Cellular activation of the self-quenched fluorescent reporter probe in tumor microenvironment

The effect of intralysosomal proteolysis of near-infrared fluorescent (NIRF) self-quenched macromolecular probe (PGC-Cy5.5) has been previously reported and used for tumor imaging. Here we demonstrate that proteolysis can be detected noninvasively in vivo at the cellular level. A codetection of GFP fluorescence (using two-photon excitation) and NIRF was performed in tumor-bearing animals injected with PGC-Cy5.5. In vivo microscopy of tumor cells in subdermal tissue layers (up to 160 microm) showed a strong Cy5.5 dequenching effect in GFP-negative cells. This observation was corroborated by flow cytometry, sorting, and reverse transcription polymerase chain reaction analysis of tumor-isolated cells. Both GFP-positive (81% total) and GFP-negative (19% total) populations contained Cy5.5-positive cells. The GFP-negative cells were confirmed to be host mouse cells by the absence of rat cathepsin mRNA signal. The subfraction of GFP-negative cells (2.5-3.0%) had seven times higher NIRF intensity than the majority of GFP-positive or GFP-negative cells (372 and 55 AU, respectively). Highly NIRF-positive, FP-negative cells were CD45- and MAC3-positive. Our results indicate that: 1) intracellular proteolysis can be imaged in vivo at the cellular level using cathepsin-sensitive probes; 2) tumor-recruited cells of hematopoetic origin participate most actively in uptake and degradation of long-circulating macromolecular probes.