Sequential, morphological, and antidonor antibody analysis in a hamster-to-rat heart transplantation model

The free radical scavenger S-PBN significantly prolongs DSG-mediated graft survival in experimental xenotransplantation

BACKGROUND

Nitrones such as 2-sulfo-phenyl-N-tert-butyl nitrone (S-PBN) are known to trap and stabilize free radicals and to reduce inflammation. Recently, S-PBN was shown to reduce infiltration of T lymphocytes and the expression of adhesion molecules on the endothelium in experimental traumatic brain injury. We hypothesized that S-PBN could reduce infiltration of T lymphocytes during cell-mediated xenograft rejection and thereby increase graft survival. The concordant mouse-to-rat heart transplantation model was used to test the hypothesis. In this model, grafts undergo acute humoral xenograft rejection (AHXR) almost invariably on day 3 and succumb to cell-mediated rejection on approximately day 8 if AHXR is inhibited by treatment with 15-deoxyspergualin (DSG).

MATERIAL AND METHODS

Hearts from Naval Medical Research Institute (NMRI) mice were transplanted to the neck vessels of Lewis rats. Recipients were treated with S-PBN (n=9), DSG (n=9), S-PBN and DSG in combination (n=10) or left untreated (n=9) for survival studies. S-PBN was given daily intraperitoneally at a dose of 150 mg/kg body weight (BW) on day -1 to 30, and DSG was given daily intraperitoneally at a dose of 10 mg/kg BW on day -1 to 4 and 5 mg/kg BW on day 5 to 21. Nine additional recipients were given S-PBN only on days -1 and 0 in combination with continuous DSG treatment. Grafts were monitored until they stopped beating. Additional recipients were treated with S-PBN (n=5), DSG (n=5), S-PBN and DSG in combination (n=6) or left untreated (n=5) for morphological, immunohistochemical and flow cytometry analyses on days 2 and 6 after transplantation.

RESULTS

S-PBN treatment in combination with DSG resulted in increased median graft survival compared to DSG treatment alone (14 vs. 7 days; P=0.019). Lower number of T lymphocytes on day 6 (P=0.019) was observed by ex vivo propagation and flow cytometry when combining S-PBN with DSG, whereas immunohistochemical analyses demonstrated a significant reduction in the number of infiltrated CD4+, but not TCR+, cells. S-PBN treatment alone had no impact on graft survival compared to untreated rats (3 vs. 3 days). No differences were seen in ICAM-1 and VCAM-1 expression or in morphology between the groups.

CONCLUSION

The combination of S-PBN and DSG treatment increases xenograft survival. The main effect of S-PBN appears to be in direct connection with the transplantation. Because of its low toxicity, S-PBN could become useful in combination with other immunosuppressants to reduce cell-mediated xenograft rejection.

Early onset of rejection in concordant hamster xeno hearts display signs of necrosis, but not apoptosis, correlating to the phosphocreatine concentration

BACKGROUND

The importance of apoptosis contra necrosis for ischemia/reperfusion (RP) and acute rejection in concordant rodent xenotransplantation is largely unknown. We explored this question by comparing rodent allo and concordant xenotransplants with different morphological methods to detect apoptosis and biochemical data on the levels of high-energy phosphates obtained with in vitro 31Phosphorous Magnetic Resonance Spectroscopy (31P MRS). More specifically, we applied a hitherto unused method in transplantation research, apoptosis specific biotin labeled oligonucleotides designed with a 10 base pair stem region and a 20 nucleotides large loop that form a hairpin like shape. The results obtained with this method were compared to results obtained with the more widely used in situ 3'-end labeling of DNA (TUNEL) assay and extraction and gel electrophoresis of labeled DNA (DNA laddering).

METHODS

Cervical heart transplantations were performed between inbred Lewis (L) (RT1l) to L, L to DA (RT1a) rats, hamster (H) to H and H to L (X) (n=5 for all groups except for X, n=9). All hearts were subjected to 30 min of cold ischemia (+4 degrees C) and 6 h of RP before explantation. In vitro 31P MRS was used to determine the phosphocreatine (PCr), beta-adenosine triphosphate (beta-ATP) concentrations and the PCr/beta-ATP ratio of the transplants. We correlated the biochemical data to haematoxylin and eosin (H & E) stained tissue slides scored for rejection, infiltration of antibodies and complement depositions, DNA extraction and gel electrophoresis of labeled DNA (DNA laddering), in situ 3'-end labeling of DNA (TUNEL) and the apoptosis specific hairpin probe assays scoring.

