Hypoxia Impact on Red Blood Cell-Mediated Microvascular Occlusion and Adhesion in Sickle Cell Disease and Sickle Cell Trait

Introduction: Sickle cell disease (SCD) is a recessively inherited anemia caused by a single gene mutation leading to sickle hemoglobin production. Sickle cell trait (SCT) is the carrier state. Abnormal hemoglobin polymerization and resultant red blood cell (RBC) sickling, decreased deformability and increased adhesion, are well-known features of homozygous SCD. However, the overall pathophysiological impact of SCT on the RBC remains incompletely characterized. Here we use microfluidic techniques designed by us, the OcclusionChip and SCD Biochip (previously published), and commercially available ektacytometry to investigate hypoxia impact on RBC biophysical properties in SCT.

Methods: Venous blood samples were collected in EDTA from subjects with homozygous HbSS, SCT (HbAS), and non-anemic controls (HbAA) under an IRB-approved protocol. OcclusionChip devices were fabricated using standard soft lithography protocols [1]. RBCs were isolated from whole blood, re-suspended in PBS at 20% hematocrit, and passed through the OcclusionChip device with a constant inlet pressure. Following a wash step, the OcclusionChip microchannel was imaged, and Occlusion Index (OI), a standardized generalizable parameter we developed, representing the overall microcapillary network occlusion, was quantified. SCD Biochip microchannels were fabricated by lamination and were functionalized with human laminin (LN-511) [2]. Undiluted whole blood was injected into the microchannel at 1 dyne/cm ², a shear stress value typically observed in the post-capillary venules. Following a wash step, the SCD Biochip microchannel was imaged, and the number of adherent RBCs in a 32-mm ² window was quantified. For hypoxia experiments, a hypoxic setup was fabricated for blood deoxygenation (pO ₂ ~45 mmHg) [3, 4]. Ektacytometry measurements were performed according to the manufacturers' specifications (Lorrca Maxsis). Data are reported as mean ± standard deviation (SD).

Results: We initially analyzed RBC-mediated microvascular occlusion under normoxia or hypoxia using the OcclusionChip (Figure 1A). Under normoxia, HbSS-containing RBCs had relatively greater OI values compared to HbAA- and HbAS-containing RBCs (Figure 1B, P = 0.057 for HbSS vs HbAA and P = 0.060 for HbSS vs HbAS). However, exposure to hypoxia led to significantly elevated OI values in the HbAS- and HbSS-containing RBCs (Figure 1B, 0.05 ± 0.02% vs 33.62 ± 18.31%, P = 0.015 for HbAS, and 0.27 ± 0.24% vs 49.37 ± 24.47%, P = 0.001 for HbSS, normoxia vs hypoxia). Negligible occlusion was observed in HbAA-containing RBCs (Figure 1B). We then analyzed RBC adhesion to LN under normoxia or hypoxia using the SCD Biochip (Figure 1C). Hypoxia led to greater number of adherent RBCs on LN in the HbSS-containing RBCs (Figure 1D, 141 ± 91 vs 497 ± 392, P = 0.089, normoxia vs hypoxia), but this effect was not present in HbAA- or HbAS-containing RBCs (Figure 1B, 2 ± 1 vs 3 ± 1, P > 0.05 for HbAA, and 10 ± 7 vs 12 ± 3, P > 0.05 for HbAS, normoxia vs hypoxia). Further, under normoxia, we found that the HbAS-containing RBCs had slightly greater number of adherent RBCs on LN compared to the HbAA-containing RBCs (Figure 1D, P = 0.057 for HbAA vs HbAS). As previously reported, HbSS-containing RBCs showed greatest adhesion to LN under normoxia compared to the HbAA- and HbAS-containing RBCs (Figure 1D, P = 0.027 for HbSS vs HbAA and P = 0.033 for HbSS vs HbAS)., Finally, we preformed Lorrca oxyscan and found that ektacytometry is less sensitive to RBC deformability change under hypoxia in SCT (Figure 1E).

Conclusions: Findings in this study suggest that although RBCs from subjects with SCT are deformable under normoxia and are able to clear narrow capillaries similar to normal RBCs, hypoxia changes deformability, presumably due to hypoxic polymer formation, and could contribute to microvascular occlusion in SCT. The OcclusionChip is a single cell-based technology, and may be more sensitive to single RBC deformability. Future studies will prospectively focus on analyzing RBC adhesion on activated microvascular endothelial cells in physiologic flow to further interrogate the impact of hypoxia on pathophysiology in SCT.

References:

[1] Man et al., LabChip, 2020, 20, 2086-2099.

[2] Kim et al., Microcirculation, 2017, 24, e12374.

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