T and B Cells in Graves' Orbitopathy

Graves orbitopathy (GO) is an inflammatory eye disease associated in 95% of patients with Graves' hyperthyroidism (GH), in \~3-4% with hypothyroid autoimmune thyroiditis, and in \~1-2% with thyroid autoimmunity in the absence of thyroid dysfunction, the former known as euthyroid GO. The pathogenesis of GO is autoimmune, with the TSH-receptor being considered the major autoantigen, thereby establishing a pathogenetic link between the thyroid and orbital tissue. Thus, TSH-receptor is expressed by orbital fibroblasts, where it forms a complex with the IGF-1 receptor. Unlike GH, which is notoriously caused by TSH-receptor stimulating autoantibodies, GO is believed to reflect cell-mediated autoimmunity, as suggested by studies showing a Th1-like pattern of cytokine release by primary cultures of orbital infiltrating lymphocytes from GO patients. On the other hand, a role of B lymphocytes has emerged in recent years based on the observation that the anti-CD20 monoclonal antibody rituximab has a beneficial effect on GO activity, as demonstrated by a recent randomized clinical trial in which rituximab was compared with intravenous glucocorticoids (GC), being the former the standard treatment of moderately-severe GO. The explanation for the findings was that B lymphocytes are involved in the pathogenesis of GO as antigen-presenting cells. However, in spite of the above mentioned promising observations, another randomized clinical trial in which rituximab was compared with placebo provided opposite results. Thus, rituximab had no effect at all on GO. Data from the two studies were confronted and major differences between the two cohorts emerged, especially concerning GO activity, leading to the conclusion that rituximab may be effective for active, but not for inactive GO. Rituximab has been employed also for autoimmune diseases other than GO, including type 1 diabetes. In the former, it was shown that the effectiveness of rituximab paralleled the presence of CD20-positive in