Dynamic Sickling Assay: Using Enzymatically Induced Hypoxia to Assess Real-Time Sickling Scalable from Large Erythrocyte Populations to Individual Cells

Abstract

Background: There is a pressing need for assays to assess both patient condition and to monitor the progress of Sickle Cell Disease (SCD) therapies. It would be beneficial for an assay to allow assessment of oxygen dissociation, hemoglobin polymerization and Red Blood Cell (RBc) sickling under both equilibrium (mimicking longer-term effects) and non-equilibrium (simulating in-vivo process) conditions. Also desirable is the CLIA-waived system suitable for use in clinical trials and ultimately for clinical management of patients on SCD therapies including those those targeting Hb oxygen affinity. Our proposed work introduces an RBC hypoxia-induced sickling assay as a clinical tool to assess SCD patient condition and to assist in development, monitoring, and optimization of SCD therapies. Methods: The system uses an enzymatic oxygen scrubbing system allowing for a wide range of deoxygenation rates and final hypoxia severity. Diluted blood samples were supplemented with Protocatechuic acid (PCA) and then with Protocatechuate 3,4-Dioxygenase (PCD) enzyme inducing hypoxia at a predetermined rate. The sample was injected into an anaerobic microfluidic chamber with the resulting Red Blood Cell (RBC) morphological changes observed through time-lapsed photography. Image analysis allows for identification and quantification of sickled RBC linked to different polymerization scenarios/mechanisms. An oxygen probe was used to track the rate of medium deoxygenation. Sickling profiles were constructed to represent a percent of sickled RBC in a total cell population measured every two seconds after the enzymatic reaction is initiated. Profiles were assessed though i) percent sickling at normoxia, ii) morphological Point of Sickling (mPoS) and iii) time delay of sickling both measured at 5% increase in sickling over the normoxia sickling baseline, iv rate of sickling, v) size of the sickling fraction and vi) Area under the Curve (AUC). The last four parameters are measured for each sickling fraction if more than one such is present. Results: In control experiments PCA/PCD system was shown to allow reliable and reproducible sample deoxygenation currently enabled in a range from seconds to hours. Comprehensive multidimensional profiles were constructed presenting fraction of sickled RBC as a function of a) time from enzymatic reaction initiation, and b) one or more of rate of oxygen consumption, end of reaction hypoxia level, or oxygen tension in the medium. Such profiling allows for fast optimization of assay parameters to target assay conditions most suitable for detection of desired effects.Significant changes in sickling profiles, with marked delay in sickling, were observed upon treatment of RBC with hemoglobin-modifying drugs like voxelotor.Sickling profiles of Sickle Trait (SCT) patients showed significant delay in sickling compared to SS-type SCD patients. The method shows the potential to differentiate SCT patients based on the risk of adverse conditions to occur due to the patient's trait status.By stopping the flow, the system allows tracking of morphological status of individual cells over the full course of medium deoxygenation. Individual cells show pronounced differences in their propensity to sickle, potentially correlated to individual cell Hb F content. Change in morphology of sickling cells from biconcave disk to crescent and then to holly leaf and granular forms had been observed upon progressively decreasing medium oxygen tension.RBC microparticle formation can be observed and tracked over time as an additional metric of erythrocyte response to developing hypoxia Conclusions: By using enzymatically induced hypoxia in a microfluidic environment, presented system allows comprehensive analysis of RBC sickling dynamics in a wide range of deoxygenation rates and conditions. It would find utility in SCD therapy development by offering a robust and high throughput method for assessing critical RBC response to therapies. With no need for cumbersome gas cylinders and low training requirements, a stand-alone system with single use disposable cartridges would also have utility in clinical trials both in the US and abroad, due to simplicity of use suitable for developing countries with limited infrastructure. When validated, the system can be adapted for Point of Care use either as complementary to other blood tests or in an independent monitoring mode.