Lodygin, D. et al. Nat. Med. 19, 784–790 (2013).

Mues, M. et al. Nat. Med. 19, 778–783 (2013).

The initial steps that lead to the activation of T cells during an immune response are currently of high interest. Two reports provide genetic tools for in vivo imaging of T-cell activation in rodents in real time. Because early activation events in lymphocytes involve changes in intracellular calcium levels, Mues et al. used a genetically encoded calcium indicator to label and then track T cells using two-photon microscopy. They found that the sensor, TN-XXL, which has been used for a range of studies in the nervous system, was not efficiently expressed in lymphocytes, so they optimized TN-XXL to improve its expression in T cells. In separate work, Lodygin et al. engineered a molecular sensor based on the protein nuclear factor of activated T cells and the histone protein H2B. The sensor labeled the initial and late phases of T-cell activation and could be used for intravital imaging of immune processes.