The 120 kDa outer membrane protein of Ehrlichia chaffeensis: preferential expression on dense-core cells and gene expression in Escherichia coli associated with attachment and entry

Molecular characterization of antibody epitopes of Ehrlichia chaffeensis ankyrin protein 200 and tandem repeat protein 47 and evaluation of synthetic immunodeterminants for serodiagnosis of human monocytotropic ehrlichiosis

Recently, major species-specific antibody epitopes in three immunoreactive tandem repeat proteins (TRPs) of Ehrlichia chaffeensis, TRP32, TRP47, and TRP120, have been identified and molecularly characterized within tandem repeat (TR) regions. In this study, we mapped the major immunodeterminants of the E. chaffeensis 200-kDa ankyrin protein (Ank200) and the minor immunodeterminants in the N- and C-terminal regions of E. chaffeensis TRP47. Major antibody epitopes of Ank200 were localized to four polypeptide regions (18-mer, 20-mer, 20-mer, and 21-mer, respectively) in terminal acidic domains, which reacted with antibodies in sera from human monocytotropic ehrlichiosis (HME) patients and an E. chaffeensis-infected dog. Two minor epitope-containing regions were identified in the N terminus and the C terminus of TRP47. The sensitivities and specificities of synthetic peptides representing these and other well-defined major immunodeterminants of E. chaffeensis were determined by enzyme-linked immunosorbent assay (ELISA). Thirty-one HME patient serum samples that had detectable E. chaffeensis antibodies (titers from 64 to 8,192) by indirect fluorescent-antibody assay (IFA) were tested. All 31 serum samples reacted with at least one E. chaffeensis peptide, 30 (96.8%) with TRP120 peptides, 27 (87.1%) with TRP32 peptides, 24 (77.4%) with TRP47 peptides, 19 (61.3%) with Ank200 peptides, and 28 (90.3%) with recombinant TRP120-TR protein. A mixture of the two most sensitive peptides from TRP120 and TRP32 did not provide enhanced analytical sensitivity compared to that provided by TRP120 alone. Our results demonstrate that the TRP120 peptide can be utilized for development of standardized sensitive point-of-care and reference laboratory immunodiagnostics for HME. This is the first study to compare analysis of molecularly defined major antibody epitopes with IFA for diagnosis of HME.

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.

Major species-specific antibody epitopes of the Ehrlichia chaffeensis p120 and E. canis p140 orthologs in surface-exposed tandem repeat regions

Ehrlichia chaffeensis and E. canis have a small subset of tandem repeat (TR)-containing protein orthologs, including p120/p140, which elicit strong antibody responses. The TR regions of these protein orthologs are immunoreactive, but the molecular characteristics of the p120/p140 epitopes have not been determined. In this study, the immunodeterminants of the E. chaffeensis p120 and E. canis p140 were identified and molecularly defined. Major antibody epitope-containing regions of both p120 and p140 were localized to the TR regions, which reacted strongly by Western immunoblotting with antibodies in sera from E. chaffeensis-infected dogs or patients and E. canis-infected dogs, respectively. Single continuous species-specific major epitopes within the E. chaffeensis p120 and E. canis p140 TRs were mapped to homologous surface-exposed glutamate/aspartate-rich regions (19 to 22 amino acids). In addition, minor cross-reactive epitopes were localized to homologous N- and C-terminal regions of p120 and p140. Furthermore, although the native and recombinant p120 and p140 proteins exhibited higher-than-predicted molecular masses, posttranslational modifications were not present on abnormally migrating p120 and p140 TR recombinant proteins as determined by matrix-assisted laser desorption ionization-time of flight mass spectrometry.