RESULTS

The rejection score of the xeno grafts differed significantly compared to their syngeneic hamster to hamster controls (2.40 +/- 0.25 vs. 1.20 +/- 0.20; P=0.005) and they had a significantly higher TUNEL score, 228 +/- 15 vs. 2.44 +/- 0.32 (P=0.009), that correlated to changes in PCr concentration (P<0.001) and to the PCr/beta-ATP ratio (P=0.01). The uptake was mainly (90-95%) located to 1-2 microm large extra cellular 'granule'. A picture resembling early necrosis was seen on the H & E stainings and reflected in the Billingham rejection score above.

CONCLUSIONS

After 6 h of RP the onset of acute rejection in the concordant hamster xeno hearts displayed features of early, possibly mitochondrial, necrosis, but not apoptosis, which correlated to changes in the PCr concentration and the PCr/beta-ATP ratio. The mechanism for the early rejection observed is unclear and might be caused by other factors in the sera apart from cellular components, antibodies and complement factors. Identification of the underlying mechanisms could enable us to design rational therapies that prevent activation of the recipient's innate immune response.

The graft content of hyaluronan is increased during xenograft rejection

BACKGROUND

Hyaluronan, a macromolecule with strong water binding capacity, is associated with interstitial oedema during rejection of allogeneic transplants. However, the involvement of hyaluronan during xenograft rejection has previously not been investigated. The aims of this study were to characterize hyaluronan content and distribution during rejection of concordant mouse-to-rat cardiac xenografts, and to explore the effects of hyaluronidase (HAse) on xenograft survival.

METHODS

Graft recipients were treated with 15-deoxyspergualin (DSG) or both HAse and DSG. Grafts were removed on day 5 from some of the animals to analyse hyaluronan and water content, while other animals were used to investigate graft survival. The hyaluronan content was measured by a radiometric assay and the distribution was analysed by histochemical staining.

RESULTS

In xenografts undergoing rejection (the DSG group) there was a strong increase of the hyaluronan [555 +/- 93 microg/g dry weight (dw)] and water (82.7 +/- 0.4%) contents compared with normal mouse heart tissue (166 +/- 10 microg/g dw; P < 0.01 and 78.6 +/- 0.5%; P < 0.001, respectively). The combined use of HAse and DSG reduced the accumulation of hyaluronan (284 +/- 43 microg/g dw; P < 0.05 vs. DSG) but did not affect the average water content. The average graft survival time did not differ between the groups; however, three grafts in the HAse + DSG-treatment group survived much longer than the longest-surviving grafts in the DSG group.

CONCLUSIONS

These data suggest that the graft content of hyaluronan considerably increases during xenograft rejection. HAse effectively reduces this accumulation, but does not affect the average water content.

T cell response in xenorecognition and xenografts: a review

Xenotransplantation has recently become a subject of interest for the transplantation community due to the current organ shortage, which could be partially or even totally solved by the development of this strategy. The humoral response, which arises as a result of species disparities, is the major obstacle to the success of xenotransplantation. However, if the use of different strategies such as plasmapheresis, immunoadsorption, the utilization of organs from transgenic pigs for complement regulatory molecules and new immunosuppressive drugs, may allow to overcome or reduce the early antibody mediated rejections (hyperacute or acute vascular rejection), delayed responses based on cellular activations will still occur. In this review, despite the fact that different cell populations have been shown to be implicated in these phenomena (NK, granulocytes, macrophages), we will focus on recent published information concerning T cell response only, in xenorecognition.

A crucial role of host CD80 and CD86 in rat cardiac xenograft rejection in mice

BACKGROUND

Graft rejection can be initiated by two primary pathways of antigen presentation: (a) direct activation of host T cells by donor-derived antigen presenting cells (APC) and (b) indirect presentation of processed graft antigens by host APC.

METHODS

We investigated the differential roles for direct and indirect antigen presentation by preventing the CD28 costimulatory pathway with monoclonal antibodies to rat or mouse CD80 and CD86 in a rat-to-mouse cardiac transplantation model.

RESULTS

Although the mouse anti-rat monoclonal antibodies to CD80 and CD86 did not significantly prolong the survival of rat cardiac xenografts in mice, the rat anti-mouse monoclonal antibodies to CD80 and CD86 did prolong the survival. Development of the anti-donor antibodies was inhibited, and the deposition of C3, IgM, and IgG on endothelium in the xenografts was mild in the anti-mouse CD80/CD86-treated mice. Infiltration of macrophages, neutrophils, and lymphocytes expressing perforin and interferon-gamma was decreased by the anti-mouse CD80/CD86 treatment.

CONCLUSIONS

These findings suggest that the indirect antigen presentation, which is mediated by CD80 and CD86 pathway on host APC, plays a crucial role in concordant cardiac xenograft rejection.

