[Introduction]

Although there is some empirical evidence that severe infectious complications provoke refractory AMR following ABO-incompatible transplantations (ABOi-Tx), its mechanism remains unclear. Here, we investigated a novel function of MyD88 in TLR signaling induced by pathogenic microbes in B cells responding to blood group antigens.

[Methods]

We examined whether TRIF and MyD88, the core adaptor molecules for TLRs signaling, regulate activation of B ells responding to blood group A antigens (A-Ags), which belong to thymus-independent type 2 (TI-2) Ags. MyD88-deficient, TRIF-deficient and wild-type (WT) Balb/c mice were immunized with human blood group A-erythrocytes with/without LPS, TLR4 ligand. The phenotype of peritoneal B cells with B cell-receptors (BCRs) for A-Ags were analyzed. The inhibitory effect of CsA on anti-A Ab production were analyzed. As an in vitro B cell activation model, resting splenic B cells from those mice were treated with anti-IgM F(ab')₂ (a-IgM), an analogue of TI-2 Ags, in the presence of various TLR-agonists.

[Results]

Immunization with A-erythrocytes resulted in the expansion of CD5⁺ B-1a cells with BCRs for A-Ags and the elevation of serum anti-A Ab in WT mice. Those were markedly inhibited by CsA. In the presence of LPS, the CD5^dim/- B-1b cells with BCRs for A-Ags were expanded and the serum levels of anti-A Abs were further increased, which were not inhibited by CsA. However, in MyD88-deficient mice, even in the presence of LPS, the B-1a cells were expanded, which were well inhibited by CsA. Such a drastic alteration was not observed in TRIF-deficient mice. On the in vitro model, the resting WT B cells differentiated into B-1a cells under the stimulus of a-IgM. In the presence of the Pam3CSK4, LPS, FSL-1 and CpG-ODN, which activate TLRs-MyD88, they differentiated into CD5^–B-1b cells. However, MyD88-deficient B cells stimulated with a-IgM and LPS differentiated into B-1a cells. TRIF-deficient B-cells stimulated with a-IgM and LPS differentiated into B-1b cells. CsA abrogated B-1a cell differentiation, but did not B-1b cell differentiation. Western blotting revealed that treating WT B cells with LPS along with a-IgM enhanced NFATc1 expression in those cells, but not in MyD88-deficient B cells.

[Conclusion]

Various pathogens enhance BCR signaling by activating NFATc1, a downstream factor of BCR-calcineurin pathways. Hence, the inhibitory effect of CsA on B cells responding to A-Ags are disabled by the activation of MyD88 elicited by pathogenic microbes in ABOi-Tx.

CITATION INFORMATION: Sakai H, Tanaka Y, Ohdan H. A Novel Role for MyD88 Signaling Downstream from Toll-Like Receptors in B Cells Responding to Blood Group Antigens. Am J Transplant. 2017;17 (suppl 3).

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