Title: Dynamics of T cell activation in vivo
Description: The rules by which naive T cells decide whether to respond to antigenic stimuli are only beginning to be fully understood. T cells are activated in secondary lymph nodes (SLOs) by the recognition of signals from antigen presenting cells (APCs), usually mature dendritic cells (DCs). We showed that CD8 + T cells are primed by DCs in three phases using multiphoton intravital microscopy (MP-IVM) in lymph nodes (LNs) of anesthetized mice. During phase one, T cells undergo brief, serial contacts with DCs for several hours and begin to upregulate activation markers. During phase two, which lasts approximately twelve hours, T cells engage in stable interactions with DCs, fully upregulate activation markers and secrete cytokines. The third phase is characterized by a return to serial, transient DC-T cell interactions and the initiation of T cell proliferation. The initial phase of serial interactions was intriguing, since previous studies had suggested that T cells stop immediately upon recognition of cognate-antigen presenting APCs. We therefore examined the influence of antigen dose on the duration of phase one by varying the number of cognate peptide-MHC (pMHC) complexes per DC and the density of cognate pMHC complex-presenting DCs per LN. The duration of phase one was inversely correlated with antigen dose. Very few pMHC complexes were needed for T cell activation and there was a sharp threshold antigen dose below which T cells did not transition to phase two, migrating until they egressed from the LN. In situations of low antigen, T cells may prolong phase one and scan more DCs to determine whether to become activated.

Finally, we also investigated the importance of stable, phase two-like, DC-T cell contacts in the differentiation of effector and memory CD8 + T cells. We showed that there is a concentration of antigenic peptide that does not seem to yield a population-wide transition to stable DC-CD8 + T cell interactions but does yield effector and memory T cell differentiation. Overall, we provide support for an integrative mechanism for T cell activation by serial encounters with DCs.