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Background: Routine liver function tests capture information about the metabolic and inflammatory condition of the liver, but we lack sensitive biomarkers of early hepatocyte stress. In humans, soluble CD46 (sCD46) levels in blood were recently identified as an accurate biomarker of hepatic steatosis. Here, we explore the diagnostic utility of sCD46 in other liver diseases., Methods: We developed, optimised and validated an ELISA that facilitates measurements of human sCD46 in plasma, serum and culture supernatants. Then, we analysed mechanisms that lead to the release of sCD46 and identified its role in various hepatic stress conditions., Results: We discovered that prostaglandin E2 (PGE2) drives upregulation of matrix metalloproteinase (MMP)-1 in fat-loaded hepatocytes, leading to proteolytic shedding of CD46. We further found that sCD46 release was increased by viral, toxic and hypoxic stresses., Conclusions: sCD46 appears to be a promising biomarker with potential applications in the detection of early liver diseases or monitoring therapeutic responses, which could complement established diagnostic algorithms because sCD46 release is uniquely responsive to hepatocyte stress., Competing Interests: University Hospital Regensburg has filed a not yet published European patent application (Registration Nr. 23 183 382.3) for sCD46 as a clinical biomarker of hepatic steatosis. J.A.H. received in-kind support from Beckman Coulter. The authors have no other conflicts of interest to declare., (© 2024 The Authors.)

Background: The increasing prevalence of metabolic dysfunction-associated steatotic liver disease (MASLD) incurs substantial morbidity, mortality and healthcare costs. Detection and clinical intervention at early stages of disease improves prognosis; however, we are currently limited by a lack of reliable diagnostic tests for population screening and monitoring responses to therapy. To address this unmet need, we investigated human invariant Natural Killer T cell (iNKT) activation by fat-loaded hepatocytes, leading to the discovery that circulating soluble CD46 (sCD46) levels accurately predict hepatic steatosis., Methods: sCD46 in plasma was measured using a newly developed immuno-competition assay in two independent cohorts: Prospective living liver donors (n = 156; male = 66, female = 90) and patients with liver tumours (n = 91; male = 58, female = 33). sCD46 levels were statistically evaluated as a predictor of hepatic steatosis., Findings: Interleukin-4-secreting (IL-4 + ) iNKT cells were over-represented amongst intrahepatic lymphocytes isolated from resected human liver samples. IL-4 + iNKT cells preferentially developed in cocultures with a fat-loaded, hepatocyte-like cell line, HepaRG. This was attributed to induction of matrix metalloproteases (MMP) in fat-loaded HepaRG cells and primary human liver organoids, which led to indiscriminate cleavage of immune receptors. Loss of cell-surface CD46 resulted in unrepressed differentiation of IL-4 + iNKT cells. sCD46 levels were elevated in patients with hepatic steatosis. Discriminatory cut-off values for plasma sCD46 were found that accurately classified patients according to histological steatosis grade., Interpretation: sCD46 is a reliable clinical marker of hepatic steatosis, which can be conveniently and non-invasively measured in serum and plasma samples, raising the possibility of using sCD46 levels as a diagnostic method for detecting or grading hepatic steatosis., Funding: F.B. was supported by the Else Kröner Foundation (Award 2016_kolleg.14). G.G. was supported by the Bristol Myers Squibb Foundation for Immuno-Oncology (Award FA-19-009). N.S. was supported by a Wellcome Trust Fellowship (211113/A/18/Z). J.A.H. received funding from the European Union's Horizon 2020 research and innovation programme (Award 860003). J.M.W. received funding from the Else Kröner Foundation (Award 2015_A10)., Competing Interests: Declaration of interests University Hospital Regensburg has filed a not yet published European patent application (Registration Nr. 23 183 382.3) for sCD46 as a clinical biomarker of hepatic steatosis. J.A.H. received in-kind support from Beckman Coulter. The authors have no other conflicts of interest to declare., (Copyright © 2024 The Author(s). Published by Elsevier B.V. All rights reserved.)