High-affinity anti-Gal immunoglobulin G in chronic rejection of xenografts

The anti-non-gal xenoantibody response to xenoantigens on gal knockout pig cells is encoded by a restricted number of germline progenitors

Antibodies directed at non-gal xenoantigens are responsible for acute humoral xenograft rejection when gal knockout (GalTKO) pig organs are transplanted into nonhuman primates. We generated IgM and IgG gene libraries using peripheral blood lymphocytes of rhesus monkeys initiating active xenoantibody responses after immunization with GalTKO pig endothelial cells and used these libraries to identify IgV(H) genes that encode antibody responses to non-gal pig xenoantigens. Immunoglobulin genes derived from the IGHV3-21 germline progenitor encode xenoantibodies directed at non-gal xenoantigens. Transduction of GalTKO cells with lentiviral vectors expressing the porcine alpha1,3 galactosyltransferase gene responsible for gal carbohydrate expression results in a higher level of binding of 'anti-non-gal' xenoantibodies to transduced GalTKO cells expressing the gal carbohydrate, suggesting that anti-non-gal xenoantibodies cross react with carbohydrate xenoantigens. The galactosyltransferase two gene encoding isoglobotriaosylceramide synthase (iGb3 synthase) is not expressed in GalTKO pig cells. Our results demonstrate that anti-non-gal xenoantibodies in primates are encoded by IgV(H) genes that are restricted to IGHV3-21 and bind to an epitope that is structurally related to but distinct from the Gal carbohydrate.

Directional selection for specific sheep cell antibody responses affects natural rabbit agglutinins of chickens

Agglutination data from generations 8 through 19 indicate that bidirectional selection for specific SRBC antibody responses was successful in a line cross of ISA x Warren medium heavy layers. After 11 generations titers of the high SRBC selected line (H line) were nearly 1:32,000; those of the low SRBC selected line (L line) were less than 1:2, but titers of the randombred control line remained stable at 1:32. Directional SRBC selection also affected levels of a naturally occurring rabbit cell agglutinating antibody (RRBC), presumably the avian form of alpha-galactose antibody (anti-Gal). This indirect response was biphasic and opposite in direction to the SRBC responses through generation 14 after which anti-Gal titers of all 3 lines increased. At generation 19, line H had the highest agglutinin titers; of both types, control line was intermediate, and line L was lowest. The correlation between SRBC and RRBC titers was 0.43 (P = 0.0). Females had higher titers than males, but the difference was only significant for the SRBC antibody (P = 0.028). Qualitative changes in anti-Gal accompanied SRBC selection. Rabbit agglutinins of 4 types were recognizable: classic, granular, annular, and one negative or very weak reaction. The score type means in line L were highest, in the control line were intermediate, and in line H were lowest, suggesting avidity differences now exist among these lines. The results show integration of natural and acquired immune systems because selection for one temporarily affected the other. Given the importance of anti-Gal in primates, our results should stimulate further study of this antibody in poultry species.

Treatment with a short course of LF 15-0195 and continuous cyclosporin A attenuates acute xenograft rejection in a rat-to-mouse cardiac transplantation model

