Extracellular purine metabolism is the switchboard of immunosuppressive macrophages and a novel target to treat diseases with macrophage imbalances

If misregulated, macrophage (M)T cell interactions can drive chronic inflammation thereby causing diseases, such as rheumatoid arthritis (RA). We report that in a proinflammatory environment, granulocyte-M (GM-CSF)- and M colony-stimulating factor (M-CSF)-dependent Ms have dichotomous effects on T cell activity. While GM-CSF-dependent Ms show a highly stimulatory activity typical for M1 Ms, M-CSF-dependent Ms, marked by folate receptor (FR), adopt an immunosuppressive M2 phenotype. We find the latter to be caused by the purinergic pathway that directs release of extracellular ATP and its conversion to immunosuppressive adenosine by co-expressed CD39 and CD73. Since we observed a misbalance between immunosuppressive and immunostimulatory Ms in human and murine arthritic joints, we devised a new strategy for RA treatment based on targeted delivery of a novel methotrexate (MTX) formulation to the immunosuppressive FR+CD39+CD73+ Ms, which boosts adenosine production and curtails the dominance of proinflammatory Ms. In contrast to untargeted MTX, this approach leads to potent alleviation of inflammation in the murine arthritis model. In conclusion, we define the M extracellular purine metabolism as a novel checkpoint in M cell fate decision-making and an attractive target to control pathological Ms in immune-mediated diseases.The research leading to these results has received funding from the European Union's Horizon 2020 Research and Innovation Program under grant agreement No 683356 - FOLSMART and from the Seventh Framework Program (FP7/2007-2013) under grant agreement NMP4-LA-2009-228827 NANOFOL. RP was supported by the Boehringer Ingelheim Fonds and the Ph.D. program Cell Communication in Health and Disease supported by the Austrian Science Fund (FWF). VL was supported by the FWF (P22908), VEGA (2/0063/14), and APVV (16-0452). JH received support from the Vienna Science and Technology Fund (WWTF) LS14-031.