Tau aggregates are abnormally phosphorylated in inclusion body myositis and have an immunoelectrophoretic profile distinct from other tauopathies

Sporadic inclusion body myositis (s‐IBM) is the most frequent progressive acquired inflammatory myopathy in people older than 50 years. Abnormal aggregates of ‘Alzheimer's proteins’, including tau proteins, have been previously demonstrated in s‐IBM. In the present study, we have investigated by immunohistochemistry and immunoblotting analysis the presence of tau proteins in muscle biopsy samples from patients with s‐IBM and other myopathies with rimmed vacuoles, using newly developed antibodies raised against tau protein epitopes found in Alzheimer's disease brain. Tau immunoreactivity was shown in rimmed vacuoles or inclusions, preferentially with antibodies directed against phosphorylated carboxy‐terminal epitopes of tau proteins. Cytoplasmic reactivity was also demonstrated in atrophic, nonvacuolated fibres, as well as in non‐necrotic fibres invaded by inflammatory cells. Abnormally phosphorylated tau aggregates were also  found  in  other  compartments  of  the  muscle  fibre  in s‐IBM and other myopathies. Tau immunoblotting showed an electrophorectic profile of a doublet within the range of 60–62 kDa isovariants, which was different from tauopathies of the central nervous system. Finally, the unique pattern of immunoreactivity of s‐IBM samples towards anti‐tau antibodies is a new clue to a possible distinct subclass of peripheral tauopathy, different from the tauopathies of the central nervous system.