The fate of dendritic cells in a mouse model of liver ischemia/reperfusion injury

Role of the immune system in liver transplantation and its implications for therapeutic interventions

Liver transplantation (LT) stands as the gold standard for treating end-stage liver disease and hepatocellular carcinoma, yet postoperative complications continue to impact survival rates. The liver's unique immune system, governed by a microenvironment of diverse immune cells, is disrupted during processes like ischemia-reperfusion injury posttransplantation, leading to immune imbalance, inflammation, and subsequent complications. In the posttransplantation period, immune cells within the liver collaboratively foster a tolerant environment, crucial for immune tolerance and liver regeneration. While clinical trials exploring cell therapy for LT complications exist, a comprehensive summary is lacking. This review provides an insight into the intricacies of the liver's immune microenvironment, with a specific focus on macrophages and T cells as primary immune players. Delving into the immunological dynamics at different stages of LT, we explore the disruptions after LT and subsequent immune responses. Focusing on immune cell targeting for treating liver transplant complications, we provide a comprehensive summary of ongoing clinical trials in this domain, especially cell therapies. Furthermore, we offer innovative treatment strategies that leverage the opportunities and prospects identified in the therapeutic landscape. This review seeks to advance our understanding of LT immunology and steer the development of precise therapies for postoperative complications.

Dendritic Cell-Mediated Regulation of Liver Ischemia-Reperfusion Injury and Liver Transplant Rejection

Liver allograft recipients are more likely to develop transplantation tolerance than those that receive other types of organ graft. Experimental studies suggest that immune cells and other non-parenchymal cells in the unique liver microenvironment play critical roles in promoting liver tolerogenicity. Of these, liver interstitial dendritic cells (DCs) are heterogeneous, innate immune cells that appear to play pivotal roles in the instigation, integration and regulation of inflammatory responses after liver transplantation. Interstitial liver DCs (recruited in situ or derived from circulating precursors) have been implicated in regulation of both ischemia/reperfusion injury (IRI) and anti-donor immunity. Thus, livers transplanted from mice constitutively lacking DCs into syngeneic, wild-type recipients, display increased tissue injury, indicating a protective role of liver-resident donor DCs against transplant IRI. Also, donor DC depletion before transplant prevents mouse spontaneous liver allograft tolerance across major histocompatibility complex (MHC) barriers. On the other hand, mouse liver graft-infiltrating host DCs that acquire donor MHC antigen via "cross-dressing", regulate anti-donor T cell reactivity in association with exhaustion of graft-infiltrating T cells and promote allograft tolerance. In an early phase clinical trial, infusion of donor-derived regulatory DCs (DCreg) before living donor liver transplantation can induce alterations in host T cell populations that may be conducive to attenuation of anti-donor immune reactivity. We discuss the role of DCs in regulation of warm and liver transplant IRI and the induction of liver allograft tolerance. We also address design of cell therapies using DCreg to reduce the immunosuppressive drug burden and promote clinical liver allograft tolerance.

Ischemia/Reperfusion

Ischemic disorders, such as myocardial infarction, stroke, and peripheral vascular disease, are the most common causes of debilitating disease and death in westernized cultures. The extent of tissue injury relates directly to the extent of blood flow reduction and to the length of the ischemic period, which influence the levels to which cellular ATP and intracellular pH are reduced. By impairing ATPase-dependent ion transport, ischemia causes intracellular and mitochondrial calcium levels to increase (calcium overload). Cell volume regulatory mechanisms are also disrupted by the lack of ATP, which can induce lysis of organelle and plasma membranes. Reperfusion, although required to salvage oxygen-starved tissues, produces paradoxical tissue responses that fuel the production of reactive oxygen species (oxygen paradox), sequestration of proinflammatory immunocytes in ischemic tissues, endoplasmic reticulum stress, and development of postischemic capillary no-reflow, which amplify tissue injury. These pathologic events culminate in opening of mitochondrial permeability transition pores as a common end-effector of ischemia/reperfusion (I/R)-induced cell lysis and death. Emerging concepts include the influence of the intestinal microbiome, fetal programming, epigenetic changes, and microparticles in the pathogenesis of I/R. The overall goal of this review is to describe these and other mechanisms that contribute to I/R injury. Because so many different deleterious events participate in I/R, it is clear that therapeutic approaches will be effective only when multiple pathologic processes are targeted. In addition, the translational significance of I/R research will be enhanced by much wider use of animal models that incorporate the complicating effects of risk factors for cardiovascular disease. © 2017 American Physiological Society. Compr Physiol 7:113-170, 2017.

