Adenosine limits the therapeutic effectiveness of anti-CTLA4 mAb in a mouse melanoma model(1)
The co-inhibitory receptor Cytotoxic T Lympho- cyte Antigen-4 (CTLA-4) interacts with members of the B7 family on antigen-presenting cells (APCs) modulates T-cell activation. Monoclonal antibodies (mAbs) that block CTLA- 4 enhance T-cell proliferation and activation and induce long-term regression of melanoma.
Adenosine is an ATP-derived nucleoside, produced in the extracellular compartment by two ectonucleotidases: CD39, which hydrolyzes ATP and ADP into AMP, and CD73, which converts AMP into adenosine. Adenosine is known to inhibit T-cell proliferation and reduce cytokine production and cytotoxicity of activated T-cells, via A2a receptor subtype activation, protecting the tumour from immune-mediated destruction.
Melanoma-bearing mice were treated with the selective CD73 inhibitor APCP (400 μg/mouse, p.t.), selective inhibitor of the adenosine-generating nucleotidase CD73, and/or anti-CTLA4 mAb (100 μg/ mouse, i.p.). Inhibition of CD73 with APCP in the tumor tissue significantly reduced melanoma growth. Anti-CTLA4 mAb did not affect tumor growth in the B16.F10 melanoma model. Mice treated with both APCP + anti-CTLA4 mAb displayed significantly decreased tumor growth compared with control, and APCP or anti-CTLA4 alone. Cytokine analysis revealed increased levels of IFN-γ in melanoma tissue of mice treated with APCP or APCP in combination with anti-CTLA4 mAb compared to control or anti-CTLA4 mAb alone.
Melanoma-bearing mice treated with A2aR antagonist, ZM241365 (40 μg/ mouse, p.t.) alone showed a marked tumor growth inhibition compared with controls. The combination therapy with anti-CTLA4 therapy showed significant tumor growth delay compared with control or either agent alone. Cytokine analysis showed that the levels of both IFN-γ and granzyme B were elevated in tumor tissue after combination therapy with ZM241365 and anti-CTLA4 mAb.
The peritumoral administration of the selective agonist of A3R, Cl-IB-MECA, significantly inhibits tumor growth in melanoma-bearing mice. In contrast with the promising results of the combinations of APCP or ZM241365 with anti-CTLA4 mAb, the combination of Cl-IB-MECA with anti-CTLA4 mAb did not cause any additional benefit in limiting melanoma growth compared with Cl-IB-MECA alone.
In other mouse tumor models, blockade of CD73 enhances the anti-tumor activity of anti-CTLA4 and anti-PD-1 mAbs. Adenosine generation by CD73 mediates immune suppression, mainly mediated by the activation of A2aR, which has the highest affinity for adenosine and is up-regulated on effector T-cells. A2aR-deficient mice reject tumor cells in a T-cell-dependent manner and show increased responsiveness to T-cell adoptive transfer and tumor vaccination. A2bR activation causes the release of pro- angiogenic factors that facilitate tumor progression. Targeting A2aR, an inhibitory receptor on T-cells, rather than A3R in tumor stroma may be a promising strategy to increase the effectiveness of CTLA4 mAb in melanoma.