Induction of molecular chimerism by gene therapy prevents antibody-mediated heart transplant rejection.

In order for xenotransplantation to become a clinical reality, and fulfill its promise of overcoming shortages of human organs and tissues, rejection mediated by the host's immune system must first be overcome. In primates, preformed natural antibodies that bind the carbohydrate antigen Galalpha1-3Galbeta1-4GIcNAc-R (alphaGal), which is synthesized by UDP galactose:beta-D-galactosyl-1,4-N-acetyl-D-glucosaminide alpha(1-3)galactosyltransferase (E.C. 2.4.1.151) or simply alphaGT, mediate rigorous rejection of transplanted pig organs and tissues. In alphaGT knockout mice (GT0 mice), which like humans contain in their serum antibodies that bind alphaGal, expression of a retrovirally transduced alphaGT in bone marrow-derived cells is sufficient to prevent production of alphaGal-reactive antibodies. Here, we demonstrate that reconstitution of lethally irradiated GT0 mice with alphaGT-transduced bone marrow cells from GT0 littermates prevents antibody-mediated rejection of cardiac transplants from wild-type mice. These data suggest that gene therapy can be used to induce immunological tolerance to defined antigens and thereby overcome transplant rejection.

Induction of B-cell tolerance by retroviral gene therapy.

The primary immunologic barrier to overcome before clinical xenotransplantation can be successful is rejection mediated by preformed natural antibodies in the host, directed toward a single carbohydrate epitope Galalpha1-3Galbeta1-4GlcNAc-R (alphaGal) present on porcine tissue, encoded for by the enzyme glucosyltransferase UDP galactose:beta-D-galactosyl-1, 4-N-acetyl-D-glucosaminide alpha(1-3)galactosyltransferase (EC 2.4.1. 151) or simply alphaGT. Although we have shown previously that a gene therapy approach could be used to prevent production of natural antibodies specific for alphaGal, the ability to induce and maintain tolerance after rigorous antigen challenge would be required if similar approaches are to be used clinically. Here, we demonstrate in alphaGT knockout mice (GT(0) mice), which, like humans, contain in their serum antibodies that bind alphaGal, that the efficient transduction and expression of a retrovirally transduced alphaGT gene in bone marrow-derived cells induces stable long-term tolerance to the alphaGal epitope. GT(0) mice reconstituted with alphaGT-transduced bone marrow cells were unable to produce antibodies that bind alphaGal after extensive immunization with pig cells. Furthermore, using ELISPOT assays, we were unable to detect the presence of B cells that produce alphaGal reactive antibodies after immunization, suggesting that such B cells were eliminated from the immunologic repertoire after gene therapy. Interestingly, after tolerance to alphaGal is induced by gene therapy, the antiporcine non-alphaGal humoral response changes from a predominantly IgM to an IgG response. This suggests that once the natural antibody barrier is eliminated by the induction of tolerance, the antipig response changes to a typical T-cell-dependent response involving isotype switching. Thus, gene therapy approaches may be used to overcome immunologic responses leading to xenograft rejection, and similar gene therapy approaches could be used to overcome autoimmunity.

Inhibition of xenoreactive natural antibody production by retroviral gene therapy.

The major barrier to transplantation across discordant species, such as from pig to human, is rejection mediated by xenoreactive natural antibodies (XNA) that bind the carbohydrate epitope Galalpha1-3Galbeta1-4GlcNAc-R (alphaGal) on donor tissues. This epitope is synthesized by the enzyme glucosyltransferase uridine 5'-diphosphate galactose:beta-D-galactosyl-1, 4-N-acetyl-D-glucosaminide alpha(1-3)galactosyltransferase (E.C. 2.4.1.151), or simply alphaGT. When a functional alphaGT gene was introduced by retroviral gene transfer into bone marrow cells, alphaGal XNA production in a murine model ceased. Thus, genetic engineering of bone marrow may overcome humoral rejection of discordant xenografts and may be useful for inducing B cell tolerance.

Tolerization of anti-Galalpha1-3Gal natural antibody-forming B cells by induction of mixed chimerism.

Xenotransplantation could overcome the severe shortage of allogeneic organs, a major factor limiting organ transplantation. Unfortunately, transplantation of organs from pigs, the most suitable potential donor species, results in hyperacute rejection in primate recipients, due to the presence of anti-Galalpha1-3Gal (Gal) natural antibodies (NAbs) in their sera. We evaluated the ability to tolerize anti-Gal NAb-producing B cells in alpha1,3-galactosyltransferase knockout (GalT KO) mice using bone marrow transplantation (BMT) from GalT+/+ wild-type (WT) mice. Lasting mixed chimerism was achieved in KO mice by cotransplantation of GalT KO and WT marrow after lethal irradiation. The levels of anti-Gal NAb in sera of mixed chimeras were reduced markedly 2 wk after BMT, and became undetectable at later time points. Immunization with Gal+/+ xenogeneic cells failed to stimulate anti-Gal antibody production in mixed chimeras, whereas the production of non-Gal-specific antixenoantigen antibodies was stimulated. An absence of anti-Gal-producing B cells was demonstrated by enzyme-linked immunospot assays in mixed KO + WT --> KO chimeras. Thus, mixed chimerism efficiently induces anti-Gal-specific B cell tolerance in addition to T cell tolerance, providing a single approach to overcoming both the humoral and the cellular immune barriers to discordant xenotransplantation.