Immune mechanisms in inclusion body myositis. Studies on the T cell receptor Va/ß repertoire and apoptosis related proteins

Abstract

Inclusion body myositis (IBM) is the most common inflammatory myopathy presenting in people over 50 years of age and has a progressive course leading to severe disability. There is no efficient treatment for the disease. One important histologic feature of IBM is that T cells invade nonnecrotic MHC class I expressing muscle fibers, suggesting that immunologic mechanisms are involved in the pathogenesis of IBM. The purpose of the present study was to investigate the T cell receptor Va/b repertoire in inflammatory cell infiltrates in muscle since this could provide insights into immunopathologic mechanisms. Another purpose was to study mechanisms for the T cell mediated muscle cell injury in IBM. Using reverse transcription polymerase chain reaction (RT-PCR) the usage of the variable (V) segment of the T cell receptor (TCR) of 22 Va and 24 Vb families were investigated in muscle infiltrating T cells of six IBM patients. In two of the patients peripheral blood lymphocytes (PBL) were investigated in parallel. There was a restricted usage of TCR Va/b families in muscle in comparison to PBL but the restriction pattern varied between the patients. The clonality of two predominating TCR Vb families, i.e. Vb3 and Vb8 bearing T cells was investigated in three patients. Cloning and sequencing the antigenbinding region, the complementarity determining region 3 (CDR3) of the TCR Vb3 and Vb8 revealed oligoclonal expansions of T cells in muscle. Using fragment length distribution analysis of the CDR3 region for each of 24 TCR Vb families in muscle infiltrating T cells and PBL, the clonality was investigated. The fragment length distribution pattern showed an oligoclonal pattern for some of the Vb families in comparison to PBL. The results indicate that T cells have expanded locally or selectively accumulated in muscle indicating an antigen driven process.The Fas/Fas ligand system is important for T cell mediated cell death and Bcl-2 proteins are a group of important apoptosis regulating proteins. Combining RT-PCR and immunohistochemistry the expression of Fas and Fas ligand in muscle was investigated. The results showed upregulation of both Fas and Fas ligand in five IBM patients. Immunohistochemical staining with Fas showed expression on muscle fibers and inflammatory cells. Immunohistochemical investigation of Bcl-2 showed upregulation in regenerating muscle fibers. Few apoptotic cells were demonstrated by the TUNEL method and most of these appeared to be inflammatory cells. Rare TUNEL positive myonuclei were identified. The results indicate that the Fas/Fas ligand system may be of importance for the inflammatory reaction and for the T cell mediated muscle cell injury.