) were generated by crossing IRE1Αf/f mice with CD19-Cre knock-in mice. Cardiac allografts from BALB/c mice were transplanted into the IRE1Α KO mice (IRE1Α f/f CD19Cre+/-) or their wild type (WT) littermates (IRE1Α f/f CD19Cre-/-). The recipients were either untreated or treated with a low-dose Rapamycin (RAPA), 0.1mg/kg/day, from day-1 to POD8. DSA levels were measured by flowcytometry. Cells from spleens and bone marrow were harvested for phenotypical and functional analyses. Results: IRE1Α deficiency does not influence B cell development, as the bone marrow of IRE1Α KOs contains comparable numbers of B lineage cells at all stages to the WT controls. In addition, numbers and phenotypes of both T cells and B cells were not altered in the spleen of IRE1Α KOs. However, when challenged by LPS in vitro, B cells purified from the IRE1Α KOs failed to differentiate into PCs, suggesting a crucial role of IRE1Α in TLR4/LPS-mediated PC differentiation. Consistent to the in vitro observation, PC numbers and DSA production were diminished in the IRE1Α KOs following cardiac allotransplantation. While the WT controls with a transient RAPA therapy rejected the cardiac allografts in 14 days, in contrast, the B cell-specific IRE1Α KOs with the same RAPA regimen had a significantly prolonged cardiac allograft survival to 37 days (P<0.001, vs. controls). Furthermore, the prolonged graft survival in the KOs was associated with an increase (>2 fold) in the number of IL-10 secreting B cells (regulatory B cells, Bregs), compared to the WT controls on POD15. Conclusions: Abrogating B cell IRE1Α inhibits antigen specific PC differentiation and DSA production, and promotes Breg expansion, resulting in a significantly prolonged cardiac allograft survival in synergy with a low-dose RAPA treatment. Therefore, IRE1Α may be a novel therapeutic target for clinical transplantation aimed at controlling DSA mediated rejection.

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