Sickle cell disease biochip: a functional red blood cell adhesion assay for monitoring sickle cell disease

Sickle cell disease (SCD) afflicts millions of people worldwide and is associated with considerable morbidity and mortality. Chronic and acute vaso-occlusion are the clinical hallmarks of SCD and can result in pain crisis, widespread organ damage, and early movtality. Even though the molecular underpinnings of SCD were identified more than 60 years ago, there are no molecular or biophysical markers of disease severity that are feasibly measured in the clinic. Abnormal cellular adhesion to vascular endothelium is at the root of vaso-occlusion. However, cellular adhesion is not currently evaluated clinically. Here, we present a clinically applicable microfluidic device (SCD biochip) that allows serial quantitative evaluation of red blood cell (RBC) adhesion to endothelium-associated protein-immobilized microchannels, in a closed and preprocessing-free system. With the SCD biochip, we have analyzed blood samples from more than 100 subjects and have shown associations between the measured RBC adhesion to endothelium-associated proteins (fibronectin and laminin) and individual RBC characteristics, including hemoglobin content, fetal hemoglobin concentration, plasma lactate dehydrogenase level, and reticulocyte count. The SCD biochip is a functional adhesion assay, reflecting quantitative evaluation of RBC adhesion, which could be used at baseline, during crises, relative to various long-term complications, and before and after therapeutic interventions.

Introduction

In mammals, the red blood cell (RBC) has uniquely evolved to lose its nucleus and organelles to become remarkably flexible.¹ RBC's adherence to vascular wall and other cells is insignificant,² whereas most other cell types depend on adhesive interactions to survive.³ RBC repeatedly deforms and squeezes through narrow capillaries that can be as small as half of its diameter.2, 4, 5 Its characteristic shape and exceptional mechanical deformability are determined by its membrane skeleton, which is located underneath the cell membrane and linked to adhesion receptors on the cell surface.6, 7, 8, 9 RBC's reduced deformability and increased adhesion have been associated with microcirculatory impairment in many diseases, including hemoglobin disorders,10, 11, 12, 13, 14 sepsis,15, 16 malaria,17, 18, 19, 20 lupus,21, 22 heavy metal exposure,23, 24 blood transfusion complications,25, 26 diabetes,27, 28 cancer,29, 30 kidney diseases,31, 32, 33, 34, 35 cardiovascular diseases,36, 37 obesity,38, 39 and neurologic disorders.40, 41, 42, 43 These diseases affect hundreds of millions of people globally with a socioeconomic burden of hundreds of billions of dollars annually.44, 45, 46, 47, 48, 49, 50, 51

In sickle cell disease (SCD), RBC adhesion has been associated with blood flow blockage,52, 53 disease severity,10, 11, 12, 13 and organ damage.⁵⁴ SCD arises from a point mutation in the β-globin gene resulting in production of hemoglobin S (HbS). Intracellular HbS molecules polymerize on deoxygenation, forming long fibers that lead to membrane damage and abnormal cellular stiffness. Membrane damage caused by HbS polymerization increases sickle RBC adhesion to vascular endothelium.55, 56 In addition, increased RBC stiffness impacts blood flow and, with abnormal cellular adhesion, results in blockage of blood vessels (vaso-occlusion).52, 53

SCD affects millions worldwide⁵⁷ and imposes significant physical, emotional, and financial burdens on its sufferers, their families and communities. Chronic and acute vaso-occlusion are the clinical hallmarks of SCD and can result in painful crises, cumulative organ damage, and early mortality.⁵⁶ SCD can cost >$8 million per patient over a 50-year life span (in the United States).⁵⁸ Even though abnormal RBC adhesion is the centerpiece of vaso-occlusion and vascular damage in SCD, there is no clinically relevant tool or method to evaluate cellular adhesion as a clinical biomarker for disease severity. Lack of such clinically applicable assays has slowed the development of new pharmaceutical and therapeutic approaches because there is no in vitro test for measuring the effects of these interventions on RBC adhesion. To address this clinically unmet need, we developed a versatile microfluidic platform for evaluation of RBC adhesion in whole blood samples. SCD may be an ideal disease with which to interrogate cellular adhesion as an indicator of disease activity and severity because point-of-care and real-time markers of disease activity are urgently needed.

In the 1980s, abnormal RBC adhesion in SCD was studied using flow chambers or ex vivo rat mesocecum.52, 59, 60, 61 However, cellular adhesion is not currently evaluated clinically because analyses of these cellular interactions are technically challenging and difficult to reproduce. Recently, microfluidic technologies have emerged as versatile platforms for diagnosing and monitoring diseases.62, 63, 64 These devices allow simple and cost-efficient fabrication, short processing times, and minimal reagent use. Furthermore, microfluidic systems can be easily adapted to mimic biophysical microenvironment of cells for a more comprehensive and accurate analysis of the subject's pathophysiological state.65, 66, 67, 68 Microfluidic platforms have been used in diagnosis and/or monitoring of several life-threatening diseases such as cancer,67, 69, 70 human immunodeficiency virus,71, 72 and thrombosis.73, 74

Here, we present a clinically relevant microfluidic device (SCD biochip) that allows serial quantitative measurements of RBC adhesion to endothelium-associated protein-immobilized microchannels (fibronectin [FN] or laminin [LN]) in a closed and preprocessing-free system. RBC deformability, when adhered, can also be evaluated using this technology. FN is a glycoprotein that circulates in plasma and is present in the endothelial cell membrane.75, 76 FN plays a role in SCD RBC adhesion, via RBC integrin α4β1 (also known as very late antigen-4 or VLA-4 integrin) interaction75, 77, 78 with the endothelial wall.75, 77, 78 LN is subendothelial and binds to an important RBC surface protein from the immunoglobulin superfamily, BCAM/LU (basal cell adhesion molecule, Lutheran antigen),59, 79, 80, 81, 82 which is phosphorylated during beta-adrenergic stimulation.59, 80, 81, 82 We have analyzed more than 100 subject blood samples using the SCD biochip, showing significant associations between RBC adhesion on FN and LN with hemoglobin (Hb) phenotype (HbSS, HbSC, and HbAA), fetal hemoglobin (HbF) levels; lactate dehydrogenase (LDH); platelet and reticulocyte counts. The SCD biochip allowed us to analyze RBC adhesion in a sizable, adult, well-phenotyped, SCD population.