RICKETTSIOLOGY AND RICKETTSIAL DISEASES-FIFTH INTERNATIONAL CONFERENCE
Type IV Secretion System of Anaplasma phagocytophilum and Ehrlichia chaffeensis Yasuko Rikihisa, Mingqun Lin, Hua Niu, and Zhihui Cheng Department of Veterinary Biosciences, College of Veterinary Medicine, The Ohio State University, Columbus, Ohio 43210, USA The intracellular bacterial pathogens Ehrlichia chaffeensis and Anaplasma phagocytophilum have evolved to infect leukocytes and hijack biological compounds and processes of these host defensive cells. Bacterial type IV secretion (T4S) system transports macromolecules across the membrane in an ATP-dependent manner and is increasingly recognized as a virulence factor delivery mechanism that allows pathogens to modulate eukaryotic cell functions for their own benefit. Genes encoding T4S system homologous to those of a plant pathogen Agrobacterium tumefaciens have been identified in E. chaffeensis and A. phagocytophilum. Upon interaction with new host cells, E. chaffeensis and A. phagocytophilum genes encoding the T4S apparatus are upregulated. The delivered macromolecules are referred to as T4S substrates, or effectors, because they affect and alter basic host cellular processes, resulting in disease development. Recently, A. phagocytophilum 160-kDa AnkA protein was to be delivered by T4S system into the host cytoplasm. Thus, dynamic signal transduction events are likely induced by T4S substrates in the host cells for successful establishment of intracellular infection. Further studies on Ehrlichia and Anaplasma T4S effectors cognate host cell molecules will undoubtedly advance our understanding of the complex interplay between obligatory intracellular pathogens and their hosts. Such data can be applied toward treatment, diagnosis, and control of ehrlichiosis and anaplasmosis. Key words: A. phagocytophilum; E. chaffeensis; type IV secretion
Introduction Ehrlichia chaffeensis and Anaplasma phagocytophilum are gram-negative bacteria belonging to the family Anaplasmataceae. E. chaffeensis is the agent of human monocytic ehrlichiosis (HME)1,2 and A. phagocytophilum is the agent of human granulocytic anaplasmosis (HGA).3,4 HME and HGA are systemic diseases characterized by fever, headache, myalgia, anorexia, and chills, and frequently are accompanied by leukopenia with a leftward shift, thrombocytopenia, anemia, and elevations in serum hepatic aminotransferases. The severity of the disease varies from asymptomatic seroconver-
Address for correspondence: Yasuko Rikihisa, Department of Veterinary Biosciences, College of Veterinary Medicine, The Ohio State University, 1925 Coffey Road, Columbus, Ohio 43210. Voice: +614-292-9677; fax: +614-292-6473.
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sion to frequently documented severe morbidity to death.5,6 HME and HGA are prevalent, life-threatening tick-borne zoonoses in North America and were designated as a nationally notifiable diseases in 1998.7 The disease continues to be a public health problem, since wild animals such as white-tailed deer and whitefooted mice are reservoirs of these bacteria.8,9 E. chaffeensis has been identified most commonly in the Lone Star tick (Amblyomma americanum).8,10 A. phagocytophilum has been found in the deer tick (Ixodes scapularis) and other Ixodes spp.9,11 Signaling Events in the Infected Host Cells A. phagocytophilum and E. chaffeensis are obligatory intracellular bacteria that replicate inside
Rickettsiology and Rickettsial Diseases-Fifth International Conference: Ann. N.Y. Acad. Sci. 1166: 106–111 (2009). c 2009 New York Academy of Sciences. doi: 10.1111/j.1749-6632.2009.04527.x
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mammalian granulocytes and monocytesmacrophages, respectively; primary immune defensive cells that normally are responsible for powerful innate antimicrobial defenses. Recently, unique strategies employed by these intravacuolar bacteria for successful parasitism have begun to be unraveled.12,13 A common mechanism shared by these bacteria is the ability to subvert multiple innate immune responses of host cells.13 Furthermore, this group of bacteria inhibits host cell apoptosis to maximize intracellular bacterial reproduction.14–17 A. phagocytophilum and E. chaffeensis induce their internalization into host cells without eliciting signals that induce microbicidal activities. For internalization, these bacteria usurp caveolae-mediated endocytosis, which directs pathogens to an intracellular compartment secluded from late endosome or lysosome markers and NADPH oxidase components.18–22 Both early and replicative E. chaffeensis and A. phagocytophilum inclusions are co-localized with tyrosine-phosphorylated proteins and PLC-γ2, which are required for infection of host cells.18,23 Despite sharing several common features as described above, E. chaffeensis and A. phagocytophilum inclusions are distinct from each other, and