Ehrlichia chaffeensis Induces Monocyte Inflammatory Responses through MyD88, ERK, and NF-κB but Not through TRIF, Interleukin-1 Receptor 1 (IL-1R1)/IL-18R1, or Toll-Like Receptors

Human monocytic ehrlichiosis, an influenza-like illness accompanied by signs of hepatitis, is caused by infection of monocytes/macrophages with a lipopolysaccharide-deficient bacterium,Ehrlichia chaffeensis . TheE. chaffeensis strain Wakulla induces diffuse hepatitis with neutrophil infiltration in mice with severe combined immunodeficiency, which is accompanied by strong CXCL2 (mouse functional homolog of interleukin-8 [IL-8]) and tumor necrosis factor alpha (TNF-α) expression in the liver. In this study, we found that expression of IL-1β, CXCL2, and TNF-α was induced by strain Wakulla in mouse bone marrow-derived macrophages; this expression was dependent on MyD88, but not on TRIF, TLR2/4, IL-1R1/IL-18R1, or endosome acidification. When the human leukemia cell line THP-1 was exposed toE. chaffeensis , significant upregulation of IL-8, IL-1β, and TNF-α mRNA and extracellular regulated kinase 2 (ERK2) activation were detected. U0126 (inhibitor of mitogen-activated protein kinase/extracellular signal-regulated kinase kinase 1/2 [MEK1/2] upstream of ERK), manumycin A (Ras inhibitor), BAY43-9006 (Raf-1 inhibitor), and NS-50 (inhibitor of NF-κB nuclear translocation) inhibited the cytokine gene expression. A luciferase reporter assay using HEK293 cells, which lack Toll-like receptors (TLRs), showed activation of both the IL-8 promoter and NF-κB byE. chaffeensis . Activation of the IL-8 promoter in transfected HEK293 cells was inhibited by manumycin A, BAY43-9006, U0126, and transfection with a dominant-negative Ras mutant. These results indicate that theE. chaffeensis Wakulla strain can induce inflammatory responses through MyD88-dependent NF-κB and ERK pathways, without the involvement of TRIF and TLRs.