Endothelial injury: cause and effect of alloimmune inflammation

Microvasculature partial endothelial mesenchymal transition in early posttransplant biopsy with acute tubular necrosis identifies poor recovery renal allografts

Acute tubular necrosis (ATN), a frequent histopathological feature in the early post-renal transplant biopsy, affects long-term graft function. Appropriate markers to identify patients at risk of no or incomplete recovery after delayed graft function are lacking. In this study, we first included 41 renal transplant patients whose biopsy for cause during the first month after transplantation showed ATN lesions. Using partial microvasculature endothelial (fascin, vimentin) and tubular epithelial (vimentin) to mesenchymal transition markers, detected by immunohistochemistry, we found a significant association between partial endothelial to mesenchymal transition and poor graft function recovery (Spearman's rho = -0.55, P = .0005). Transforming growth factor-β1 was strongly expressed in these phenotypic changed endothelial cells. Extent of ATN was also correlated with short- and long-term graft dysfunction. However, the association of extensive ATN with long-term graft dysfunction (24 months posttransplant) was observed only in patients with partial endothelial to mesenchymal transition marker expression in their grafts (Spearman's rho = -0.64, P = .003), but not in those without. The association of partial endothelial to mesenchymal transition with worse renal graft outcome was confirmed on 34 other early biopsies with ATN from a second transplant center. Our results suggest that endothelial cell activation at the early phase of renal transplantation plays a detrimental role.

VEGF-C, VEGF-A and related angiogenesis factors as biomarkers of allograft vasculopathy in cardiac transplant recipients

BACKGROUND

Cardiac allograft vasculopathy (CAV), the major cause of late allograft loss after cardiac transplantation, results from donor-directed cellular and humoral alloimmune responses. Graft vascular endothelial cells (EC) are primary targets of these destructive responses, suggesting that factors associated with endothelial injury and repair could serve as biomarkers of CAV.

METHODS

Using a protein profiler array platform, we measured the levels of 55 angiogenesis-related proteins in sera from 33 adult heart transplant recipients, including 17 with angiographically documented CAV and 16 age- and gender-matched controls without CAV. All patients were >2 years after heart transplant.

RESULTS

The study population was 75% male with a mean age of 62 ± 11 years. On average, patients were 12 ± 5 years after heart transplantation. We found that vascular endothelial growth factor (VEGF)-C, VEGF-A, angiopoietin-2, artemin, urokinase-type plasminogen activator and vasohibin were strongly associated with established CAV (all p < 0.01). Multivariable modeling identified VEGF-C, VEGF-A and platelet factor-4 (PF-4) as significant independent biomarkers of CAV. Furthermore, receiver-operating characteristic curve analysis demonstrated that the combination of all 3 molecules provided outstanding performance for the diagnosis of CAV (area under the curve [AUC] = 0.98; p < 0.001).

CONCLUSIONS

Serum levels of VEGF-C, VEGF-A and PF-4 demonstrate strong associations with established CAV and, together with related angiogenesis factors, may serve as a reliable, non-invasive diagnostic test for CAV in cardiac transplant recipients.

Concise review: immunologic lessons from solid organ transplantation for stem cell-based therapies

Clinical organ transplantation became possible only after powerful immunosuppressive drugs became available to suppress the alloimmune response. After decades of solid organ transplantation, organ rejection is still a major challenge. However, significant insight into allorecognition has emerged from this vast experience and should be used to inform future stem cell-based therapies. For this reason, we review the current understanding of selected topics in transplant immunology that have not been prominent in the stem cell literature, including immune responses to ischemia/reperfusion injuries, natural killer cells, the adaptive immune response, some unresolved issues in T-cell allorecognition, costimulatory molecules, and the anticipated role of regulatory T cells in graft tolerance.

Antibodies to MHC class II molecules induce autoimmunity: critical role for macrophages in the immunopathogenesis of obliterative airway disease

Previous studies have shown that intrabronchial administration of antibodies (Abs) to MHC class I resulted in development of obliterative airway disease (OAD), a correlate of chronic human lung allograft rejection. Since development of Abs specific to mismatched donor HLA class II have also been associated with chronic human lung allograft rejection, we analyzed the role of Abs to MHC class II in inducing OAD. Administration of MHC class II Abs (M5/114) to C57BL/6 mice induced the classical features of OAD even though MHC class II expression is absent de novo on murine lung epithelial and endothelial cells. The induction of OAD was accompanied by enhanced cellular and humoral immune responses to self-antigens (Collagen V and K- α1Tubulin). Further, lung-infiltrating macrophages demonstrated a switch in their phenotype predominance from MΦ1 (F4/80⁺CD11c⁺) to MΦ2 (F4/80⁺CD206⁺) following administration of Abs and prior to development of OAD. Passive administration of macrophages harvested from animals with OAD but not from naïve animals induced OAD lesions. We conclude that MHC class II Abs induces a phenotype switch of lung infiltrating macrophages from MΦ1 (F4/80⁺CD11c⁺) to MΦ2 (F4/80⁺CD206⁺) resulting in the breakdown of self-tolerance along with an increase in autoimmune Th17 response leading to OAD.

