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Timing Anti-PD-L1 Checkpoint Blockade Immunotherapy to Enhance Tumor Irradiation
by
,
Steve Seung-Young Lee
1,*,
Joanna Pagacz
2,
Sera Averbek
2,
David Scholten
2,
Yue Liu
2 and
Stephen J. Kron
2,* 1
Department of Pharmaceutical Sciences, College of Pharmacy, University of Illinois Chicago, Chicago, IL 60612, USA
2
Department of Molecular Genetics and Cell Biology, University of Chicago, Chicago, IL 60637, USA
*
Authors to whom correspondence should be addressed.
Cancers 2025, 17(3), 391; https://doi.org/10.3390/cancers17030391
Submission received: 19 December 2024
/
Revised: 14 January 2025
/
Accepted: 21 January 2025
/
Published: 24 January 2025
(This article belongs to the Special Issue Feature Paper in Section “Cancer Therapy” in 2024)
Simple Summary
In this study, we performed anti-PD-L1 antibody therapy in preclinical mouse tumor models at different time points following single-dose ionizing radiation exposure to local tumors. We observed significant suppression of tumor growth in the group treated with anti-PD-L1 antibody at 5 days post-tumor irradiation. Longitudinal molecular and imaging assays of irradiated tumor tissues showed the largest number of tumor-infiltrating cytotoxic T lymphocytes (CTLs) and the highest expression of PD-L1 at 3 and 5 days post-10 Gy radiation, respectively. Systemic anti-PD-L1 therapy at 5 days post-radiation offered enhanced delivery of the therapeutic antibody to the tumor through damaged, leaky blood vessels and upregulated PD-L1 expression on cancer cells, resulting in the induction of strong anti-cancer immune response. The in vivo and in vitro works suggest that the IFNgamma from the CTL infiltrates is responsible for most of the PD-L1 expression at 5 days, leading to rebound immunosuppression. The anti-PD-L1 antibody therapy is thus able to interrupt the negative feedback and allow the CTLs to persist and eliminate the tumors.
Abstract
Background: The ability of radiotherapy (RT) to drive anti-tumor immunity is limited by adaptive resistance. While RT induces inflammation and recruits activated tumor-infiltrating lymphocytes (TILs), including cytotoxic T lymphocytes (CTLs), the resulting radiation- and IFNγ-dependent PD-L1 expression restores an immunosuppressed tumor microenvironment. Unleashing an effective anti-tumor response may require the precise sequencing of RT and checkpoint blockade immunotherapy (CBI) to block PD-L1 signaling before it can mediate its suppressive effects. Methods: Flank tumors formed in BALB/c mice with syngeneic CT26 colon or 4T1 mammary carcinoma cells were treated with otherwise ineffective doses of ionizing radiation (10 Gy) followed by CBI (0.2 mg anti-PD-L1, i.v.) after 0, 1, 3, 5, or 7 days, comparing tumor response. Anti-PD-L1 delivery was measured by fluorescence, TILs by flow cytometry and immunofluorescence, PD-L1 expression by immunohistochemistry, and tumor size by calipers. Results: In both CT26 and 4T1 tumors, 10 Gy alone resulted in a transient growth delay associated with infiltrating CTLs peaking at 3 days and PD-L1 at 5 days. CTLs returned to baseline after 7 days, consistent with adaptive resistance. Anti-PD-L1 failed to potentiate radiation except when injected 5 days after 10 Gy, which prevented CTL depletion and led to tumor elimination. Potentially contributing to compound effects, anti-PD-L1 penetrated tumors and bound PD-L1 more efficiently after irradiation. Conclusions: Optimal timing to exploit radiation-induced permeability to enhance CBI delivery and interrupt adaptive resistance by blocking PD-L1 as it peaks may offer a general strategy to enhance external beam radiotherapy by protecting activated TILs and potentiating anti-tumor immune response.
