Patients with alcoholic hepatitis have dysfunctional T-cell and natural killer cell responses that lead to overwhelming bacterial infections. Researchers report in the March issue of Gastroenterology that the immune impairment is mediated by endotoxin-induced over-expression of proteins called programmed cell death 1 (PD1) and mucin domain–containing protein 3 (TIM3), which suppress the immune response. Importantly, the researchers show that this process can be reversed through blockade of these molecules.
Susceptibility to bacterial infection is a feature of alcohol-related liver disease. Patients with alcoholic hepatitis develop bacterial infections that ultimately lead to multiple organ failure and death, despite increased immune activity and inflammatory disease. It seems that although their immune cells are primed, their antibacterial effector functions are switched off.
The immune response is tightly regulated by positive and negative regulators. PD1 and TIM3 (also known as hepatitis A virus cellular receptor 2), and their respective ligands—PD ligand 1 (PDL1) and galectin-9—are inhibitory receptors that regulate the balance between protective immunity and immune-mediated damage. However, their sustained hyperexpression promotes immune exhaustion and paralysis.
Lee J.L. Markwick et al investigated whether these processes function correctly during alcohol-associated liver disease. They analyzed immune cells from 20 patients with alcoholic hepatitis, 16 patients with stable advanced alcohol-related cirrhosis, and 12 healthy individuals (controls).
They found that anti-bacterial innate and adaptive immune responses were greatly reduced in cells from patients with alcoholic hepatitis, compared with controls. Patients with alcohol-related cirrhosis had less severe dysfunctions in innate immune effector cells and preserved functional T-cell responses. Fewer T cells from patients with alcoholic hepatitis produced interferon gamma (IFNG) in response to lipopolysaccharide (LPS or endotoxin), compared with controls.
In addition, patients with alcoholic hepatitis had greater numbers of interleukin 10 (IL10)–producing T cells, and reduced levels of neutrophil phagocytosis and oxidative burst in response to stimulation with Escherichia coli, compared with cells from controls. T cells from patients with alcoholic hepatitis, but not alcohol-related cirrhosis, expressed higher levels of PD1 and PDL1, as well as TIM3 and galectin-9, than T cells from controls.
Interestingly, antibodies against PD1 and TIM3 restored T-cell production of IFNG, reduced the numbers of IL10–producing T cells, and increased the antimicrobial activities of neutrophils from the patients with alcoholic hepatitis.
Markwick et al explain that researchers have been hesitant to test immune therapies in patients with alcoholic hepatitis for fear of exacerbating inflammation. However, since blocking PD1 and TIM3 did not alter expression of the inflammatory factors associated with alcoholic hepatitis, this approach might be developed as a therapeutic strategy.
How are these immune inhibitory mechanisms activated in patients with alcoholic hepatitis? The authors showed that circulating levels of LPS in plasma from patients with alcoholic hepatitis caused over-expression of immune inhibitory receptors on T cells via Toll-like receptor 4 (TLR4) binding to CD14+ monocytes, via production of IL10 and tumor necrosis factor (see diagram).
The authors explain that the balance between IL10 and IFNG is an important regulator of immunity and defense against pathogens. IL10 impedes pathogen clearance by inhibiting T-cell, monocyte, and neutrophil functions, whereas IFNG activates these activities.
Overproduction of IL10 establishes an immune-suppressive state that allows for chronic infection by increasing expression of immune inhibitors such as PD1. This imbalance was not observed in patients with alcohol-related cirrhosis, controls, or in patients with hepatitis B-associated cirrhosis.
In an editorial that accompanies the article, Ramon Bataller and Pranoti Mandrekar say that strategies to alter bacterial dysbiosis and increase the function of the intestinal barrier, to reduce translocation of endotoxin into the portal circulation, could benefit patients with alcoholic hepatitis.
Furthermore, the study approach used by Markwick et al might be used to test other anti-inflammatory strategies and increase interest in immune dysfuction in patients with liver disease.