Cellular-specific role of toll-like receptor 4 in hepatic ischemia-reperfusion injury in mice

Ischemia-reperfusion (I/R) injury is a process whereby an initial hypoxic insult and subsequent return of blood flow leads to the propagation of innate immune responses and organ injury. The necessity of the pattern recognition receptor, Toll-like receptor (TLR)4, for this innate immune response has been previously shown. However, TLR4 is present on various cell types of the liver, both immune and nonimmune cells. Therefore, we sought to determine the role of TLR4 in individual cell populations, specifically, parenchymal hepatocytes (HCs), myeloid cells, including Kupffer cells, and dendritic cells (DCs) subsequent to hepatic I/R. When HC-specific (Alb-TLR4(-/-) ) and myeloid-cell-specific (Lyz-TLR4(-/-) ) TLR4 knockout (KO) mice were subjected to warm hepatic ischemia, there was significant protection in these mice, compared to wild type (WT). However, the protection afforded in these two strains was significantly less than global TLR4 KO (TLR4(-/-) ) mice. DC-specific TLR4(-/-) (CD11c-TLR4(-/-) ) mice had significantly increased hepatocellular damage, compared to WT mice. Circulating levels of high-mobility group box 1 (HMGB1) were significantly reduced in Alb-TLR4(-/-) mice, compared to WT, Lyz-TLR4(-/-) , CD11c-TLR4(-/-) mice and equivalent to global TLR4(-/-) mice, suggesting that TLR4-mediated HMGB1 release from HCs may be a source of HMGB1 after I/R. HCs exposed to hypoxia responded by rapidly phosphorylating the mitogen-activated protein kinases, c-Jun-N-terminal kinase (JNK) and p38, in a TLR4-dependent manner; inhibition of JNK decreased release of HMGB1 after both hypoxia in vitro and I/R in vivo.

CONCLUSION

These results provide insight into the individual cellular response of TLR4. The parenchymal HC is an active participant in sterile inflammatory response after I/R through TLR4-mediated activation of proinflammatory signaling and release of danger signals, such as HMGB1.

Sterile inflammation in hepatic ischemia/reperfusion injury: present concepts and potential therapeutics

Ischemia and reperfusion (I/R) injury is an often unavoidable consequence of major liver surgery and is characterized by a sterile inflammatory response that jeopardizes the viability of the organ. The inflammatory response results from acute oxidative and nitrosative stress and consequent hepatocellular death during the early reperfusion phase, which causes the release of endogenous self-antigens known as damage-associated molecular patterns (DAMPs). DAMPs, in turn, are indirectly responsible for a second wave of reactive oxygen and nitrogen species (ROS and RNS) production by driving the chemoattraction of various leukocyte subsets that exacerbate oxidative liver damage during the later stages of reperfusion. In this review, the molecular mechanisms underlying hepatic I/R injury are outlined, with emphasis on the interplay between ROS/RNS, DAMPs, and the cell types that either produce ROS/RNS and DAMPs or respond to them. This theoretical background is subsequently used to explain why current interventions for hepatic I/R injury have not been very successful. Moreover, novel therapeutic modalities are addressed, including MitoSNO and nilotinib, and metalloporphyrins on the basis of the updated paradigm of hepatic I/R injury.

