T lymphocytes orient against the direction of fluid flow during LFA-1-mediated migration

Abstract

As they leave the blood stream and travel to lymph nodes or sites of inflammation, T lymphocytes are captured by the endothelium and migrate along the vascular wall to permissive sites of transmigration. These processes take place under the influence of hemodynamic shear stress; therefore, we investigated how migrational speed and directionality are influenced by variations in shear stress. We examined human effector T lymphocytes on intercellular adhesion molecule 1 (ICAM-1)-coated surfaces under the influence of shear stresses from 2 to 60 dyn.cm(-2). T lymphocytes were shown to respond to shear stress application by a rapid (30 s) and fully reversible orientation of their migration against the fluid flow without a change in migration speed. Primary T lymphocytes migrating on ICAM-1 in the presence of uniformly applied SDF-1α were also found to migrate against the direction of shear flow. In sharp contrast, neutrophils migrating in the presence of uniformly applied fMLP and leukemic HSB2 T lymphocytes migrating on ICAM-1 alone oriented their migration downstream, with the direction of fluid flow. Our findings suggest that, in addition to biochemical cues, shear stress is a contributing factor to leukocyte migration directionality.

Figure 1

Cell tracking and data analysis. (A) Bright-field images of the migration of a single cell shown at different time points (0 s, 270 s, 400 s, 500 s, 580 s, and 650 s). (B) Bright-field images of T lymphocytes and their trajectories recorded during 16 min. (C) Cell trajectories with initial coordinates (0,0). (D) Parameter definitions. The initial coordinates of the cell are (0,0). Flow is directed from top to bottom, Y_i,end is the distance the cell number i traversed in the direction of flow, and d_i,accum is the contour length of the cell trajectory. We define the Y forward migration index, or Y_FMI, as Y_i,end/d_i,accum, which is positive if the net translation is in the direction of the flow. θ_i is defined as the angle between the X axis and the axis joining the initial and final positions of the cell number i. It is counted positively in counterclockwise rotation.

Figure 2

Angle histogram for one representative experiment in four different leukocyte cell populations (primary T cell, effector T cells, HSB2 T cells, and neutrophils) in static (upper panels) and flow (lower panels) conditions showing the distribution of θ in ≤40 angle bins. The length of each bin reflects the fraction of cells with a given angle that fall within a group. Under the flow condition, the shear stress was set to a value of 8 dyn.cm⁻² and the direction of the flow was along the Y axis from top to bottom, as indicated by the arrow. The numbers of cells analyzed were respectively 192, 234, 148, and 134 in the static condition and 23, 73, 93, and 60 under the flow condition for primary T cells, effector T cells, HSB2 T cells, and neutrophils.

Figure 3

Expression of LFA-1 and MAC-1 on T lymphocytes and neutrophils. (A) The expression levels of the two integrin α subunits, αL/CD11a (solid colored lines) and αM/CD11b (dotted colored lines) are depicted in the left panels, and the integrin β2 chain is shown in the right panels (solid colored line) for primary T lymphocytes (red), effector T lymphocytes (blue), HSB2 T cell line (green), and neutrophils (black). All negative controls are depicted with solid gray lines. (B) Angle histograms of a representative experiment of neutrophils migrating at 8 dyn.cm⁻² after pretreatment with an isotype control (upper histogram) or 50 μg/mL blocking antibodies directed against either CD11a (middle histogram) or CD11b (bottom histogram). Histograms represent tracking of 117, 50, and 31 cells for the control, anti-CD11a, and anti-CD11b, respectively.

Figure 4

Leukocyte response to shear stress rate. (A and B) Y_FMI (A) and speed (B) for a representative experiment in four different leukocyte cell populations (primary T cells (triangles), effector T cells (solid circles), HSB2 T cells (open circles), and neutrophils (squares)). Number of cells analyzed: N > 50 except for primary cells at σ = 8 dyn.cm⁻², where N = 23. Data are mean ± SE. (C and D) Y_FMI (C) and speed (D) were calculated for four independent experiments in effector T cells. Box-and-whisker plots show the smallest and largest, lower and upper quartiles, and median observations. Number of cells analyzed: N = 100–300. Data for Y_FMI in the absence of flow are not significantly different from zero. One-way ANOVA followed by the Tukey multiple-comparison test confirmed the existence of significant differences in Y_FMI for different values of shear stress (all p-values < 0.0001, except between 8 and 12 dyn.cm⁻², for which the difference is not significant (p > 0.05)). One-way ANOVA followed by the Tukey multiple-comparison test confirmed the absence of significant differences in speed for different values of shear stress (all p-values > 0.05).

Figure 5

Effector T lymphocytes at high shear stress. (A) Evolution as a function of the shear stress of the fraction of adherent cells relative to the number of cells that initially adhered in the absence of flow (black bars), and of the fraction of immobile cells among adherent cells at a given shear stress (gray bars). Initial cell numbers: N = 150. (B) Y_FMI versus shear stress. Green circles correspond to the average Y_FMI of all cells at a given shear stress, and violet circles indicate the average Y_FMI of the remaining migrating cells only. Number of cells analyzed ranged between 171 in static conditions and 16 at highest shear stress. Data are mean ± SE. (C) Cumulative distance versus time of individual cells (each color corresponds to a different unique cell) with a shear stress of 40 dyn.cm⁻² between 0 and 500 s, and no flow between 500 and 1000 s. (D) Average speed of individual cells as a function of the total force F exerted by the flow, where F is estimated as the product of the projected cell area S_p of individual cells and the applied shear stress σ, F = S_pxσ.

Figure 6

Effector T lymphocyte response to a periodic square wave shear stress. (A and B) Y_FMI (A) and speed (B) are reported (circles) versus the time in response to a periodic square wave of length 33 min and of shear stress intensity of 8 dyn.cm⁻² (bars). The speed and Y_FMI of 10 frames were calculated (100 s).