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Sequential Analysis of Anti‐αGal Natural Antibody‐Producing B Cells in GalT Knockout Mice in Cyclophosphamide‐Induced Tolerance

Previously, we have shown that cyclophosphamide (CP)‐induced tolerance, marked by permanent acceptance of donor skin graft and establishment of donor mixed chimerism, was readily induced with treatment with donor spleen cells (SC), CP, busulfan (BU) and donor bone marrow cells (BMC). Here, we investigated the mechanism of anti‐donor natural antibody (nAb) producing B‐cell tolerance in our CP‐induced tolerance systems in α1,3‐galactosyltransferase‐deficient knockout mice (GalT KO; GalT–/–, H‐2b/d). After induction of tolerance using donor AKR SC and BMC, survival of donor heart and skin grafts and production of anti‐Galα1‐3Galβ1‐4GlcNAc (anti‐αGal) Ab in recipient GalT KO mice were analyzed. In addition, the production of anti‐αGal Ab and the presence of Gal‐BSA binding B cells in GalT KO mice were analyzed by flow cytometry (FCM) after treatments with rabbit red blood cells (RRBC) and CP. Permanent acceptance of donor skin and heart grafts and abrogation of anti‐αGal Ab were achieved in GalT KO mice treated with donor SC + CP/BU + BMC. However, in the GalT KO mice treated with donor SC and CP, donor skin grafts were acutely rejected, even though anti‐αGal Ab was undetectable. Similarly, anti‐αGal Ab was undetectable in GalT KO mice treated with RRBC and CP. Our data strongly indicated the following mechanisms: the clonal destruction in the early stage and the clonal anergy or ignorance in the late stage after conventional conditioning with RRBC and CP. In conclusion, our drug‐induced tolerance protocols are effective to induce tolerance in recipients that produce anti‐donor nAb.