Reciprocal transmission of activating and inhibitory signals and cell fate in regenerating T cells.

The ability of activated progenitor T cells to self-renew while producing differentiated effector cell descendants may underlie immunological memory and persistent responses to ongoing infection. The nature of stem-like T cells responding to cancer and during treatment with immunotherapy is not clear. The subcellular organization of dividing progenitor CD8+ T cells from mice challenged with syngeneic tumors is examined here. Three-dimensional microscopy reveals an activating hub composed of polarized CD3, CD28, and phosphatidylinositol 3-kinase (PI3K) activity at the putative immunological synapse with an inhibitory hub composed of polarized PD-1 and CD73 at the opposite pole of mitotic blasts. Progenitor T cells from untreated and inhibitory checkpoint blockade-treated mice yield a differentiated TCF1- daughter cell, which inherits the PI3K activation hub, alongside a discordantly fated, self-renewing TCF1+ sister cell. Dynamic organization of opposite activating and inhibitory signaling poles in mitotic lymphocytes may account for the enigmatic durability of specific immunity.

PD-1 blockade increases the self-renewal of stem-like CD8 T cells to compensate for their accelerated differentiation into effectors.

PD-1+TCF-1+ stem-like CD8 T cells act as critical resource cells for maintaining T cell immunity in chronic viral infections and cancer. In addition, they provide the proliferative burst of effector CD8 T cells after programmed death protein 1 (PD-1)-directed immunotherapy. However, it is not known whether checkpoint blockade diminishes the number of these stem-like progenitor cells as effector cell differentiation increases. To investigate this, we used the mouse model of chronic lymphocytic choriomeningitis virus (LCMV) infection. Treatment of chronically infected mice with either αPD-1 or αPD-L1 antibody not only increased effector cell differentiation from the virus-specific stem-like CD8 T cells but also increased their proliferation so their numbers were maintained. The increased self-renewal of LCMV-specific stem-like CD8 T cells was mTOR dependent. We used microscopy to understand the division of these progenitor cells and found that after PD-1 blockade, an individual dividing cell could give rise to a differentiated TCF-1- daughter cell alongside a self-renewing TCF-1+ sister cell. This asymmetric division helped to preserve the number of stem-like cells. Moreover, we found that the PD-1+TCF-1+ stem-like CD8 T cells retained their transcriptional program and their in vivo functionality in terms of responding to viral infection and to repeat PD-1 blockade. Together, our results demonstrate that PD-1 blockade does not deplete the stem-like population despite increasing effector differentiation. These findings have implications for PD-1-directed immunotherapy in humans.

Self-Renewing CD8+ T-cell Abundance in Blood Associates with Response to Immunotherapy.

Treatment with immune checkpoint blockade (ICB) often fails to elicit durable antitumor immunity. Recent studies suggest that ICB does not restore potency to terminally dysfunctional T cells, but instead drives proliferation and differentiation of self-renewing progenitor T cells into fresh, effector-like T cells. Antitumor immunity catalyzed by ICB is characterized by mobilization of antitumor T cells in systemic circulation and tumor. To address whether abundance of self-renewing T cells in blood is associated with immunotherapy response, we used flow cytometry of peripheral blood from a cohort of patients with metastatic non-small cell lung cancer (NSCLC) treated with ICB. At baseline, expression of T-cell factor 1 (TCF1), a marker of self-renewing T cells, was detected at higher frequency in effector-memory (CCR7-) CD8+ T cells from patients who experienced durable clinical benefit compared to those with primary resistance to ICB. On-treatment blood samples from patients benefiting from ICB also exhibited a greater frequency of TCF1+CCR7-CD8+ T cells and higher proportions of TCF1 expression in treatment-expanded PD-1+CCR7-CD8+ T cells. The observed correlation of TCF1 frequency in CCR7-CD8+ T cells and response to ICB suggests that broader examination of self-renewing T-cell abundance in blood will determine its potential as a noninvasive, predictive biomarker of response and resistance to immunotherapy.

Cutting Edge: Promoting T Cell Factor 1+ T Cell Self-Renewal to Improve Programmed Cell Death Protein 1 Blockade.

Immune checkpoint blockade is limited by resistance to treatment, with many patients not achieving durable antitumor responses. Self-renewing (T cell factor 1+ [TCF1+]) CD8+ T cells have recently been implicated in efficacy of anti-programmed cell death protein 1 (anti-PD-1). Mice challenged with syngeneic tumors were treated with anti-PD-1 and/or a reversible inhibitor of PI3K δ, designed to promote T cell self-renewal. Growth of tumors in untreated mice was characterized by waning proportions of TCF1+ T cells, suggesting self-renewing T cells become limiting for successful immunotherapy. Higher proportions of TCF1+ T cells in tumor and blood correlated with better control of tumor growth. Combining anti-PD-1 and inhibitor of PI3K δ conferred superior protection compared with either monotherapy and was associated with higher frequency of TCF1+ T cells in tumor and blood compared with anti-PD-1 alone. These findings reveal predictive importance of self-renewing T cells in anti-tumor immunity and suggest that resistance-directed strategies to enhance T cell self-renewal could potentiate the efficacy of PD-1 blockade.