Binding and Cellular Activation Studies Reveal That Toll-like Receptor 2 Can Differentially Recognize Peptidoglycan from Gram-positive and Gram-negative Bacteria

Abstract

Although much progress has been made toward the identification of innate immune receptors, far less is known about how these receptors recognize specific microbial products. Such studies have been hampered by the need to purify compounds from microbial sources and a reliance on biological assays rather than direct binding to monitor recognition. We have employed surface plasmon resonance (SPR) binding studies using a wide range of well defined synthetic muropeptides derived from Gram-positive (lysine-containing) and Gram-negative (diaminopimelic acid (DAP)-containing) bacteria to demonstrate that Toll-like receptor 2 can recognize peptidoglycan (PGN). In the case of lysine-containing muropeptides, a limited number of compounds, which were derived from PGN remodeled by bacterial autolysins, was recognized. However, a wider range of DAP-containing muropeptides was bound with high affinity, and these compounds were derived from nascent and remodeled PGN. The difference in recognition of the two classes of muropeptides is proposed to be a strategy by the host to respond appropriately to Gram-negative and -positive bacteria, which produce vastly different quantities of PGN. It was also found that certain modifications of the carboxylic acids of isoglutamine and DAP can dramatically reduce binding, and thus, bacterial strains may employ such modifications to evade innate immune detection. Cellular activation studies employing highly purified PGN from Bacillus licheniformis, Bacillus subtilis, Escherichia coli, Lactobacillus plantarum, Micrococcus luteus, and Staphylococcus aureus support the structure binding relationship. The data firmly establish Toll-like receptor 2 as an innate immune sensor for PGN and provides an understanding of host-pathogen interactions at the molecular level.

Antimony-resistant but not antimony-sensitiveLeishmania donovaniup-regulates host IL-10 to overexpress multidrug-resistant protein 1

“…Previously, it has been shown that TLR2 can differentially recognize peptidoglycans from Gram-positive and Gram-negative bacteria (35). Moreover, recent reports suggest that Sb S LD (AG83) may deactivate the host TLR2 signaling pathway (36).…”

Subversion of Host Recognition and Defense Systems by Francisella spp

“…Known Francisella TLR2 agonists include the uncharacterized lipoproteins LpnA/Tul4 and FTT_1103 (86,220). To our knowledge, TLR2 recognition of Francisella PGN has not been reported, and the status of PGN as a TLR2 ligand is still under debate (12,73,222). Several reports have shown that TLR2 is essential for the early inflammatory response to Francisella infection in macrophages in vitro, as well as a critical component of the host response to in vivo Francisella infection as demonstrated by its requirement for control of pulmonary and intradermal infection (1,16,55).…”

“…These transcriptional regulators induce the expression of inflammatory cytokines and type I interferons, resulting in the activation of innate and adaptive immune cells (109,115). Numerous TLRs recognize bacterial PAMPs, including TLR2, which senses bacterial lipoproteins (BLPs) and peptidoglycan (PGN) (12,73), TLR4, which signals in response to LPS from Gramnegative bacteria, TLR5, which recognizes flagellin, and TLR9, which senses bacterial CpG DNA (115).…”

Hydrolysis of peptidoglycan is modulated by amidation of meso‐diaminopimelic acid and Mg2+ in Bacillus subtilis

“…A canonical member of this family is the asparagine synthase of Escherichia coli , AsnB (Huang et al ., 2001). PG recognition by the mammalian host innate immune system has been shown to be affected by amidation of mDAP (Asong et al ., 2009; Girardin et al ., 2003) but the physiological role of mDAP amidation in the homeostasis of the bacterial cell wall remains unknown. PG synthesizing enzymes seem to work as efficiently on amidated as in non‐amidated mDAP in both L. plantarum and C. glutamicum (Bernard et al ., 2011b; Levefaudes et al ., 2015), suggesting that mDAP amidation may not be necessary for efficient cell wall synthesis (Levefaudes et al ., 2015; Hirasawa et al ., 2000).…”