The expression and function of Nod-like receptors in neutrophils

Neutrophils make up an essential part of the innate immune system, and are involved both in the initial responses to pathogens, and in orchestrating later immune responses. Neutrophils recognize pathogens through pattern-recognition receptors (PRRs), which are activated by microbial motifs. The Nod-like receptors (nucleotide-binding domain leucine-rich repeat containing family; NLRs) constitute a recently discovered group of PRRs whose role in the neutrophil immune responses is not yet characterized. The present study aimed to investigate the expression and function of NLRs in neutrophils. Neutrophils were isolated from human peripheral blood, and the presence of nucleotide-binding oligomerization domain 1 (NOD1), NOD2 and NACHT-LRR-PYD-containing protein 3 (NLRP3) was evaluated with flow cytometry and immunohistochemistry. The expression of NOD1, NOD2 and NLRP3 messenger RNA was determined using real-time reverse transcription-polymerase chain reaction. Changes in neutrophil cytokine secretion, phenotype and migration following agonist-induced activation were studied using enzyme-linked immunosorbent assay, flow cytometry and a chemotaxis assay, respectively. No expression of NOD1 was found in isolated neutrophils and stimulation with the NOD1 ligand gamma-d-glutamyl-meso-diaminopimelic acid induced no signs of activity. In contrast, a marked expression of NOD2 and NLRP3 was found. NOD2 activation with MurNAc-l-Ala-d-isoGln (MDP) resulted in interleukin-8 secretion, CD62 ligand down-regulation, CD11b up-regulation and increased migration towards an inflammatory stimulus. NLRP3 activation with alum caused interleukin-1beta secretion and facilitated migration. Altogether, this suggests that NLRs may be a previously unknown pathway for neutrophil activation.