An Ehrlichia chaffeensis tandem repeat protein interacts with multiple host targets involved in cell signaling, transcriptional regulation, and vesicle trafficking

Ehrlichia chaffeensis is an obligately intracellular bacterium that exhibits tropism for mononuclear phagocytes forming cytoplasmic membrane-bound microcolonies called morulae. To survive and replicate within phagocytes, E. chaffeensis exploits the host cell by modulating a number of host cell processes, but the ehrlichial effector proteins involved are unknown. In this study, we determined that p47, a secreted, differentially expressed, tandem repeat (TR) protein, interacts with multiple host proteins associated with cell signaling, transcriptional regulation, and vesicle trafficking. Yeast two-hybrid analysis revealed that p47 interacts with polycomb group ring finger 5 (PCGF5) protein, Src protein tyrosine kinase FYN (FYN), protein tyrosine phosphatase non-receptor type 2 (PTPN2), and adenylate cyclase-associated protein 1 (CAP1). p47 interaction with these proteins was further confirmed by coimmunoprecipitation assays and colocalization in HeLa cells transfected with p47-green fluorescent fusion protein (AcGFP1-p47). Moreover, confocal microscopy demonstrated p47-expressing dense-cored (DC) ehrlichiae colocalized with PCGF5, FYN, PTPN2, and CAP1. An amino-terminally truncated form of p47 containing TRs interacted only with PCGF5 and not with FYN, PTPN2, and CAP1, indicating differences in p47 domains that are involved in these interactions. These results demonstrate that p47 is involved in a complex network of interactions involving numerous host cell proteins. Furthermore, this study provides a new insight into the molecular and functional distinction of DC ehrlichiae, as well as the effector proteins involved in facilitating ehrlichial survival in mononuclear phagocytes.

Total, membrane, and immunogenic proteomes of macrophage- and tick cell-derived Ehrlichia chaffeensis evaluated by liquid chromatography-tandem mass spectrometry and MALDI-TOF methods

Ehrlichia chaffeensis, a tick-transmitted rickettsial, is the causative agent of human monocytic ehrlichiosis. To examine protein expression patterns, we analyzed total, membrane, and immunogenic proteomes of E. chaffeensis originating from macrophage and tick cell cultures. Total proteins resolved by one-dimensional gel electrophoresis and subjected to liquid chromatography-electrospray ionization ion trap mass spectrometry allowed identification of 134 and 116 proteins from macrophage- and tick cell-derived E. chaffeensis, respectively. Because a majority of immunogenic proteins remained in the membrane fraction, individually picked total and immunogenic membrane proteins were also surveyed by liquid chromatography-tandem mass spectrometry and matrix-assisted laser desorption ionization-time of flight methods. The analysis aided the identification of 48 additional proteins. In all, 278 genes of the E. chaffeensis genome were verified as functional genes. They included genes for DNA and protein metabolism, energy metabolism and transport, membrane proteins, hypothetical proteins, and many novel proteins of unknown function. The data reported in this study suggest that the membrane of E. chaffeensis is very complex, having many expressed proteins. This study represents the first and the most comprehensive analysis of E. chaffeensis-expressed proteins. This also is the first study confirming the expression of nearly one-fourth of all predicted genes of the E. chaffeensis genome, validating that they are functionally active genes, and demonstrating that classic shotgun proteomic approaches are feasible for tick-transmitted intraphagosomal bacteria. The identity of novel expressed proteins reported in this study, including the large selection of membrane and immunogenic proteins, will be valuable in elucidating pathogenic mechanisms and developing effective prevention and control methods.

A variable-length PCR target protein of Ehrlichia chaffeensis contains major species-specific antibody epitopes in acidic serine-rich tandem repeats

Ehrlichia chaffeensis and E. canis have a small subset of tandem repeat (TR)-containing proteins that elicit strong host immune responses and are associated with host-pathogen interactions. In a previous study, we molecularly characterized a highly conserved 19-kDa major immunoreactive protein (gp19) of E. canis and identified the corresponding TR-containing ortholog variable-length PCR target (VLPT) protein in E. chaffeensis. In this study, the native 32-kDa VLPT protein was identified and the immunodeterminants defined in order to further understand the molecular basis of the host immune response to E. chaffeensis. Synthetic and/or recombinant polypeptides corresponding to various regions of VLPT were used to localize major antibody epitopes to the TR-containing region. Major antibody epitopes were identified in three nonidentical repeats (R2, R3, and R4), which reacted strongly with antibodies in sera from an E. chaffeensis-infected dog and human monocytotropic ehrlichiosis patients. VLPT-R3 and VLPT-R2 reacted most strongly with antibody, and the epitope was further localized to a nearly identical proximal 17-amino-acid region common between these repeats that was species specific. The epitope in R4 was distinct from that of R2 and R3 and was found to have conformational dependence. VLPT was detected in supernatants from infected cells, indicating that the protein was secreted. VLPT was localized on both reticulate and dense-core cells, and it was found extracellularly in the morula fibrillar matrix and associated with the morula membrane.

