Programmed Death-1 (PD-1) is a co-inhibitory receptor that down-regulates the activity of tumor-infiltrating lymphocytes (TIL) in cancer and of virus-specific T cells in chronic infection. The molecular mechanisms driving high PD-1 expression on TIL have not been fully investigated. We demonstrate that transforming growth factor-β1 (TGF-β1) directly enhances antigen-induced PD-1 expression through Smad3-dependent, Smad2-independent transcriptional activation in T cells in vitro and in TIL in vivo. The PD-1hi subset seen in CD8+ TIL is absent in Smad3-deficient tumor-specific CD8+ TIL, resulting in enhanced cytokine production by TIL and in draining lymph nodes and of anti-tumor activity. In addition to TGF-β1’s previously known effects on T cell function, our findings suggest that TGF-β1 mediates T cell suppression via PD-1 upregulation in the TME. They highlight bidirectional crosstalk between effector TIL and TGF-β-producing cells that upregulates multiple components of the PD-1 signaling pathway to inhibit anti-tumor immunity.
In the proper context, radiotherapy can promote antitumor immunity. It is unknown if elective nodal irradiation (ENI), a strategy that irradiates tumor-associated draining lymph nodes (DLN), affects adaptive immune responses and combinatorial efficacy of radiotherapy with immune checkpoint blockade (ICB). We developed a preclinical model to compare stereotactic radiotherapy (Tumor RT) with or without ENI to examine immunologic differences between radiotherapy techniques that spare or irradiate the DLN. Tumor RT was associated with upregulation of an intratumoral T-cell chemoattractant chemokine signature (CXCR3, CCR5-related) that resulted in robust infiltration of antigen-specific CD8 effector T cells as well as FoxP3 regulatory T cells (Tregs). The addition of ENI attenuated chemokine expression, restrained immune infiltration, and adversely affected survival when combined with ICB, especially with anti-CLTA4 therapy. The combination of stereotactic radiotherapy and ICB led to long-term survival in a subset of mice and was associated with favorable CD8 effector-to-Treg ratios and increased intratumoral density of antigen-specific CD8 T cells. Although radiotherapy technique (Tumor RT vs. ENI) affected initial tumor control and survival, the ability to reject tumor upon rechallenge was partially dependent upon the mechanism of action of ICB; as radiotherapy/anti-CTLA4 was superior to radiotherapy/anti-PD-1. Our results highlight that irradiation of the DLN restrains adaptive immune responses through altered chemokine expression and CD8 T-cell trafficking. These data have implications for combining radiotherapy and ICB, long-term survival, and induction of immunologic memory. Clinically, the immunomodulatory effect of the radiotherapy strategy should be considered when combining stereotactic radiotherapy with immunotherapy. .
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