Pathogenesis of and potential therapies for delayed xenograft rejection:

Study of the impact of liver transplantation on the outcome of intestinal grafts in children

BACKGROUND

Successful small bowel transplantation remains a challenge due to the septic and immune content of the gut. The possible beneficial role of the liver was assessed in pediatric recipients of isolated intestinal and liver intestinal combined transplantation, receiving the same immunosuppressive therapy.

METHODS

Fifteen children who underwent small bowel transplantation (seven SbTx) or combined liver-small bowel transplantation (eight LSbTx) at a single center between 1994 and 1998 were retrospectively reviewed and compared with fifteen controls (eight normal and seven appendicitis as inflammatory control). Transplant and patient survival, acute rejection episodes were analyzed and compared. Epithelial apoptotic body counts (ABC) and NF-kB (p65), Caspase-3 and Bax intestinal immunostaining from days 0 to 20 after transplantation were assessed.

RESULTS

Graft and patient survivals at 5 years were respectively 75% and 75% in LSbTx; 43% and 57% in SbTx (NS). Histological analysis showed higher ABC in LSbTx intestinal mucosa (P = 0.05 on day 5, P < 0.01 thereafter). Immunostaining of biopsies on day 0 after reperfusion showed different expression of NF-kB, Caspase-3 and Bax on endothelial (P < 0.05 for NF-kB and Bax), mononuclear (P < 0.05 for Bax) and epithelial cells in LSbTx and SbTx.

CONCLUSIONS

Our results suggest a protective role of the liver toward intestinal transplantation even in absence of significative difference, probably due to the small number of children. Early changes in NF-kB immunostaining in the biopsies sampled on day 0, pointed to a possible beneficial effect of the liver in the very early phase following transplantation, perhaps through the differential control of ischemia-reperfusion.

Heme oxygenase-1 (HO-1), a protective gene that prevents chronic graft dysfunction

Heme oxygenase-1 (HO-1) is a stress-responsive enzyme that acts during inflammatory reactions as the rate-limiting step in the catabolism of heme, yielding equimolar amounts of iron (Fe), biliverdin, and the gas carbon monoxide (CO). Expression of HO-1 regulates inflammatory and immune responses, such as those involved in the rejection of transplanted organs. We will discuss here accumulating evidence supporting the notion that expression of HO-1 in a transplanted organ can prevent its rejection. We will argue that the protective effects exerted by HO-1 are mediated to a large extent by the end products that it generates via the catabolism of heme. Better knowledge of how to enhance these protective effects is likely to help create new therapeutic strategies to improve the outcome of transplanted organs.

Different patterns of TCR beta chain regulation following allo- and xeno-transplantation

BACKGROUND

In the concordant hamster-to-rat cardiac xenograft model, recipients treated with cobra venom factor for the first 10 days following transplantation and daily with Cyclosporine A (CsA) do not reject their grafts. However, when CsA is withdrawn on day 40, an acute cellular rejection occurs within 4 +/- 1 days. Allografts performed in the same conditions are rejected within 18 +/- 4 days.

METHODS

In this model, we have compared graft infiltrating T cells through both a quantitative (number of Vbeta transcripts) and qualitative (CDR3 length distribution) assessment of the T cell receptor (TCR) beta chain transcriptome in allo- and xeno-transplantations.

RESULTS

We report striking differences in TCR usage at day 15 following allo- and xeno-transplantation as well as during rejection following CsA withdrawal. The number of Vbeta transcripts was high in both rejected allo- and xenografts. However, whereas in xenografts acute rejection occurred without skewing of Vbeta CDR3 length distribution, T cells infiltrating allografts during rejection after CsA interruption had a highly altered CDR3 length distribution pattern. In addition, using a correspondence factor analysis of the beta chain transcriptome, we show that some families can clusterize and can discriminate allo- or xeno-patterns at the level of both the number of Vbeta transcripts and the CDR3 length distribution.

CONCLUSIONS

Our data show that, in vivo, even in the hamster-to-rat concordant combination, the anti-xenograft T cell response is strong and will likely represent another challenge for xenotransplantation.

Carbon monoxide generated by heme oxygenase-1 suppresses the rejection of mouse-to-rat cardiac transplants

Mouse-to-rat cardiac transplants survive long term after transient complement depletion by cobra venom factor and T cell immunosuppression by cyclosporin A. Expression of heme oxygenase-1 (HO-1) by the graft vasculature is critical to achieve graft survival. In the present study, we asked whether this protective effect was attributable to the generation of one of the catabolic products of HO-1, carbon monoxide (CO). Our present data suggests that this is the case. Under the same immunosuppressive regimen that allows mouse-to-rat cardiac transplants to survive long term (i.e., cobra venom factor plus cyclosporin A), inhibition of HO-1 activity by tin protoporphyrin, caused graft rejection in 3--7 days. Rejection was associated with widespread platelet sequestration, thrombosis of coronary arterioles, myocardial infarction, and apoptosis of endothelial cells as well as cardiac myocytes. Under inhibition of HO-1 activity by tin protoporphyrin, exogenous CO suppressed graft rejection and restored long-term graft survival. This effect of CO was associated with inhibition of platelet aggregation, thrombosis, myocardial infarction, and apoptosis. We also found that expression of HO-1 by endothelial cells in vitro inhibits platelet aggregation and protects endothelial cells from apoptosis. Both these actions of HO-1 are mediated through the generation of CO. These data suggests that HO-1 suppresses the rejection of mouse-to-rat cardiac transplants through a mechanism that involves the generation of CO. Presumably CO suppresses graft rejection by inhibiting platelet aggregation that facilitates vascular thrombosis and myocardial infarction. Additional mechanisms by which CO overcomes graft rejection may involve its ability to suppress endothelial cell apoptosis.