Molecular Pathology of Rickettsial Lung Infections

Rickettsial infections of humans comprise a diverse group of infections caused by pathogens that are obligate intracellular bacteria with a genetic relationship, including the genera Rickettsia, Orientia, Ehrlichia, and Anaplasma. The host cells of these pathogens largely belie the systemic clinical manifestations, because Rickettsia and Orientia infect endothelial cells, and Ehrlichia and Anaplasma infect circulating leukocytes (monocytes and neutrophils, respectively). Thus, the predominant manifestations (fever, headache, myalgia, with or without rash) do not usually focus attention on the respiratory system; however, the underlying pathogenesis of these infections involves degrees of vascular compromise either by direct injury and inflammation or by the action of vasoactive proinflammatory molecules such as cytokines, chemokines, and prostaglandins. Given that the lung possesses the largest vascular bed in the human body, it is not surprising that pulmonary involvement is periodically identified and, when severely affected, is considered a potentially life-threatening complication.1,2

Ehrlichia canis gp200 contains dominant species-specific antibody epitopes in terminal acidic domains

Species-specific antibody epitopes within several major immunoreactive protein orthologs of Ehrlichia species have recently been identified and molecularly characterized. In this study, dominant B-cell epitopes within the acidic (pI 5.35) ankyrin repeat-containing 200-kDa major immunoreactive protein (gp200) of Ehrlichia canis were defined. The E. canis gp200 gene (4,263 bp; 1,421 amino acids) was cloned and expressed as four (N-terminal, 1,107 bp; N-internal, 910 bp; C-internal, 1,000 bp; and C-terminal, 1,280 bp) overlapping recombinant proteins. The N-terminal, C-internal, and C-terminal polypeptides (369, 332, and 426 amino acids, respectively) were strongly recognized by antibody, and the major epitope(s) in these polypeptides was mapped to four polypeptide regions (40 to 70 amino acids). Smaller overlapping recombinant polypeptides (14 to 15 amino acids) spanning these regions identified five strongly immunoreactive species-specific epitopes that exhibited conformational dependence. The majority of the epitopes (four) were located in two strongly acidic (pI 4 to 4.9) domains in the distal N- and C-terminal regions of the protein flanking the centralized ankyrin domain-containing region. The amino acid content of the epitope-containing domains included a high proportion of strongly acidic amino acids (glutamate and aspartate), and these domains appear to have important biophysical properties that influence the antibody response to gp200.

The cytoplasmic tails of Uukuniemi Virus (Bunyaviridae) G(N) and G(C) glycoproteins are important for intracellular targeting and the budding of virus-like particles

Functional motifs within the cytoplasmic tails of the two glycoproteins G(N) and G(C) of Uukuniemi virus (UUK) (Bunyaviridae family) were identified with the help of our recently developed virus-like particle (VLP) system for UUK virus (A. K. Overby, V. Popov, E. P. Neve, and R. F. Pettersson, J. Virol. 80:10428-10435, 2006). We previously reported that information necessary for the packaging of ribonucleoproteins into VLPs is located within the G(N) cytoplasmic tail (A. K. Overby, R. F. Pettersson, and E. P. Neve, J. Virol. 81:3198-3205, 2007). The G(N) glycoprotein cytoplasmic tail specifically interacts with the ribonucleoproteins and is critical for genome packaging. In addition, two other regions in the G(N) cytoplasmic tail, encompassing residues 21 to 25 and 46 to 50, were shown to be important for particle generation and release. By the introduction of point mutations within these two regions, we demonstrate that leucines at positions 23 and 24 are crucial for the initiation of VLP budding, while leucine 46, glutamate 47, and leucine 50 are important for efficient exit from the endoplasmic reticulum and subsequent transport to the Golgi complex. We found that budding and particle generation are highly dependent on the intracellular localization of both glycoproteins. The short cytoplasmic tail of UUK G(C) contains a lysine at position -3 from the C terminus that is highly conserved among members of the Phlebovirus, Hantavirus, and Orthobunyavirus genera. Mutating this single amino acid residue in G(C) resulted in the mislocalization of not only G(C) but also G(N) to the plasma membrane, and VLP generation was compromised in cells expressing this mutant. Together, these results demonstrate that the cytoplasmic tails of both G(N) and G(C) contain specific information necessary for efficient virus particle generation.

