Abstract
Bacterial peptidoglycan-derived muramyl dipeptide (MDP) and derivatives have long-recognized antiviral properties but their mechanism of action remains unclear. In recent years, the pattern-recognition receptor NOD2 has been shown to mediate innate responses to MDP. Here, we show that MDP treatment of mice infected with Influenza A virus (IAV) significantly reduces mortality, viral load and pulmonary inflammation in a NOD2-dependent manner. Importantly, the induction of type I interferon (IFN) and CCL2 chemokine was markedly increased in the lungs following MDP treatment and correlated with a NOD2-dependent enhancement in circulating monocytes. Mechanistically, the protective effect of MDP could be explained by the NOD2-dependent transient increase in recruitment of Ly6Chigh “inflammatory” monocytes and, to a lesser extent, neutrophils to the lungs. Indeed, impairment in both Ly6Chigh monocyte recruitment and survival observed in infected Nod2-/- mice treated with MDP was recapitulated in mice deficient for the chemokine receptor CCR2 required for CCL2-mediated Ly6Chigh monocyte migration from the bone marrow into the lungs. MDP-induced pulmonary monocyte recruitment occurred normally in IAV-infected and MDP-treated Ips-1-/- mice. However, IPS-1 was required for improved survival upon MDP treatment. Finally, mycobacterial N-glycolyl MDP was more potent than N-acetyl MDP expressed by most bacteria at reducing viral burden while both forms of MDP restored pulmonary function following IAV challenge. Overall, our work sheds light on the antiviral mechanism of a clinically relevant bacterial-derived compound and identifies the NOD2 pathway as a potential therapeutic target against IAV.
Highlights
Extending from the concept of vaccination employed by Edward Jenner more than two centuries ago, early studies performed with Freund’s complete adjuvant have contributed to establish the notion that microbial organisms could be decomposed and still retain immunostimulatory properties
Since we observed a NOD2-dependent increase in CCL2, type I IFNs and circulating monocytes following muramyl dipeptide (MDP) treatment of Influenza A virus (IAV)-infected mice (Figure 2E–G and Figure 3), we investigated whether such treatment affected populations of monocytes, neutrophils and macrophages in the lungs during the course of infection
The first direct link between NOD2 and antiviral immunity has recently been provided by Sabbah and colleagues, who have identified a role for NOD2 in detection of viral single-stranded RNA (ssRNA) [15]
Summary
Extending from the concept of vaccination employed by Edward Jenner more than two centuries ago, early studies performed with Freund’s complete adjuvant have contributed to establish the notion that microbial organisms could be decomposed and still retain immunostimulatory properties. The smallest bacterial cell wall constituent that retained immunogenic properties was identified as the peptidogylcan N-acetylmuramyl-Lalanyl-D-isoglutamine substructure, commonly referred to as MDP [2]. This molecule was synthesized and found to have antibacterial, antiviral and tumoricidal activity [3,4,5].
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