Previous attempts to achieve mixed chimerism across the xenogeneic barrier of pig-to-primate have allfailed, largely due to high levels of primate natural antibodies to the Gal antigen, expressed on the cellsurface of all mature pig hematopoeitic cells. Thus, following mixed chimerism induction protocols innon-human primates, we have had evidence for long-term engraftment of pig hematopoietic stem cells(HSC), which do not appear to express Gal. However, despite antibody absorptions, inhibition ofcomplement and various drug treatments, natural anti-Gal antibodies persisted and/or recurred, andappeared to play an important role in inhibiting the progeny of pig HSC from repopulating primaterecipients. The recent availability of galactosyltransferase (Gal-T) knockout (GalT-KO) swine, derivedfrom our most highly inbred line of miniature swine, has now made it possible to study pig-to-baboonxenotransplantation in the absence of effects of natural anti-Gal antibodies. The goal of thisproposal is to utilize these new GalT-KO animals as hematopoietic cell donors for theestablishment of mixed xenogeneic chimerism in the pig-to-baboon combination. Ourpreviousstudies and our preliminary data all suggest the feasibility of applying the mixed chimerism approachto induction of transplantation tolerance across this barrier, an approach already used successfully inthis laboratory for allografts and concordant xenografts in rodents, pigs and cynomolgus monkeys.Specifically, we will: 1) Optimize the engraftment of HC from GalT-KO miniature swine inconditioned baboons; 2) Examine the role of selected innate immune and non-immunefactors in promoting and/or resisting engraftment of GalT-KO HCin baboons; and 3) Test therole of GalT-KO pig HC engraftment in baboons enabling the induction of tolerance inrecipients of subsequent organ xenografts.
Aim 3, in particular, will apply the findings of Aims 1and 2, as well as strategies developed in the other projects of this Program Project, to the mixedchimerism approach for inducing transplantation tolerance to organ xenografts in this preclinical,discordant species combination. In addition, the experiments planned will provide basic information onxenogeneic stem cell engrafment and on the immunologic pathways responsible for xenogeneicrejection and tolerance induction in primates. As such, these studies should have both theoretical andpractical implications for the eventual application of xenotransplantation as a clinical modality.
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