Surface-exposed proteins of Ehrlichia chaffeensis

The surface proteins of Ehrlichia chaffeensis provide an important interface for pathogen-host interactions. To investigate the surface proteins of E. chaffeensis, membrane-impermeable, cleavable Sulfo-NHS-SS-Biotin was used to label intact bacteria. The biotinylated bacterial surface proteins were isolated by streptavidin-agarose affinity purification. The affinity-captured proteins were separated by electrophoresis, and five relatively abundant protein bands containing immunoreactive proteins were subjected to capillary-liquid chromatography-nanospray tandem mass spectrometry analysis. Nineteen out of 22 OMP-1/P28 family proteins, including P28 (which previously was shown to be surface exposed), were detected in E. chaffeensis cultured in human monocytic leukemia THP-1 cells. For the first time, with the exception of P28 and P28-1, 17 OMP-1/P28 family proteins were demonstrated to be expressed at the protein level. The surface exposure of OMP-1A and OMP-1N was verified by immunofluorescence microscopy. OMP-1B was undetectable either by surface biotinylation or by Western blotting of the whole bacterial lysate, suggesting that it is not expressed by E. chaffeensis cultured in THP-1 cells. Additional E. chaffeensis surface proteins detected were OMP85, hypothetical protein ECH_0525 (here named Esp73), immunodominant surface protein gp47, and 11 other proteins. The identification of E. chaffeensis surface-exposed proteins provides novel insights into the E. chaffeensis surface and lays the foundation for rational studies on pathogen-host interactions and vaccine development.

Restriction and expansion of Ehrlichia strain diversity

Ehrlichia are tick-borne gram negative, obligately intracellular bacteria. The 16S rRNA gene DNA sequences are highly conserved among strains of each Ehrlichia species. The 28-kDa/Map-1 outer membrane protein genes are highly diversified among strains of Ehrlichia chaffeensis and E. ruminantium, but are highly conserved among E. canis isolates. The diversity of the immunodominant proteins of E. chaffeensis and E. ruminantium in contrast with the conservation of the immunodominant proteins of E. canis suggests that E. chaffeensis and E. ruminantium face more host immune pressure than E. canis or that E. chaffeensis and E. ruminantium evolved earlier than E. canis and have diverged.

The developmental cycle of Ehrlichia chaffeensis in vertebrate cells

Ehrlichia chaffeensis, an obligatory intracellular bacterium, has two forms in mammalian cells: small dense-cored cells (DC) with dense nucleoid and larger reticulate cells (RC) with uniformly dispersed nucleoid. We have determined by electron microscopy that DC but not RC attaches to and enters into the host cells and RC but not DC multiples inside the host cells. Analysis of outer membrane protein expression by confocal microscopy showed that RC expressed the 28 kDa outer membrane protein (p28), the intermediate form, which were transforming from RC to DC, expressed both gp120 and p28, and the mature DC expressed gp120 only. The TCID50 of DC is 6 log10 higher than RC. We conclude that E. chaffeensis has a developmental cycle, in which the DC attaches to and enters into the host cells, and transforms into RC and the RC multiplies by binary fission for 48 h and then matures into DC at 72 h.

Generation and analysis of infectious virus-like particles of uukuniemi virus (bunyaviridae): a useful system for studying bunyaviral packaging and budding