Targeting the intragraft microenvironment and the development of chronic allograft rejection

In this review, we discuss a paradigm whereby changes in the intragraft microenvironment promote or sustain the development of chronic allograft rejection. A key feature of this model involves the microvasculature including (a) endothelial cell (EC) destruction, and (b) EC proliferation, both of which result from alloimmune leukocyte- and/or alloantibody-induced responses. These changes in the microvasculature likely create abnormal blood flow patterns and thus promote local tissue hypoxia. Another feature of the chronic rejection microenvironment involves the overexpression of vascular endothelial growth factor (VEGF). VEGF stimulates EC activation and proliferation and it has potential to sustain inflammation via direct interactions with leukocytes. In this manner, VEGF may promote ongoing tissue injury. Finally, we review how these events can be targeted therapeutically using mTOR inhibitors. EC activation and proliferation as well as VEGF-VEGFR interactions require PI-3K/Akt/mTOR intracellular signaling. Thus, agents that inhibit this signaling pathway within the graft may also target the progression of chronic rejection and thus promote long-term graft survival.

The effects of Castanospermine, an oligosaccharide processing inhibitor, on mononuclear/endothelial cell binding and the expression of cell adhesion molecules

INTRODUCTION

In this study we aimed to determine whether Castanospermine, a transplant immunosuppressive agent, impaired mononuclear/endothelial cell binding and expression of their cell adhesion molecules.

METHODS

The binding of human umbilical vein endothelial cells with peripheral blood mononuclear cells was measured by a binding assay using Chromium 51 label; the membrane expression of cell adhesion molecules was measured by flow cytometry expressed as mean fluorescence intensity ratios.

RESULTS

Castanospermine decreased mononuclear/endothelial cell binding if and only if both cell types were treated with Castanospermine: this impairment occurred if endothelial cells were treated with a range of doses of Castanospermine and mononuclear cells were treated with a constant dose of Castanospermine (p<0.001 versus untreated p=0.978) or vice versa (p=0.004 versus untreated p=0.582). Upon human umbilical vein endothelial cells Castanospermine reduced the mean fluorescence intensity ratios of E-selectin (p=0.003), ICAM-1 (p<0.001), ICAM-2 (p=0.004) and PECAM-1 (p<0.001) but increased it for P-selectin (p<0.001). Upon peripheral blood mononuclear cells Castanospermine reduced the mean fluorescence intensity ratios of L-selectin (P<0.001), LFA-1α (p<0.001), VLA-4 (p<0.001), Mac-1 (P<0.001) and CR4 (p<0.001) but increased the mean fluorescence intensity ratios of PSGL-1 (p<0.001) and PECAM-1 (p=0.001). Similar changes in mean fluorescence intensity ratios were found in the subset of lymphocytes and monocytes but the reductions in LFA-1α and VLA-4 on lymphocytes and Mac-1 and CR4 on monocytes were greater.

CONCLUSIONS

The reduction in mononuclear/endothelial cell binding mediated by CAST and the reduction in expression of multiple cell adhesion molecules on these cell types help to explain the mechanism of its established immunosuppressive effect.

Key Features of the Intragraft Microenvironment that Determine Long-Term Survival Following Transplantation

In this review, we discuss how changes in the intragraft microenvironment serve to promote or sustain the development of chronic allograft rejection. We propose two key elements within the microenvironment that contribute to the rejection process. The first is endothelial cell proliferation and angiogenesis that serve to create abnormal microvascular blood flow patterns as well as local tissue hypoxia, and precedes endothelial-to-mesenchymal transition. The second is the overexpression of local cytokines and growth factors that serve to sustain inflammation and, in turn, function to promote a leukocyte-induced angiogenesis reaction. Central to both events is overexpression of vascular endothelial growth factor (VEGF), which is both pro-inflammatory and pro-angiogenic, and thus drives progression of the chronic rejection microenvironment. In our discussion, we focus on how inflammation results in angiogenesis and how leukocyte-induced angiogenesis is pathological. We also discuss how VEGF is a master control factor that fosters the development of the chronic rejection microenvironment. Overall, this review provides insight into the intragraft microenvironment as an important paradigm for future direction in the field.

