Background: Inflammation is characterized by leukocyte recruitment. Macrophages and neutrophils contribute to tissue damage and organ dysfunction. Modulating leukocyte invasion can protect from these adverse effects. Leukocyte recruitment critically depends on the urokinase-type plasminogen activator receptor (u-PAR). We here use a novel technique to longitudinally quantify cell trafficking in inflammatory models in live animals.
Methods: Near-infrared fluorophore-labeled leukocytes were adoptively transferred to mice with thioglycollate peritonitis to study leukocyte trafficking to sites of inflammation. Macrophage and neutrophil trafficking was followed with three-dimensional fluorescence-mediated-tomography. u-PAR-/- and wild-type macrophage recruitment was studied by cross-over adoptive cell transfer to elucidate the role of leukocytic versus u-PAR expressed on other cells. Endotoxic shock-induced pulmonary inflammation was used to study u-PARs role for pulmonary neutrophil recruitment.
Results: Mice experiencing peritonitis showed a significant increase in mean fluorescence intensity because of enhanced macrophage (315%, n=9-10), P<0.05) or neutrophil (194%, n=6, P<0.02) recruitment. Fluorescence-mediated-tomography uncovered a macrophage recruitment defect in the peritonitis model for u-PAR-/- mice (147% of baseline) compared with control mice (335% of baseline, n=8-9, P<0.05). When u-PAR-/--macrophages were transferred to wild-type mice fluorescence intensity increased to 145% while wild-type macrophage transfer into u-PAR-/- resulted in 192% increase compared with baseline (n=6, P<0.05). Reduced neutrophil recruitment in pulmonary inflammation in u-PAR-/- mice was accompanied by improved pulmonary gas exchange.
Conclusion: Using noninvasive in vivo fluorescence-mediated tomography to image leukocyte recruitment in inflammatory mouse models, we describe a novel macrophage recruitment defect in u-PAR-/- mice. Targeting u-PAR for modulation of leukocyte recruitment is a promising therapeutic strategy to ameliorate leukocyte induced tissue damage.