Abstract 526: Tumor-draining lymph node irradiation reduces tumor-infiltrating stem-like CD8+T-cells and abrogates the abscopal effect
Authors: Buchwald Zachary S. , Nasti Tahseen H. , Eberhardt Christiane S. , Wieland Andreas , Lawson David , Curran Walter , Ahmed Rafi , Khan Mohammad K. 
Published: August 2019

Abstract
Purpose: PD-1 and PD-L1 antagonists are efficacious because cancer induces T-cell exhaustion via upregulation of PD-L1 and persistent antigen exposure. Our lab, has shown that the proliferative burst following αPD-L1 therapy of exhausted CD8+PD-1+T-cells is restricted to a “stem-like” CD8+T-cell subset in a murine chronic viral infection model. The role of these stem-like CD8+T-cells in malignancies and whether these cells respond to other immuno-stimulation including radiotherapy (RT) is unknown. RT’s immuno-stimulation includes acting as an in-situ vaccine by liberating cryptic tumor neo-antigens, generating a potent anti-tumor CD8+T-cell response, synergizing with αPD-1/L1 and leading to control at distant sites of disease outside the radiation field (abscopal effect). In initial studies, we found these stem-like CD8+T-cells in B16F10 tumors and at high frequencies in the tumor-draining lymph nodes (TDLN) of immunocompetent mice. Here, we investigated: (1) the impact of tumor-directed RT on this stem-like CD8+T-cell population in the tumor and TDLN in the context of an abscopal response, and (2) due to the high frequency of stem-like CD8+T-cells in the TDLN, whether depletion of lymphocytes from the TDLN via RT influenced the abscopal effect.
Experimental Design: We developed a preclinical model of oligo-metastatic melanoma to evaluate the role of stem-like CD8+ T-cells and the TDLN in the abscopal effect. This was done with unilateral tumor-directed RT with or without TDLN-directed RT. This was also done in the presence and absence of αPD-L1.
Results: Tumor-directed RT improved local tumor control and induced an abscopal response with a concomitant increase in tumor infiltrating tumor-specific stem-like CD8+T-cells. Tumor-specific stem-like CD8+T-cells were also observed in the TDLN of tumors on both sides. Importantly, the tumor-directed RT increased tumor-specific T-cell proliferation in the TDLNs bilaterally despite only being targeted at one tumor. Given this robust proliferative response and the high frequency of stem-like CD8+T-cells in the TDLN, we next evaluated the role of the TDLN in mediating the abscopal effect. We found that the abscopal effect is impaired if the TDLN is lymphocyte depleted with TDLN-directed RT in the presence or absence of αPD-L1. Additionally, the tumor-directed RT mediated increase in stem-like T-cells in the irradiated and unirradiated tumor was abrogated with TDLN-directed RT.
Conclusion: Our results demonstrate slowed distant tumor growth following irradiation of a local site and that this correlates with an increase in tumor-infiltrating stem-like CD8+T-cells which is reduced with TDLN-directed RT. These data suggest a critical role for both the stem-like CD8+T-cells and the TDLN in mediating the abscopal effect.
Citation Format: Zachary S. Buchwald, Tahseen H. Nasti, Christiane S. Eberhardt, Andreas Wieland, David Lawson, Walter Curran, Rafi Ahmed, Mohammad K. Khan. Tumor-draining lymph node irradiation reduces tumor-infiltrating stem-like CD8+T-cells and abrogates the abscopal effect [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 526.