Addition of simvastatin to cold storage solution prevents endothelial dysfunction in explanted rat livers

Pathophysiological alterations in the endothelial phenotype result in endothelial dysfunction. Flow cessation, occurring during organ procurement for transplantation, triggers the endothelial dysfunction characteristic of ischemia/reperfusion injury, partly due to a reduction in the expression of the vasoprotective transcription factor Kruppel-like Factor 2 (KLF2). We aimed at (1) characterizing the effects of flow cessation and cold storage on hepatic endothelial phenotype, and (2) ascertaining if the consequences of cold stasis on the hepatic endothelium can be pharmacologically modulated, improving liver graft function. Expression of KLF2 and its vasoprotective programs was determined in (i) hepatic endothelial cells (HEC) incubated under cold storage conditions with or without the KLF2-inducer simvastatin, and (ii) rat livers not cold stored or preserved in cold University of Wisconsin solution (UWS) supplemented with simvastatin or its vehicle. In addition, upon warm reperfusion hepatic vascular resistance, endothelial function, nitric oxide vasodilator pathway, apoptosis, inflammation, and liver injury were evaluated in not cold stored livers or livers preserved in cold UWS supplemented with simvastatin or vehicle. Expression of KLF2 and its vasoprotective programs decrease in HEC incubated under cold storage conditions. Cold-stored rat livers exhibit a time-dependent decrease in KLF2 and its target genes, liver injury, increased hepatic vascular resistance, and endothelial dysfunction. The addition of simvastatin to the storage solution, maintained KLF2-dependent vasoprotective programs, prevented liver damage, inflammation, and oxidative stress and improved endothelial dysfunction.

CONCLUSION

Our results provide a rationale to evaluate the beneficial effects of a vasoprotective preservation solution on human liver procurement for transplantation.

Mechanisms involved in antibody- and complement-mediated allograft rejection

Antibody-mediated rejection has become critical clinically because this form of rejection is usually unresponsive to conventional anti-rejection therapy, and therefore, it has been recognized as a major cause of allograft loss. Our group developed experimental animal models of vascularized organ transplantation to study pathogenesis of antibody- and complement-mediated endothelial cell injury leading to graft rejection. In this review, we discuss mechanisms of antibody-mediated graft rejection resulting from activation of complement by C1q- and MBL (mannose-binding lectin)-dependent pathways and interactions with a variety of effector cells, including macrophages and monocytes through Fcgamma receptors and complement receptors.

Leukocyte accumulation in graft blood vessels during self-limiting acute rejection of rat kidneys

During self-limiting acute rejection preceding chronic vasculopathy, large amounts of leukocytes, predominantly monocytes, interact with the endothelium of renal allografts. We aim to characterize them and to identify targets for functional and interventional studies. Leukocytes were harvested by vascular perfusion from Fischer 344 to Lewis renal allografts or Lewis isografts, followed by flow cytometry, quantitative RT-PCR and genome-wide transcriptional profiling. Leukocyte accumulation peaked in allografts on day 9. The percentage of monocytes expressing MHC class II and CD161 was increased whereas CD4, CD11a, CD43, and CD71 expression remained unchanged. IFN-γ, IL-1β, IL-2, IL-10, TNF-α, and iNOS mRNA increased in allograft leukocytes but IL-4, IL-6, IL-12, TGF-β, and tissue factor did not. During acute rejection, 1783 genes were differentially expressed. In conclusion, graft blood leukocytes display a unique state of partial activation during self-limiting rejection. Numerous differentially expressed genes deserve further investigation as potential factors in deciding the fate of the allograft.

Flow cessation triggers endothelial dysfunction during organ cold storage conditions: strategies for pharmacologic intervention

BACKGROUND

Vascular pathologies constitute a major cause of graft rejection after organ transplantation. Recent studies have documented an improvement in transplant outcome when organs are preserved through pulsatile perfusion; however, the underlying mechanisms of these observations are poorly characterized. We hypothesized that the temporary absence of flow occurring in the context of organ cold storage conditions disrupts endothelial vasoprotective programs, and that this consequence of stasis may be a target for pharmacological modulation.

METHODS

The expression of the transcription factor Kruppel-like factor 2 (KLF2) and its vasoprotective target genes were assessed during cold storage conditions in cultured human endothelial cells and murine aortic segments. In addition, we evaluated the effect of simvastatin used as a supplement in a cold preservation solution on the expression of vasoprotective genes, and on endothelial activation and apoptosis.

