Antibodies Anti-Smooth Muscle (ASMA)

Autoimmune liver diseases are a group of rare chronic liver disorders caused by immune dysregulation that leads the immune system to attack hepatic structures. Autoimmune hepatitis (AIH) is a chronic inflammatory liver condition characterised by persistent hypertransaminasemia, hypergammaglobulinemia (high levels of IgG) and circulating autoantibodies. AIH may occur in association with or overlap with other liver diseases, including primary biliary cholangitis (PBC), primary sclerosing cholangitis (PSC), drug-induced liver injury (DILI) or viral hepatitis, complicating the clinical and diagnostic picture.

Autoimmune hepatitis is classified into three subtypes based on the autoantibody profile:

A. Type 1 AIH: the most common form, can affect both children and adults; defined by the presence of antinuclear antibodies (ANA) and/or anti-smooth muscle antibodies (ASMA);
B. Type 2 AIH: more frequent in paediatric/adolescent patients; characterized by antibodies to LKM1 (liver–kidney microsomal type 1) and/or LC1 (liver cytosol type 1);
C. Type 3 AIH: more recently proposed; defined by anti-SLA/LP (soluble liver antigen/liver–pancreas antigen) antibodies. There is ongoing debate as to whether this should be considered as a separate entity or a variant of type 1 AIH.

Anti-smooth muscle antibodies (ASMA) are one of the main serological markers of type 1 AIH, detected in 70–85% of patients, often in association with ANA. ASMAs recognise a heterogeneous group of smooth-muscle cytoskeletal antigens, including F-actin (polymerised actin filaments), vimentin, and desmin. ASMAs against F-actin are the most specific for type 1 AIH, and their detection is useful for diagnostic and classification purposes.

The gold standard for ASMA screening is indirect immunofluorescence (IIFT) on rat stomach sections to reveal the typical fibrillar/perivascular pattern.