Figure 2. Characterization of the CD3-induced pushing phase.

(A) The dynamic probe-protocol used to measure T cell protrusion growth consists in stepping back the right pipette along the x-axis as soon as a RBC compression is detected (above), in order to relax this compression (below). The residual force exerted on the T cell protrusion after stepping back the pipette, F_residual , is kept below 25 pN. (B) Representative experiment (Movie S3) using the dynamic-probe protocol (Left: brightfield images; Right: fura2 ratio). Bar is 5 µm. (C) Above: Protrusion length L_protrusion versus time. During the latency phase, L_protrusion is constant and equal to 0. The pushing phase starts at t = t_push, when L_protrusion starts increasing with time. L_protrusion increases initially at a constant velocity v_protrusion. The protrusion reached a maximal length L_max. Below: fura2 ratio versus time. The ratio increases abruptly at t = t_Calcium>t_push, with a maximal amplitude Δr_fura2 relative to the base level. (D) Distribution of L_max over N = 37 cells. L_max = 8.2+/−0.7 µm. (E) Distribution of v_protrusion over N = 37 cells. v_protrusion = 0.12+/−0.01 µm/s.