In the present report we describe an infectious virus-like particle (VLP) system for the Uukuniemi (UUK) virus, a member of the Bunyaviridae family. It utilizes our recently developed reverse genetic system based on the RNA polymerase I minigenome system for UUK virus used to study replication, encapsidation, and transcription by monitoring reporter gene expression. Here, we have added the glycoprotein precursor expression plasmid together with the minigenome, nucleoprotein, and polymerase to generate VLPs, which incorporate the minigenome and are released into the supernatant. The particles are able to infect new cells, and reporter gene expression can be monitored if the trans-acting viral proteins (RNA polymerase and nucleoprotein) are also expressed in these cells. No minigenome transfer occurred in the absence of glycoproteins, demonstrating that the glycoproteins are absolutely required for the generation of infectious particles. Moreover, expression of glycoproteins alone was sufficient to produce and release VLPs. We show that the ribonucleoproteins (RNPs) are incorporated into VLPs but are not required for the generation of particles. Morphological analysis of the particles by electron microscopy revealed that VLPs, either with or without minigenomes, display a surface morphology indistinguishable from that of the authentic UUK virus and that they bud into Golgi vesicles in the same way as UUK virus does. This infectious VLP system will be very useful for studying the bunyaviral structural components required for budding and packaging of RNPs and receptor binding and may also be useful for the development of new vaccines for the human pathogens from this family.

Overcoming barriers to the transformation of the genus Ehrlichia

While bacterial transformation has evolved since the early 20th century to allow for the genetic manipulation of a variety of microbial agents, rickettsial organisms have proved resistant to such advances until only recently. The Ehrlichia are small, gram-negative, obligately intracellular bacterial parasites, which belong to the family Anaplasmataceae and cause a variety of infections in human and animal hosts. E. chaffeensis is the causative agent of human monocytotropic ehrlichiosis and is transmitted by Amblyomma americanum, the Lone Star tick. In this work, we describe the first report of successful transformation of a closely related ehrlichial species, the murine monocytotropic species E. muris. Application of these techniques should allow for a wide variety of molecular studies to be performed that were previously impossible. This heralds the beginning of a new era in ehrlichial research.

Co-infection of White-Tailed Deer with Multiple Strains ofEhrlichia chaffeensis

We investigated the effect of exposing deer to multiple strains of Ehrlichia chaffeensis that differed in number of tandem repeats in either the variable-length PCR target (VLPT) gene or 120 kDa antigen gene. We hypothesized that infection with one strain would provide immunity to infection with other strains of E. chaffeensis. All deer initially exposed to strain A (604-2) became PCR and culture positive by 10 days post-infection (DPI). Three deer infected with strain A and subsequently inoculated with strain B (623-4) became infected with strain B. Two deer infected with strain A and subsequently inoculated with strain C (125B) became infected with strain C. Of three deer, each infected with strain B and subsequently inoculated with strain C, one was PCR positive for strain C. Of three deer previously inoculated with both strains A and B, and subsequently inoculated with strain C, one showed delayed evidence of strain C. Western blot analysis demonstrated that deer sera reacted differently to antigens from each exposed strain. A complementary in vitro study demonstrated that exposure to two strains differing in VLPT repeats may lead to co-infection of DH82 cells. These results complement a previous study and further show that deer can become sequentially infected with up to three strains of E. chaffeensis. This suggests that competitive exclusion, a phenomenon described in related organisms such as Anaplasma marginale whereby infection with one strain precludes subsequent infection by a second, distinct strain of the same species, may not occur with E. chaffeensis.

Ehrlichia under our noses and no one notices

Ehrlichia chaffeensis, an obligately intracellular bacterium, resides within a cytoplasmic vacuole in macrophages, establishes persistent infection in natural hosts such as white-tailed deer and canids, and is transmitted transstadially and during feeding by ticks, particularly Amblyomma americanum. Ehrlichial cell walls contain glycoproteins and a family of divergent 28 kDa proteins, but no peptidoglycan or lipopolysaccharide. The dense-cored ultrastructural form preferentially expresses certain glycoproteins, including a multiple repeat unit-containing adhesin. Ehrlichiae attach to L-selectin and E-selectin, inhibit phagolysosomal fusion, apoptosis, and JAK/STAT activation, and downregulate IL-12, IL-15, IL-18, TLR2 and 3, and CD14. Mouse models implicate overproduction of TNF-alpha by antigen-specific CD8 T lymphocytes in pathogenesis and strong type 1 CD4 and CD8 T lymphocyte responses, synergistic activities of IFN-gamma and TNF-alpha, and IgG2a antibodies in immunity. Human monocytotropic ehrlichiosis (HME) manifests as a flu-like illness that progresses in severity to resemble toxic shock-like syndrome, with meningoencephalitis or adult respiratory distress syndrome in some patients, and requires hospitalization in half. In immunocompromised patients, HME acts as an overwhelming opportunistic infection. In one family physician's practice, active surveillance for three years revealed an incidence of 1000 cases per million population. Diagnosis employs serology or polymerase chain reaction, which are not utilized sufficiently to establish the true impact of this emerging virus-like illness.

