Cell-Specific Variation in E-Selectin Ligand Expression among Human Peripheral Blood Mononuclear Cells: Implications for Immunosurveillance and Pathobiology

Abstract

Both host defense and immunopathology are shaped by the ordered recruitment of circulating leukocytes to affected sites, a process initiated by binding of blood-borne cells to E-selectin displayed at target endothelial beds. Accordingly, knowledge of the expression and function of leukocyte E-selectin ligands is key to understanding the tempo and specificity of immunoreactivity. Here, we performed E-selectin adherence assays under hemodynamic flow conditions coupled with flow cytometry and western blot analysis to elucidate the function and structural biology of glycoprotein E-selectin ligands expressed on human peripheral blood mononuclear cells (PBMCs). Circulating monocytes uniformly express high levels of the canonical E-selectin binding determinant sLeX and display markedly greater adhesive interactions with E-selectin than do circulating lymphocytes, which exhibit variable E-selectin binding among CD4+ and CD8+ T-cells but no binding by B-cells. Monocytes prominently present sLeX decorations on an array of protein scaffolds including PSGL-1, CD43, and CD44 (rendering the E-selectin ligands CLA, CD43E, and HCELL, respectively), and B-cells altogether lack E-selectin ligands. Quantitative PCR gene expression studies of glycosyltransferases that regulate display of sLeX reveal high transcript levels among circulating monocytes and low levels among circulating B-cells, and, commensurately, cell surface α(1,3)-fucosylation reveals that acceptor sialyllactosaminyl glycans convertible into sLeX are abundantly expressed on human monocytes yet are relatively deficient on B-cells. Collectively, these findings unveil distinct cell-specific patterns of E-selectin ligand expression among human PBMCs, indicating that circulating monocytes are specialized to engage E-selectin and providing key insights into the molecular effectors mediating recruitment of these cells at inflammatory sites.

E-Selectin Ligands in the Human Mononuclear Phagocyte System: Implications for Infection, Inflammation, and Immunotherapy

“…Among the cells of MPS, human circulating monocytes and, to a lesser extent, human blood cDCs and moDCs are the most comprehensively analyzed group in terms of E-selectin ligand activity. In our studies, human classical monocytes (CD14 ++ CD16 − ) showed significantly higher levels of sLe x determinants as compared to intermediate monocytes (CD14 ++ CD16 + ), whereas non-classical monocytes (CD14 + CD16 ++ ) were almost devoid of sLe x expression ( 123 ). Another study compared the trafficking capacity of human monocyte subsets by analyzing their ability to bind to activated endothelial monolayers, and commensurately, classical monocytes showed noticeably higher capability of adhering to reactive endothelium than did non-classical/intermediate monocytes ( 137 ).…”

“…While E-selectin receptor/ligand interactions prominently mediate Step 1 events in transmigration for all leukocytes, L-selectin-dependent binding interaction have also been observed to potently mediate human peripheral blood monocyte binding to activated vascular endothelium under shear stress ( 142 – 144 ). Thus, even though the majority of the reports indicates that initial monocyte adhesion to activated endothelial cells is most critically dependent on E-selectin receptor/ligand interactions ( 123 , 145 – 150 ), distinct interactions were also reported by other authors. The differences have to do with differences in the leukocyte populations under study, variations in the assay conditions employed (i.e., shear stress levels employed, rotatory shear versus fluid shear conditions, temperature, etc.…”

“…These additional FTs provide for a much wider capacity to create sLe X ; in addition, the expression of FTIII and FTV in primates uniquely drives creation of sLe A determinants. Human circulating monocytes express all the α(1,3)-FTs, with the exception of FTV, heightening the potential for variability in glycoconjugates bearing sLe x among human and mouse cells ( 123 ). Notably, sLe A is not expressed on any primate leukocytes as these cells do not synthesize Type 1 lactosamines ( 3 ).…”

“…Several diverse and structurally singular glycostructures with E-selectin binding activity have been identified on human classical monocytes or human blood DCs. Human classical monocytes greatly display sLe x decorations on an array of protein scaffolds, consisting of P-selectin glycoprotein ligand-1 (PSGL-1), CD43 and CD44, and GSLs ( 123 ), whereas human circulating DCs appear to display sLe x solely on PSGL-1 ( 129 , 155 ).…”

Distinct human α(1,3)-fucosyltransferases drive Lewis-X/sialyl Lewis-X assembly in human cells

“…In all mammals, E-selectin is constitutively expressed in microvessels in the bone marrow and skin and is inducibly expressed in endothelial beds at inflammatory sites in response to the cytokines tumor necrosis factor and interleukin-1 (15). Expression of cell-surface sLe X is therefore a prerequisite for extravasation of all mammalian leukocytes and for migration of mammalian hematopoietic stem/progenitor cells to marrow (15)(16)(17). Importantly, whereas sLe X plays a critical role in controlling leukocyte and hematopoietic stem/progenitor cell migration, aberrant expression of this tetrasaccharide by human malignant cells is a critical mediator of cancer metastasis (18 -20).…”