Sequence and expression analysis of virB9 of the type IV secretion system of Ehrlichia canis strains in ticks, dogs, and cultured cells

Role and Function of the Type IV Secretion System in Anaplasma and Ehrlichia Species

The obligatory intracellular pathogens Anaplasma phagocytophilum and Ehrlichia chaffeensis proliferate within membrane-bound vacuoles of human leukocytes and cause potentially fatal emerging infectious diseases. Despite the reductive genome evolution in this group of bacteria, genes encoding the type IV secretion system (T4SS), which is homologous to the VirB/VirD4 system of the plant pathogen Agrobacterium tumefaciens, have been expanded and are highly expressed in A. phagocytophilum and E. chaffeensis in human cells. Of six T4SS effector proteins identified in them, roles and functions have been described so far only for ankyrin repeat domain-containing protein A (AnkA), Anaplasma translocated substrate 1 (Ats-1), and Ehrlichia translocated factor 1 (Etf-1, ECH0825). These effectors are abundantly produced and secreted into the host cytoplasm during infection, but not toxic to host cells. They contain eukaryotic protein motifs or organelle localization signals and have distinct subcellular localization, target to specific host cell molecules to promote infection. Ats-1 and Etf-1 are orthologous proteins, subvert two important innate immune mechanisms against intracellular infection, cellular apoptosis and autophagy, and manipulate autophagy to gain nutrients from host cells. Although Ats-1 and Etf-1 have similar functions and roles in obligatory intracellular infection, they are specifically adapted to the distinct membrane-bound intracellular niche of A. phagocytophilum and E. chaffeensis, respectively. Ectopic expression of these effectors enhances respective bacterial infection, whereas intracellular delivery of antibodies against these effectors or targeted knockdown of the effector with antisense peptide nucleic acid significantly impairs bacterial infection. Thus, both T4SSs have evolved as important survival and nutritional virulence mechanism in these obligatory intracellular bacteria. Future studies on the functions of Anaplasma and Ehrlichia T4SS effector molecules and signaling pathways will undoubtedly advance our understanding of the complex interplay between obligatory intracellular pathogens and their hosts. Such data can be applied toward the treatment and control of anaplasmosis and ehrlichiosis.

VirB10 vaccination for protection against Anaplasma phagocytophilum

Background

Human granulocytic anaplasmosis (HGA) is a tick-borne disease caused by the etiologic agent Anaplasma phagocytophilum. HGA was designated a nationally notifiable disease in the United States in 1998. Currently there are no vaccines available against HGA. Conserved membrane proteins that are subdominant in Anaplasma species, such as VirB9 and VirB10, may represent better vaccine targets than the variable immunodominant surface proteins. VirB9 and VirB10 are constituents of the Type 4 secretion system (T4SS) that is conserved amongst many intracellular bacteria and performs essential functions for invasion and survival in host cells.

Results

Immunogenicity and contribution to protection, provided after intramuscular vaccination of plasmid DNA encoding VirB9-1, VirB9-2, and VirB10 followed by inoculation of homologous recombinant proteins, in a prime-boost immunization strategy was evaluated in a murine model of HGA. Recombinant VirB9-1-, VirB9-2-, and VirB10-vaccinated mice developed antibody responses that specifically reacted with A. phagocytophilum organisms. However, only the mice vaccinated with VirB10 developed a significant increase in IFN-γ CD4⁺ T cells and partial protection against challenge with A. phagocytophilum.

Conclusions

This work provides evidence that A. phagocytophilum T4SS VirB10 is partially protective in a murine model against infection in an IFN-γ-dependent fashion and suggests that this protein may be a potential vaccine candidate against this and possibly other pathogenic bacteria with a T4SS.

