Pace of macrophage recruitment during different stages of soft tissue infection: semi-quantitative evaluation by in vivo magnetic resonance imaging.

We describe the pace of recruitment of iron-oxide-labeled macrophages to the site of different stages of infection by in vivo magnetic resonance (MR) imaging. Peritoneal macrophages were labeled with superparamagnetic iron oxide ex vivo and administered through the tail vein 6 (acute) or 48 (subacute) h after bacterial inoculation. The legs of the mice were imaged sequentially on a 4.7-T MR unit before and 3, 6, 12, 18, 24, 48 and 72 h after macrophage administration. The band-shaped lower signal intensity zone around the abscess on T2*-weighted GRE images became more obvious due to recruited macrophages up until 24 h after injection in the subacute and 48 h after injection in the acute group, indicating that the relative SI of the abscess wall decreased more rapidly and the pace of recruitment of macrophages was faster in the subacute than in the acute group. Chemokine antibody arrays of mouse sera detected increased concentration of granulocyte-colony-stimulating factor and tissue inhibitor of metalloproteinase-1 beginning at 12 h and increased interleukin-13 at 18 h. Monocyte chemoattractant protein-1 and macrophage-colony-stimulating factor began to increase at 96 h after infection. This difference in pace of recruitment may result from the release of chemokines.