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Stimulation of macrophages and neutrophils by complexes of lipopolysaccharide and soluble CD14
E Hailman1, T Vasselon, M Kelley
1Laboratory of Cellular Physiology and Immunology, The Rockefeller University, New York, New York 10021, USA.
Abstract:
Sensitive responses of monocytes, macrophages, and neutrophils to bacterial LPS require membrane-bound CD14 (mCD14) and a plasma protein called LPS-binding protein (LBP). Cells lacking mCD14 respond to complexes of LPS and soluble CD14 (sCD14); these responses do not require LBP. To determine whether LBP is necessary for responses of mCD14-bearing cells to LPS, we measured responses of macrophages and neutrophils to complexes of LPS and sCD14 formed in the absence of LBP. We found that the amount of LPS needed to induce adhesive responses of neutrophils or cytokine production by macrophages was the same whether LPS was added with LBP or as LPS-sCD14 complexes, and was >100-fold less than when LPS was added alone. This result supports the view that LBP transfers LPS to CD14, but is not directly involved in responses of CD14-bearing cells to LPS. Responses of neutrophils to LPS-sCD14 complexes could be inhibited partially by blocking mCD14, suggesting that LPS may move rapidly from sCD14 to mCD14. Additionally, we found that responses of neutrophils to LBP and smooth LPS were made 30 to 100 times more sensitive when sCD14 was added. Our findings show that LBP is not necessary for the activation of CD14-bearing cells with LPS, and suggest that LPS-sCD14 complexes are an important intermediate in the inflammatory responses of leukocytes to LPS.
Insights
LPS-binding protein (LBP) is not essential for activating immune cells like macrophages and neutrophils with lipopolysaccharide (LPS). Soluble CD14 (sCD14) forms complexes with LPS, enabling cell responses without LBP.
Area of Science:
- Immunology
- Cellular Biology
- Microbiology
Background:
- Immune cell activation by bacterial lipopolysaccharide (LPS) typically requires membrane-bound CD14 (mCD14) and LPS-binding protein (LBP).
- Cells deficient in mCD14 can respond to LPS complexed with soluble CD14 (sCD14), a process independent of LBP.
Purpose of the Study:
- To investigate the necessity of LBP for the activation of mCD14-bearing cells by LPS.
- To elucidate the role of LBP in the context of LPS-sCD14 complexes.
Main Methods:
- Macrophages and neutrophils were stimulated with LPS-sCD14 complexes formed in the absence of LBP.
- Cellular responses, including neutrophil adhesion and macrophage cytokine production, were measured.
- The effect of blocking mCD14 on neutrophil responses was assessed.
- Neutrophil responses to LBP and LPS were evaluated in the presence and absence of sCD14.
Main Results:
- The dose of LPS required for neutrophil adhesion and macrophage cytokine production was significantly reduced (>100-fold) when LPS was complexed with sCD14, irrespective of LBP presence.
- Blocking mCD14 partially inhibited neutrophil responses to LPS-sCD14 complexes, indicating LPS transfer from sCD14 to mCD14.
- The addition of sCD14 enhanced neutrophil sensitivity to LBP and LPS by 30-100 fold.
Conclusions:
- LBP is not required for the activation of CD14-bearing cells by LPS.
- LPS-sCD14 complexes serve as a crucial intermediate in mediating leukocyte inflammatory responses to LPS.
- These findings highlight the significant role of sCD14 in LPS recognition and immune activation pathways.
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