Bacillus anthracis Edema Toxin Inhibits Efferocytosis in Human Macrophages and Alters Efferocytic Receptor Signaling

Abstract

The Bacillus anthracis Edema Toxin (ET), composed of a Protective Antigen (PA) and the Edema Factor (EF), is a cellular adenylate cyclase that alters host responses by elevating cyclic adenosine monophosphate (cAMP) to supraphysiologic levels. However, the role of ET in systemic anthrax is unclear. Efferocytosis is a cAMP-sensitive, anti-inflammatory process of apoptotic cell engulfment, the inhibition of which may promote sepsis in systemic anthrax. Here, we tested the hypothesis that ET inhibits efferocytosis by primary human macrophages and evaluated the mechanisms of altered efferocytic signaling. ET, but not PA or EF alone, inhibited the efferocytosis of early apoptotic neutrophils (PMN) by primary human M2 macrophages (polarized with IL-4, IL-10, and/or dexamethasone) at concentrations relevant to those encountered in systemic infection. ET inhibited Protein S- and MFGE8-dependent efferocytosis initiated by signaling through MerTK and αVβ5 receptors, respectively. ET inhibited Rac1 activation as well as the phosphorylation of Rac1 and key activating sites of calcium calmodulin-dependent kinases CamK1α, CamK4, and vasodilator-stimulated phosphoprotein, that were induced by the exposure of M2(Dex) macrophages to Protein S-opsonized apoptotic PMN. These results show that ET impairs macrophage efferocytosis and alters efferocytic receptor signaling.

Geranylgeraniol Restores Zoledronic Acid-Induced Efferocytosis Inhibition in Bisphosphonate-Related Osteonecrosis of the Jaw

“…However, only heterozygous deletion of gene Rac1, but not Cdc42 and RhoA, could reverse inflammation in mouse models for arthritis, indicating the special role of Rac1 in chronic inflammation diseases ( Akula et al, 2019 ). As a membrane protein, Rac1 needs to rearrange distribution and interact with downstream signaling effectors to form phagocytotic cup when encountering ACs ( Tao et al, 2015 ; Pan et al, 2019 ). We suggest that ZA might impair macrophage efferocytosis via inhibiting Rac1 prenylation.…”

Role of Apoptotic Cell Clearance in Pneumonia and Inflammatory Lung Disease

“…Once F. novicida infects a macrophage, the efferocytic receptor CD36 is downregulated via an unknown mechanism, leading to the accumulation of necrotic debris in the lung and continued infection [ 236 ]. Another pathogen that inhibits signaling through efferocytic receptors is Bacillus anthracis , which has been shown to inhibit the activation and phosphorylation of Rac1 downstream of the MERTK and α V β 5 integrin signaling via its virulence factor edema toxin ( Figure 3 B [ 105 , 237 ]). Thus, the efferocytic uptake of infected apoptotic cells is inhibited as the requisite actin cytoskeletal rearrangements needed to engulf an apoptotic cell are inhibited.…”

Efferocytosis and Anthrax: Implications for Bacterial Sepsis?

“…Given the important role of efferocytosis in regulating host inflammation, our article asked whether ET, a factor known to contribute to systemic pathology during inhalational anthrax, alters the capacity of macrophages to perform efferocytosis [ 28 ]. To summarize these findings, primary human macrophages treated with dexamethasone developed a pro-efferocytic M2 phenotype, noted by increased expression of MerTK, CD163, CD206, and αVβ5.…”