Differential Expression of Adhesion Molecules Within theHuman Thymus

Development of a diverse, MHC-restricted yet self-tolerant T-cell repertoire occurs within the thymus, and requires contact between developing T cells and their stromal microenvironment. Such interactions are likely to depend on the combinatorial effect of specific adhesion molecules. As a preliminary step to determining their role in T-cell development, we have studied the distribution of LFA-1/ICAM-1, CD2/LFA-3, VLA-4/VCAM-1, and HECA 452-antigen/E-Selectin ligand pairs on frozen sections of human thymus. Using two color-immunohistochemistry, and a variety of cell-lineage markers that reveal the nature of the cells on which these adhesion molecules are located, we find a differential distribution of adhesion molecules, with some being shared by both endothelial and epithelial cells. We also identify the VCAM-1-positive subpopulation as cortical macrophages. The relevance of these findings to thymopoiesis is discussed.