Role of Btp proteins in the pathogenesis of Brucella infection acquired through the airways

Abstract

Dear Editor,We have read with interest the article by Li et al. recently publishedin Microbial Pathogenesis [1]. In this paper the authors show that,during B. suis infection of primary goat alveolar macrophages, the TIRdomain-containing protein B (BtpB) expressed by this pathogen inhibitsTLR2 and TLR4 expression, reduces the activation of the NLRP3inflammasome, and suppresses the caspase-1-mediated secretion ofIL-1β. They also show that BtpB mediates a reduction in the nucleartranslocation of NF-kB and in the secretion of the proinflammatory cytokinesIL-1β, IL-6, IFN-γ and TNF-α in infected alveolar macrophageswhile up-regulating IL-10 expression. These results were obtained bycomparing the expression of these immune receptors and effectors betweencells infected with either the wild type strain, a ΔbtpB mutant, orthe complemented strain. These results are of interest as Brucellainfection is frequently acquired through the respiratory route and, asshown by animal and clinical studies, can rapidly disseminate systemicallyto colonize diverse organs. In fact, several in vitro and in vivostudies have been performed in recent years to try to shed light on theinteraction of Brucella with lung cells and the role of innate and immuneresponses in the control of the respiratory infection. Most of these previousstudies were cited in the paper by Li et al. However, we weresurprised to find that our previous study regarding the role of BtpA andBtpB proteins in the modulation of the lung immune response to respiratoryB. abortus infection in mice was completely overlooked [2]. Inthat study, mice were intratracheally infected with either wild type(WT) B. abortus or a double mutant for both Btp proteins (btpAbtpB-). Weshowed that proinflammatory mediators (IL-12, KC, MCP-1) weresignificantly increased in lungs from mice infected with the mutant ascompared to WT-infected mice, while a similar but non-significanttendency was observed for IL-1β and IFN-γ. Moreover, lungs frombtpAbtpB- infected mice presented a higher level of inflammatory infiltratethan those from WT-infected mice at both 2 days and 7 dayspost-infection. Notably, these results obtained in vivo in lungs ofinfected mice agree with those reported by Li et al. for in vitro-infectedgoat alveolar macrophages. In our study, in addition, we found that thestronger proinflammatory profile in lungs of mice infected with the Btpmutant did not result in a reduction of B. abortus CFU counts ascompared to infection with the WT strain. These findings suggested that,regardless of the expression of Btp proteins, the immune responsemounted in the lung during the first stages of infection is not enough toreduce the pulmonary load of Brucella. To test whether a stronger innate immune response in the lungs could reduce Brucella CFU numbers andwhether Btps may have a role in Brucella survival in this situation, micewere inoculated intratracheally with E. coli LPS 24 hours beforeB. abortus infection by the same route. Whereas LPS pretreatment did notmodify the survival of B. abortus WT, it reduced the survival of thebtpAbtpB mutant. Overall, these results suggest that Btp proteins do notconfer a survival advantage to B. abortus in the context of the weak inflammatoryenvironment elicited in the lungs during the first days ofinfection, but may confer such advantage within a stronger inflammatoryenvironment. To our best knowledge, this was the first study toaddress the role of Btp proteins in mice infected through a natural routefor Brucella species.Given the important role of the airways as a route for Brucellainfection and the ability of this pathogen to disseminate systemicallyfrom this portal of entry, the characterization of the host-pathogeninteraction at the lung level is of utmost interest for the understandingof the infection process and for the rational design of vaccines againstbrucellosis. Elucidating the role of bacterial factors such as Btps,involved in the modulation of the host immune response to favor Brucellainfectivity, may contribute to achieve these goals.