Bacterial polyphosphates induce CXCL4 and synergize with complement anaphylatoxin C5a in lung injury

Abstract

Polyphosphates are linear polymers of inorganic phosphates that exist in all living cells and serve pleiotropic functions. Bacteria produce long-chain polyphosphates, which can interfere with host defense to infection. In contrast, short-chain polyphosphates are released from platelet dense granules and bind to the chemokine CXCL4. Here, we report that long-chain polyphosphates induced the release of CXCL4 from mouse bone marrow-derived macrophages and peritoneal macrophages in a dose-/time-dependent fashion resulting from an induction of CXCL4 mRNA. This polyphosphate effect was lost after pre-incubation with recombinant exopolyphosphatase (PPX) Fc fusion protein, demonstrating the potency of long chains over monophosphates and ambient cations. In detail, polyphosphate chains >70 inorganic phosphate residues were required to reliably induce CXCL4. Polyphosphates acted independently of the purinergic P2Y1 receptor and the MyD88/TRIF adaptors of Toll-like receptors. On the other hand, polyphosphates augmented LPS/MyD88-induced CXCL4 release, which was explained by intracellular signaling convergence on PI3K/Akt. Polyphosphates induced Akt phosphorylation at threonine-308. Pharmacologic blockade of PI3K (wortmannin, LY294002) antagonized polyphosphate-induced CXCL4 release from macrophages. Intratracheal polyphosphate administration to C57BL/6J mice caused histologic signs of lung injury, disruption of the endothelial-epithelial barrier, influx of Ly6G⁺ polymorphonuclear neutrophils, depletion of CD11c⁺SiglecF⁺ alveolar macrophages, and release of CXCL4. Long-chain polyphosphates synergized with the complement anaphylatoxin, C5a, which was partly explained by upregulation of C5aR1 on myeloid cells. C5aR1^-/- mice were protected from polyphosphate-induced lung injury. C5a generation occurred in the lungs and bronchoalveolar lavage fluid (BALF) of polyphosphate-treated C57BL/6J mice. In conclusion, we demonstrate that polyphosphates govern immunomodulation in macrophages and promote acute lung injury.

Keywords: acute respiratory distress syndrome; immunology; infection; innate immunity; platelet factor 4; sepsis.