POLARITY OF T-CELL SHAPE, MOTILITY, AND SENSITIVITY TO ANTIGEN

T cell activation requires contact with APCs. We used optical techniqu es to demonstrate T cell polarity on the basis of shape, motility, and localized sensitivity to antigen. An intracellular Ca2+ clamp showed that T cell shape and motility are extremely sensitive to changes in [ Ca2+](i) (K-d = 200 nM), with immobilization and rounding occurring vi a a calcineurin-independent pathway. Ca2+-dependent immobilization pro longed T cell contact with the antigen-presenting B cell; buffering th e [Ca2+](i) signal prevented the formation of stable cell pairs. Optic al tweezers revealed spatial T cell sensitivity to antigen by controll ing placement on the T cell surface of either B cells or alpha-CD3 MAb -coated beads. T cells were 4-fold more sensitive to contact made at t he leading edge of the T cell compared with the tail. We conclude that motile T cells are polarized antigen sensors that respond physically to [Ca2+](i) signals to stabilize their interaction with APCs.