CD28 engagement inhibits CD73-mediated regulatory activity of CD8 + T cells.

CD28 is required for T cell activation as well as the generation of CD4 ⁺ Foxp3 ⁺ Treg. It is unclear, however, how CD28 costimulation affects the development of CD8 ⁺ T cell suppressive function. Here, by use of Hepa1.6.gp33 in vitro killing assay and B16.gp33 tumor mouse model we demonstrate that CD28 engagement during TCR ligation prevents CD8 ⁺ T cells from becoming suppressive. Interestingly, our results showed that ectonucleotidase CD73 expression on CD8 ⁺ T cells is upregulated in the absence of CD28 costimulation. In both murine and human tumor-bearing hosts, CD73 is upregulated on CD28 ^- CD8 ⁺ T cells that infiltrate the solid tumor. UPLC-MS/MS analysis revealed that CD8 ⁺ T cells activation without CD28 costimulation produces elevated levels of adenosine and that CD73 mediates its production. Adenosine receptor antagonists block CD73-mediated suppression. Our data support the notion that CD28 costimulation inhibits CD73 upregulation and thereby prevents CD8 ⁺ T cells from becoming suppressive. This study uncovers a previously unidentified role for CD28 costimulation in CD8 ⁺ T cell activation and suggests that the CD28 costimulatory pathway can be a potential target for cancer immunotherapy.