Expansion of Tumor-Infiltrating CD8+ T Cell Clonotypes Occurs in the Spleen in Response to Immune Checkpoint Blockade

Immune checkpoint blockade (ICB) enhances tumor-reactive T cell responses against cancer, leading to long-term tumor control and survival in a fraction of patients. Given the increasingly recognized complexity of CD8+ T cell differentiation that occurs in response to chronic antigen stimulation, it remains unclear precisely which T cell differentiation states are critical for the response to ICB, as well as the anatomic sites at which ICB-mediated reinvigoration of these T cells occurs. We used paired single-cell RNA and T cell receptor (TCR) sequencing to profile endogenous, tumor-reactive CD8+ T cells isolated from tumors, tumor-draining lymph nodes, and spleens of mice treated with ICB. We identified an intermediate-exhausted population of T cells in the spleen which underwent the greatest expansion in response to ICB and gave rise to the majority of tumor-infiltrating clonotypes. Increasing concentrations of systemic antigen perturbed the differentiation of this phenotype towards an exhausted_KLR state, while a lack of cross-presentation of tumor-derived antigen blunted differentiation in the spleen. We identified an analogous population of exhausted_KLR CD8+ T cells in matched human tumor and blood samples that exhibited diminished tumor-trafficking ability. Collectively, our data demonstrate the critical role of antigen density within the spleen in coordinating the differentiation and expansion of tumor-infiltrating T cell clonotypes in response to ICB.