B CELLS AND ANTIBODY RESPONSE

Y. Igarashi, T. Irei, H. Ohdan
Transplant Surgery, Hiroshima University Hospital, hiroshima/JAPAN

Body: Introduction We have recently demonstrated that a tolerant state among B cells responding to blood group-A antigens develops following blood group A-to-O pediatric liver transplantation. Blood group antigen-reactive B cells might be tolerized through their interaction with the liver sinusoidal endothelial cells (LSECs), which exclusively express blood group antigens together with Fas-L and PDL1. Methods In order to address this possibility, we have used α1,3-galactosyltransferase-deficient (GalT^–/–) mice, since the α-Gal epitope is very similar in structure for blood groups A and B. Immune fluorescence staining of the wild-type mouse livers reveals that Gal epitopes predominantly express on the LSECs, resembling blood group antigens in human livers. The α-Gal-expressing LSECs isolated from wild-type GalT^+/+ B6 mice were adoptively transferred via the portal vein into the congeneic GalT^–/– mice (4 × 10⁶ cells/mouse); they were intraperitoneally injected 2 days before the transplantation with the pyrrolizidine alkaloid monocrotaline (3–400 mg/kg), a genotoxin, which impaired host LSECs, conferring proliferative advantage to the transplanted LSECs. Three to four weeks after the adoptive transfer of LSECs, the recipient mice underwent myeloablative radiation and reconstitution with bone marrow cells (BMCs) with/without splenocytes from GalT^–/– mice. Four weeks after the immune reconstruction, the recipient mice were immunized with α-Gal-expressing rabbit erythrocytes. Results After the immunization, high levels of anti-Gal Abs were detected in the sera of the GalT^–/– mice that had not received LSECs but were repopulated with BMCs from GalT^–/– mice. In contrast, anti-Gal Abs were persistently undetectable in the sera of the GalT^–/– mice that had received LSECs from GalT^+/+ mice and were consequently repopulated with BMCs from GalT^–/– mice. To rule out the possibility that anti-Gal Abs were merely absorbed by the Gal epitope expressed on the engrafted GalT^+/+ LSECs, the presence of anti-Gal–producing cells was assessed by ELISPOT assay. Cells producing anti-Gal Abs were undetectable in the bone marrow of those GalT^–/– mice that had received LSECs from GalT^+/+ mice. In contrast, in the GalT^–/– mice repopulated with both BMCs and splenocytes from GalT^–/– mice, anti-Gal Abs were highly detectable, despite LSEC engraftment from GalT^+/+ mice. In addition, when the LSECs from GalT^+/+ B6 mice were adoptively transferred into the splenectomized congeneic GalT^–/– mice, these GalT^–/– mice lost their ability to produce anti-αGal Abs. Even when the LSECs from FasL-deficient (c-gld) GalT^+/+ B6 mice were adoptively transferred into the splenectomized congeneic GalT^–/– mice, the GalT^–/– recipient mice still lost their ability to produce anti-αGal Abs. Conclusion These findings suggest that after ABO-incompatible liver transplantation, the LSECs tolerize immature B cells but do not tolerize mature B cells specific for blood group carbohydrate antigens. For inducing such immune-regulatory effects of the LSECs on B cells, Fas/FasL pathway might be dispensable.

Disclosure: All authors have declared no conflicts of interest.