Christiane Ferran

Accommodation of vascularized xenografts: Expression of “protective genes” by donor endothelial cells in a host Th2 cytokine environment

Organ xenografts under certain circumstances survive in the presence of anti-graft antibodies and complement, a situation referred to as “accommodation.” We find that the endothelial cells (ECs) in hamster hearts that accommodate themselves in rats express genes, such as A20 and bcl-2, that in vitro protect ECs from apoptosis and prevent upregulation in those cells of proinflammatory genes such as cytokines, procoagulant and adhesion molecules. Hearts that are rejected do not express these genes. In addition, vessels of rejected hearts show florid transplant arteriosclerosis whereas those of accommodated hearts do not. Accommodated xenografts have an ongoing T helper cell type 2 (Th2) cytokine immune response, whereas the rejected grafts have a Th1 response. We propose a model for factors that contribute to the survival of xenografts and the avoidance of transplant arteriosclerosis.

Delayed xenograft rejection

The triumph of genetic engineering in overcoming hyperacute rejection (HAR) of a discordant organ xenograft is clear, but the promise of clinical application of xenotransplantation remains unfulfilled as further immunologic barriers are defined that lead to rejection of a vascularized xenograft within days of transplantation. This report describes the features of this second set of immunologic responses, collectively termed delayed xenograft rejection (DXR). DXR is a syndrome seen in xenograft recipients in which HAR has been avoided or suppressed by antibody depletion or blockade of complement activation. DXR may result, at least in part, from the persisting activation of those pathways first encountered during the HAR phase. Serial studies over several days after transplant show that, histologically, xenografts undergoing DXR demonstrate varying combinations of (1) progressive infiltration by activated macrophages and natural killer (NK) cells, (2) platelet aggregation and fibrin deposition throughout the microvasculature, and (3) endothelial activation. In various experimental models, DXR is T cell-independent and can occur in the absence of demonstrable xenoreactive antibodies. Hence DXR is probably best regarded as arising from the activation of innate host defense mechanisms coupled with failure of normal regulatory mechanisms due to manifold molecular incompatibilities. Although DXR-like features can be seen in concordant models, T cell involvement in the latter is probably requisite. Similarly, in a much muted form, aspects of a DXR-like process may contribute to numerous inflammatory processes, including allograft rejection. The importance of DXR in xenotransplantation is that its development appears resistant to all but the most dense and toxic forms of immunosuppression, which prolong xenograft survival at the expense of inducing host leukopenia, thrombocytopenia, and coagulopathies. It is likely that until the basis of DXR is more clearly understood there can be no further significant progress toward clinical xenotransplantation. However, as the mechanisms responsible for DXR are dissected and understood, still further genetic engineering of donor pigs, involving the introduction of additional or multiple genes to regulate macrophage and NK cell responses, local coagulation, and endothelial cell activation, may once again prove to be an attractive, practical, powerful therapeutic option.

A20 Blocks Endothelial Cell Activation through a NF-κB-dependent Mechanism

The A20 gene product is a novel zinc finger protein originally described as a tumor necrosis factor α (TNF)-inducible early response gene in human umbilical vein endothelial cells (HUVEC). Its described function is to block TNF-induced apoptosis in fibroblasts and B lymphocytes, but more recently it has also been shown to play a role in lymphoid cell maturation. The mechanism of action of A20 is unknown. The aim of our study was to assess the effect of A20 upon endothelial cell activation. By transfecting bovine aortic endothelial cells (BAEC) with A20 as well as reporter constructs consisting of the promoters of genes known to be up-regulated during endothelial cell activation, i.e. E-selectin, interleukin (IL)-8, tissue factor (TF), and inhibitor of nuclear factor κBα (IκBα), we demonstrate that A20 expression inhibits gene up-regulation associated with TNF, lipopolysaccharide (LPS), phorbol 12-myristate 13-acetate (PMA), and hydrogen peroxide (H2O2)-induced endothelial cell (EC) activation. The mechanism of action of A20 is in part, or totally, due to the blockade of nuclear factor κB (NF-κB), as shown by its ability to suppress the activity of a NF-κB reporter. This effect is specific, as A20 does not block a noninducible, constitutively expressed reporter, Rous sarcoma virus-luciferase (RSV-LUC); nor does it block the c-Tat-inducible, NF-κB-independent reporter, human immunodeficiency virus-chloramphenicol acetyltransferase (HIV-CAT). How A20 blocks NF-κB is unclear, although we demonstrate that it does not affect p65 (RelA)-mediated gene transactivation. The inhibition of endothelial cell activation by A20 is a novel function for A20.

