Redefining Clinical Viscosity in Sickle Cell Diseaseby Leveraging Microfluidic Technologies

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Project Summary/Abstract Sickle cell disease (SCD) is a devastating monogenic disease in which mutant hemoglobin polymerizes into rigid fibers leading to red cell (RBC) stiffening, and, canonically, to increased blood viscosity and to the pathologic process of vaso-occlusion. The concept of blood viscosity is clinically important, as physicians are instructed to use blood transfusions judiciously to avoid “hyperviscosity” but are also hampered by clinical transfusion guidelines that are scientifically oversimplified and not evidence-based. This overly simplified view of blood viscosity is problematic for several reasons. First, the guidelines overlook the reality that blood viscosity depends on blood vessel size, shear rate, and oxygen tension (which directly affects RBC stiffness) in SCD, in addition to hemoglobin (Hb) concentrations. Furthermore, in the microcirculation, where SCD pathophysiology takes place and the caliber of the blood vessel approaches the size of the blood cells, a complex fluid such as blood cannot be described by its “bulk” viscosity. Finally, the last several decades of research have revealed that SCD also involves endothelial dysfunction and aberrant adhesion and a multitude of cell-cell interactions involving reticulocytes, platelets, and leukocyte subpopulations, all of which are further modulated by hemolytic byproducts, coagulation proteins, and inflammatory cytokines. Therefore, the multifactorial interactions of these complex biophysical and biological characteristics synergize to alter the “effective” viscosity of blood, especially in the microcirculation. These complex processes that contribute to effective viscosity in SCD cannot be quantitatively studied in in vivo animal models, and no existing in vitro assays can integrate all of these variables. To that end, for this MPI R01 grant, Drs. Wood and Lam, who both have extensive and complementary expertise in microsystems engineering and experimental hematology, in close collaboration with Dr. Kemp, a systems biologist, will apply a multi-disciplinary experimental and computational approach to develop an in vitro model of the vasculature that incorporates all of the relevant physical, biological, and biochemical variables that contribute to increased effective blood viscosity and therefore, vaso-occlusion in SCD. The vast amounts of data generated by our experiments will then be computationally and statistically modeled to construct a comprehensive understanding of effective blood viscosity in the context of SCD vaso-occlusion. Successful completion of this project will also serve as an analytical platform that will ultimately lead to patient-specific transfusion regimens catered towards each patient’s individual hematologic profile. Moreover, the approach and methods developed here will be the basis to developing new therapeutic strategies for SCD.