Background Cytotoxic (CD8⁺) T-cells are required for tumor eradication and durable anti-tumor immunity.¹ The induction of tumor-reactive CD8⁺ T-cells is predominately attributed to a subset of dendritic cells (DC) called Batf3-driven DC1, given their robust ability to cross-present antigens for T-cell priming and their role in effector T-cell recruitment.^2–4 Presence of the DC1 signature in tumors correlates with improved survival and response to immunotherapies.^5–7 Yet, most tumors with a DC1 infiltrate still progress, suggesting that while DC1 can initiate tumor-reactive CD8⁺ T-cell responses, they are unable to sustain them. Therefore, there is a critical need to identify and engage additional stimulatory DC subsets to strengthen anti-tumor immunity and boost immunotherapy responses.

Methods To identify DC subsets that drive poly-functional CD8⁺ T-cell responses, we compared the DC infiltrate of a spontaneously regressing tumor with a progressing tumor. Multicolor flow immunophenotyping and single-cell RNA-sequencing were used to profile the DC compartment of both tumors. IFNγ-ELISpot was performed on splenocytes to assess for systemic tumor-reactive T-cell responses. Sorted DC subsets from tumors were co-cultured with TCR-transgenic T-cells ex vivo to evaluate their stimulatory capacity. Cross-dressing (in vivo/ex vivo) was assayed by staining for transfer of tumor-derived H-2^b MHC complexes to Balb/c DC, which express the H-2^d haplotype. Protective systemic immunity was assayed via contralateral flank tumor outgrowth experiments.

Results Regressor tumors were infiltrated with more cross-presenting DC1 than progressor tumors. However, tumor-reactive CD8⁺ T-cell responses and tumor control were preserved in Batf3^-/- mice lacking DC1, indicating that anti-tumor immune responses could be induced independent of DC1. Through functional assays, we established that anti-tumor immunity against regressor tumors required CD11c⁺ DC and cGAS/STING-independent type-I-interferon-sensing. Single-cell RNA-sequencing of the immune infiltrate of regressor tumors revealed a novel CD11b⁺ DC subset expressing an interferon-stimulated gene signature (ISG⁺ DC). Flow studies demonstrated that ISG⁺ DC were more enriched in regressor tumors than progressor tumors. We showed that ISG⁺ DC could activate CD8⁺ T-cells by cross-dressing with tumor-derived peptide-MHC complexes, thereby bypassing the requirement for cross-presentation to initiate CD8⁺ T-cell-driven immunity. ISG⁺ DC highly expressed cytosolic dsRNA sensors (RIG-I/MDA5) and could be therapeutically harnessed by exogenous addition of a dsRNA analog to drive protective CD8⁺ T-cell responses in DC1-deficient mice.

Conclusions The DC infiltrate in tumors can dictate the strength of anti-tumor immunity. Harnessing multiple stimulatory DC subsets, such as cross-presenting DC1 and cross-dressing ISG⁺ DC, provides a therapeutic opportunity to enhance anti-tumor immunity and increase immunotherapy responses.

References

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2. Hildner K, et al. Batf3 deficiency reveals a critical role for CD8alpha+ dendritic cells in cytotoxic T cell immunity. Science 2008;322(5904):p. 1097–100.

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5. Broz ML, et al. Dissecting the tumor myeloid compartment reveals rare activating antigen-presenting cells critical for T cell immunity. Cancer Cell 2014;26(5): p. 638–52.

6. Salmon H., et al., Expansion and activation of CD103(+) dendritic cell progenitors at the tumor site enhances tumor responses to therapeutic PD-L1 and BRAF inhibition. Immunity, 2016. 44(4): p. 924–38.

7. Sánchez-Paulete AR, et al., Cancer immunotherapy with immunomodulatory anti-CD137 and Anti-PD-1 monoclonal antibodies requires BATF3-dependent dendritic cells. Cancer Discov, 2016;6(1):p. 71–9.