Samson Hennessy-Strahs

Introduction. Coxiella burnetii, an intracellular bacterium and the causative agent of Q fever, can evade the immune system leading to acute or chronic infections.  C. burnetii virulence is aided by its type IV secretion system (T4SS) to deliver effector proteins into the host cytoplasm.  These proteins modulate host cell survival, direct formation of the C. burnetii-containing vacuole, and mediate immune signaling¹.  Although nearly 150 C. burnetii genes encoding candidate effector proteins have been identified², the majority are not functionally characterized.  This project reviews a selection of recently defined strategies of immune evasion by C. burnetii effector proteins. Methods. Caspase 1 activation was assessed using western blotting and production of IL-1beta.  Flagellin mutants of Legionella pneumophila were used as a surrogate host to identify the effector mediating the Caspase 1 block.  Inflammasome activation was measured after co-infection experiments with IcaA- or wt C. burnetii followed by infection with Escherichia coli or L. pneumophila to demonstrate IcaA is responsible for the inhibition³.  Transcriptomic analysis by RNA-seq and ELISA of IL-17 related gene products in wt or T4SS- C. burnetii.-infected macrophages were performed⁴.  Candidate effector proteins were identified by searching for eukaryotic-like genes in C. burnetii genome.  Candidate genes were cloned into cells and compared against knock downs.  Candidate proteins were tagged and visualized by immunofluorescence microscopy⁵. Separately, NLRP3-related protein levels were assessed in C. burnetii +/- macrophages to determine pathway activation⁶.  Finally, western blot, immunohistochemistry and ELISA of MAPK and Caspase-3 proteins were performed in wt or CB-infected neutrophils to determine potential of C burnetii to infect neutrophils⁷. Results. Genetic screening revealed a novel C. burnetii gene (IcaA) inhibits Caspase 1 activation through inhibition of non-canonical inflammasome. Expression of IcaA in L. pneumophila inhibited the caspase-11 activation in macrophages.  Furthermore, IcaA- mutants of C. burnetii failed to suppress the caspase-11-mediated inflammasome activation induced by L. pneumophila³.  Coxiella T4SS effector proteins downregulated the expression of IL-17 pathway genes and this phenotype was confirmed using ELISA for IL-17 regulated chemokines⁴.  NopA reduced the nuclear levels of transcription factors involved in the innate immune sensing of pathogens⁵.  C. burnetii primes but avoids cytosolic detection by NLRP3 inflammasomes⁶.  Finally, C. burnetii inhibits Caspase 3 cleavage and increases MAPK in neutrophils⁷. Conclusions. Recently, new ways in which C. burnetii modulates cell signaling have been described.  These involve inhibition of inflammasome formation and cell death, as well as modulation of IL-17 related signaling pathway.

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