The role of HMGB1 in heart transplantation

Immunometabolism of Liver Xenotransplantation and Prospective Solutions

End-stage liver diseases, such as hepatocellular carcinoma or acute liver failure, critically necessitate liver transplantation. However, the shortage of available organ donors fails to meet the rapidly growing transplantation demand. Due to the high similarity of liver tissue structure and metabolism between miniature pigs and humans, xenotransplantation of pig livers is considered as a potentially viable solution to organ scarcity. In the 2024, teams from China first time have successfully transplanted a genetically modified Bama miniature pig liver into a clinically brain-dead man lasting for 10 days. This milestone in human xenotransplantation research not only confirms the feasibility of clinical application of xenotransplantation, but also underscores the daunting and protracted nature of this pathway. Despite advanced gene-editing technologies theoretically circumventing the occurrence of most transplant rejection reactions, patients still face challenges such as chronic immune rejection, coagulation disorders, and thrombotic microangiopathy after receiving xenografts. Moreover, prolonged use of immunosuppressive drugs may induce irreversible immune dysfunction, leading to opportunistic infections and metabolic disorders. This article compares the similarities and differences in livers between humans and pigs, summarizes the immunometabolism of xenotransplantation based on current findings, and provides research perspectives on pre-transplantation and post-transplantation strategies for prolonging the survival time of xenografts.

HMGB1-mediated CIITA super-enhancers are critical for DC trained immunity in acute-to-chronic progression of allograft rejection

Chronic allograft rejection is mainly mediated by indirect recognition. Dendritic cells (DCs), as the major antigen-presenting cells in indirect recognition, exhibit an enhanced antigen-presenting ability in chronic rejection, but the specific mechanism is still unclear. Here, we found that pretreatment with high mobility group box-1 protein (HMGB1) in vivo can induce trained immunity in DCs. These trained DCs demonstrated an enhanced ability to present alloantigen, accelerating allograft rejection in a CTLA4-Ig-induced chronic rejection model by upregulating the expression of MHC-II and class II major histocompatibility complex transactivator (CIITA) molecules. Mechanistically, we found that HMGB1 promoted the formation of super-enhancers (SEs) of CIITA, epigenetically reprogramming DCs and promoting trained immunity. The SEs inhibitor JQ1 reduced the expression of CIITA and MHC-II in DCs, thereby delaying the occurrence of chronic rejection. Interestingly, we identified HMGB1 as a specific inducer of SE formation in a newly named SEa region of CIITA. Targeted knockout of the CIITA's SEa region inhibited HMGB1-induced trained immunity in DCs. Taken together, our data confirm that HMGB1 can induce the formation of the SEs of CIITA, promote trained immunity in DCs, and accelerate allograft rejection, thus offering a new potential target for the treatment of chronic rejection.

Advances in Innate Immunity to Overcome Immune Rejection during Xenotransplantation

Transplantation is an effective approach for treating end-stage organ failure. There has been a long-standing interest in xenotransplantation as a means of increasing the number of available organs. In the past decade, there has been tremendous progress in xenotransplantation accelerated by the development of rapid gene-editing tools and immunosuppressive therapy. Recently, the heart and kidney from pigs were transplanted into the recipients, which suggests that xenotransplantation has entered a new era. The genetic discrepancy and molecular incompatibility between pigs and primates results in barriers to xenotransplantation. An increasing body of evidence suggests that innate immune responses play an important role in all aspects of the xenogeneic rejection. Simultaneously, the role of important cellular components like macrophages, natural killer (NK) cells, and neutrophils, suggests that the innate immune response in the xenogeneic rejection should not be underestimated. Here, we summarize the current knowledge about the innate immune system in xenotransplantation and highlight the key issues for future investigations. A better understanding of the innate immune responses in xenotransplantation may help to control the xenograft rejection and design optimal combination therapies.