Impact of Three Different Mutations in Ehrlichia chaffeensis in Altering the Global Gene Expression Patterns

The rickettsial pathogen Ehrlichia chaffeensis causes a tick-borne disease, human monocytic ehrlichiosis. Mutations within certain genomic locations of the pathogen aid in understanding the pathogenesis and in developing attenuated vaccines. Our previous studies demonstrated that mutations in different genomic sites in E. chaffeensis caused variable impacts on their growth and attenuation in vertebrate and tick hosts. Here, we assessed the effect of three mutations on transcriptional changes using RNA deep-sequencing technology. RNA sequencing aided in detecting 66-80% of the transcripts of wildtype and mutant E. chaffeensis. Mutation in an antiporter gene (ECH_0379) causing attenuated growth in vertebrate hosts resulted in the down regulation of many transcribed genes. Similarly, a mutation downstream to the ECH_0490 coding sequence resulted in minimal impact on the pathogen's in vivo growth, but caused major changes in its transcriptome. This mutation caused enhanced expression of several host stress response genes. Even though the ECH_0660 gene mutation caused the pathogen's rapid clearance in vertebrate hosts and aids in generating a protective response, there was minimal impact on the transcriptome. The transcriptomic data offer novel insights about the impact of mutations on global gene expression and how they may contribute to the pathogen's resistance and/or clearance from the host.

Cloning, expression and purification of virB10 protein of Brucella melitensis and evaluation of its role as a serological marker for Brucella infection in experimental and natural host

Brucellosis is a zoonotic disease caused by various species of the genus Brucella. The control of this disease mainly depends on its accurate and early diagnosis. Culture methods employed for diagnosis are time consuming and require well equipped biosafety level 3 laboratories and hence serological tests are favored alternative for brucellosis diagnosis. At present serological diagnosis is based on LPS (lipopolysaccharide) which is less specific as it shows cross reactivity with other gram-negative bacteria. There is a need to develop serological diagnostic assay based on purified recombinant antigen of Brucella. T4SS (Type IV Secretion System) is an important virulent factor of Brucella and required for infection suggesting their expression in vivo and can be targeted as serological marker for infection. To test this concept, the present study is designed to clone, express and purify virB10 gene of Brucella T4SS under denaturing conditions and to evaluate its use as serological marker of Brucella infection. The immunoreactivity of this recombinant antigen was checked with antisera collected after experimental infection in Balb/C mice with B. melitensis 16M, BR31 (human clinical isolate) and Y. enterocolitica O:9. The recombinant protein was also tested against a panel of 46 bovine sera samples collected from Leh, India. Antibody response against VirB10 was detected in experimental and natural host suggesting that it can be explored as potential target for serodiagnosis of Brucella infection.

Tick-Borne Emerging Infections: Ehrlichiosis and Anaplasmosis

Human ehrlichiosis and anaplasmosis are acute febrile tick-borne infectious diseases caused by various members from the genera Ehrlichia and Anaplasma. Ehrlichia chaffeensis is the major etiologic agent of human monocytotropic ehrlichiosis (HME), while Anaplasma phagocytophilum is the major cause of human granulocytic anaplasmosis (HGA). The clinical manifestations of HME and HGA ranges from subclinical to potentially life-threatening diseases associated with multi-organ failure. Macrophages and neutrophils are the major target cells for Ehrlichia and Anaplasma, respectively. The threat to public health is increasing with newly emerging ehrlichial and anaplasma agents, yet vaccines for human ehrlichioses and anaplasmosis are not available, and therapeutic options are limited. This article reviews recent advances in the understanding of HME and HGA.

Comparative Proteomic Profiling of Ehrlichia ruminantium Pathogenic Strain and Its High-Passaged Attenuated Strain Reveals Virulence and Attenuation-Associated Proteins