Searching for a novel immunosuppressive agent to effectively prevent acute vascular rejection (AVR) is essential for success in clinical xenotransplantation. We previously reported that Lewis rat hearts transplanted into BALB/c mice developed typical AVR in 6 days. The present study was undertaken to determine the efficacy of LF 15-0195, a new immunosuppressive analog of 15-deoxyspergualin in the prevention of AVR in a rat-to-mouse cardiac xenograft model. We transplanted 2-week old Lewis rat hearts into BALB/c mice. Four groups were included in this study: untreated recipients and cyclosporin A (CsA) treated recipients were controls; LF 15-0195 treated recipients or LF 15-0195 combined with CsA treated recipients were experimental groups. Mouse recipients received either LF 15-0195 2 mg/kg subcutaneously from day-1 to post-operative day 14, or CsA 15 mg/kg subcutaneously daily, from day 0 to endpoint rejection, or the two drugs in combination. We observed that high dose CsA did not inhibit AVR and the graft was rejected in 11.3 +/- 1.9 days. Graft histology and immunohistology showed typical AVR, characterized by interstitial hemorrhage, intravascular fibrin deposition, thrombosis, and massive deposition of anti-rat immunoglobulin G (IgG) and immunoglobulin M (IgM). Serum xenoreactive antibodies (xAbs) were markedly elevated in these animals as well. In contrast, we observed that treatment with LF 15-0195 alone significantly prolonged graft survival to 19.3 +/- 0.7 days. Notably, xAbs were significantly decreased and the rejection pattern of these grafts was cell-mediated rejection (CMR), instead of AVR. When CsA was combined with LF 15-0195, the graft mean survival time was further increased to 58.5 +/- 17.3 days. Antibody production and T-cell infiltration were significantly inhibited at the terminal stages of graft survival and pathology showed striking attenuation of both AVR and CMR. Sequential studies on days 6 and 14 demonstrated that LF 15-0195 either alone or combined with CsA completely inhibited antibody production. However, intragraft infiltration by Mac-1 positive cells including natural killer cells, macrophages and granulocytes in LF 15-0195 treated recipients was similar to that of untreated recipients. We conclude that LF 15-0195 effectively prevented AVR by markedly inhibiting the production of anti-donor IgG xAbs. Also, treatment with short course LF 15-0195 and continuous CsA significantly reduced T-cell infiltration. Studies to test this therapy in inhibiting AVR in a pig-to-non-human primate xenotransplantation model are underway.

Delayed rejection of porcine cartilage is averted by transgenic expression of alpha1,2-fucosyltransferase

The use of xenogeneic cells or tissues for tissue engineering applications may lead to advances in biomedical research. Hyperacute and delayed rejection are immunologic hurdles that must be addressed to achieve xenograft survival in the pig-to-primate setting. Expression of human alpha1,2-fucosyltransferase (HT) in the donor cell or tissue protects from hyperacute rejection (HAR) by reducing expression of Galalpha1,3-Gal epitope, the major xenoantigen recognized by human natural antibodies. We hypothesized that Galalpha1,3-Gal antigen contributes to delayed tissue rejection. To test this hypothesis, we transplanted control or HT-transgenic engineered porcine cartilage s.c. into alpha1,3-galactosyltransferase knockout (Gal KO) mice. Control porcine cartilage grafted in Gal KO mice was not susceptible to HAR but was rejected in several wk by a prominent cellular immune infiltrate and elevated antibody titers. In contrast, Gal KO mice receiving the HT engineered cartilage showed a markedly reduced anti-pig antibody response and no anti-Galalpha1,3-Gal-elicited antibody response. The HT implants had a mild cellular infiltrate that was confined to the graft periphery. Our study demonstrates that a marked reduction of Galalpha1,3-Gal antigen in HT-transgenic porcine cartilage confers resistance to a process of delayed rejection. Further development of tissue engineering applications that use genetically modified porcine tissues is encouraged.

Characterization of baboon anti-porcine IgG antibodies during acute vascular rejection of porcine kidney xenograft

In the pig-to-baboon model, the removal of anti-porcine natural antibodies abrogates hyperacute vascular rejection (HAVR), but the xenograft then undergoes an acute vascular rejection (AVR) concomitantly to the appearance of newly formed anti-porcine antibodies. The use of anti-IgM monoclonal antibody (mAb) in baboons allowed to avoid HAVR of pig-to-baboon renal xenografts, but, at post-operative day 6, AVR occurred because of a rapid return of anti-porcine antibodies. The aim of this work was to characterize the anti-porcine antibodies during AVR. Sera from anti-IgM-treated animals were assessed prior to the graft and at the time of AVR by enzyme linked immunosorbent assay (ELISA) to determine anti-porcine antibodies concentration as well as the IgG subtypes. The same sera were tested on confluent cultures of porcine aortic endothelial cells (PAECs) to assess (i) the cytolytic complement-dependent activity and (ii) the E-selectin expression. The K affinity of anti-Gal IgG antibodies was measured by ELISA. Anti-porcine (Gal and non-Gal) IgG antibodies were tested on PAECs by flow cytometry to discriminate the presence of Gal epitopes from the recognition of other porcine epitopes. We found that both anti-porcine IgM and IgG antibodies presented a significantly increased cytolytic activity and E-selectin expression on PAECs during AVR. These characteristics are related to an important increase of the antibody (Ab) titer (especially anti-galactosyl) and a switch to anti-galactosyl IgG1 subclass production, whereas the K affinity remained unchanged. The deleterious effects of both IgM and IgG antibodies observed during AVR showed the crucial need for treatment controlling the cells producing anti-porcine antibodies.

