Ca2+ signals in CD4+ T cells during early contacts with antigen-bearing dendritic cells in lymph node

T cell activation by APC requires cytosolic Ca2+ ([Ca2+](i)) elevation. Using two-photon microscopy, we visualized Ca2+ signaling and motility of murine CD4(+) T cells within lymph node (LN) explants under control, inflammatory, and immunizing conditions. Without Ag under basal noninflammatory conditions, T cells showed infrequent Ca2+ spikes associated with sustained slowing. Inflammation reduced velocities and Ca2+ spiking in the absence of specific Ag. During early Ag encounter, most T cells engaged Ag-presenting dendritic cells in clusters, and showed increased Ca2+ spike frequency and elevated basal [Ca2+](i). These Ca2+ signals persisted for hours, irrespective of whether T cells were in contact with visualized dendritic cells. We propose that sustained increases in basal [Ca2+](i) and spiking frequency constitute a Ca2+ signaling modality that, integrated over hours, distinguishes immunogenic from basal state in the native lymphoid environment.