Xenotransplantation is a potential solution to the organ shortage in clinical transplantation. Pigs are regarded as the most suitable donors. Studies in pig-to-nonhuman primates have demonstrated the formidable immunologic barriers to successful transplantation of pig xenografts. Humoral responses mediated by B cells play a critical role in rejecting pig xenografts. While GalT knockout pigs prevent humoral responses to the Gal antigen, responses to non-Gal antigens expressed by pig tissues remain a major problem. We previously demonstrated in a rat-to-mouse model that induction of mixed xenogeneic chimerism led to robust tolerance of host mouse T cell-independent B cells of all specificities to donor rat cells in addition to T cell tolerance. It is currently unknown whether mixed pig/human xenogeneic hematopoietic chimerism could likewise lead to tolerance of human B cells to all pig specificities. We address this issue in a humanized mouse model.

Pig/human mixed chimeric mice were generated by injection of pig bone marrow cells to irradiated pig hematopoietic cytokine transgenic NSG mice followed by injection of human fetal liver-derived CD34+ cells 3-7 days later. Control non-chimeric mice received human CD34+ cells only. Anti-pig antibodies in the serum of chimeric and non- chimeric mice were determined by complement-dependent cytotoxicity assay at 14-18 weeks following transplantation.

Anti-pig natural cytotoxic antibodies were detected in serum of a large fraction of non-chimeric humanized mice. These antibodies were mainly IgM. Pooled data from three independent experiments showed that significantly higher percentages of non-chimeric mice (68%, 17 out of 25) than mixed chimeric mice (10%, 2 out of 20) demonstrated anti-pig antibodies in serum. In addition, among mice with anti-pig natural cytotoxic antibodies, serum from non-chimeric mice mediated significantly higher cytotoxicity than that from chimeric mice. Phenotypic analysis showed no significant differences in the subsets of B cells, including naïve, memory, immature and transitional B cells, in lymphoid tissues between chimeric and non-chimeric mice.

Our data suggest that induction of mixed pig/human xenogeneic hematopoietic chimerism results in tolerization of human B cells to pig xenoantigens.

CITATION INFORMATION: Li H, Shao S, Vishwasrao P, Holzl M, Sykes M. Impact of Mixed Xenogeneic Chimerism on Human B Cell Response to Pig Antigens. Am J Transplant. 2016;16 (suppl 3).

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