An inflammatory, familial, inclusion body myositis with autoimmune features and a phenotype identical to sporadic inclusion body myositis. Studies in three families.

We describe the occurrence of an inflammatory inclusion body myositis in siblings of a single generation in three separate families. The disease in this total of seven patients was characterized by selective and early involvement of forearm and finger flexors, confirmed by MRI, and weakness of the quadriceps, triceps and foot extensors. Muscle biopsies in at least two members from each family showed endomysial inflammation, red-rimmed vacuoles, intracellular amyloid deposition and 15-18-nm tubulo-filaments within the vacuolated muscle fibres. Immunocytochemistry on serial muscle biopsy sections demonstrated an abundance of CD8+ cells invading non-necrotic, MHC-1-expressing muscle fibres. Immunogenetic studies showed the presence of the DR3 allele (DRB1*0301/0302) in all seven patients. The combination of the clinical, histological, immunopathological and immunogenetic features indicate that these patients have a disease identical to sporadic inclusion body myositis (s-IBM). We conclude that the classic, inflammatory, s-IBM can also occur in families (familial inclusion body myositis), in a pattern analogous to the familial occurrence of other autoimmune neurological diseases such as myasthenia gravis and multiple sclerosis. These observations strengthen the view that s-IBM behaves like other autoimmune diseases and has disease susceptibility linked to the DR3 allele.