Anti-carbohydrate antibodies associated with hyperacute rejection in a vascularized mouse heart-to-rat xenotransplantation model

IRAK-4 and MyD88 deficiencies impair IgM responses against T-independent bacterial antigens

IRAK-4 and MyD88 deficiencies impair interleukin 1 receptor and Toll-like receptor (TLR) signaling and lead to heightened susceptibility to invasive bacterial infections. Individuals with these primary immunodeficiencies have fewer immunoglobulin M (IgM)(+)IgD(+)CD27(+) B cells, a population that resembles murine splenic marginal zone B cells that mount T-independent antibody responses against bacterial antigens. However, the significance of this B-cell subset in humans is poorly understood. Using both a 610 carbohydrate array and enzyme-linked immunosorbent assay, we found that patients with IRAK-4 and MyD88 deficiencies have reduced serum IgM, but not IgG antibody, recognizing T-independent bacterial antigens. Moreover, the quantity of specific IgM correlated with IgM(+)IgD(+)CD27(+) B-cell frequencies. As with mouse marginal zone B cells, human IgM(+)CD27(+) B cells activated by TLR7 or TLR9 agonists produced phosphorylcholine-specific IgM. Further linking splenic IgM(+)IgD(+)CD27(+) B cells with production of T-independent IgM, serum from splenectomized subjects, who also have few IgM(+)IgD(+)CD27(+) B cells, had reduced antibacterial IgM. IRAK-4 and MyD88 deficiencies impaired TLR-induced proliferation of this B-cell subset, suggesting a means by which loss of this activation pathway leads to reduced cell numbers. Thus, by bolstering the IgM(+)IgD(+)CD27(+) B-cell subset, IRAK-4 and MyD88 promote optimal T-independent IgM antibody responses against bacteria in humans.

Gal/non-Gal antigens in pig tissues and human non-Gal antibodies in the GalT-KO era

Our knowledge regarding Gal and non-Gal antigens in GalT-KO pig tissues can be summarized as α3Galactosyl-tranferase gene knock out eliminates the Galα3Galβ4GlcNAc-R antigen expression in pig tissues as well as anti-Gal antibody binding. Other Galα-terminating saccharides (e.g. iGb3 glycolipids and Galα2 determinants) may be present but have not been documented. α3Galactosyl-tranferase gene knock out slightly changes the carbohydrate antigen expression but no "new" antigens recognized by the human immune system have been found. Non-Gal antigens are both of protein and carbohydrate nature but their exact chemical structures are poorly defined. Regarding human non-Gal antibodies our knowledge is as Non-Gal antibodies exist naturally and increase in humans/non-human primate (NHP) receiving WT or GalT-KO pig grafts. Non-Gal antibodies with new antigen epitope recognition can be induced in humans/NHP after challenge by WT or GalT-KO pig grafts. Non-Gal antibodies react with both carbohydrates and proteins. Part of the protein reactivity is directed to glycoprotein carbohydrates chains. Non-Gal antibodies reacting with neuraminic acid terminated saccharides (both N-Acetyl and N-Glycoloyl variants) are present in humans/NHP. Anti-neuraminic acid antibodies are increased, as well as induced, after grafting pig organs into humans/NHP. Non-Gal antibodies does not cause hyperacute xenorejection but can be cytotoxic and cause xenoorgan damage. If humans sensitized to HLA antigens are at a higher risk of rejecting pig xenograft compared with non-sensitized individuals is not fully clarified. Clinical trials are needed to evaluate the relevance of non-Gal antigens/antibodies and for the xenofield to advance.

Antibody responses to xenogenic antigens?a study in the mouse-to-rat system

Antibodies play a crucial role in the rejection of an organ that has been transplanted between different animal species, i.e. xenotransplantation. In previous work, we have induced a state of humoral tolerance where mouse-to-rat heart grafts continued to beat under ciclosporine A monotherapy. Initially, a combined treatment with ciclosporine A and 15-deoxyspergualin was given. This state of tolerance could not be reproduced when the vascularised heart graft was replaced with a free tissue graft or xenogeneic blood transfusions. To gain further insight into the humoral response against mouse antigens, we studied the antibody production in naive rats and rats challenged with heart transplants, heart cells, mononuclear cells (MNC) and erythrocytes from mice. Rats not challenged with any mouse cells or organs had a moderate amount of antibodies targeted against mouse MNC as well as rosette-forming cells in the spleen targeted against mouse erythrocytes. A challenge with either mouse MNC or erythrocytes lead to immunisation with antibodies of both IgM and IgG subtype directed against both MNC and erythrocytes. Antibody titres against mouse erythrocytes in animals challenged with MNC were not detectable until day 7, whereas antibody titres against mouse MNC in animals challenged with erythrocytes were detected on day 1. Immunisation with mouse erythrocytes raised the titre of rosette-forming cells in the spleen compared with naive rats (P < 0.05). Our data indicate that different xenogeneic antigens in the mouse-to-rat system are shared between heart cells, MNC and erythrocytes; however, the immunisation patterns differ regarding the time when antibodies are first detected.

