This proposal is to request support for a Keystone Symposia meeting entitled ?Leukocyte Trafficking?,? organized by Ulrich von Andrian, Federica Sallusto and Klaus Ley, which will be held in Keystone, Colorado? from January 13 ? 18, 2008. Remarkable progress has been made in understanding the cellular and? molecular mechanisms that govern the intravascular recruitment, extravascular migration and functional? regulation of immune cells by visualizing cell-cell interactions using powerful intravital microscopy and flow? cytometry techniques. The identification of peptide- and lipid-based regulators of immune cell entry and exit? and the structure-function relationship of leukocyte adhesion molecules at atomic resolution have resulted in a? refined understanding of how leukocyte subset differentiation modifies migratory ability. Early clinical results? with drugs such as natalizumab, which interferes with leukocyte trafficking, have demonstrated not only? marked therapeutic efficacy in autoimmune diseases, but also potentially fatal immunosuppression that? remains poorly understood. This meeting will bring together experts from a variety of fields to present and? synthesize our current mechanistic insights into the regulation of protective and pathological immune? responses and to determine how this knowledge can be translated to better and safer therapies.