In vivo molecular imaging for immunotherapy using ultra-bright near-infrared-IIb rare-earth nanoparticles

The near-infrared-IIb (NIR-IIb) (1,500-1,700 nm) window is ideal for deep-tissue optical imaging in mammals, but lacks bright and biocompatible probes. Here, we developed biocompatible cubic-phase (a-phase) erbium-based rare-earth nanoparticles (ErNPs) exhibiting bright downconversion luminescence at similar to 1,600 nm for dynamic imaging of cancer immunotherapy in mice. We used ErNPs functionalized with cross-linked hydrophilic polymer layers attached to anti-PD-L1 (programmed cell death-1 ligand-1) antibody for molecular imaging of PD-L1 in a mouse model of colon cancer and achieved tumor-to-normal tissue signal ratios of similar to 40. The long luminescence lifetime of ErNPs (similar to 4.6 ms) enabled simultaneous imaging of ErNPs and lead sulfide quantum dots emitting in the same similar to 1,600 nm window. In vivo NIR-IIb molecular imaging of PD-L1 and CD8 revealed cytotoxic T lymphocytes in the tumor microenvironment in response to immunotherapy, and altered CD8 signals in tumor and spleen due to immune activation. The cross-linked functionalization layer facilitated 90% ErNP excretion within 2 weeks without detectable toxicity in mice.