The obligate intracellular bacterium Ehrlichia ruminantium (ER) causes heartwater, a fatal tick-borne disease in livestock. In the field, ER strains present different levels of virulence, limiting vaccine efficacy, for which the molecular basis remains unknown. Moreover, there are no genetic tools currently available for ER manipulation, thus limiting the knowledge of the genes/proteins that are essential for ER pathogenesis and biology. As such, to identify proteins and/or mechanisms involved in ER virulence, we performed the first exhaustive comparative proteomic analysis between a virulent strain (ERGvir) and its high-passaged attenuated strain (ERGatt). Despite their different behaviors in vivo and in vitro, our results from 1DE-nanoLC-MS/MS showed that ERGvir and ERGatt share 80% of their proteins; this core proteome includes chaperones, proteins involved in metabolism, protein-DNA-RNA biosynthesis and processing, and bacterial effectors. Conventional 2DE revealed that 85% of the identified proteins are proteoforms, suggesting that post-translational modifications (namely glycosylation) are important in ER biology. Strain-specific proteins were also identified: while ERGatt has an increased number and overexpression of proteins involved in cell division, metabolism, transport and protein processing, ERGvir shows an overexpression of proteins and proteoforms (DIGE experiments) involved in pathogenesis such as Lpd, AnkA, VirB9 and B10, providing molecular evidence for its increased virulence in vivo and in vitro. Overall, our work reveals that ERGvir and ERGatt proteomes are streamlined to fulfill their biological function (maximum virulence for ERGvir and replicative capacity for ERGatt), and we provide both pioneering data and novel insights into the pathogenesis of this obligate intracellular bacterium.

Occurrence of Ehrlichia canis and Anaplasma platys in household dogs from northern Parana

Canine monocytic ehrlichiosis caused primarily by Ehrlichia canis and canine thrombocytic anaplasmosis induced by Anaplasma platys are important emerging zoonotic tick-borne diseases of dogs. There is evidence that these pathogens can also affect humans. This study evaluated the presence of E. canis and A. platys in blood samples collected from 256 domiciled dogs in the municipality of Jataizinho, located in north region of the State of Parana, Brazil, by PCR assay. The occurrence of E. canis and A. platys was 16.4% (42/256) and 19.4% (49/256), respectively; while 5.47% (14/256) of the dogs evaluated were co-infected by these two organisms. The presence of E. canis and A. platys was not significantly associated with the variables evaluated (sex, age, outdoor access, and presence of ticks during blood collection). Infection of dogs by E. canis was associated with anemia and thrombocytopenia, while infection induced by A. platys was related only to thrombocytopenia. Canine monocytic ehrlichiosis and canine thrombocytic anaplasmosis should be included in the differential diagnoses when these hematological alterations are observed during routine laboratory evaluation of dogs.

Evaluation of an attenuated strain of Ehrlichia canis as a vaccine for canine monocytic ehrlichiosis

Canine monocytic ehrlichiosis is an important tick-borne disease worldwide. No commercial vaccine for the disease is currently available and tick control is the main preventive measure against the disease. The aim of this study was to evaluate the potential of a multi-passaged attenuated strain of Ehrlichia canis to serve as a vaccine for canine monocytic ehrlichiosis, and to assess the use of azithromycin in the treatment of acute ehrlichiosis. Twelve beagle dogs were divided into 3 groups of 4 dogs. Groups 1 and 2 were inoculated (vaccinated) with an attenuated strain of E. canis (#611A) twice or once, respectively. The third group consisted of naïve dogs which served as controls. All 3 groups were challenged with a wild virulent strain of E. canis by administering infected dog-blood intravenously. Transient thrombocytopenia was the only hematological abnormality observed following inoculation of dogs with the attenuated strain. Challenge with the virulent strain resulted in severe disease in all 4 control dogs while only 3 of 8 vaccinated dogs presented mild transient fever. Furthermore, the mean blood rickettsial load was significantly higher in the control group (27-92-folds higher during days 14-19 post challenge with the wild the strain) as compared to the vaccinated dogs. The use of azithromycin was assessed as a therapeutic agent for the acute disease. Four days treatment resulted in further deterioration of the clinical condition of the dogs. Molecular comparison of 4 genes known to express immunoreactive proteins and virulence factors (p30, gp19, VirB4 and VirB9) between the attenuated strain and the challenge wild strain revealed no genetic differences between the strains. The results of this study indicate that the attenuated E. canis strain may serve as an effective and secure future vaccine for canine ehrlichiosis.

