Immune cell traffic control and the central nervous system

That lymphocytes traffic to the central nervous system (CNS) is a relatively new concept. From studies on lymphocyte trafficking to lymphoid tissue and systemic organs, it is known that small lymphocytes recirculate continually from blood to lymphoid tissue and back again to blood, and that lymphocyte activation leads to changes in trafficking patterns with rerouting to areas of antigen deposition. Control of lymphocyte trafficking involves the induction and expression of a series of cell-surface molecules on circulating immune cells and target tissue vasculature. Lymphocyte trafficking into the CNS preferentially involves activated lymphocytes. This may be secondary to elaboration by activated immune cells of cytokines which are capable of stimulating CNS endothelium and may also relate to their production of enzymes which degrade cytoskeletal elements, aiding transmigration. Based upon observations of systemic homing receptors specific for certain organs, the existence of CNS-specific homing receptors with corresponding ligands on CNS vasculature is postulated.