Immune mechanisms in acetaminophen-induced acute liver failure

Abstract

An overdose of acetaminophen (N-acetyl-p-aminophenol, APAP), also termed paracetamol, can cause severe liver damage, ultimately leading to acute liver failure (ALF) with the need of liver transplantation. APAP is rapidly taken up from the intestine and metabolized in hepatocytes. A small fraction of the metabolized APAP forms cytotoxic mitochondrial protein adducts, leading to hepatocyte necrosis. The course of disease is not only critically influenced by dose of APAP and the initial hepatocyte damage, but also by the inflammatory response following acetaminophen-induced liver injury (AILI). As revealed by mouse models of AILI and corresponding translational studies in ALF patients, necrotic hepatocytes release danger-associated-molecular patterns (DAMPs), which are recognized by resident hepatic macrophages, Kupffer cell (KC), and neutrophils, leading to the activation of these cells. Activated hepatic macrophages release various proinflammatory cytokines, such as TNF-α or IL-1β, as well as chemokines (e.g., CCL2) thereby further enhancing inflammation and increasing the influx of immune cells, like bone-marrow derived monocytes and neutrophils. Monocytes are mainly recruited via their receptor CCR2 and aggravate inflammation. Infiltrating monocytes, however, can mature into monocyte-derived macrophages (MoMF), which are, in cooperation with neutrophils, also involved in the resolution of inflammation. Besides macrophages and neutrophils, distinct lymphocyte populations, especially γδ T cells, are also linked to the inflammatory response following an APAP overdose. Natural killer (NK), natural killer T (NKT) and T cells possibly further perpetuate inflammation in AILI. Understanding the complex interplay of immune cell subsets in experimental models and defining their functional involvement in disease progression is essential to identify novel therapeutic targets for human disease.

Keywords: Acetaminophen (APAP); acute liver failure (ALF); immune response; liver injury; macrophages.

Figures

Figure 1

Formation of NAPQI-protein adducts. N-acetyl-p-aminophenol (APAP) is metabolized by cytochrome P450 isoforms (mainly CYP2E1) into the toxic metabolite N-acetyl-p-benzoquinone imine (NAPQI). Low levels of NAPQI can be fully converted into the inactive reduced form by the glutathione S-transferase and glutathione. If glutathione is depleted due to an overdose of APAP NAPQI is no longer fully reduced and starts binding to mitochondrial proteins. The formation of NAPQI protein adducts then leads to hepatocyte necrosis, either by directly causing oxidative stress and mitochondrial dysfunction or by the presentation of protein adducts via human leukocyte antigen (HLA). N-Acetylcysteine (NAC) is a precursor of glutathione and is administered therapeutically after an APAP overdose to enhance hepatocytic NAPQI clearance.

Figure 2

Progression of hepatocellular necroses following acetaminophen-induced acute liver injury. (A) In homeostasis, fenestrated sinusoidal endothelial cells line the liver sinusoids while KC is located at the luminal side, constantly sensing for PAMPs and DAMPs. The main metabolic cells of the human body, the hepatocytes, are constantly supplied with nutrients from the intestine and metabolic byproducts from other organs. (B) An overdose of APAP leads to hepatocyte necrosis, followed by the release of DAMPs. DAMPs are recognized by KC and MoMF, leading to their activation. Activated KC secrete chemokines, to attract monocytes and neutrophils, and cytokines, which enhance inflammation and sensitize hepatocytes to undergo apoptosis. Neutrophils and monocyte-derived macrophages phagocytose apoptotic bodies and cell debris. Macrophages also secrete cytokines and pro-angiogenic factors. MoMF mature under the influence of the inflammatory milieu. KC-activated γδ T-cells are involved in the activation of neutrophils, thereby further enhancing inflammation. Data from different models of acute liver injury suggest that NK, NKT and T cells might also contribute to hepatotoxicity, but the exact mechanisms have not been fully clarified yet. DAMPs, danger-associated-molecular patterns; MoMF, monocyte-derived macrophages; KC, Kupffer Cell(s); NK, natural killer; NKT, natural killer T cell(s).

Figure 3

Involvement of different immune cells in acetaminophen-induced acute liver failure. KC and neutrophils recognize DAMPs (HMGB1, Hsp70, free DNA), released from necrotic hepatocytes, via pattern recognition receptors such as TLRs. This leads to the activation of KC and neutrophils and subsequently to their activation. Activated KC secretes TNF-α, IL-1β, IL-6 and IL-10, from which TNF-α sensitizes hepatocytes to apoptosis. Chemokines released from KC (CCL2, CXCL1, CXCL2, CXCL8) promote the accumulation of monocytes and neutrophils into areas of necrosis. Infiltrating monocytes (Ly-6Chigh) likely mature under the influence of IL-6 and CCL2, into MoMF, which are distinct from resident KC. Mature MoMF show a restorative phenotype with increased expression of IL-10. IL-23 secreted by KC activates γδ T cells, ultimately leading to activation of neutrophils, thereby aggravating hepatotoxicity. The role of NKT, NK and T cells has not been fully elucidated yet. NKT cells might influence neutrophils via IL-4 whereas IFN-γ secreted by NK cells and possibly Th cells could mediate hepatocyte necrosis. DAMPs, danger-associated-molecular patterns; MoMF, monocyte-derived macrophages; KC, Kupffer Cell(s); NK, natural killer; NKT, natural killer T cell(s).