Evasion of IFN-γ Signaling by Francisella novicida Is Dependent upon Francisella Outer Membrane Protein C




Nallaparaju, Kalyan C.
Yu, Jieh-Juen
Rodriguez, Stephen A.
Zogaj, Xhavit
Manam, Srikanth
Guentzel, M. Neal
Seshu, Janakiram
Murthy, Ashlesh K.
Chambers, James P.
Klose, Karl E.

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Public Library of Science


Background: Francisella tularensis is a Gram-negative facultative intracellular bacterium and the causative agent of the lethal disease tularemia. An outer membrane protein (FTT0918) of F. tularensis subsp. tularensis has been identified as a virulence factor. We generated a F. novicida (F. tularensis subsp. novicida) FTN_0444 (homolog of FTT0918) fopC mutant to study the virulence-associated mechanism(s) of FTT0918.

Methods and Findings: The ΔfopC strain phenotype was characterized using immunological and biochemical assays. Attenuated virulence via the pulmonary route in wildtype C57BL/6 and BALB/c mice, as well as in knockout (KO) mice, including MHC I, MHC II, and µmT (B cell deficient), but not in IFN-γ or IFN-γR KO mice was observed. Primary bone marrow derived macrophages (BMDM) prepared from C57BL/6 mice treated with rIFN-γ exhibited greater inhibition of intracellular ΔfopC than wildtype U112 strain replication; whereas, IFN-γR KO macrophages showed no IFN-γ-dependent inhibition of ΔfopC replication. Moreover, phosphorylation of STAT1 was downregulated by the wildtype strain, but not the fopC mutant, in rIFN-γ treated macrophages. Addition of NG-monomethyl-L-arginine, an NOS inhibitor, led to an increase of ΔfopC replication to that seen in the BMDM unstimulated with rIFN-γ. Enzymatic screening of ΔfopC revealed aberrant acid phosphatase activity and localization. Furthermore, a greater abundance of different proteins in the culture supernatants of ΔfopC than that in the wildtype U112 strain was observed.

Conclusions: F. novicida FopC protein facilitates evasion of IFN-γ-mediated immune defense(s) by down-regulation of STAT1 phosphorylation and nitric oxide production, thereby promoting virulence. Additionally, the FopC protein also may play a role in maintaining outer membrane stability (integrity) facilitating the activity and localization of acid phosphatases and other F. novicida cell components.




Nallaparaju, K. C., Yu, J.-J., Rodriguez, S. A., Zogaj, X., Manam, S., Guentzel, M. N., . . . Arulanandam, B. P. (2011). Evasion of IFN-γ Signaling by Francisella novicida Is Dependent upon Francisella Outer Membrane Protein C. PLOS ONE, 6(3), e18201. doi:10.1371/journal.pone.0018201


Molecular Microbiology and Immunology
Neuroscience, Developmental and Regenerative Biology