CD45 pre-exclusion from the tips of microvilli establishes a phosphatase-free zone for early TCR triggering

The tyrosine phosphatase CD45 is a major gatekeeper for restraining T cell activation. Its exclusion from the immunological synapse (IS) is crucial for TCR signal transduction. Here, we used expansion super-resolution microscopy to reveal that CD45 is pre-excluded from the tips of microvilli on primary T cells prior to antigen encounter. This pre-exclusion was diminished by depleting cholesterol or by engineering the transmembrane domain of CD45 to increase its membrane integration length, but was independent of the CD45 extracellular domain. We further show that brief microvilli-mediated contacts can induce Ca2+ influx in mouse antigen-specific T cells engaged by antigen-pulsed APCs. We propose that the absence of CD45 phosphatase activity at the tips of microvilli enables or facilitates TCR triggering from brief T cell-APC contacts before formation of a stable IS, and that these microvilli-mediated contacts represent the earliest step in the initiation of a T cell adaptive immune response.