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The innate immune recognition of bacterial lipopolysaccharide (LPS) is mediated by Toll-like receptor 4 (TLR4) and results in activation of proinflammatory signaling including NF-κB and MAPK pathways. Heterotrimeric G proteins have been previously implicated in LPS signaling in macrophages and monocytes. In the present study, we show that pertussis toxin sensitive heterotrimeric G proteins (Gαi/o) are involved in the activation of MAPK and Akt downstream of TLR2, TLR3, and TLR4 in endothelial cells. Gαi/o are also required for full activation of interferon signaling downstream of TLR3 and TLR4 but are not required for the activation of NF-κB. We find that Gαi/o-mediated activation of the MAPK is independent of the canonical MyD88, interleukin-1 receptor-associated kinase, and tumor necrosis factor receptor-associated factor 6 signaling cascade in LPS-stimulated cells. Taken together, the data presented here suggest that heterotrimeric G proteins are widely involved in TLR pathways along a signaling cascade that is distinct from MyD88-TRAF6. [ABSTRACT FROM AUTHOR]

Regulatory T cells (Treg) have been found to be central for host defense regulation against microbial antigens, the prevention of allergic and autoimmune diseases and the suppression of effective tumor immune responses. However, the influence of the microenvironment and the mechanisms leading to their activation in the periphery still remain unclear. In vitro infection models revealed that survival and suppressive function of Treg is improved when they are confronted with lipopolysaccharide (LPS). Because LPS initiates signalling via the receptor Toll-like receptor 4 (TLR4) and the consequent activation of the transcription factor nuclear factor-kappaB (NF-κB), we investigated TLR4 expression and NF-κB regulation in human Treg. We demonstrated that LPS in combination with IL-2 induces human CD25+FoxP3+ T cells in vitro. FoxP3 expression of purified natural Treg increased and suppressive capacity was markedly improved compared with unstimulated Treg upon stimulation with LPS/IL-2. Furthermore, blockade of the NF-κB pathway by a selective inhibitor of IκB kinase (IKK)β abrogated the upregulation of FoxP3 expression. Taken together, our results suggest an important role of the NF-κB signalling pathway for the induction and modulation of suppressive function of natural Treg, if they are confronted with TLR4-stimulating agents such as Gram-negative bacteria. [ABSTRACT FROM AUTHOR]

Cultured primary human keratinocytes were screened for their expression of various members of the toll-like receptor (TLR) family. Keratinocytes were found to constitutively express TLR1, TLR2, TLR3, TLR5, and TLR9 but not TLR4, TLR6, TLR7, TLR8, or TLR10 as shown by polymerase chain reaction analysis. The expression of the crucial receptor for signaling of staphylococcal compounds TLR2 was also confirmed by immunohistochemistry, in contrast to TLR4, which showed a negative staining pattern. Next, we analyzed the activation of the proinflammatory nuclear transcription factor κB by Staphylococcus aureus strain 8325-4. Using nuclear extract gel shifts, RelA staining, and luciferase reporter transfection plasmids we found a clear induction of nuclear factor κB translocation by the bacteria. This translocation induced the transcription of nuclear factor κB controlled genes such as inducible nitric oxide synthetase, COX2, and interleukin-8. Transcription of these genes was followed by production of increased amounts of interleukin-8 protein and NO. Inhibition experiments using monoclonal antibodies and the specific platelet activating factor receptor inhibitor CV3988 showed that nuclear factor κB activation by S. aureus was TLR2 but not TLR4 or platelet activating factor receptor dependent. In line, the purified staphylococcal cell wall components lipoteichoic acid and peptidoglycan, known to signal through TLR2, also showed nuclear factor κB translocation in human keratinocytes, indicating a crucial role of the staphylococcal cell wall in the innate immune stimulation of human keratinocytes. These results help to explain the complex activation of human keratinocytes by S. aureus and its cell wall components in various inflammatory disorders of the skin. [ABSTRACT FROM AUTHOR]

