Expansion of tumor-reactive CD8+ T cell clonotypes occurs in the spleen in response to immune checkpoint blockade

Immune checkpoint blockade (ICB) enhances T cell responses against cancer, leading to long-term survival in a fraction of patients. CD8+ T cell differentiation in response to chronic antigen stimulation is highly complex and it remains unclear precisely which T cell differentiation states at which anatomic sites are critical for the response to ICB. We identified an intermediate-exhausted population in the white pulp of the spleen which underwent significant expansion in response to ICB and gave rise to the majority of tumor-infiltrating clonotypes. Increased systemic antigen perturbed differentiation of this population towards a most circulatory exhausted_KLR state, while a lack of cross-presented tumor-antigen blunted its differentiation in the spleen. An analogous population of exhausted_KLR CD8+ T cells in human blood samples exhibited diminished tumortrafficking ability. Collectively, our data demonstrate the critical role of antigen density within the spleen for the differentiation and expansion of T cell clonotypes in response to ICB.