Commensal bacteria boost serum IgG production in response to oral immunization with antigen and cholera toxin(CT)in a manner that depends on Nod2(nucleotide-binding oligomerization domain-containing protein 2).In this study,we examined the role of intestinal lysozyme(Lyz1)in adjuvant activity of CT.We found that Lyz1 released Nod2 ligand(s) from bacteria.Lyz1 deficiency reduced the level of circulating Nod2 ligand in mice.Lyz1 deficiency also reduced the production of IgG and T-cell-specific cytokines after oral immunization in mice.Supplementing Lyz1-deficient mice with MDP restored IgG production.Furthermore,overexpression of Lyz1 in intestinal epithelium boosted the antigen-specific IgG response induced by CT.Col-lectively,our results indicate that Lyz1 plays an important role in mediating the immune regulatory effect of commensal bacteria through the release of Nod2 ligand(s).