IN VIVO CELL ACTIVATION FOLLOWING OKT3 ADMINISTRATION: SYSTEMIC CYTOKINE RELEASE AND MODULATION BY CORTICOSTEROIDS

A massive and self-limited release of tumor necrosis factor and interferon gamma was detected in the systemic circulation in 35 consecutive renal allograft recipients by specific radioimmunoassays very soon following the first injection of the monoclonal antibody OKT3 (anti-CD3). Peak serum TNF and IFNγ levels were reached, respectively, at 1 and 4 hr following the first OKT3 injection. Abnormally high serum interleukin 2 levels were also observed 4 hr following the first OKT3 injection in a minority of patients (5 cases). OKT3 had no effect on interleukin 1 beta, interferon alpha, and granulocyte/macrophage colony stimulating factor serum levels, which in all patients remained within the normal range throughout the study. This selective OKT3-induced cytokine release, which only followed the first injection, was transient (i.e., lasting a few hours). It tightly paralleled the spontaneously reversible clinical syndrome characterized by high fever, headaches, and gastrointestinal symptoms that is invariably associated with the first OKT3 administration. Importantly, when administered in adequate dosages and with adequate timing, corticosteroids influenced both the cytokine release and the systemic reaction. Thus, the highest TNF, IFNγ, and IL-2 serum levels were detected in patients who did not received corticosteroids. Patients who received high-dose corticosteroids (1 g solumedrol bolus) concomitantly with the first OKT3 injection still had high TNF and IFNγ levels. Conversely, when the same corticosteroid dose was injected 15–60 min prior to the first OKT3 injection, in all cases the increase of serum TNF and IFNγ was significantly lower as compared with the above-described groups; IL-2 levels did not rise. These data offer a direct explanation for one major side effect of OKT3 and thus provide the basis for devising means to prevent its occurrence.

Protective genes expressed in endothelial cells: a regulatory response to injury

Endothelial cells (ECs) have evolved to guard against insults that incite inflammation. Response to injury is an active process that, if uncontrolled, can progress to EC death (apoptosis). Here Fritz Bach and colleagues suggest that ECs have a balancing component to their proinflammatory response: they upregulate a set of protective genes, including anti-apoptotic genes, that serve to limit the activation process and thereby regulate the response to injury.

Endothelial Cell Activation and Thromboregulation during Xenograft Rejection

Xenoreactive natural antibodies (XNA) and complement (C) are thought to be the two major inciting factors that result in hyperacute rejection (HAR) of an immediately vascularized, discordant xenograft within minutes to a very few hours, with destruction and infarction of the transplanted organ. If recipients are modified by various experimental modalities, such as removal and suppression of XNAand C-mediated responses, thus avoiding HAR, the process of delayed xenograft rejection (DXR) with a significant vascular component still occurs after a delay of several days or, at the most, a few weeks (Bach et al. 1993). The end result in both instances is the invariable and unacceptable loss of xenografts, which currently limits application of xenotransplantation beyond experimental protocols. The mechanisms underlying DXR are far from clear but appear not necessarily to involve XNAand C-mediated responses as those noted in HAR. Moreover, DXR can occur without the prominent participation of T lymphocytes. One of us (FHB) has suggested that the final common pathogenic mechanisms underly-

Bcl-2 and Bcl-XL serve an anti-inflammatory function in endothelial cells through inhibition of NF-κB

To maintain the integrity of the vascular barrier, endothelial cells (EC) are resistant to cell death. The molecular basis of this resistance may be explained by the function of antiapoptotic genes such as bcl family members. Overexpression of Bcl-2 or Bcl-XL protects EC from tumor necrosis factor (TNF)–mediated apoptosis. In addition, Bcl-2 or Bcl-XL inhibits activation of NF-κB and thus upregulation of proinflammatory genes. Bcl-2–mediated inhibition of NF-κB in EC occurs upstream of IκBα degradation without affecting p65-mediated transactivation. Overexpression of bcl genes in EC does not affect other transcription factors. Using deletion mutants of Bcl-2, the NF-κB inhibitory function of Bcl-2 was mapped to bcl homology domains BH2 and BH4, whereas all BH domains were required for the antiapoptotic function. These data suggest that Bcl-2 and Bcl-XL belong to a cytoprotective response that counteracts proapoptotic and proinflammatory insults and restores the physiological anti-inflammatory phenotype to the EC. By inhibiting NF-κB without sensitizing the cells (as with IκBα) to TNF-mediated apoptosis, Bcl-2 and Bcl-XL are prime candidates for genetic engineering of EC in pathological conditions where EC loss and unfettered activation are undesirable.

