Liver fibrosis is a common final stage of almost all chronic liver diseases, and if not treated in a timely manner, it progresses to cirrhosis and carries a high risk of developing hepatocellular carcinoma (HCC). Therefore, it is crucial to detect the stage of fibrosis early.

The gold standard in assessing fibrosis is liver biopsy. However, despite its high diagnostic accuracy, biopsy is invasive and accompanied by risks (bleeding, pain, and rarely fatal outcome), which has prompted the search for reliable non-invasive alternatives. Serological indices such as FIB-4 and APRI, and imaging methods such as transient elastography (FibroScan), have reduced the reliance on biopsy in practice. However, none of these approaches measure the activity of the fibrogenic process, but only its indirect consequences.

In this context, Mac-2 binding protein glycosylation isomer (M2BPGi) – a glycoprotein biomarker that reflects the activity of hepatic stellate cells (HSCs) as key drivers of fibrosis – enters the clinic.

The biological basis of M2BPGi: from HSC activation to serum

M2BP (Mac-2 binding protein, also known as galectin-3-binding protein) is a secretory glycoprotein present in the extracellular matrix. It is found in low concentrations in a healthy liver; however, during liver damage, HSCs are activated into myofibroblasts that proliferate, produce collagen, and secrete M2BP. Advances in glycoproteomics have shown that the N-glycan chains of M2BP change with the progression of fibrosis. The lectin Wisteria floribunda agglutinin (WFA) recognizes specific terminal N-acetylgalactosamine residues that form on these altered glycosylated forms. The sandwich immunochemical test that combines WFA-lectin and antiM2BP antibody measures exactly that glycosylated isomer – WFA⁺-M2BP, i.e., M2BPGi.

A key biological feature of M2BPGi is that it does not reflect only the amount of accumulated fibrotic tissue, but primarily the activity of the HSCs themselves. M2BPGi secreted by HSCs induces Mac-2 (galectin-3) expression in Kupffer cells, which in turn activates HSCs and upregulates α-smooth muscle actin (α-SMA) expression. Thus, M2BPGi is not only a passive indicator of fibrosis but an active participant in the fibrogenic process.

Method of measurement and expression of results

M2BPGi is measured in serum by chemiluminescent enzyme immunoassay (CLEIA). The result is expressed as a cutoff index (COI), a dimensionless unit normalized to a reference population of healthy subjects. A COI value of 1.0 corresponds to the level measured in the 95th percentile of healthy subjects. Higher COI values correspond to higher stages of fibrosis: mean COI values for stages F1, F2, F3, and F4 are approximately 0.88–2.23; 1.81–3.86; 2.3–3.53, and 3.12–7.86, respectively. It should be noted that optimal cutoff thresholds vary depending on the etiology of liver disease and the presence of antiviral treatment, which is a clinically important limitation in interpretation.

Diagnostic value and comparison with existing methods

A meta-analysis published in 2024, which included multiple studies, showed that M2BPGi achieves a sensitivity of 74% and a specificity of 84% in predicting fibrosis ≥F3. Compared with FIB-4 and APRI, M2BPGi shows similar or slightly higher diagnostic accuracy, with the advantage of being a direct biomarker of active fibrogenic activity, rather than a calculated index that depends on age, platelets, and liver enzymes.

Particularly promising are the results in cases of fatty liver, an increasingly common etiology of chronic liver disease in the Western world. A study using magnetic resonance elastography (MRE) as a reference showed that M2BPGi accurately assessed the stage of fibrosis independent of body mass index, making it applicable even in obese patients. In addition, the combined algorithm of FIB-4 and M2BPGi showed higher diagnostic accuracy than either marker alone.

In addition to diagnosis, M2BPGi has value in monitoring therapeutic response and assessing HCC risk. M2BPGi levels decline in parallel with fibrosis regression after HCV eradication with direct-acting antivirals. In patients with chronic hepatitis B receiving entecavir, higher baseline M2BPGi levels are independently associated with a higher risk of developing HCC, making it a potential prognostic tool.

Limitations and challenges in clinical implementation

Despite promising results, M2BPGi has not yet entered routine clinical practice outside of Japan, where it has been in use since 2013. Key limitations include: the lack of international standardization and universal cutoff values, as COI depends on disease etiology and treatment status; limited availability of analyzers outside specialized centers; and the possible influence of extraneous fibrotic conditions (e.g., idiopathic pulmonary fibrosis) on M2BPGi levels. Future studies should define optimal algorithms that combine M2BPGi with elastographic and clinical parameters, and determine the value in monitoring fibrosis therapies currently in clinical trials.

Translated by: Mirta Maršić