T cell subsets and disease mechanisms in inflammatory myopathies

Abstract: The idiopathic inflammatory myopathies (IIMs) are a heterogeneous group of chronic muscle disorders, typically displaying infiltrating T cells in skeletal muscle tissue and classified into polymyositis, dermatomyositis and sporadic inclusion body myositis. Several studies involving both humans and animal models point towards a role for T cells in the pathogenesis of IIMs, however, the precise phenotype, functionality and specificity of pathogenic T cells remain elusive. Increased frequencies of a subset of T cells, known as CD28null T cells, in peripheral blood and affected organs in various chronic inflammatory disorders, are reported by several studies. Such CD28null T cells are highly differentiated T cells lacking the co-stimulatory molecule CD28, which acquire expression of other receptors commonly associated with natural killer cells, and display proinflammatory, cytotoxic and apoptosis resistant features. In contrast to CD28null T cells, regulatory T cells are T cell subset critical for maintaining immune tolerance and also described to assist in the muscle repair process. The aims of this thesis were to investigate CD28null T cell subsets in both muscle tissue and peripheral blood of patients with IIMs, by evaluating frequencies, phenotype, function and clinical relevance of these cells. The cytotoxic mechanisms of CD28null T cells towards autologous muscle cells were investigated using in vitro T cell - muscle cell co-cultures. Muscle tissues of patients were investigated for the effects of conventional immunosuppressive therapies on CD28null and regulatory T cell subsets. Glucocorticoid and regulatory T cells mediated immunosuppressive effects on circulating CD28nulls T cells were evaluated using in vitro assays. We demonstrate that muscle-infiltrating T cells are predominantly CD4+CD28null and CD8+CD28null T cells in patients with polymyositis and dermatomyositis. Also in sporadic inclusion body myositis, where the role of immune system is controversial, T cell infiltrates in muscle tissue are dominated by CD28null T cells. Circulating CD28null T cell subsets of both CD4 and CD8 lineage were more common in patients compared to healthy controls, and were associated with human cytomegalovirus infection. These cells displayed oligoclonal expansions, proinflammatory cytokine secretion and degranulation potential, and also contained perforin. Using autologous in vitro co-cultures, we showed that the cytotoxic effects of CD28null T cells towards muscle cells are mediated largely via perforin-dependent mechanisms and regulated by IFNγ-induced HLA expression on muscle cells. Interestingly, poor clinical response in patients following immunosuppressive therapy was linked to persistence of CD28null T cells in muscle tissue. CD4+CD28null T cells were also found to be resistant towards glucocorticoid and regulatory T cell mediated immunosuppression in in vitro assays. These findings imply that CD28null T cells represent clinically important effector cells in IIMs, capable of attacking muscle fibers and inducing chronic inflammation mediated pathogenesis. Ineffectiveness of current immunosuppressive therapies appears to be linked with persistent CD28null T cells in muscle tissue as well as their immunosuppression resistant properties; therefore, these cells represent potential target candidates for future therapies.