VSIG4
A second cell-surface complement receptor implicated in autophagy induction is V-set and immunoglobulin domain containing 4 (VSIG4), also known as Complement Receptor of the Immunoglobulin superfamily (CRIg), which binds to both C3b and iC3b (Helmy et al., 2006). VSIG4 is expressed on macrophages and contributes to complement-dependent clearance of circulating particles by liver Kupffer cells. VSIG4 also contributes to phagocytosis and subsequent phagosome acidification, leading to enhanced killing of complement-opsonised bacteria (Kim et al., 2013). In addition to this, a role for VSIG4 has also been shown in the killing of intracellular bacteria via autophagy induction.
Listeria monocytogenes is an intracellular pathogen that is able to escape from phagosomes into the cytosol, therefore avoiding killing. Kim et al found that once Listeria had escaped into the cytosol, cross-linking of cell-surface VSIG4 by specific antibodies led to formation of LC3-II-positive autophagosomes, which contained labeled bacteria (Kim et al., 2016). As a result, fewer viable bacteria were isolated from cell lysates. In comparison, induction of autophagy in infected cells by serum starvation did not result in decreases in bacterial numbers compared to control cells, showing that VSIG4-triggered autophagy increased targeted capture and killing of intracellular bacteria via xenophagy. This was attributed to the increased polyubiquitinylation of intracellular bacteria found after VSIG4 stimulation, which recruits autophagy ubiquitin-binding receptor proteins such as P6214. Although VSIG4 signaling is not fully understood, the adaptor protein MyD88 was implicated, which phosphorylates and activates Beclin 1 (Shi & Kehrl, 2008), therefore triggering autophagosome formation. Overexpression of VSIG4 in HeLa cells also conferred the same function, allowing them to kill intracellular bacteria by xenophagy, while conversely, macrophages from VSIG4-/- mice supported greater intracellularListeria growth in the presence of VSIG4 receptor stimulation, compared to WT macrophages. While these results clearly show the ability of VSIG4 to trigger autophagy and targeted xenophagy, the use of cross-linking antibodies is not physiological. While complement-opsonised Listeria triggered autophagy in macrophages more efficiently than unopsonised bacteria, once bacteria have entered the intracellular environment, one would not expect continued surface VSIG4 signaling. However, it is likely that during an ongoing infection, the presence of extracellular bacteria or DAMPs would provide complement activation and VSIG4 stimulation, resulting in increased autophagy to clear potential intracellular pathogens.