The urokinase plasminogen activator receptor is crucially involved in host defense during acute pyelonephritis

The urokinase plasminogen activator receptor (uPAR) is expressed at the cell surface of inflammatory cells and plays an important role in neutrophil migration. To investigate the in vivo role of uPAR during urinary tract infection, acute pyelonephritis was induced in uPAR-/- and wild-type (WT) mice by intravesical inoculation with 1 × 109 colony-forming units (CFU) of uropathogenic Escherichia coli. Mice were killed after 24 and 48 h, after which bacterial outgrowth and cytokine levels in kidney homogenates were determined. Influx of neutrophils was quantified by myeloperoxidase-enzyme-linked immunosorbent assay. uPAR-/- kidneys had significantly higher numbers of E. coli CFU, accompanied by higher levels of interleukin-1β (IL-1β), IL-6, keratinocyte-derived chemokine (KC), macrophage inflammatory protein-2 (MIP-2), and tumor necrosis factor-α (TNF-α). However, the number of infiltrating neutrophils was similar in uPAR-/- and WT mice at both time points, suggesting that uPAR-/- neutrophils have a lower ability to eliminate E. coli. To further investigate this, neutrophil oxidative burst and phagocytosis was measured. The generation of reactive oxygen species upon stimulation with E. coli was not diminished in uPAR-/- neutrophils compared with WT. Interestingly, uPAR-/- neutrophils displayed significantly impaired phagocytosis of E. coli organisms compared with WT neutrophils. We conclude that uPAR is crucially involved in host defense through phagocytosis during E. coli induced acute pyelonephritis.