CRIg signals induce anti‐intracellular bacterial phagosome activity in a chloride intracellular channel 3‐dependent manner

Abstract

Macrophages provide a first line of defense against bacterial infection by engulfing and killing invading bacteria, but intracellular bacteria such as Listeria monocytogenes (LM) can survive in macrophages by various mechanisms of evasion. Complement receptor of the immunoglobulin (CRIg), a C3b receptor, binds to C3b on opsonized bacteria and facilitates clearance of the bacteria by promoting their uptake. We found that CRIg signaling induced by agonistic anti-CRIg mAb enhanced the killing of intracellular LM by macrophages, and that this occurred in LM-containing phagosomes. Chloride intracellular channel 3 CLIC3, an intracellular chloride channel protein, was essential for CRIgmediated LM killing by directly interacting with the cytoplasmic domain of CRIg, and the two proteins colocalized on the membranes of LM-containing vacuoles. CLIC3 −/− mice were as susceptible to LM as CRIg −/− mice. These findings identify a mechanism embedded in the process by which macrophages take up opsonized bacteria that prevents the bacteria from evading cell-mediated killing.Keywords: CLIC3 r CRIg r Listeria r Phagosome r Signaling Additional supporting information may be found in the online version of this article at the publisher's web-site

Extracellular stimulation of VSIG4/complement receptor Ig suppresses intracellular bacterial infection by inducing autophagy

“…We also showed that transgenic expression of VSIG4 in cells of the nonphagocytic HeLa cell line cells allows them to take up opsonized LM and selectively target autophagosomes to LM. Our findings demonstrate that VSIG4 signals provide 2 defense mechanisms against intracellular bacterial infections: 1) phagosome-dependent defense involving uptake of the circulating complement-opsonized pathogens and rapid acidification of the phagosomes, as previously reported, 20 and 2) xenophagy-dependent defense by selectively inducing autophagosomes directed against LM that have escaped from phagosomes.…”

“…6 Since we treated J774 cells with anti-VSIG4 mAb 1 h after infection with LM, 20 we suspected that many of the phagocytosed LM would already have escaped to the cytosol. Since nevertheless the anti-VSIG4 mAb suppressed LM growth, 20 this implied that VSIG4 triggering also eliminated the escaped LM.…”

“…6 Since we treated J774 cells with anti-VSIG4 mAb 1 h after infection with LM, 20 we suspected that many of the phagocytosed LM would already have escaped to the cytosol. Since nevertheless the anti-VSIG4 mAb suppressed LM growth, 20 this implied that VSIG4 triggering also eliminated the escaped LM. To determine when the LM escaped from LM vacuoles, J774 cells were infected with FITC-labeled LM and stained with phalloidin to visualize actin-nucleated motile LM at times after LM infection.…”

“…18 In addition, VSIG4 cross-linked by an agonistic antibody rapidly translocates to bacteria-containing phagosomes and enhances the acidification of the phagosomes before the bacteria can escape, and this process requires CLIC3 (chloride intracellular channel protein 3). 20 VSIG4 has been reported to regulate T-cell proliferation in vitro and in vivo, 21 and a soluble form of HsVSIG4 displays therapeutic potential in mouse models of arthritis by inhibiting autoreactive immune responses. 22 We have previously reported that VSIG4 signaling eliminates phagocytosed LMs by rapidly inducing CLIC3-dependent acidification of LM-containing phagosomes.…”

“…22 We have previously reported that VSIG4 signaling eliminates phagocytosed LMs by rapidly inducing CLIC3-dependent acidification of LM-containing phagosomes. 20 However, we also suspected that VSIG4 signaling might inhibit the intracellular growth of LMs that escaped into the macrophage cytosol, because some phagocytosed LMs manage to escape from phagosomes before VSIG4 was triggered and yet are eliminated. Indeed, we demonstrate here that some phagocytosed LMs escape from the phagosomes during the first hour of infection of macrophages, and VSIG4 triggering efficiently removes these cytosolic LMs by inducing the formation of LM-targeting autophagosomes.…”