RESULTS

The expression of endothelial KLF2 and its vasoprotective transcriptional targets were rapidly lost during cold preservation in vitro and ex vivo. Importantly, simvastatin treatment blocked the decay of KLF2, sustaining a vasoprotective phenotype, and preventing endothelial activation and apoptosis.

CONCLUSIONS

Flow stasis leads to acute endothelial dysfunction and apoptosis in the context of cold storage conditions. Supplementation of organ preservation solutions with pharmacologic agents that restore endothelial vasoprotective programs, by upregulating KLF2, may represent a significant advancement of current organ preservation techniques.

mTOR-understanding the clinical effects

The target of rapamycin (TOR) is a highly conserved serine/threonine kinase that controls cell growth and metabolism in response to nutrients, growth factors, cellular energy, and stress. The TOR kinase, which was originally discovered in yeast, is also expressed in human cells as mammalian TOR (mTOR). In this review, we focus on how mTOR-inducible signals function in cell protection and cell survival of effector and regulatory T cells as well as its role in endothelial cell biology. We evaluate how signaling is important for vascular endothelial cell growth, survival, and proliferation; and we consider how the function of mTOR in endothelial cells may be clinically important in the rejection process. Understanding the biology of mTOR allows clinicians to use mTOR inhibitors optimally as therapeutics following solid organ transplantation.

Low immunogenicity of endothelial derivatives from rat embryonic stem cell-like cells

Embryonic stem cells (ESC) are suggested to be immune-privileged, but they carry the risk of uncontrolled expansion and malignancy. Upon differentiation they lose their tumor-forming capacity, but they become immunogenic by the expression of a normal set of MHC molecules. This immunogenicity might trigger rejection after application in regenerative therapies. In this study MHC expression of and immune responses to endothelial derivatives of rat embryonic stem cell-like cells (RESC) under inflammatory conditions were determined in comparison to primary rat aortic endothelial cells (ECs). Cellular as well as humoral allo-recognition was analyzed in vitro. In addition, immune reactions in vivo were assessed by allo-antibody production and determination of interferon-gamma (IFNgamma)-secreting allo-reactive T cells. RESC derivatives expressed low but significant levels of MHC class I, and no MHC class II. In response to IFNgamma MHC class I expression was enhanced, while class II transactivator induction failed completely in these cells; MHC class II expression remained consistently absent. Functionally, the RESC derivatives showed a reduced allo-stimulatory capacity, protection against humoral allo-recognition in vitro and a slightly diminished susceptibility to cytotoxic T cell lysis. Furthermore, in vivo experiments demonstrated that these cells do not trigger host immune reactions, characterized by no allo-antibody production and no induction of allo-reactive memory T cells. Our results show that endothelial derivatives of RESC have a distinctive reduced immunogenic potency even under inflammatory conditions.

The involvement of FcR mechanisms in antibody-mediated rejection

BACKGROUND

Antibody-mediated rejection is characterized by macrophage margination against vascular endothelium. The potential interactions triggered by antibodies between endothelial cells (EC) and macrophages have not been examined thoroughly in transplants. We used in vivo and in vitro models of antibody-mediated rejection.

METHODS

Passive transfer of monoclonal alloantibodies (Allo-mAbs) to donor major histocompatibility complex-class I antigens was used to restore acute rejection of B10.A (H-2a) hearts to C57BL/6 (H-2b) immunoglobulin knockout (IgKO) recipients. Intragraft cytokine mRNA expression was measured by real-time polymerase chain reaction. In vitro, mouse EC were cultured in the presence of Allo-mAbs to donor major histocompatibility complex class I antigens and mononuclear cells. Levels of cytokines in culture supernatants were determined in enzyme-linked immunosorbent assay.

RESULTS

Expression of MCP-1, IL-6 and IL-1alpha mRNA was higher in rejecting transplants from recipients treated with Allo-mAbs compared to non-rejecting transplants. EC sensitized with Allo-mAbs produced high levels of MCP-1 and KC. The addition of macrophages to sensitized EC stimulated high levels of IL-6 in addition to MCP-1, KC, Rantes, and TIMP-1. The levels of MCP-1 and IL-6 were significantly lower in co-cultures of EC sensitized with IgG1 Allo-mAbs in the presence of mononuclear cells from Fcgamma-Receptor III KO (FcgammaRIII-KO) graft recipients compared to co-cultures with wild-type cells. The levels of both cytokines were also lower in co-cultures of EC stimulated with F(ab')2 fragments of antibody.