Decreasing ratios of phosphocreatine to beta-ATP correlates to progressive acute rejection in a concordant mouse heart to rat xenotransplantation model

Biopsies are difficult to perform in rodent heart transplant models without compromising the graft function and therefore other means to evaluate the grafts repeatedly and noninvasively are warranted. The goal of the present study was to measure changes in ratios of high energy phosphorus containing metabolites detected with in vivo 31Phosphorous Magnetic Resonance Spectroscopy ((31)P MRS) in a xenotransplantation model and to investigate if these ratios correlated to histological signs of acute xenograft rejection. Thirty-five heart transplantations were performed (NMRI-mice to Lewis (RT1(1)) rats). Thirteen heart transplants underwent repeated daily in vivo (31)P MRS measurements and 22 grafts were measured on any of 4 postoperative days and thereafter sacrificed for histology. A modified scoring system based on Billingham's criteria was used to stage the rejection process. The median graft survival was 3.0 +/- 0.44 (median +/- SD) days (n = 17). Significant differences, both overall and interday, could be calculated for the phosphocreatine (PCr)/beta-adenosine triphosphate (beta-ATP) ratios and for the rejection score. The decreases in PCr/beta-ATP ratios correlated significantly to the progressive acute rejection process in the sacrificed grafts (P = 0.01). Further studies are indicated to establish the potential of (31)P MRS in immunosuppressed recipients of vascularized xenotransplants with prolonged graft survival.

Characterization of Forssman and other antigen/antibody systems in vascularized mouse heart to rat xenotransplantation

In the present study, the nature of hyperacute xenograft rejection was closely studied in a vascularized mouse-to-rat transplantation model. Antibodies against mouse heart, erythrocytes and lymphocytes and against the Forssman antigen were raised in the rat. Upon heterotopic heart transplantation the respective antisera were intravenously (i.v.) injected. Passive transfer of antiheart, antierythrocyte or antilymphocyte serum resulted in hyperacute rejection of the transplanted mouse heart. Subfractionation of the antiheart serum showed that the capacity to induce hyperacute rejection was carried by the immunoglobulin (Ig)G fraction. When antierythrocyte serum adsorbed with mouse erythrocytes was administered the cardiac grafts remained beating. To the contrary, antilymphocyte serum adsorbed with erythrocytes still had the capacity to induce hyperacute rejection. None of the rats that had previously been challenged with the Forssman antigen rejected their grafts hyperacutely. Subsequent investigations by electron microscopy revealed that the Forssman antigen is expressed on dendritic cells (DC) adjacent to the vessels, but not on the vascular endothelium, thus explaining the inability of the anti-Forssman serum to induce hyperacute rejection. Taken together, we have demonstrated the existence of several xenoantigens that can be targets for antibody-mediated rejection, suggesting that more than one relevant xenoantigen exists also in more distantly related combinations, such as the pig-to-human combination.

Concordant xenotransplantation--non-vascularized pancreatic islets are more difficult to regraft than the vascularized heart

We have previously demonstrated that it is possible to perform retransplantation of a xenogeneic heart (mouse-to-rat) using cyclosporine A as monotherapy, provided that the first heart is transplanted under a short course of deoxyspergualin (DSG). If DSG is omitted, the first heart is rejected within four days and the second heart succumbs to hyperacute rejection within minutes. A mouse heart as first graft does not protect a consecutive pancreatic islet graft, although the heart continues to function after rejection of the cellular graft. One explanation for this discrepancy may be the fact that cellular grafts, as pancreatic islets, lack an endothelial lining. We have, therefore, further investigated possible differences between vascularized and non-vascularized xenografts regarding their capacity to induce unresponsiveness. The use of pancreatic islets as primary graft neither accelerated nor decelerated the speed of rejection of the vascularized heart used as secondary graft. Furthermore, hemagglutinating and cytotoxic antibody titres responded in the same manner as in naive rats transplanted with a mouse heart. Retransplantation with pancreatic islets also resulted in complete rejection of both the primary and secondary grafts. Thus, the lack of unresponsiveness cannot simply be explained by differences, between the pancreatic and cardiac tissues, in antigen expression. In addition, intraperitoneal transplantation of mouse heart cells as primary graft resulted in rejection of a secondary cardiac graft after three days. However, it cannot be totally excluded that the time of antigen exposure had an impact on these results. In conclusion, our previous and present studies suggest that the presence of an intact vascular bed, both in the first and second graft, is necessary to create a state of unresponsiveness. Because the pancreatic islets lack an endothelial lining, they do not benefit from an unresponsiveness of the immune system. Neither are they able to induce such an unresponsiveness.