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.

Primary and Secondary Infection withEhrlichia chaffeensisin White-Tailed Deer (Odocoileus virginianus)

White-tailed deer (Odocoileus virginianus) are the principal reservoir host for Ehrlichia chaffeensis, causative agent of human monocytic ehrlichiosis (HME). Because white-tailed deer maintain a long-term infection with E. chaffeensis and because deer can be naturally exposed to multiple strains of E. chaffeensis, we evaluated the response to secondary infection of E. chaffeensis in deer. For primary infection, six white-tailed deer were injected with 5.4 x 10(6) DH82 cells infected with the Arkansas strain of E. chaffeensis (Ark) and two control deer were injected with noninfected DH82 cells. On post-infection day 54, three E. chaffeensis (Ark) infected deer and one naive deer were injected with 4.2 x 10(6) cells infected with strain WTD-6045B E. chaffeensis, which differs from the Arkansas strain by number of nucleotide repeats in the variable length PCR target (VLPT) gene; three other Arkansas strain infected deer were injected with noninfected DH82 cells. All animals were monitored for 31 additional days. All deer in the primary infection became positive by PCR amplification of the 16S rRNA or VLPT genes and/or cell culture by DPI-8. PCR amplification of the VLPT gene on whole blood, cell culture, and tissues detected primary and/or secondary strains in all deer exposed to both primary and secondary strains; in one deer, the primary strain was cultured from the lymph node. Our culture results demonstrated that both strains were present; however, PCR detection suggests that the secondary strain may have been circulating in blood at higher levels. In conclusion, this study provides evidence that primary infection of deer with E. chaffeensis does not protect against subsequent exposure and confirms that deer can be simultaneously coinfected with at least two different strains of E. chaffeensis.

Invasion of the central nervous system by intracellular bacteria

Infection of the central nervous system (CNS) is a severe and frequently fatal event during the course of many diseases caused by microbes with predominantly intracellular life cycles. Examples of these include the facultative intracellular bacteria Listeria monocytogenes, Mycobacterium tuberculosis, and Brucella and Salmonella spp. and obligate intracellular microbes of the Rickettsiaceae family and Tropheryma whipplei. Unfortunately, the mechanisms used by intracellular bacterial pathogens to enter the CNS are less well known than those used by bacterial pathogens with an extracellular life cycle. The goal of this review is to elaborate on the means by which intracellular bacterial pathogens establish infection within the CNS. This review encompasses the clinical and pathological findings that pertain to the CNS infection in humans and includes experimental data from animal models that illuminate how these microbes enter the CNS. Recent experimental data showing that L. monocytogenes can invade the CNS by more than one mechanism make it a useful model for discussing the various routes for neuroinvasion used by intracellular bacterial pathogens.

Identification and molecular characterization of a gene encoding a protective Leishmania amazonensis Trp-Asp (WD) protein

Several Leishmania proteins have been identified and characterized in pursuit of understanding pathogenesis and protection in cutaneous leishmaniasis. In the present study, we utilized sera from infected BALB/c mice to screen a Leishmania amazonensis amastigote cDNA expression library and obtained the full-length gene that encodes a novel Trp-Asp (WD) protein designated LAWD (for Leishmania antigenic WD protein). The WD family of proteins mediates protein-protein interactions and coordinates the formation of protein complexes. The single-copy LAWD gene is transcribed as a approximately 3.1-kb mRNA in both promastigotes and amastigotes, with homologues being detected in several other Leishmania species. Immunoelectron microscopy revealed a predominant localization of the LAWD protein in the flagellar pocket. Analyses of sera from human patients with cutaneous and mucocutaneous leishmaniasis indicated that these individuals mounted significant humoral responses against LAWD. Given that recombinant LAWD protein elicited the production of high levels of gamma interferon, but no detectable levels of interleukin-10 (IL-10), in CD4(+) cells of L. amazonensis-infected mice, we further examined whether LAWD could elicit protective immunity. DNA vaccination with the LAWD and IL-12 genes significantly delayed lesion development, which correlated with a dramatic reduction in parasite burdens. Thus, we have successfully identified a promising vaccine candidate and antigenic vehicle to aid in the dissection of the complicated pathogenic immune response of L. amazonensis.

