Type 3 Deiodinase Expression in Inflammatory Spinal Cord Lesions in Rat Experimental Autoimmune Encephalomyelitis

Background: We have shown substantial expression of type 3 deiodinase (D3, a major enzyme involved in the inactivation of thyroid hormone) in infiltrating leukocytes in several models of inflammation. Recently, thyroid hormone has been shown to improve remyelination in experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis. As induction of D3 may play an important role in decreasing local bioavailability of thyroid hormone at inflammation sites, we hypothesized that D3 is induced in spinal cord inflammatory lesions in EAE. Methods: The aim of the study was to evaluate D3 expression in spinal cord inflammatory lesions of EAE Dark Agouti rats and to investigate D3 induction in activated monocytes. Results: Here, we show marked expression of D3 by granulocytes and macrophages in spinal cord inflammatory lesions of EAE rats. We further confirm induction of D3 expression in vitro in monocytes that were activated toward proinflammatory or immunomodulatory phenotypes. Conclusions: We observed increased D3 expression both in spinal cord inflammatory lesions during EAE and in activated monocytes. Although increased D3 expression theoretically results in decreased triiodothyronine availability, it is unknown at present whether reduced local triiodothyronine concentrations are involved in impaired remyelination as observed during EAE.