Genetic Engineering of a Suboptimal Islet Graft with A20 Preserves β Cell Mass and Function

Transplantation of an excessive number of islets of Langerhans (two to four pancreata per recipient) into patients with type I diabetes is required to restore euglycemia. Hypoxia, nutrient deprivation, local inflammation, and the β cell inflammatory response (up-regulation of NF-κB-dependent genes such as inos) result in β cell destruction in the early post-transplantation period. Genetic engineering of islets with anti-inflammatory and antiapoptotic genes may prevent β cell loss and primary nonfunction. We have shown in vitro that A20 inhibits NF-κB activation in islets and protects from cytokine- and death receptor-mediated apoptosis. In vivo, protection of newly transplanted islets would reduce the number of islets required for successful transplantation. Transplantation of 500 B6/AF1 mouse islets into syngeneic, diabetic recipients resulted in a cure rate of 100% within 5 days. Transplantation of 250 islets resulted in a cure rate of only 20%. Transplantation of 250 islets overexpressing A20 resulted in a cure rate of 75% with a mean time to cure of 5.2 days, comparable to that achieved with 500 islets. A20-expressing islets preserve functional β cell mass and are protected from cell death. These data demonstrate that A20 is an ideal cytoprotective gene therapy candidate for islet transplantation.

Corticosteroid inhibition of the OKT3-induced cytokine-related syndrome : dosage and kinetics prerequisites

The data presented extend to a larger series of 27 consecutive renal allograft recipients treated prophylactically with OKT3 our previous observation that the acute OKT3-induced clinical syndrome is related to massive release in the circulation of some cytokines, among which are tumor necrosis factor and interferon gamma. In addition, a pilot randomized study was set up including 12 consecutive patients receiving high-dose corticosteroid treatment (0.5 g solumedrol) either before or at the same time as the first OKT3 injection. Results confirm that when corticosteroids are given in sufficient amount and, importantly, 1 hr before the first OKT3 injection, they significantly decrease the release of both tumor necrosis factor and interferon gamma. In addition, the pretreatment with corticosteroids may totally abolish the IL-2 release induced by OKT3. Given the key role the massive although transient cytokine release plays in determining the OKT3-induced acute syndrome, these results provide the biological basis supporting a precise kinetics of administration of high-dose corticosteroids to better decrease the severity of the clinical reaction.

Forkhead transcription factors inhibit vascular smooth muscle cell proliferation and neointimal hyperplasia

Vascular smooth muscle cell (VSMC) proliferation and migration contribute significantly to atherosclerosis, postangioplasty restenosis, and transplant vasculopathy. Forkhead transcription factors belonging to the FoxO subfamily have been shown to inhibit growth and cell cycle progression in a variety of cell types. We hypothesized that forkhead proteins may play a role in VSMC biology. Under in vitro conditions, platelet-derived growth factor (PDGF)-BB, tumor necrosis factor-α, and insulin-like growth factor 1 stimulated phosphorylation of FoxO in human coronary artery smooth muscle cells via MEK1/2 and/or phosphatidylinositol 3-kinase-dependent signaling pathways. PDGF-BB, tumor necrosis factor-α, and insulin-like growth factor 1 treatment resulted in the nuclear exclusion of FoxO, whereas PDGF-BB alone down-regulated the FoxO target gene, p27kip1, and enhanced cell survival and progression through the cell cycle. These effects were abrogated by overexpression of a constitutively active, phosphorylation-resistant mutant of the FoxO family member, TM-FKHRL1. The anti-proliferative effect of TM-FKHRL1 was partially reversed by small interfering RNA against p27kip1. In a rat balloon carotid arterial injury model, adenovirus-mediated gene transfer of FKHRL1 caused an increase in the expression of p27kip1 in the VSMC and inhibition of neointimal hyperplasia. These data suggest that FoxO activity inhibits VSMC proliferation and activation and that this signaling axis may represent a therapeutic target in vasculopathic disease states.