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.