Physical mechanisms of red blood cell splenic filtration

Abstract

Interendothelial slits in the spleen fulfill the major physiological function of continuously filtering red blood cells (RBCs) from the bloodstream to remove abnormal and aged cells. To date, the process of passage of 8 um RBCs through 0.3-um wide slits remains enigmatic. Should the slits increase their caliber during RBC passage as sometimes proposed in the literature? Here, we elucidated the mechanisms that govern the passage dynamics or retention of RBCs in slits by combining multiscale modeling, live imaging, and microfluidic experiments on an original device with slits of defined physiological dimensions, including submicron width. We observed that healthy RBCs pass through 0.28-um wide rigid slits at body temperature. To achieve this tour de force, 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, they must be able to locally unfold their spectrin cytoskeleton inside the slits. In contrast, activation of the mechanosensitive PIEZO1 channel is not required. The RBC transit time through slits scales with in-slit pressure drop and slit width to the -1 and -3 power, respectively. This transit dynamics is similar to that of a Newtonian fluid in a 2D Poiseuille flow, thus showing that it is controlled by the RBC cytoplasmic viscosity. Altogether, our results clearly 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.

Red blood cell passage through deformable interendothelial slits in the spleen: Insights into splenic filtration and hemodynamics

“…Recently, Alexis et al . clarified the mechanisms influencing the dynamics of RBC passage or retention within a narrow slit through the use of multiscale modeling, and their findings also indicate a noticeable increase in the minimum pressure gradient when the sphericity of RBCs exceeds 0.76 [23]. Figure 8B depicts changes in the minimum pressure gradient over RBC stiffness for both rigid and passive models.…”

“…The minimum pressure gradient required for cells to pass through IES exhibits an exponential growth pattern as the slit width decreases. This implies that in non-deformable models, cells would require a significantly large pressure difference to pass through IES when the slit width is below a certain threshold, posing a risk of cell rupture [23]. Therefore, there is an imminent need for a model that better reflects the mechanical characteristics of RBCs in the spleen environment to study and analyze cell mechanics in the spleen.…”

“…While significant research has been dedicated to understanding the filtration process of RBCs by the spleen [15, 16, 22, 29, 24, 26], current investigations encounter two primary limitations. Firstly, existing in vitro and in vivo studies often treat endothelial cells as non-deformable when examining individual IES [25, 24], leading to a higher pressure differential required for cell traversal through these rigid slits [23]. This elevated pressure difference can potentially result in cellular damage as cells navigate through narrower slits.…”

Peeling fingers in an elastic Hele-Shaw channel

“…Thin-film flows confined by elastic boundaries occur in a plethora of applications related to industrial, geophysical and biological processes (Modarres-Sadeghi 2021), e.g. from roll coating (Carvalho & Scriven 1997) to splenic filtration of red blood cells (Moreau et al 2023). In microfluidics, there is considerable interest in incorporating thin membranes and other soft boundaries into devices to harness flow-induced functionality and enable passive flow control (Stone 2009;Alvarado et al 2017;Gomez, Moulton & Vella 2017;Christov 2022).…”

“…from roll coating (Carvalho & Scriven 1997) to splenic filtration of red blood cells (Moreau et al. 2023). In microfluidics, there is considerable interest in incorporating thin membranes and other soft boundaries into devices to harness flow-induced functionality and enable passive flow control (Stone 2009; Alvarado et al.…”