Phototheranostics of Splenic Myeloid-Derived Suppressor Cells and Its Impact on Spleen Metabolism in Tumor-Bearing Mice

SIMPLE SUMMARY: Myeloid-derived suppressor cells (MDSCs) create an immune suppressive tumor microenvironment making them an important target in cancer. Here we demonstrated, in proof-of-principle studies, the ability of near-infrared photoimmunotherapy (NIR-PIT) to visualize and selectively eliminate MDSCs that are significantly expanded in the spleen of tumor-bearing mice using an anti-Gr1 antibody conjugated to the NIR dye IR700. Characterization of spleen extracts, using (1)H magnetic resonance spectroscopy (MRS), identified metabolic changes indicative of MDSC elimination. ABSTRACT: (1) Background: MDSCs play an active role in the immune surveillance escape of cancer cells. Because MDSCs in mice are CD11b(+)Gr1(+), near-infrared photoimmunotherapy (NIR-PIT) using the NIR dye IR700 conjugated to an MDSC-binding antibody provides an opportunity for targeted elimination of MDSCs. (2) Methods: The efficacy of Gr1-IR700-mediated NIR-PIT was evaluated in vitro using magnetically separated CD11b(+)Gr1(+) MDSCs from spleens of 4T1-luc tumor-bearing (TB) mice. For in vivo evaluation, spleens of Gr1-IR700-injected 4T1-luc TB mice were irradiated with NIR light, and splenocyte viability was determined using CCK-8 assays. Metabolic profiling of NIR-PIT-irradiated spleens was performed using (1)H MRS. (3) Results: Flow cytometric analysis confirmed a ten-fold increase in splenic MDSCs in 4T1-luc TB mice. Gr1-IR700-mediated NIR-PIT eliminated tumor-induced splenic MDSCs in culture. Ex vivo fluorescence imaging revealed an 8- and 9-fold increase in mean fluorescence intensity (MFI) in the spleen and lungs of Gr1-IR700-injected compared to IgG-IR700-injected TB mice. Splenocytes from Gr1-IR700-injected TB mice exposed in vivo to NIR-PIT demonstrated significantly lower viability compared to no light exposure or untreated control groups. Significant metabolic changes were observed in spleens following NIR-PIT. (4) Conclusions: Our data confirm the ability of NIR-PIT to eliminate splenic MDSCs, identifying its potential to eliminate MDSCs in tumors to reduce immune suppression. The metabolic changes observed may identify potential biomarkers of splenic MDSC depletion as well as potential metabolic targets of MDSCs.