The intrathymic microenvironment: expression of lectin receptors and lectin-like molecules of differentiation antigens and MHC gene products.

The expression of MHC products, differentiation antigens and lectin receptors has been investigated in the various cell types populating different compartments of the thymus. The ultrastructural classification of suspended thymic epithelial cells was facilitated by using a technique that preserves cortical nursing cells or medullary epithelial cell clusters. A subset of peanut lectin positive lymphocytes could be distinguished by their ability to bind soybean lectin also. This subset corresponds to the large proliferating lymphocytes that populate the area between the thymic capsule and the cortex. Ia and H-2 D/K antigens could be detected on nearly all epithelial and lymphoid cells. Expression of H-2 antigens, however, is more pronounced on medullary epithelial cells. T-cell differentiation antigens such as Thy-1 and Lyt-1 could be demonstrated not only on lymphocytes, but, interestingly enough, on cortical epithelial cells as well. These latter cells, in addition, exhibit a cell membrane-bound lectin with a specificity for D-galactose which might well be the structure responsible for binding the galactosyl residues of the peanut lectin receptor of thymic lymphocytes. Binding sites for a large set of lectins could be demonstrated on both, thymic lymphocytes and epithelium. The intrathymic differentiation pathway of T-lineage cells is discussed with regard to those lymphocytic and epithelial cell surface structures considered to enable cellular interaction.