Association and evidence for linked recognition of type IV secretion system proteins VirB9-1, VirB9-2, and VirB10 in Anaplasma marginale

Like several other bacterial pathogens, Anaplasma marginale has an outer membrane that induces complete protection from infection and disease. However, the proteins that confer protective immunity and whether protection requires interacting proteins and/or linked T-cell and immunoglobulin G epitopes are not known. Our goal is to target the conserved type IV secretion system (T4SS) to identify conserved, immunogenic membrane proteins that are interacting and linked recognition candidates. Linked recognition is a process by which a B cell is optimally activated by a helper T cell that responds to the same, or physically associated, antigen. A. marginale T4SS proteins VirB2, VirB4-1, VirB4-2, VirB6-1, VirB7, VirB8-2, VirB9-1, VirB9-2, VirB10, VirB11, and VirD4 were screened for their ability to induce IgG and to stimulate CD4+ T cells from outer membrane-vaccinated cattle. VirB9-1, VirB9-2, and VirB10 induced the strongest IgG and T-cell responses in the majority of cattle, although three animals with major histocompatibility complex class II DRB3 restriction fragment length polymorphism types 8/23, 3/16, and 16/27 lacked T-cell responses to VirB9-1, VirB9-1 and VirB9-2, or VirB9-2 and VirB10, respectively. For these animals, VirB9-1-, VirB9-2-, and VirB10-specific IgG production may be associated with T-cell help provided by responses to an interacting protein partner(s). Interacting protein partners indicated by far-Western blotting were confirmed by immunoprecipitation assays and revealed, for the first time, specific interactions of VirB9-1 with VirB9-2 and VirB10. The immunogenicity and interactions of VirB9-1, VirB9-2, and VirB10 justify their testing as a linked protein vaccine against A. marginale.

Type IV secretion in the obligatory intracellular bacterium Anaplasma phagocytophilum

Anaplasma phagocytophilum is an obligatory intracellular bacterium that infects neutrophils, the primary host defence cells. Consequent effects of infection on host cells result in a potentially fatal systemic disease called human granulocytic anaplasmosis. Despite ongoing reductive genome evolution and deletion of most genes for intermediary metabolism and amino acid biosynthesis, Anaplasma has also experienced expansion of genes encoding several components of the type IV secretion (T4S) apparatus. Two A. phagocytophilum T4S effector molecules are currently known; Anaplasma translocated substrate 1 (Ats-1) and ankyrin repeat domain-containing protein A (AnkA) have C-terminal positively charged amino acid residues that are recognized by the T4S coupling protein, VirD4. AnkA and Ats-1 contain eukaryotic protein motifs and are uniquely evolved in the family Anaplasmataceae; Ats-1 contains a mitochondria-targeting signal. They are abundantly produced and secreted into the host cytoplasm, are not toxic to host cells, and manipulate host cell processes to aid in the infection process. At the cellular level, the two effectors have distinct subcellular localization and signalling in host cells. Thus in this obligatory intracellular pathogen, the T4S system has evolved as a host-subversive survival factor.

Phylogenomics reveals a diverse Rickettsiales type IV secretion system

With an obligate intracellular lifestyle, Alphaproteobacteria of the order Rickettsiales have inextricably coevolved with their various eukaryotic hosts, resulting in small, reductive genomes and strict dependency on host resources. Unsurprisingly, large portions of Rickettsiales genomes encode proteins involved in transport and secretion. One particular transporter that has garnered recent attention from researchers is the type IV secretion system (T4SS). Homologous to the well-studied archetypal vir T4SS of Agrobacterium tumefaciens, the Rickettsiales vir homolog (rvh) T4SS is characterized primarily by duplication of several of its genes and scattered genomic distribution of all components in several conserved islets. Phylogeny estimation suggests a single event of ancestral acquirement of the rvh T4SS, likely from a nonalphaproteobacterial origin. Bioinformatics analysis of over 30 Rickettsiales genome sequences illustrates a conserved core rvh scaffold (lacking only a virB5 homolog), with lineage-specific diversification of several components (rvhB1, rvhB2, and rvhB9b), likely a result of modifications to cell envelope structure. This coevolution of the rvh T4SS and cell envelope morphology is probably driven by adaptations to various host cells, identifying the transporter as an important target for vaccine development. Despite the genetic intractability of Rickettsiales, recent advancements have been made in the characterization of several components of the rvh T4SS, as well as its putative regulators and substrates. While current data favor a role in effector translocation, functions in DNA uptake and release and/or conjugation cannot at present be ruled out, especially considering that a mechanism for plasmid transfer in Rickettsia spp. has yet to be proposed.

