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M. L. Bernardini - Role of Shigella surface components in immunomodulation of the inflammatory response
derivatives we have rationally mutagenized S. flexneri
5 in two genes, ampG and mppA, in the attempt to
impair PGN recycling and to augment the release of
muropeptides by shigellae in host cells and tissues.
AmpG is a transmembrane protein that acts as a specific
permease for intact muropeptides (tri or tetra)
whereas the periplasmic binding protein MppA binds
murein tripeptides and utilizes general oligopeptide
permease (Opp) to transfer its bound ligand into the
cytoplasm where tripeptides are recycled.
Our aim was to analyze whether and how these
released muramylpeptides might influence host cell
sensing by PRMs and ultimately Shigella virulence.
Our results demonstrate that Shigella spontaneously
releases PGN fragments and that this process can be
greatly increased by inactivating either ampG or
mppA genes.
We found that the increase of muramylpeptide shedding
corresponds to significant Nod1 activation in
epithelial cells. In fact, the Shigella ampG and mppA
mutants trigger Nod1-mediated NF-κB activation to
a greater extent than the wild-type strain. Likewise,
sterile purified supernatants (SPS) added from
“inside” to host cells were able to stimulate Nod1 to
varying degrees, depending on their relative amount
and composition. In particular, muramylpeptides
shed by the S. flexneri mutant ∆mppA, whose composition
differs significantly from that of the wild-type
and the ampG mutant, were especially efficient in
mediating Nod1 stimulation in HEK293 cells
expressing ectopic Nod1.
Our findings support the idea that muramylpeptides
released by pathogens during natural infection could
modulate the immune response through qualitative
and quantitative differences in Nod protein activation.
Shigella ∆mppA was strongly attenuated. In lungs of
mice infected intranasally with this strain, IL-6 production
was comparable to that induced by wild-type
bacteria, while the level of IFN-γ was higher.
Similarly, the liver of animals infected intravenously
with Shigella ∆mppA contained larger microgranulomas
(Martino MC et al., Cell Microbiol. 2005, 7:115-
18
27), than those induced by the wild-type, in which a
significant presence of IFN-γ expressing cells might
facilitate Shigella clearance. IFN-γ could contribute to
pathogen clearance as shown during natural and
experimental shigellosis (Way SS et al., Infect
Immun.1998, 66:1342-8) thus supporting the attenuation
of this strain.
Recent studies have stressed the role of bacterial
PGN composition in immune evasion of various
pathogens and have unveiled bacterial strategies to
modulate the PGN structure or to inject
muramylpeptides into host cells (Viala J et al., C R
Biol. 2004, 327:551-5). Our results show that the
immunopotential of Shigella is significantly influenced
by quantitative and qualitative alterations in
muramylpeptide shedding, thus suggesting that the
regulation of PGN release could represent a further
sophisticated bacterial strategy to survive in host tissues
and to evade immune defenses.
Selected publications
Hachani A, Biskri L, Rossi G, Marty A, Menard R,
Sansonetti P, Parsot C, Tran Van Nhieu G,
Bernardini ML, Alloaoui A. IpgB1 and IpgB2, two
homologous effectors secreted via the Mxi-Spa type
III secretion apparatus, cooperates to mediate polarized
cell invasion and inflammatory potential of
Shigella flexneri. Microb Infect. 2008, 10:260-8.
Nigro G, Lembo Fazio L, Martino MC, Rossi G,
Tattoli T, Liparoti V, De Castro C, Molinaro A,
Philpott D, Bernardini ML. Muramylpeptide shedding
modulates cell sensing of Shigella flexneri. Cell
Microbiol. 2008, 10:682-95.
Tattoli I, Lembo Fazio L, Nigro G, Carneiro LAM,
Ferraro E, Rossi G, Martino MC, de Stefano ME,
Cecconi F, Girardin SE, Philpott D, Bernardini ML.
Intracellular bacteriolysis triggers a massive apoptotic
cell death in Shigella-infected epithelial cells.
Microb Infect. 2008, 10:1114-23.