Identification of commensals and molecular components mediating trypsin degradation in the large intestine

Abstract The gastrointestinal tract is constitutively exposed to proteases including trypsin, a serine protease originating from the pancreas1. Elevated trypsin levels in the large intestine have been implicated in pathological conditions including infectious and inflammatory bowel disease2-4. Here we show that trypsin is regulated via degradation by members of the gut microbiota. After passing through the small intestine, trypsin activity is markedly reduced in the caecum of specific pathogen-free (SPF) mice, whereas germ-free (GF) mice have high luminal trypsin levels. We have successfully identified and isolated Paraprevotella strains from the faecal microbiome of healthy human donors as potent trypsin-degrading commensals. Mechanistically, Paraprevotella recruit trypsin to the bacterial surface through type IX secretion system-dependent polysaccharide-anchoring proteins and promote trypsin autolysis. Paraprevotella colonization protects IgA from trypsin degradation and enhances the effectiveness of oral vaccines against Citrobacter rodentium. Moreover, Paraprevotella colonization inhibits lethal infection with murine hepatitis virus, a mouse coronavirus dependent on trypsin and trypsin-like proteases for entry into host cells5,6. Congruently, carriage of putative genes involved in trypsin degradation in the gut microbiome was associated with reduced diarrhoea severity in patients with SARS-CoV2 infection. Therefore, trypsin-degrading commensal colonization may contribute to the maintenance of intestinal homeostasis and protection from pathogen infection.