The recognition of lipopolysaccharides present in Gram-negative bacteria by receptors of the immune system

Invasive bacterial infections cause severe systemic complications including a life-threatening condition known as septic shock. Sepsis results in a dysregulated systemic inflammatory immune response with systemic consequences. The function of the immune system requires efficient communication between immune cells and this is achieved partly through specific cell surface receptors, some of which are involved in bacterial recognition. One virulent pathogen that binds cell surface receptors and triggers immune cells to overproduce pro-inflammatory cytokines is gram-negative bacteria. This pathogen is covered with lipopolysaccharides (LPS), which are recognized by macrophages through the cluster of differentiation receptor 14 (CD14). CD14 functions with the co-receptor known as Toll-like receptor 4 (TLR-4), which in the presence of an accessory receptor known as myeloid differentiation 2 (MD-2) triggers immunity against gram-negative bacteria. Experiments were designed to investigate the effect of purified exogenous LPS on the expression of CD14 and TLR-4 and cytokines IL-1β and TNF-α. In order to mimic in vivo responses against LPS an in vitro cellular model based on THP-1, a continuous cell line, was used. The cell model resembles macrophage-like cells and was previously isolated from a patient suffering from myelomonocytic leukemia. The central hypothesis for testing in this study was that blocking CD14, TLR-4 and MD-2 binding sites expressed on THP-1 cells would impair secretion of IL-1β and TNF-α, this was indeed observed. From a mechanistic point of view, and using an improved confocal bioimaging technique it was observed that physical proximity and co-localization of CD14, TRL-4, MD2, and HMGB1 generates a receptor complex that is most likely part of the functional activation of macrophages. Future studies employing macrophages isolated from normal donors and patients suffering from septic shock would validate and yield information on the medical significance of these findings.