Abstract 678: Investigating the mechanisms involved in HMGB1-dependent DNA uptake and STING activation in dendritic cells

Background: The cGAS-STING pathway is critical for the development of anti-tumor immunity. Activation of host STING can occur by sensing of extracellular DNA by intratumoral dendritic cells (DCs). After chemotherapy and under conditions of chromosomal instability, tumor cells release double-stranded DNA. We have previously shown DCs are able to internalize DNA in a dynamin-dependent manner, suggesting that a membrane receptor might mediate DNA endocytosis. DNA uptake also requires the presence of the high mobility group box protein 1 (HMGB1). HMGB1 has multiple known receptors such as TLR2, TLR4, RAGE and CD24. However, whether these receptors are involved in DNA engulfment by DCs remains unknown. Here, we sought to elucidate the mechanisms facilitating DNA uptake and subsequent STING activation in DCs.

Methods: Bone marrow derived (BM)DCs obtained from wild type (WT), RAGE, TLR2 or TLR4-deficient mice, as well as and WT or CD24-deficient MutuDC1940 cells, were cultured in the presence or absence of Cy5-labeled plasmid DNA, a recombinant Flag-tagged HMGB1 protein (rHMGB1) and/or DMXAA. Cy5-DNA uptake, Flag-HMGB1 binding and IRF3 phosphorylation was analyzed by flow cytometry or immunofluorescence microscopy. MutuDC1940 cells were also treated with plasmid DNA, rHMGB1, tumor debris (HS) and/or L-Leucyl-L-Leucine methyl ester (LLOMe) and galectin-3 clustering was quantified by immunofluorescence. Statistically significant differences were determined by ANOVA test.

Results: DCs take-up Cy5-DNA when rHMGB1 was added, and it was HMGB1 dose-dependent, supporting HMGB1 as a key factor for DNA uptake. We found that TLR2, TLR4, RAGE or CD24 deficient DCs were capable of internalizing Cy5-DNA in the presence of rHMGB1 to the same extent as WT DCs, indicating that none of these receptors are necessary for HMGB1-mediated DNA uptake. DNA-HMGB1 stimulation triggered IRF3 phosphorylation in DCs, suggesting extracellular DNA enters the cytosol and is sensed by cGAS, leading to STING activation. In support of this, DNA-HMGB1 treatment induced an increase in lysosomal membrane permeabilization, as measured by the extent of galectin-3 clusters within cells.

Conclusion: HMGB1 promotes DNA uptake by DCs independently of TLR2, TLR4, RAGE or CD24, suggesting other receptor(s) mediate the process of HMGB1-dependent DNA internalization. Further, DNA-HMGB1 induced lysosomal membrane destabilization which may facilitate DNA release into the cytosol for sensing and trigger activation of the cGAS-STING pathway.

Citation Format: Daiana P. Celias, Kay Hänggi, Brian Ruffell. Investigating the mechanisms involved in HMGB1-dependent DNA uptake and STING activation in dendritic cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 678.

Detection of exogenous DNA uptake by murine dendritic cells

This protocol has been developed to measure exogenous DNA uptake by murine dendritic cells (DCs) using supernatant containing cellular debris, which allows for DNA uptake in the absence of transfection reagents. Inhibitors or antibodies that alter the process can be added, and either flow cytometry or fluorescent microscopy can be used to measure DNA uptake. This is intended to mimic the exposure of DCs to dying cells in the tumor microenvironment or other pathological conditions of high cellular death.

For complete details on the use and execution of this protocol, please refer to de Mingo Pulido et al. (2021).

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In vitro assay to measure exogenous DNA uptake by primary murine dendritic cells

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Designed to mimic conditions of high cell death such as those within tumors

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DNA uptake occurs spontaneously without the use of transfection reagents

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Useful for studying DNA uptake and searching for activators or inhibitors

Publisher’s note: Undertaking any experimental protocol requires adherence to local institutional guidelines for laboratory safety and ethics.

The inhibitory receptor TIM-3 limits activation of the cGAS-STING pathway in intra-tumoral dendritic cells by suppressing extracellular DNA uptake

Blockade of the inhibitory receptor TIM-3 shows efficacy in cancer immunotherapy clinical trials. TIM-3 inhibits production of the chemokine CXCL9 by XCR1⁺ classical dendritic cells (cDC1), thereby limiting antitumor immunity in mammary carcinomas. We found that increased CXCL9 expression by splenic cDC1s upon TIM-3 blockade required type I interferons and extracellular DNA. Chemokine expression as well as combinatorial efficacy of TIM-3 blockade and paclitaxel chemotherapy were impaired by deletion of Cgas and Sting. TIM-3 blockade increased uptake of extracellular DNA by cDC1 through an endocytic process that resulted in cytoplasmic localization. DNA uptake and efficacy of TIM-3 blockade required DNA binding by HMGB1, while galectin-9-induced cell surface clustering of TIM-3 was necessary for its suppressive function. Human peripheral blood cDC1s also took up extracellular DNA upon TIM-3 blockade. Thus, TIM-3 regulates endocytosis of extracellular DNA and activation of the cytoplasmic DNA sensing cGAS-STING pathway in cDC1s, with implications for understanding the mechanisms underlying TIM-3 immunotherapy.