Phase-directed therapy and cardiac xenograft survival

Xenotransplant rejection is facilitated not only by T cell upregulation but also by endothelial activation and B cell/antibody mechanisms, which standard immunosuppression is unable to overcome and xenorejection ensues. However, therapy directed specifically at each phase of xenorejection may improve xenograft survival. To study this we used a heterotopic cardiac xenotransplant mode (Syrian hamster to Lewis rat). Controls had no immunotherapy. Xenorecipients received cyclosporine to restrict T cellular response/development or cyclophosphamide, an antiproliferative, to reduce xenoreactive clones and antibody/complement injury, or anti-TNF antibody to alter cytokine cascades and endothelial activation/inflammation. Further xenorecipients received combinations. While single modalities alone did not enhance survival, combinations appeared to be at least additive in vivo, suggesting that therapy directed at specific phases of xenorejection may prove useful.

The influence of concordant xenografts on the humoral and cell-mediated immune responses to subsequent allografts in primates

The humoral and cell-mediated immune responses to subsequent allografts were determined in primate recipients after concordant xenotransplantation as a bridge to allotransplantation. Heterotopic heart transplants (n = 4) were performed from cynomolgus monkeys into ABH type-matched olive baboons followed 2 weeks later by allotransplantation from ABH type-matched baboon donors. Allografts were explanted at 8 weeks. All recipients underwent splenectomy at the time of xenotransplantation and received immunosuppression with cyclosporine, azathioprine, and methylprednisolone. Concordant xenotransplantation in these primates did not induce humoral or cell-mediated immune responses that jeopardized subsequent allografts. The degree of xenospecific immune reactivity, as determined by specific cytotoxicity of recipient T-cell lines derived from the xenograft and extent of histologic xenograft rejection, did not predict the severity of subsequent allograft rejection. In two of the four recipients, xenotransplantation induced an alloreactive humoral response against antigens expressed by the B cells of more than 50% of members from a panel of 12 unrelated baboons. In all recipients, priming with xenogeneic splenocytes in vitro induced an accelerated proliferative T-cell response to allogeneic lymphocytes from 16% of this panel. This study affirms the role of concordant xenografts as appropriate biologic bridges to human allotransplantation. However, our results suggest that xenoreactive baboon memory CD4 T cells may recognize major histocompatibility complex class II--like structures shared between the xenogeneic and allogeneic targets. The potential allorecognition induced by a xenograft may affect the process of subsequent allograft donor selection.

Cell and extracellular matrix rejection in arterial concordant and discordant xenografts in the rat

Vascularized xenografts are rejected acutely and hyperacutely in concordant or discordant combinations, respectively. We investigated the impact of the donor-recipient combination on the rejection of arterial xenografts, analyzing the cellular and extracellular matricial compartments. Aortic xenografts were performed in a concordant (hamster) and a discordant (guinea pig) combination with Lewis rat. Graft cells and immune effectors were characterized by immunohistochemistry after 15 min and up to 30 days postimplantation. Macroscopic and microscopic structure of the grafts was studied at 60 days. IgC in the concordant combination and C3, C5b9, and IgM in the discordant combination deposited on endothelial cells, acutely and hyperacutely, respectively. The same immune effectors deposited on medial smooth muscle cells, but later than on endothelial cells. In both combinations the medial extracellular matrix was covered by IgM and IgC and infiltrated by monocytes (90%) and T lymphocytes (10%), with elastinolysis in the vicinity of monocytes. However, elastin resorption in the media at day 60 differed in concordant and discordant xenografts(75+/-10% and 99+/-1%, respectively). Intimal thickening and aneurysm developed in concordant and discordant combinations, respectively. Unlike arterial allografts, arterial xenografts are not a homogeneous group. The donor-recipient combination determines the mechanism and the timing of graft cell rejection, as well as the magnitude of medial elastin injury. As a consequence, chronic graft remodeling differs in the two combinations.

Hamster to rat kidney transplantation: technique, functional outcome and complications

Hamster to rat kidney transplantation has only recently been introduced as model of concordant xenografting. The kidney model offers unique possibilities for studying both immunological and functional aspects of xenografts as opposed to the widely used heterotopic heart model. This article provides a detailed description of surgical technique as well as data on functional outcome and complications. The renal artery with a small segment of the aorta is sutured end-to-side to the abdominal aorta of the recipient, and the renal vein is anastomosed end-to-side to the inferior vena cava. The urinary system is reconstructed by bladder-to-bladder anastomosis. Xenografts will maintain close to normal serum-creatinine levels for 2-3 days, after which they are rejected. Complications occurred in 22% of xenografts. Postrenal obstruction due to severe hematuria or ureter stenosis was the most frequent problem encountered.

