Stem-like PD-1⁺TCF-1⁺ CD8⁺ T cells result from helpless priming and rely on CD4⁺ T-cell help to complete their...

Antigen-specific CD8⁺ T cells can be in a stem-like programmed cell death protein-1 (PD-1)⁺TCF-1⁺ differentiation state that progresses into terminal exhaustion in cancer and chronic infection. These stem-like cells are important, since they are the responders to PD-1 targeted immunotherapy and a potential resource for antitumor immunity.

We use a mouse vaccination model to delineate by spectral flow cytometry and single-cell RNA sequencing the effects of CD4⁺ T-cell help during priming on the differentiation fate of stem-like CD8⁺ T cells. We use bioinformatic analysis to extrapolate our data to mouse models of cancer and chronic infection. We next explore CD8⁺ T-cell differentiation states and delivery of CD4⁺ T-cell help in the immunogenic MC38 tumor model.

Upon vaccination in the absence of help signals, stem-like CD8⁺ T cells do not further differentiate and accumulate in the draining lymph node. When help signals are delivered, stem-like CD8⁺ T cells proliferate and differentiate into circulating cytotoxic effector cells. Stem-like CD8⁺ T cells raised by vaccination in presence or absence of CD4⁺ T-cell help have an identical transcriptome, which they share with stem-like CD8⁺ T cells defined in mouse models of cancer and chronic infection. The immunogenic MC38 tumor harbors endogenous helper epitopes, but primes stem-like CD8⁺ T cells rather than helped cytotoxic effectors. Therapeutic vaccination with endogenous helper epitopes does not improve MC38 tumor control. Intratumoral expression of strong, exogenous helper epitopes as present in our vaccine improves tumor control, but does not efficiently convert stem-like tumor-specific CD8⁺ T cells into helped cytotoxic effectors.

Our data argue that stem-like CD8⁺ T cells are helpless cells that lie at the bifurcation point of CD8⁺ T-cell effector and exhaustion trajectories. Even though the immunogenic MC38 tumor expresses helper epitopes, it primarily raises stem-like CD8⁺ T cells, indicating that help delivery is impaired in this tumor context. Promoting the efficient delivery of help signals to stem-like tumor-specific CD8⁺ T cells to drive their expansion and differentiation into cytotoxic effectors is therefore an important therapeutic challenge in cancer and other conditions that lead to T-cell exhaustion.