(A) Formation of TCR MCs or CD45 exclusion does not require F-actin foci. AND CD4 T cell blasts labeled with Alexa568-H57 Fab (to assess TCR clustering), or Alexa488-CD45 Fab (for CD45 exclusion) at 4⁰C, were then incubated with bilayers containing ICAM1 and MHCp for 2 min, fixed (to assess TCR clustering) or visualized live (for CD45 exclusion) using TIRF microscopy. The images were processed to assess TCR clustering, or CD45 and TCR colocalization using rank filter based filtering, as described in ‘Materials and methods’ section. For the bars showing TCR cluster intensities per cell, n1 = 45, n2 = 28, p = 0.6; for CD45 co-localization, n1 = 26, n2 = 13, p = 0.09. (B) Phosphorylation of TCR-proximal molecule Zap70 is not reduced in the cells treated with CK666. T cells were treated with DMSO or CK666 for 10 min, then incubated with surface containing ICAM1/anti-CD3 for 3 min, and processed for Y319-phospho-Zap70 and imaged using TIRF. The images were quantified to obtain the synaptic levels phospho-Zap70, and plotted as normalized to mean value of the ‘control’ cells. In the graph shown here, n1 = 34, n2 = 38, p = 0.16. (C) Synaptic phospho-PLCγ1 levels are reduced in cells lacking F-actin foci. DMSO (control, top panel) or CK666 (bottom panel) treated AND CD4 T cell blasts were incubated with bilayer containing anti-CD3 and ICAM1 for 2 min, fixed and stained with Alexa488-phalloidin (green) and phospho-Y783-PLCγ1 (red), and visualized using TIRF microscopy. In these images, total synaptic levels of phospho-PLCγ1 were assessed (top left, graph). These images were also analyzed using integrated morphometry and total number of phospho-PLCγ1 events per cell (c, top right, graph) or intensity distribution of the events across the population of T cells (bottom right, histogram) were measured. Note that CK666 treatment leads to a uniform reduction in phospho-PLCγ1 events across different intensity ranges (c, bottom histogram). In all graphs in c, n1 = 50, n2 = 67, p < 0.0001 (D) Total cellular levels of phospho-PLCγ1 and PLCγ1 with or without treatment of cells with CK666. CD4 T cell blasts were treated with 100 μM CK666 or DMSO (control), and then incubated with anti-CD3/CD28 beads for 5 min in the presence of the inhibitor, lysed and analyzed using western blotting. (E) In CK666-treated cells, there is a substantial reduction in synaptic phospho-PLCγ1 co-localized with F-actin foci. Colocalization analysis was performed on images from (C), to estimate phospho-PLCγ1 and F-actin colocalization, as described in ‘Materials and methods’. (F) Pan- PLCγ1 levels at the synapse are reduced in CK666-treated cells. T cells were processed for PLCγ1 immunofluorescence and imaged as described above. In the graph, each dot represents PLCγ1 intensity per cell in the TIRF images. n1 = 29, n2 = 34, p < 0.0001. (G) Defective calcium ion mobilization in CK666 treated cells. AND CD4 T cell blasts were loaded with Fluo4, and treated with DMSO or 100 µM CK666 for 1 min, and then imaged live on anti-CD3 and ICAM1, to monitor calcium ion flux (‘Materials and methods’). Each point on the graph (far right) represents mean value of the baseline corrected fluorescence ±SEM (n = 30 cells). (H) Nuclear translocation of NFAT1 in cells treated with CK666. DMSO (control, top panels) or CK666-treated (bottom panels) AND CD4 T cell blasts were incubated with glass coverslips coated with ICAM1 alone (left), or ICAM1 and anti-CD3 (right) for 10 min at 37°C, fixed and immunostained for NFAT1 (red), and stained with Alexa488-phalloidin (green). Cells were subsequently imaged using confocal microscopy. The middle sections from the z-stack of the images of the cells were used to quantitate nuclear levels of NFAT1, by outlining phalloidin-free central area (nucleus) of the cell. The graph (right) shows nuclear NFAT intensity in the nuclear area, in a single section per cell. n1 = 98, n2 = 40, n3 = 156, n4 = 31. p-values, p1 = 0.92, p2, p3 < 0.0001. (I) PLCγ1 dynamics at TCR microdots. Human CD4 T cells transiently expressing PLCγ1-YFP (PLC, green) were incubated with glass coverslips patterned with Alexa647 tagged anti-CD3 microdots (TCR, red) and coated with ICAM1 and imaged live using TIRF microscopy. The graphs on the bottom show linescan intensity profiles of PLCγ1 and anti-CD3 (TCR) across the pixels marked in the corresponding image in the top panels. The middle panel shows the intensity profiles during the addition of CK666, and the right panel shows the profiles 30 s after the addition of CK666. The graph in (J) shows anti-CD3 (TCR) and PLCγ1-YFP profiles obtained from at least 30 different microdots from >12 cells in each control and CK666 treatment background. PLCγ1-YFP expressing human T cells were incubated with patterned substrate for (1 + 4) min in the presence of DMSO (Control) or CK666, fixed and imaged using TIRF microscope. The CK666 was added after 1 min of incubation of cells with the substrate. The graph shows mean value of pixel intensities calculated from the linescan profiles. The pixel intensities within a linescan were normalized to lowest pixel intensity within that linescan prior to calculating mean value across different linescans. For a given treatment background and linescan profile, TCR and PLCγ1 intensities were obtained from the identical pixel positions. Scale bar, 5 µm.