The consequence of autoantigen presentation by thyroid cells is dependent on the magnitude of expression of both HLA class II antigens (mainly HLA-DR) and costimulatory molecules, such as B7 (CD80 and CD86). Autoimmune thyrocytes are induced to express HLA-DR by interferon-gamma (IFN-gamma). The costimulatory signal leading to autoantibody production or cytotoxic T-cell immune response could be provided by antigen presenting cells (APCs) attracted to the thyroid by the primary autoimmune stimulus. Malignant thyrocytes can express HLA-DR antigens either constitutively, as a result of a nonimmunologic stimulus, or on induction with IFN-gamma after triggering of an immune response. However, their ability to express B7 molecules, which may determine enhanced antitumoral immune response mainly in the absence of intrathyroidal macrophages, has not yet been studied. The regulation of HLA-DR gene expression in APCs, such as B cells, is mediated by a series of short DNA consensus sequences located in the promoter, termed the W, X, and Y boxes, which bind several known transcription factors. We have previously characterized the expression of HLA-DR in four human thyroid carcinoma cell lines and found differences between constitutive and high- or moderate-induced expression of the protein and mRNA. Evaluation of B7 expression on the surface of thyroid cancer cells and understanding the mechanisms of HLA-DR gene expression may help in designing efficient immune response to thyroid tumors. Using the electrophoretic mobility shift assay (EMSA), we have demonstrated differences between the four thyroid cell lines in the binding of transcription factors to each of the three boxes. The binding to the promoter in each of the cell lines resulted in a single band, probably representing a complex of proteins formed via protein-protein interactions. Using flow cytometry we have shown that the B7 molecule was absent in the four thyroid cell lines and could not be induced by IFN-gamma. The absence of surface B7 molecules from the malignant thyroid cells may lead to either suppression of antitumoral cytotoxic T cell response or demand the cooperation of infiltrating APCs to favor immune response. Differences previously found in HLA-DR expression in the four human malignant thyroid cell lines may be explained by the variation in the binding of transcription factors to the boxes in the HLA-DRalpha promoter. The binding patterns of nuclear proteins derived from the four thyroid cell lines or from the B lymphocyte cell line--Raji--to each of the boxes or to the whole promoter exhibit similarities, thus suggesting similar DNA-protein interactions.