Introduction
Cirrhosis is one of the leading causes of global mortality, resulting in over 1 million deaths annually
[1]. In the progression of cirrhosis, inflammation plays a pivotal role in the progression from chronic liver disease to cirrhosis. Inflammation can be triggered by the recognition of distinctive molecules derived from pathogen production and released from dying or injured cells, known as pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs), respectively
[2]. Monocytes/macrophages serve as key initiators in this pathophysiological process. DAMPs and PAMPs recognize pattern recognition receptors in monocytes/macrophages, activating the NF-κb signaling pathway and leading to inflammatory cytokine production
[3]. Functional activation and quantity alteration of monocytes/macrophages were reported to be closely related to cirrhosis severity from Child‒Pugh A to C and acute-on-chronic liver failure (ACLF). Monocytes enhance surface expression of HLA-DR along with costimulatory molecules CD80 and CD86, while also producing significant amounts of proinflammatory cytokines such as IL-6, TNF-α, and IL-1β
[4]. The macrophage activation markers sCD206 and sCD163 show elevated levels corresponding to disease severity and exhibit good predictive value for adverse outcomes
[5]. Growing evidence indicates that monocytes and macrophages are a heterogeneous population of myeloid cells
[6]. In patients with chronic liver disease, a proinflammatory subset known as CD14
+CD16
+ monocytes, has increased both in peripheral blood and liver
[7]. Additionally, an intrahepatic CD14
+CD206
+HLA-DR
hi myeloid subset was reported in pathologic liver in patients with end-stage liver disease, which spontaneously secreted proinflammatory mediators
[8].
CD169, also known as sialoadhesin or Siglec-1, defines a unique macrophage subset
[9]. CD169
+ macrophages have been reported to play a critical role in various immunological regulation processes, such as antigen presentation, antitumor immunity, immunological tolerance and inflammation
[10]. CD169 is constitutively expressed at low levels in monocytes. However, its expression increases dramatically in patients with an elevated IFN-α signature, due to pathogen-induced inflammation (e.g., virus infection) or nonpathogen-induced inflammation (e.g., systemic lupus erythematosus, coronary artery disease and cancer)
[11‐
15]. Nevertheless, the precise role of CD169
+ monocytes/macrophages has not been fully depicted in liver disease.
In this study, we observed that CD169+ monocytes progressively increased with the advancement of liver cirrhosis and exhibited heightened production of inflammatory mediators. Additionally, we found that CD169+ monocytes/macrophages displayed activated phenotypes and prominently expressed chemokine receptors. Repeated liver injury and fibrin deposition are the pathophysiological basis of cirrhosis. We showed that depletion of CD169+ cells in CD169-diphtheria toxin receptor (CD169-DTR) transgenic mice resulted in attenuated inflammatory response and tissue necrosis during liver injury.
Discussion
In this study, we confirmed an increase in the CD169+ monocyte subset with the severity of cirrhosis, which robustly contributes to systemic and local inflammation. This was characterized by elevated expression of the inflammatory cytokines IL-6, IL-1β, TNF-α and IL-10, as well as enhanced neutrophil recruitment and survival. Depletion of CD169+ cells in CD169-DTR transgenic mice effectively attenuated the inflammatory response and hepatic necrosis in an acute liver injury model.
Systemic inflammation plays a critical role in the progression of cirrhosis
[23]. Previous studies have shown that cirrhotic patients with bacterial infection exhibit excessive production of the inflammatory cytokines TNF-α and IL-6, which is associated with the development of an episode of extrahepatic organ failure and in-hospital mortality
[24]. Studies which stimulated monocytes or PBMCs from patients with cirrhosis or healthy controls with LPS ex vivo have indicated higher level of TNF-α, IL-1β and IL-6 production in the former
[25,
26]. Despite extensive research, the mechanism underlying the excessive proinflammatory response to LPS in patients with cirrhosis remains largely unclear. The gram-negative bacterial component LPS can induce “antiviral” IFN-stimulated gene (ISG) expression even without a viral pathogen. Evidence indicates that during the progression of liver fibrosis, the gut microbiota and bacterial degradation products are delivered via portal venous blood to the liver and circulation, triggering tonic type I IFN expression, which results in dysfunction of innate immune responses and an inability to control bacterial infection
[27]. Increased activation of the IFN signaling pathway can result in excessive inflammation and tissue damage. In patients with alcoholic cirrhosis, higher baseline expression of ISGs was associated with a higher risk of death
[28]. Expression of CD169 on circulating monocytes as an indicator of type I IFN activation
[13]. A high level of IFN-α in systemic lupus erythematosus (SLE) patients demonstrated high expression of CD169 on monocytes and contributed to disease activation
[29]. Furthermore, exposure of healthy monocytes to IFN-α robustly induces CD169 expression
[30]. Our findings confirmed that the CD169-expressing monocyte population expanded with the progression of liver cirrhosis. The high level of CD169 expression may indicate the activation of the IFN signaling pathway. It is worth noting that CD169
+ monocytes from patients with cirrhosis highly express both IL-10 and IL-6. On the one hand, IL-6 is a sensitive marker of systemic inflammation and is closely related to mortality
[23,
31]. On the other hand, IL-10, a key immunosuppressive mediator, causes corruption of antibacterial immunity, leading to loss of infection control and infection-associated mortality
[27]. Therefore, increased levels of both pro- and anti-inflammatory cytokines may help to explain the immunodeficiency and systemic inflammation that are present in cirrhosis. Interestingly, we also found that the intrahepatic and circulating CD169
+ CD14
+ myeloid cells in cirrhotic patients displayed “mixed” phenotypes, highly expressing both M1 and M2 macrophage markers. Considering the high degree of plasticity of macrophages, the simple M1/M2 dichotomy cannot properly reflect the complex phenotypic changes in cirrhosis. It is not uncommon to find macrophages with both pro- and anti-inflammatory phenotypes simultaneously in the same tissue
[32]. A chronic alcohol feeding mouse model indicated an increased frequency of CD206
+CD163
+ macrophages and led to increased expression of M1 (TNF-α, MCP1, and IL-1β) and M2 (Arg1, Mrc1, and IL-10) genes
[33]. In both a mouse model of HBV-induced liver inflammation and patients with advanced viral-related chronic liver disease, the intrahepatic CD14
+HLA-DR
highCD206
+ myeloid population is proinflammatory and spontaneously produces high levels of TNF-α
[8]. The antibiotic-mediated deletion of the intestinal microbiota caused a pronounced reduction in these cells in the liver
[8]. The accumulation of CD14
+HLA-DR
highCD206
+ in pathological liver is directly linked to gut intestinal microbiota but not viral infection. Our data supported that the CD169
+CD14
+ subset may share the same phenotypes and functions as the intrahepatic myeloid subset reported by Alfonso Tan-Garcia and colleagues
[8]. Elevated CD169
+CD14
+ myeloid cells help to further explain that the mechanisms of type I IFN are critical in inducing systemic inflammation and immune deficiency in patients with cirrhosis. However, how the activation of the IFN signaling pathway by bacterial products shifts monocytes/macrophages toward “mixed” features in this course needs further study.