Anti-CD4 monoclonal antibody treatment in combination with total lymphoid irradiation and cyclosporin A in hamster-to-rat cardiac transplantation. Morphological features of heart grafts, recipient spleens and lymph nodes

Significantly prolonged graft survival (GS) of hamster hearts transplanted heterotopically into rats can be achieved by different immunosuppressive treatment strategies. The exact mechanism of graft rejection is unclear, but it seems to be a primarily humoral, antibody-mediated type of rejection. The histopathology of long-term surviving grafts is controversial and the morphology of lymphoid tissue in spleens and lymph nodes as the possible site of anti-donor antibody formation has not previously been investigated. This report demonstrates a significantly prolonged GS in hamster-to-rat cardiac transplantation after combined treatment with total lymphoid irradiation (TLI), cyclosporin A (CyA) and anti-CD4 monoclonal antibodies (MAb), where long-term GS (> 100 days) could be achieved in a few animals. The histopathology of heart grafts showed predominantly chronic vascular changes with endothelial proliferation, intimal thickening and vessel obliteration. No substantial cellular reactivity in terms of mononuclear/lymphoid cell infiltration could be demonstrated in rejected grafts. Spleens and lymph nodes were characterized by a profound global reduction in lymphoid tissue after preoperative TLI. Although subsequent lymphoid regeneration was depressed due to postoperative immunosuppression, a significant increase in IgM-positive plasma cells was observed, supporting evidence of an antibody-mediated mechanism of graft rejection. The role of CD4+ cells is unclear, but anti-donor antibody formation might involve T-cell help.

Xenotransplantation rejection is antibody-mediated in both sensitized and nonsensitized recipients

This study analyzes the mechanisms involved in xenotransplantation rejection between closely related species. Hamster hearts were transplanted heterotopically into both normal rats and rats previously sensitized by the transfusion of donor blood. Sequential ultrastructural and immunohistochemical analyses were performed on the grafts, spleens, and sera. The data obtained support the view that induced antibodies directed against the xenograft endothelium play a very important role in producing graft damage. Moreover, the demonstration of antibodies against myocyte determinants suggests that it is possible, in this particular model, that the antiendothelial antibodies are not the only ones involved in the injury process.

Immunosuppressive effect of total body irradiation and cyclosporine A on graft survival and the lymphatic system in a cardiac hamster-to-rat transplantation model

The aim of the study was to evaluate the effect of total body irradiation (TBI) and cyclosporine A (CyA) on graft survival and the lymphatic system in a concordant hamster-to-rat heart transplantation model, and to compare these effects with those of total lymphoid irradiation (TLI). Preoperatively TBI was given as a single dose of 5 Gy, CyA was given intramuscularly at a dose of 10 mg/kg/day. TBI prolonged graft survival to seven days. Combined TBI and CyA prolonged graft survival to ten days. The effect of TBI on graft survival, total white blood cell count (WBC) and differential counts was reproducible but not as distinct as the effect of TLI. Analysis of changes in WBC and differential counts combined with the morphology of the grafts at rejection and of spleens from TBI- and CyA-treated animals indicates a reproducible immunosuppressive effect of TBI and a severe type of acute humoral rejection with vasculitis and cellular infiltrates dominated by macrophages and neutrophilic granulocytes. In conclusion, we find TBI a simple pretreatment which may be useful in combination with other immunosuppressive treatment as preoperative induction and depletion therapy.

Evidence for a primarily humoral rejection mechanism in concordant xenogeneic heart transplantation. A sequential immunohistological study in a hamster-to-rat model

Heterotopic heart transplantations in an unmodified hamster-to-rat model were studied sequentially by immunohistochemical analysis. Monoclonal mouse anti-rat antibodies against B cells, T cells, macrophages and neutrophilic granulocytes (MRC OX-19, MRC OX-38, MRC OX-8, MRC OX-22, MRC OX-33, MRC OX-41 and MRC OX-42) were used in an indirect immunoperoxidase technique and monoclonal mouse anti-rat IgM and IgG were used for immunofluorescence. In grafts investigated after 6 h (N = 8) minimal infiltration of macrophages was demonstrated with MRC OX-41+ and MRC OX-42+ cells. No T- or B cells were seen. In a few cases, deposition of IgG and IgM was seen related to the endothelium of larger vessels. In grafts examined 24 h after transplantation (N = 10) the number of MRC OX-41+ and MRC OX-42+ cells had increased and in half of the cases IgM and IgG were located in relation to endothelial cells of larger vessels. In grafts investigated 48 h after transplantation (N = 8) the infiltration with MRC OX-41+ and MRC OX-42+ cells had further increased and a few scattered MRC OX-19+ and MRC OX-8+ cells appeared. At this time all but one heart had deposition of IgG and IgM in the vessel walls. Upon complete rejection (N = 8) diffuse infiltration of MRC OX-41+ and MRC OX-42+ cells was seen, but still only a few scattered T cells could be demonstrated. At this time IgG an IgM deposition appeared in all vessels and was also located in relation to the capillaries. These results further support our hypothesis that acute xenograft rejection in this animal model is primarily of the humoral type.