The human microbiota provides tonic signals that calibrate the host immune response1,2, but their identity is unknown. Bacterial peptidoglycan (PGN) subunits are likely candidates since they are well-known immunity-enhancing adjuvants, released by most bacteria during growth, and have been found in the blood of healthy people3,4,5,6,7. We developed a monoclonal antibody (mAb), 2E7, that targets muramyl-l-alanyl-d-isoglutamine (MDP), a conserved and minimal immunostimulatory structure of PGN. Using 2E7-based assays, we detected PGN ubiquitously in human blood at a broad range of concentrations that is relatively stable in each individual. We also detected PGN in the serum of several warm-blooded animals. However, PGN is barely detectable in the serum of germ-free mice, indicating that its origin is the host microbiota. Neutralization of circulating PGN via intraperitoneal administration of 2E7 suppressed the development of autoimmune arthritis and experimental autoimmune encephalomyelitis in mice. Arthritic NOD2−/− mice lacking the MDP sensor did not respond to 2E7, indicating that 2E7 dampens inflammation by blocking nucleotide-binding oligomerization domain-containing protein 2 (NOD2)-mediated pathways. We propose that circulating PGN acts as a natural immune potentiator that tunes the host immune response; altering its level is a promising therapeutic strategy for immune-mediated diseases.
Subscribe to Journal
Get full journal access for 1 year
only $5.17 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Rent or Buy article
Get time limited or full article access on ReadCube.
All prices are NET prices.
All data that support the findings of this study are either included in this published article and its Supplementary Information or available from the corresponding author upon request.
Schroeder, B. O. & Bäckhed, F. Signals from the gut microbiota to distant organs in physiology and disease. Nat. Med. 22, 1079–1089 (2016).
Grigg, J. B. & Sonnenberg, G. F. Host-microbiota interactions shape local and systemic inflammatory diseases. J. Immunol. 198, 564–571 (2017).
Kotani, S., Watanabe, Y., Kinoshita, F., Shimono, T. & Morisaki, I. Immunoadjuvant activities of synthetic N-acetyl-muramyl-peptides or -amino acids. Biken J. 18, 105–111 (1975).
Löwy, I., Bona, C. & Chedid, L. Target cells for the activity of a synthetic adjuvant: muramyl dipeptide. Cell. Immunol. 29, 195–199 (1977).
Xu, X. L. et al. Bacterial peptidoglycan triggers Candida albicans hyphal growth by directly activating the adenylyl cyclase Cyr1p. Cell Host Microbe 4, 28–39 (2008).
Behr, M. A. & Divangahi, M. Freund’s adjuvant, NOD2 and mycobacteria. Curr. Opin. Microbiol. 23, 126–132 (2015).
Alexander, K. L., Targan, S. R. & Elson, C. O. 3rd Microbiota activation and regulation of innate and adaptive immunity. Immunol. Rev. 260, 206–220 (2014).
Erny, D. et al. Host microbiota constantly control maturation and function of microglia in the CNS. Nat. Neurosci. 18, 965–977 (2015).
Rogers, G. B. Germs and joints: the contribution of the human microbiome to rheumatoid arthritis. Nat. Med. 21, 839–841 (2015).
Lee, W. J. & Hase, K. Gut microbiota-generated metabolites in animal health and disease. Nat. Chem. Biol. 10, 416–424 (2014).
den Besten, G. et al. The role of short-chain fatty acids in the interplay between diet, gut microbiota, and host energy metabolism. J. Lipid Res. 54, 2325–2340 (2013).
Nicholson, J. K. et al. Host-gut microbiota metabolic interactions. Science 336, 1262–1267 (2012).
Dworkin, J. The medium is the message: interspecies and interkingdom signaling by peptidoglycan and related bacterial glycans. Annu. Rev. Microbiol. 68, 137–154 (2014).