Neutrophils are the largest population of circulating myeloid cells and a key component of local inflammation. During local inflammation, neutrophils are recruited from the circulation as first-action leucocytes and perform a series of antibacterial functions, including degranulation, ROS generation, phagocytosis, and the formation of neutrophil extracellular traps (NETs)
[34]. However, massive accumulation of neutrophils aggravates liver inflammation and tissue damage by increasing ROS, proteases, and inflammatory mediators
[35]. Ex vivo culture with or without LPS stimulation also suggests different chemokine expressions between CD169
+ and CD169
− monocytes. Among these differences, CXCL1 and IL-8, the major neutrophil-recruiting chemokines, were secreted at higher levels in CD169
+ monocytes than in CD169
− monocytes, which may attract massive infiltration of neutrophils in the inflammatory liver. Binxia Chang and colleagues proved that overproduced CXCL1 exacerbated steatohepatitis in HFD-fed mice, whereas reducing the level of CXCL1 relieved HFD plus ethanol-induced hepatic neutrophil infiltration and injury
[36]. In patients with HBV-ACLF, serum CXCL1 was positively correlated with organ failure and can be a predictive marker of short-term mortality
[37]. In addition, our data also revealed that CD169
+ monocytes prolonged neutrophil survival in vitro via high expression of G-CSF and GM-CSF. Diminished neutrophil apoptosis could augment the respiratory burst and increase inflammatory cytokine production, further worsening tissue damage and sustaining chronic inflammation. The production of GM-CSF by CD169
+ monocytes can in turn prevent LPS tolerance and maintain TNF-α during a state of chronic inflammation
[8].
Liver inflammation and repeated liver injury lead to fibrosis, cirrhosis and the development of hepatocellular carcinoma. Upon removal of the etiological source of chronic injury, hepatic fibrosis can be reversed in patients and experimental rodent models
[38]. CD169-DTR transgenic mice were used to explore the roles of CD169
+ cells in immune responses to liver injury. Depleting CD169
+ cells dramatically attenuated liver injury and the local inflammatory response in hepatitis models. The infiltration of MoMFs is a main change for augmenting liver inflammation, while loss of KCs is responsible for reducing immune tolerance
[39‐
41]. Our results may help to illustrate that CD169
+ MoMFs are a key proinflammatory subset of liver-infiltrated MoMFs. In addition, depleting CD169
+ cells attenuated neutrophil recruitment in the necrotic area of the liver. Neutrophils are recruited to sites of inflammation or injured tissue as first-action myeloid cells, where they cause tissue damage though cytokine and granule protein release. Reducing neutrophil infiltration helps to ameliorate acute liver injury
[42,
43].
Our findings also strengthen that pre-existed inflammation may service an indispensable role in the process of fibrosis resolution. After depleting of CD169, the spontaneous resolution of liver fibrosis was also impaired. Neutrophils are key proinflammation subset in acutes injury phase but also facilitates liver scar degradation in resolution phase after liver injury
[44]. The diminished recruitment of neutrophils after CD169 cells depletion may attenuate the fibric degradation. Besides, the restorative monocyte/macrophage subset may derive from the proinflammatory monocyte/macrophage subset. Previous observations indicated in liver fibrosis resolution, the proinflammatory Ly6C
hi monocytes converted to restorative Ly6Clo monocytes, which exhibited highly expression of matrix-degrading metalloproteinase enzymes and enhanced phagocytosis
[45]. Interestingly, we found the mice with CD169 cells depletion were prone to die by DT treatment during the fibrosis formation phase of CCl
4 administration (Data not shown). The possible mechanisms are deserved to be explored further. The phenomenon may reveal depleting CD169-positive cells affect the hepatic drug metabolism upon CCl
4 administration, amplifying radical peroxidation of hepatocyte lipids.
In conclusion, we identified the expansion of CD169+ myeloid cells along with the progression of cirrhosis in the blood and liver. The special subset expressed both M1- and M2-like macrophage phenotypes and was critical in promoting systemic inflammation in patients with cirrhosis with high expression of inflammatory cytokines and chemokines. Special depletion of CD169+ cells in a mouse model dramatically ameliorated acute liver injury, but impaired fibrosis resolution.
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