Tissue distribution of very late activation antigens-1/6 and very late activation antigen ligands in the normal thymus and in thymoma

The expression of very late activation antigens (VLAs)-1/6 was correlated with that of the VLA ligands fibronectin, laminin, collagen, and vascular cell adhesion molecule-1 in sections of normal thymus, in thymocyte suspensions, and in 10 cases of thymoma. Capsular epithelial cells are VLA- 2⁺, VLA-3⁺, and VLA-6⁺ and face the thymic basement membrane, which is rich in fibronectin, laminin, and collagen type IV. Cortical epithelial cells are VLA-2⁺ and are embedded in a reticular meshwork of nonorganized extracellular matrix (ECM) that is rich in fibronectin. Cortical thymocytes, identified as CD3(dim) cells by using immunofluorescence in suspension, are highly positive for VLA-4, a fibronectin ligand. Most cortical macrophages are positive for vascular cell adhesion molecule-1, a molecule recognized by VLA-4. Medullary epithelial cells are VLA-2⁺/VLA-3⁺ and are codistributed with fibrous strands of organized ECM that are positive for fibronectin, collagen, and laminin. Medullary thymocytes, identified as CD3(bright) cells, are positive for VLA-4 and VLA-6, a ligand for laminin. Our findings suggest that intrathymic thymocyte maturation is associated with changes in expression of VLA molecules, which are apparently correlated with the presence of VLA ligands in the tissue microenvironment. Thymomas were classified as cortical (three), common (five), or medullary (two) type. Expression of VLA molecules and distribution of ECM in the three histological subtypes were reminiscent of those observed in the respective regions of the normal thymus. All cases of thymoma were characterized by overexpression of VLA molecules on neoplastic cells, which was associated with increased deposition of organized ECM rich in fibronectin, laminin, and collagen.