Anaplasma marginale type IV secretion system proteins VirB2, VirB7, VirB11, and VirD4 are immunogenic components of a protective bacterial membrane vaccine

Anaplasma and related Ehrlichia spp. are important tick-borne, Gram-negative bacterial pathogens of livestock and humans that cause acute infection and disease and can persist. Immunization of cattle with an Anaplasma marginale fraction enriched in outer membranes (OM) can provide complete protection against disease and persistent infection. Serological responses of OM vaccinees to the OM proteome previously identified over 20 antigenic proteins, including three type IV secretion system (T4SS) proteins, VirB9-1, VirB9-2, and VirB10. Subsequent studies showed that these three proteins also stimulated CD4(+) T-cell responses in OM vaccinees. The T4SS, composed of a complex of proteins spanning the inner and outer membranes of certain bacteria, is an important virulence factor but is relatively unexplored as a vaccine target. The goal of this study was to determine if additional T4SS proteins are immunogenic for animals immunized with the protective OM fraction of A. marginale. T4SS proteins expressed by in vitro transcription and translation were screened for stimulating proliferation of T cells from OM vaccinees, and immunogenic proteins were expressed as recombinant proteins in Escherichia coli and their immunogenicity was verified. VirB2, a putative VirB7, VirB11, and VirD4 were immunogenic for OM vaccinees expressing several common major histocompatibility complex (MHC) class II haplotypes. VirB2 is encoded by multiple genes that share a conserved central region, and epitope mapping revealed T-cell epitopes in this region. The discovery of novel immunogenic T4SS proteins recognized by outbred individuals with common MHC haplotypes further justifies evaluating the T4SS as a potential vaccine candidate for pathogenic bacteria.

Anaplasma phagocytophilum and Ehrlichia chaffeensis type IV secretion and Ank proteins

The obligatory intracellular bacterial pathogens Anaplasma and Ehrlichia infect leukocytes by hijacking host-cell components and processes. The type IV secretion system is up-regulated during infection. Among type IV secretion candidate substrates, an ankyrin repeat protein of Anaplasma phagocytophilum, AnkA, is delivered into the host cytoplasm via a complex that includes VirD4. AnkA is highly tyrosine phosphorylated and binds to the Abl interactor 1, SHP-1, and nuclear DNA fragments. Ehrlichia chaffeensis AnkA was recently reported to be translocated into host-cell nucleus. The recent discovery of several ankyrin repeat proteins secreted via the type IV secretion system of different intracellular bacteria suggests that a common strategy evolved to subvert host-cell functions.

Biological diversity of prokaryotic type IV secretion systems

Type IV secretion systems (T4SS) translocate DNA and protein substrates across prokaryotic cell envelopes generally by a mechanism requiring direct contact with a target cell. Three types of T4SS have been described: (i) conjugation systems, operationally defined as machines that translocate DNA substrates intercellularly by a contact-dependent process; (ii) effector translocator systems, functioning to deliver proteins or other macromolecules to eukaryotic target cells; and (iii) DNA release/uptake systems, which translocate DNA to or from the extracellular milieu. Studies of a few paradigmatic systems, notably the conjugation systems of plasmids F, R388, RP4, and pKM101 and the Agrobacterium tumefaciens VirB/VirD4 system, have supplied important insights into the structure, function, and mechanism of action of type IV secretion machines. Information on these systems is updated, with emphasis on recent exciting structural advances. An underappreciated feature of T4SS, most notably of the conjugation subfamily, is that they are widely distributed among many species of gram-negative and -positive bacteria, wall-less bacteria, and the Archaea. Conjugation-mediated lateral gene transfer has shaped the genomes of most if not all prokaryotes over evolutionary time and also contributed in the short term to the dissemination of antibiotic resistance and other virulence traits among medically important pathogens. How have these machines adapted to function across envelopes of distantly related microorganisms? A survey of T4SS functioning in phylogenetically diverse species highlights the biological complexity of these translocation systems and identifies common mechanistic themes as well as novel adaptations for specialized purposes relating to the modulation of the donor-target cell interaction.