Cell biology of ischemia/reperfusion injury

Disorders characterized by ischemia/reperfusion (I/R), such as myocardial infarction, stroke, and peripheral vascular disease, continue to be among the most frequent causes of debilitating disease and death. Tissue injury and/or death occur as a result of the initial ischemic insult, which is determined primarily by the magnitude and duration of the interruption in the blood supply, and then subsequent damage induced by reperfusion. During prolonged ischemia, ATP levels and intracellular pH decrease as a result of anaerobic metabolism and lactate accumulation. As a consequence, ATPase-dependent ion transport mechanisms become dysfunctional, contributing to increased intracellular and mitochondrial calcium levels (calcium overload), cell swelling and rupture, and cell death by necrotic, necroptotic, apoptotic, and autophagic mechanisms. Although oxygen levels are restored upon reperfusion, a surge in the generation of reactive oxygen species occurs and proinflammatory neutrophils infiltrate ischemic tissues to exacerbate ischemic injury. The pathologic events induced by I/R orchestrate the opening of the mitochondrial permeability transition pore, which appears to represent a common end-effector of the pathologic events initiated by I/R. The aim of this treatise is to provide a comprehensive review of the mechanisms underlying the development of I/R injury, from which it should be apparent that a combination of molecular and cellular approaches targeting multiple pathologic processes to limit the extent of I/R injury must be adopted to enhance resistance to cell death and increase regenerative capacity in order to effect long-lasting repair of ischemic tissues.

Dendritic cells and damage-associated molecular patterns: endogenous danger signals linking innate and adaptive immunity

Dendritic cells (DCs) are potent antigen-presenting cells critical in regulating the adaptive immune response. The role of DCs is dichotomous; they may both present antigens and the appropriate stimulatory molecules to initiate an adaptive immune response, or they may induce tolerance and release anti-inflammatory signals. The activation of immature DCs, required for the expression of the necessary costimulatory T cell molecules, is dependent on pattern recognition receptors. In addition to the pathogen-derived ligands of pattern recognition receptors, several damage-associated molecular patterns (DAMPs) have recently been shown to interact with DCs and dramatically affect their ultimate function. The complex interplay of DAMPs on DCs is clinically important, with implications for transplantation, tumor immunity, autoimmunity, chronic inflammation and other conditions of sterile inflammation such as ischemia reperfusion injury. In this review, we will focus on the role of DAMPs in DC function.

Toll-like receptor 4 knockout mice are protected from endothelial overactivation in the absence of Kupffer cells after total hepatic ischemia/reperfusion

Kupffer cells (KCs) have been shown to be critical mediators of ischemia/reperfusion (I/R) injury in the murine liver. Using liposomal clodronate (LC), we found that KCs were protective in models of total hepatic ischemia with bowel congestion. We investigated the role of toll-like receptor 4 (TLR4) in the damage that occurs after I/R in KC-depleted livers. We injected 8-week-old C57BL/10J mice and C57BL/10ScN [toll-like receptor 4 knockout (TLR4KO)] mice with LC 48 hours before 35 minutes of warm hepatic ischemia with bowel congestion, which was followed by either 6 or 24 hours of reperfusion. The KC-depleted animals had increased mortality as well as a 10-fold increase in their aminotransferase levels that correlated with increases in centrilobular necrosis. These changes were absent in the TLR4KO animals. Lipopolysaccharide was bound extensively to endothelial cells after I/R, and this binding was diminished in the TLR4KO animals. In conjunction with this, there was an up-regulation of endothelial cell adhesion molecules in the LC-treated animals that was absent in the TLR4KO animals. Finally, there was a dramatic increase in the proinflammatory cytokine levels of the LC-treated animals, and the TLR4KO animals were protected against this increase. In conclusion, TLR4 promotes endothelial overactivation after I/R in the absence of KCs.

Antigen-presenting cell function in the tolerogenic liver environment

The demands that are imposed on the liver as a result of its function as a metabolic organ that extracts nutrients and clears gut-derived microbial products from the blood are met by a unique microanatomical and immunological environment. The inherent tolerogenicity of the liver and its role in the regulation of innate and adaptive immunity are mediated by parenchymal and non-parenchymal antigen-presenting cells (APCs), cell-autonomous molecular pathways and locally produced factors. Here, we review the central role of liver APCs in the regulation of hepatic immune function and also consider how recent insights may be applied in strategies to target liver tolerance for disease therapy.

