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CD137-targeting immune therapy, which activates anti-tumor T effector cell responses, seems to be an attractive concept in clinical oncology. Recent evidence has demonstrated that tumor cells besides T cells and antigen-presenting cells are able to express CD137 and CD137L. Here we aimed to identify CD137/CD137L expression in established colon cancer cell lines and primary tumors (UICC stages I-IV) from patients with documented long-term follow-up. CD137/CD137L expression was highly upregulated in early to late-stage tumors while the inverse was observed in patient-derived peripheral blood mononuclear cells. High CD137L expression within primary tumors was mediated by tumor cells and significantly correlated with the occurrence of distant metastases and shortened survival in advanced stages of disease (UICC stage IV). Interestingly, induced tumor cell signaling via CD137L on its surface in vitro resulted in dual effects: (i) reduced tumor cell proliferation suggesting inhibitory signaling in all investigated cancers and (ii) increased epithelial-to-mesenchymal transition signaling events. Taken together CD137/CD137L expression was stage-dependently upregulated with shortened survival in patients with highly CD137L-expressing tumors. Our clinical and experimental data suggest that colon cancer cells predominantly express CD137L and thereby have negative impact on overall survival through a process of reverse signaling. Beside agonistic CD137 antibody therapy to foster T effector cell responses, CD137L-mediated intervention strategies may become instrumental to circumvent relapsed tumor growth through induced epithelial-to-mesenchymal transition and consecutive metastases formation.

Background: CD137L is expressed on antigen presenting cells and CD137/CD137L interaction co-stimulates activation and proliferation of CD137 expressing T cells. CD137L expression has also been detected on various cancer cells. While signaling in immune cells showed both inhibiting and activating effects the impact of CD137L induction on tumor cells is poorly understood. In this study, the expression of CD137 and CD137L and the effects of CD137L activation were analyzed in human colorectal cancer (CRC). Methods: Expression of CD137 and CD137L was examined in human colon cancer cell lines HT29, SW480 and SW620 treated with or without hypoxia using RT-qPCR, Western blot and FACS analysis. Its differentiated expression was further analyzed in human CRCs by immunofluorescent double-staining. To dissect the effects of CD137L activation on cancer cells, MTS proliferation assays were additionally performed. Results: All three human colon cancer cell lines demonstrated high cellular (FACS) and protein (Western Blot) CD137L expression. CD137 was found only in SW480 cells at weak expression. Additionally, both CD137 and CD137L were significantly upregulated in cells treated with hypoxia. In human CRCs both CD137 and CD137L expressing tumor cells were detected by immunofluorescent double-staining. Interestingly, activation of CD137L by immobilized CD137-Fc resulted in reduced cancer cell proliferation compared to IgG1-Fc controls. Conclusion: Our results demonstrate tumor cell mediated expression of the costimulatory molecule CD137 and its ligand CD137L in both colon cancer cell lines and human colorectal cancer. Moreover, CD137L reverse signaling was found to reduce cancer cell proliferation in vitro. Based on these data, the CD137/CD137L signaling pathway seems to present a promising target for cancer therapy in CRC. Note: This abstract was not presented at the meeting. Citation Format: Tanja Grimmig, Simone Callies, Hannes Naegele, Martin Wagner, Romana Moench, Buelent Polat, Christoph-Thomas Germer, Martin Gasser, Ana Maria Waaga-Gasser. Activation of CD137/CD137L results in reduced tumor cell proliferation in human colon cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5616. doi:10.1158/1538-7445.AM2017-5616