Assessing the Physiologic Relevance of Red Blood Cell Deformability in Iron Deficiency Anemia

Background: Iron deficiency anemia (IDA), which affects individuals of all ages worldwide, is an often overlooked and undertreated component of chronic disease, despite data correlating its association with adverse outcomes in patients with cardiovascular disease (von Haehling, Nat Rev Cardiol, 2015). While red blood cells (RBCs) in IDA are known to be smaller and contain less hemoglobin than healthy RBCs, how RBC deformability is altered in IDA remains poorly understood; some ektacytometry studies have observed impaired deformability in iron deficient RBCs (idRBCs), while others described either unchanged or increased deformability (Brandão, Clin Hemorheol Microcirc, 2009). Here we ask: can single cell biophysical techniques definitively determine whether idRBCs are less deformable than healthy RBCs and how heterogenous that phenomena may be? Recent investigations into IDA's role in cardiovascular disease have generally focused on the myocardium and coronary vasculature, yet much regarding other physiologic implications remains unknown, including whether idRBCs cause microvascular obstruction or vasculopathy. To address such questions, we leveraged a suite of microvascular models we developed.

Methods: We first coupled our microfluidic capillary model with μEXACT, our customized automated particle tracking program for hematologic cell-based assays, to collect high-throughput velocity tracking of single RBCs from a healthy control and 2 IDA patients (anemic for age, ferritin <10 ng/mL) to create a single cell deformability index (sDI) for each RBC (Fig 1). Next, whole blood samples collected in EDTA tubes from the control and IDA patients were perfused into both straight 100μm wide channels (mimicking large venules) and branching 30μm wide microfluidic devices (mimicking smaller venules) at a constant shear rate for 30 minutes to observe if any occlusions or obvious alterations in flow were observed (Fig 2). Finally, using the straight 100μm channel microfluidic devices, human umbilical vein endothelial cells (HUVECs) were cultured throughout each microchannel and RBCs from a healthy control and 3 IDA patients were perfused in parallel microchannels for 4 hours. The endothelialized models were then fixed, permeabilized, and immunostained with antibodies against VCAM-1 and E-selectin, known markers of endothelial inflammation. Mean fluorescence intensity was measured to quantify endothelial inflammation (Fig 3).

Results: sDI distribution histograms were obtained for healthy and IDA patient RBCs. The mean sDIs for IDA patient RBCs were decreased in comparison to the healthy RBCs. Additionally, both IDA patient's RBCs lacked a subpopulation of highly deformable RBCs, likely reticulocytes, seen in the healthy RBCs (Fig 1C). There was no evidence of microchannel occlusion for the healthy control or IDA patient whole blood samples in either the straight 100μm microchannels or branching 30μm microfluidic devices (Fig 2D). Finally, in our endothelialized microfluidic model, endothelium exposed to IDA patient RBCs exhibited increased VCAM-1 and E-selectin expression over endothelium exposed to healthy RBCs (Fig 3B).

Conclusions: By utilizing an array of microfluidic models we can develop a more comprehensive understanding of the role idRBCs play systemically on microvasculature. Our combined microfluidic and image analysis system demonstrated decreased deformability in idRBCs and can offer detection of subpopulation differences that cannot be fully characterized with bulk techniques such as ektacytometry. So far, our data demonstrates that while no microvascular occlusion occurs, idRBCs contribute to endothelial inflammation. Additionally, the observation that physical interactions between endothelial cells and idRBCs are sufficient to cause endothelial inflammation warrants further investigation, as generally idRBCs had not been viewed as pro-inflammatory. Ongoing studies will couple unique sDI distribution curves with the degree of endothelial inflammation, as well as elucidate how these changes are associated with the degree of IDA or clinical events such as the initiation of iron supplementation. Utilizing atomic force microscopy to better understand how the idRBC membrane impacts deformability and developing biophysical computer simulations to determine if increased idRBC-endothelium interactions are observed in silico are also planned.

Figure 1

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Figure 1

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