Questions: Immunological Synapse and T Cell-APC Interaction

Question 1

A researcher treats T cells with a blocking antibody against LFA-1 (a T cell adhesion molecule) before mixing them with antigen-presenting cells. Based on your understanding of synapse formation, what would you expect?

• A. T cells would activate normally because LFA-1 is only involved in T cell migration, not activation
• B. T cells would fail to form stable contact with APCs, preventing the sustained interaction needed for synapse assembly and robust activation
• C. T cells would form synapses normally but with a disorganized cSMAC lacking CD28 costimulatory signals
• D. T cell activation would be enhanced because LFA-1 normally competes with TCR for signaling space

Answer: (B)

LFA-1/ICAM-1 adhesion is the first step in stabilizing T cell-APC contact. When a T cell encounters an APC, initial TCR-peptide-MHC interactions are individually weak and transient. LFA-1 binding to ICAM-1 slows the interaction, giving TCRs time to scan the APC surface and accumulate enough engagements to trigger synapse organization. Without LFA-1, contact is too brief and unstable to allow the cytoskeletal reorganization and SMAC formation needed for robust signaling. Blocking LFA-1 is therefore an effective way to suppress T cell responses, and this principle is exploited therapeutically (e.g., efalizumab targets LFA-1 in autoimmune disease).

Question 2

Large phosphatases like CD45 are actively excluded from the cSMAC during immunological synapse formation. Why is this spatial exclusion important for T cell activation?

• A. CD45 would compete with TCR for peptide-MHC binding if it were in the cSMAC
• B. Excluding CD45 from the center allows sustained phosphorylation of TCR signaling molecules without immediate dephosphorylation, amplifying the weak activation signal
• C. CD45 is needed at the periphery to recruit adhesion molecules that maintain synapse stability
• D. CD45 exclusion prevents premature apoptosis of the T cell during prolonged APC contact

Answer: (B)

The cSMAC concentrates TCR complexes, kinases (like Lck and ZAP-70), and costimulatory molecules that drive phosphorylation-dependent signaling. CD45 is a phosphatase that would immediately reverse these phosphorylation events. By excluding CD45 to the distal SMAC (dSMAC), the synapse creates a protected signaling compartment where phosphorylated signaling intermediates can accumulate and persist. This spatial segregation — kinases at center, phosphatases at periphery — is the molecular mechanism by which many individually weak TCR-peptide-MHC interactions (each contributing small, brief phosphorylation events) are integrated into a sustained, threshold-crossing activation signal.

Question 3

The immunological synapse allows a T cell to commit to activation based on many individually weak TCR-peptide-MHC interactions, rather than requiring a single high-affinity binding event.

• T. True
• F. False

Answer: True

This integration function is the central purpose of the synapse. A T cell may have only 30,000 TCRs, and each TCR-peptide-MHC interaction has relatively low affinity (lasting seconds to minutes). Without some mechanism for integrating these signals, the T cell could not achieve the sustained signaling threshold needed for activation. The synapse solves this by concentrating TCR complexes in the cSMAC, excluding phosphatases that would quench signals, and persisting for hours — enabling the accumulation of weak signals into a decisive activation response. This is why disrupting synapse architecture (not just TCR binding) impairs T cell activation.

Question 4

The cSMAC is the primary site of sustained TCR signaling throughout the duration of the immunological synapse.

• T. True
• F. False

Answer: False

This is a common misconception. TCR microclusters form at the periphery (dSMAC) and are the primary sites of active signaling as they stream centripetally toward the cSMAC. By the time TCR complexes reach the cSMAC, signaling is partially winding down — the cSMAC functions significantly as a site of signal termination and TCR internalization (downregulation), not purely signal amplification. The pSMAC (adhesion ring) seals the interface, and the dSMAC is where new TCR activation events initiate. The synapse is a dynamic signaling conveyor belt, not a static amplifier.

Question 5

How does the spatial organization of the immunological synapse — with signaling molecules concentrated in the center and phosphatases excluded to the periphery — solve the problem of individually weak TCR-peptide-MHC interactions?

Model answer: Individual TCR-peptide-MHC interactions are short-lived and each produces only a brief, small phosphorylation event — insufficient alone to trigger activation. The synapse solves this by concentrating many TCR complexes in the cSMAC while actively excluding the phosphatase CD45 from this region. In this protected compartment, each weak phosphorylation event persists longer before being reversed, and many such events accumulate simultaneously. Over the hours that the synapse persists, thousands of individually insufficient signals are integrated into the sustained, high-level signaling cascade required for T cell commitment to activation.

This question targets the functional logic of synapse architecture — why the bull's-eye organization is not arbitrary but mechanistically explains how T cells solve a real biophysical problem. Students who understand this can connect synapse structure to activation thresholds, to why disrupting adhesion molecules suppresses immunity, and to why some immunosuppressive drugs target synapse assembly rather than TCR binding directly.