Tumor perfusion enhancement by ultrasound stimulated microbubbles potentiates PD-L1 blockade of MC38 colon cancer in mice

Cancer immunotherapy holds tremendous promise as a strategy for eradicating solid tumors, and its therapeutic effect highly relies on sufficient CD8+ T cells infiltration. Here, we demonstrate that ultrasound stimulated microbubble cavitation (USMC) promotes tumor perfusion, thereby increasing CD8+ T cells infiltration and anti-PD-L1 antibody delivery, then further enhancing the PD-L1 blockade of MC38 colon cancer in mice. Firstly, we optimized the mechanic index (MI) of ultrasound, and found that USMC with MI of 0.4 (equal to peak negative pressure of 0.8 MPa) significantly improved the peak intensity and area under curve of tumor contrast-enhanced ultrasound. Also, flow cytometry exhibited higher percentage of infiltrating CD8+ T cells in the USMC (MI = 0.4)-treated tumors than that of the control. We further explored the combination therapy of optimized USMC with anti-PD-L1 antibody. The combination therapy enhanced tumor perfusion and even led to the tumor vascular normalization. More importantly, flow cytometry showed that the combination not only increased the percentage and absolute number of tumor infiltrating CD8+ T cells, but also promoted the expression of Ki67 as well as the secretions of IFN γ and granzyme B, therefore, the combination therapy achieved greater tumor growth inhibition and longer survival than that of the monotherapies. These suggest that USMC is a promising therapeutic modality for combining immune checkpoint blockade against solid tumors.