Human blood leukocytes continually flux between blood and tissues. The first step in extravasation of blood-borne cells involves shear-resistant binding to the endothelial surface, a process mediated by adhesive interactions between an endothelial lectin, E-selectin, and a tetrasaccharide known as sialyl Lewis X (sLeX) displayed on circulating cells. Human neutrophils and monocytes express high levels of "E-selectin ligands" (glycoconjugates that contain sLeX) and are thereby operationally primed to extravasate at inflammatory loci. However, the capacity of human blood dendritic cells (DCs) to similarly engage E-selectin is unknown. To address this question, we performed multiparameter flow cytometry, biochemical analyses, and measurements of transcripts encoding key glycosyltransferases programming sLeX biosynthesis. Our results reveal that a unifying feature of all subsets of human blood DCs is the display of carbohydrates that endow robust E-selectin binding. This feature is also characteristic of their ontogenic precursors. Notably, while bulk RNA sequencing analysis is mechanistically informative, single-cell RNA sequencing does not provide insights into the critical chemoenzymatic effectors of sLeX expression by DCs. These results thus reconceptualize our understanding of an entire class of human immune cells that converge the innate and adaptive immune systems, providing key insights into the molecular basis of their tissue recruitment in both steady-state and at the onset of an inflammatory response.
