The molecular cues regulating immune cell trafficking.

Lymphocyte recirculation between the blood and the lymphoid/non-lymphoid tissues is an essential homeostatic mechanism that regulates humoral and cellular immune responses in vivo. This system promotes the encounter of naïve T and B cells with their specific cognate antigen presented by dendritic cells, and with the regulatory cells with which they need to interact to initiate, maintain, and terminate immune responses. The constitutive lymphocyte trafficking is mediated by particular types of blood vessels, including the high endothelial venules (HEVs) in lymph nodes and Peyer's patches, and the flat-walled venules in non-lymphoid tissues including the skin. The lymphocyte migration across HEVs involves tethering/rolling, arrest/firm adhesion/intraluminal crawling, and transendothelial migration. On the other hand, relatively little is known about how lymphocytes and other types of cells migrate across the venules of non-lymphoid tissues. Here we summarize recent findings about the molecular mechanisms that govern immune cell trafficking, including the roles of chemokines and lysophospholipids in regulating immune cell motility and endothelial permeability.