Virulence Potential of Ehrlichia chaffeensis Strains of Distinct Genome Sequences

Abstract

Human monocytic ehrlichiosis, one of the most frequent life-threatening tick-borne zoonoses, is caused by Ehrlichia chaffeensis that lacks endotoxin and peptidoglycan. While sequence polymorphisms in several genes in E. chaffeensis strains have been reported, global genomic divergence and biological differences among strains are unknown. The objectives of the present study were to compare the genome sequences of strains of E. chaffeensis and to examine the virulence potentials of the strains with defined genome sequences. Genomic DNA was extracted from purified E. chaffeensis strains Wakulla and Liberty, and comparative genome hybridization was performed using a densely tiled microarray of 147,027 chromosome positions of the E. chaffeensis strain Arkansas genome. The results revealed that 4,663 and 5,325 positions in the chromosomes of strains Wakulla and Liberty, respectively, were different from those in the chromosome of strain Arkansas, including three common major polymorphic chromosomal regions. Of various functional categories, the differences were most concentrated in genes predicted to encode cell envelope proteins. Of all the open reading frames (ORFs), 21 omp-1 (p28 gene) paralogs, nine genes encoding hypothetical proteins, two genes encoding ankyrin repeat proteins, and hemE contained the most differences. Several highly polymorphic ORFs were confirmed by sequencing. When the E. chaffeensis strains were inoculated into severe combined immunodeficiency mice, the order of the severity of clinical signs and the bacterial burden detected in mice was Wakulla > Liberty > Arkansas. Severe diffuse inflammation and granulomatous inflammation were evident in the livers of mice infected with strains Wakulla and Arkansas, respectively, but not in the livers of mice infected with strain Liberty. These results revealed distinct virulence phenotypes of E. chaffeensis strains with defined genome sequences.

Liver Transcriptome Profiles Associated with Strain-SpecificEhrlichia chaffeensis-Induced Hepatitis in SCID Mice

“…The genes that vary between the two strains are more likely to encode virulence factors than the genes that are highly conserved between them. Our previous study of comparative genome hybridization showed genes varying among three E. chaffeensis strains of diverse virulence (36). Importantly, most of these genes encode putative membrane proteins and hypothetical proteins, suggesting a novel pathogenic mechanism.…”

“…Dissection of the relative roles of the pathogen strain versus host factors is very difficult with human infections due to the relatively low number of clinical cases and laboratory tissue specimens with a full data set and unknowns regarding the strain type in most infections. The use of these three previously characterized E. chaffeensis strains (36) in a single experimental host species allows testing of whether there are intrinsic differences in the response to each strain.…”

“…We previously selected the Wakulla (group II) and Liberty (group III) strains of E. chaffeensis and compared their pathogenesis and genomic sequences with those of the Arkansas strain (group I) (group designation is based on the work of Cheng et al [10]) (36). To compare pathogenesis, we inoculated these isolates into SCID mice.…”

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

“…Arkansas and Wakulla strains of E. chaffeensis were propagated in DH82 cells as previously described (33). Antibodies used were rabbit anti-extracellular regulated kinase (anti-ERK) antibody, mouse anti-phosphorylated ERK monoclonal antibody (both from Cell Signaling, Danvers, MA), and mouse anti-tubulin monoclonal antibody (Santa Cruz, Santa Cruz, CA).…”

“…HME is caused by Ehrlichia chaffeensis, a monocyte-tropic obligatory intracellular bacterium (9). Previously, we demonstrated that the Wakulla strain is more virulent than the type strain Arkansas in mice with severe combined immunodeficiency (SCID mice) (33). The Wakulla strain of E. chaffeensis causes a fatal illness in SCID mice; the mice develop fulminant hepatitis and show upregulation of tumor necrosis factor alpha (TNF-␣) and interleukin-1␤ (IL-1␤), several chemokines, including CXCL2 (Mip2, a mouse homolog of human IL-8), and chemokine receptors in the inflammatory liver (32).…”