Aim: Bacterial infection following ABO-incompatible organ-transplantation is associated with refractory Ab-mediated rejection. Here, we investigated the influence of toll-like receptor (TLR)-signaling, via the binding of bacteria-derived ligands, on B-cell activation.

Methods: We immunized Balb/c mice with human blood group A-erythrocytes with or without LPS administration (1 or 10 μg/body). We analyzed inhibitory effects of calcineurin inhibitor (CsA) on proliferating B-cells and anti-A Ab production. Either purified sIgM⁺CD5⁺B-1a cells or sIgM⁺CD5^dim/-B-1b cells with anti-A receptors from the peritoneal cavity of CsA-treated Balb/c mice immunized with A-erythrocytes were transferred intraperitoneally to NOD-Rag1^null IL2rγ^null double mutant mice, followed by second stimulation with A-erythrocytes. Anti-A Ab levels in sera was determined by ELISA, and the frequency of anti-A Ab-producing cells in the spleen was quantified by ELISPOT.

Results: In mice immunized with only A-erythrocytes, peritoneal B-cells with anti-A receptors showed sIgM⁺CD5⁺B-1a phenotype and CsA-treatment significantly inhibited anti-A Ab production. In contrast, in mice immunized with A-erythrocytes and LPS, those cells showed sIgM⁺CD5^dim/-B-1b phenotype. Peritoneal sIgM^dimCD5^dim/-B-1b cells that ubiquitously expressed MyD88, an adaptor protein of the TLR-signaling pathway, were specifically activated by LPS. Anti-A Ab production augmented by LPS was not inhibited by CsA administration. Anti-A Ab-producing cells were detected in the spleens of NOD-Rag1^null IL2rγ^null mice that received sIgM⁺CD5^dim/-B-1b cells with anti-A receptors from CsA-treated wild-type mice by ELISPOT. However, anti-A Ab-producing cells were undetectable in mice receiving sIgM⁺CD5⁺B-1a cells with anti-A receptors, validating the difference in sensitivity between B-cell subsets to CsA.

Conclusions: B-cells responding to blood group A-antigens, in TLR-signaling, show sIgM⁺CD5^dim/-B-1b phenotype, losing susceptibility to inhibition by CsA. Calcineurin/NFAT factors control recombination-activating genes (RAG) expression, suggesting that B-1a cell activity is solely due to BCR-signaling, inducing RAG expression that can be inhibited by CsA. However, a pathogen (LPS) directly induces signals in B-1b cells by activating innate receptors such as TLR4, independent of calcineurin/NFAT factors, suggesting that CsA cannot inhibit B-1b cell activities. TLR4 pathway blockade might restore the sensitivity to CsA in B-cells responding to A-antigens.

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