Granzyme B-expressing treg cells are enriched in colorectal cancer and present the potential to eliminate autologous T conventional cells

In addition to expressing inhibitory cytokines and suppressive molecules, Treg cells could downplay inflammation by releasing cytotoxic molecules and eliminating proinflammatory immune cells. Colorectal cancer (CRC) is a common malignancy that has led to many cancer-related deaths. In this study, we investigated the cytotoxic aspect of Treg cells in CRC patients. Data showed that tumor-infiltrating FOXP3+ Treg cells expressed granzyme B immediately following resection, indicating that granzyme B-expressing Treg cells were present directly ex vivo. In the tumor-associated lymph nodes (LNs) and circulating lymphocytes, however, granzyme B-expressing Treg cells were only scarcely found. We then attempted to stimulate granzyme B expression in circulating Treg cells. Granzyme B upregulation in Treg cells could not be activated by standard T cell receptor (TCR) activation through anti-CD3/CD28 and IL-2 but required stimulation with bacterial products, such as with heat-killed Staphylococcus aureus. Interestingly, granzyme B expression was highly concentrated in TIM-3+ Treg cells, a Treg subset previously shown to be enriched in the tumor microenvironment and presented increased suppressive capacity. These TIM-3+ Treg cells presented higher cytolytic capacity toward autologous T conventional cells than the TIM-3- Treg cells, in a manner that was dependent on granzyme B but not TIM-3. Overall, we found that granzyme B-expressing Treg cells were enriched in the tumors from CRC patients and had the potential to eliminate autologous T conventional cells.