Résumé
S100A9 is a calcium-binding protein known as an alarmin for its pro-inflammatory extracellular role in the form of a S100A8/ S100A9 heterodimer called calprotectin. However, we recently evidenced that S100A9 also exerts an intracellular antiviral effect against HIV-1 infection of Langherans cells, a cutaneous- and mucosal-resident subset of dendritic cells acting as a first line of defense against HIV-1. Divalent cation-binding capacity of S100A9 is required for its immune-related functions which led us to investigate this further in a context of HIV-1 infection of human myeloid cells. We transduced a homemade THP-1-KOS100A9 cellular model to re-establish stable expression of S100A9 wild-type (THP-1A9WT) or a defective Ca2+-/Mg2+-binding mutant (THP-1A9Mut). Our results show that the antiviral effect observed in THP-1A9WT challenged with HIV-GFP-pseudo-typed lentivectors or infected by wild-type HIV-1 is lost in THP-1A9Mut cells, suggesting a divalent cation-dependent antiviral mechanism. The reverse transcription (RT) step of HIV-1 replication was inhibited in THP-1A9WT cells while similar to control cells in THP-1A9Mut. Moreover, drug-mediated release of either Ca2+ or Mg2+ into the cytosol shortly after virus entry overcomes the antiviral effect of S100A9 in THP-1A9WT cells and human monocyte-derived Langherans cells (MoLC) while having no effect in THP-1A9Mut cells or in siRNA-S100A9-MoLC. Additionally, purified cellular S100A9WT , but not S100A9Mut, inhibits HIV-1-RTase enzymatic activity in vitro, while addition of Ca2+ or Mg2+ restores HIV-1 RTase activity in a dose-dependent manner. Finally, S100A9Mut exhibits impaired higher order oligomer formation compared with S100A9WT, suggesting that divalent cation-dependent formation of S100A9-containing protein complexes are required for antiviral activity. Together, these results unveil a novel HIV-1 restriction mechanism in myeloid cell subsets controlled by a tight regulation of alarmins by alkaline metals metabolism.