Induction of anti-Forssman antibodies in the hamster-to-rat xenotransplantation model

BACKGROUND

In the hamster-to-rat heart xenotransplantation model, the serum response of the host contributes to determine whether the xenograft is accommodated or rejected.

METHODS

To further characterize the serum response in this model, we compared anti-hamster antibodies found in naive LEW-1A rats, or in LEW-1A rats rejecting or accommodating a hamster heart, using a combination of cobra venom factor (CVF) and cyclosporin A (CsA) given for 10 days, and then CsA alone.

RESULTS

Hamster hearts grafted into rat recipients contained IgG and IgA deposits to the same extent whether the xenograft was rejected or accommodated. Only immunoglobulins of the IgM isotype were found to be more abundant in recipients rejecting their graft. A significant part of this IgM response was directed toward the Forssman antigen, a sphingolipid present in the hamster but not in the rat. However, although anti-Forssman antibodies bind in situ to hamster tissues, this binding was not able to induce hyperacute rejection after antibody transfer. Furthermore, depletion of anti-Forssman antibodies from a rejecting serum did not modify its rejection properties.

CONCLUSION

Unlike the pig-to-primate discordant xenotransplantation model, in which preexisting anti-carbohydrate antibodies are directly responsible for hyperacute rejection, in the concordant hamster-to-rat situation, the evoked IgM anti-Forssman carbohydrate antibodies do not appear to be the main cause of the vascular rejection.

Failure of anti-Forssman antibodies to induce rejection of mouse heart xenografts

The Forssman antigen has been proposed to be a target for the xenograft reaction in selected species combinations, including the rat and mouse, which are Forssman-negative and -positive species respectively. The mouse represents an important experimental model for a variety of immune-mediated disease processes, and the availability of a simple, inexpensive target antigen could provide an important tool for studying a selected portion of the immunologic basis for the rejection of xenografts. We have examined the potential that antibodies directed against mouse Forssman antigen could cause the hyperacute rejection of mouse heart xenografts in naive rat recipients. The Forssman antibodies tested included rat anti-mouse (R-anti-M) antiserum, R-anti-M antiserum depleted of anti-Forssman (anti-F) antibodies, rat anti-sheep red blood cell (SRBC) antiserum containing anti-F antibodies and a rat monoclonal anti-F IgM antibody. Our results demonstrate that the R-anti-M antiserum at day 4 post transplantation displayed significant titers (1:512-4096) of hemagglutinating antibodies for SRBC and mild to moderate levels of IgM that specifically binds to Forssman glycolipid (GalNAcalpha1-3GalNAcbeta1-3Galalpha1-4Galbeta1- 4Glcbeta1-1ceramide) as measured by an enzyme-linked immunosorbent assay (ELISA). Passive transfer of the R-anti-M serum to rats receiving mouse cardiac grafts immediately after transplantation caused hyperacute rejection of the xenografts. Sequential immunoabsorption of R-anti-M sera with SRBCs resulted in total removal of the anti-Forssman activity (as defined by negative hemagglutination titer and minimal binding to Forssman glycolipid in ELISA). The anti-F Ab-depleted R-anti-M antisera, however, retained the capacity to induce hyperacute rejection of the mouse hearts [n = 6, median survival time (MST) 13 min] when passively transferred to rat recipients. Anti-Forssman antibodies induced by immunization of LEW rats with SRBCs or a rat anti-Forssman monoclonal antibody, mAb M.1.22.25, exhibited substantial anti-Forssman activity (hemagglutinating titer 1:512-4096 and moderate-to-strong binding to Forssman glycolipid in ELISA respectively). These antibodies also failed, however, to trigger hyperacute rejection of mouse cardiac xenografts. In conclusion, our results suggest that the rat anti-Forssman antibodies, including those stimulated by mouse cardiac xenografts, do not appear to play a role in the immediate (hyperacute) rejection of mouse heart xenografts.

