Cyanine Phototruncation Enables Cell Labeling with Spatiotemporal Control

: Photoconvertible tracking strategies examine the dynamic migration of various cell populations. Here we de-velop p hototruncation- a ssisted c ell t racking (PACT) and apply it to evaluate the migration of immune cells into tumor-draining lymphatics. This approach is enabled by a recently discovered cyanine photoconversion reaction that leads to the two-carbon truncation and consequent blue-shift of these commonly used probes. By examining substituent effects on the heptamethine cyanine chromophore, we find that introduction of a single methoxy group increases the yield of the phototruncation reaction in neutral buffer by almost 8-fold. The resulting cell-track-ing probes are applied in a series of in vitro and in vivo experiments, including quantitative, time-dependent measurements of the migration of immune cells from tumors to tumor-draining lymph nodes. Unlike previously reported cellular photoconversion approaches, this method does not require genetic engineer-ing. Overall, PACT provides a straightforward approach to labeling cell populations with precise spatiotemporal control. Cell Tumor-draining lymph nodes (TDLNs) downstream an important cancer immunology 3- and TDLNs quan-titatively