Study of lymphocyte dynamics in the ocular circulation: technique of labeling cells

Individual cell motion in healthy human skin microvasculature by reflectance confocal video microscopy

OBJECTIVE

To describe upper dermal microvasculature of healthy human skin in terms of density and size of cutaneous blood vessels, leukocyte velocity, and leukocyte interactions with the endothelium.

METHODS

We used a reflectance confocal microscope, the VivaScope 1500, to acquire videos of individual cell motion.

RESULTS

We found no rolling leukocytes in the upper microvasculature of ten healthy subjects. We observed "paused" leukocytes, that is, leukocytes that temporarily stop, coinciding with the simultaneous stopping of the rest of the blood flow. We imaged more paused (median: 1.0 per subject) and adherent (1.5) leukocytes in the forearm than in the chest (median 0 paused and 0 adherent per subject) per 5 minutes of videos per body site. Leukocytes were paused for a median of 7 seconds in the forearm and 3 seconds in the chest, and we found no correlation between this parameter and the blood vessel or leukocyte size. We visualized blood flow change direction. Flowing leukocyte velocities followed a lognormal distribution and were on average higher in the chest (117 µm/s) than in the forearm (66 µm/s).

CONCLUSION

The proposed method and reported values in healthy skin provide new insights into intact human skin microcirculation.

Noninvasive in vivo characterization of erythrocyte motion in human retinal capillaries using high-speed adaptive optics near-confocal imaging

The flow of erythrocytes in parafoveal capillaries was imaged in the living human eye with an adaptive optics near-confocal ophthalmoscope at a frame rate of 800 Hz with a low coherence near-infrared (NIR) light source. Spatiotemporal traces of the erythrocyte movement were extracted from consecutive images. Erythrocyte velocity was measured using custom software based on the Radon transform. The impact of imaging speed on velocity measurement was estimated using images of frame rates of 200, 400, and 800 Hz. The NIR light allowed for long imaging periods without visually stimulating the retina and disturbing the natural rheological state. High speed near-confocal imaging enabled direct and accurate measurement of erythrocyte velocity, and revealed a distinctively cardiac-dependent pulsatile velocity waveform of the erythrocyte flow in retinal capillaries, disclosed the impact of the leukocytes on erythrocyte motion, and provided new metrics for precise assessment of erythrocyte movement. The approach may facilitate new investigations on the pathophysiology of retinal microcirculation with applications for ocular and systemic diseases.

Fluorescent Dye Labeling of Erythrocytes and Leukocytes for Studying the Flow Dynamics in Mouse Retinal Circulation

The retinal and choroidal blood flow dynamics may provide insight into the pathophysiology and sequelae of various ocular diseases, such as glaucoma, diabetic retinopathy, age-related macular degeneration (AMD) and other ocular inflammatory conditions. It may also help to monitor the therapeutic responses in the eye. The proper labeling of the blood cells, coupled with live-cell imaging of the labeled cells, allows for the investigation of the flow dynamics in the retinal and choroidal circulation. Here, we describe the standardized protocols of 1.5% indocyanine green (ICG) and 1% sodium fluorescein labeling of mice erythrocytes and leukocytes, respectively. Scanning laser ophthalmoscopy (SLO) was applied to visualize the labeled cells in the retinal circulation of C57BL/6J mice (wild type). Both methods demonstrated distinct fluorescently labeled cells in the mouse retinal circulation. These labeling methods can have wider applications in various ocular disease models.

Scanning laser ophthalmoscope-particle tracking method to assess blood velocity during hypoxia and hyperoxia

The main objective was to evaluate a Scanning Laser Ophthalmoscope (SLO) based particle tracking method as a means of quantitative assessment of retinal blood velocity and vessel diameter changes in response to hypoxia and hyperoxia. Retinal blood velocities were measured by tracking fluorescent microspheres (1.0 microm diameter) in anesthetized adult pigmented rats. Velocities were calculated based on microsphere position changes and the recording frame rate. Hypoxia was induced by inspiring a mixture of nitrogen and air and hyperoxia was induced by inspiring 100% oxygen. Average blood velocities during hypoxia obtained for arteries, veins, and small vessels (diameter < 40 microm) were 39.9 +/- 9.9, 34.9 +/- 2.7, and 8.8 +/- 1.8 mm/sec, respectively, whereas during hyperoxia, the average blood velocities obtained were 23.7 +/- 6.2, 28.2 +/- 2.7, and 7.6 +/- 0.7 mm/sec. Hypoxia was found to increase the diameters of arteries by 25% but did not change the diameters of veins; whereas, hyperoxia was found to decrease their diameters by 25% and 18%. Changes detected in vessel diameter and blood velocity suggest that the level of oxygen tension alters retinal hemodynamics. Dynamics of retinal hemodynamics in response to hypoxia and hyperoxia can be assessed using the SLO imaging method.

