Innate immunity and alcoholic liver disease

Abstract

Innate immunity provides the primary response to danger signals from pathogens or injured host cells and tissues. The cells of the innate immune system include monocytes, macrophages, dendritic cells, neutrophils, NK cells and NKT cells that orchestrate innate immune and initiate adaptive immune responses via cell interactions, cytokines, chemokines and other mediators. The most robust and common response of the innate immune system to danger signals is inflammation. In the multifactorial pathophysiology of alcoholic liver disease (ALD), activation of innate immune cells and the inflammatory cascade play a central role. Recent studies have demonstrated that Toll-like receptors (TLRs), the sensors of microbial and endogenous danger signals, are expressed and activated in innate immune cells as well as in parenchymal cells in the liver, and thereby contribute to ALD. The importance of gut-derived endotoxin and its recognition by TLR4 expressed on innate immune cells and liver parenchymal cells and the specificity of TLR4-induced downstream signaling via the interferon regulator factor 3 (IRF3) has recently been investigated. We have shown that mice deficient in IRF3 or TLR4 expression are protected from alcohol-induced liver steatosis, inflammation and hepatocyte injury. In addition to pathogen-derived danger molecules, the inflammatory cascade can also be activated by endogenous danger signals released from damaged cells. The inflammasome, a multiprotein complex, senses endogenous danger molecules to result in caspase-1-mediated cleavage of IL-1β. Our recent results suggest that inflammasome and caspase-1 activation occur in ALD and that IL-1 significantly contributes to both steatosis and inflammation in the liver in ALD.

Figure 1.

Innate immune activation in ALD

Figure 2.. Inflammasome components are upregulated in alcoholic liver disease in mice

Wild-type mice were fed with control (pair-fed) or alcohol (EtOH-fed) diet and sacrificed 4 week later. The expression of inflammasome components pro-Caspase-1, ASC and NLRP3 in the liver was measured using qPCR. N=5–10 mice per group. Numbers in graph denote p values.

Figure 3.. Schematic of micro-RNAs in alcohol-related liver injury

Alcohol-induced increase in gut permeability results in increased LPS exposure and sensitization of Kupffer cells to produce pro-inflammatory cytokines such as TNFα. Micro-RNA-155 is a key regulator of TNFα production, Alcohol increases miR-155 in KCs and this results in augmentation of LPS-induced KC TNFα production. Alcohol also sensitizes hepatocytes to TNFα-induced cell injury. Micro-RNA-122, a hepatocye-specific miRNA as well as miR-155 are increased in the serum in alcohol-induced liver damage in mice.

Figure 4.. Induction of miRNA-155 in Kupffer cells of alcohol-fed mice

Kupffer cells isolated either from pair-fed or alcohol-fed mice (n=8–10/group, cells from two mice were pooled) after 5 weeks of Leiber DeCarli diet were plated onto 24 well plate. Nonadherent cells were removed after 2–3h of plating, fresh medium was added to adherent cells and rested overnight. Next day, cells were stimulated or not with 100ng/ml LPS for 6h, washed with PBS and lysed in Qiazole (Qiagen). Total RNA was isolated using miRNeasy kit (Qiagen) and used to quantify miRNA-155 expression by real time PCR using TaqMan miRNA Assays (Applied Biosystems). SnoRNA202 was used for normalization and fold increase over the pairfed control group is shown. Data represent mean values ± S.E.M. Non-parametric MannWhitney test was employed for statistical analysis.