Mechanisms Responsible for and Strategies To Overcome Bone Marrow (BM) Rejection in Allosensitized Recipients.
Authors: Taylor Patricia A. , Ehrhardt Michael J. , Roforth Matthew M. , Swedin Jessica M. , Panoskaltsis-Mortari Angela , Serody Jonathan S. , Blazar Bruce R. 
Published: October 2019

Abstract
Although allosensitization results in the priming of both cellular and humoral immunity, our data indicate that preformed antibody is the major barrier to successful bone marrow transplantation (BMT) in the sensitized recipient. A single priming event led to rapid, high, long-lived antibody levels that practically precluded waiting for sufficient waning of antibody titer for successful BMT. C57BL/6 (B6) mice, whether primed against BALB/c alloantigen 7 days or 4 months before BMT, died of graft failure by 2 wks after transplantation. Antibody-mediated BM rejection was not entirely antigen specific as B6 mice primed against BALB/c also rejected B10.BR BM albeit more slowly than BALB/c BM. However, B6 mice primed against B10.BR rejected B10.BR BM but not BALB/c BM. Examination of serum from primed B6 mice indicated that BALB/c priming resulted in a more vigorous and broadly cross-reactive antibody response than did B10.BR priming perhaps due to the multiple minor antigen differences (as well as MHC disparity) that exist between BALB/c and B6 mice but not between B10.BR and B6 mice. Moreover, BM incubated with serum from primed mice was rejected in non-primed recipients indicating that ex vivo antibody-coating of donor BM was sufficient for its elimination and that continual in vivo exposure to high titer antibody was not essential for rejection. Antibody-mediated rejection was formally shown to be dependent upon host Fc receptor+ cells. We hypothesized that if donor BM cells could be delivered directly to the bone marrow cavity they might bypass destruction by the RES system or a BM homing defect, but direct intra-BM infusion of donor BM did not abrogate rejection in primed recipients. Imaging studies using B6 GFP+ BM indicated that antibody-mediated BM rejection was complete by 18 hrs in B6-primed BALB/c mice. In contrast, T cell-mediated rejection in non-primed mice took >6 days with donor BM first eliminated from lymphoid organs and later, from the BM cavity and parenchymal organs. We hypothesized that mega-BM doses might overcome primed rejection however, increasing the BM cell dose 10-fold was sufficient to overcome T cell priming in B cell-deficient muMT mice but not antibody-mediated rejection in WT mice. Despite the formidable barrier to engraftment, high donor chimerism levels were achieved in a proportion of primed mice by the combination of high dose murine immunoglobulin (mIg) infusion pre-BMT, in vivo T cell-depletion peri-BMT and mega-BM dose. The addition of splenectomy pre-BMT to this protocol only modestly added to the efficacy of this combination strategy. Although mIg, especially in combination with splenectomy, reduced antibody levels, the reduction was unlikely to be of sufficient magnitude to account for the increased survival and engraftment seen in mice when used in conjunction with in vivo TCD. The efficacy of mIg was more likely due to inhibition of Fc receptor-mediated clearance of antibody-bound cells. The heretofore unappreciated rapidity of antibody-mediated BM rejection in allosensitized recipients indicates there is no window of opportunity for clinical intervention after BMT and dictates the need for greater patient evaluation and therapeutic strategies prior to BMT.