Sophie Brouard

Modulation of Endothelial Cell Apoptosis by Heme Oxygenase-1-Derived Carbon Monoxide

It is well established that expression of heme oxygenase-1 (HO-1) acts in a cytoprotective manner in a variety of cell types, including in endothelial cells (EC). We have recently shown that HO-1 expression protects EC from undergoing apoptosis. We have also shown that the antiapoptotic effect of HO-1 is mediated through heme catabolism into the gas carbon monoxide (CO). In this review, we discuss the possible molecular mechanisms by which HO-1-derived CO suppresses EC apoptosis. We will review data suggesting that the antiapoptotic effect of CO acts through the activation of the p38 mitogen-activated protein kinase signal transduction pathway and requires the activation of the transcription factor nuclear factor-kappa B (NF-kappa B), as well as the expression of a subset of NF-kappa B-dependent antiapoptotic genes.

Heme oxygenase‐1, a protective gene that prevents the rejection of transplanted organs

Summary: Endothelial cells (EC) play a pivotal role in regulating inflammatory reactions such as those involved in the rejection of transplanted organs. This occurs through the expression of a series of pro‐ and anti‐inflammatory genes that are associated with the activation of these cells. Presumably, the expression of pro‐inflammatory genes promotes events that lead to graft rejection, while expression of anti‐inflammatory (protective) genes suppresses those events and thus contributes in sustaining graft survival. Understanding how the expression of these genes is regulated and their mechanism of action are important issues for the development of new therapeutic strategies to suppress graft rejection. We have studied this phenomenon using experimental models of transplantation in rats. We discuss here data that supports the concept that grafts can express anti‐inflammatory (protective) genes that mitigate inflammatory reactions leading to graft rejection. The data reviewed focus on the role of one of such genes, the stress responsive gene heme oxygenase‐1, and of its byproduct carbon monoxide, which can suppress graft rejection and lead to long‐term graft survival.

Highly Altered Vβ Repertoire of T Cells Infiltrating Long-Term Rejected Kidney Allografts

Chronic rejection represents a major cause of long-term kidney graft loss. T cells that are predominant in long-term rejected kidney allografts (35 ± 10% of area infiltrate) may thus be instrumental in this phenomenom, which is likely to be dependant on the indirect pathway of allorecognition only. We have analyzed the variations in T cell repertoire usage of the Vβ chain at the complementary determining region 3 (CDR3) level in 18 human kidney grafts lost due to chronic rejection. We observed a strongly biased intragraft TCR Vβ usage for the majority of Vβ families and also a very high percentage (55%) of Vβ families exhibiting common and oligoclonal Vβ-Cβ rearrangements in the grafts of patients with chronic rejection associated with superimposed histologically acute lesions. Furthermore, Vβ8 and Vβ23 families exhibited common and oligoclonal Vβ-Jβ rearrangements in 4 of 18 patients (22%). Several CDR3 amino acid sequences were found for the common and oligoclonal Vβ8-Jβ1.4 rearrangement. Quantitative PCR showed that biased Vβ transcripts were also overexpressed in chronically rejected kidneys with superimposed acute lesions. In contrast, T lymphocytes infiltrating rejected allografts with chronic rejection only showed an unaltered Gaussian-type CDR3 length distribution. This pattern suggests that late graft failure associated with histological lesions restricted to Banff-defined chronic rejection does not involve T cell-mediated injury. Thus, our observation suggests that a limited number of determinants stimulates the recipient immune system in long-term allograft failure. The possibility of a local response against viral or parenchymatous cell-derived determinants is discussed.

T-cell-mediated rejection of vascularized xenografts in the absence of induced anti-donor antibody response.

T cells are considered to play a major indirect role in the pathogenesis of xenograft vascular rejection, by promoting the induction of anti‐donor antibodies that trigger complement‐ and antibody‐dependent cell cytotoxicity. However, how vigorous the T cell xenoresponse is in vivo, and whether, besides their helper function, T cells are capable of directly affecting the graft is still unclear. We have previously shown that cyclosporine A (CsA) withdrawal in accommodated cardiac xenograft recipient allows for a rapid and dense T‐cell infiltration, concomitant to an acute graft rejection. In this paper we further characterize the role of T cells in this rejection process and we demonstrate that adoptive transfer of CD4+ T cells in irradiated recipients of long‐term cardiac xenografts is sufficient to trigger acute rejection, in the absence of any detectable induced anti‐hamster antibody response. Therefore, our data suggest that unusually strong T‐cell response will be another major barrier to xenotransplantation, even if antibody‐mediated vascular rejection is controlled.

Expression of heme oxygenase-1 by endothelial cells: a protective response to injury in transplantation

Endothelial cells (EC) play a pivotal role in the regulation of inflammation by expressing a series of pro- and anti-inflammatory genes that are associated with the activation of these cells. The nature of these genes and the regulation of their expression may be particularly important for the outcome of immediately vascularised transplants. We refer to the set of anti-inflammatory genes that are expressed during EC activation as protective genes because they can block the expression of pro-inflammatory genes associated with EC activation and prevent EC apoptosis. In this review we discuss data that supports the hypothesis that expression of these protective genes in a transplanted organ can promote its survival. We will focus on the description of one such protective gene, heme oxygenase-1 (HO-1). The first part of the review discusses the potential role of EC activation in regulating inflammatory responses such as those associated with the rejection of transplanted organs. The second part discusses the molecular mechanisms that regulate the expression of HO-1 in EC as well as the molecular mechanism by which the expression of this gene can regulate EC activation. The third part discusses potential mechanisms by which HO-1 may contribute to suppress different phases of the rejection of transplanted organs, e.g., ischaemia reperfusion injury, acute rejection and chronic failure. In the last part we discuss the role of HO-1 in establishing long-term survival of organs that are transplanted across different species, an approach referred to as xenotransplantation.

Tolerance in Kidney Transplantation

Advances in immunosuppressive treatments led us to better control acute rejection and improve graft survival in organ transplantation. However, immunosuppressive drugs, due to their toxicity, are also responsible for many side effects as opportunistic infections, renal failure, cardiovascular disease and malignancy (Stegall et al, 1997; Soulillou et al, 2001; Ojo et al, 2003; Fishman et al, 2007). Then, establishing long-term graft acceptance without the continuous utilisation of immunosuppression, also called “tolerance” is a highly desirable therapeutic goal. Many strategies have been developed to achieve this goal in transplantation but whereas achievable in rodent models (Tomita et al,. 1994), it remains very difficult in human because of many differences between their immune system. The definition of true tolerance has been proposed by Billingham et al in 1953, as a well-functioning graft lacking histological lesions of rejection, in the absence of immunosuppression in an immunocompetent host accepting a second graft of the same donor, while able to reject a third-party graft. In clinic some cases of spontaneous tolerance, who stopped their immunosuppressive treatments and display a good graft function, were reported in the last decades in 20 % of liver transplanted recipients (Leruta et al., 2006) but also in kidney transplantation (Roussey-Kesler et al., 2006) suggesting that tolerance exist in human. Because several keys elements of transplant tolerance in rodents cannot be demonstrated in humans, this state has been referred to as “operational tolerance”. Thanks to these patients, the scientific community aims to identify prognostic and diagnostic biomarkers that could help physicians to detect tolerance (Brouard et al, 2007; Newell et al, 2010; Sagoo et al) to safely reduce immunosuppression in transplanted recipients.