Role of dendritic cells in cardiovascular diseases

Dendritic cells (DCs) are potent antigen-presenting cells that bridge innate and adaptive immune responses. Recent work has elucidated the DC life cycle, including several important stages such as maturation, migration and homeostasis, as well as DC classification and subsets/locations, which provided etiological insights on the role of DCs in disease processes. DCs have a close relationship to endothelial cells and they interact with each other to maintain immunity. DCs are deposited in the atherosclerotic plaque and contribute to the pathogenesis of atherosclerosis. In addition, the necrotic cardiac cells induced by ischemia activate DCs by Toll-like receptors, which initiate innate and adaptive immune responses to renal, hepatic and cardiac ischemia reperfusion injury (IRI). Furthermore, DCs are involved in the acute/chronic rejection of solid organ transplantation and mediate transplant tolerance as well. Advancing our knowledge of the biology of DCs will aid development of new approaches to treat many cardiovascular diseases, including atherosclerosis, cardiac IRI and transplantation.

Conventional DCs reduce liver ischemia/reperfusion injury in mice via IL-10 secretion

TLRs are recognized as promoters of tissue damage, even in the absence of pathogens. TLR binding to damage-associated molecular patterns (DAMPs) released by injured host cells unleashes an inflammatory cascade that amplifies tissue destruction. However, whether TLRs possess the reciprocal ability to curtail the extent of sterile inflammation is uncertain. Here, we investigated this possibility in mice by studying the role of conventional DCs (cDCs) in liver ischemia/reperfusion (I/R) injury, a model of sterile inflammation. Targeted depletion of mouse cDCs increased liver injury after I/R, as assessed by serum alanine aminotransferase and histologic analysis. In vitro, we identified hepatocyte DNA as an endogenous ligand to TLR9 that promoted cDCs to secrete IL-10. In vivo, cDC production of IL-10 required TLR9 and reduced liver injury. In addition, we found that inflammatory monocytes recruited to the liver via chemokine receptor 2 were downstream targets of cDC IL-10. IL-10 from cDCs reduced production of TNF, IL-6, and ROS by inflammatory monocytes. Our results implicate inflammatory monocytes as mediators of liver I/R injury and reveal that cDCs respond to DAMPS during sterile inflammation, providing the host with protection from progressive tissue damage.

Reoxygenation of hypoxia-differentiated dentritic cells induces Th1 and Th17 cell differentiation

Dendritic cells (DCs) are often exposed to various oxygen tensions under physiological and pathological conditions. However, the effects of various oxygen tensions on DC functions remain unclear. In this study, we showed that hypoxia-differentiated DCs expressed lower levels of MHC-II molecule, co-stimulatory molecules (CD80, CD86) and proinflammatory cytokines (IL-1beta, IL-6, and TNF-alpha), but higher levels of immunoregulatory cytokine transforming growth factor-beta (TGF-beta) than normoxia-differentiated DCs. Unexpectedly, re-exposure of hypoxia-differentiated DCs to saturated oxygen (reoxygenation) completely restored their mature phenotype and function. Specifically, the reoxygenated DCs induced naïve CD4(+) T cells to differentiate into Th1 and Th17 effector cells, but deceased the generation of CD4(+)CD25(+)Foxp3(+) regulatory T cells (Tregs). The data indicate that hypoxic microenvironment suppresses the maturation and function of murine DCs. Reoxygenation of hypoxia-differentiated DCs however results in complete recovery of their mature phenotype and function, and has strong ability to drive immune response toward a proinflammatory direction, suggesting reoxygenated DCs may contribute to inflammation of ischemia-reperfusion injury.

Distinct contributions of CD4+ T cell subsets in hepatic ischemia/reperfusion injury

Helper T cells are known to mediate hepatic ischemia/reperfusion (I/R) injury. However, the precise mechanisms and subsets of CD4(+) T cells that contribute to this injury are still controversial. Therefore, we sought to determine the contributions of different CD4(+) T cell subsets during hepatic I/R injury. Wild-type, OT-II, or T cell receptor (TCR)-delta-deficient mice were subjected to 90 min of partial hepatic ischemia followed by 8 h of reperfusion. Additionally, wild-type mice were pretreated with anti-CD1d, -NK1.1, or -IL-2R-alpha antibodies before I/R injury. OT-II mice had diminished liver injury compared with wild-type mice, implicating that antigen-dependent activation of CD4(+) T cells through TCRs is involved in hepatic I/R injury. TCR-delta knockout mice had decreased hepatic neutrophil accumulation, suggesting that gammadelta T cells regulate neutrophil recruitment. We found that natural killer T (NKT) cells, but not NK cells, contribute to hepatic I/R injury via CD1d-dependent activation of their TCRs, as depletion of NKT cells by anti-CD1d antibody or depletion of both NKT cells and NK cells by anti-NK1.1 attenuated liver injury. Although regulatory T cells (Treg) are known to suppress T cell-dependent inflammation, depletion of Treg cells had little effect on hepatic I/R injury. The data suggest that antigen-dependent activation of CD4(+) T cells contributes to hepatic I/R injury. Among the subsets of CD4(+) T cells, it appears that gammadelta T cells contribute to neutrophil recruitment and that NKT cells directly injure the liver. In contrast, NK cells and Treg have little effects on hepatic I/R injury.

