A pan-immunotherapy signature to predict intratumoral CD8 T cell expansions.

Effective cancer immunotherapy relies on the clonal proliferation and expansion of CD8 T cells in the tumor. However, our insights into clonal expansions are limited, owing to an inability to track the same clones in tumors over time. Here, we develop a multi-site tumor mouse model system to track hundreds of expanding and contracting CD8 T cell clones over multiple timepoints in tumors of the same individual. Through coupling of clonal expansion dynamics and single-cell RNA/TCR-seq data, we identify a transcriptomic signature in PD-1Ly108 precursor exhausted cells that strongly predicts rates of intratumoral clone expansion. The signature correlates with expansion in mice, both with and without immunotherapies, and in patients undergoing PD-1 blockade therapy. Expression of the signature during treatment corresponds with positive clinical outcomes. Downregulation of the signature precedes clone contraction-a phase in which clones contract but maintain revivable precursor exhausted cells in the tumor. LAG-3 blockade re-activates the expansion signature, re-expanding pre-existing clones, including previously contracted clones. These findings reveal how the study of clonal expansion dynamics provide a powerful 'pan-immunotherapy' signature for monitoring immunotherapies with implications for their future development.