Thymic nurse cells: entry of exogenous horseradish peroxidase via a transcytotic route.

Abstract

In order to address the question whether thymocytes inside thymic nurse cells (TNCs) are secluded against exogenous proteins, TNCs were isolated and kept in the presence of horseradish peroxidase. The distribution of horseradish peroxidase reaction product (HRP) within the TNCs was studied by light (LM) and electron microscopy (EM). Stained and unstained TNCs could be distinguished by LM. At the EM level, stained TNCs had a widened lumen and HRP lined the luminal surfaces. In unstained TNCs the lumen appeared normal and contained almost no HRP. In both types of TNCs the epithelial cell contained HRP in endosomes and, together with cellular debris, in larger vacuoles. Exocytosis of the latter was observed. Conglomerates of HRP were found inside the lumen and also within thymocytes. Data suggested that TNCs can shield secluded thymocytes against exogenous proteins except those which are provided by a transcytotic pathway. Such a pathway has not been demonstrated before in TNCs.