High-mannose intercellular adhesion molecule-1 (ICAM-1) enhances CD16+ monocyte adhesion to the endothelium.

Human monocytes have been classified into three distinct groups - classical (anti-inflammatory; CD14⁺/CD16^-), non-classical (patrolling; CD14⁺/CD16⁺⁺), and intermediate (pro-inflammatory; CD14⁺⁺/CD16⁺). Adhesion of non-classical / intermediate monocytes with the endothelium is important for innate immunity, and also vascular inflammatory disease. However, there is an incomplete understanding of the mechanisms that regulate CD16⁺ vs. CD16^- monocyte adhesion to the inflamed endothelium. Here, we tested the hypothesis that a high mannose (HM) N-glycoform of intercellular adhesion molecule-1 (ICAM-1) on the endothelium mediates the selective recruitment of CD16⁺ monocytes. Using TNFα treatment of human umbilical vein endothelial cells (HUVECs), and using proximity ligation assay (PLA) for detecting proximity of specific N-glycans and ICAM-1, we show that TNFa induces HM-ICAM-1 formation on the endothelial surface in a time-dependent manner. Next, we measured CD16^- or CD16⁺ monocyte rolling and adhesion to TNFα treated HUVECs in which HM- or hybrid ICAM-1 N-glycoforms were generated using the a-mannosidase class I and II inhibitors, Kifunensine and Swainsonine, respectively. Expression of HM-ICAM-1 selectively enhanced CD16⁺ monocyte adhesion under flow with no effect on CD16^- monocytes noted. CD16⁺ monocyte adhesion was abrogated by blocking either HM-epitopes or ICAM-1. A critical role for HM-ICAM-1 in mediating CD16⁺ monocyte rolling and adhesion was confirmed using Cos-1 cells engineered to express HM- or complex ICAM-1 N-glycoforms. These data suggest that HM-ICAM-1 selectively recruits non-classical/intermediate CD16⁺ monocytes to the activated endothelium.