Constitutive Expression of FasL in Thyrocytes

Abstract

C. Giordano et al. (1) reported evidence that the ligand for Fas antigen (FasL) is constitutively expressed on thyroid follicular cells from both normal and Hashimoto's thyroiditis (HT) tissue, and that normal thyrocytes express Fas antigen only after induction with interleukin-1β (IL-1β). Giordano et al. conclude that their results suggested a possible mechanism for thyrocyte cytotoxicity in autoimmune thyroiditis. The absence of Fas antigen expression on the surface of normal thyrocytes has been supported by one study (2); however, other studies have found Fas expression by normal thyrocytes both in situ (3) and in primary cultured cells (4,5). The findings by Giordano et al. raised the possibility of the expression of both Fas antigen and FasL on normal thyrocytes. To clarify this issue, we examined the expression of both FasL and Fas antigen mRNA in primary cultured thyrocytes with the use of reverse-transcriptase polymerase chain reaction (RT-PCR) (6) and ribonuclease protection (7) techniques. Neither assay, performed on RNA isolated from normal human thyrocytes, demonstrated mRNA for FasL (Fig. 1, A and B). To assure that this result was not unique to this sample, RNA samples from five different normal and Graves's diseased thyrocytes were screened by ribonuclease protection assay; these also did not show mRNA for FasL (Fig. 1C). In contrast, Fas antigen mRNA was detected in all five specimens (Fig. 1C) and has also been detected by RT-PCR (5). Treating the thyroid cells with TSH, IL-1β, or γINF for up to 48 hours (before harvest and RNA isolation) also did not induce the expression of FasL mRNA or alter the expression of Fas antigen mRNA.