Hypoxia-induced soluble CD137 in malignant cells blocks CD137L-costimulation as an immune escape mechanism

ABSTRACT

Hypoxia is a common feature in solid tumors that has been implicated in immune evasion. Previous studies from our group have shown that hypoxia upregulates the co-stimulatory receptor CD137 on activated T lymphocytes and on vascular endothelial cells. In this study, we show that exposure of mouse and human tumor cell lines to hypoxic conditions (1% O₂) promotes CD137 transcription. However, the resulting mRNA is predominantly an alternatively spliced form that encodes for a soluble variant, lacking the transmembrane domain. Accordingly, soluble CD137 (sCD137) is detectable by ELISA in the supernatant of hypoxia-exposed cell lines and in the serum of tumor-bearing mice. sCD137, as secreted by tumor cells, is able to bind to CD137-Ligand (CD137L). Our studies on primed T lymphocytes in co-culture with stable transfectants for CD137L demonstrate that tumor-secreted sCD137 prevents co-stimulation of T lymphocytes. Such an effect results from preventing the interaction of CD137L with the transmembrane forms of CD137 expressed on T lymphocytes undergoing activation. Indeed, silencing CD137 with shRNA renders more immunogenic tumor-cell variants upon inoculation to immunocompetent mice but which readily grafted on immunodeficient or CD8⁺ T-cell-depleted mice. These mechanisms are interpreted as a molecular strategy deployed by tumors to repress lymphocyte co-stimulation via CD137/CD137L.

KEYWORDS:

• Cancer
• hypoxia
• immunotherapy
• 4-1BB (CD137)

Disclosure of potential conflicts of interest

IM is a consultant for Bristol-Myers Squibb, Merck Serono, Boehringer Ingelheim, Roche-Genentech, Takeda and Miltenyi Biotec. MJK is a Full time employee in Bristol-Myers Squibb.

Acknowledgments

We are grateful to Dr Paul Miller for English editing and to Dr Ana Rouzaut for fruitful scientific discussions and critical reading. Eneko Elizalde is acknowledged for excellent animal facility work.

Funding

Financial support was from MICINN (SAF2008-03294, SAF2011-22831 to IM). IM was also funded by the Departamento de Educación del Gobierno de Navarra and Departamento de Salud del Gobierno de Navarra, Redes temáticas de investigación cooperativa RETICC (RD06/0020/0065), European commission 7th framework program (ENCITE and IACT), and “UTE for project FIMA’’. PB is a recipient of a Miguel Servet contract from ISCIII. AM-K and SL are recipients of pre-doctoral scholarships from Ministerio de Economia.

Author contributions

Conception and design: SL, AP, IM, Development of methodology: SL, AP, EB, AA, AR, AMK, JIQ, AG, MRR and MAA, Acquisition of data: SL, AP, EB, AA, Provided critical reagents and transgenic mice: MJK, PB, Analysis and interpretation of data: SL, IM, AP, PB, MJK, JLP and AG, Writing, review and/or revision of the manuscript: SL, IM.