thus, the 2 species never co-localize in the same inclusions even after co-infection of the same HL-60 cell.21 E. chaffeensis inclusions retain the early endosome characteristics including rab5 and early endosome antigen 1 (EEA1), and fuse with transferrin receptor (TfR) endosomes.20,21 Exogenous iron-loaded transferrin (Tf) actually enters into the E. chaffeensis replicative inclusion through the TfR-Tf endosome recycling pathway.20 In contrast, the inclusions of A. phagocytophilum are negative for these endosomal markers.21 Recently, several hallmarks of early autophagosomes were detected in A. phagocytophilum replicative inclusions, including a double lipid bilayer membrane, and colocalization with GFP-tagged LC3 and Beclin 1, the human homologs of Saccharomyces cerevisiae autophagy-related proteins Atg8 and Atg6, respectively.24 Stimulation of autophagy by ra-
pamycin favors A. phagocytophilum infection. Inhibition of the autophagosomal pathway by 3-methyladenine does not inhibit A. phagocytophilum internalization, but reversibly arrests its growth.24 Although autophagy is considered part of the innate immune response that clears a variety of intracellular pathogens, A. phagocytophilum subverts this system to establish itself in an early autophagosome-like compartment, segregated from lysosomes, to facilitate its proliferation. How do these bacteria down-regulate a number of critical innate immune responses and modify vesicular traffic to create a sheltered niche in host cells? Our central hypothesis is that the bacterial type IV secretion (T4S) system has an important role in these processes. For analysis of A. phagocytophilum and E. chaffeensis factors, we now have much better tools than ever before, since several members of genus Rickettsia and family Anaplasmataceae have been sequenced. Complete genome sequences of A. phagocytophilum (1.47 Mbp) and E. chaffeensis (1.18 Mbp) were obtained and compared to each other and to the published Rickettsiales genome sequences. E. chaffeensis and A. phagocytophilum genomes are syntenic. The 2 species share approximately 640 genes25 with approximately 470–580 genes that are unique to each species. These latter genes are mostly of unknown function and presumably are responsible for the important phenotypic differences between these 2 organisms. T4S System T4S system transports macromolecules across the membrane in an ATP-dependent manner and is ancestrally related to the conjugation system of gram-negative bacteria. The T4S system is increasingly recognized as a virulence factor delivery mechanism that allows pathogens to modulate eukaryotic cell functions for their own benefit.26 There are at least 2 ancestral lineages for the T4S system: the virB/virD system of Agrobacterium tumefaciens
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Figure 1. virB/D gene loci on the E. chaffeensis genome. The genome map of E. chaffeensis is shown as a circle. The putative origin of replication is shown as oriC . The virB/D gene loci are shown as black arrows. The length of each arrow is enlarged to approximately 10 times of the size in the genome.
and the dot/icm system of Legionella pneumophila, sometimes referred as T4aS and T4bS systems, respectively. For facultative intracellular bacteria, including Legionella, Bartonella, and Brucella species, the T4S system is essential for their intracellular survival.27–29 In the most extensively studied T4aS system from Ag. tumefaciens, the single virB operon, along with virD4, encodes 12 membrane-associated proteins that form a transmembrane channel complex.30 Genes encoding T4aS system: vir orthologs (virB2, B3, B4, B6, B8, B9, B10, B11, and D4), have been identified in E. chaffeensis and A. phagocytophilum.25,31,32 These virB/D genes are split into 2 major operons: sodB-virB3–virB4-virB6– 1-virB6–2-virB6–3-virB6–4 and virB8–1-virB9– 1-virB10-virB11–virD4. Between these 2 operons, there are virB4–2, multiple virB2, virB8–2, and virB9–2 (A genomic map of E. chaffeensis virB/D is shown in Fig. 1). Analysis of recent whole-genome sequence databases indicates conservation of this split operon structure in other members of the order Rickettsiales.33–37 The A. phagocytophilum T4aS system is expected to function during mammalian leukocyte infection in vitro and in vivo, since 1) both virB8–
Annals of the New York Academy of Sciences
