Adenosinergic Regulation of the Expansion and Immunosuppressive Activity of CD11b+Gr1+ Cells

Abstract

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 A_2B adenosine receptors on hematopoietic cells play an important role in accumulation of intratumoral CD11b⁺Gr1^high cells in a mouse Lewis lung carcinoma model in vivo and demonstrated that these receptors promote preferential expansion of the granulocytic CD11b⁺Gr1^high subset of MDSCs in vitro. Flow cytometry analysis of MDSCs generated from mouse hematopoietic progenitor cells revealed that the CD11b⁺Gr-1^high subset had the highest levels of CD73 (ecto-5′-nucleotidase) expression (Δmean fluorescence intensity [MFI] of 118.5 ± 16.8), followed by CD11b⁺Gr-1^int (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-1^high 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.