THYMIC NURSE CELLS - ENTRY OF EXOGENOUS HORSERADISH-PEROXIDASE VIA A TRANSCYTOTIC ROUTE

In order to address the question whether thymocytes inside thymic nurs e cells (TNCs) are secluded against exogenous proteins, TNCs were isol ated and kept in the presence of horseradish peroxidase. The distribut ion of horseradish peroxidase reaction product (HRP) within the TNCs w as studied by light (LM) and electron microscopy (EM). Stained and uns tained TNCs could be distinguished by LM. At the EM level, stained TNC s had a widened lumen and HRP lined the luminal surfaces. In unstained TNCs the lumen appeared normal and contained almost no HRP. In both t ypes of TNCs the epithelial cell contained HRP in endosomes and, toget her with cellular debris, in larger vacuoles. Exocytosis of the latter was observed. Conglomerates of HRP were found inside the lumen and al so within thymocytes. Data suggested that TNCs can shield secluded thy mocytes against exogenous proteins except these which are provided by a transcytotic pathway. Such a pathway has not been demonstrated befor e in TNCs.