EXPRESSION OF T-LYMPHOCYTE ADHESION MOLECULES - REGULATION DURING ANTIGEN-INDUCED T-CELL ACTIVATION AND DIFFERENTIATION

The pattern of lymphocyte traffic and migration in vivo is a composite of constitutive recirculation and transient changes induced by intera ction with antigen. Naive T lymphocytes in their basal, unstimulated s tate continuously recirculate throughout the entire host, poised to re act to specific antigens that they are programmed to recognize. After interaction with antigen, T cell traffic changes, first with the trapp ing of reactive cells in antigen-containing lymphoid tissue. Subsequen tly, the effector cells responding to antigen, accompanied by nonspeci fic T cells and monocytes, traffic in large numbers to sites of antige n localization, resulting in the localized inflammatory response. Then , as the immune response wanes, memory T cells develop, many of which exhibit still different routes of recirculation. The traffic and tissu e localization of leukocytes is regulated by a series of cell surface adhesion molecules that recognize specific ligands on endothelial cell s and in the extracellular matrix. Modulation of the expression of the se adhesion molecules results in the changes in T cell traffic that ar e characteristic of each stage of T cell differentiation. Thus, during T cell activation and differentiation, the downregulation of adhesion receptors specific for lymphoid tissue endothelium and up-regulation of integrins facilitate the targeting of effector cells to sites of in flammation. Subsequent changes in adhesion receptors regulate the traf fic of the antigen-specific memory cells. T cell adhesion molecule exp ression is therefore regulated as a function of the stage of activatio n and differentiation ana, in addition, is influenced by cytokines and the local lymphoid microenvironment.