Tolerization of Gal alpha 1,3Gal-reactive B cells in pre-sensitized alpha 1,3-galactosyltransferase-deficient mice by nonmyeloablative induction of mixed chimerism

Using a alpha 1,3-galactosyltransferase wild-type (GalT(+/+)) to deficient (GalT(-/-)) mouse bone marrow transplantation model, we have previously demonstrated that a non-myeloablative conditioning regimen is capable of permitting induction of allogeneic and xenogeneic mixed chimerism. Chimerism is associated with the rapid and lasting tolerization of anti-Gal alpha 1,3Gal (Gal) natural antibody (Ab)-producing B cells. However, one limitation of this model is that anti-Gal natural Ab levels are lower in GalT(-/-) mice than in humans and other primates. To overcome this limitation, we have now investigated the possibility of inducing such tolerance in GalT(-/-) mice that produce much higher levels of anti-Gal Abs due to presensitization with Gal-bearing xenogeneic cells. B6 GalT(-/-) mice that were pre-sensitized with rabbit red blood cells received non-myeloablative conditioning with depleting anti-CD4 and CD8 mAbs, 3Gy whole body and 7Gy thymic irradiation, and infusion of BALB/c GalT(+/+) bone marrow cells (BMC). Although engraftment of standard marrow doses was inhibited by the presensitization, long-lasting mixed chimerism could be induced in recipients of a high dose [160 x 10(6)] of allogeneic wild-type BMC. Achievement of persistent chimerism was associated with high levels of anti-Gal IgG(1) pretransplant, suggesting an inhibitory effect of non-complement-fixing IgG(1) Ab on anti-Gal-mediated marrow rejection. Induction of mixed chimerism was associated with a rapid disappearance of serum anti-Gal and tolerization of anti-Gal Ab-producing cells. B cells with anti-Gal receptors became undetectable in mixed chimeras. Mixed chimeras accepted subsequently transplanted donor-type GalT(+/+) hearts (> 140 days), whereas rapid (within 2 days) rejection of GalT(+/+) hearts occurred in conditioned control GalT(-/-) mice. In conclusion, when a high dose of GalT(+/+) BMC was administered to pre-sensitized GalT(-/-) mice, chimerism and tolerance were achieved. The absence of B cells with receptors recognizing Gal in mixed chimeras suggests a role for clonal deletion/receptor editing in the maintenance of B cell tolerance.

The immunological hurdles to cardiac xenotransplantation

The main hurdle to clinical application of cardiac xenotransplantation is the immune response of the recipient against the graft. Although all xenografts arouse an intense immune response, the effect of that response depends very much on whether the graft consists of isolated cells or an intact organ, such as the heart. Intact organs, which are transplanted by primary vascular anastomosis, are subject to severe vascular injury owing to the reaction of immune elements with the endothelial lining of donor blood vessels. Vascular injury leads to hyperacute rejection, acute vascular rejection, and chronic rejection. The immunological basis for these types of rejection and potential therapies, which might be used to avert them, are discussed.