Dendritic cells, the liver, and transplantation

Interstitial liver dendritic cells (DCs) exhibit phenotypic diversity and functional plasticity. They play important roles in both innate and adaptive immunity. Their comparatively low inherent T cell stimulatory ability and the outcome of their interactions with CD4(+) and CD8(+) T cells, as well as with natural killer (NK) T cells and NK cells within the liver, may contribute to regulation of hepatic inflammatory responses and liver allograft outcome. Liver DCs migrate in the steady state and after liver transplantation to secondary lymphoid tissues, where the outcome of their interaction with antigen-specific T cells determines the balance between tolerance and immunity. Systemic and local environmental factors that are modulated by ischemia-reperfusion injury, liver regeneration, microbial infection, and malignancy influence hepatic DC migration, maturation, and function. Current research in DC biology is providing new insights into the role of these important antigen-presenting cells in the complex events that affect liver transplant outcome.

Ischemia-reperfusion and immediate T cell responses

The pathogenesis of ischemia-reperfusion injury (IRI) is complex and not well understood. Inflammation plays an important role in IRI, with involvement of leukocytes, adhesion molecules, chemokines and cytokines. Emerging data suggest a role of T cells as mediators of IRI both in renal and extra-renal organs. Divergent roles of T cell subsets have also been elucidated, suggesting a more complicated role of T cells in the different phases of IRI. This review presents recent evidence from various animal models that advances our understanding of the role T cells play in IRI. These findings entertain the possibility of using immunotherapeutic agents for the prevention and treatment of IRI.

Organ transplantation in rodents: novel applications of long-established methods

Rodent models of solid organ transplantation have been used for many decades. Standardized operative techniques resulting in highly reproducible survival rates have been developed for several organs. This allowed scientists to investigate many clinically relevant problems, test new drugs and establish novel treatment regimens. Recently, many studies used these models to explore novel issues such as graft modification by pharmaceutical, surgical or genetic engineering methods, post-transplant regeneration, leukocyte trafficking or interactions between the innate and allo-specific arms of the immune response. The results from these studies clearly facilitate a more complex and comprehensive understanding of existing problem. The long-established methods of rodent organ transplantation, combined with the newest achievements in surgical techniques, biotechnology and imaging, will remain indispensable tools of transplantation biology.

New cellular and molecular immune pathways in ischemia/reperfusion injury

Ischemia/reperfusion injury (IRI) is a multi-factorial antigen-independent inflammatory condition that profoundly affects both early and long-term function of the allograft as suggested by both clinical and experimental data. In recent years, the acute phase of IRI has been increasingly viewed as part of the innate immune response. Identification of novel molecular pathways and new insights into the mechanisms of known mediators of IRI have established links among innate immunity, adaptive immune responses and organ regeneration, and thus long-term graft function. This review approaches these novel aspects of IRI in the context of solid organ transplantation, presenting data on new observations with kidney, liver and heart allografts.

