Lipopolysaccharide Could Be Internalized into Human Peripheral Blood Mononuclear Cells and Elicits TNF-α Release, but not via the Pathway of Toll-Like Receptor 4 on the Cell Surface

Lipopolysaccharide (LPS), the principal component of the outer membrane of gram-negative bacteria, stimulate various cell types to release numerous proinflammatory mediators such as TNF-alpha, IL-6 and IL-12, which may damage cells and lead to organ injury, even sepsis and septic shock. Toll-like receptor 4 (TLR4) has been identified as the receptor involved in the recognition of LPS, but the role of LPS uptake in activating signal transduction remains controversial. In the present study, TNF-alpha was used as a marker of macrophages/monocytes activated by LPS, and CQ was used as an inhibitor of endosome mature in order to definitude what stage the signal transduction elicited by LPS was interrupted. We found that there indeed existed internalization of LPS and internalization partially participated in LPS signaling since CQ inhibited cytokine release, and decreased accumulation of FITC-LPS in hPBMC. In contrast, anti-hTLR4 antibody could decrease cytokines' release, but no inhibition on accumulation of FITC-LPS. This result revealed that inhibition of cytokine release was related to reduction of FITC-LPS accumulation in the cells. But TLR4 on the cell surface didn't possibly participated in internalization of LPS. Thus, LPS signaling and internalization cannot be viewed as mutually independent processes.