Biophysical characteristics of anti-Gal(alpha)1-3Gal IgM binding to cell surfaces: implications for xenotransplantation

BACKGROUND

Natural antibodies directed against cell surface carbohydrates are thought to be vital to host defense and to initiate the rejection of xenografts and ABO-incompatible allografts. The biophysical properties underlying the association and dissociation of these antibodies from cell surfaces is incompletely understood. We investigated those properties for the binding of Galalpha1-3Gal antibodies to porcine endothelial cell surfaces, because such interactions might be relevant to the clinical application of xenotransplantation.

RESULTS AND CONCLUSIONS

The initial rate of binding of anti-Galalpha1-3Gal antibodies to endothelial cells was found to depend on antibody concentration, antibody diffusion, and antigen concentration. The presence of an intact glycocalyx had a greater impact on antibody binding than mobility of antigen in cell membranes. Disruption of glycocalyx increased the amount of antibody bound at equilibrium by more than 50%. Although the binding of anti-Galalpha1-3Gal antibodies to cell surfaces could be inhibited by soluble Galalpha1-3Gal, once bound, some anti-Galalpha1-3Gal could not be dissociated by competitive inhibitors of binding or by denaturation of the bound Ig with chaotropic reagents, but could be dissociated by reduction of disulfide bonds, suggesting that attachment to cell surfaces was, at least in part, by means other than specific reaction with the epitope.

IgG2 anti-Galalpha1-3Gal does not induce porcine aortic endothelial cell accommodation in vitro

BACKGROUND

Xenografts that have been protected from hyperacute rejection (HAR) are termed accommodated if they are not then rejected despite the presence of xenoantibody. It has been proposed that IgG may confer resistance to complement dependent cytotoxicity (CDC), a conventional in vitro marker of accommodation. We hypothesized that noncytotoxic IgG2 anti-Galalpha1-3Gal was responsible for this effect.

METHODS AND RESULTS

We purified IgG anti-Galalpha1-3Gal from pooled human normal immunoglobulin and three sera, by elution from protein G and Galalpha1-3Gal-R immunoadsorbents. The eluates were IgM free and > or =95% IgG2. They bound to Galalpha1-3Gal, porcine aortic endothelial cells (PAEC) and lymphocytes. It was not possible to block IgM binding to PAEC or lymphocytes using IgG anti-Galalpha1-3Gal (200 microg/ml). The eluates were noncytotoxic in micro-CDC assays. To investigate accommodation, PAEC were cultured with subsaturating doses of the four IgG eluates for up to 144 hr. Resistance of nontrypsinized PAEC to CDC by human serum was measured in a cell viability assay. PAEC were not rendered resistant to CDC in any of the experiments. To investigate the possibility that accommodation might be induced by non-Galalpha1-3Gal IgG, the experiments were repeated using HNIg, again with no protection demonstrated.

CONCLUSIONS

Using primary PAEC monolayers, we were unable to induce resistance to CDC with human normal immunoglobulin and its IgG2 anti-Gabeta1-3Gal subset. This contradicts previous experiments using trypsinized, immortalized cells. Although resistance to CDC is not an ideal marker of accommodation, the detrimental effects of IgG make it unlikely that it will become a useful clinical means of inducing accommodation.

Differential expression of alpha-GAL epitopes (Galalpha1-3Galbeta1-4GlcNAc-R) on pig and mouse organs

BACKGROUND

Expression of the alpha-gal epitope in mice can be completely eliminated by disruption of the alpha1,3 galactosyltransferase gene. As an initial step for assessing the feasibility of this approach in the pig, it was of interest to compare the expression of alpha-gal epitopes in pig and mouse organs.

METHODS

Membranes from pig and mouse organ homogenates were analyzed for alpha-gal epitope expression by Western blots, enzyme-linked immunosorbent assay (ELISA), immunostaining of tissues, and ELISA inhibition assay.

RESULTS

Immunostaining of Western blots with human anti-Gal detected alpha-gal epitopes on glycoproteins from pig organs but not on glycoproteins from the corresponding mouse organs. ELISA with membrane homogenates and immunostaining of tissue sections demonstrated a much higher binding of human anti-Gal to alpha-gal epitopes on pig membranes than on mouse membranes. ELISA inhibition assay with monoclonal anti-Gal indicated that alpha-gal epitope expression in pig organs is up to 500-fold higher than in mouse organs.

