HU reverses the effects of Hb on endothelial cell activation and neutrophil adhesion in vitro. The expression of adhesion molecules on the HUVEC surface was determined by flow cytometry after incubation with 10 mg/mL Hb in the presence or absence of HU (100 µM) or vehicle for 4 hours. (A) VCAM-1 (anti-CD106-fluorescein isothiocyanate), (B) ICAM-1 (anti-CD54- phycoerythrin), and (C) E-selectin (anti-CD62E-allophycocyanin); n = 9 assays. Cytokines in HUVEC-conditioned culture medium were quantified by enzyme-linked immunosorbent assay; (D) MCP-1, (E) IL-6, and (F) IL-8; n = 4 to 14 assays. Adhesion of human neutrophils to ICAM-1 ligand (10 µg/mL) evaluated after incubation with autologous RBC lysate (Hb 1.5 mg/mL) in the presence or absence of HU (100 µM) (G) by static adhesion. After 5 minutes of treatment, neutrophil adhesion was quantified by using a colorimetric assay (n = 8 subjects), or (H) by using a microfluidic platform after 4 hours of treatment. For the microfluidic assay, neutrophil adhesion was quantified in a 400-µm-width channel with an applied shear stress of 0.5 dynes/cm²; data were analyzed by using a DucoCell analysis program, recording the mean number of neutrophils adhered to an area of 0.08 mm² (n = 13 to 17 subjects). TNF-α (10 ng/mL) was used as a positive control. *P < .05, **P < .01, and ***P < .001 compared to basal without HU; #P < .05 and ^##P < .01 compared with Hb or RBC lysate without HU.