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

Molecular genetic basis of the human Forssman glycolipid antigen negativity

Forssman heterophilic glycolipid antigen has structural similarity to the histo-blood group A antigen, and the GBGT1 gene encoding the Forssman glycolipid synthetase (FS) is evolutionarily related to the ABO gene. The antigen is present in various species, but not in others including humans. We have elucidated the molecular genetic basis of the Forssman antigen negativity in humans. In the human GBGT1 gene, we identified two common inactivating missense mutations (c.688G>A [p.Gly230Ser] and c.887A>G [p.Gln296Arg]). The reversion of the two mutations fully restored the glycosyltransferase activity to synthesize the Forssman antigen in vitro. These glycine and glutamine residues are conserved among functional GBGT1 genes in Forssman-positive species. Furthermore, the glycine and serine residues represent those at the corresponding position of the human blood group A and B transferases with GalNAc and galactose specificity, respectively, implicating the crucial role the glycine residue may play in the FS α1,3-GalNAc transferase activity.

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.

Oral feeding with pig peripheral lymphocytes decreases the xenogeneic delayed type hypersensitivity reaction in galactosyltransferase knockout mice

PURPOSE

Oral tolerance induction has shown promising results in experimental allotransplantation models but is not well investigated in xenotransplantation. We investigated the possibility to induce tolerance against pig peripheral lymphocytes (pPBL) in galactosyltransferase knockout mice (gal -/-), which produce antibodies against Galalpha1-3Gal.

MATERIAL AND METHODS

Female (gal -/-) mice 6 to 8 weeks old weighing 35 to 40 g (n = 10) were fed orally every third day five times with 2 x 10(7) isolated, viable pPBL, or with phosphate-buffered saline (PBS) only (n = 7). They were then immunized subcutaneously on day 0 with a subcellular lysate from 4 x 10(7) isolated, viable pPBL. On day 13, 25 microL of a subcellular lysate corresponding to 1 x 10(7) isolated, viable pPBL was injected in the right dorsal foot pad, and the delayed type hypersensitivity (DTH) reaction was calculated after 24 hours by subtracting the swelling response from 25 microL PBS in the left footpad. Anti-Galalpha1-3Gal immunoglobulin IgG and IgM antibody titers were measured in the serum before oral feeding and at day 14.

RESULTS

The DTH reaction of the pPBL fed mice was 0.07 +/- 0.05 mm vs 0.57 +/- 0.23 mm for the controls (P < .001). No significant differences in anti Gal alpha1-3 Gal IgG and IgM antibody titers were seen.

CONCLUSIONS

This study demonstrates for the first time that oral delivery of pPBL can counteract the indirect T-cell reaction against xenogeneic subcellular antigens from pPBL. These observations warrant further investigation in immunologically modified mice and perhaps in primate models of xenotransplantation.

ORAL ADMINISTRATION OF XENOGENEIC ERYTHROCYTES INDUCES PRODUCTION OF ANTIBODIES THAT ARE CAPABLE OF INDUCING HYPERACUTE REJECTION OF CONCORDANT VASCULARIZED XENOGRAFTS

Oral tolerance induction has proven to be an effective approach for inducing antigen-specific unresponsiveness in several models for allogeneic transplantation and autoimmune diseases. The authors' preliminary studies, however, indicated that xenospecific antibodies are produced when rats are given mouse erythrocytes orally. This response was further examined. Mouse erythrocytes were administered to rats orally or intravenously during one or two episodes, and sera were obtained on day 9 or day 29, respectively. Rat sera containing a positive hemagglutinating titer against mouse antigens were injected into rats that had recently undergone xenotransplantation to study graft survival. Oral administration of xenogeneic cells induced a powerful antibody response consisting mainly of xenospecific immunoglobulin (Ig) M and IgG. This antibody response also induced hyperacute rejection as powerfully as sera from intravenously immunized rats. The authors' study thus indicates that oral administration of xenogeneic cells is a powerful immunization pathway that induces an antibody response capable of rejecting concordant vascularized xenografts.

