Polyphosphates attenuate interleukin-12 production in macrophages infected with Legionella pneumophila

Polyphosphates are evolutionarily conserved linear chains of phosphate residues present in all living cells. Bacteria accumulate polyphosphates under stress and starvation for energy and phosphate storage, protein folding, and stress adaptation. During infection, bacteria release polyphosphates that may impair host responses, although the exact mechanisms remain elusive. In this study, polyphosphates were found to be elevated in bronchoalveolar lavage fluids from patients with Legionnaires’ disease, in Legionella pneumophila cultured alone, or during infection of bone marrow-derived macrophages (BMDMs) from C57BL/6J mice. We performed RNA-sequencing of infected BMDMs co-incubated with or without long-chain, bacterial-type polyphosphates. Among nearly 500 differentially expressed genes, Il12b (p40) showed the strongest suppression among highly expressed genes. IL-12p40 protein release was dose-dependently reduced by long-chain polyphosphates (~Pi 700), but not by short-chain, mammalian-type polyphosphates (Pi 70), indicating a specific bacterial mechanism targeting innate immune signaling. In contrast, IL-18, processed by inflammasome activation and synergizing functionally with IL-12, was not consistently suppressed. Polyphosphates predominantly inhibited LPS/TLR4 signaling, with Legionella-induced IL-12 relying on the MyD88 pathway, but not TRIF. RAGE and P2Y1, previously implicated in polyphosphate biology, were not required for IL-12 suppression. However, PI3K/AKT signaling appeared to mediate polyphosphate effects, which were reversed by the PI3K inhibitors, LY294002, copanlisib and eganelisib. Finally, long-chain polyphosphates suppressed IL-12 release also from human monocyte-derived macrophages exposed to L. pneumophila or LPS. In summary, our findings identify a selective inhibition of IL-12 by long-chain bacterial polyphosphates, suggesting that these molecules act as bacterial effectors capable of suppressing protective innate immune responses.