Vaso-occlusion by sickle cells: evidence for selective trapping of dense red cells

We have characterized the type of red cells from sickle cell patients that were trapped in the course of sickle-cell vaso-occlusion. In addition, the perfusion conditions (arterial perfusion pressure [Pa] and oxygen tension [PO2]) leading to experimentally induced vaso-occlusion in the artificially perfused, innervated mesocecum microvascular preparation were determined. Microvascular obstruction was induced by decrease in Pa; the lower the Pa, the greater the peripheral resistance as well as the extent of obstruction. The cells involved in the obstruction were recovered by vasodilation (secondary to denervation) and increase in Pa. Densitometric analysis of density gradient-separated infused and trapped cells was supplemented with morphological analysis to ascertain the involvement of density classes as well as morphological types seen in oxy and deoxy sickle blood. The trapping phenomenon was sensitive to PO2. Percentage of densest gradient classes, ie, fraction 3 (F3; mainly dense unsicklable SS discocytes [USDs]) and fraction 4 (F4; irreversibly sickled cells [ISCs] and the densest discocytes), showed a significant increase in trapping when perfusion was switched from oxy to deoxy perfusate. Morphological analysis revealed that unsicklable SS discocytes are more effectively trapped when deoxygenated. The deoxygenation of infused cells did not further change the percentage of ISCs trapped, suggesting that ISCs are equally capable of sequestration in the oxy and the deoxy states. The venous effluent showed a selective and significant depletion of dense cells (F4) and ISC counts at all Pa. We conclude that the progressive obstruction of the microcirculation by sickle cells involves selective sequestration of the densest classes of cells and that this mechanism might explain their partial disappearance during painful sickle cell crisis.