A functional and phenotypic study on immune accessory cells isolated from the thyroids of Wistar and autoimmune-prone BB-DP rats

Dendritic cells (DCs) comprise a small population of cells in the normal th yroid. These excellent antigen-presenting cells (APCs) are thought to be in volved in the initiation of thyroid autoimmune reactions. However it is not known whether the APCs involved in this process are indeed DCs, or thyrocy tes. Our aims were as follows: (1) to isolate DCs from the thyroid of normal Wis tar rats and BB-DP rats prior to the development of lymphocytic thyroiditis ; (2) to determine the T-cell stimulatory capability of such isolated thyro id DCs and to compare this capability to that of BB-DP thyrocytes and splen ic DCs; and (3) to investigate the phenotype of isolated thyroid DCs and to compare it to that of splenic DCs; and (4) to investigate the capability o f such thyroid DCs to regulate thyrocyte growth and function, and to compar e it to our earlier reports demonstrating such capability with splenic DCs. Leukokcytic cell fractions were isolated from the thyroids of BB-DP and con trol Wistar rats of 7-20 weeks of age. The isolation steps included gentle enzymatic tissue disruption, the collection of non-plastic adherent cells a nd density gradient centrifugation of these cells to yield a low and a high density non-adherent fraction. The low density cell (LDC) fraction was composed of 50-75% leukocytes in bo th strains. These leukocytes were almost exclusively ED1+ monocytes or MHC- class II+ DC. The high density cell (HDC) fractions of both strains were co mposed of about 70% MHC-class II-negative thyrocytes and 30% ED1+ monocytes . The thyroid LDCs of both strains had an APC capability in syngeneic(syn)- MLR comparable to that of splenic DCs. However, the HDCs were extremely poo r in syngeneic T cell stimulation. There was a difference in composition be tween the Wistar and the BB-DP LDC fractions: The Wistar LDCs were composed of 30-35% ED1+ monocytes and 15-20% typical MHC-II+ DCs, while BB-DP LDC f ractions contained more ED1+ monocytes (about 70%), but fewer DCs (5-10%). In comparison to splenic DCs, thyroid DCs had a low CD80 and CD86 expressio n in both strains (i.e., an 'immature' phenotype). The LDCs of both animal strains were shown to decrease both basal and TSH-stimulated thyrocyte prol iferation and T, release by about half. This report shows that a cell fraction enriched for monocytes and DCs can b e isolated from the thyroids of both Wistar and BB-DP rats. The cells in th is fraction were as capable as splenic DCs to act as T cell stimulators in syn-MLR. Since the thyroid HDCs (predominantly thyrocytes) were very poor a t such T cell stimulation, thyroid monocytes and DCs (and not thyrocytes) a re the prime candidates to act as immune accessory cells in the initiation of thyroid autoimmunity in the rat. Wistar thyroid LDCs differed in phenoty pe from BB-DP LDCs. The latter contained a lower percentage of DCs and a hi gher percentage of their precursors, the monocytes. Interestingly, a defect in the transition of monocytes to DCs has been described in another animal model of autoimmune thyroiditis/insulitis (the NOD mouse), as well as in t hyroiditis and diabetic patients. (C) 2000 Academic Press.