Peptidoglycan: microbiota’s messenger to host immunity
Microbiota-derived peptidoglycan, a component of the bacterial cell wall , is ubiquitously present in human blood and plays an important role in host immune response and autoimmune disease.
So often in science, discoveries are made when we are trying to solve one puzzle but stumble upon something unexpected. During the early years of the 2000s, my laboratory was painstakingly fractionating liters after liters of human serum in hopes to decipher the ‘magic potion’ that trigger the invasive growth of the human fungal pathogen Candida albicans (1,2). We discovered a molecule that carries the hallmark of bacterial peptidoglycan (PGN). This finding surprised everyone and, understandably, aroused suspicion of sample contamination. At that time, well before the start of the Human Microbiome Project, the existence of bacterium-derived substances in the blood of healthy people was not yet readily accepted.
A decade later, it is now widely accepted that the human body is home to trillions of bacteria that continuously release small molecules that can enter the host circulatory system (3). PGN subunits, released by most bacteria during cell wall remodeling and recycling (4), are especially interesting because they have long been known to possess diverse signaling activities such as immunostimulation, sleep promotion, and fever induction (5). Our findings of PGN in the blood of healthy people raised fundamental questions regarding its possible roles in influencing host physiology. Also, the circulating PGN could potentially provide a mechanism by which the host microbiota extends its impact to sterile extra-intestinal organs.
To develop tools to investigate the roles of PGN in mammalian hosts, we successfully produced a high-affinity monoclonal antibody, named 2E7, that targets muramyl-L-alanyl-D-isoglutamine (called MDP in short), a conserved and minimal immunostimulatory structure of PGN (6). This antibody allows us to detect, quantify, and neutralize PGN. Applying a 2E7-based ELISA to the analysis of serum samples from hundreds of healthy donors led to two illuminating findings. First, PGN is ubiquitously present in the blood; and second, the concentration of circulating PGN varies over a broad range between individuals while remaining relatively stable in each person.
To investigate the role of the circulating PGN, we increased its level in mice by implanting an osmotic pump that slowly released PGN subunits and reduced it by intraperitoneal injection with 2E7. To see the consequence, we examined the effect on the development of autoimmune arthritis. While increasing the level of circulating PGN accelerated disease development as we expected, a single injection of 2E7 before disease induction completely suppressed the appearance of the symptoms. 2E7 exhibited a similar effect on experimental autoimmune encephalomyelitis in mice. The results strongly indicate that circulating PGN plays a critical role in determining the intensity of host immune response.
We hypothesize that through eons of symbiotic interaction and coevolution of bacteria and their hosts, circulating PGN subunits have become an integral regulator of the host immunity. In the future, systemic PGN could serve as a biomarker as well as a therapeutic target for autoimmune disease. Strategies to alter PGN levels may lead to new therapeutic approaches against immune-mediated disorders.
(1) 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).
(2) Y Wang and Xu XL. Bacterial peptidoglycan-derived molecules activate Candida albicans hyphal growth. Commun Integr Biol 1, 137-139 (2008).
(3) Schroeder, B. O. & Backhed, F. Signals from the gut microbiota to distant organs in physiology and disease. Nat Med 22, 1079-1089, 1079-1089 (2016).
(4) Johnson, J. W., Fisher, J. F. & Mobashery, S. Bacterial cell-wall recycling. Ann N Y Acad Sci 1277, 54-75 (2013).
(5) Cloud-Hansen, K. A. et al. Breaching the great wall: peptidoglycan and microbial interactions. Nat Rev Microbiol 4, 710-716 (2006).
(6) 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).