Nod2 and Rip2 contribute to innate immune responses in mouse neutrophils

Nod-like receptors are a family of innate immune receptors that link cytosolic sensing of microbial and danger stimuli to the activation of immune responses. Two Nod-like receptor family members, Nod1 and Nod2, recognize bacterial peptidoglycan and activate immune responses via nuclear factor-κB (NF-κB) and mitogen-activated protein kinase (MAPK). The function of Nod1 and Nod2 has been largely studied in macrophages, but the role of these receptors in other innate immune cells remains unclear. In this study, we examined the function of Nod1 and Nod2 in innate immune responses of neutrophils. Mice were injected intraperitoneally with thioglycollate, and then peritoneal neutrophils were isolated 4 hr after injection. Tri-DAP and muramyl-dipeptide (MDP) were used as Nod1 and Nod2 agonists, respectively. The level of cytokines [interleukin-6 (IL-6) and tumour necrosis factor-α (TNF-α)] and chemokines (CXCL1 and CCL2) was increased by MDP, but not Tri-DAP in wild-type (WT) neutrophils. Increased production of cytokines and chemokines with MDP was abolished in Nod2- and Rip2-deficient neutrophils. MDP also induced the activation of NF-κB and MAPK in WT neutrophils, but not in Nod2- and Rip2-deficient cells. Flow cytometry analysis showed that L-selectin shedding was induced by MDP in WT neutrophils, but not in Nod2- and Rip2-deficient cells. MDP and Toll-like receptor (TLR) agonists (Pam3 CSK4 and lipopolysaccharide) exerted synergistic effects on the production of IL-6 and CXCL1 in neutrophils. Moreover, Nod2 and TLR4 cooperated to produce IL-6, TNF-α, CXCL1 and CCL2 in neutrophils in response to Gram-negative bacteria. Our findings suggest that the Nod2-Rip2 axis may contribute to the innate immune response of neutrophils against bacterial infection.