Experimental Ehrlichia chaffeensis infection in beagles

A canine model for human monocytic ehrlichiosis was used to assess persistent infection and antigenic variation of Ehrlichia chaffeensis. Two beagle dogs were infected subcutaneously with E. chaffeensis Arkansas strain. The dogs were observed for 6 months after inoculation for clinical signs, blood chemistry changes, antibodies to E. chaffeensis and presence of E. chaffeensis in the blood. Both dogs developed thrombocytopenia, but exhibited normal body temperatures during the entire course of infection. In one dog, E. chaffeensis was cultivated for up to 74 days post-inoculation and E. chaffeensis DNA was detected in the dog's blood for up to 81 days. In the other dog, E. chaffeensis was cultured for up to 102 days and E. chaffeensis DNA was detected in the blood for up to 117 days. PCR amplification and DNA sequence analysis indicated that there was no genetic variation in the 120 kDa outer-membrane glycoprotein gene of E. chaffeensis during infection of the dogs. The dogs developed antibodies to the immunodominant proteins of E. chaffeensis, including the 175, 140, 120, 80, 50 and 28 kDa proteins, starting in the fifth week post-inoculation. The dogs maintained high antibody titres throughout the 6-month study period. These results indicate that dogs become carriers of E. chaffeensis for 2-4 months after infection without exhibiting signs of clinical disease, suggesting that dogs may serve as a natural host for E. chaffeensis.

Molecular variation in the variable-length PCR target and 120-kilodalton antigen genes of Ehrlichia chaffeensis from white-tailed deer (Odocoileus virginianus)

Genes encoding two surface-expressed antigens of Ehrlichia chaffeensis, the variable-length PCR target (VLPT) and the 120-kDa antigen, which contain variable numbers of tandem repeats, were characterized for E. chaffeensis from white-tailed deer (Odocoileus virginianus). Both genes from infected deer contained numbers of repeats similar to those reported in genes from humans and ticks, although a new variant of the 120-kDa antigen gene containing five repeat units and coinfection with multiple VLPT and 120-kDa antigen gene genetic types were detected. Sequence analysis of both genes revealed more nucleotide variation than previously reported for E. chaffeensis from infected humans or ticks. This is the most extensive study of E. chaffeensis VLPT and 120-kDa antigen gene genetic variation to date and is the first to examine genetic variation in E. chaffeensis from a nonhuman vertebrate host.

Anaplasma phagocytophilummajor surface protein-2 (Msp2) forms multimeric complexes in the bacterial membrane

Anaplasma phagocytophilum 44-kDa major surface protein-2 (Msp2) mediates partial neutrophil adhesion and interactions. Since A. phagocytophilum 44-kDa monoclonal antibodies also react with 160- and 100-kDa bands, a putative adhesin complex was studied. After separate excision/immunoprecipitation of these three bands, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) resolved each into three bands again with increased 44-kDa protein under reducing conditions suggesting oligomerization of Msp2 44-kDa monomers. With 9 M urea, each separately excised band was resolved only into 44-kDa monomers with three different pIs. With protein cross-linking, immunoblots showed four additional bands and increased high molecular mass band intensity, suggesting homo- and hetero-polymerization with other A. phagocytophilum proteins. Recognition of Msp2 complexes facilitates understanding of A. phagocytophilum-neutrophil adhesion.

