Background: The present study was undertaken to examine the ability of lipopolysaccharide-containing outer membrane vesicles (OMV-LPS) and purified LPS (P-LPS) from the same meningococcal strain to induce the expression of Toll-like receptors (TLR2 and TLR4) and TNF-alpha production in leukocytes, and further to study the involvement of TLRs, and CD14 in monocyte TNF- alpha production in an ex vivo human whole blood system.
Material/methods: OMV-LPS or P-LPS were added to human whole blood and expression of TLR2/4 and production of TNF- alpha in leukocytes were measured by flow cytometry. To study involvement of TLRs and CD14 in monocyte cytokine production we used monoclonal antibodies against TLR2/4 and CD14.
Results: OMV-LPS and P-LPS induced surface expression (maximal at 120 min) of TLR2 and TLR4 on granulocytes and monocytes. LPS incorporated in OMV was less potent (weight basis) than P-LPS in inducing monocyte TNF- alpha production. When inducing monocyte TNF-alpha by OMV-LPS, antibodies directed against TLR2 and TLR4 caused 45 and 78% inhibition, respectively. When inducing TNF- alpha by P-LPS, antibodies against TLR2 had no effect, whereas anti-TLR4 antibodies caused 63% inhibition. Antibodies against CD14 inhibited nearly completely the monocyte TNF- alpha response induced by meningococcal LPS irrespective of whether LPS was presented in purified form or incorporated in membrane vesicles.
Conclusions: OMV-LPS and P-LPS from the same meningococcal strain induced expression of TLR2/4 on monocytes and granulocytes. Surface receptors TLR2/4 and CD14 are essential for in vitro cellular activation induced by OMV-LPS and P-LPS, but the functional significance of these receptors during meningococcal infections remains elusive.