Extracellular ADP facilitates monocyte recruitment in bacterial infection via ERK signaling

As the most prominent clinical drug targets for the inhibition of platelet aggregation, P2Y₁₂ and P2Y₁₃ have been found to be highly expressed in both platelets and macrophages. However, the roles and function of P2Y_12/13 in the regulation of macrophage-mediated innate immune responses remain unclear. Here, we demonstrate that adenosine 5'-diphosphate (ADP), the endogenous ligand of P2Y₁, P2Y₁₂ and P2Y₁₃, was released both in E. coli-infected mice and from macrophages treated with either lipopolysaccharide (LPS) or Pam3CSK4. Furthermore, the expression of P2Y₁₃ was clearly increased in both LPS-treated macrophages and tuberculosis patients. ADP protected mice from E. coli 0111-induced peritonitis by recruiting more macrophages to the infected sites. Consistent with this, ADP and ADP-treated cell culture medium attracted more macrophages in the transwell assay by enhancing the expression of MCP-1. Nevertheless, P2Y₁ is dispensable for ADP-mediated protection against bacterial infection. However, either P2Y₁₂/P2Y₁₃ deficiency or blocking the downstream signaling of P2Y₁₂/P2Y₁₃ blocked the ADP-mediated immune response and allowed more bacteria to persist in the infected mice. Furthermore, extracellular signal-regulated kinase (ERK) phosphorylation was clearly increased by ADP, and this type of activation could be blocked by either forskolin or analogs of cyclic AMP (cAMP) (for example, 8-bromo-cAMP). Accordingly, ADP-induced MCP-1 production and protection against bacterial infection could also be reduced by U0126, forskolin and 8-bromo-cAMP. Overall, our study reveals a relationship between danger signals and innate immune responses, which suggests the potential therapeutic significance of ADP-mediated purinergic signaling in infectious diseases.