Objective  To investigate the effect of ^99mTc-labeled RGD peptide (^99mTc-RGD) on regulating the tumor immune microenvironment and enhancing the anti-tumor effect of immunotherapy when combined with anti-programmed death-ligand 1 (PD-L1) antibody.

  Methods  The expression of PD-L1 on tumor cells was detected by flow cytometry, and the distribution of ^99mTc-RGD in MC38 tumor mice was evaluated using single photon emission computed tomography(SPECT) imaging. The combination therapy was performed to monitor the anti-tumor effect.

  Results  The expression of PD-L1 on CT26, MC38, 4T1 and B16F10 tumor cells was upregulated after being induced by ^99mTc-RGD, and there was a statistical significance between ^99mTc-RGD treated and untreated groups. Subsequently, SPECT imaging was performed by injecting ^99mTc-RGD into MC38 tumor-bearing mice, and significant tumor uptake was observed. Based on this, the combination of ^99mTc-RGD and anti-PD-L1 antibody in MC38 tumor mice was studied, and the results showed that different doses and time windows had significant effects on the efficacy of the combination therapy. The combination of 18.5 MBq or 37 MBq ^99mTc-RGD with an anti-PD-L1 antibody led to the best therapeutic effect at 4 h intervals.

  Conclusions  ^99mTc-RGD could induce the upregulation of tumor PD-L1 expression, and when combined with an anti-PD-L1 antibody, it could enhance the anti-tumor effect.