Abstract
The obligatory intracellular pathogen Ehrlichia chaffeensis lacks most factors that could respond to oxidative stress (a host cell defense mechanism). We previously found that the C terminus of Ehrlichia surface invasin, entry-triggering protein of Ehrlichia (EtpE; EtpE-C) directly binds mammalian DNase X, a glycosylphosphatidylinositol-anchored cell surface receptor and that binding is required to induce bacterial entry and simultaneously to block the generation of reactive oxygen species (ROS) by host monocytes and macrophages. However, how the EtpE-C-DNase X complex mediates the ROS blockade was unknown. A mammalian transmembrane glycoprotein CD147 (basigin) binds to the EtpE-DNase X complex and is required for Ehrlichia entry and infection of host cells. Here, we found that bone marrow-derived macrophages (BMDM) from myeloid cell lineage-selective CD147-null mice had significantly reduced Ehrlichia-induced or EtpE-C-induced blockade of ROS generation in response to phorbol myristate acetate. In BMDM from CD147-null mice, nucleofection with CD147 partially restored the Ehrlichia-mediated inhibition of ROS generation. Indeed, CD147-null mice as well as their BMDM were resistant to Ehrlichia infection. Moreover, in human monocytes, anti-CD147 partially abrogated EtpE-C-induced blockade of ROS generation. Both Ehrlichia and EtpE-C could block activation of the small GTPase Rac1 (which in turn activates phagocyte NADPH oxidase) and suppress activation of Vav1, a hematopoietic-specific Rho/Rac guanine nucleotide exchange factor by phorbol myristate acetate. Vav1 suppression by Ehrlichia was CD147 dependent. E. chaffeensis is the first example of pathogens that block Rac1 activation to colonize macrophages. Furthermore, Ehrlichia uses EtpE to hijack the unique host DNase X-CD147-Vav1 signaling to block Rac1 activation.IMPORTANCEEhrlichia chaffeensis is an obligatory intracellular bacterium with the capability of causing an emerging infectious disease called human monocytic ehrlichiosis. E. chaffeensis preferentially infects monocytes and macrophages, professional phagocytes, equipped with an arsenal of antimicrobial mechanisms, including rapid reactive oxygen species (ROS) generation upon encountering bacteria. As Ehrlichia isolated from host cells are readily killed upon exposure to ROS, Ehrlichia must have evolved a unique mechanism to safely enter phagocytes. We discovered that binding of the Ehrlichia surface invasin to the host cell surface receptor not only triggers Ehrlichia entry but also blocks ROS generation by the host cells by mobilizing a novel intracellular signaling pathway. Knowledge of the mechanisms by which ROS production is inhibited may lead to the development of therapeutics for ehrlichiosis as well as other ROS-related pathologies.
Highlights
The obligatory intracellular pathogen Ehrlichia chaffeensis lacks most factors that could respond to oxidative stress
Upon E. chaffeensis binding to DNase X, the transmembrane protein CD147 is recruited to the entrytriggering protein of Ehrlichia (EtpE)-CϪDNase X complex, which results in a relay of the extracellular signal (i.e., E. chaffeensis binding) to the cytoplasm to trigger actin polymerization [1]
We measured levels of reactive oxygen species (ROS) generated in response to phorbol myristate acetate (PMA) using WT and bsgflox/flox-lyz2-Cre mouse bone marrow-derived macrophages (BMDM) preincubated for 30 min with isolated E. chaffeensis or with lysate of canine macrophage DH82 cells
Summary
The obligatory intracellular pathogen Ehrlichia chaffeensis lacks most factors that could respond to oxidative stress (a host cell defense mechanism). In human monocytes, antiCD147 partially abrogated EtpE-C-induced blockade of ROS generation Both Ehrlichia and EtpE-C could block activation of the small GTPase Rac (which in turn activates phagocyte NADPH oxidase) and suppress activation of Vav, a hematopoietic-specific Rho/Rac guanine nucleotide exchange factor by phorbol myristate acetate. To infect host monocytes and macrophages, E. chaffeensis uses the C terminus of its unique outer membrane invasin, entry-triggering protein of Ehrlichia (EtpE; EtpE-C), to directly bind the host cell DNase X, a cell surface glycosylphosphatidylinositol-anchored receptor This binding drives E. chaffeensis entry by engaging the type I transmembrane glycoprotein CD147 (basigin/extracellular matrix metalloproteinase inducer) and cytoplasmic heterogeneous nuclear ribonucleoprotein K (hnRNPK), which leads to the neuronal WiskottAldrich syndrome protein (N-WASP)-dependent polymerization of actin [1]. CD147-Dependent Suppression of ROS by Ehrlichia findings provide important molecular insights into how an obligatory intracellular pathogen may subvert NADPH oxidase-related signaling during its entry to facilitate subsequent colonization of phagocytes
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