Certain anticancer treatments have been revolutionized by the ability to harness a person’s own immune cells for therapeutic purposes1. Such immunotherapy can result in lasting anticancer responses in people with advanced-stage blood cancers or solid tumours. But not everyone responds. For a variety of cancers, the presence of cytotoxic T cells — immune cells that can kill cancer cells — in a tumour correlates with, but does not predict, an anticancer response and survival. And it is unclear why robust tumour infiltration by T cells occurs in some people, but not in others. Writing in Nature, Jansen et al.2 reveal a previously unknown source of tumour-infiltrating T cells.
Because tumour cells can proliferate continuously, tumour-targeting T cells must have a similar ability to persist and divide until the last remaining tumour cell is eradicated. In people undergoing immunotherapy, a greater longevity of antitumour T cells correlates with a better therapeutic outcome3. Therefore, for effective immunotherapy, it is crucial to understand the factors that influence T-cell persistence and infiltration of tumours. Some clues already exist4 about these factors, such as the presence of long versions of structures called telomeres, found at the ends of chromosomes, and high expression levels of the protein CD27 in T cells.
In addition to these factors, another clue came from the identification of a subset of stem-cell-like T cells called memory T cells, which can provide long-lasting immune responses5,6, and which express high levels of TCF7 (previously known as TCF-1). This protein is important for maintaining a stem-cell-like state in T cells that also express the protein CD8 (known as CD8 T cells)7,8. Such stem-cell-like cells can self-renew and give rise to different types of T cell, including a type of CD8 T cell called a cytotoxic CD8 T cell. The presence of stem-cell-like T cells in people who have cancer was reported previously5; however, the anatomical location of these cells had not been elucidated. Jansen and colleagues now show that human kidney tumours contain stem-cell-like T cells that reside in the tumour in niches that support them (Fig. 1).
The authors investigated how the level of tumour-infiltrating cytotoxic CD8 T cells varied. They analysed samples of human kidney tumours obtained from people who had undergone tumour-removal surgery, and noted a wide variation in the level of T-cell infiltration between the samples. In people who had tumours in which CD8 T cells accounted for fewer than 2.2% of cells in the sample, the cancer continued to grow, indicating that the surgery and the person’s immune response to the residual cancer cells were insufficient to halt disease progression. By contrast, above this threshold of 2.2% infiltration, cancer growth after surgery was four times slower.
Jansen and colleagues then studied the composition and type of T cell in the tumour samples using a technique called flow cytometry, and identified two distinct sets of T cell. One set consisted of cytotoxic CD8 T cells that express high levels of cancer-killing molecules but that also express ‘immune-checkpoint’ molecules. Expression of checkpoint molecules can drive cytotoxic T cells to enter a dysfunctional state known as exhaustion, which can occur in the tumour microenvironment after prolonged exposure to cancer cells recognized by the T cells. The other set consisted of stem-cell-like T cells that Jansen et al. demonstrate give rise to cytotoxic CD8 T cells that help to promote an effective antitumour immune response. Stem-cell-like T cells were present only at very low levels in tumours with low levels of T-cell infiltration, whereas tumours with high levels of T-cell infiltration had high levels of the stem-cell-like T cells.
To gain further insight, the authors assessed cellular gene-expression profiles, and analysed epigenetic modifications — types of modification to DNA and its associated proteins that can affect gene expression. They found that, compared with the exhausted cytotoxic CD8 T cells, the stem-cell-like T cells express distinctive immune-signalling molecules called chemokines that are correlated with better patient survival, along with higher levels of key co-stimulatory molecules (which are essential for T-cell differentiation into cytotoxic T cells). Previous analyses9,10 of T cells revealed a pattern of progressive steps in epigenetic modification as stem-cell-like T cells give rise to cytotoxic CD8 T cells and then eventually become exhausted.
The epigenetic-modification profile of T cells in tumours can be profoundly influenced by factors in the tumour microenvironment, which can affect the ability of T cells to function as stem cells11,12. For example, the concentration of potassium ions in a tumour modulates epigenetic modifications that influence whether T cells are in the stem-cell-like state that is needed for them to give rise to cytotoxic CD8 T cells11,12. The effect of the tumour microenvironment on the development of cancer-targeting T cells is unclear, and should be a subject for future studies.
Jansen and colleagues noted that the higher than normal expression of chemokines and chemokine-binding receptors in the stem-cell-like T cells is similar to that seen in cells in the microenvironment of lymph vessels — structures through which immune cells move and which support T-cell activation and survival. The authors’ analyses demonstrate that stem-cell-like T cells are located in niches in tumours near lymph vessels (Fig. 1), and are confined to dense zones of antigen-presenting cells, which can prime T cells to target tumours. The discovery of these niches by Jansen and colleagues now reveal how stem-cell-like T cells can be maintained in tumours in a functional state capable of generating cytotoxic T cells.
The authors observed a correlation between the presence of protein markers of stem-cell-like T-cell niches and longer, progression-free survival of the people assessed in the study. By contrast, other common ways of assessing an immune response in tumours, such as the expression of the immune-checkpoint protein PD-L1, did not reveal a correlation with progression-free cancer survival.
Previous research13 identified stem-cell-like T cells that express rising levels of immune-checkpoint molecules as they progress towards forming cytotoxic CD8 T cells that eventually become exhausted14. In one example13, approaches to block the immune-checkpoint protein PD-1 caused a burst of proliferation in stem-cell-like T cells that express the TCF7 protein. Similarly, in a skin cancer called melanoma, people whose CD8 T cells express TCF7 have a better clinical outcome if they receive immunotherapy to block immune-checkpoint proteins15. These results suggest that people whose tumours cannot be removed by surgery might benefit from therapy that blocks immune-checkpoint molecules, if their tumours contain stem-cell-like T cells.
Jansen and colleagues’ work raises questions about how the stem-cell niches are generated and maintained, and whether tumours might act on them to evade destruction by the immune system. The discovery that resident stem-cell-like T cells exist in specialized niches in tumours suggests that clinical leveraging of such cells to increase the immune infiltration of tumours, together with immunotherapy to boost exhausted T cells, might unleash T-cell responses to aid the success of anticancer treatment.
Nature 576, 385-386 (2019)
Competing Financial Interests
The authors are employees of Lyell Immunopharma and hold equity.