Red blood cell aggregation within a blood clot causes platelet-independent clot shrinkage.

Platelet-driven blood clot contraction (retraction) is important for hemostasis and thrombosis. Red blood cells (RBCs) occupy approximately half of the clot volume, but their possible active contribution to contraction is unknown. The work was aimed at elucidating the ability of RBCs to promote clot shrinkage. To distinguish the effects of platelets and RBCs, we formed thrombin-induced clots from reconstituted human samples containing platelet-free plasma and platelet-depleted RBCs, followed by tracking the clot size. The clots before and after RBC-induced shrinkage were analyzed using histology and scanning electron microscopy. Tension developed in the RBC-containing plasma clots was measured with rheometry, and theoretical modeling was used to elucidate the clot shrinkage mechanisms. Platelet-depleted clots formed in the presence of RBCs exhibited >20% volume shrinkage within one hour. This process was insensitive to blebbistatin, latrunculin A, and abciximab. At a higher RBC count, clot shrinkage increased, whereas in the absence of RBCs no plasma clot shrinkage was observed. At low platelet counts, RBCs stimulated clot contraction proportionately to the platelet level. Inside the shrunken clots, RBCs formed aggregates. The average tensile force per 1 RBC was ∼120 ± 100 pN. Clots from purified fibrinogen formed in the presence of RBCs did not change in size, but underwent shrinkage in the presence of osmotically active dextran. Blood clot shrinkage can be caused by RBCs alone, and this effect is because of the RBC aggregation driven mainly by osmotic depletion. The RBC-induced clot shrinkage may reinforce platelet-driven blood clot contraction and promote clot compaction when there are few and/or dysfunctional platelets.