Abstract 4080: Expansion of CX3CR1+ cytotoxic CD8+ T cells provides a rationale for IL-15 in combination cancer therapy with PD-1 antibody
Authors: Zhang Henan , Liu Xin , Cao Siyu , Chen Lingling , Harrington Susan , Li Ying , Mansfield Aaron , Park Sean , Yan Yiyi , Dronca Roxana , Dong Haidong 
Published: August 2019

Abstract
Introduction:
IL-15 has been used in clinical trials with PD-1 antibody to overcome cancer resistance to PD-1 monotherapy in cancer patients. However, it is not clear what T cell population would be expanded to mediate tumor control in IL-15 in combination with PD-1 antibody. We recently found CX3CR1+ cytotoxic CD8+ T cells, which increased in responders compared with non-responders after anti-PD-1 in melanoma patients, demonstrated an increased transcription of CD122 (IL2RB). Since CD122 is a subunit of IL-15 receptor, we hypothesized that IL-15 in combination with anti-PD-1 therapy may have the ability to expand CX3CR1+ cytotoxic CD8+ T cells for tumor control.
Methods and findings:
To examine whether IL-15 plus anti-PD-1 expand CX3CR1+ cytotoxic CD8+ T cells in tumor tissues, we treated tumor-bearing mice with IL-15/IL-15Rα complexes alone or with PD-1 antibody and measured the frequency of CX3CR1+Granzyme B+ CD8+ T cells. As only the combination therapy induced dramatic tumor regression, we found the highest increase of CX3CR1+Granzyme B+ CD8 T cells in tumor tissues in the group treated with IL-15 plus anti-PD-1 (28.87±3.19% vs. 7.91±1.32%, p&lt;0.01) compared with groups treated with either IL-15 alone or anti-PD-1 alone. In contrast to wild type mice, IL-15 plus anti-PD-1 did not suppress tumor growth in CX3CR1-KO mice. To determine whether IL-15 alone can also expand human CX3CR1+ cytotoxic CD8+ T cells, we cultured peripheral blood mononuclear cells (PBMCs) isolated from healthy donors (n = 5) with IL-15 for 24 hours in vitro. We found IL-15 (5 ng/mL) only modestly increased the frequency of CX3CR1+ Granzyme B+ CD8+ T cells than those cultured with no IL-15 (39.9±6.44% vs. 27.56±4.98%). To test whether this increase is due to enhanced proliferation of this subset of CD8+ T cells, we measured the expression of Ki-67, a cell proliferative marker, in CX3CR1+ CD8+ T cells by intranuclear staining. We found IL-15 modestly induced more proliferating CX3CR1+ CD8+ T cells in vitro culture compared with control group without IL-15 (6.25±0.54% vs. 0.91±0.25%).
Summary:
Taken together, we found IL-15 combined with anti-PD-1 therapy has the ability to significantly expand CX3CR1+ cytotoxic CD8+ T cells, however IL-15 alone may have limited ability to significantly expand this effector cell population. Our studies provide a new rational for combination therapy of IL-15 and PD-1 antibody to overcome cancer resistance for patients who did not respond to initial PD-1 therapy.
Citation Format: Henan Zhang, Xin Liu, Siyu Cao, Lingling Chen, Susan Harrington, Ying Li, Aaron Mansfield, Sean Park, Yiyi Yan, Roxana Dronca, Haidong Dong. Expansion of CX3CR1+ cytotoxic CD8+ T cells provides a rationale for IL-15 in combination cancer therapy with PD-1 antibody [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 4080.