Highly Purified Lipoteichoic Acid Induced Pro-inflammatory Signalling in Primary Culture of Rat Microglia through Toll-like Receptor 2: Selective Potentiation of Nitric Oxide Production by Muramyl Dipeptide

Abstract

In contrast to lipopolysaccharide (LPS) from Gram-negative bacteria, the role of Gram-positive bacterial components in inducing inflammation in the CNS remains controversial. We studied the potency of highly purified lipoteichoic acid (LTA) and muramyl dipeptide isolated from Staphylococcus aureus to activate primary cultures of rat microglia. Exposure of pure microglial cultures to LTA triggered a significant time- and dose-dependent production of proinflammatory cytokines (TNF-α, IL-1β, IL-6) and nitric oxide (NO). Muramyl dipeptide strongly and selectively potentiated LTA-induced iNOS expression and NO production. However, it did not have any significant influence on the production of proinflammatory cytokines. As bacterial components are recognized by the innate immunity through Toll-like receptors (TLRs) we showed that LTA was recognized in microglia by the TLR2, while LPS by the TLR4, since cells isolated from mice lacking TLR2 or TLR4 did not produce proinflammatory cytokines and nitric oxide upon LTA or LPS stimulation, respectively. LTA-induced glia activation was mediated by p38 and ERK1/2 MAP kinases, since pre-treatment with inhibitor of p38 or ERK1/2 decreased LTA-induced cytokine release, iNOS mRNA expression and NO production. The observed proinflammatory response induced by LTA-activated microglia could play a major role in the inflammatory response of CNS induced by Gram-positive bacteria.