Role of chemokines in trafficking of lymphocytes and dendritic cells.

Lymphocytes continuously recirculate between the vascular system and tissues. Furthermore, different lineages and subsets are capable of migrating to different tissues and tissue microenvironments. Upon proinflammatory stimulation or antigen loading, immature dendritic cells in the skin and mucosa migrate into the T-cell zones of regional lymph nodes, where they become fully differentiated dendritic cells capable of activating naive T cells. These migratory properties of lymphocytes and dendritic cells, which are essential for the homeostasis and function of the immune system, are regulated by various cell-adhesion molecules and by a group of chemokines collectively called "immune chemokines." These chemokines are expressed constitutively in specific tissue microenvironments within the primary and secondary lymphoid organs, act via highly specific G protein-coupled receptors, and regulate the migration of lymphocytes and dendritic cells to specific tissue microenvironments. Immune chemokines are thus key elements in the genesis, maintenance, and function of the immune system.