The Mechanical Impact of Rigid Red Blood Cells in Sickle Cell Disease on Leukocyte Adhesion Performance in Blood Flow

Introduction: One of the significant outcomes of sickle cell disease (SCD) is the alteration of red blood cell (RBC) membrane rigidity. Although known as a remarkable characteristic of the hematological disease, knowledge of the extent to which increased RBC rigidity influences white blood cells' adhesion capability (WBCs) to inflammation in sickle cell disease is limited. Despite the known role of white blood cells in sickle cell disease pathogenesis, there is little work fully exploring the mechanism of their adhesive interactions under flow conditions with patient blood or representative models of human blood. This study seeks to quantify the impact RBC rigidity has on immune response related to sickle cell disease. Furthermore, we describe a series of artificially constructed blood models matching key characteristics of sickle cell disease patient blood and comparing their immune cell adhesion capability in a microfluidic blood flow model. We find that the presence of rigid RBCs in the blood flow reduces leukocyte adhesion capability to inflammation. Additionally, we investigate how the controlled increase in hematocrit, mimicking infusion therapy, alters leukocyte adhesion performance; we find a non-linear response.

Methods: Blood draw protocols have been approved by the University of Michigan Internal Review Board (IRB-MED). SCD donors and legal guardians are informed of the study and give written consent before blood collection. Two different artificial blood models are denoted in this study to model/mimic SCD donor blood: the Rigid Model and Non-Rigid Model. More specifically, fresh blood from healthy non-SCD blood donors is separated, compositionally altered, and reconstituted to match unique SCD donor blood characteristics. The Rigid Model uses reconstituted healthy donor blood to match the hematocrit, WBC count, rigid RBC fraction, and approximate rigidity of the rigid population. Alternatively, the Non-Rigid Model matches only the hematocrit and WBC count of the SCD donor blood sample, excepting the rigidity attributes. Healthy RBCs are treated with specific concentrations of tert-butyl hydroperoxide to induce the stiffening of the RBC membranes. Coverslips with confluent monolayer were held by vacuum to the parallel plate flow chamber (PPFC, Glycotech, Inc.) height: 127 µm. Individual blood samples, both actual SCD donor blood and artificially constructed Rigid Model and Non-Rigid Model blood samples perfused through the PPFC in a laminar flow profile for 5 min at physiological WSRs of 200 s^-1, 500 s^-1, 1,000 s^-1 by controlling the volumetric flow rate. Healthy RBCs are spiked into pre-infusion SCD Patient blood samples at controlled volumes to raise hematocrit. Subsequently, altered samples are perfused through the PPFC. In both cases, WBC adhesion to the inflamed endothelial layer is quantified as an adhesion density.

Conclusions: The knowledge uncovered from this work highlights the intricate impact RBC membrane rigidity has on reducing the capacity for WBC to properly adhere to an inflammatory challenge in a vessel wall model (Figure 1A). Also, further evidence is shown that highlights the interactions between immune cells and RBCs. Having a high count of WBCs does not necessarily translate to high adhesion to inflammation. Additionally, we find that leukocyte adhesion to inflammation varies non-linearly with an infusion mimic process (Figure 1B). Undoubtedly, the presence of rigid RBCs alters the ability of WBCs to adhere to an inflamed vessel wall. The insights gained from this study could have a profound impact on better understanding the pathophysiology of SCD and offering insights for the optimization of infusion therapy in SCD.