Induction of specific transplantation tolerance across xenogeneic barriers in the T-independent immune compartment

After transplantation of primarily vascularized xenografts (Xgs), T-independent mechanisms may lead to Xg rejection before T-cell activation even takes place. The possibility of achieving T-independent xenotolerance was evaluated in nude rats that normally reject hamster cardiac Xgs within 4 days by non-T cell-mediated mechanisms. After donor antigen infusion, temporary NK-cell depletion and a 4-week administration of Leflunomide, hamster heart grafts survived even after withdrawal of immunosuppression. Tolerant rats accepted second hamster hearts, but promptly rejected mouse heart Xgs. In vivo immunization and in vitro cytotoxicity assays indicated that this species-specific tolerance was based on B-lymphocyte and NK-cell tolerance respectively.

Changes in cell surface glycosylation in alpha1,3-galactosyltransferase knockout and alpha1,2-fucosyltransferase transgenic mice

BACKGROUND

Inactivation of the alpha1,3-galactosyltransferase (GalT) gene by homologous recombination (knockout [KO] mice) and competition for the enzyme's N-acetyllactosamine substrate by transgenically expressed alpha1,2-fucosyltransferase (H-transferase) are two genetic approaches to elimination of the Gal alpha1,3Gal (alphaGal) epitope, which is the major xenoantigen in pigs against which humans have preformed antibodies. Such genetic manipulations often have unpredictable results.

METHODS

A panel of 19 selected lectins was used to characterize the changes in cell surface glycosylation in GalT KO and H-transferase transgenic mice, compared with nontransgenic littermate controls.

RESULTS

GalT KO mice showed complete elimination of the alphaGal epitope, as reported previously. Surprisingly, however, this was associated with only a modest increase in N-acetyllactosamine residues and had little other effect on the pattern of lectin binding. In contrast, the pattern of lectin binding to H-transferase transgenic mouse cells was more profoundly disturbed and indicated, in addition to the expected expression of H substance and suppression of the alphaGal epitope, that there was a marked reduction in alpha2,3-sialylation and exposure of the normally cryptic antigens, sialylated Tn and Forssman antigens. Similar changes in lectin reactivity with porcine aortic endothelial cells were induced by neuraminidase treatment.

CONCLUSIONS

Lectins were able to bind underlying carbohydrate structures (sialylated Tn and Forssman antigens) that are normally cryptic antigens on H-transferase transgenic mouse spleen and cardiac endothelial cells, probably as a consequence of the reduction in the electronegativity of the cell surface due to reduced sialylation. As humans have preformed anti-Tn and anti-Forssman antibodies, it is possible that these structures may become targets of the xenograft rejection process, including hyperacute rejection.

Successful retransplantation of mouse-to-rat cardiac xenografts under immunosuppressive monotherapy with cyclosporine

The present study was undertaken to investigate whether retransplantation with a second xenograft, from the same species as the primary graft, is possible to achieve using only moderate immunosuppression. Heterotopic mouse-to-rat cardiac transplantations were performed, and the recipients were treated with 15-deoxyspergualin (DSG) and cyclosporine (CsA) at high doses for days -1 to 4 and at moderate doses for days 5 to 28. From day 29 and onward, the immunosuppressive protocol consisted of daily oral administration of CsA 10 mg/kg as monotherapy. Animals that had beating grafts when DSG treatment was stopped were retransplanted 56-154 days after the primary transplantation, either with a vascularized graft (heart) or with nonvascularized graft (pancreatic islets), under continued therapy with CsA. Six of 10 secondary cardiac xenografts functioned for more than 50 days and were harvested beating after 60-100 days. In contrast, nonimmunosuppressed or DSG-treated rats are known to reject a second cardiac mouse graft hyperacutely. The unresponsiveness was confined to cardiac tissue, as the pancreatic islets, transplanted under the kidney capsule, were totally rejected after 14 days. Long-term functioning cardiac xenografts, primary and secondary, had a well-preserved morphology, and infiltrating mononuclear cells were found just in the periphery of the grafts. A majority of these cells were macrophages expressing the ED1, but not the ED2, antigen. No deposition of IgG or complement was seen in any of the graft vessels, whereas a slight deposition of IgM was observed in some vessels of both primary and secondary grafts. In conclusion, we have demonstrated that unresponsiveness can be induced by effective immunosuppression of the recipient at the time of the initial transplantation, so that retransplantation with a second xenograft can be performed successfully under single-drug immunosuppressive therapy with CsA.