High Mobility Group Box 1 Contributes to the Acute Rejection of Liver Allografts by Activating Dendritic Cells

Acute rejection induced by the recognition of donor alloantigens by recipient T cells leads to graft failure in liver transplant recipients. The role of high mobility group box 1 (HMGB1), an inflammatory mediator, in the acute allograft rejection of liver transplants is unknown. Here, rat orthotopic liver transplantation was successfully established to analyze the expression pattern of HMGB1 in liver allografts and its potential role in promoting the maturation of dendritic cells (DCs) to promote T cell proliferation and differentiation. Five and 10 days after transplantation, allografts showed a marked upregulation of HMGB1 expression accompanied by elevated levels of serum transaminase and CD3⁺ and CD86⁺ inflammatory cell infiltration. Furthermore, in vitro experiments showed HMGB1 increased the expressions of co-stimulatory molecules (CD80, CD83, and MHC class II) on bone marrow-derived DCs. HMGB1-pulsed DCs induced naive CD4⁺ T cells to differentiate to Th1 and Th17 subsets secreting IFN-γ and IL-17, respectively. Further in vivo experiments confirmed the administration of glycyrrhizic acid, a natural HMGB1 inhibitor, during donor liver preservation had therapeutic effects by reducing inflammation and hepatocyte damage reflected by a decline in serum transaminase and prolonged allograft survival time. These results suggest the involvement of HMBG1 in acute liver allograft rejection and that it might be a candidate therapeutic target to avoid acute rejection after liver transplantation.

Extracellular HMGB1 Contributes to the Chronic Cardiac Allograft Vasculopathy/Fibrosis by Modulating TGF-β1 Signaling

Cardiac allograft vasculopathy (CAV) charactered with aberrant remodeling and fibrosis usually leads to the loss of graft after heart transplantation. Our previous work has reported that extracellular high-mobility group box 1 (HMGB1) participated in the CAV progression via promoting inflammatory cells infiltration and immune damage. The aim of this study was to investigate the involvement of HMGB1 in the pathogenesis of CAV/fibrosis and potential mechanisms using a chronic cardiac rejection model in mice. We found high levels of transforming growth factor (TGF)-β1 in cardiac allografts after transplantation. Treatment with HMGB1 neutralizing antibody markedly prolonged the allograft survival accompanied by attenuated fibrosis of cardiac allograft, decreased fibroblasts-to-myofibroblasts conversion, and reduced synthesis and release of TGF-β1. In addition, recombinant HMGB1 stimulation promoted release of active TGF-β1 from cardiac fibroblasts and macrophages in vitro, and subsequent phosphorylation of Smad2 and Smad3 which were downstream of TGF-β1 signaling. These data indicate that HMGB1 contributes to the CAV/fibrosis via promoting the activation of TGF-β1/Smad signaling. Targeting HMGB1 might become a new therapeutic strategy for inhibiting cardiac allograft fibrosis and dysfunction.

Recombinant high‑mobility group box 1 induces cardiomyocyte hypertrophy by regulating the 14‑3‑3η, PI3K and nuclear factor of activated T cells signaling pathways

High-mobility group box 1 (HMGB1) is released by necrotic cells and serves an important role in cardiovascular pathology. However, the effects of HMGB1 in cardiomyocyte hypertrophy remain unclear. Therefore, the aim of the present study was to investigate the potential role of HMGB1 in cardiomyocyte hypertrophy and the underlying mechanisms of its action. Neonatal mouse cardiomyocytes (NMCs) were co-cultured with recombinant HMGB1 (rHMGB1). Wortmannin was used to inhibit PI3K activity in cardiomyocytes. Subsequently, atrial natriuretic peptide (ANP), 14-3-3 and phosphorylated-Akt (p-Akt) protein levels were detected using western blot analysis. In addition, nuclear factor of activated T cells 3 (NFAT3) protein levels were measured by western blot analysis and observed in NMCs under a confocal microscope. The results revealed that rHMGB1 increased ANP and p-Akt, and decreased 14-3-3η protein levels. Furthermore, wortmannin abrogated the effects of rHMGB1 on ANP, 14-3-3η and p-Akt protein levels. In addition, rHMGB1 induced nuclear translocation of NFAT3, which was also inhibited by wortmannin pretreatment. The results of this study suggest that rHMGB1 induces cardiac hypertrophy by regulating the 14-3-3η/PI3K/Akt/NFAT3 signaling pathway.

