LPS is a major factor in the modulation of macrophage functions and is capable of affecting the macrophage physiology even at very low concentrations (reviewed in ref. 93). LPS contamination during purification steps may result in marked effects on the adhesion properties of these cells. For example, it has been shown that the presence of low levels of LPS results in the recovery of weakly adherent subpopulations exhibiting a high cytolytic capacity (94). Low concentrations of LPS also may result in the production of some macrophage-derived cytokines in unstimulated or interferon (IFN) -y primed macrophages (95, 96) that in turn can influence macrophage activities. The response of monocytes/macrophages to LPS is also influenced by the state of differentiation (95, 96) and appears to be different depending on specific monocyte subsets (80). Finally, LPS-stimulated monocytes may exhibit a down-modulation of chemokine receptors (97), which might result in an impaired chemotactic response of these cells. In consideration of these well-known effects of LPS on macrophage physiology, it is extremely important to avoid LPS contamination during all stages of purification and culture by paying special attention to the use of endotoxin-free reagents and materials.
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