Evaluation of the effect of α-defensin human neutrophil peptides on neutrophil apoptosis.

Peptide antibiotics possess potent antimicrobial activities against invading micro-organisms and contribute to the innate host defense. Antimicrobial α-defensin human neutrophil peptides (HNPs) not only exhibit potent bactericidal activities against Gram-negative and -positive bacteria but also function as immunomodulatory molecules by inducing cytokine and chemokine production, as well as inflammatory and immune cell activation. Neutrophil is a critical effector cell in host defense against microbial infection, and its lifespan is regulated by various pathogen- and host-derived substances. Here, in order to further evaluate the role of HNPs in innate immunity, we investigated the action of HNPs-1 to -3 on neutrophil apoptosis. Neutrophil apoptosis was assessed using human blood neutrophils based on the morphological changes. Of note, HNP-1 most potently suppressed neutrophil apoptosis among HNPs-1 to -3, accompanied by the down-regulation of truncated Bid (a pro-apoptotic protein), the up-regulation of Bcl-xL (an anti-apoptotic protein), and the inhibition of mitochondrial membrane potential change and caspase 3 activity. It should be noted that, a selective P2Y6 antagonist, MRS2578, abolished the suppression of neutrophil apoptosis elicited by HNP-1 as well as UDP (a P2Y6 ligand). Collectively, these observations suggest that HNPs, especially HNP-1, can not only destroy bacteria but also modulate (suppress) neutrophil apoptosis via the P2Y6 signaling pathway. The suppression of neutrophil apoptosis results in the prolongation of their lifespan and could be advantageous for the host defense against bacterial invasion.