HLA-DR antigens, the human equivalent of mouse I region-associated or Ia products, are polymorphic cell surface sialoglycoproteins involved in initiation of the immune response1. Their expression is normally restricted to B lymphocytes, macrophages, dendritic and other antigen-presenting cells and vascular endothelium2 and possibly some cells of the mucosa lining body cavities3,4. HLA-DR expression can be modified during cell differentiation; B lymphocytes become negative on maturing to plasma cells and human T lymphocytes acquire these antigens when activated in vitro or in vivo5. We report here that human thyroid follicular cells which are normally negative for HLA-DR molecules, can be induced to express these antigens when cultured with phytohaemagglutinin (PHA), concanavalin A (Con A) or pokeweed mitogen (PWM). These lectins exert their action directly on the thyroid cells with no concomitant mitogenic effect.
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