78 Most studies on the location of effector/memory T cells in non-lymphoid tissues have focused on entry (homing) or proliferation and survival as determining factors of lymphocyte content in a given tissue. Recent findings have shown that exit from the tissue is an find more active process controlled by chemotactic mechanisms.

The chemokine receptor CCR7 was shown to be required for T-cell exit from inflamed peripheral tissue.79,80 Another chemotactic agent, sphingosin-1-phosphate (S1P), and its receptors are required for the exit from lymph nodes, a finding emerging from studies with the drug FTY 720, which displays immunosuppressive effects. Both CCR7 and S1P receptors are modulated in the course of T-cell activation, and thereby might cause the transient retention of recently activated T cells in the lymph node.81 When CCR7 is knocked out, the number of T cells retained in an inflamed tissue doubles, confirming its importance for continuous circulation.82 For technical reasons, quantification of exit rates for specific subsets of cells and specific tissues is

more difficult. However, a variety of data are available from early studies in which cannulation of the thoracic duct or even single lymph nodes was applied, Ku-0059436 molecular weight which provided clear evidence not only that naïve cells entering a lymph node via the high endothelium pass through the tissue within half a day and exit it, but also that large numbers of effector/memory cells attracted to an inflamed tissue, or generated by local proliferation, exit the tissue via the efferent lymph.83 It is conceivable that

the process of emigration also underlies a variety of regulatory influences; T-cell activation upon antigen encounter within the tissue may be one factor, but an influence of inflammation-generated mediators such as prostaglandins has also been described.84 The directional movement towards a chemical Casein kinase 1 compound plays a major role in the recruitment as well as the egress of T cells from the site of an immune response. Leucocytes are able to integrate signals from multiple chemoattractants in their migration.85 In fact, cells migrating away from a local chemoattractant source actually chemotax towards distant attractants. The ability to navigate through chemoattractant arrays may be sufficient to explain entry and egress of T cells during an immune response. However, recent evidence supports the existence of both chemoattractants and chemorepellents that guide the directed movement of leucocytes into and out of tissues. Chemorepulsion is defined as the migration away from peak concentrations of a chemokine and was initially studied in the context of axonal guidance, where the same molecule may act as a chemoattractive or chemorepulsive cue depending on the receptor expressed on the cell surface.