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Extracellular adenosine and purine nucleotides are elevated in many pathological situations associated with the expansion of CD11b+Gr1+ myeloid-derived suppressor cells (MDSCs). Therefore, we tested whether adenosinergic pathways play a role in MDSC expansion and functions. We found that A2B adenosine receptors on hematopoietic cells play an important role in accumulation of intratumoral CD11b+Gr1high cells in a mouse Lewis lung carcinoma model in vivo and demonstrated that these receptors promote preferential expansion of the granulocytic CD11b+Gr1high subset of MDSCs in vitro. Flow cytometry analysis of MDSCs generated from mouse hematopoietic progenitor cells revealed that the CD11b+Gr-1high subset had the highest levels of CD73 (ecto-5′-nucleotidase) expression (Δmean fluorescence intensity [MFI] of 118.5 ± 16.8), followed by CD11b+Gr-1int (ΔMFI of 57.9 ± 6.8) and CD11b+Gr-1−/low (ΔMFI of 12.4 ± 1.0) subsets. Even lower levels of CD73 expression were found on Lewis lung carcinoma tumor cells (ΔMFI of 3.2 ± 0.2). The high levels of CD73 expression in granulocytic CD11b+Gr-1high cells correlated with high levels of ecto-5′-nucleotidase enzymatic activity. We further demonstrated that the ability of granulocytic MDSCs to suppress CD3/CD28-induced T cell proliferation was significantly facilitated in the presence of the ecto-5′-nucleotidase substrate 5′-AMP. We propose that generation of adenosine by CD73 expressed at high levels on granulocytic MDSCs may promote their expansion and facilitate their immunosuppressive activity.

In our recent publication in The Journal of Immunology , we reported that the granulocytic CD11b+Gr1high subset of myeloid-derived suppressor cells (MDSCs) is significantly decreased in tumors growing in animals lacking A2B adenosine receptors, suggesting that this adenosine receptor subtype plays a