Molecular events involved in cellular invasion by Ehrlichia chaffeensis and Anaplasma phagocytophilum

Ehrlichia chaffeensis and Anaplasma phagocytophilum are obligatory intracellular bacteria that preferentially replicate inside leukocytes by utilizing biological compounds and processes of these primary host defensive cells. These bacteria incorporate cholesterol from the host for their survival. Upon interaction with host monocytes and granulocytes, respectively, these bacteria usurp the lipid raft domain containing GPI-anchored protein to induce a series of signaling events that result in internalization of the bacteria. Monocytes and neutrophils usually kill invading microorganisms by fusion of the phagosomes containing the bacteria with granules containing both antimicrobial peptides and lysosomal hydrolytic enzymes and/or through sequestering vital nutrients. However, E. chaffeensis and A. phagocytophilum alter vesicular traffic to create a unique intracellular membrane-bound compartment that allows their replication in seclusion from lysosomal killing. These bacteria are quite sensitive to reactive oxygen species (ROS), so in order to survive in host cells that are primary mediators of ROS-induced killing, they inhibit activation of NADPH oxidase and assembly of this enzyme in their inclusion compartments. Moreover, host phagocyte activation and differentiation, apoptosis, and IFN-gamma signaling pathways are inhibited by these bacteria. Through reductive evolution, lipopolysaccharide and peptidoglycan that activate the innate immune response, have been eliminated from these gram-negative bacteria at the genomic level. Upon interaction with new host cells, bacterial genes encoding the Type IV secretion apparatus and the two-component regulatory system are up-regulated to sense and adapt to the host environment. Thus dynamic signal transduction events concurrently proceed both in the host cells and in the invading E. chaffeensis and A. phagocytophilum bacteria for successful establishment of intracellular infection. Several bacterial surface-exposed proteins and porins are recently identified. Further functional studies on Ehrlichia and Anaplasma effector or ligand molecules and cognate host cell receptors will undoubtedly advance our understanding of the complex interplay between obligatory intracellular pathogens and their hosts. Such data can be applied towards treatment, diagnosis, and control of ehrlichiosis and anaplasmosis.

Type IV secretion system of Anaplasma phagocytophilum and Ehrlichia chaffeensis

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.

VirB12 is a serological marker of Brucella infection in experimental and natural hosts

The Brucella species type IV secretion system, encoded by the virB1-12 locus, is required for intracellular replication and persistent infection in vivo. The requirement of VirB proteins for infection suggests that they are expressed in vivo and may therefore represent serological markers of infection. To test this idea, we purified recombinant VirB1, VirB5, VirB11, and VirB12 and tested for their recognition by antibodies in sera from experimentally infected mice and goats by using an indirect enzyme-linked immunosorbent assay. Antibody responses to VirB12 but not to VirB1, VirB5, or VirB11 were detected in 20/20 mice experimentally inoculated with Brucella abortus and 12/12 goats experimentally infected with Brucella melitensis. The potential use of VirB12 as a serological tool for the diagnosis of brucellosis was evaluated in the natural bovine host. Serum samples from 145 cattle of known serology (29% negative and 71% positive) were analyzed for the production of antibody responses to VirB12. One hundred two cattle samples (70.3%) were positive for antibodies to VirB12, while 43 samples were negative (29.7%). A positive serological response to VirB12 correlated with positive serology to whole B. abortus antigen in 99% of samples tested. These results show that VirB12 is expressed during infection of both experimental and natural hosts of Brucella species, and they suggest that VirB12 may be a useful serodiagnostic marker for brucellosis.

