Molecular Ultrasound Imaging of PD-L1 Expression on Cancer Endothelial Cells

Tumor endothelial cells (ECs) can regulate tumor immunogenicity by overexpressing immunosuppressive molecules that block immune cell infiltration into cancer tissues. Identifying EC-specific markers in vivo could help clinicians select candidates for immune checkpoint inhibitor (ICI) therapy. This study evaluated the use of molecular ultrasound (MUS) to characterize EC-specific PD-L1 expression in a colon cancer mouse model.

Target-ready Micromarker© Microbubbles (MB) conjugated to PD-L1 (MB_PD-L1), or isotype (MB_Iso) were used for all experiments. Cell experiments were carried out to verify EC expression of PD-L1 and MB binding. All in vivo experiments used a syngeneic colon cancer CT26 mouse tumor model implanted into the flank of Balb/c wild-type immunocompetent mice. Baseline and changes in EC PD-L1 expression were evaluated by modulating the expression of EC PD-L1 using anti-PD-L1 antibodies or interferon-γ (IFNγ) in vivo.

In vitro validation through flow cytometry confirmed the presence of PD-L1 on ECs and demonstrated that MB_PD-L1 can specifically bind to these cells. In vivo results suggest that MB_PD-L1 can be used to image PD-L1 expression exclusively on ECs using MUS, and that the expression of PD-L1 is highly variable, mouse-to-mouse, in our syngeneic tumor model. Furthermore, experiments where EC PD-L1 expression was modulated confirmed the sensitivity of MUS to these changes. Specifically, a significant increase in the signal from MB_PD-L1 (dTE_PD-L1 = 7.4 ± 4.12) compared to its isotype control (dTE_Iso = 2.7 ± 2.32), p-value = 0.0089, was observed. In addition, quantified MUS of MB_PD-L1 demonstrated significantly higher PD-L1 expression in IFNγ treated mice than in the PD-L1-blocked group using MB_PD-L1 (p-value = 0.0017).

MUS using MB_PD-L1 enables the exclusive characterization of PD-L1 expression on ECs and facilitates the longitudinal detection of subtle changes in EC PD-L1 expression.