UC Irvine

T-cell contact with antigen-presenting B cells initiates an activation cascade which includes an increase in T-cell [Ca 2+] and leads to T-cell differentiation and proliferation. We evaluated cell-cell contact requirements for T-cell activation by using an optical trap to control the orientation of T-cell/B-cell pairs and fluorescence microscopy to measure subsequent T-cell [Ca 2+] responses. B cells were trapped with a titanium-sapphire laser tuned to 760 nm and placed at various locations along the T cells, which had a polarized appearance defined by shape and the direction of crawling. T-cell intracellular [Ca 2+] was detected as an emission shift from the combination of fura-red and fluo-3, two cytoplasmic [Ca 2+] indicators. T cells which were presented antigen at the leading edge had a higher probability of responding (84% vs. 31%) and a shorter latency of response (42 s vs. 143 s) than those contacting B cells with their trailing end. Similar results were obtained using beads coated with antibodies to the T-cell receptor. These findings demonstrate a role for initial T-cell/B-cell contact geometry in T-cell activation by showing that the T cell is a polarized antigen sensor.