Immunogenicity of Anaplasma marginale type IV secretion system proteins in a protective outer membrane vaccine

Rickettsial pathogens in the genera Anaplasma and Ehrlichia cause acute infection in immunologically naive hosts and are major causes of tick-borne disease in animals and humans. Immunization with purified outer membranes induces protection against acute Anaplasma marginale infection and disease, and a proteomic and genomic approach recently identified 21 proteins within the outer membrane immunogen in addition to the well-characterized major surface proteins MSP1 to MSP5. Among the newly described proteins were the type IV secretion system (TFSS) proteins VirB9, VirB10, and conjugal transfer protein (CTP). In other gram-negative bacteria, TFSS proteins form channels, facilitate secretion of molecules, and are required for intracellular survival. However, TFSS proteins have not been explored as vaccine antigens. In this study we demonstrate that in Anaplasma marginale outer membrane-vaccinated cattle, VirB9, VirB10, and CTP are recognized by serum immunoglobulin G2 (IgG2) and stimulate memory T-lymphocyte proliferation and gamma interferon secretion. VirB9 induced the greatest proliferation in CD4+ T-cell lines, and VirB9-specific CD4+ T-cell clones responded to three A. marginale strains, confirming the VirB9-specific T-cell responses are directed against epitopes in the native protein. The three TFSS proteins are highly conserved with orthologous proteins in Anaplasma phagocytophilum, Ehrlichia chaffeensis, and Ehrlichia canis. Recognition of TFSS antigens by CD4+ T cells and by IgG2 from cattle immunized with the protective outer membrane fraction provides a rationale for including these proteins in development of vaccines against A. marginale and related pathogens.

Human Infection with Ehrlichia Canis Accompanied by Clinical Signs in Venezuela

A total of 20 human patients with clinical signs compatible with human monocytic ehrlichiosis (HME), who were admitted to the emergency clinic in Lara State, Venezuela, were studied. Thirty percent (6/20) patients were positive for Ehrlichia canis 16S rRNA on gene-specific polymerase chain reaction (PCR). Compared with the U.S. strains, 16S rRNA gene sequences from all six patients had the same base mutation as the sequence of the E. canis Venezuelan human Ehrlichia (VHE) strain previously isolated from an asymptomatic human. This study is the first report of E. canis infection of human patients with clinical signs of HME.

Cytokine Gene Expression by Peripheral Blood Leukocytes in Dogs Experimentally Infected with a New Virulent Strain of Ehrlichia canis

Ehrlichia canis (E. canis) is a lipopolysaccharide (LPS)-deficient obligatory intracellular bacterium that causes canine monocytic ehrlichiosis, a chronic febrile disease accompanied with hematological abnormality. This study analyzed temporal expression levels of IL-1beta, IL-2, IL-6, IL-8, IFN-gamma, and TNF-alpha mRNA by peripheral blood leukocytes from dogs experimentally infected with a new virulent strain of E. canis by using real-time RT-PCR. Relative levels of IL-1beta and IL-8 transcripts normalized by the beta-actin transcript levels, were significantly upregulated, whereas those of TNF-alpha and IFN-gamma transcripts were only weakly upregulated in all three infected dogs, starting from 2 days up to 52 days post inoculation. The expressions of IL-2 and IL-6 genes were extremely low compared with the positive control (ConA-stimulated canine peripheral blood leukocytes). This study showed that E. canis can induce chronic expression of a subset of proinflammatory cytokine genes: balance, timing, and duration of these cytokine generations may contribute to the progression of canine ehrlichiosis.

