T cell homing and exercise.

Lymphocytes recirculate between the blood and lymph moving by routes that take them through various lymphoid and non-lymphoid organs in order to search for their cognate antigen. Naïve and effector/memory T cells provide distinct repertoires of receptors and ligands that constitute their ability to interact with the microvessels of different anatomical compartments and, consequently, have distinct patterns of migration. Lymphocyte migration from vascular to extravascular sites is a tightly controlled cascade of events, initiated by tethering and rolling interactions of lymphocytes on the endothelial surface. Local chemokines initiate in the activation of integrin adhesiveness, followed by firm arrest and endothelial transmigration. Environmental stress induces a substantial re-distribution of T-cells within lymphoid and non-lymphoid organs. A uniform response pattern seems to exist with a decrease in lymphocyte numbers in the spleen which is accompanied by an increase in lymphocytes in lung, bone marrow and Peyer's patches. The alterations of the migration properties could be partially explained by adrenergic mechanisms which influence surface expression of adhesion molecules. Furthermore exercise and environmental stress result in a decreased expression of adhesion molecules, which might be the result of a selective mobilization of cells. In conclusion, exercise stress induces a substantial re-distribution of T-cells within lymphoid and non-lymphoid organs. It can be hypothesized that these stress-induced effects on lymphocyte trafficking might enhance immune surveillance and vigilance. However, further investigations are crucially needed to gain more insights into the underlying mechanisms.