CONCLUSIONS

Our findings indicate that IgG1 Allo-mAbs to major histocompatibility complex class I antigens can augment graft injury by stimulating EC to produce MCP-1 and by activating mononuclear cells through their Fc receptors.

Effects of mycophenolic acid on endothelial cells

Mycophenolate mofetil (MMF) is a potent immunosuppressant that inhibits the activity of inosine monophosphate dehydrogenase (IMPDH), the rate-limiting enzyme in de novo synthesis of guanosine nucleotides. MMF has been used widely in solid-organ transplantation. Increased evidence indicated that MMF exhibited beneficial effects on various types of vasculitis, for reasons that were not fully understood. Endothelial cells play a pivotal role in the pathogenesis of vasculitis. Endothelium may not only be the main target for injury, but also be able to amplify the inflammatory response by adhesion molecule expression, leukocyte adhesion, cytokine production and angiogenesis. In the present study, the effect of mycophenolic acid (MPA), the active metabolite of MMF, on human umbilical vein endothelial cells (HUVECs) was investigated. MPA markedly inhibited tumor necrosis factor-alpha (TNFalpha)-induced intercellular adhesion molecule-1 (ICAM-1) mRNA and surface expression, suppressed TNFalpha-induced neutrophils adhesion to endothelial cells, and reduced TNFalpha-induced interleukin-6 (IL-6) secretion. The inhibitory effects of MPA on ICAM-1 surface expression and IL-6 secretion were not attenuated by addition of guanosine, implying that inhibition of these processes were not due to intracellular guanosine nucleotides depletion. MPA also decreased angiogenesis of endothelial cells in three-dimensional collagen gel culture system, reduced the migration in a wounded monolayer of endothelial cells, and inhibited the proliferation of endothelial cells. In conclusion, MPA exhibited multifarious effects on endothelial cells including inhibition of ICAM-1 expression, neutrophil attachment, IL-6 secretion, and the process of angiogenesis, which might contribute to the efficacy of MMF in the treatment of vasculitis.

The role of nitric oxide in renal transplantation

This review discusses the concept that nitric oxide synthase (NOS) may orchestrate both the inflammatory response to the renal allograft and anti-inflammatory defense in the graft itself. NO is produced by endothelial, epithelial, as well as inflammatory cells. In the setting of transplantation, the endothelium is the first lining to be subjected to the early response to injury. In turn, activated endothelial cells facilitate leukocyte recruitment, immune-mediated injury, and angiogenesis. On activation by inflammatory stimuli, endothelial cells up-regulate multiple vasoactive substances, oxygen radicals, cytokines, chemokines, and growth factors. Therefore, endothelial integrity, especially the expression of protecting vasoactive agents, such as NO, may be a key factor in resistance or sensitivity to transplantation-mediated injury. Thus, evaluating the mechanisms by which NO is involved in either protecting or injuring the transplanted allogeneic kidney is important for our understanding of renal allograft rejection. This review focuses on the role of NO in the inflammatory endothelial-leukocyte interactions, which are implicated in acute and chronic rejection of the transplanted kidney.

Non-complement- and complement-activating antibodies synergize to cause rejection of cardiac allografts

Alloantibodies (AlloAbs) are a clinically significant component of the immune response to organ transplants. In our experimental model, B10.A (H-2a) cardiac transplants survived significantly longer in C57BL/6 (H-2b) immunoglobulin knock-out (IgKO) recipients than in their wild-type (WT) counterparts. Passive transfer of a single 50-200-microg dose of complement-activating IgG2b AlloAbs to IgKO recipients reconstituted acute rejection of cardiac allografts. Although passive transfer of a subthreshold dose of 25 microg of IgG2b or a single 100-200-microg dose of non-complement-activating IgG1 AlloAbs did not restore acute rejection to IgKO recipients, a combination of these AlloAbs did cause acute graft rejection. Histologically, rejection was accompanied by augmented release of von Willebrand factor from endothelial cells. IgG1 AlloAbs did not activate complement on their own and did not augment complement activation by IgG2b AlloAbs. However, IgG1 AlloAbs stimulated cultured mouse endothelial cells to produce monocyte chemotactic protein 1 (MCP-1) and neutrophil chemoattractant growth-related oncogene alpha (KC). TNF-alpha augmented IgG1 induced secretion of MCP-1 and KC. These findings indicate that non-complement-activating AlloAbs can augment injury to allografts by complement-activating AlloAbs. Non-complement-activating AlloAbs stimulate endothelial cells to produce chemokines and this effect is augmented in the milieu of proinflammatory cytokines.