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Coexistence of CD14-dependent and independent pathways for stimulation of human monocytes by gram-positive bacteria
1Rockefeller University, New York, New York 10021, USA.
Abstract:
The cell wall is a key inflammatory agent of gram-positive bacteria. Possible receptors mediating cell wall-induced inflammation include CD14 and platelet-activating factor (PAF) receptor. To delineate the conditions under which these various receptors might be used, human monocytic THP-1 cells and heparinized whole human blood were stimulated with lipopolysaccharide (LPS), intact Streptococcus pneumoniae bacteria, or purified pneumococcal cell wall. THP-1 culture supernatant or cell-free plasma was analyzed for the presence of tumor necrosis factor, interleukin-1beta (IL-1beta), and IL-6. For the cultured monocytes, anti-CD14 inhibited induction of the inflammatory cytokines by the cell wall and LPS but not by intact pneumococcal bacteria. Despite the difference in CD-14 usage, the intracellular pathways induced by the three agents demonstrated similarities, as revealed in the presence of specific signal transduction inhibitors such as cholera toxin, pertussis toxin, and genistein. Cytokine production in whole human blood indicated that anti-CD14 failed to block responses to cell wall and intact pneumococci, whereas while LPS-induced responses were inhibited. PAF receptor antagonist had no effect under any conditions in both assays. These results indicate that although cell walls bind to both CD14 and PAF receptor, only CD14 appears to engender a cytokine response under restricted conditions. Furthermore, host cell responses to intact pneumococci are consistently independent of CD14 and PAF receptor.
Insights
Gram-positive bacterial cell walls trigger inflammation. While CD14 mediates responses to cell walls and LPS in some conditions, intact bacteria trigger inflammation independently of CD14 and the platelet-activating factor receptor.
Area of Science:
- Immunology
- Microbiology
- Cell Biology
Background:
- Gram-positive bacterial cell walls are potent inflammatory agents.
- CD14 and platelet-activating factor (PAF) receptor are potential mediators of cell wall-induced inflammation.
Purpose of the Study:
- To investigate the roles of CD14 and PAF receptor in mediating inflammatory responses to bacterial components.
- To delineate the conditions under which CD14 and PAF receptor are utilized.
Main Methods:
- Human monocytic THP-1 cells and whole human blood were stimulated with lipopolysaccharide (LPS), intact Streptococcus pneumoniae, or purified pneumococcal cell wall.
- Cytokine levels (TNF, IL-1beta, IL-6) were measured in cell culture supernatant and plasma.
- The effects of anti-CD14 and PAF receptor antagonist were assessed.
Main Results:
- Anti-CD14 inhibited cytokine induction by cell wall and LPS in THP-1 cells, but not by intact bacteria.
- In whole blood, anti-CD14 blocked LPS responses but not those to cell wall or intact pneumococci.
- PAF receptor antagonist had no effect on cytokine production under any tested condition.
Conclusions:
- Bacterial cell walls bind to both CD14 and PAF receptor, but only CD14 mediates cytokine response under specific conditions.
- Host cell responses to intact Streptococcus pneumoniae are consistently independent of CD14 and PAF receptor involvement.