Video-rate two-photon imaging of mouse footpad - a promising model for studying leukocyte recruitment dynamics during inflammation.

Leukocyte recruitment is a key host defense mechanism to infection and a salient feature of autoimmune diseases such as arthritis. The cell dynamics of these processes are difficult to study due to the challenge of tracking cells flowing in the circulation and migrating through light scattering tissues. Here, we describe a noninvasive two-photon (2P) microscopy approach to study leukocyte homing in the mouse footpad. In the absence of inflammation, cells moved > several hundred microm/s in vessels and only rarely adhered to endothelium or entered the tissue parenchyma. In response to bacterial infection, neutrophils moved in small capillaries at reduced speeds of (14-45 microm/min) and rolled in larger vessels at 5-60 microm/min. Within minutes of adoptive transfer, neutrophils entered the connective tissue and crawled with a median velocity of 7.3 microm/min. 2P imaging has excellent spatiotemporal resolution and is a promising in vivo approach to study the cellular basis of inflammation.