  Red blood cells (RBCs) are a major component of blood clots, which form
physiologically as a response to injury or pathologically in thrombosis. The
active participation of RBCs in thrombus solidification has been previously
proposed but not yet experimentally proven. Holographic optical tweezers and
single-cell force spectroscopy were used to study potential cell-cell adhesion
between RBCs. Irreversible intercellular adhesion of RBCs could be induced by
stimulation with lysophosphatidic acid (LPA), a compound known to be released
by activated platelets. We identified Ca2+ as an essential player in the
signaling cascade by directly inducing Ca2+ influx using A23187. Elevation of
the internal Ca2+ concentration leads to an intercellular adhesion of RBCs
similar to that induced by LPA stimulation. Using single-cell force
spectroscopy, the adhesion of the RBCs was identified to be approximately 100
pN, a value large enough to be of significance inside a blood clot or in
pathological situations like the vasco-occlusive crisis in sickle cell disease
patients.
