Improving the efficacy of immune checkpoint therapies will require a better understanding of how immune cells are recruited and sustained in tumors. Here, we used the photoconversion of the tumor immune cell compartment to identify newly entering lymphocytes, determine how they change over time, and investigate their egress from the tumor. Combining single-cell transcriptomics and flow cytometry, we found that while a diverse mix of CD8 T cell subsets enter the tumor, all CD8 T cells retained within this environment for more than 72 h developed an exhausted phenotype, revealing the rapid establishment of this program. Rather than forming tumor-resident populations, non-effector subsets, which express TCF-1 and include memory and stem-like cells, were continuously recruited into the tumor, but this recruitment was balanced by concurrent
egress to the tumor-draining lymph node. Thus, the TCF-1+ CD8 T cell niche in tumors is highly dynamic, with the circulation of cells between the tumor and peripheral lymphoid tissue to bridge systemic and intratumoral responses.
Introduction
T cell infiltration is predictive of patient survival and the response to immunotherapy for many cancers (Azimi et al., 2012; Galon et al., 2006; Herbst et al., 2014; Naito et al., 1998; Tumeh et al., 2014). A defining feature of intratumoral T cells is an altered or exhausted phenotype, first characterized in chronic viral infections (Wherry et al., 2007), where persistent exposure to antigen (Ag) results in defective T cell responses (Gallimore
et al., 1998; Zajac et al., 1998). Compared with conventional effector cells, exhausted CD8 T cells have impaired functions, highly express multiple inhibitory receptors including PD-1,
LAG-3, and TIM-3, and have distinct transcriptional and metabolic profiles (McLane et al., 2019). Crucially, antibodies (Abs) against PD-1 reinvigorated the CD8 T cell response during
chronic lymphocytic choriomeningitis virus (LCMV) infection demonstrating that exhaustion could be overcome (Barber et al., 2006; Pauken et al., 2016), and preclinical cancer models established the efficacy of targeting the PD-1:PD-L1 pathway to limit tumor growth (Blank et al., 2004; Iwai et al., 2005). Targeting the PD-1:PD-L1 pathway or CTLA-4 to enhance anti-tumor T cell activity, termed immune checkpoint blockade (ICB) therapy, results in striking clinical responses in some tumors including melanoma, renal cell carcinoma, and non-small cell lung cancer (Brahmer et al., 2012; Hodi et al., 2010; Topalian et al., 2012). However, only a minority of patients robustly and durably respond to these therapies, and while combined targeting of PD-1 and CTLA-4 enhances tumor regression, it is also associated with significantly more adverse events (Chae et al., 2018; Curran et al., 2010; Postow et al., 2015).
While T cell infiltration predicts responsiveness to ICB (Daud et al., 2016; Jiang et al., 2018; Teng et al., 2015), the origin and fate of tumor-infiltrating lymphocytes (TILs) and how specific therapies impact this remain incompletely understood. Analysis of chromatin accessibility revealed two dysfunctional CD8 T cell states, an early “plastic” state and a second more “locked” state that then persisted and was resistant to reprogramming (Philip
et al., 2017). The effector T cell response appears to be sustained by a “stem-like” population of TCF-1+ PD-1+ CD8 T cells that are required for tumor control in response to immunotherapy (Brummelman et al., 2018; He et al., 2016; Im et al., 2016; Jansen
et al., 2019; Kurtulus et al., 2019; Sade-Feldman et al., 2018; Siddiqui et al., 2019; Utzschneider et al., 2016). While considered resident within lymphoid tissues following LCMV infection (Im et al., 2020), the presence of TCF-1+ PD-1+ cells within tumors indicates different migratory potential during anti-tumor responses. Notably, these cells were not evenly distributed throughout human tumors; rather, they were principally located within a specific niche (Jansen et al., 2019). Whether TCF-1+ PD-1+ CD8 T cells become resident within tumors is unclear; however, current data indicate that intratumoral expansion of
TCF-1+ PD-1+ CD8 T cells is critical for tumor regression.
