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

Abstract

SummaryMonocytes 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 97Á5% and 2Á5% 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 2Á5% 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.

Proteomic Analysis of ABCA1-Null Macrophages Reveals a Role for Stomatin-Like Protein-2 in Raft Composition and Toll-Like Receptor Signaling

“…Indeed, GM1 up-regulation has itself been linked to mislocalization of raft proteins, reduced membrane fluidity, and abnormal raft-dependent signaling (35,55,56). Exogenous GM1 inhibits LPS signaling (57), and a naturally occurring GM1 hi human monocyte subset has recently been shown to display a reduced LPS response compared with GM1 lo counterparts (58). It is thus possible that SLP-2 deficiency may attenuate LPS signaling in rafts via leading to increased GM1 in raft microdomains.…”

Activation of ganglioside GM3 biosynthesis in human blood mononuclear cells in atherosclerosis

“…Например, показано, что в мембранах лимфоцитов рафты содержат почти все поверхностные антигены и в 20 раз по сравнению с суммарной мембраной обогащены ганглиозидом GM3 [9]. В мембранах моноцитов также установлено присутствие рафтов [10].…”

Rafts and the battleships of defense: The multifaceted microdomains for positive and negative signals in immune cells

“…Ganglioside expression correlates with functional differences in terms of endocytic activity and response to LPS. The CD14 + GM1 low population (97.5%) is more responsive to LPS, whereas the CD14 + GM1 high cells are more endocytic [72]. In resting monocytes, CD14 can only bind LPS in the presence of lipopolysaccharide-binding protein (LBP) [73].…”