Deficiency of neutrophil extracellular traps increases susceptibility to colonic inflammation

Abstract

Abstract Neutrophil extracellular traps (NETs), DNA structures released from neutrophils, are primarily enriched with bactericidal proteins. We envision that the antimicrobial nature of NETs would be an appropriate immune response in the gut, which is continuously exposed to trillions of bacteria. NETs formation is peptidyl arginine deiminase-4 (PAD4)-dependent, therefore, we employed Pad4KO mice and their WT littermates in two well-established models of murine colitis, i.e., loss of IL-10 signaling (immune hyperactivation) and chemical-induced injury (DSS). Upon treatment with αIL-10R mAb, Pad4KO mice developed robust chronic colitis as exemplified by colomegaly, colonic crypt elongation, and increased immune cell infiltration. In comparison, αIL-10R-treated WT mice exhibited modest colitis. We theorized that NETs may function as a ‘safety net’ which confine the pro-inflammatory neutrophil granule proteins (NGP) and prevent them to cause further damage. Indeed, the concentration and activity of NGP [i.e. lipocalin 2 (Lcn2), myeloperoxidase (MPO) and neutrophil elastase (NE)], in serum and in colons, were strikingly elevated in colitic Pad4KO mice, when compared to WT mice. The level of colonic secretory leukocyte protease inhibitor (SLPI) was reduced in the colitic Pad4KO mice. Such impaired SLPI response coupled with the ‘spillover’ of NGPs may, in part, explain the aggravated colitis in mice lacking NETs. Similarly, upon DSS treatment Pad4KO mice developed more severe chronic colitis than WT mice. Taken together, our findings advance the notion that gastrointestinal NETs are beneficial, and elucidation of NETs-mediated signaling, during colitis, may yield a novel therapeutic approach to mitigate intestinal inflammation.