Written by Nick Trompeter Ph.D., Cole Toohey, Payton Olson, Peter Lee, and Kenneth Dorko
Functional Physiology of the Liver: Liver Sinusoidal Endothelial Cells, the first line of defense.
In the context of NAFLD, multiple studies indicate that the disruption of the equilibrium of the gut-liver axis induces liver dysfunction and metabolic.(11,27–29) Intestinal barrier dysfunction, dysbiosis, and immune dysregulation converge to exacerbate liver pathology, highlighting the intricate connections between gut health and liver disease. The influence of gut signaling and the consequence on liver health is multifactorial in nature, differentially effecting hepatic parenchymal (hepatocytes) and nonparenchymal cells (Liver Sinusoidal, Hepatic Stellate Cells, and Kupffer Cells).
Figure 1: Diagram of the liver lobule during health and MASH. Generated using BioRender Software.
The liver lobule is the functional unit of the hepatic system. Venous blood from the intestines enters the liver through the portal vein. The hepatic portal vein then branches and connects to liver lobules, perfusing these functional units with nutrients and digested products from the gut. Transportation of metabolites, nutrients, drugs, bacterial postbiotics, and xenobiotics occurs from the portal vein into the liver sinusoid, culminating in the central vein. Lining the hepatic sinusoid are a specialized endothelial layer composed of the Liver Sinusoidal Endothelial Cells (LSECs). The unique morphology of LSECs, hallmarked by their elongated shape with fenestrations ranging between 50-300 nm in diameter, permit bidirectional transport of lipoproteins, drugs, nutrients, waste products, and xenobiotics between the liver sinusoid and the space of Dissé. While the fenestrations of LSECs combine to form sieve plates -, porous structures that allow for transport -, the endocytic capacity of these endothelial cells endows an integral function for the homeostasis of the hepatic system in both normal physiology and pathophysiology.
The formation of sieve plates by the fenestrated LSECs allows for passive transport of the contents of the blood to reach the space of Dissé. Utilizing molecular machinery like the GVB, LSECs form cell-cell junctions from coordinated actions of JAM-A, claudins, ZO-1 + 2, and VE-Cadherin. However, paracellular transport occurs in the cluster of fenestrae formed by multiple LSECs. Like typical endothelial cells, LSECs express markers and proteins that allow them to integrate signals from physiologic cues including external stimulivia the expression of markers/proteins of stereotypical endothelial cells, allowing these cells to integrate signals from external stimuli (paracrine/ digestive/ xenobiotics) and autocrine signals. Cell Adhesion Molecules, including PECAM, VCAM, and ICAM-1/3, which coat the membrane of LSECs to permit attachment of immune cells during inflammatory processes. Furthermore, the expression of LYVE-1, L-SIGN, and von Willebrand Factor in LSECs, which are found in other endothelial populations, promote adhesion and/or uptake of both liver and extrahepatic molecules during physiologic and pathophysiologic turnover within the body. Like other endothelial cells, LSECs respond to Vasoactive agents, such as Nitric Oxide and VEGF that are derived from either autocrine or paracrine sources to drive phenotypic preservation of healthy LSECs.(30,31)
LSECs form the first line of defense in the liver as scavengers because of their endocytic and pinocytic capacity. The expression of Fc-gamma receptor IIb2 (FcγRIIb2), combined with receptors that recognize foreign pattern sequences (PRRs) -, Stabilin- 1, Stabilin- and 2, and Toll-like Receptors (TLRs) - convey a unique function to these endothelial cells. Expression of FcγRIIb2 on LSECs allows for endocytosis, and the eventual clearance by Kupffer cells, of IgG antibodies.(31) With approximately 70% of expression of FcγRIIb2 found within the liver, capture of IgG-based antibody therapies by LSECs is a key concern during drug development. Meanwhile, the presence of scavenger receptors, Stabilin-1 and Stabilin-2, permit for the endocytosis of various endogenous ligands from the body, including hyaluronan, oxidized/acetylated LDL, and advanced glycation end products by LSECs. Clearance of thisthese products maintains tissue homeostasis, with excessive concentrations of these ligands implicated in various pathologies.(32,33)
The expression of TLRs on the membrane of LSECs suggests these cells play a role in adaptive immunity, which was confirmed allogeneic T-Cells were stimulated by LSECs when LSECs were stimulated by agents against TLR 1/2/6.(34) LSECs can also resist viral infection, through the pinocytic uptake of adenoviruses and polyomavirus-like particles.(35,36) In response to the activation of TLRs, Stabilin, and other pattern recognition receptors, LSECs stimulate innate immune cells via the production of inflammatory cytokines (IL-1, TNFα, interferon-β). These observations highlight the diverse roles and functions in adaptive and innate immunity displayed by LSECs.
Functional Physiology of the Liver: Kupffer Cells, the sentinel of the liver.
The liver requires complex functional crosstalk between various cell populations to protect the body from xenobiotics, pathogens, and toxic compounds. The macrophagic Kupffer Cells (KCs) prowl across the apical membrane of LSECs, scavenging for bacteria, bacterial post-biotics, xenobiotics, and metabolites. Scientists initially postulated that KCs were the primary contributors to clearance of substances within the hepatic circulation. However, the discovery of the endocytic capacity of LSECs highlights a concordant effort to protect the body from foreign substances. Furthermore, KCs appear to be a primary effector of hepatic immune tolerance to maintain homeostatic function during infection and hepatic.(37) During administration of LPS, cellular debris, cytokines, or other endogenous immunogens, KCs polarize into either immunogenic (M1-like) or tolerogenic (M2-like) macrophages. Polarization to the M1 phenotype promotes inflammatory cytokine and chemokine production, whereas the M2-like KCs provide a protective and pro-regenerative response.(37,38) The fate of KCs dependss on a litany of factors, including genetics, the source and magnitude of the stimulus, and current health status of the liver.
