Ehrlichial strategy for survival and proliferation in leukocytes

Analysis of involvement of the RecF pathway in p44 recombination in Anaplasma phagocytophilum and in Escherichia coli by using a plasmid carrying the p44 expression and p44 donor loci

Anaplasma phagocytophilum, the etiologic agent of human granulocytic anaplasmosis, has a large paralog cluster (approximate 90 members) that encodes the 44-kDa major outer membrane proteins (P44s). Gene conversion at a single p44 expression locus leads to P44 antigenic variation. Homologs of genes for the RecA-dependent RecF pathway, but not the RecBCD or RecE pathways, of recombination were detected in the A. phagocytophilum genome. In the present study, we examined whether the RecF pathway is involved in p44 gene conversion. The recombination intermediate structure between a donor p44 and the p44 expression locus of A. phagocytophilum was detected in an HL-60 cell culture by Southern blot analysis followed by sequencing the band and in blood samples from infected SCID mice by PCR, followed by sequencing. The sequences were consistent with the RecF pathway recombination: a half-crossover structure, consisting of the donor p44 locus connected to the 3' conserved region of the recipient p44 in the p44 expression locus in direct orientation. To determine whether the p44 recombination intermediate structure can be generated in a RecF-active Escherichia coli strain, we constructed a double-origin plasmid carrying the p44 expression locus and a donor p44 locus and introduced the plasmid into various E. coli strains. The recombination intermediate was recovered in an E. coli strain with active RecF recombination pathway but not in strains with deficient RecF pathway. Our results support the view that the p44 gene conversion in A. phagocytophilum occurs through the RecF pathway.

Differential expression of VirB9 and VirB6 during the life cycle of Anaplasma phagocytophilum in human leucocytes is associated with differential binding and avoidance of lysosome pathway

Anaplasma phagocytophilum, an obligate intracellular bacterium, is the aetiologic agent of human granulocytic anaplasmosis (HGA). A. phagocytophilum virB/D operons encoding type IV secretion system are expressed in cell culture and in the blood of HGA patients. In the present study, their expression across the A. phagocytophilum intracellular developmental cycle was investigated. We found that mRNA levels of both virB9 and virB6 were upregulated during infection of human neutrophils in vitro. The antibody against the recombinant VirB9 protein was prepared and immunogold and immunofluorescence labelling were used to determine the VirB9 protein expression by individual organisms. Majority of A. phagocytophilum spontaneously released from the infected host cells poorly expressed VirB9. At 1 h post infection, VirB9 was not detectable on most bacteria associated with neutrophils. However, VirB9 was strongly expressed by A. phagocytophilum during proliferation in neutrophils. In contrast, with HL-60 cells, approximately 80% of A. phagocytophilum organisms associated at 1 h post infection expressed VirB9 protein and were colocalized with lysosome-associated membrane protein-1 (LAMP-1), whereas, VirB9-undetectable bacteria were not colocalized with LAMP-1. These results indicate developmental regulation of expression of components of type IV secretion system during A. phagocytophilum intracellular life cycle and suggest that bacterial developmental stages influence the nature of binding to the hosts and early avoidance of late endosome-lysosome pathway.

Ehrlichia chaffeensis and Anaplasma phagocytophilum lack genes for lipid A biosynthesis and incorporate cholesterol for their survival

Ehrlichia chaffeensis and Anaplasma phagocytophilum are agents of human monocytic and granulocytic ehrlichioses, respectively. They are extremely sensitive to mechanical stress and are pleomorphic gram-negative bacteria. Membrane incorporation of cholesterol from the eukaryotic host is known to be essential for other fragile and pleomorphic bacteria and mycoplasmas that lack a cell wall. Thus, we tested whether cholesterol is required for E. chaffeensis and A. phagocytophilum. Using a freeze fracture technique and biochemical analysis, these bacteria were found to contain significant levels of membrane cholesterol. These bacteria lack genes for cholesterol biosynthesis or modification. However, host cell-free bacteria had the ability to take up directly exogenous cholesterol or NBD-cholesterol, a fluorescent cholesterol derivative. Treatment of the bacteria with cholesterol extraction reagent methyl-beta-cyclodextrin caused their ultrastructural changes. Furthermore, pretreatment of the bacteria with methyl-beta-cyclodextrin or NBD-cholesterol deprived these bacteria of the ability to infect leukocytes, thus killing these obligate intracellular bacteria. Analysis of E. chaffeensis and A. phagocytophilum genome sequences revealed that these bacteria lack all genes for the biosynthesis of lipid A and most genes for the biosynthesis of peptidoglycan, which confer structural strength to gram-negative bacteria. Taken together, these results suggest that human ehrlichiosis agents became cholesterol dependent due to the loss of these genes. As the first report of gram-negative bacteria incorporating cholesterol for survival, these findings offer insight into the unique nature of their parasitism and imply that cholesterol is important in the control of human ehrlichioses.

Mechanisms to create a safe haven by members of the family Anaplasmataceae

Members of the family Anaplasmataceae are obligatory intracellular bacteria with unique host cell specificities. Depending on each bacterial species, granulocytes, platelets, endothelial cells, monocytes, macrophages, red blood cells, and cells of invertebrates are specifically infected. This unique host cell specificity has been the major hurdle to overcome in order to cultivate this group of bacteria. Because these bacteria cannot survive outside host cells, once released from a host cell, they need to rapidly induce signals for their own internalization into another host cell unique to each species. How these bacteria enter and continue to survive and replicate within the host milieu, then exit the host cell is largely unknown. Recently, however, unique strategies employed by some of these bacteria for successful parasitism of mammalian leukocytes have begun to be uncovered. When these bacteria interact with host cells, signals are transduced both inside the host cells and inside the bacteria. These signals disable the alarm system, as well as microbicidal mechanisms, of the leukocytes and condition the host cells to accept these intruders to share space and nutrient resources. Signals transduced inside the bacteria allow them to finely tune their metabolism and physiology in the new host cell environment and to disguise themselves as "insiders" so that their sojourn does not upset the host cell physiology until they have sufficiently multiplied. This paper discusses our recent findings on these topics.

Ehrlichia chaffeensis: a prototypical emerging pathogen

Ehrlichia chaffeensis is an obligately intracellular, tick-transmitted bacterium that is maintained in nature in a cycle involving at least one and perhaps several vertebrate reservoir hosts. The moderate to severe disease caused by E. chaffeensis in humans, first identified in 1986 and reported for more than 1,000 patients through 2000, represents a prototypical "emerging infection." Knowledge of the biology and natural history of E. chaffeensis, and of the epidemiology, clinical features, and laboratory diagnosis of the zoonotic disease it causes (commonly referred to as human monocytic ehrlichiosis [HME]) has expanded considerably in the period since its discovery. In this review, we summarize briefly the current understanding of the microbiology, pathogenesis, and clinical manifestations associated with this pathogen but focus primarily on discussing various ecological factors responsible for the recent recognition of this important and potentially life-threatening tick-borne disease. Perhaps the most pivotal element in the emergence of HME has been the staggering increases in white-tailed deer populations in the eastern United States during the 20th century. This animal serves as a keystone host for all life stages of the principal tick vector (Amblyomma americanum) and is perhaps the most important vertebrate reservoir host for E. chaffeensis. The contributions of other components, including expansion of susceptible human populations, growth and broadening geographical distributions of other potential reservoir species and A. americanum, and improvements in confirmatory diagnostic methods, are also explored.