On the regulation of Brucella melitensis virulence through flagellin synthesis and self-quorum quenching

(eng) The mammalian innate immune system uses pattern recognition receptors (PRRs), including Toll-like receptors (TLRs) and Nod-like receptors (NLRs) to detect microbial components during infection and subsequently trigger an appropriate immune response. These microbial features are often referred to as pathogen-associated molecular patterns (PAMPs) and are considered as part of the molecular signature of the invading pathogen. Brucella is an intracellular bacterium that causes the worldwide zoonotic disease brucellosis. A key characteristic of Brucella infection is its chronic nature. The ability of Brucella to cause long-lasting infections relies at least on its survival, replication and persistence within immune phagocytic cells while avoiding sustained recognition by PRRs due to non-canonical surface molecules that lack marked PAMP-activities. However, reaching a chronic state of infection implies a balance between pathogen virulence and host resistance. Accordingly, brucellae are not entirely invisible to the immune system, which can yet detect them and shape an immune response. Production of IFN-γ, the mediator of the T helper 1 (TH1) immune response, and granuloma formation are crucial for host resistance against Brucella melitensis infection. Bacterial flagellin, the monomeric subunit of flagellar filament is a PAMP. Here, we show that tight regulation of flagellin FliC synthesis during infection is crucial for the equilibrated relationship between Brucella and its experimental murine host. Indeed, ectopic flagellin production attenuates B. melitensis virulence, while the lack of FliC enhances bacterial persistence, which results in severe splenic pathology. These findings suggest that Brucella flagellin is an important immune target, and our work provides first insights into the mechanisms that enable the host to detect this protein. Indeed, while Brucella most likely evades recognition by the extracellular flagellin sensor TLR5, we show that cytosolic FliC is a PAMP for NLRC4 that plays a role, along with caspase-1 in controlling B. melitensis in vivo. However, we propose that a still unidentified flagellin-dependent but NLRC4- and caspase-1-independent pathway could also contribute to host resistance to B. melitensis infection. Data presented here suggest that the failure of mice to control flagellin-deficient B. melitensis mutants would not be due to a defect in the IFN-γ response, but rather to alteration of the granulomatous response. In the second part of this work, we show that regulation of flagellin production in Brucella occurs through quorum sensing (QS). QS is a regulatory system that allows genetic reprogramming in response to a threshold concentration of small diffusible signalling molecules that are produced and released by bacteria. Here, we provide data showing that B. melitensis produces long-chain N-acyl-homoserine lactones (AHLs) as QS cues, and controls their concentration by means of an AHL-inactivating protein called AiiD. Both AHLs production and inactivation occur in B. melitensis during in vitro growth and macrophage infection. Furthermore, we show that quenching self-produced AHLs regulates expression of virulence genes, including the virB operon coding for a type IV secretion system and flagellar genes. Importantly, flagellin production is defective in the absence of AiiD, and a B. melitensis strain deleted for aiiD shows similar phenotypes than a flagellin-deficient mutant in vivo.