Team TIME

Keeping something in reserve

CD8 T cell responses are dynamic processes that are regulated by the signals that T cells receive from their environment. In the context of acute responses, these signals are delivered around the time of initial priming by the dendritic cell, but in chronic responses like cancer, T cells continue to receive signals from their environment and this can drive their terminal differentiation. One critical signal for this is the signal from antigen, which both drives proliferation and differentiation. Unfortunately for the host, unchecked proliferation and function of T cells can lead to lethal immunopathology, so T cell responses are highly regulated through both the expression of inhibitory “checkpoint” receptors on T cells and by the trade-off of proliferative capacity for effector function & migration into tissues that accompanies terminal differentiation. These mechanisms largely prevent lethal immunopathology, but can hinder potentially effective response against cancer.

Not all T cells are thought to reach the terminal effector stage in an immune response. Indeed, in most cases, a subset of less-differentiated stem-like T cells can be identified that retain high proliferative and functional capacity. These T cells are often quiescent, likely due to the need to protect these cells signals that would promote their terminal differentiation. Absent this protection, the entire antigen-specific CD8 T cell pool could become terminally differentiated, which would yield a “hole” in the T cell repertoire that future pathogens could exploit. This phenomenon is observed in extreme cases of chronic infection (e.g., CD4-deficient Clone 13 infected mice), but remains poorly understood. Given the potential for chronic antigen exposure to “exhaust” entire populations of antigen-specific CD8 T cells, we wondered how T cell responses are sustained in the tumor microenvironment, which contains high amounts of these differentiation-promoting signals.

Using the KP-NINJA mouse, we analyzed the differentiation of tumor-specific CD8 T cells as tumors developed and noted that the overall population became more differentiated over the five months of tumor development, but there was also a sustained population of less differentiated T cells at all timepoints analyzed. This suggested a mechanism for maintaining this population. To uncover this mechanism, we looked outside the tumor at the tumor-draining lymph node and found a large population of stem-like tumor-specific CD8 present in even late tumor-bearing animals. Single cell RNA sequencing showed that this population was remarkably stable over time and, using shared TCR clonality and motif analysis, we found evidence that the same stem-like T cells themselves may be maintained throughout the course of tumor development. We also found that migration of stem-like T cells from the lymph node to the tumors was required to maintain the less differentiated T cells in the tumor. Finally, through single cell analysis of CD8 T cells in human lung cancer patients, we found less-differentiated stem-like populations were enriched in lung-draining, non-metastatic lymph nodes.

Thus, our data supported the idea that tumor-draining lymph nodes contained a reservoir of stem-like CD8 T cells that was important in the maintenance of T cell responses over the course of tumor development.

This work has many implications for the field, including the fact that relevant T cell populations may be located outside of the tumor microenvironment and that these populations may be relevant for therapy. Work from other groups has supported the hypothesis that these cells are present in patients, although whether these cells are engaged by current standard of care immunotherapies is less clear. Ongoing work in the lab focuses on studying the biology of stem-like T cells in tumors, tumor-associated TLS and in tumor-draining lymph nodes.