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Human macrophages respond to LPS in a serum-independent, CD14-dependent manner
Abstract:
Two crucial mediators of monocyte activation by lipopolysaccharide (LPS) are the acute phase plasma factor, lipopolysaccharide binding protein (LBP) and cell-surface-expressed CD14. Whether macrophage (M phi) recognized and respond to LPS in a similar manner is unknown. Here we show that human monocyte-derived M phi respond to LPS by tumor necrosis factor-alpha release and procoagulant activity upregulation by a similar dose response curve in the presence or absence of serum, suggesting that humoral factors such as LBP are relatively unimportant in the activation of M phi. Both serum-dependent and serum-independent activation of M phi by LPS require cellular CD14, as evidence by blocking studies with CD14-specific antibodies. Clones from the monocytoid cell line Mono Mac-6 selected for high LPS sensitivity displayed similar properties. When washed free of serum and cultured in the presence of calcitriol, they responded to LPS in a similar manner, regardless of the presence or absence of serum, and this response was inhibited by anti-CD14. It is hypothesized that during their differentiation. M phi acquire a functional substitute for the serum factor LBP, thereby being able to recognize low LPS concentrations in a milieu low in LBP concentration. It will be of interest to determine whether this is a high-affinity LBP receptor, LBP itself, or another cell surface constituent.
Insights
Macrophages recognize lipopolysaccharide (LPS) independently of serum factors like lipopolysaccharide-binding protein (LBP). Cellular CD14 is essential for this LPS recognition and subsequent activation in macrophages.
Area of Science:
- Immunology
- Cell Biology
Background:
- Lipopolysaccharide (LPS) is a potent activator of monocytes, primarily mediated by lipopolysaccharide-binding protein (LBP) and CD14.
- The role of these factors in LPS recognition by macrophages (M phi) remains unclear.
Purpose of the Study:
- To investigate whether macrophages recognize and respond to LPS similarly to monocytes.
- To elucidate the role of serum factors and CD14 in macrophage activation by LPS.
Main Methods:
- Human monocyte-derived macrophages and Mono Mac-6 cell line were used.
- LPS-induced tumor necrosis factor-alpha release and procoagulant activity were measured.
- Blocking studies with CD14-specific antibodies were performed.
- Experiments were conducted in the presence and absence of serum and calcitriol.
Main Results:
- Macrophages responded to LPS with similar dose-response curves irrespective of serum presence, indicating minimal LBP importance.
- Both serum-dependent and serum-independent LPS activation of macrophages required cellular CD14.
- CD14-dependent LPS response in Mono Mac-6 cells was observed even without serum.
Conclusions:
- Macrophages possess a mechanism to recognize LPS without relying on serum factors like LBP.
- Cellular CD14 is critical for both serum-dependent and serum-independent LPS recognition by macrophages.
- Macrophages may develop an LBP functional substitute during differentiation to detect low LPS concentrations.