The genome of the obligately intracellular bacterium Ehrlichia canis reveals themes of complex membrane structure and immune evasion strategies

Ehrlichia canis, a small obligately intracellular, tick-transmitted, gram-negative, alpha-proteobacterium, is the primary etiologic agent of globally distributed canine monocytic ehrlichiosis. Complete genome sequencing revealed that the E. canis genome consists of a single circular chromosome of 1,315,030 bp predicted to encode 925 proteins, 40 stable RNA species, 17 putative pseudogenes, and a substantial proportion of noncoding sequence (27%). Interesting genome features include a large set of proteins with transmembrane helices and/or signal sequences and a unique serine-threonine bias associated with the potential for O glycosylation that was prominent in proteins associated with pathogen-host interactions. Furthermore, two paralogous protein families associated with immune evasion were identified, one of which contains poly(G-C) tracts, suggesting that they may play a role in phase variation and facilitation of persistent infections. Genes associated with pathogen-host interactions were identified, including a small group encoding proteins (n = 12) with tandem repeats and another group encoding proteins with eukaryote-like ankyrin domains (n = 7).

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.

Differentially expressed and secreted major immunoreactive protein orthologs of Ehrlichia canis and E. chaffeensis elicit early antibody responses to epitopes on glycosylated tandem repeats

Ehrlichia canis major immunoreactive proteins of 36 and 19 kDa elicit the earliest detectable antibody responses during the acute phase of canine monocytic ehrlichiosis. Genes encoding the major immunoreactive 36-kDa protein of E. canis and the corresponding ortholog of E. chaffeensis (47 kDa) were identified and the proteins characterized. The molecular masses of the strongly immunoreactive recombinant proteins were larger than predicted (26.7 and 32.9 kDa, respectively) but were consistent with those of the corresponding native proteins (36 and 47 kDa). Similar to other reported ehrlichial immunoreactive glycoproteins, carbohydrate was detected on the recombinant expressed proteins, indicating that they were glycoproteins. Both glycoproteins (gp36 and gp47) have carboxy-terminal serine/threonine-rich tandem repeat regions containing repeats that vary in number (4 to 16 repeats) and amino acid sequence among different isolates of each species. E. canis gp36 was recognized by early acute-phase antibodies (day 14), and species-specific antibody epitopes were mapped to C-terminal nonhomologous repeat units of gp36 and gp47. Periodate treatment of recombinant gp36 reduced the antibody reactivity, and nonglycosylated synthetic peptide repeat units from E. canis gp36 and E. chaffeensis gp47 were substantially less immunoreactive than corresponding recombinant peptides, demonstrating that glycans are important epitope determinants that are structurally conserved on the recombinant proteins expressed in Escherichia coli. E. canis gp36 and E. chaffeensis gp47 were differentially expressed only on the surface of dense-cored ehrlichiae and detected in the Ehrlichia-free supernatants, indicating that these proteins are released extracellularly during infection.

Some lessons from Rickettsia genomics

Sequencing of the Rickettsia conorii genome and its comparison with its closest sequenced pathogenic relative, i.e., Rickettsia prowazekii, provided powerful insights into the evolution of these microbial pathogens. However, advances in our knowledge of rickettsial diseases are still hindered by the difficulty of working with strict intracellular bacteria and their hosts. Information gained from comparing the genomes of closely related organisms will shed new light on proteins susceptible to be targeted in specific diagnostic assays, by new antimicrobial drugs, and that could be employed in the generation of future rickettsial vaccines. In this review we present a detailed comparison of the metabolic pathways of these bacteria as well as the polymorphisms of their membrane proteins, transporters and putative virulence factors. Environmental adaptation of Rickettsia is also discussed.

The evolution of chronic infection strategies in the alpha-proteobacteria

Many of the alpha-proteobacteria establish long-term, often chronic, interactions with higher eukaryotes. These interactions range from pericellular colonization through facultative intracellular multiplication to obligate intracellular lifestyles. A common feature in this wide range of interactions is modulation of host-cell proliferation, which sometimes leads to the formation of tumour-like structures in which the bacteria can grow. Comparative genome analyses reveal genome reduction by gene loss in the intracellular alpha-proteobacterial lineages, and genome expansion by gene duplication and horizontal gene transfer in the free-living species. In this review, we discuss alpha-proteobacterial genome evolution and highlight strategies and mechanisms used by these bacteria to infect and multiply in eukaryotic cells.