T cell receptor-proximal signals are sustained in peripheral microclusters and terminated in the central supramolecular activation cluster

Abstract

T cell receptor (TCR) signaling is initiated and sustained in microclusters; however, it's not known whether signaling also occurs in the TCR-rich central supramolecular activation cluster (cSMAC). We showed that the cSMAC formed by fusion of microclusters contained more CD45 than microclusters and is a site enriched in lysobisphosphatidic acid, a lipid involved in sorting ubiquitinated membrane proteins for degradation. Calcium signaling via TCR was blocked within 2 min by anti-MHCp treatment and 1 min by latrunculin-A treatment. TCR-MHCp interactions in the cSMAC survived these perturbations for 10 min and hence were not sufficient to sustain signaling. TCR microclusters were also resistant to disruption by anti-MHCp and latrunculin-A treatments. We propose that TCR signaling is sustained by stabilized microclusters and is terminated in the cSMAC, a structure from which TCR are sorted for degradation. Our studies reveal a role for F-actin in TCR signaling beyond microcluster formation.

Figure 1

cSMAC Formation and CD45 Exclusion from Microclusters (A-E) TCRs were visualized by TIRFM of Alexa-488-H57-Fab-labeled AND T cells forming IS on glass-supported bilayers containing ICAM-1 and 10 agonist MHCp/μm². (B)-(E) represent TIRFM images of the box around the cSMAC in (A). The cSMAC region starts out as a collection of micro-clusters (B), and these microclusters fuse to form the cSMAC structure. Images in (B) and (C) were scaled relative to (D) and (E) for visualization of microclusters. (F-I) Each image in these panels represent three sequential frames in a time course observing cSMAC formation color coded in red, green, and blue and overlayed that are 6 s apart. Structures that appear white are those that are not undergoing motion, while portions that appear colored are those that are undergoing substantial motion. 5-6 min after contact formation (F and G), there is more relative motion among clusters than at 10-12 min (H and I). Scale bar in (E) equals 2 μm. Similar data were obtained from at least four independent experiments. (J-O) CD45 and TCR were visualized by TIRFM of Alexa-488-I3/2.3-Fab and Alexa-568-H57-Fab-labeled AND T cells forming IS on glass-supported bilayers containing ICAM-1 and 10 agonist MHCp/μm². (J-L) Representative images of cells forming TCR microclusters (K) 30 s post contact formation that exclude CD45. (M-O) Representative images of cells 30 min post synapse formation. Microclusters formed at 30 min (N) continue to exclude CD45 (M); however, 93% of the cSMACs (see insets) are now enriched in CD45 (three independent experiments, total n = 45 cSMACs measured). Scale bar in (L) equals 4 μm.

Figure 2

Localization of LBPA in the IS AND T cells interacting with bilayers containing the indicated amounts of agonist-MHCp were fixed at 20 min and stained for TCR and LBPA. Enrichment of LBPA in the cSMAC is observed at 2.0 (A-D) and 10.0 agonist (E-H) MHCp/μm². AND T cells interacting with bilayers containing ICAM-1 did not show any staining in the TIRF field (I-L). The specificity of the antibody was determined with a isotype control antibody (M-P). Similar results were obtained from at least three independent experiments. Scale bar equals 4 μm.

Figure 3

MHCp Antibodies and Latrunculin-A Inhibit Sustained Calcium Signaling in T Cells Forming IS AND TCR Tg T cells were labeled with Fura-2-AM and were incubated with supported planar bilayers containing ICAM-1 and 10 agonist MHCp/μm². 25 min after IS formation, the 340/380 ratio was determined by wide-field fluorescence microscopy every 5 s. The low and high calcium ratios corresponding to cells in EGTA Mg²⁺ (+) Ca²⁺ (-) buffer and ionomycin were also determined. Injections of anti-MHCp (A and B) or latrun-culin-A (C and D) are indicated as dark gray bar in graphs. Individual cell values (A and C) and population averages (B and D) show that cyto-plasmic Ca²⁺ concentration reduced in 2 min after anti-MHCp treatment and 1 min after latrunculin-A treatment. The blue line in each graph represents calcium concentration of cells treated with EGTA Mg²⁺ (+) Ca²⁺ (-) buffer. Error bars in (B) and (D) represent standard deviation. Similar results were obtained from two independent experiments.

Figure 4

MHCp Antibody Treatment Does Not Disrupt the cSMAC or Microclusters, but Prevents New Microcluster Formation Images of AND T cell forming an IS on bilayers containing ICAM-1 and 10 agonist MHCp/μm² before (A, D) and after (B, E) treating cells with MHCp antibodies (red; Cy5-Icam-1, green; Oregon Green I-E^K in [A] and [B] and Alexa-488-H57-Fab in [D] and [E]). Scale bar equals 4 μm. TCR-MHCp interactions in the cSMAC are at 80% of initial levels at 10 min after addition of anti-MHCp antibodies based on retention of I-E^k fluorescence (C) and TCR fluorescence (F). A line connects data points for each cell before and after treatment. Some cells have no reduction in TCR-MHCp interaction, yet all cells show reduced Ca²⁺. Microclusters continue to form up to 30-60 min in the mature IS (G, H). Treatment with anti-MHCp blocked formation of new microclusters; however, the existing microclusters were chased into the cSMAC (I, J). Zap-70-GFP was introduced into AND T cells via retroviral transduction. Confocal imaging was performed on these cells interacting with bilayers containing ICAM-1 and 10 agonist MHCp/μm². Microclusters recruit Zap-70 and fade as they approach the cSMAC (K, L). Treatment with anti-MHCp blocked the recruitment of Zap-70 to the microclusters, and the existing ones were chased into the cSMAC by 1-2 min. Tracks shown start in frames (G), (I), (K), and (M) and end in frames (H), (J), (L), and (N). Similar results were obtained from three independent experiments. Scale bar equals 4 μm.

Figure 5

F-Actin Depolymerization Arrests Microclusters but Does Not Disrupt Microclusters or the cSMAC Images of AND T cell forming an IS on bilayers containing ICAM-1 and 10 agonist MHCp/μm² before (A, D) and after (B, E) treating cells with latrunculin-A (red; Cy5-Icam-1, green; Oregon Green I-E^k in [A] and [B] and Alexa-488-H57-Fab in [D] and [E]). Scale bar equals 4 μm. TCR-MHCp interactions in the cSMAC are at 80% of initial levels at 10 min after addition of latrunculin-A based on retention of I-E^k fluorescence (C) and TCR fluorescence (F). A line connects data points for each cell before and after treatment. The translocation of microclusters toward the cSMAC in the mature IS was imaged with TIRFM and tracked at 30 min (G-J). Latrunculin-A prevented the formation of new microclusters and many microclusters in the periphery were dispersed, while some near the cSMAC persisted and their translocation was blocked. Similar results were obtained from two independent experiments. Scale bar equals 4 μm.

Figure 6

Microclusters Are High-Avidity Structures Based on Resistance to Anti-MHCp and Latrunculin-A TCRs were visualized by TIRFM of Alexa-488-H57-Fab-labeled AND T cells forming IS on glass-supported bilayers containing ICAM-1 and 10 agonist MHCp/μm². Cells were treated with anti-MHCp (A-E) or latrunculin-A (F-J) when they formed an immature synapse or were in the expansion phase. Anti-MHCp was added between (B) and (C) and latrunculin-A was added between (G) and (H). Microclusters showed resistance to both these treatments, as shown in (C)-(E) and (H)-(J). Similar results were obtained from two independent experiments. Scale bar equals 4 μm.