Intrinsic nitric oxide-stimulatory activity of lipoteichoic acids from different Gram-positive bacteria

Lipoteichoic acid (LTA) is a structural component of the cell walls of Gram-positive bacteria. Similar to lipopolysaccharide (LPS) which is expressed in Gram-negative bacteria, LTA exhibits immunostimulatory properties. Frequently observed positive response of LTA in the Limulus amebocyte lysate (LAL) assay has been interpreted as a sign of LPS contamination, raising doubts about the intrinsic immune activities of LTA. Regarding many similarities in immunobiological and physicochemical properties of LTA and LPS, we hypothesized that similar to LPS, the LAL reactivity of LTA might be due to its ability to bind to LAL. Our data confirm the positivity of Bacillus subtilis, Staphylococcus aureus, Streptococcus faecalis and Streptococcus pyogenes LTAs in the LAL test. The estimates of suspected LPS content were 605, 10.3, 6.2 and 127 pg/μg LTA, respectively. The effectiveness of LTAs to induce the NO production in rat peritoneal cells was remarkably higher than that of equivalent concentrations of reference LPS (Escherichia coli). The LPS-induced NO was inhibited by polymyxin B (PMX), the IC50 of PMX:LPS concentration ratio (pg:pg) being 1050:1. Many fold higher concentrations of PMX were needed to partially suppress the NO-augmenting effects of LTAs, applied at concentrations representing the equivalents of LPS. Transposed to the concentrations of LTAs per se, the IC50s of the PMX:LTA ratios (μg:μg) ranged from 0.3:1 (S. aureus) to 7.5:1 (B. subtilis). It is concluded that LTA is not necessarily contaminated with LPS. The results prove the intrinsic immunostimulatory properties of LTAs of Gram-positive bacteria. The positive response of LTA in the LAL assay results from its capacity to bind to LAL. In addition, LTA binds with high affinity to PMX.