Background Intratumoral delivery of peptides that reactivate virus-specific CD8⁺ memory T cells triggers clearance of poorly immunogenic tumors in mice and is phenocopied in human tumor explants.^1–3

Methods Investigations into the mechanisms of this ‘peptide alarm therapy’ (PAT) were conducted using a mouse model of melanoma. Genetic, transcriptomic, and antibody-based methods were utilized to identify cellular and molecular mechanisms of treatment efficacy.

Results Upon reactivation, intratumoral antiviral CD8⁺ T cells upregulate cytotoxic protein expression, however, we found that neither perforin-dependent cytotoxicity of antiviral T cells nor viral peptide presentation by cancer cells was required for treatment efficacy. We also observed that activated antigen presenting cells (APCs) acquired tumor-derived antigen and accumulated in tumor-draining lymph nodes. Surprisingly, in genetic models lacking tumor-specific T cells, antiviral memory T cell reactivation with PAT remained sufficient to clear tumors, but mice were susceptible to tumor recurrence. Single-cell transcriptomic analysis uncovered cytokine-associated, T cell-driven activation of inducible nitric oxide synthase (iNOS)-expressing monocyte/macrophages, which were further correlated with expression of acute phase reactants (TNFα, IL-1, IL-6) and signatures of oxidative stress among tumor cells. Following PAT, antiviral T cells rapidly upregulate interferon-γ (IFNγ), tumor necrosis factor-α (TNFα), and interleukin-2 (IL-2). When IFNγ, TNFα, and IL-2 were collectively blocked, treatment efficacy was abrogated.

Conclusions Thus, local reactivation of antiviral memory T cells, which infiltrate tumors and possess known specificities, drives a multi-cytokine response that effectuates tumor clearance. iNOS-expressing monocyte/macrophages are implicated in amplifying inflammatory signaling and tumor cell killing. PAT demonstrates the potent anti-tumor potential of intratumoral bystander memory T cells and more broadly uncovers immunologic mechanisms of intratumoral therapies that leverage adaptive immune responses.

References

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