Outside in: A role for C3 in intracellular detection of
pathogens.
As well as extracellular complement being able to induce autophagy via
cell surface receptors and MAC, evidence exists for C3 being able to
trigger autophagy from within the cell. This would not be the first case
of a major serum protein inducing anti-pathogen responses when it is
brought into the cytosolic environment; multiple papers have shown how
antibody bound to pathogen surfaces triggers cell intrinsic immunity,
cytokine production, and pathogen restriction via proteasomal and
autophagic mechanisms, via binding to the high-affinity cytosolic
Fc-receptor and E3 ubiquitin ligase, tripartite-motif containing protein
21 (TRIM21) (Foss, Watkinson, Sandlie, James, & Andersen, 2015; Mallery
et al., 2010; McEwan et al., 2013). TRIM21 is able to restrict both
bacterial and viral infections in a strictly antibody-dependent fashion,
and is recruited to the surface of antibody-opsonised pathogens when
they invade into the cytosol, by binding to Fc domains. TRIM21 not only
ubiquitinylates the surface of the pathogen, targeting viral particles
to degradation in the proteasome, and bacteria to autophagosomes, but
also triggers a signaling cascade leading to NF-kB and interferon
response factor (IRF) signaling.
Similar to internalised antibody, C3 itself can be brought into cells on
the surface of opsonised non-enveloped viruses and invading bacteria,
triggering cell intrinsic immunity in a manner dependent on
mitochondrial anti-viral signaling protein (MAVS) (Tam, Bidgood, McEwan,
& James, 2014), activation of which also leads to IRF and NF-kB
signaling, and expression of type 1 interferons (Vazquez & Horner,
2015). Serum opsonisation of adenovirus before cellular invasion led to
triggering of cell intrinsic immunity and restriction of adenovirus
replication, via proteasomal function. This was only partially dependent
on serum antibodies and TRIM21 (Tam et al., 2014). The remaining
activity was heat labile, an important fact as antibodies are heat
stable, while serum complement activity is easily destroyed by heat
inactivation. C3 dependency was confirmed when viral particles were
opsonized using purified complement proteins of the alternative pathway,
leading to C3b deposition, which specifically triggered the same
response. This pathway of intracellular C3 detection was highly
conserved, with responses in human cells being triggered by adenovirus
exposed to heat labile activity of serum from multiple mammalian
species, and was present in all tested non-immune cell types. Although
complement-mediated adenoviral restriction was dependent on MAVS, C3 and
MAVS were not found to interact directly, suggesting that there is still
as-yet undiscovered intracellular C3 detection machinery. Of note, the
autophagy receptor protein P62 was required for NF-kB induction by
C3-opsonised particles.