Morphological study of thymus stromal cells (TEL-2 cell) which play a role in the elimination of double positive immature thymocytes by phagocytosis

Background The process of the selective elimination of immature double positive thymocytes, during culture with TEL-2 cells (an epithelial cell line from the thymus stroma), is initiated by the contact between thymocytes and TEL-2 cells (Nakashima et al. 1990 Eur. J. Immunol., 20:47–53; Hirata et al. 1991 Anat. Rec., 230: 524–530). Methods Our approach was to follow the process of thymocyte internalization as a sequelae of this interaction with scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron microscopic cytochemistry for the hydrolytic enzyme acid phosphatase (Ac Pase). Results With SEM, numerous thymocytes lay underneath the TEL-2 cells. An enhanced activity of the TEL-2 cell membrane, consisting of several thin thread-like projections either with or without an expanded termination, was involved with contacting the thymocytes. With TEM, close appositions were noted either at adjacent segments of the cellular membrane or between thick plump local protrusions of the plasma membrane. Blunt pseudopodia and deep invaginations on indented portions of the TEL-2 surface engulfed one or more thymocytes. With Ac Pase histochemistry: 1) the incorporation of thymocytes into a phagosome was indicated by the Ac Pase negative thymocytes enclosed in a membrane beneath the protuberance of a TEL-2 cell surface without fusion to the TEL-2 lysosomes; 2) most of these thymocytes were morphologically intact, whereas the rest were already damaged, having changes similar to apoptosis; and 3) a few Ac Pase positive dense bodies of the TEL-2 cell, mostly with the morphology of secondary lysosomes, fused with the thymocyte-enclosing membrane after which the digestion of several thymocytes proceeded. Conclusions With the prominent activity of the plasma membrane involving the initial attachment and subsequent phagocytosis, a murine thymic epithelial cell line TEL-2 plays a demonstrating role in the mechanism of the elimination of double positive thymocytes. © 1996 Wiley-Liss, Inc.