Cell wall‐anchored 5′‐nucleotidases in Gram‐positive cocci

5′‐nucleotidases (5′‐NTs) are enzymes that catalyze the hydrolysis of nucleoside monophosphates to produce nucleosides and phosphate. Since the identification of adenosine synthase A (AdsA) in Staphylococcus aureus in 2009, several other 5′‐NTs have been discovered in Gram‐positive cocci, mainly in streptococci. Despite some differences in substrate specificity, pH range and metal ion requirements, all characterized 5′‐NTs use AMP and ADP, and in some cases ATP, to produce the immunosuppressive adenosine, which dampens pro‐inflammatory immune responses. Several 5′‐NTs are also able to use dAMP as substrate to generate deoxy‐adenosine which is cytotoxic for macrophages. A synergy between 5′‐NTs and exonucleases which are commonly expressed in Gram‐positive cocci has been described, where the nucleases provide dAMP as a cleavage product from DNA. Some of these nucleases produce dAMP by degrading the DNA backbone of neutrophil extracellular traps (NETs) resulting in a “double hit” strategy of immune evasion. This Micro Review provides an overview of the biochemical properties of Gram‐positive cell wall‐anchored 5′‐NTs and their role as virulence factors. A potential use of 5′‐NTs for vaccine development is also briefly discussed. (a) ATP and ADP are released in response to cellular damage and apoptosis and activate P2 purinergic receptors (P2R) on the surface of immune cells. Mammalian CD39 generates AMP by phosphohydrolysis of extracellular ATP and ADP, which then serves as a substrate for the ecto‐5′‐nucleotidase CD73 to produce adenosine (Ado), which leads to an increase in signaling via the anti‐inflammatory P1 purinergic receptors (P1R), shifting toward an anti‐inflammatory response. The 5′‐NTs found in Gram‐positive cocci hydrolyze AMP to generate Ado similar to CD73. Several species, like S. aureus, S. equi subsp. zooepidemicus, S. pyogenes and S. iniae produce 5′‐NTs that are also able to generate Ado from ATP/ADP, combining CD39 and CD73 activities. (b) Synergy between 5′‐NTs and secreted nucleases. NETs are released from the activated neutrophils during an infection and consist of backbone DNA molecules spiked with antimicrobial proteins to entrap and kill the invading organism. Extracellular nucleases, either secreted or cell wall‐anchored, degrade the DNA to produce dNMPs, including dAMP which serves as substrate for the cell wall‐anchored 5′‐NT. Hydrolysis of dAMP results in deoxy‐adenosine (dAdo) which triggers caspase‐3‐dependent apoptosis in m