Differential expression of homing-associated adhesion molecules by T cell subsets in man

The ability of lymphocyte populations to recognize and bind high endothelial venules during homing into lymphoid tissues and sites of chronic inflammation is critically dependent on their expression of certain homing-associated adhesion molecules known as homing receptors (HR). In animal models, certain lymphocyte populations, particularly subsets of memory or previously activated lymphoyctes, demonstrate tissue-selective homing behavior, and it has been hypothesized that differential expression of HR accounts for this selective migration. In this study, we analyzed expression of human HR - the Dreg 56/Leu 8-defined peripheral lymph node (PLN) HR (also known as LECAM-1), H-CAM (CD44), and α₄-integrins (CD49d; VLA-4) - among subsets of thymocytes and peripheral blood T cells to identify populations with differential homing potential. In the thymus, these three HR classes are differentially regulated relative to phenotypically defined maturational stages, but are all expressed on the mature, surface CD3(high) subset. In the peripheral blood, virgin T cells (LFA-3/CD58(low)) show uniform high expression of the PLN HR, uniform relatively low expression of H-CAM and α₄-integrin, and lack markers of tissue association - the mucosal and cutaneous lymphocyte associated Ag (MLA and CLA Ags) defined by mAb Ber ACT8 and HECA-452, respectively. In contrast, circulating memory T cells (LFA-3/CD58(high)) are bimodal with respect to PLN HR expression, show uniform high expression of H-CAM and α₄-integrin, and contain essentially all the CLA and MLA Ag-bearing T cells. The circulating skin-associated T cell subset (CLA Ag⁺; 10 to 15% of total T cells) is predominantly PLN HR⁺, and shows high levels of both the α₄- and β₁-integrin chains. The distinct mucosa-associated T cell subset (MLA Ag⁺; 1 to 3% of peripheral blood T cells) is predominantly PLN HR^-; and is α₄(high), but β₁(low). These findings indicate the independent regulation of homing-associated adhesion molecules among populations of memory/previously activated T cells, and suggest that the expression patterns of these molecules contribute to, or perhaps determine, the tissue distribution of these subsets.