Tacrolimus pretreatment attenuates preexisting xenospecific immunity and abrogates hyperacute rejection in a presensitized hamster to rat liver transplant model

Hepatocyte growth factor mediates a novel form of hepatic stem/progenitor cell-induced tolerance in a rat xenogeneic liver rejection model

We have previously identified novel neural/glial antigen 2-expressing hepatic stem/progenitor cells (NG2+ HSPs) that are beneficial for tissue repair by inhibiting the immune cell response. In this in vivo study, we investigated the use of hepatocyte growth factor (HGF)-secreting NG2+ HSPs as a tolerogen in the well-established Syrian golden hamster (SGH) to Lewis (LEW) xenogeneic rat acute liver rejection (ARJ) model. Liver and blood cells were collected for histology and functional analyses using immunofluorescence staining, western blot, ELISA, and TUNEL assays. All recipient rats were randomly divided into 5 groups (n = 14 rats/group) and treated with: (1) ARJ + PBS: (2) ARJ + NG2: tail vein injection of NG2+ HSPs; (3) ARJ + tacrolimus (FK506, oral administration); (4) ARJ + an anti-cMet functional blocking antibody (a-cMet-Ab, I.V) 24 h before the injection of NG2+ HSPs; (5) ARJ + cHGF (clinically used HGF). LEW to LEW syngeneic rats were considered "normal" (n = 14, namely Syn). Significantly prolonged mean survival times (MSTs) and improved graft functions were observed after NG2+ HSP transplantation. An anti-cMet Ab significantly blocked the effect of NG2+ HSPs, suggesting that the effects were likely associated with HGF secreted from NG2+ HSPs. Notably, when intravenously injected into the xenogeneic rat model, the injected cHGF not only prolonged the MST of recipient rats but also increased the number of TUNEL-expressing xenoreactive cytotoxic T lymphocytes (CD8+ T cells). Based on these results, HGF-secreting NG2+ HSPs may specifically target recipient CD8+ T cells by inducing their apoptosis.

Pre-emptive immunosuppression using tacrolimus monotherapy does not reduce the rate of early acute rejection in renal transplantation from live donors: a comparative cohort study

The planned nature of live donor kidney transplantation allows for immunosuppression to be initiated in the pretransplant period. The aim of this study was to determine the effect of pre-emptive immunosuppression on acute rejection rates after live donor kidney transplantation. In two consecutive cohorts of live donor kidneys transplants, 99 patients received pre-emptive immunosuppression with tacrolimus monotherapy for 2 weeks prior to transplantation (PET group - first era) and 100 patients received tacrolimus-based immunosuppression commencing on the day of transplantation (control group - second era). The main outcome measure was the incidence of biopsy-proven acute rejection (BPAR) in the first 3 months post-transplantation. Tacrolimus levels were significantly higher in the PET group at day 4 post-transplant (PET 9.08 ± 4.57 vs. control 5.92 ± 3.64 ng/ml; P < 0.0001), but there were no significant differences in tacrolimus levels at day 7 (PET 8.22 ± 3.58 vs. control 7.63 ± 3.56 ng/ml; P = 0.2452). BPAR was numerically higher in the PET group, but this difference did not reach statistical significance (PET 13/99 vs. control 6/100; P = 0.097). There were no differences in allograft function measured by serum creatinine at 1 year (PET 130 ± 36 vs. control 142 ± 69 μmol/l; P = 0.6829). Graft survival at 1 year was equivalent in both groups (PET 96.9 vs. control 97.0%; P = 0.9915). This study suggests that there is little role for the use of pre-emptive tacrolimus monotherapy in ABO blood group and HLA-compatible live donor kidney transplantation in patients on triple maintenance immunosuppression.

Auxiliary xenotransplantation as an in vivo bioreactor-Development of a transplantable liver graft from a tiny partial liver

BACKGROUND

We established a completely novel method of auxiliary xenogeneic partial liver transplantation and examined whether liver grafts procured from Syrian hamsters regenerated in nude rats, which were used as in vivo bioreactors.

