ULTRASTRUCTURAL ANALYSIS OF THYMIC NURSE CELL EPITHELIUM

Thymic nurse cells (TNC), a paradigmatic cell type of cortical epithel ium, are large lymphoid-epithelial cell complexes of thymocytes enclos ed within vacuoles lined by the epithelial cell membrane. TNC express major histocompatibility complex (MHC) class I and class II molecules on their surface and vacuole-lining membranes at high density and it w as suggested that TNC provide an optimal microenvironment for positive selection of T cells. In this report we present electron microscopica l data demonstrating that chicken TNC display morphological structures of exocytosis previously shown for hormone-secreting cells. In TNC, h owever, exocytosis is restricted to the capillary cleft between the ep ithelial cell and engulfed thymocytes. Thus, besides physical contact between the epithelial cell and enclosed thymocytes, TNC may additiona lly influence the development of thymocytes through release of soluble factors in a restricted microenvironment. By employing the 3-(2,4-din itroanilino)-3'-amino-N-methyl propylamine technique which at the ultr astructural level detects acidic organelles involved in processing of antigens presented by MHC class II molecules,we also show that TNC con tain acidic compartments similar to classical antigen-presenting cells , i.e. early and late endosomes and lysosomes, albeit in a lower amoun t than in thymic dendritic cells. This fact provides evidence that TNC not only are capable of antigen presentation but also possess the int racellular machinery for antigen processing.