Intragraft cytokine mRNA expression in rejecting and non-rejecting vascularized xenografts

BACKGROUND

The aim of the present study was to further investigate the characteristics of both graft-infiltrating cells and splenocytes during acute vascular rejection (AVR), cell-mediated rejection and non-rejection of vascularized concordant xenografts, by analysing both proinflammatory [interleukin-1beta (IL-1beta) and tumour necrosis factor (TNF-alpha)] and more specific [(IL-2, IL-4, IL-10, IL-12p40 and interferon-gamma (IFN-gamma)] cytokines. A parallel investigation was made of the antibody response of IgM and IgG to the xenografts.

METHODS

Mouse hearts were heterotopically transplanted to the neck vessels of recipient rats. Grafts, spleens and sera were collected from untreated (AVR) and cyclosporin A (CyA) treated animals on day 2 after transplantation. Organs from rats treated with 15-deoxyspergualin (DSG) or CyA and DSG in combination were harvested on both day 2 and day 8. Grafts from DSG-treated rats undergo cell-mediated rejection and stop beating on day 9 and forth, while CyA + DSG treatment results in long-term graft survival. Real-time quantitative reverse transcriptase polymerase chain reaction (RT-PCR) was applied for analysis of intragraft and splenic cytokine messenger RNA (mRNA) expression. The phenotypes of the graft infiltrating cells were characterized by immunohistochemistry. The antibody response was investigated by means of immunofluorescence, haemagglutination and flow cytometry.

RESULTS

All the studied cytokines (IL-1beta, IL-2, IL-4, IL-10, IL-12p40, IFN-gamma and TNF-alpha) were up-regulated in the grafts from rejecting untreated (day 2) and DSG-treated animals (day 8) in comparison with grafts from CyA + DSG treated animals (day 8). On day 2 under immunosuppression with CyA, DSG or CyA + DSG no or low cytokine mRNA levels were found. The mRNA levels of IL-2, IL-4 and IFN-gamma in the spleens were suppressed under both DSG- and CyA + DSG treatment on day 8. Immunofluorescence showed deposits of both IgM and IgG in grafts from untreated, CyA-treated (day 2) and DSG-treated (day 8) animals, while CyA + DSG treatment diminished these deposits on both day 2 and day 8. No circulating antibodies were identified in either group.

CONCLUSION

We hereby conclude that both AVR on day 2 and cell-mediated rejection on day 8 (under DSG treatment) in a concordant cardiac mouse-to-rat xenotransplantation model are associated with an increase of proinflammatory cytokines, T helper 1 (Th1)-associated cytokines as well as IL-10, while immunosuppression with CyA + DSG diminishes the levels of all examined cytokines. Grafts undergoing AVR or cellular rejection are subjected to deposits of both IgM and IgG, although circulating donor specific antibodies are undetectable in serum.

Pretransplant xenogeneic blood transfusions reduce the humoral response in a mouse-to-rat heart transplantation model

A mouse heart transplanted to a rat is rejected promptly 3 days after transplantation, independent of whether cyclosporin A (CyA) is used as an immunosuppressant or not. Adding a short course of deoxyspergualin (DSG) initially, in addition to continuous CyA treatment, results in long-term graft survival and permits retransplantation during CyA monotherapy. In this paper, we have explored the possibility of substituting the initial heart transplant with blood transfusions. Lymphocyte-enriched blood transfusions combined with CyA and an initial course of DSG proved to lower or eliminate the haemagglutinating antibody titre normally seen in acute vascular xenorejection. The therapy, however, did not prolong the mean survival of the cardiac xenograft, but the same treatment protocol could result in either hyperacute rejection or prolonged survival of up to 11 days. In conclusion, this and earlier studies propose that a humoral unresponsiveness can be induced if the recipient vascular circulation is exposed to a xenoantigen in a mouse-to-rat combination.