CONCLUSION

Expression of alpha-gal epitopes in pig organs is many fold higher than in mouse organs. The abundance of these epitopes in pigs raises the question of whether pigs can properly develop without expression of alpha-gal epitopes.

The Human Antibody Response to Porcine Xenoantigens Is Encoded by IGHV3-11 and IGHV3-74 IgVH Germline Progenitors

Preformed and induced Ab responses present a major immunological barrier to the use of pig organs for human xenotransplantation. We generated IgM and IgG gene libraries established from lymphocytes of patients treated with a bioartificial liver (BAL) containing pig hepatocytes and used these libraries to identify IgVH genes that encode human Ab responses to pig xenoantigens. Genes encoded by the VH3 family are increased in expression in patients following BAL treatment. cDNA libraries representing the VH3 gene family were generated, and the relative frequency of expression of genes used to encode the Ab response was determined at days 0, 10, and 21. Ig genes derived from the IGHV3-11 and IGHV3-74 germline progenitors increase in frequency post-BAL. The IGHV3-11 gene encodes 12% of VH3 cDNA clones expressed as IgM Abs at day 0 and 32.4–39.0% of cDNA clones encoding IgM Abs in two patients at day 10. IGHV3-11 and IGHV3-74 genes encoding IgM Abs in these patients are expressed without evidence of somatic mutation. By day 21, an isotype switch occurs and IGHV3-11 IgVH progenitors encode IgG Abs that demonstrate somatic mutation. We cloned these genes into a phagemid vector, expressed these clones as single-chain Abs, and demonstrated that the IGHV3-11 gene encodes Abs with the ability to bind to the gal α (1,3) gal epitope. Our results demonstrate that the xenoantibody response in humans is encoded by IgVH genes restricted to IGHV3-11 and IGHV3-74 germline progenitors. IgM Abs are expressed in germline configuration and IgG Abs demonstrate somatic mutations by day 21.

Naturally occurring anti-alpha-galactosyl antibodies: relationship to xenoreactive anti-alpha-galactosyl antibodies

Antibodies produced by an individual without a known history of sensitization to the relevant antigen are called "natural" antibodies. Some natural antibodies, called xenoreactive antibodies, react with the cells of foreign species. Most xenoreactive antibodies in humans and higher primates bind to a nonreducing terminal galactose expressed by pigs and other lower mammals. Although human natural antibodies which bind to one or more of a variety of terminal alpha-galactosyl structures have been identified previously, the antigen recognized by anti-alpha-galactosyl antibodies on the cells of foreign species is thought to be exclusively Galalpha1-3Gal. Thus, anti-alpha-galactosyl antibodies which do not react with Galalpha1-3Gal are thought to be nonxenoreactive. Here, we identify natural antibodies in human serum which bind to Galalpha1-6Hexosepyrranosides but not Galalpha1-3Gal, indicating that these antibodies are not xenoreactive. Various lower mammals were found to have natural anti-Galalpha1-2Gal antibodies in their sera, suggesting that at least some anti-Galalpha1-2Gal antibodies might not be xenoreactive and indicating, surprisingly, that anti-alpha-galactosyl antibodies are much more phylogenetically disperse than previously known. Also surprising was the finding that some natural antibodies which bind to Galalpha1-3Gal in vitro do not bind to porcine xenografts. These studies show that naturally occurring anti-alpha-galactosyl antibodies in mammalian serum include antibodies with a greater variety of reactivities than previously thought, only some of which would bind to a porcine xenograft. Further, these studies show that the methods used to detect anti-alpha-galactosyl antibodies of relevance in xenotransplantation must be carefully evaluated to avoid detection of anti-alpha-galactosyl antibodies which would not bind to a porcine organ and which therefore are not involved in xenograft rejection.

