Physical mechanisms of red blood cell splenic filtration

The splenic interendothelial slits fulfill the essential function of continuously filtering red blood cells (RBCs) from the bloodstream to eliminate abnormal and aged cells. To date, the process by which 8 {micro}m RBCs pass through 0.3 {micro}m-wide slits remains enigmatic. Does the slit caliber increase during RBC passage as sometimes suggested? Here, we elucidated the mechanisms that govern the RBC retention or passage dynamics in slits by combining multiscale modeling, live imaging, and microfluidic experiments on an original device with sub-micron wide physiologically calibrated slits. We observed that healthy RBCs pass through 0.28 {micro}m-wide rigid slits at 37{degrees}C. To achieve this feat, they must meet two requirements. Geometrically, their surface area-to-volume ratio must be compatible with a shape in two tether-connected equal spheres. Mechanically, the cells with a low surface area-to-volume ratio (28 % of RBCs in a 0.4 {micro}m-wide slit) must locally unfold their spectrin cytoskeleton inside the slit. In contrast, activation of the mechanosensitive PIEZO1 channel is not required. The RBC transit time through the slits follows a -1 and -3 power law with in-slit pressure drop and slip width, respectively. This law is similar to that of a Newtonian fluid in a 2D Poiseuille flow, showing that the dynamics of RBCs is controlled by their cytoplasmic viscosity. Altogether, our results show that filtration through submicron-wide slits is possible without further slit opening. Furthermore, our approach addresses the critical need for in-vitro evaluation of splenic clearance of diseased or engineered RBCs for transfusion and drug delivery. Significance StatementSplenic filtration of red blood cells through narrow interendothelial slits remains poorly understood despite its physiological significance as experiments and imaging of red cells passing through the slits are lacking. Here, we coupled live imaging, biomimetic submicron-fluidics, and multiscale modeling to quantify passage conditions. Remarkably, healthy 8-{micro}m cells can pass through 0.28-{micro}m slits at body temperature. This event is conditioned to cells being able to deform into two tether-connected equal spheres and, in limiting cases, to unfold their spectrin cytoskeleton. We showed that cells behave like a Newtonian fluid and that their dynamics is controlled by the inner fluid viscosity. We thus propose an in-vitro and in-silico approach to quantify splenic clearance of diseased cells and cells engineered for transfusion and drug delivery.