Liver pathophysiology

Definition of the utility of immune checkpoint receptors as therapeutic targets in ACLF

The issue

Acute-on-chronic liver failure (ACLF) represents a paradoxical immunological landscape characterized by a highly activated immune response with damaging systemic inflammatory responses that is simultaneously unable to defend against bacterial pathogens; immune cells are primed and stimulated but their antibacterial effector functions are switched off. This suggests a disrupted balance between protective anti-pathogen immunity and host-induced immunopathology. Membrane bound checkpoint receptors (memb-CR) on immune effector cells control this equilibrium serving as pivotal regulators of immunity. Best known for their involvement in suppressing anti-tumour immunity, blockade with Nivolumab (anti-PD1) has obtained FDA approval in multiple cancers including hepatocellular carcinoma.

It is now known that the expression and involvement of multiple memb-CRs can modulate immune effector cell functions in a range of chronic liver diseases. We have shown previously that in alcoholic hepatitis prolonged exposure of immune cells to bacterial antigens as a consequence of increased gut permeability results in hyper-expression of two immune inhibitory checkpoint receptors on the surface of bacteria specific T-cells and impairs their function. We also show that ex-vivo blockade of immune checkpoint inhibitors using neutralising antibodies can lead to reconstitution of bacteria-specific adaptive immunity in patients with alcoholic hepatitis. Recent discoveries have shown that these cell surface bound CRs (memb-CRs) can be released into the systemic circulation through alternative mRNA splicing or protease-mediated shedding as soluble CRs. These soluble CRs, which have been recognized for some time have been “hiding in plain sight” are secreted by immune cells into the serum, are functional parts of membrane immune checkpoints and evidence has demonstrated that these soluble checkpoints are involved in positive (stimulatory) or negative (inhibitory) immune regulation and that changes in their levels affect the development, prognosis and treatment of cancer. They are also increasingly being considered as diagnostic/prognostic biomarkers of disease as well as potential therapeutic agents and drug targets. The role and importance of soluble CRs and their relationship with bacterial translocation in ACLF is not well understood.

The approach

We have measured levels of several immune checkpoints in both the CANONIC cohort and a smaller longitudinal cohort. Several immune checkpoints are elevated with increasing ACLF) grade, and we have found higher levels of some of these factors can discriminate clinical outcomes. Results from our study have also demonstrated that ACLF grade increases markers of gut translocation, suggesting this contributes to the pathological state; different patterns of soluble immune checkpoints associate with different predisposition and injury in ACLF; levels of some galectins, important immune checkpoints, are elevated in ACLF liver tissue at the transcriptional level and are also produced by stressed liver tissue in an explant model; levels of immune checkpoints are dynamic in ACLF; and further experiments from this longitudinal cohort have shown that blockade of some factors can affect the immune response in vitro.

Soluble-CRs can act as agonists or antagonistic molecular decoys and can orchestrate host immunity distally, performing paracrine tasks similar to stimulatory or inhibitory cytokines. Systemic concentrations of several soluble-CRs rise during inflammation, autoimmunity, infectious disease and cancer often mirroring immune dysfunction, disease progression and increased mortality. Measurements of systemic soluble-CRs are useful both as potential diagnostic/prognostic biomarker but also to expose mechanisms underlying immunopathogenesis of disease. The defining contribution of soluble-CRs in ACLF remains unclear and was the aim of this investigation. Understanding the role of soluble CRs in the immune paresis observed in ACLF could potentially reresent innovative immune-based therapeutic approaches to rescue antibacterial defences in these patients