Signaling by Toll-Like Receptor 2 and 4 Agonists Results in Differential Gene Expression in Murine Macrophages

Abstract

Lipopolysaccharide (LPS) derived from the periodontal pathogen Porphyromonas gingivalis has been reported to differ structurally and functionally from enterobacterial LPS. These studies demonstrate that in contrast to protein-free enterobacterial LPS, a similarly purified preparation of P. gingivalis LPS exhibited potent Toll-like receptor 2 (TLR2), rather than TLR4, agonist activity to elicit gene expression and cytokine secretion in murine macrophages and transfectants. More importantly, TLR2 stimulation by this P. gingivalis LPS preparation resulted in differential expression of a panel of genes that are normally induced in murine macrophages by Escherichia coli LPS. These data suggest that (i) P. gingivalis LPS does not signal through TLR4 and (ii) signaling through TLR2 and through TLR4 differs quantitatively and qualitatively. Our data support the hypothesis that the shared signaling pathways elicited by TLR2 and by TLR4 agonists must diverge in order to account for the distinct patterns of inflammatory gene expression.Lipopolysaccharides (LPS) are among the most potent inflammatory bacterial mediators and have been strongly implicated in the inflammatory response associated with gram-negative sepsis. Most LPS signaling studies have used LPS preparations derived from species within the Enterobacteriaceae, which possess relatively well-conserved lipid A structures (reviewed in reference 36). A convergence of data suggest that these prototypic LPS preparations, when highly purified, elicit LPS responses that are restricted in the use of TLR4 as the principal signal-transducing molecule (reviewed in reference 21), which is strongly supported by the finding that synthetic E. coli lipid A activated Toll-like receptor 4 (TLR4) and not TLR2 transfectants (8). However, the lipid A of nonenterobacterial species, e.g., Porphyromonas gingivalis, which has been implicated in the inflammation associated with chronic periodontitis (reviewed in reference 9), differs both structurally and functionally from enterobacterial lipid A. Specifically, the major species of P. gingivalis lipid A is composed of unique branched fatty acids, with longer carbon chains than in enterobacterial lipid A, the absence of a phosphoryl group at position 4Ј of the nonreducing glucosamine, as well as other modifications ( Fig. 1) (1). Consistent with these structural differences is the finding that P. gingivalis LPS activity is poorly inhibited by polymyxin B (12), which has been postulated to inactivate LPS by binding electrostatically to negatively charged phosphate groups, leading to a subsequent interaction of polymyxin B with the hydrophobic fatty acids (25, 33). Although P. gingivalis-induced signaling was shown some time ago to be CD14 dependent (34), site-specific mutagenesis of CD14 suggests that the substitution of certain charged amino acids differentially affects the abilities of Escherichia coli and P. gingivalis LPS to bind CD14 (4, 5). In addition, binding of P. gingivalis LPS to LPS binding protein has been reported to be 100-fo...

Toll-Like Receptors in Health and Disease: Complex Questions Remain

“…Contrary to expectations, TLR4 Ϫ/Ϫ mice also showed a role for TLR4 in driving Th2 cytokine generation through modulation of DC costimulatory molecule expression (72), and LPS stimulation amplified Th2 responses in a mouse model of allergic airway inflammation (73). Some data also suggest that TLR2 activation can favor Th2 phenotypes by induction of IL-12 p40 homodimers and a reduced capacity for production of bioactive IL-12 p70 (74,75). Nonetheless, the general trend for TLR activation to favor Th1 suggests that activation of these receptors in the context of allergen presentation may be exploited to augment allergen desensitization therapies, or perhaps anti-tumor responses.…”

Bacterial lipopolysaccharide induces increased expression of toll-like receptor (TLR) 4 and downstream TLR signaling molecules in bovine mammary epithelial cells

“…It may also be an evolutionarily acquired adaptive attribute of the mammary gland to help it fight infections since it is constantly under attack by invading pathogens. Furthermore, TLR2 has been shown to recognize LPS preparations from sources like Leptospira interrogans, Porphyromonas gingivalis, and Helicobacter pyroli, all non-enterobacteria [21,36,43]. Another study demonstrated TLR2/CD14 complex as the exogenous mediators of Trypanosoma cruzi induced inflammation [8].…”

Human dendritic cells respond to Porphyromonas gingivalis LPS by promoting a Th2 effector response in vitro

“…TLR4 is the principal TLR species involved in LPS signaling, whereas TLR2 is the paradigmatic receptor for bacterial components other than LPS, including lipoteichoic acid, peptidoglycan and lipoproteins [8]. Recent studies suggest that TLR2 might be the primary signal-transducing molecule for LPS from certain nonenterobacterial Gramnegative organisms, including P. gingivalis [9,11,12] and Leptospira interrogans [32]. However, controversy still exists in the case of P. gingivalis LPS, as it appears to also have activity for TLR4 [12,[33][34][35].…”

“…Our recent studies in mice [9] suggest that LPS from the oral mucosal pathogen Porphyromonas gingivalis, which does not require TLR4 [10] and purportedly signals through TLR2 [11,12], can skew the murine response to ovalbumin towards a Th2-response. It is extremely important, however, to establish whether P. gingivalis LPS induces a Th2-effector response in the human system, in which the disease chronic periodontitis (CP) occurs.…”