The capacity of KCs to link innate and adaptive immunity within the hepatic system occurs through via the presence of transmembrane receptors that respond to external stimuli. The repertoire of external sensors on KCs includes pattern recognition receptors (PRRs), complement receptors (CRs), scavenger receptors (SRs), and toll-like receptors (TLRs). As the name suggests, PRRs function to recognize specific motifs from bacteria and bacterial components (DNA, cell walls, etc.) known as pathogen-associated molecular patterns (PAMPs). Furthermore, Damage-associated molecular patterns (DAMPs), molecules and DNA secreted by cells in response to damage or stress, are phagocytosed by KCs through the coordinated signaling of PRRs, such as TLRs, and CD-14b.(39)
Complement receptors, such as CRIg, anchored to the membrane of KCs recognize antigen-: antibody immune complexes, initiating phagocytosis of labeled foreign objects. These complement receptors and other SRs and PRRs allowsallow for the liver to account for approximately 90% of pathogen clearance within the body. However, expression and internalization of these receptors, specifically CRIg on KCs, can also elicits regulatory effector function of T-cell activation and proliferation. In concordance with B7/CD28, CRIg induces inhibitory functions on T-cell activation, namely decreased cytokine production and proliferation within T-cells.(40) While suppression of T-cells promotes immune tolerance in healthy tissues, regulation of T-cells but can be co-opted by viral pathogens to promote viral replication, such as during chronic hepatitis B.(41) Complement receptors additionally contribute to the recruitment and activation of immune cells, including neutrophils, NK cells, effector T-cells and T regulatory regulatory T-cells through both antigen presentation and inflammatory cytokine secretion.(42)
.TLRs link the clearance of PAMPs and DAMPs to innate immunity through inflammasome complexes and the production of inflammatory cytokines and chemokines. For example, TLR-4 on KCs binds to LPS from the gut to induce inflammasome complex formation and the generation of reactive oxygen species (ROS).(43–45) Inflammasome formation and ROS generation induces nuclear translocation of the pro-inflammatory nuclear factor kappa b (NF-κb) to generate IL-1β, IL-6, IL-18 and TNF-α. In mice, Wu and colleagues discovered that TLR-1 and TLR-8 binding and activation enhances the antigen presentation capacity of KCs, while stimulation of TLR-1,. -2, -4, and -6 promote T-cell effector function and proliferation.(34) Furthermore, agonism of TLR-3 and TLR-4 initiates the production and secretion of antiviral agents from KCs, as assessed by encephalomyocarditis virus killing of L929 cells. KCs utilize TLRs to monitor the liver sinusoid for the appearance of bacterial products, bacterial DNA, bacteria, viruses, free fatty acids, and as well as endogenous molecules and damageds cellular byproducts, such as mitochondrial DNA (mtDNA). This is exemplified during the injury to hepatocytes that leads to the release of mtDNA, which in turn, binds to TLR-9 on KCs to stimulate TNF-α production. Ultimately, the activation of KCs by TLR-ligand binding culminates in the infiltration of blood monocytes to the liver sinusoid, which enhances pro-inflammatory signaling cascades initiated by KCs.(46)
While polarization of KCs towards an M1-like phenotype alerts the liver and host of the appears from harmful substances, KCs leverage autocrine signaling and integration of anti-inflammatory cytokines, such as IL-4, IL-10, and IL-13 as transducers to attenuate pro-inflammatory signals. In response to LPS treatment, KCs produce prostaglandin E2, functioning as an autocrine negative feedback loop of TNF-α, IL-1β, and IL-6 production.(47) Furthermore, the negative regulation of M1- polarized KCs by M2 KCs may include the induction of KC apoptosis, as highlighted in mice resistant to alcohol-induced liver damage, which appear to be mediated by an IL-10/Arginase dependent mechanism.48 This finding was translationally significant in humans, where patients with daily alcohol intakes with lower grade liver steatosis and transaminase levels showed higher expression of M2 markers CD206 and CD163, when compared to peers with daily alcohol intake and liver damage.(48) KCs appear to require either a phenotypic change to the M2 phenotype or the induction of apoptosis in M1-polarized KCs to resolve chronic inflammation and liver regeneration.(49) Another mechanism by which KCs promote an anti-inflammatory and pro-regenerative environment occurs through the activation of the cholesterol and oxysterol activated Liver X Receptor (LXR) family. The mechanism of liver protection by LXR activation stems from both paracrine signaling and anti-inflammatory events. Excessive cholesterol within the liver activates LXRs to stimulate reverse cholesterol transports, signaling for hepatocytes to enhance cholesterol catabolism and the secretion within the biliary tract.(50)
While KCs play an integral role in recognizing foreign particles that enter the hepatic portal vein, intercellular communication with other cells of the liver and the immune system are required to elicit the pro-inflammatory and tolergenic effects required for liver homeostasis.
Learn more about Hepatic Stellate Cells and Hepatocytes in Part 4!:
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[CT1]Can you add what the three cytokines do?
[PL2]Unclear sentence
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