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Bronchoalveolar macrophage CD14 expression: shift between membrane-associated and soluble pools
J D Hasday1, W Dubin, S Mongovin
1Department of Medicine, University of Maryland Medical School, Baltimore, USA.
Abstract:
The bacterial endotoxin [lipopolysaccharide (LPS)]-binding protein CD14 modulates the host response to LPS, but membrane-associated and soluble forms of the molecule exert different biological effects. CD14 anchored to the mononuclear phagocyte membrane (mCD14) enhances response to LPS. Soluble CD14 (sCD14) may block LPS stimulation of CD14-bearing cells while supporting LPS presentation to non-CD14-bearing cells. We analyzed cell mCD14 and sCD14 expression in simultaneously collected human bronchoalveolar macrophages (BAM) and peripheral blood monocytes (PBM). Expression of mCD14 in freshly isolated BAM was only 9% as high as in PBM. Levels of sCD14 in 48 h in BAM culture supernatants were 19% as high as in PBM cultures. Interleukin (IL)-6 increased CD14 expression in both BAM and PBM but exerted different effects on CD14 distribution in these cell types. IL-6 increased only sCD14 release (2.5-fold) in BAM while increasing only mCD14 expression (2.5-fold) in PBM. IL-4 reduced both mCD14 (> 40%) and sCD14 (> 60%) expression in both cell types. We speculate that the balance between sCD14 and mCD14 expression influences the response to aspirated or inhaled LPS in the bronchoalveolar compartment. Cytokine expression and monocyte recruitment may influence this process by modulating CD14 expression.
Insights
The study reveals differing roles for membrane-bound CD14 (mCD14) and soluble CD14 (sCD14) in host response to lipopolysaccharide (LPS). Cytokines like IL-6 and IL-4 differentially regulate mCD14 and sCD14 expression in macrophages and monocytes.
Area of Science:
- Immunology
- Cell Biology
- Respiratory Medicine
Background:
- CD14 protein binds bacterial lipopolysaccharide (LPS), modulating host immune responses.
- Membrane-bound CD14 (mCD14) enhances LPS response, while soluble CD14 (sCD14) has complex roles, potentially blocking or facilitating LPS presentation.
- Bronchoalveolar macrophages (BAM) and peripheral blood monocytes (PBM) are key immune cells involved in LPS recognition.
Purpose of the Study:
- To compare the expression of mCD14 and sCD14 in human BAM and PBM.
- To investigate the effects of interleukin-6 (IL-6) and interleukin-4 (IL-4) on CD14 expression and distribution in BAM and PBM.
- To understand how the balance of CD14 forms influences the lung's response to inhaled LPS.
Main Methods:
- Simultaneous collection of human BAM and PBM.
- Quantification of mCD14 and sCD14 expression levels.
- Analysis of CD14 expression following stimulation with IL-6 and IL-4 in cell cultures.
Main Results:
- Freshly isolated BAM expressed significantly less mCD14 (9%) compared to PBM.
- BAM culture supernatants showed lower sCD14 levels (19%) than PBM cultures.
- IL-6 selectively increased sCD14 release in BAM and mCD14 expression in PBM, while IL-4 reduced both forms in both cell types.
Conclusions:
- The differential expression and regulation of mCD14 and sCD14 in BAM versus PBM suggest distinct roles in pulmonary immune responses.
- The balance between mCD14 and sCD14 is crucial for modulating the host's reaction to LPS in the bronchoalveolar space.
- Cytokine signaling pathways significantly impact CD14 expression, influencing monocyte recruitment and immune cell activation in response to LPS.
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