Background Resistance to immune checkpoint inhibitors (ICIs) is a significant barrier to improving cancer immunotherapy.¹ To this end, we interrogated ICI-induced inflammation. One type of inflammation, driven by nuclear factor-kB (NF-kB) activation in the tumor microenvironment (TME), can power tumor-promoting mechanisms including immunosuppression to limit ICI efficacy.² Conversely, the current paradigm predicts that another type of inflammation, characterized by T cell infiltration, improves ICI responsiveness.^3,4 How these two units of inflammation can co-exist in the TME yet direct divergent responses to ICIs requires reconciliation. In particular, it is unknown whether conventional CD4 or CD8 T cells can themselves instruct NF-kB inflammation in the TME, shaping the immunologic setpoint and, unexpectedly, limiting ICI efficacy.

Methods We generated new mouse syngeneic microsatellite instability-high (MSI-H) tumor models to study ICI response and resistance in the context of ample T cell inflammation. High-neoantigen-burden cell lines were implanted into syngeneic mice to assess responses to combination anti-PD-1 and -CTLA4 treatment.

Results Consistent with response in human tumors⁵, 50% of mouse MSI-H tumors were resistant to combination ICIs. Analysis of the TME in the resistant AT3 breast cancer model and responsive B16F10 melanoma model by mass cytometry revealed that ICIs increased tumor T cell infiltration in each model, but CD8 T cells in AT3 tumors assumed a non-cytotoxic but early-activated phenotype characterized by CD69 expression. T cell expansion in AT3 MSI-H tumors was accompanied by recruitment of T cell-suppressive polymorphonuclear myeloid-derived suppressor cells requiring cytokines G-CSF and CXCL1, absent in B16F10 tumors. Inhibition of the inflammatory NF-kB circuit, which drives expression of these cytokines, via IL-1 receptor neutralization increased ICI efficacy against AT3 MSI-H tumors. Unexpectedly, in vivo depletion of T cells also abolished the NF-kB circuit in AT3 MSI-H tumors, and activated CD8 T cells were sufficient to instruct tumor cells to increase G-CSF and CXCL1 expression. We found that TNFa, increased in T cells and neutrophils upon ICIs, perpetuated the NF-kB circuit and contributed to ICI resistance in AT3 MSI-H tumors. Finally, analysis of single-cell RNA sequencing data from ICI-treated breast cancer patients revealed candidate human T cell correlates.

Conclusions We report a surprising, novel mechanism of ICI resistance whereby treatment-induced T cell infiltration and activation can paradoxically exacerbate a TNFa- and IL-1-dependent resistance circuit in tumors with active NF-kB inflammation. Our findings refine the current models of ICI response and resistance with important therapeutic implications.

Acknowledgements We thank E. Engleman and J. Bluestone for cell lines. We thank A. Marson and T. Roth for CRISPR-Cas9 reagents, protocols and equipment. We thank the UCSF Clinical Cancer Genomics Laboratory, Molecular Oncology Initiative, and D. Raleigh for access to the UCSF cBioPortal dataset.

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