Molecular characterization of rat leukocyte P-selectin glycoprotein ligand-1 and effect of its blockade: protection from ischemia-reperfusion injury in liver transplantation

P-selectin glycoprotein ligand-1 (PSGL-1) mediates the initial tethering of leukocytes to activated platelets and endothelium. We report molecular cloning and characterization of the rat PSGL-1 gene. A neutralizing Ab was generated, and its binding epitope was mapped to the N-terminal binding region of rat PSGL-1. We examined the effects of early PSGL-1 blockade in rat liver models of cold ischemia, followed by ex vivo reperfusion or transplantation (orthotopic liver transplantation (OLT)) using an anti-PSGL-1 Ab with diminished Fc-mediated effector function. In the ex vivo hepatic cold ischemia and reperfusion model, pretreatment with anti-PSGL-1 Ab improved portal venous flow, increased bile production, and decreased hepatocellular damage. Rat pretreatment with anti-PSGL-1 Ab prevented hepatic insult in a model of cold ischemia, followed by OLT, as assessed by 1) decreased hepatocellular damage (serum glutamic oxaloacetic transaminase/glutamic-pyruvic transaminase levels), and ameliorated histological features of ischemia/reperfusion injury, consistent with extended OLT survival; 2) reduced intrahepatic leukocyte infiltration, as evidenced by decreased expression of P-selectin, ED-1, CD3, and OX-62 cells; 3) inhibited expression of proinflammatory cytokine genes (TNF-alpha, IL-1beta, IL-6, IFN-gamma, and IL-2); and 4) prevented hepatic apoptosis accompanied by up-regulation of antiapoptotic Bcl-2/Bcl-xL protective genes. Thus, targeting PSGL-1 with a blocking Ab that has diminished Fc-mediated effector function is a simple and effective strategy that provides the rationale for novel therapeutic approaches to maximize the organ donor pool through the safer use of liver transplants despite prolonged periods of cold ischemia.

Renal ischemia/reperfusion injury inhibits differentiation of dendritic cells derived from bone marrow monocytes in rats

Dendritic cells (DCs) are impacted by surgical injury, exercise, and other physiological stressors. This study aims to determine whether renal I/R injury affects 1) the differentiation of myeloid DCs from bone marrow monocytes (BMMos) and the maturation and activation state of these DCs and 2) DC infiltration of kidney. Sprague-Dawley rats were subjected to I/R injury or sham-operated. Creatinine clearance was monitored daily during the 14 d of reperfusion that followed the ischemic insult. At 2 and 14 d of reperfusion, the following were assessed 1) properties of BMMo-derived DCs (i.e., the amount of generated DCs, differentiation state markers [CD11c, CD80, CD86, and Ia], and functional state [MLR and amount of IL-12 produced]), and 2) the presence of DCs in the kidney. Numbers of BMMo-derived DCs were significantly decreased in the I/R injured group (compared with the sham-operated group) at 2 d but not 14 d. A comparison of the their functionality found mixed lymphocyte response [MLR] and IL-12 production were similar in the two groups at both time points. Also, immunohistochemistry showed infiltrating DCs in the outer medulla of the I/R injured kidney at 2 d but not 14 d of reperfusion. Thus, I/R stress reduces the number of DCs differentiated from BMMos but not the functional activity of these DCs. This decrease may reflect a stress-induced downshift in the capacity of BMMos to differentiate into DCs and a parallel upshift in the capacity of DCs to infiltrate the kidney.

Unexpected effects of viral interleukin-10-secreting dendritic cells in vivo: preferential inhibition of TH2 responses

Viral interleukin (IL)-10 (vIL-10) has been widely described as an immunoregulatory cytokine that does not possess the T-cell costimulatory activities of cellular IL-10; it was therefore believed to be a more potent tolerogenic mediator. The immunosuppressive properties of this cytokine are partly attributed to its capacity to render dendritic cells (DCs) unable to undergo full maturation and to activate T cells. We reported here that myeloid DCs retrovirally transduced with vIL-10 had an impaired production of IL-12 and a decreased expression of MHC class II molecules but had minor defects in costimulatory molecule expression and no alteration on CCR5 and CCR7 expression. In mixed leukocyte reaction, vIL-10-transduced C57BL/6 bm12 (MHC class II mismatch) DCs had a reduced capacity to stimulate C57BL/6 wild-type CD4+ T-cell proliferation. We show that bm12 vIL-10-transduced DC administration in CD8-/- C57BL/6 mice promoted IFN-gamma production, down-regulated TH2-type cytokine production, and did not induce skin graft tolerance. These findings suggest that vIL-10-transduced DC may surprisingly facilitate Th1-type inflammatory responses in vivo.