METHODS

The hamsters and the rats were all males (n = 10). Partial liver grafts from hamsters were transplanted into nude rats in an auxiliary manner. We evaluated liver graft injury, rejection, and regeneration during 7 days after auxiliary xenogeneic partial liver transplantation.

RESULTS

All rats survived until sacrifice on post-operative day (POD) 1, 3, and 7. HE-staining showed normal at POD1, mild periportal edema, and slight bile duct and venous endothelial inflammation at POD3, and moderate acute cellular rejection at POD7 without parenchymal necrosis. The liver regeneration rates at POD3 and 7 were 1.54 ± 0.23 and 2.54 ± 0.43, respectively. The Ki-67 labeling index was also elevated at POD3 (27.5 ± 4.1%). Serum HGF and VEGF were elevated at POD1 and 3. ATP levels of liver grafts recovered at POD7.

CONCLUSIONS

These results revealed that with appropriate immunosuppressive therapy, partial liver graft regeneration occurred in a xenogeneic animal, which suggests liver grafts regenerated in xenogeneic environments, such as an in vivo bioreactor, have potential to be transplantable liver grafts for humans.

Xenogeneic transplantation

This review summarizes the clinical history and rationale for xenotransplantation; recent progress in understanding the physiologic, immunologic, and infectious obstacles to the procedure's success; and some of the strategies being pursued to overcome these obstacles. The problems of xenotransplantation are complex, and a combination of approaches is required. The earliest and most striking immunologic obstacle, that of hyperacute rejection, appears to be the closest to being solved. This phenomenon depends on the binding of natural antibody to the vascular endothelium, fixation of complement by that antibody, and finally, activation of the endothelium and initiation of coagulation. Therefore, these three pathways have been targeted as sites for intervention in the process. The mechanisms responsible for the next immunologic barrier, that of delayed xenograft/acute vascular rejection, remain to be fully elucidated. They probably also involve multiple pathways, including antibody and/or immune cell binding and endothelial cell activation. The final immunologic barrier, that of the cellular immune response, involves mechanisms that are similar to those involved in allograft rejection. However, the strength of the cellular immune response to xenografts is so great that it is unlikely to be controlled by the types of nonspecific immunosuppression used routinely to prevent allograft rejection. For this reason, it may be essential to induce specific immunologic unresponsiveness to at least some of the most antigenic xenogeneic molecules.

IgG, but not IgM, mediates hyperacute rejection in hepatic xenografting

We reported previously that no classical features of hyperacute rejection (HAR) could be found in liver grafts in the guinea-pig (GP)-to-rat model and that recipients died shortly after transplantation of non-immunologic causes. Thus, the GP-to-rat model is not suitable for studying the mechanisms of discordant liver xenograft rejection. In the hamster to rat model, long-term survival of a liver graft is possible, but extremely low levels of xenoreactive natural antibodies are present. To mimic a discordant situation with pre-formed IgM and IgG antibodies, we sensitized rats 1 or 5 weeks before grafting. Specific anti-hamster IgM antibodies were found in recipients sensitized at week -1 but not week -5. Anti-hamster IgG was present in all recipients, albeit considerably higher in animals sensitized 5 weeks before grafting. In these two models, we examined the mechanism of HAR of liver grafts and compared this with heart xenografts. Control heart and liver grafts were rejected 4 and 7 days after transplantation respectively. Liver grafts in recipients sensitized at week -5 showed venous congestion and bleeding after reperfusion, indicating HAR, however this was not observed after sensitization at week -1. This surprising finding was confirmed by histology. Massive extravasation, edema, and acute liver cell degradation were noticed in grafts subjected to HAR. Liver grafts of recipients sensitized at week -1 showed only minimal changes. Heart grafts were rejected hyperacutely in both sensitization models. IgG antibodies could be detected on liver grafts in the group sensitized at week -5 but not in the group sensitized at week -1. Minimal IgM depositions were found on liver grafts of animals sensitized 1 week before grafting. Rejected heart grafts from similar sensitization groups showed identical antibody depositions; only IgM depositions were massive. Complement depositions were found in all groups. These results indicate that IgG, but not IgM, mediates HAR in hepatic xenografting. Such a predominance of IgG over IgM does not exist for heart grafts.