An ultrastructural analysis of concordant and discordant cardiac xenografts in unmodified recipients

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.

Acute xenograft rejection, late xenograft rejection and long term survival xenografts in the hamster-to-rat heart transplantation model: histological characterisation under low-dose of FK506

BACKGROUND

Long-term survival studies have been conducted in hamster-to-rat cardiac models with a range of immunosuppressive treatments, but the histological pattern of Late Xenograft Rejection (LXR) has not been outlined. This study offers a detailed description of the histological changes in cardiac xenografts under three different immunological responses.

MATERIALS AND METHODS

Heterotopic hamster-to-Lewis rat cardiac transplant. Recipients were administered an antiproliferative drug (MMF, 25 mg/kg, or CyP, 10 mg/kg, from day -7 to +7 or from day 0 to +7, according to group) and FK506 (0.2 mg/kg; from day 0 to +30 or continuously). Unmodified recipients were used as controls. Conventional histology and indirect immunofluorescence of IgM, IgG and C3 deposits were performed.

RESULTS

In our study, xenografted rats that did not receive treatment developed a pattern of Acute Xenograft Rejection (AXR), with substantial tissue breakdown. Pretreated and treated animals until day 30 post-transplant developed LXR that may present two different histological patterns: one with vascular damage and predominant interstitial haemorrhage, and the other with extensive myocardial fibrosis. Long-term surviving rats (LTS) showed a morphology that was almost normal, with mild fibrosis and vascular endothelium preserved.

CONCLUSIONS

AXR, LXR and LTS in the hamster-to-rat heart transplantation model present a common humoral mechanism although their histopathological patterns are different depending on the length of immunosuppressive treatment but not on the type of antiproliferative drug administered. Pretreatment exerts an effect on fibrosis formation.

Importance of natural killer cells in the rejection of hamster skin xenografts

BACKGROUND

In the hamster to rat xenogeneic combination, antibodies, T cells, and natural killer (NK) cells have all been implicated in the process of rejection. 3.2.3 is a mouse IgG1kappa monoclonal antibody (mAb) directed against NKR-P1A on rat NK cells. The purpose of this study was to evaluate the effect of this mAb independently and in combination with other immunosuppressive agents in a hamster to rat skin graft model in order to elucidate the mechanisms involved in xenograft rejection.

METHODS

Lewis rats were recipients of hamster skin grafts. Various groups received antilymphocyte serum (ALS) (days -1, 0, and +2), rapamycin (3 mg/kg; alternate days from day +1 through day +13), and 3.2.3 mAb (days 0, +1, and +2). Anti-hamster antibody production was determined serially with a complement-dependent cytotoxicity assay. Lewis anti-hamster mixed lymphocyte reaction and cell-mediated lympholysis assays were performed within 7 days after rejection of the skin graft. NK cell function was tested using a cytotoxicity assay versus YAC-1 target cells on day 14 or day 15 after skin grafting.

RESULTS

Median graft survival in untreated animals was 7 days. There was only modest prolongation in rats treated with rapamycin alone (median survival time [MST]=9 days) or ALS alone (MST=10 days). The use of 3.2.3 mAb in untreated rats (3.2.3 alone MST=7 days) and in ALS-treated rats (ALS+3.2.3 MST=9.5 days) did not improve graft survival. The combination of ALS+rapamycin substantially improved graft survival (MST=13 days), and even greater prolongation was seen with the addition of 3.2.3 mAb (ALS+rapamycin+3.2.3 MST=18.5 days). Cytotoxic antibodies, secondary mixed lymphocyte reaction responses, cytotoxic T cells, and normal NK activity were seen at the time of rejection in untreated rats as well as those treated with 3.2.3 mAb alone, ALS alone, ALS+3.2.3 mAb, and rapamycin alone. ALS+rapamycin completely blocked the formation of anti-hamster antibodies and cytotoxic T cells but did not suppress NK activity. The use of 3.2.3 mAb produced a marked but transient suppression of NK activity in all groups.

CONCLUSION

Hamster skin xenografts can be rejected by Lewis rats in the absence of cytotoxic antibodies and cytotoxic T cells. ALS, rapamycin, and ALS+rapamycin do not suppress NK activity in Lewis rats, although their use produces a modest prolongation of hamster skin graft survival. The administration of 3.2.3 mAb to Lewis rats results in a marked but transient suppression of NK cell function, which substantially prolongs hamster skin graft survival only when antibody and cytotoxic T-cell production have also been suppressed.

Assessment of hyperacute rejection in a rat-to-primate cardiac xenograft model

We studied a rat-to-cynomolgous monkey model for xenotransplantation of vascularized organs and found that a rat heart was rejected in 5.5 +/- 1.4 min (n = 10). This hyperacute rejection (HAR) was consistent with kinetic experiments in vitro that showed damage to rat endothelial cells (ECs) after 3 min of incubation with primate serum. Histopathology and ultrastructural analysis of rejected hearts showed marked EC damage and early adherence of platelets and polymorphonuclear leukocytes to the endothelium. Immunohistochemical analysis revealed deposition along endothelial surfaces of IgG, IgM, and complement (C) components of the classical but not the alternative pathway, suggesting that, as in the pig-to-primate model, HAR is mediated by the binding of recipient xenogeneic natural antibodies and C activation. The effect of C depletion on xenograft survival was evaluated in two recipients that were treated with cobra venom factor (CVF). CVF caused complete C inactivation, demonstrated by lack of serum hemolytic activity and C-dependent EC cytotoxicity at engraftment and until the animals died. The rat cardiac transplants survived for at least 9 hr and 77 hr. Histology showed massive interstitial hemorrhage, edema, and cellular infiltration with scanty fibrin deposits. These results in CVF-treated recipient monkeys indicate that C activation mediates the development of HAR in this rat-to-primate model. We suggest that the model may be of interest as an alternative to the more expensive and time-consuming pig-to-primate model for testing the efficacy of transgenic modification of donor organs to prolong xenograft survival and for studying mechanisms of discordant xenograft rejection.