Microwave ablation combined with α-PD-L1 enhances abscopal effect and promotes CTL activation and intratumoral homing by a cytokine network involving IFN-γ and CXCL9.

BACKGROUND AND PURPOSE: Microwave ablation (MWA) has demonstrated promising potential in instigating an anti-tumor immune response, potentially resulting in the regression of distant tumors. Nevertheless, the observed abscopal effect remains modest, and its underlying mechanism remains poorly elucidated. This study aims to systematically examine the utilization of α-PD-L1 to enhance the abscopal effect induced by MWA and to comprehensively investigate the associated mechanisms.

METHODS: The Lewis lung cancer model was employed to investigate the abscopal effect of MWA and α-PD-L1 co-treatment. The maturation status of bone marrow-derived dendritic cells (BMDCs) was observed after stimulating with the fragments of tumor cells harvested by microwave treatment. Flow cytometry analysis was employed to scrutinize the ratio of T cells and dendritic cells, along with the quantification of IFN-γ and GzmB expression levels. The expression of CXCL9 or PD-L1 was detected by immunofluorescence.

RESULTS: Local MWA treatment on one site of tumor did not yield a significant reduction in the volume of distant tumors. However, when combined with intraperitoneal α-PD-L1 injection, a notable abscopal effect was induced, resulting in decreased distant tumor volume and prolonged survival in mice. Flow cytometry analysis revealed an increase in infiltrating and activated CD8+ T cells in distant tumors, with up-regulation of IFN-γ and GzmB expression. Notably, the expression of CXCR3 was increased in CD8+ T cells. The mRNA and protein levels of CXCL9 were elevated within distant tumors, concomitant with increased infiltration of macrophages. Moreover, dendritic cells (DCs) in tumor-draining lymph nodes (TDLNs) exhibited enhanced maturation. Additionally, in vitro experiments showed enhanced maturation of BMDCs following stimulation with fragments of Lewis cells harvested via microwave treatment.

CONCLUSIONS: MWA exhibited the potential to stimulate systemic immune responses, it did not lead to a significant abscopal effect. However, the combination of MWA with α-PD-L1 treatment effectively induced the occurrence of the abscopal effect. This phenomenon appears to be linked to the activation of cytotoxic T lymphocytes (CTLs), phagocytosis by macrophages, and the maturation of DCs, facilitated by a cytokine network involving IFN-γ and CXCL9.