UC Irvine

Lymphocytes are the central players in the human adaptive immune response. In the body, individual T helper lymphocytes need to be activated first by physical contact with antigen-presenting cells (APC). T-cell contact with APCs initiates an activation cascade, which includes an increase in T-cell intracellular calcium, leading to T-cell proliferation, differentiation and lymphokine production. Calcium imaging are combined with optical manipulation to investigate the physical properties of T-cell activation. We study cell-cell contact requirements for T-cell activation using optical tweezers to control the orientation of T-cell/APC pairs and fluorescence microscopy to measure the subsequent T-cell intracellular calcium level ([Ca2+]i) response. APCs or beads coated with antibodies to the T-cell receptor (TCR) are trapped with a near-infrared titanium-sapphire laser and placed at different locations along the T-cell, which has a polarized appearance defined by the shape and direction of crawling. T cells which are presented with antigen at the leading edge have a higher probability of responding and a shorter latency of response than those contacting APCs or beads with their trailing end. Alterations in antibody density and bead size are used to determine the spatial requirements for T cell activation and the minimum number of receptors which must be engaged in order to transmit a positive signal.