Lipoteichoic acid is a potent inducer of cytokine production in rat and human Kupffer cells in vitro.

BACKGROUND: Kupffer cells have been proposed to be a major cellular origin of pro-inflammatory mediators in sepsis. However, the cytokine response of Kupffer cells to gram-positive bacteria and their endotoxins peptidoglycan (PepG) and lipoteichoic acid (LTA) has never previously been studied. MATERIALS AND METHODS: Primary cultures of rat and human Kupffer cells were exposed to live Staphylococcus aureus (S. aureus) (4.0 x 10(1) to 4.0 x 10(7) CFU/mL culture medium), as well as highly purified PepG and LTA (0-100 microg/mL). Lipopolysaccharide (LPS) at 1 microg/mL was used for control. In parallel experiments, whole blood obtained from the same rats was stimulated in a similar manner. Accumulation of the proinflammatory cytokines tumor necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6) in plasma or culture supernatants were assessed by enzyme immuno assays. TNF-alpha and IL-6 mRNA were analyzed by real-time RT-PCR. RESULTS: PepG and LTA, as well as live S. aureus, induced the production of TNF-alpha and IL-6 in Kupffer cells from both species in a time- and dose-dependent manner. Whereas PepG was a more potent inducer of TNF-alpha and IL-6 in whole blood, the opposite seemed to be the case in Kupffer cells. In fact, a 100-fold lower concentration of LTA (1 microg/mL) than of PepG (100 microg/mL) was sufficient to induce a substantial production of both TNF-alpha and IL-6 in the Kupffer cells. TNF-alpha and IL-6 mRNA were induced correspondingly. CONCLUSION: Our results support the contention that gram-positive bacteria may activate cytokine production in Kupffer cells during bacteremia and suggest that LTA is important in this interaction.

The phosphatidylinositol 3-kinase/protein kinase B signaling pathway is activated by lipoteichoic acid and plays a role in Kupffer cell production of interleukin-6 (IL-6) and IL-10.

Sepsis caused by gram-positive bacteria lacking lipopolysaccharide (LPS) has become a major and increasing cause of mortality in intensive-care units. We have recently demonstrated that the gram-positive-specific bacterial cell wall component lipoteichoic acid (LTA) stimulates the release of the proinflammatory cytokines in Kupffer cells in culture. In the present study, we have started to assess the signal transduction events by which LTA induces the production of tumor necrosis factor alpha (TNF-alpha), interleukin-6 (IL-6), and the anti-inflammatory cytokine IL-10 in rat Kupffer cells. LTA was found to trigger phosphorylation of mitogen-activated protein kinases (MAPK) (p38 MAPK and ERK 1/2) and protein kinase B (PKB). Compared to LPS, LTA was more potent in inducing PKB phosphorylation after 40 min, although we found that the cytokine responses were similar. For both bacterial molecules, blocking phosphatidylinositol 3-kinase (PI3-K; Ly294002) or Janus kinase 2 (JAK-2; AG490) particularly affected the induction of IL-6 and IL-10 release, whereas TNF-alpha levels were strongly reduced by inhibition of Src family tyrosine kinases (PP2). All three cytokines were reduced by inhibition of p38 MAPK (SB202190) or the broad-range tyrosine kinase inhibitor genistein, whereas IL-6 release was particularly blocked by inhibition of ERK 1/2 (PD98059). Divergences in the regulatory pathways controlling TNF-alpha, IL-10, and IL-6 production in Kupffer cells following LPS or LTA stimulation may create a basis for understanding how the balance between pro- and anti-inflammatory cytokines is regulated in the liver following infections by gram-positive or gram-negative bacteria.

Organ injury and cytokine release caused by peptidoglycan are dependent on the structural integrity of the glycan chain.

Several studies have implicated a role of peptidoglycan (PepG) as a pathogenicity factor in sepsis and organ injury, in part by initiating the release of inflammatory mediators. We wanted to elucidate the structural requirements of PepG to trigger inflammatory responses and organ injury. Injection of native PepG into anesthetized rats caused moderate but significant increases in the levels of alanine aminotransferase, aspartate aminotransferase, gamma-glutamyl transferase, and bilirubin (markers of hepatic injury and/or dysfunction) and creatinine and urea (markers of renal dysfunction) in serum, whereas PepG pretreated with muramidase to digest the glycan backbone failed to do this. In an ex vivo model of human blood, PepG containing different amino acids induced similar levels of the cytokines tumor necrosis factor alpha (TNF-alpha), interleukin-6 (IL-6), IL-8, and IL-10, as determined by plasma analyses (enzyme-linked immunosorbent assay). Hydrolysis of the Staphylococcus aureus cross-bridge with lysostaphin resulted in moderately reduced release of TNF-alpha, IL-6, IL-8, and IL-10, whereas muramidase digestion nearly abolished the ability to induce cytokine release and IL-6 mRNA accumulation in CD14(+) monocytes compared to intact PepG. However, additional experiments showed that muramidase-treated PepG synergized with lipopolysaccharide to induce TNF-alpha and IL-10 release in whole blood, despite its lack of inflammatory activity when administered alone. Based on these studies, we hypothesize that the structural integrity of the glycan chain of the PepG molecule is very important for the pathogenic effects of PepG. The amino acid composition of PepG, however, does not seem to be essential for the inflammatory properties of the molecule.