Peptidoglycan and Lipoteichoic Acidinduced Cell Activation

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This can eventually lead to sepsis (Alberts et al 2002a). Previous work showed that in Gram negative bacteria lipopolysaccharide (LPN), a component of the outer membrane, activated a cell’s immune response in this manner. It interacted with the Toll-like receptors types 2 and 4 (TLR2 and TLR4) present on host cell membranes (Kirschning et al 1998, Poltorak et al 1998, Quereshi et al 1999). The authors in this paper demonstrate how the components of Gram positive bacterial cell membranes stimulate cell activation through TLR2.
The invasion of an organism’s blood stream with bacteria often results in sepsis. Invasive bacteria will activate the host cell’s immune system. Sepsis results from the inability of the immune system to limit bacterial spread during an infection. The inhibitory mechanisms controlling inflammation are over-ridden by the huge bacterial load on the cell. Inflammation during sepsis will then develop into a systemic syndrome with a number of clinical symptoms such as tissue injury, increased vascular permeability, dilation of blood vessels, loss of plasma volume and blood clotting, and, eventually, multi-organ failure and shock (Alberts et al 2002b, Decker 2004)).
Initially bacteria bind to host cell membranes and this stimulates the systemic release of cytokines and other inflammatory signalling molecules into the blood. (Alberts et al 2002b). …
The mechanism for Gram negative bacterial mediated activation of the cell’s immune response has been well documented (Alberts et al 2002). LPN within the bacterial cell wall binds to LPN binding protein present in serum and this complex in turn binds to the CD14 receptor (figure 4). The CD14 receptor is either soluble in serum or tethered to the host cell membrane through a GPI (glycophosphatidylinositol) anchor. Either way the CD14 receptor does not have an intracellular domain so it cannot transmit its signalling messages intracellularly. This observation suggested that another "co-receptor" acted in conjunction with the CD14 receptor to allow the transmission of an extracellular signal to the inside of the cell. The Toll-like receptors (TLRs) have an intracellular domain linking to the IL-1 signalling pathway. The Toll-like receptor acts to phosphorylate a cascade of kinases TRAF6, TAK-1, IKK and eventually the transcription factor NFB. NFB then translocates to the nucleus where it activates the transcription of a number of genes involved in immune and inflammatory responses (Alberts et al 2002a, figure 5). In Gram negative bacteria the Toll-like receptors 2 and 4 has been shown to be intrinsic in cell activation (Kirschning et al 1998, Poltorak et al 1998, Quereshi et al 1999).
In this paper the authors attempted to determine if components isolated from Gram positive bacteria activated cells in a TLR dependent manner. HEK293 cells were transfected with a number of Toll-like receptors alongside a luciferase reporter gene. A NFB transcription element was placed upstream of the luciferase reporter gene, such that activaiton of NFB would initiate transcription of luciferase. When luciferase