Innate immune surveillance in the blood is executed mostly by circulating monocytes, which recognise conserved bacterial molecules such as peptidoglycan and lipopolysaccharide. Toll-like receptors (TLR) play a central role in microbe-associated molecular pattern detection. Here, we compared the differences in TLR expression and cytokine production after stimulation of peripheral blood cells with heat-killed Gram-negative and Gram-positive human pathogens Neisseria meningitidis, Escherichia coli, Staphylococcus aureus and Streptococcus pneumoniae. We found that TLR2 expression is up-regulated on monocytes after stimulation with S. aureus, S. pneumoniae, E. coli and N. meningitidis. Moreover, TLR2 up-regulation was positively associated with increasing concentrations of Gram-positive bacteria, whereas higher concentrations of Gram-negative bacteria, especially E. coli, caused a milder TLR2 expression increase compared with low doses. Cytokines were produced in similar dose-dependent profiles regardless of the stimulatory pathogen; however, Gram-negative pathogens induced higher cytokine levels than Gram-positive ones at same concentrations. These results indicate that Gram-positive and Gram-negative bacteria differ in their dose-dependent patterns of induction of TLR2 and TLR4, but not in cytokine expression.

Department of Infectious and Tropical Diseases 1st Faculty of Medicine Charles University Prague and University Hospital Bulovka Praha

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