The innate immune recognition of bacterial lipopolysaccharide (LPS) is mediated by Toll-like receptor 4 (TLR4) and results in activation of proinflammatory signaling including NF-κB and MAPK pathways. Heterotrimeric G proteins have been previously implicated in LPS signaling in macrophages and monocytes. In the present study, we show that pertussis toxin sensitive heterotrimeric G proteins (Gα(i/o)) are involved in the activation of MAPK and Akt downstream of TLR2, TLR3, and TLR4 in endothelial cells. Gα(i/o) are also required for full activation of interferon signaling downstream of TLR3 and TLR4 but are not required for the activation of NF-κB. We find that Gα(i/o)-mediated activation of the MAPK is independent of the canonical MyD88, interleukin-1 receptor-associated kinase, and tumor necrosis factor receptor-associated factor 6 signaling cascade in LPS-stimulated cells. Taken together, the data presented here suggest that heterotrimeric G proteins are widely involved in TLR pathways along a signaling cascade that is distinct from MyD88-TRAF6.

Regulatory T cells (Treg) have been found to be central for host defense regulation against microbial antigens, the prevention of allergic and autoimmune diseases and the suppression of effective tumor immune responses. However, the influence of the microenvironment and the mechanisms leading to their activation in the periphery still remain unclear. In vitro infection models revealed that survival and suppressive function of Treg is improved when they are confronted with lipopolysaccharide (LPS). Because LPS initiates signalling via the receptor Toll-like receptor 4 (TLR4) and the consequent activation of the transcription factor nuclear factor-kappaB (NF-kappaB), we investigated TLR4 expression and NF-kappaB regulation in human Treg. We demonstrated that LPS in combination with IL-2 induces human CD25(+)FoxP3(+) T cells in vitro. FoxP3 expression of purified natural Treg increased and suppressive capacity was markedly improved compared with unstimulated Treg upon stimulation with LPS/IL-2. Furthermore, blockade of the NF-kappaB pathway by a selective inhibitor of IkappaB kinase (IKK)beta abrogated the upregulation of FoxP3 expression. Taken together, our results suggest an important role of the NF-kappaB signalling pathway for the induction and modulation of suppressive function of natural Treg, if they are confronted with TLR4-stimulating agents such as Gram-negative bacteria.

Cultured primary human keratinocytes were screened for their expression of various members of the toll-like receptor (TLR) family. Keratinocytes were found to constitutively express TLR1, TLR2, TLR3, TLR5, and TLR9 but not TLR4, TLR6, TLR7, TLR8, or TLR10 as shown by polymerase chain reaction analysis. The expression of the crucial receptor for signaling of staphylococcal compounds TLR2 was also confirmed by immunohistochemistry, in contrast to TLR4, which showed a negative staining pattern. Next, we analyzed the activation of the proinflammatory nuclear transcription factor kappaB by Staphylococcus aureus strain 8325-4. Using nuclear extract gel shifts, RelA staining, and luciferase reporter transfection plasmids we found a clear induction of nuclear factor kappaB translocation by the bacteria. This translocation induced the transcription of nuclear factor kappaB controlled genes such as inducible nitric oxide synthetase, COX2, and interleukin-8. Transcription of these genes was followed by production of increased amounts of interleukin-8 protein and NO. Inhibition experiments using monoclonal antibodies and the specific platelet activating factor receptor inhibitor CV3988 showed that nuclear factor kappaB activation by S. aureus was TLR2 but not TLR4 or platelet activating factor receptor dependent. In line, the purified staphylococcal cell wall components lipoteichoic acid and peptidoglycan, known to signal through TLR2, also showed nuclear factor kappaB translocation in human keratinocytes, indicating a crucial role of the staphylococcal cell wall in the innate immune stimulation of human keratinocytes. These results help to explain the complex activation of human keratinocytes by S. aureus and its cell wall components in various inflammatory disorders of the skin.