Mixed chimerism induced without lethal conditioning prevents T cell- and anti-Gal alpha 1,3Gal-mediated graft rejection

Gal alpha 1,3Gal-reactive (Gal-reactive) antibodies are a major impediment to pig-to-human xenotransplantation. We investigated the potential to induce tolerance of anti-Gal-producing cells and prevent rejection of vascularized grafts in the combination of alpha 1,3-galactosyltransferase wild-type (GalT(+/+)) and deficient (GalT(-/-)) mice. Allogeneic (H-2 mismatched) GalT(+/+) bone marrow transplantation (BMT) to GalT(-/-) mice conditioned with a nonmyeloablative regimen, consisting of depleting CD4 and CD8 mAb's and 3 Gy whole-body irradiation and 7 Gy thymic irradiation, led to lasting multilineage H-2(bxd) GalT(+/+) + H-2(d) GalT(-/-) mixed chimerism. Induction of mixed chimerism was associated with a rapid reduction of serum anti-Gal naturally occurring antibody levels. Anti-Gal-producing cells were undetectable by 2 weeks after BMT, suggesting that anti-Gal-producing cells preexisting at the time of BMT are rapidly tolerized. Even after immunization with Gal-bearing xenogeneic cells, mixed chimeras were devoid of anti-Gal-producing cells and permanently accepted donor-type GalT(+/+) heart grafts (>150 days), whereas non-BMT control animals rejected these hearts within 1-7 days. B cells bearing receptors for Gal were completely absent from the spleens of mixed chimeras, suggesting that clonal deletion and/or receptor editing may maintain B-cell tolerance to Gal. These findings demonstrate the principle that induction of mixed hematopoietic chimerism with a potentially relevant nonmyeloablative regimen can simultaneously lead to tolerance among both T cells and Gal-reactive B cells, thus preventing vascularized xenograft rejection.

Chimerism and xenotransplantation. New concepts

In both transplant and infectious circumstances, the immune response is governed by migration and localization of the antigen. If the antigenic epitopes of transgenic xenografts are sufficiently altered to avoid evoking the destructive force of innate immunity, the mechanisms of engraftment should be the same as those that permit the chimerism-dependent immunologic confrontation and resolution that is the basis of allograft acceptance. In addition to "humanizing" the epitopes, one of the unanswered questions is whether the species restriction of complement described in 1994 by Valdivia and colleagues also necessitates the introduction of human complement regulatory genes in animal donors. Because the liver is the principal or sole source of most complement components, the complement quickly is transformed to that of the donor after hepatic transplantation. Thus, the need for complementary regulatory transgenes may vary according to the kind of xenograft used. Much evidence shows that physiologically important peptides produced by xenografts (e.g., insulin, clotting factors, and enzymes) are incorporated into the metabolic machinery of the recipient body. To the extent that this is not true, xenotransplantation could result in the production of diseases that are analogous to inborn errors of metabolism. In the climate of pessimism that followed the failures of baboon to human liver xenotransplantation in 1992-1993, it seemed inconceivable that the use of even more discordant donors, such as the pig, could ever be seriously entertained; however, this preceded insight into the xenogeneic and allogeneic barriers that has brought transplantation infectious immunity to common ground. With this new insight and the increasing ease of producing transgenic donors, the goal of clinical xenotransplantation may not be so distant.

Discordant organ xenotransplantation in primates: world experience and current status

The pig-to-primate model is increasingly being utilized as the final preclinical means of assessing therapeutic strategies aimed at allowing discordant xenotransplantation. We review here the world experience of both pig-to-human and pig-to-nonhuman primate organ transplantation. Eight whole organ transplants using discordant mammalian donors have been carried out in human recipients; only one patient was reported (in 1923) to have survived for longer than 72 hr. Therapeutic approaches in the experimental laboratory setting have included pharmacologic immunosuppression, antibody and/or complement depletion or inhibition, the use of pig organs transgenic for human complement regulatory proteins, and conditioning regimens aimed at inducing a state of tolerance or specific immunologic hyporesponsiveness. The greatest success to date has been obtained with methods that inhibit complement-mediated injury, either by the administration of cobra venom factor or soluble complement receptor I to the recipient (with organ survival up to 6 weeks) or by the use of donor organs transgenic for human decay-accelerating factor (with organ survival up to 2 months). The future of xenotransplantation may lie in the judicious combination of current approaches.

