Direct targeting of autophagy by intracellular C3
It is therefore established that C3 entering the cytosol on the surface
of invading pathogens is capable of triggering cell intrinsic immune
responses. It was also recently shown that this response can include
direct induction of xenophagy, via a direct interaction of C3 with
ATG16L1. A yeast 2-hybrid assay was used to show that C3 is a ligand for
ATG16L1 (Sorbara et al., 2018). ATG16L1 is involved in several stages of
autophagosome biogenesis, including determining the site of
autophagosome initiation (Fujita et al., 2008), and the lipidation of
LC3-I into the autophagosome-associated form LC3-II (Fletcher et al.,
2018). Considering its role in defining the site of LC3 lipidation,
ATG16L1 is a key component of the autophagic response to invading
pathogens that reach the cytosol64. Sorbara et
al. went on to show that C3 opsonisation of cyto-invasive bacteria
impacts the host response to infection through increased autophagy
(Sorbara et al., 2018). Indeed, intracellular C3-opsonisedListeria monocytogenes and adherent-invasive E. coli(AIEC) were targeted by autophagy significantly more than unopsonised
bacteria, and autophagy induction restricted intracellular bacterial
growth via the C3/ATG16L1 interaction; the C3-mediated reduction in
recovered viable bacteria required expression of ATG16L1. C3 was also
shown to play a role in vivo in restricting Listeriainvasion in a murine model of intra-gastric infection. In this model, C3
expression was increased in cecal and colonic tissue in response to
infection, as well as to dextran sodium sulfate-induced colitis, showing
that mucosal inflammation could increase C3 expression, deposition of
which can then protect mucosal barriers against infection. After 24 hrs
of infection, C3-/- mice had higher numbers ofListeria colony forming units per gram of cecal and colonic
tissue, as well as lower levels of autophagic turnover as assessed by
LC3-II conversion.
Listeria expresses several virulence factors that participate in
autophagy escape: ActA and InlK recruit host cell proteins -
cytoskeletal proteins or Major Vault Protein respectively - at the
bacterial surface, to disguise themselves and escape autophagic
recognition (Birmingham et al., 2007; Dortet et al., 2011), while
phospholipases PlcA and PlcB reduce autophagic flux and
phosphatidylinositol 3-phosphate (PI3P) levels (Tattoli et al., 2013).
Despite these autophagy escape mechanisms, Listeria growth can be
restricted by autophagy in the presence of C3, perhaps due to the
C3-dependent recruitment of ATG16L1, which acts downstream of the PI3K
complex. In contrast, Sorbara et al. showed that two other
cyto-invasive bacteria, Shigella flexneri and Salmonellaenterica serovar Typhimurium, escaped C3-dependent
autophagy-mediated growth restriction. They first noticed that
intracellular Shigella rapidly shed C3 upon invasion of
epithelial cells. The omptin proteases IscP in Shigella and PgtE
in Salmonella are partly responsible for C3 cleavage, that
enables these bacteria to escape from C3-dependent autophagic
restriction. Unlike Listeria , these two bacteria have therefore
evolved C3-specific defence mechanisms in addition to their more general
autophagy escape mechanisms. Loss of IscP expression in Shigella ,
or PgtE in Salmonella , prevented C3 shedding and sensitised the
bacteria to ATG16L1-dependent intracellular killing. This paper
therefore not only established the existence of an intracellular C3
detection mechanism directly targeting intracellular pathogens for
xenophagy, but revealed that some pathogenic bacteria have evolved
mechanisms to resist it. Similarly, some viruses have also evolved
anti-C3 defenses to avoid intracellular detection: both human rhinovirus
and poliovirus express cytosolic 3C proteases that cleave C3 and reduce
intracellular C3-mediated MAVS-dependent NF-kB induction (Tam et al.,
2014). Pathogen mechanisms for preventing C3 opsonization therefore also
protect against these ’novel’ intracellular detection mechanisms,
including anti-microbial intracellular signaling and autophagy-mediated
bacterial growth restriction.