Erythrocyte and leukocyte dynamics in the retinal capillaries of diabetic mice

The purpose of this study was to analyse the dynamics of red blood cells (RBC) and white blood cells (WBC) in the retinal capillaries of C57BL/KS db/db mice, a genetic model of type 2 diabetes, and control mice, at different ages. Modified epifluorescence microscopy was used to analyse the capillary velocity of FITC-labeled RBC and rhodamine-labeled WBC in the retina. C57BL/KS db/db diabetic mice were compared to heterozygous non-diabetic mice at ages 8 and 18 weeks (n=6 in each group). At 8 weeks, when hyperglycemia begins in db/db mice, no significant difference was found between average RBC and WBC velocity of the 2 groups. At 18 weeks, RBC velocity was significantly higher in diabetic mice compared to controls (1.21+/-0.29 versus 1.08+/-0.28 mm sec(-1) p=0.0003). No significant difference was found between WBC velocities (0.87+/-0.3 versus 0.85+/-0.3 mm sec(-1)) even when normalized by RBC velocity values. Temporal and spatial coefficients of variation were significantly higher for WBC than RBC velocities (p<0.0001) but were not significantly different in diabetic and control mice. Direct measurement of RBC velocity with this new method showed that it was higher in the retinal capillaries of diabetic than control mice after 10-12 weeks of hyperglycemia, but not at the onset of hyperglycemia. This suggests that enhanced RBC velocity is not an immediate effect of hyperglycemia but a consequence of persistent hyperglycemia. The above results are in line with the hypothesis that microvascular flow increases in diabetes, as one of the first microvascular alterations. In contrast, WBC velocity was not different in diabetic and control mice.

Improved leukocyte tracking in mouse retinal and choroidal circulation

The purpose of this study is to develop a new method with which to visualize leukocyte dynamics in murine choroidal and retinal circulation. Both pigmented (B10.RIII) and non-pigmented (BALB/c) mice were used in this study. One hundred microl of 0.05% sodium fluorescein was injected via the mice tail vein to outline the vessel, followed by 150 microl (10(7) cells) C-AM labelled leukocytes. Fundus images were obtained with a confocal scanning laser ophthalmoscope. The dynamic image sequences were recorded simultaneously on videotape (S-VHS) and digitally at 25 frames per sec. The digital images were later analysed with a custom-made personal computer-based image analysis system. Both the choroidal and retinal circulation can be visualized in non-pigmented mice in the first few seconds of fluorescein angiography. However, the view of the choroidal and the retinal capillary circulation is soon blurred due to the rapid fluorescein leakage in the choroid. In contrast, in pigmented mice, retinal circulation is clear against the dark background of the choroid, while choroidal circulation is masked behind the pigment epithelial layer and cannot be seen at all. C-AM labelled leukocytes were clearly seen in the retinal circulation of all experimental mice and in the choroidal circulation of non-pigmented mice for as long as 30 min. The number of labelled circulating cells decreased as time clasped. Cells moved rapidly in the retinal arteries, slowing down or even stopping for a few seconds in the capillary system, and then moved slightly faster again through the postcapillary venules and veins. In non-pigmented mice, significant number of cells were seen to have arrested in the choroidal circulation. There was no difference between B10.RIII mice and BALB/c mice in vessel diameters, leukocyte velocities and shear stresses. This method allows the visualization of leukocytes and provides data on their behavior as they move through the choroidal and retinal circulation of non-pigmented mice, and in the retinal circulation of pigmented mice. It provides a valuable new tool for the investigation of real time leukocyte dynamics in murine retinal and choroidal microcirculations both under physiological conditions and during the development of ocular disease.

Ocular fundus images with confocal scanning laser ophthalmoscopy in the dog, monkey and minipig

Confocal scanning laser ophthalmoscopy (CSLO) is a new technique that enables ocular fundus image recording and retinal dynamic angiography to be performed. The ocular fundus image is acquired sequentially, point by point, and is reconstructed on a video monitor at the rate of 25 images per second. The feasibility of performing both ocular fundus image recordings and retinal angiography image recordings were tested on two dogs, two monkeys and two minipigs using a 40 degrees field I + Tech CSLO. Fundus area of each dog, monkey and minipig were examined without any additional optical devices. The ocular fundus and angiography images were recorded, stabilized and analyzed under the same conditions. For each species, all images were easily recorded without any additional optical device in a lighted room and the morphology of the retinal images generated was similar to those obtained with a camera or angiography of higher resolution. Capillary phase or venous times are presented. Image recording at 25 frames/second enabled more retinal dynamics to be demonstrated than with use of regular angiography. This technique is noninvasive and easy to perform if the eye is fixed and eyelids maintained open. It also allows exploration of retinal microvascularization and could be utilized for clinical, pharmacologic and toxicologic investigations as well.