The Role of IL-33 in Experimental Heart Transplantation

Interleukin-33 (IL-33) is a member of the IL-1 family of proteins that are produced by a variety of cell types in multiple tissues. Under conditions of cell injury or death, IL-33 is passively released from the nucleus and acts as an "alarmin" upon binding to its specific receptor ST2, which leads to proinflammatory or anti-inflammatory effects depending on the pathological environment. To date, numerous studies have investigated the roles of IL-33 in human and murine models of diseases of the nervous system, digestive system, pulmonary system, as well as other organs and systems, including solid organ transplantation. With graft rejection and ischemia-reperfusion injury being the most common causes of grafted organ failure or dysfunction, researchers have begun to investigate the role of IL-33 in the immune-related mechanisms of graft tolerance and rejection using heart transplantation models. In the present review, we summarize the identified roles of IL-33 as well as the corresponding mechanisms by which IL-33 acts within the progression of graft rejection after heart transplantation in animal models.

MSU Crystals induce sterile IL-1β secretion via P2X7 receptor activation and HMGB1 release

BACKGROUD

The mechanism by which monosodium urate (MSU) crystals induce inflammation is not completely understood. Few studies have shown that MSU is capable of stimulating the release of IL-1β in the absence of LPS treatment. The purinergic P2X7 receptor is involved in the release of IL-1β in inflammatory settings caused by crystals, as is the case in silicosis.

METHODS

We investigated the role of P2X7 receptor in sterile MSU-induced inflammation by evaluating peritonitis and paw edema. In in vitro models, we performed the experiments using peritoneal macrophages and THP-1 cells. We measured inflammatory parameters using ELISA and immunoblotting. We measured cell recruitment using cell phenotypic identification and hemocytometer counts.

RESULTS

Our in vivo data showed that animals without P2X7 receptors generated less paw edema, less cell recruitment, and lower levels of IL-1β release in a peritonitis model. In the in vitro model, we observed that MSU induced dye uptake by the P2X7 receptor. In the absence of the receptor, or when it was blocked, MSU crystals induced less IL-1β release and this effect corresponded to the concentration of extracellular ATP. Moreover, MSU treatment induced HMGB1 release; pre-treatment with P2X7 antagonist reduced the amount of HMGB1 in cell supernatants.

CONCLUSIONS

IL-1β secretion induced by MSU depends on P2X7 receptor activation and involves HMGB1 release.

GENERAL SIGNIFICANCE

We propose that cell activation caused by MSU crystals induces peritoneal macrophages and THP-1 cells to release ATP and HMGB1, causing IL-1β secretion via P2X7 receptor activation.

A potential role of TLR2 in xenograft rejection of porcine iliac endothelial cells: An in vitro study

BACKGROUND

Porcine vascular endothelial cells are a major participant in xenograft rejection. The Toll-like receptor 2 (TLR2) pathway plays an important role in both innate and adaptive immunity. The specific role of TLR2 in the response to a xenograft has not been reported. Whether the TLR2 pathway in pig vascular endothelial cells is involved in acute rejection needs to be investigated, and the mechanism is explored.

METHODS

We used a modified antibody-dependent complement-mediated cytotoxicity (ADCC) assay to conduct in vitro experiments. In porcine iliac artery endothelial cells (PIECs), siRNA was used to knock down the expression of TLR2, CXCL8, and CCL2. The effect of human serum or inactivated human serum on the expression of TLR2 was analyzed by real-time PCR and Western blotting, and transwell assays were used to assess the chemotactic efficiency of PIECs on human monocyte-macrophages (THP-1 cells) and human neutrophils. The downstream signaling pathways activated by human serum were detected by Western blotting, and the regulation of proinflammatory chemokines and cytokines by TLR2 signaling was assessed by real-time PCR and ELISA.

RESULTS

TLR2 was significantly upregulated in PIECs after exposure to human serum, and porcine proinflammatory chemokines, CXCL8 and CCL2, were induced, at least partially, in a TLR2-dependent pattern; the upregulated chemokines participated in the chemotaxis of human neutrophils and THP-1 cells across the species barrier.

CONCLUSIONS

(i) TLR2 is significantly upregulated in PIECs by human serum, (ii) the elevated TLR2 participates in the chemotaxis of inflammatory cells through the secretion of chemokine CCL2 and CXCL8, and (iii) blockade of TLR2 would be beneficial for xenograft survival.