A Cell-Based Adhesion Molecule Bioassay Detects Unexpected Properties of Plasma From Patients with Sickle Cell Disease with Documented Pulmonary Hypertension

Abstract 2120

Pulmonary hypertension (PH) is a common complication in adults with sickle cell disease (SCD) associated with early mortality. Several mechanistic pathways appear to be involved in PH in SCD, one of them being activation of pulmonary endothelium and increased adherence of circulation blood cells. In the past, levels of soluble adhesion molecules in the plasma of patients with SCD have been found to correlate with severity of pulmonary hypertension and risk of mortality.

We investigated the association between endothelial-cell based adhesion molecules and markers of PH. We developed a new cell-based ELISA assay and evaluated the induction of cell surface expression of adhesion molecules on cultured microvascular endothelium cells by plasma from subjects with SCD who had undergone right heart catheterization. We found no difference in baseline Intercellular Cell Adhesion Molecule-1 (ICAM-1), Vascular Cell Adhesion Molecule-1 (VCAM-1) and P-selectin induction by SCD plasma compared to healthy controls. Surprisingly, we found an inverse relationship of cell surface VCAM-1 induction with diagnosis and severity of PH, as indicated by mean pulmonary artery pressure (mPAP) on right heart catheterization. Patients who fell into the upper quartile of VCAM-1 induction had mPAP of 27.6 ± 3.2 mmHg, compared to the middle two quartiles 32 ± 2.3 mmHg, and lower quartile 38.2 ± 4.0 mmHg, (p=0.034). The prevalence of abnormally high pulmonary vascular resistance (>2 standard deviations above the mean) in the high, medium or low VCAM-1 induction groups was 20%, 35% and 80%, respectively (p=0.0066). We also found statistically significant correlations of cell surface VCAM-1 to cardiac output, transpulmonary gradient, pulse pressure, Doppler echocardiography tricuspid regurgitation velocity (TRV) and a marker of systemic iron overload, serum ferritin. Induced cell surface VCAM-1 expression did not correlate significantly in the same subjects with the plasma level of soluble VCAM-1, a previously documented marker associated with high TRV. We found very similar patterns of induction of cell surface expression of P-selectin. These results indicate that the ability of plasma to induce cell surface expression of cell adhesion molecules is a new marker predictive of the diagnosis of catheterization-proven PH in SCD, but it is independent of the levels of the soluble ectodomains of these cell adhesion molecules. These results are consistent with recent publications in the cell adhesion molecule field indicating that independent inflammation-mediated mechanisms regulate adhesion molecule expression and its ectodomain shedding via sheddases.

Our findings lead us to speculate that increased sheddase activity may contribute to the high levels of soluble adhesion molecules found in PH, simultaneously reducing the level of cell surface adhesion molecules. Future studies of sheddase activity in SCD PH would help to elucidate this interesting observation.