Toll‐like receptor 2 (TLR2) mediates astrocyte activation in response to the Gram‐positive bacterium Staphylococcus aureus

Abstract

Astrocytes play an important role in initiating and regulating CNS immune responses through the release of proinflammatory cytokines and chemokines. Here we demonstrate that primary astrocytes are capable of recognizing the Grampositive bacterium Staphylococcus aureus and its cell wall product peptidoglycan (PGN) and respond by producing numerous proinflammatory mediators including interleukin-1b (IL-1b), tumor necrosis factor-a (TNF-a), macrophage inflammatory protein-1b (MIP-1b), MIP-2, and monocyte chemoattractant protein (MCP-1). Astrocytes have recently been shown to express Toll-like receptor 2 (TLR2), a pattern recognition receptor important for recognizing structural components of various Gram-positive bacteria, fungi, and protozoa. However, the functional significance of TLR2 in mediating astrocyte activation remains unknown. Primary astrocytes from TLR2 knockout mice were used to evaluate the role of TLR2 in astrocyte responses to S. aureus and PGN. The results demonstrate that TLR2 is essential for maximal proinflammatory cytokine and chemokine production, but not phagocytosis, in primary astrocytes following S. aureus and PGN exposure. In addition, both stimuli led to a significant increase in TLR2 mRNA expression in wild-type astrocytes as assessed by real-time quantitative RT-PCR. These findings suggest that astrocytes may play a key role in the initial antibacterial immune response in the CNS through engagement of TLR2.

Microglia are required for astroglial toll‐like receptor 4 response and for optimal TLR2 and TLR3 response

“…Effective depletion typically required 3-4 rounds of shaking. Microglia also can be removed by adding reagents such as L-leucine methyl ester, a lysosomotrophic agent destroying phagocytic cells (Esen et al, 2004;Thiele et al, 1983). This is sometimes combined with antimitotic drugs such as cytosine arabinoside (Ara-C), preventing further proliferation of microglia (Hamby et al, 2006).…”

“…In spite of this, astrocytes in culture have been reported to respond to LPS (Bowman et al, 2003;Carpentier et al, 2005;Esen et al, 2004;Park and Murphy, 1994;Simmons and Murphy, 1992). Such findings, however, are complicated by the fact that even a few residual microglia can affect the response of almost pure astrocyte cultures (Saura, 2007).…”

Modulation of connexin expression and gap junction communication in astrocytes by the gram‐positive bacterium S. aureus

“…Neuroinflammation is a hallmark of various CNS pathologies such as trauma, bacterial meningitis, brain abscess, Alzheimer's disease, and multiple sclerosis, which share a general feature of reactive gliosis characterized, to varying degrees, by the proliferation and hypertrophy of activated astrocytes (Eikelenboom et al, 2002;Griffin and Mrak, 2002;Kielian, 2004;Koedel et al, 2002;McGeer and McGeer, 2002;Nau and Bruck, 2002;Scheld et al, 2002). When activated by an appropriate stimulus, astrocytes have the capacity to produce robust amounts of proinflammatory mediators which may have profound effects on GJC (Dong and Benveniste, 2001;Esen et al, 2004;Kim et al, 2005;Smits et al, 2001). Indeed, the gram-negative bacterial cell wall component lipopolysaccharide (LPS) has been shown to attenuate GJC in primary rat astrocytes, which was attributed, in part, to the autocrine/paracrine action of nitric oxide (NO), since the iNOS inhibitor N G -monoethyl-L-arginine (NMMA) was capable of restoring gap junction coupling (Bolanos and Medina, 1996).…”

Both TLR2 and TLR4 Are Required for the Effective Immune Response in Staphylococcus aureus-Induced Experimental Murine Brain Abscess

“…However, some recent data from in vitro research illustrate that TLR2 plays a pivotal role in recognition of peptidoglycan from S. aureus in microglial cells 26 and that TLR2 was required for a maximal cytokine response of primary astrocytes stimulated with peptidoglycan and S. aureus. 27 Additionally, Kielian et al 28 have postulated that, because of similar mortality rates, bacterial titers, and blood-brain barrier permeability after S. aureus-induced brain abscess in TLR2 Ϫ/Ϫ and wild-type (WT) mice, alternative pattern recognition receptors may play a role. However, early publications on the role of TLR2 in recognition of S. aureus after systemic infection underscore the importance of dose dependency in detecting a significant phenotypic reaction pattern.…”