Johnson, J. W., Fisher, J. F. & Mobashery, S. Bacterial cell-wall recycling. Ann. N. Y. Acad. Sci. 1277, 54–75 (2013).
Clarke, T. B. et al. Recognition of peptidoglycan from the microbiota by Nod1 enhances systemic innate immunity. Nat. Med. 16, 228–231 (2010).
Caruso, R., Warner, N., Inohara, N. & Núñez, G. NOD1 and NOD2: signaling, host defense, and inflammatory disease. Immunity 41, 898–908 (2014).
Philpott, D. J., Sorbara, M. T., Robertson, S. J., Croitoru, K. & Girardin, S. E. NOD proteins: regulators of inflammation in health and disease. Nat. Rev. Immunol. 14, 9–23 (2014).
Girardin, S. E. et al. Nod1 detects a unique muropeptide from gram-negative bacterial peptidoglycan. Science 300, 1584–1587 (2003).
Girardin, S. E. et al. Nod2 is a general sensor of peptidoglycan through muramyl dipeptide (MDP) detection. J. Biol. Chem. 278, 8869–8872 (2003).
Rosenberg-Hasson, Y., Hansmann, L., Liedtke, M., Herschmann, I. & Maecker, H. T. Effects of serum and plasma matrices on multiplex immunoassays. Immunol. Res. 58, 224–233 (2014).
Li, Y. et al. Inter-individual variability and genetic influences on cytokine responses to bacteria and fungi. Nat. Med. 22, 952–960 (2016).
Brand, D. D., Kang, A. H. & Rosloniec, E. F. The mouse model of collagen-induced arthritis. Methods Mol. Med. 102, 295–312 (2004).
Khachigian, L. M. Collagen antibody-induced arthritis. Nat. Protoc. 1, 2512–2516 (2006).
Nakae, S., Nambu, A., Sudo, K. & Iwakura, Y. Suppression of immune induction of collagen-induced arthritis in IL-17-deficient mice. J. Immunol. 171, 6173–6177 (2003).
Miller, S. D., Karpus, W. J. & Davidson, T. S. Experimental autoimmune encephalomyelitis in the mouse. Curr. Protoc. Immunol. Chapter 15, Unit 15.1 (2010).
Nachbur, U. et al. A RIPK2 inhibitor delays NOD signalling events yet prevents inflammatory cytokine production. Nat. Commun. 6, 6442 (2015).
Cloud-Hansen, K. A. et al. Breaching the great wall: peptidoglycan and microbial interactions. Nat. Rev. Microbiol. 4, 710–716 (2006).
Dagil, Y. A. et al. The dual NOD1/NOD2 agonism of muropeptides containing a meso-diaminopimelic acid residue. PLoS ONE 11, e0160784 (2016).
Girardin, S. E. et al. Peptidoglycan molecular requirements allowing detection by Nod1 and Nod2. J. Biol. Chem. 278, 41702–41708 (2003).
Hnasko, R. M. & Stanker, L. H. Hybridoma technology. Methods Mol. Biol. 1318, 15–28 (2015).
We thank N.A.R. Gow, J. Heitman, and D. Ko for suggestions on the manuscript and helpful discussions. We thank N. Kaliaperumal and J. Connolly for their kind help with the Luminex and flow cytometry analyses, and W.E. Goldman for providing TCT. This work was supported by National Medical Research Council grant no. BMRC/BnB/0001b/2012 awarded to Y.W., L.C., and N.P.
The authors declare no competing interests.
Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
About this article
Current Opinion in Pharmacology (2019)
Nature Reviews Rheumatology (2019)
The gut microbiota – a modulator of endothelial cell function and a contributing environmental factor to arterial thrombosis
Expert Review of Hematology (2019)
Trace levels of peptidoglycan in serum underlie the NOD-dependent cytokine response to endoplasmic reticulum stress
Journal of Biological Chemistry (2019)
Frontiers in Immunology (2019)