The purine nucleoside adenosine (9-β-d-ribofuranosyladenine) inhibits a number of lymphocyte functions in vitro, including the ability of activated T lymphocytes and natural killer cells to adhere to and kill tumor targets. Solid tumors, such as adenocarcinomas of the lung and colon, are frequently hypoxic and are, therefore likely to exhibit increased adenine nucleotide breakdown through the 5′-nucleotidase pathway, yielding adenosine. We examined whether the concentration of adenosine in the extracellular fluid of such tumors is adequate to cause immunosuppression. Murine tumors grown in syngeneic hosts or human tumors grown in immunodeficient nu/nu mice were subjected to microdialysis, and adenosine levels in the microdialysate were measured by high-performance liquid chromatography. Treatment of the tumor microdialysates with adenosine deaminase eliminated the adenosine peak. Recovery of adenosine ranged from 15 to 29%, depending on the microdialysis probe, and concentrations of adenosine in tumors ranged from 0.2 to 2.4 µm with a mean of 0.5 µm. In contrast, the adenosine concentration measured s.c. at the same location was 30 ± 5 nm (mean ± SE). Inclusion of the adenosine deaminase inhibitor coformycin (10 µm) and the adenosine kinase inhibitor 5′-iodotubercidin (0.1 µm) in the microdialysis perfusion buffer increased extracellular adenosine concentration in tumors to as high as 13 µm. These data show that extracellular adenosine levels in solid tumors are sufficient to suppress the local antitumor immune response and that interference with pathways of adenosine metabolism causes marked increases in tumor extracellular adenosine concentration.
This research was supported by a grant from the Dalhousie Faculty of Medicine Fund for Cancer Research (to J. B. and D. W. H.).