virD4 and sodB–virB6 operons are polycistronically transcribed in A. phagocytophilum replicating in HL-60 promyelocytic leukemia cells,32 2) A. phagocytophilum virB9 gene is transcribed in peripheral blood leukocytes from HGA patients and from experimentally infected animals,32 3) in the closely related monocyte-tropic Ehrlichia canis, virB9 is expressed in the blood from infected dogs and infected canine monocyte DH82 cell cultures,38 and 4) A. phagocytophilum AnkA protein can be delivered into the host cell cytoplasm in a VirB/D-dependent manner.39 The expression of T4S system is not constitutive, but is regulated during the A. phagocytophilum and E. chaffeensis intracellular life cycle.31,40 Both virB9 and virB6 of A. phagocytophilum are up-regulated at the mRNA level and VirB9 at the protein level during infection of human neutrophils in vitro31 and 5 virB/D loci (except VirB2s) were up-regulated during the exponential growth stage of E. chaffeensis synchronously cultured in THP-1 human monocytic leukemia cells.40 In contrast, the majority of A. phagocytophilum spontaneously released from infected host cells poorly expresses the VirB9 protein.31 Transcription of 5 virB/D loci is down-regulated prior to the release of E. chaffeensis from host THP-1 cells.40 In fact, proteomic analysis identified a unique DNA binding protein, EcxR, which coordinately regulates 5 virB/D loci of E. chaffeensis.40 Thus, this modulation of bacterial virB/D gene expression during the establishment of bacterial infection may promote intracellular survival and rapid replication in a safe haven and resistance when exposed to the extracellular environment, and/or the next round of infection of host cells. In addition, the T4aS system is expected to function during tick infection as well, since virB9 is expressed by E. canis in tick tissues.38 Some of duplicated virB paralogs of these bacteria may be reserved to function specifically during tick stages. Recently, differential transcription of several A. phagocytophilum VirB2 paralogs in mammalian and ISE6 tick cells were reported.41
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The delivered macromolecules are referred to as T4S substrates or effectors because they affect and alter basic host cellular processes, resulting in disease development. Two T4aS effector molecules, CagA of Helicobacter pyroli 42 and pertussis toxin,43 have been shown to play a major role in mammalian disease pathogenesis. The T4S system has been demonstrated as being essential for the creation of cytoplasmic replicative compartments unique to each of several facultative intracellular bacteria such as Brucella, Bartonella, and Legionella.44,45 There is direct evidence that over 30 effector molecules are translocated into the host cell during Legionella infection.45 Particularly, a large and diverse family of proteins containing ankyrin-repeat homology domains of L. pneumophila and Coxiella burnetii are translocated into eukaryotic cells by pathogen-associated T4bS system. The L. pneumophila AnkX protein prevented microtubule-dependent vesicular transport to interfere with fusion of the L. pneumophila-containing vacuole with late endosomes after infection of macrophages.46 Recently, we demonstrated that A. phagocytophilum 160-kDa ankyrin repeat protein, AnkA is delivered by a VirB/D-dependent manner into the host leukocyte cytoplasm and subsequently is tyrosine phosphorylated.39 AnkA binds to Abl-interactor 1 that interacts with Abl-1 tyrosine kinase, thus mediating AnkA phosphorylation. AnkA and Abl-1 are critical for A. phagocytophilum infection, as infection was inhibited upon host cytoplasmic delivery of antiAnkA antibody, Abl-1 knockdown with targeted siRNA, or treatment with a specific pharmacological inhibitor of Abl-1.39 Studies showed that A. phagocytophilum specifically induces tyrosine phosphorylation of a 190kDa AnkA (AnkA molecular size is bacterial strain-dependent), which then interact with the host cell tyrosine phosphatase SHP-1.47 AnkA also was reported to localize within nuclei of infected HL-60 cells and bind to the internucleosomal region of HL-60 cell DNA and DNA fragments containing genes encoding ATPase, tyrosine phosphatase and
NADH dehydrogenase-like functions, and nuclear proteins in a cell-free system.48,49 Ag. tumefaciens VirD4, a membrane-associated ATPase, is involved in the recognition of substrates of Ag. tumefaciens T4aSV system.50,51 Our recent study identified several additional T4S effector candidates in A. phagocytophilum and E. chaffeensis via bacterial 2-hybrid system using A. phagocytophilum and E. chaffeensis VirD4 as bait. One of them is an A. phagocytophilum hypothetical protein, named Anaplasma Translocation Substrate 1 (Ats-1). Ats-1 is expressed by A. phagocytophilum in the inclusion, translocated across the inclusion membrane, and localized in the mitochondria of infected human neutrophils and HL-60 cells. Ats-1 in mitochondria inhibits etoposideinduced apoptosis and cytochrome c release from mitochondria.24,52 Further research dissecting the functional role of these candidates will lead to a better understanding of the host processes that facilitate obligatory intracellular bacterial survival and infection. Acknowledgments
Some of studies in the authors’ laboratory reported in this review were supported by grants R01 AI054476 from the National Institutes of Health. Conflicts of Interest
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