Neutrophil extracellular traps protect the kidney from ascending infection and are required for a positive leukocyte dipstick test.

Lower urinary tract infection (UTI) is common but only rarely complicated by pyelonephritis. However, the mechanisms preventing extension to the kidney are unclear. Here, we identified neutrophil extracellular traps (NETs) in healthy human urine that provide an antibacterial defense strategy within the urinary tract. In both in vivo murine models of UTI where uropathogenic E. coli are inoculated into the bladder and ex vivo human urine models, NETs interacted with uromodulin to form large webs that entrapped the bacteria. Peptidyl arginine deiminase 4 (PADI4) inhibition in mice blocked NETosis and resulted in progression of cystitis into pyelonephritis, suggesting that NETosis of urinary neutrophils acts to prevent bacterial ascent into the kidney. Analysis of UK Biobank data revealed that genetic variants in PADI4 that associated with increased risk of rheumatoid arthritis in multiple genome-wide association studies were consistently associated with reduced susceptibility to UTI. Last, we showed that urine dipstick testing for leukocyte esterase was negative in the presence of intact blood neutrophils but became positive when neutrophils were stimulated to NET, and this could be prevented by selective PADI4 inhibition, demonstrating that this test does not detect absolute neutrophil count, as has long been assumed, but specifically detects neutrophils that have undergone NETosis. These findings highlight the role of NETosis in preventing ascending infections in the urinary tract and improve our understanding of one of the most common clinical tests in medicine. Editor's summary: Lower urinary tract infections (UTIs) rarely ascend to the kidney, but the mechanisms underlying this protection are not fully understood. Here, Stewart and colleagues identified neutrophil extracellular traps (NETs) in the urine of healthy individuals that interacted with uromodulin to form webs and entrap bacteria. Inhibition of peptidyl arginine deiminase type 4 (PADI4) reduced NETosis and increased pyelonephritis in mice challenged with uropathogenic E. coli, and an analysis of UK Biobank data revealed that variants in PADI4 were associated with reduced incidence of UTI. The authors also showed that the leukocyte esterase urine dipstick test specifically detects the presence of NETosing neutrophils, highlighting the importance of NETs in both the prevention and diagnosis of UTI. —Melissa L. Norton [ABSTRACT FROM AUTHOR]