Polymorphism in the human anti-pig natural antibody repertoire: implications for antigen-specific immunoadsorption

BACKGROUND

Anti-Galalpha1-3Gal antibodies cause hyperacute rejection (HAR) in pig-to-primate xenotransplantation. Long-term graft survival has not been achieved despite abrogation of HAR using transgenic pigs. IgG and IgM anti-Galalpha1-3Gal also play a role in the events following abrogation of HAR. Characterizing these antibodies and developing a system for their removal is therefore crucial to future success in xenotransplantation.

METHODS AND RESULTS

We have developed a neoglycoprotein enzyme-linked immunosorbent assay to probe the precise antigenic requirements for the binding of anti-Galalpha1-3Gal and have analyzed 77 normal sera. Sixty-six percent of individuals have IgG that recognizes the Galalpha1-3Gal di-, tri-, and pentasaccharides (D, T, and P, respectively), termed DTP phenotype. The frequency of other phenotypes was - -P, 13%; -TP, 12%; D-P, 8%; and DT-, 1%. The IgG subclasses found were IgG2 (95%), IgG3 (34%), IgG1 (31%), and IgG4 (17%). IgM in 91% of individuals recognized all three antigens. Further antibody heterogeneity was demonstrated when immunoadsorbents derived from Galalpha1-3Galbeta1-4GlcNAcbeta1-3Galbeta1-4Glc (PENTA) were tested. Galalpha1-3Galbeta1-4Glc (TRI 6) or PENTA agarose were effective for IgG removal in all individuals. For IgM removal, two deoxy derivatives were completely successful in 73% of individuals. Combining the Galalpha1-3Gal (DI) and TRI 6 agarose produced an adsorbent that completely removed anti-Galalpha1-3Gal IgG and IgM in all individuals tested.

CONCLUSIONS

Although the polymorphism in the anti-Galalpha1-3Gal repertoire, which we have demonstrated, represents a major obstacle to the development of an effective immunoadsorbent, the combination of DI and TRI 6 agarose appears sufficient for pig-to-human xenotransplantation.

Xenoantigens and xenoantibodies

Major efforts are being directed towards determining and modifying the glycosylated epitopes on pig vascular endothelial cells, against which human natural antibodies are directed. Genetic engineering techniques are being used in an effort to knock out or replace the major alpha galactosyl (alpha Gal) epitopes in mice, but to-date these have been only modestly successful in prolonging functional survival of such modified organs. Competitive glycosylation involving insertion of the gene for alpha 1,2 fucosyltransferase results in reduction of alpha Gal expression but also of presentation of hitherto cryptic antigens against which natural human antibodies are directed or could develop. The introduction of the gene for N-acetylglucosaminyltransferase III has been demonstrated to significantly reduce alpha Gal expression, and the intracellular expression of single chain Fv antibodies against alpha 1,3 galactosyltransferase also represses this enzyme activity. Several other carbohydrate antigens have been identified that could act as targets for human natural antibodies, and these include Gal alpha 1-3Le(x), Hanganutziu-Deicher, Tn, and Forssman antigens. The alternative approach, namely, the depletion of the recipient's natural antibodies, is proving difficult, but techniques for inducing B cell tolerance are being explored. The induction of a state of mixed hematopoietic chimerism in alpha Gal knockout mice has resulted in tolerance to the alpha Gal antigen. Tolerance to the SLA antigens of miniature swine is also being attempted in baboons by the transfer of SLA Class II genes into baboon bone marrow cells. It is hoped that one or a combination of these approaches may overcome the problem created by the presence of pig antigens against which humans have xenoreactive antibodies.