Novel Aspects Of Leukocyte Trafficking.

The inflammatory response to the Toll‐like receptor (TLR) 4 ligand lipopolysaccharide (LPS) involves a sequence of intravascular leukocyte capture, rolling and adhesion, followed by their emigration into surrounding tissue which is accompanied by plasma protein extravasation (PPE). Typically, such events are observed hours after LPS injection, but we have chosen to investigate the earlier stages of leukocyte trafficking during murine experimental endotoxaemia. Using intravital microscopy of the mesentery, we observed a rapid increase in neutrophil emigration and PPE after i.p. LPS injection, reaching a plateau after 2h. The same response was seen with the TLR2 ligand zymosan, but not the non‐specific neutrophil activator ammonium thioglycolate, implicating TLR‐specific mechanisms in early leukocyte trafficking. LPS‐induced trafficking was associated with activation of peritoneal mast cells, shown by selective uptake of the dye ruthenium red from the superfusion buffer, and was inhibited by pre‐treatment with cromolyn sodium. To conclude, our data suggest that mast cells play a role in LPS‐induced peritoneal inflammation in terms of leukocyte emigration. Further work is underway to determine the sequential mechanisms of cell types and mediators involved in these hyperacute stages of LPS‐induced inflammation. Work supported by the BBSRC.