Thyrocyte Targets and Effectors of Autoimmunity: A Role for Death Receptors?

In Hashimoto's thyroiditis, thyrocytes die by apoptosis. Whether this is the result of impaired antiapoptotic gene expression or hyperexpression of proapoptotic signals or other mechanisms is not fully established. Following the suggestion that thyrocytes from Hashimoto's glands die by a fratricidal killing mediated by Fas/Fas ligand, we have investigated whether thyroid cells from different clinical conditions are able to kill Fas-expressing target cells. We have studied whether this effector ability was mediated by Fas/Fas ligand, perforin or other death receptors/ligands, i.e., tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)/tumor necrosis factor-related apoptosis-inducing ligand receptor (TRAIL-R). We have confirmed that thyroid preparations can kill Fas-expressing HUT78 targets through apoptosis. Cell death was only partially dependent on Fas/Fas ligand but it was trypsin-sensitive. Blocking perforin did not affect Fas-expressing target killing while caspase inhibitors had a consistent although limited effect. Thyroid cells were not sensitive to TRAIL/TRAIL-R. We have also found that both thyrocytes and lymphocytes from Graves' disease thyroids were effective at killing autologous and heterologous Fas-expressing targets. Conversely, killing of these targets could be shown only with lymphocytes (but not with thyrocytes) from Hashimoto's glands. In Hashimoto's thyroiditis, thyrocytes were poorly functional while lymphocytes were able to operate as effectors. It is envisaged that thyrocyte death in Hashimoto's would result from autologous thyrocyte killing perpetrated by lymphocytes. Death receptors/ligands would appear to play a role. However, a caspase-independent mechanism may also coexist and contribute to cell death in Hashimoto's thyroiditis.