Probing single‐cell oxygen reserve in sickled erythrocytes via in vivo photoacoustic microscopy

To the Editor Individuals with sickle cell disease (SCD) face ongoing risk of multiorgan ischemia resulting in chronic disability, frequent hospitalizations, and early mortality. The relationship between hemoglobin (Hb) S polymerization, erythrocyte sickling, and tissue ischemia has been of great interest. Oxygen off-loading and increasing deoxy-Hb concentration promote HbS polymerization, the latter, which has been linked to early erythrocyte deformation or “reversible” sickling. Eventually, severe polymerization weakens the cell membrane, leading to “irreversibly” sickled cells. Whether the degree of polymerization and the cell's morphologic state, in turn, influence oxygen binding and thus, tissue oxygen availability has been of interest, but technically challenging to study in patients. Over two decades, Wang and colleagues developed an imaging platform, photoacoustic microscopy (PAM), which offers two unique aspects compared to other intravital microscopy systems: (1) high resolution in vivo human imaging using the cuticle as the window to a highly organized vascular bed capable of imaging single capillary loops, and (2) measurements of oxyand deoxy-Hb levels within single capillaries and single erythrocytes. In this study, we aimed to: (1) characterize capillary morphology and hemodynamic/oxygen metabolic properties in the cuticle nailbed of individuals with SCD compared to healthy controls and (2) track single erythrocytes along the capillary loop to obtain measurements of erythrocyte elongation (ellipticity index, EI) and oxygen saturation before and after tissue oxygen exchange. We hypothesized that erythrocyte EI, as an index of HbS polymerization, would be associated with decreased arteriolar oxygen saturation and/or increased oxygen extraction fraction (OEF) across capillaries—representing compromised “oxygen reserve. ” Adult participants with SCD (HbSS) and controls (HbAA) were prospectively enrolled and excluded for recent hospitalization, chronic transfusion therapy, and history of stem cell transplant. Controls were excluded for any chronic medical disorder. Hb type was confirmed by peripheral blood electrophoresis. Written informed consent was obtained from all participants. PAM is a dual-wavelength optical resolution system with 3 μm lateral and 15 μm axial resolution (Figure S1). The blood absorption spectra from individuals with SCD previously have been found to be similar to that of healthy controls. The nailbed cuticle imaging procedure consisted of both wide-field and high-speed dynamic imaging. Capillary measurements included density, diameter, and tortuosity. Number and duration of erythrocyte pauses were measured from the spatiotemporal image and its frequency domain image. Multiple hemo-metabolic parameters, calculated from both timeaveraged, capillary measurements and single erythrocytes, included: blood velocity, oxygen saturation (sO2), OEF, and relative metabolic rate of oxygen utilization (MRO2). Single-cell PAM additionally yielded measurements of arteriolar (sO2 in) and venular (sO2 out) oxygen saturation, from which single-cell OEF was calculated. Elongation of single-cells, ellipticity index (EI), was measured as the mean EI of six frames for each flowing erythrocyte (Figure S2). Details of PAM imaging and statistical methods are described in the Appendix S1. Ten adults with SCD (HbSS) and healthy controls (HbAA) underwent PAM cuticle imaging sessions, totaling 97 capillaries and 180 erythrocytes imaged (Table S1). Hb and hematocrit measured from peripheral blood correlated with cuticle Hb and hematocrit using PAM (Hb: ρ = 0.825, p = .002; Hct: ρ = 0.800, p = .003). Capillary diameter, density, and tortuosity were statistically increased in SCD versus healthy controls. Time-averaged capillary blood velocity was decreased in SCD versus controls: 62.5 μm/s (51.0, 74.3) versus 69.8 μm/s (63.6, 77.4), respectively (p = .013); capillary OEF was increased in SCD versus controls: 0.205 (0.150, 0.246) versus 0.147 (0.121, 0.188), respectively (p = .049); and capillary MRO2 was similar in SCD versus controls: 49.9 arbitrary units (a.u.) (21.3, 60.9) versus 39.0 a.u. (26.1, 50.6), respectively (p = .394) (Figure S3). Animal models of SCD have demonstrated the presence of red blood cell pauses, although the etiologies of pauses such as mechanical obstruction or endothelial adhesion are incompletely understood. Erythrocyte pause count was higher in SCD versus controls: 2 (2, 3) versus 1.5 (1, 2), respectively, (p < .0001). Pause duration was also higher in SCD versus controls: 14.6 s (10.3, 19.9) versus 7.0 s (4.5, 9.0), respectively, (p < .0001). Using a linear mixed-model to account for repeated measures within individuals, we evaluated the relationship between pause duration and capillary morphological and hemo-metabolic properties. Pause duration was inversely associated with blood velocity (p = .009), but not associated with OEF or capillary diameter. We also observed a direct association between pause duration and capillary tortuosity that approached significance (p = .09) (Figure S4). PAM imaging can resolve individual erythocytes, permitting a metric of elongated erythrocytes using “EI” (Figure S2). EI was increased in SCD versus controls: 0.201 (0.133, 0.245) versus 0.112 (0.075, 0.153), respectively (p < .0001) (Figure 1A). While the SCD cohort demonstrated a wider dynamic range of EI values, substantial overlap was seen across Received: 16 June 2021 Revised: 16 October 2021 Accepted: 20 October 2021