A study published in Nature Medicine (2019) investigates the clonal replacement of tumor-specific T cells following PD-1 blockade in patients with basal cell carcinoma (BCC) and squamous cell carcinoma (SCC). The research used paired single-cell RNA sequencing (scRNA) and T cell receptor (TCR) sequencing on 79,046 cells from pre- and post-treatment tumor samples. The findings reveal that PD-1 blockade leads to the expansion of CD8+ CD39+ T cells, which express markers of chronic activation and exhaustion. However, this expansion does not originate from pre-existing tumor-infiltrating lymphocytes (TILs) but rather from novel clonotypes not previously observed in the same tumor. Clonal replacement was preferentially observed in exhausted CD8+ T cells and was evident in both BCC and SCC patients. These results suggest that pre-existing tumor-specific T cells may have limited reinvigoration capacity, and that the T cell response to checkpoint blockade derives from a distinct repertoire of T cell clones that may have just recently entered the tumor. The study also found that clonal replacement of T cells is consistent with prior findings, including limited reinvigoration of exhausted T cells due to broad epigenetic remodeling, proliferation of CXCR5+ CD8+ T cells in lymphoid organs but not other tissues, and loss of anti-tumor T cell responses following chemical inhibition of T cell migration. The study did not identify the source of novel T cell clones, which could derive from tumor-extrinsic sources, including lymphoid organs, or rare unexpanded clones within the TME or tumor periphery. The results suggest that it may be feasible to monitor the clonal tumor-specific T cell response to checkpoint blockade in the blood, and that novel TIL clones may be recruited from peripheral sources. The study also highlights the importance of understanding the clonal T cell response to checkpoint blockade in human cancer, which has important implications for the design of checkpoint blockade immunotherapies.A study published in Nature Medicine (2019) investigates the clonal replacement of tumor-specific T cells following PD-1 blockade in patients with basal cell carcinoma (BCC) and squamous cell carcinoma (SCC). The research used paired single-cell RNA sequencing (scRNA) and T cell receptor (TCR) sequencing on 79,046 cells from pre- and post-treatment tumor samples. The findings reveal that PD-1 blockade leads to the expansion of CD8+ CD39+ T cells, which express markers of chronic activation and exhaustion. However, this expansion does not originate from pre-existing tumor-infiltrating lymphocytes (TILs) but rather from novel clonotypes not previously observed in the same tumor. Clonal replacement was preferentially observed in exhausted CD8+ T cells and was evident in both BCC and SCC patients. These results suggest that pre-existing tumor-specific T cells may have limited reinvigoration capacity, and that the T cell response to checkpoint blockade derives from a distinct repertoire of T cell clones that may have just recently entered the tumor. The study also found that clonal replacement of T cells is consistent with prior findings, including limited reinvigoration of exhausted T cells due to broad epigenetic remodeling, proliferation of CXCR5+ CD8+ T cells in lymphoid organs but not other tissues, and loss of anti-tumor T cell responses following chemical inhibition of T cell migration. The study did not identify the source of novel T cell clones, which could derive from tumor-extrinsic sources, including lymphoid organs, or rare unexpanded clones within the TME or tumor periphery. The results suggest that it may be feasible to monitor the clonal tumor-specific T cell response to checkpoint blockade in the blood, and that novel TIL clones may be recruited from peripheral sources. The study also highlights the importance of understanding the clonal T cell response to checkpoint blockade in human cancer, which has important implications for the design of checkpoint blockade immunotherapies.