Expression of GM1, a marker of lipid rafts, defines two subsets of human monocytes with differential endocytic capacity and lipopolysaccharide responsiveness

Monocytes constitute 5-10% of total human peripheral blood leucocytes and remain in circulation for several days before replenishing the tissue macrophage populations. Monocytes display heterogeneity in size, granularity and nuclear morphology, and in the expression of cell membrane molecules, such as CD14, CD16, CD32, CD64, major histocompatibility complex class II, CCR2, CCR5, among others. This has led to the suggestion that individual monocyte/macrophage populations have specialized functions within their microenvironments. This study provides evidence for the occurrence of two peripheral blood monocyte subpopulations on the basis of their differential expression of GM1, a sphingolipid found mostly in lipid rafts, a CD14⁺ GM1^low population and a CD14⁺ GM1^high population comprising about 97.5% and 2.5% of total CD14⁺ cells, respectively. GM1 expression correlates with functional differences in terms of endocytic activity, susceptibility to mycobacterial infection, and response to lipopolysaccharide (LPS) (modulation of Toll-like receptor-4 expression). CD14⁺ GM1 ^low cells proved to be less endocytic and more responsive to LPS, whereas CD14⁺ GM1^high cells are more endocytic and less responsive to LPS. In addition, during monocyte to macrophage differentiation in vitro, the percentage of CD14⁺ GM1^high cells increases from about 2.5% at day 1 to more than 50% at day 7 of culture. These results suggest that GM1^low and GM1^high monocytes in peripheral blood, represent either different stages of maturation or different subsets with specialized activities. The expression of CD16 on GM1^high favours the first possibility and, on the other hand that up-regulation of GM1 expression and probably lipid rafts function is involved in the monocyte to macrophage differentiation process.