Protein and mRNA expression of CD56/N-CAM on follicular epithelial cells of the human thyroid

In order to confirm CD56/N-CAM antigen prevalence in the human thyroid and to compare its expression on thyrocytes and NK cells, an expression of CD56/N-CAM antigen was searched for on isolated thyroid follicular cells and NK cells by flow cytometry. In addition, mRNA for CD56 was searched for in RNA isolated from human thyroid samples and few other organs using dot blot hybridization assay to prove the existence of mechanisms for active synthesis of the protein in question. The isolated cells from the follicular epithelium of 22 various pathological thyroid tissue specimens were examined for the expression of CD56/N-CAM in terms of the percentage of positive cells and the mean fluorescence intensity (MFI). Blood lymphocytes were tested in parallel. The total RNA isolated from thyroid and control tissue specimens was subjected to dot-blot hybridization assay using CD56/N-CAM cDNA probe. All thyroid specimens expressed CD56/N-CAM, but the obtained values differed depending on the tissue examined and the CD56 antibody used. There were no significant differences between the non-malignant thyroid cells of various histology, while cells in the carcinoma group had a much lower MFI, especially the median value. CD56 expression on NK cells from the donors' blood had a homogeneous distribution but the mean and median values of FI were almost three times lower than those on the thyroid cells. Dot blot hybridization came out positive with the RNA isolated from the thyroid specimens and also from the RNA isolated from the tonsil and lymph nodes, but came out negative with the RNA isolated from human and rat kidneys. These results strongly suggest that thyroid follicular epithelial cells express both protein and mRNA of CD56/N-CAM, thus being able to synthesis the relevant antigen. The protein expression seems to be affected by the malignant transformation of the thyroid cells. NK cells have apparently lower CD56/NCAM expression than thyroid cells.