A Standardized Clinical Flow Adhesion Assay of Whole Blood Adhesion to VCAM-1 Predicts Severe SCD Phenotypes in a 2-Yr Prospective Observational Follow-up of the Original Elipsis Cohort

Sickle cell disease (SCD) is a complex, multi-organ system condition characterized by frequent and unpredictable vaso-occlusive episodes (VOEs) and significant phenotypic variability. Red blood cell (RBC) adhesion contributes to vaso-occlusive pathology, thus we have developed and validated a standardized clinical whole blood flow adhesion assay to VCAM-1 (FA-WB-VCAM) to serve as a clinical biomarker of RBC health in sickle cell disease and other conditions^1-4. Blood for the FA-WB-VCAM assay is drawn in sodium citrate tubes and can be stored at 4^oC for up to 48hrs. Whole blood samples are perfused through VCAM-1-coated channels and adherent blood cells are quantified manually to generate a clinical adhesion index (cells/mm²). We previously reported longitudinal steady-state FA-WB-VCAM adhesion indices established from every 3 weeks blood sampling over 6 months (mean=368.3 ±198.4 cells/mm², range=83.50 - 917.0), and the correlation of steady state adhesion indices to a lifetime historical SCD severity index (SCDI) (r=0.5851, p=0.0003) ^5,6. The lifetime historical SCDSI was calculated by quantifying the total number of objectively, documentable, vaso-occlusive end-organ events (VEEs), including acute chest syndrome/pneumonia, stroke, priapism, splenic sequestration, hepatic sequestration, and cholelithiasis, indexed over the total years of life (# VEEs/age; range=0.0 to 0.38 in ELIPSIS). The objective of this study was to determine if the FA-WB-VCAM could predict clinical SCD severity prospectively using various metrics of disease severity, including the SCDI over a 2-year period post ELIPSIS. First, we classified study subjects as "severe" or "mild/moderate" adhesion phenotypes defined by mean steady state adhesion indices acquired from every 3 week sampling over 6 months of the ELIPSIS study (severe adhesion phenotype: > 75^th percentile, 455.9 cells/mm²; mild/moderate adhesion phenotype: < 75^thpercentile). VEEs were verified by retrospective review of medical records, and a SCDSI over the 2-year period following the ELIPSIS study was established. Our data show that individuals with severe adhesive SCD phenotypes experienced significantly more VEEs compared to individuals with low/moderate severe adhesive SCD phenotypes (mean=55.67 ±69.78 compared to mean=17.04 ±20.58 respectively; p=0.01). SCD patients with severe adhesive phenotypes also had more hospital admissions (mean=5.67 ±2.29 compared to mean=2.88 ±3.41, p=0.001), ER visits (mean=36.00 ±63.42 compared to mean=9.20 ±15.65, p=0.02), transfusions (mean=4.33 ±3.74 compared to mean=1.60 ±2.55, p=0.01), acute chest syndrome/pneumonia (mean=1.56 ±0.73 compared to mean=0.20 ±0.65, p<0.001), and priapism (mean=2.56 ±5.57 compared to mean=0.00 ±0.00, p=0.003) when compared to low/moderate adhesive phenotypes. These data suggest that the FA-WB-VCAM assay may serve as a predictive biomarker for impending VEEs, and a monitoring biomarker to assess response to SCD-modifying therapies. Additional studies in a larger prospective cohort are required to definitively establish the clinical utility of the FA-WB-VCAM assay.