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How long a T cell lingers when sampling the surface of antigen-presenting cells (APCs) can determine how it responds. Recognition of cognate antigen through the T-cell receptor (TCR) sends a
stop signal to the T cell, leading to reduced T-cell motility and prolonged engagement with the APC, a process that is necessary for efficient T-cell activation. The co-receptor CTLA4
(cytotoxic T-lymphocyte antigen 4) is known to regulate the threshold of signals during T-cell activation, but how it does so has been unclear. Now, Christopher Rudd and colleagues report in
Science that CTLA4 overrides the TCR-mediated stop signal by increasing T-cell motility, thereby inhibiting T-cell activation.
Previously, this group had observed that CTLA4 promotes clustering of the integrin LFA1 (lymphocyte function-associated antigen 1) and cell adhesion, so they assessed whether CTLA4 also
affects integrin-dependent motility. They found that pre-activated T cells that were treated with CTLA4-specific antibody had more movement on plates coated with the LFA1 ligand ICAM1
(intercellular adhesion molecule 1) than did untreated cells. Moreover, co-ligation of CTLA4 and CD3 on T cells reversed the arrest in movement that occurred when the T cells were exposed to
CD3-specific antibody alone, indicating that CTLA4 ligation overrides the stop signal induced by TCR ligation.
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