Novel Immunoreactive glycoprotein orthologs of Ehrlichia spp

We previously identified a strongly immunoreactive 43 kDa protein (p43) of Ehrlichia canis. As an immunodiagnostic antigen, the p43 had a 96% accuracy as compared with IFA and provided species-specific diagnosis of E. canis infections. Further investigation has revealed that the E. canis p43 represents the N-terminal portion of the largest immunoreactive protein described in Ehrlichia spp. with a predicted molecular mass of 153 kDa. Analysis of the recombinant N-terminal region (p43) of the p153 by protein gel electrophoresis demonstrated a larger than predicted molecular mass (approximately 30%), and presence of carbohydrate glycans, indicating that the p153 is a glycoprotein. A BLASTn search was performed on the E. chaffeensis genome sequence (95%), and the gene encoding the p153 ortholog was identified in E. chaffeensis. The E. canis p153 (4,263 bp) and E. chaffeensis p156 (4,389 bp) genes had similar chromosomal locations, downstream of the homologous (approximately 87%) deoxyguanosine-triphosphate triphosphohydrolase genes, and homologous (approximately 90%) intergenic sequences preceding the open reading frames. Nucleic acid sequence homology (52%) observed between the glycoprotein genes supported previous findings with regard to genetic divergence of the p43 gene fragment, and the p153 and p156 proteins had amino acid similarity of 32%. A native E. canis protein with a molecular mass of 200 kDa reacted with antisera produced against the N-terminal region (p43) of the p153, suggesting that the native protein was posttranslationally modified. Similarly, recombinant constructs of E. chaffeensis p156 migrated larger than predicted (approximately 200 kDa), and carbohydrate was detected on the recombinant proteins. The chromosomal location, amino acid homology, and biophysical properties support the conclusion that the p153 and p156 glycoproteins (designated gp200s) are species-specific immunoreactive orthologs.

The MPB83 antigen from Mycobacterium bovis contains O-linked mannose and (1-->3)-mannobiose moieties

Mycobacterium tuberculosis and Mycobacterium bovis, the causative agents of human and bovine tuberculosis, have been reported to express a range of surface and secreted glycoproteins, although only one of these has been subjected to detailed structural analysis. We describe the use of a genetic system, in conjunction with lectin binding, to characterize the points of attachment of carbohydrate moieties to the polypeptide backbone of a second mycobacterial glycoprotein, antigen MPB83 from M. bovis. Biochemical and structural analysis of the native MPB83 protein and derived peptides demonstrated the presence of 3 mannose units attached to two threonine residues. Mannose residues were joined by a (1 --> 3) linkage, in contrast to the (1 --> 2) linkage previously observed in antigen MPT32 from M. tuberculosis and the (1 --> 2) and (1 --> 6) linkages in other mycobacterial glycolipids and polysaccharides. The identification of glycosylated antigens within the M. tuberculosis complex raises the possibility that the carbohydrate moiety of these glycoproteins might be involved in pathogenesis, either by interaction with mannose receptors on host cells, or as targets or modulators of the cell-mediated immune response. Given such a possibility characterization of mycobacterial glycoproteins is a step toward understanding their functional role and elucidating the mechanisms of mycobacterial glycosylation.

Differential expression of the enolase gene under in vivo versus in vitro growth conditions of Aeromonas hydrophila

Aeromonas hydrophila is an emerging human pathogen that leads to gastroenteritis and other invasive diseases. By using a murine peritoneal culture (MPC) model, we identified via restriction fragment differential display PCR (RFDDPCR) five genes of A. hydrophila that were differentially expressed under in vivo versus in vitro growth conditions. The gene encoding enolase was among those five genes that were differentially up regulated. Enolase is a glycolytic enzyme and its surface expression was recently shown to be important in the pathogenesis of a gram-positive bacterium Streptococcus pyogenes. By Western blot analysis and Immunogold staining, we demonstrated secretion and surface expression of enolase in A. hydrophila. We also showed that the whole cells of A. hydrophila had strong enolase activity. Using an enzyme-linked immunosorbant assay and sandwich Western blot analysis, we demonstrated binding of enolase to human plasminogen, which is involved in the fibrinolytic system of the host. We cloned the A. hydrophila enolase gene, which exhibited 62% homology at the DNA level and 57% homology at the amino acid level when compared to S. pyogenes enolase. This is a first report describing the increased expression of enolase gene in vivo that could potentially contribute to the pathogenesis of A. hydrophila infections.

Characterization of the functional domain of major surface protein 1a involved in adhesion of the rickettsia Anaplasma marginale to host cells

The major surface protein (MSP) 1a of the genus type species Anaplasma marginale (Rickettsiales: Anaplasmataceae) has been shown to mediate adhesion, infection and transmission of the organism, as well as to contribute to protective immunity in cattle. MSP1a contains a variable number of tandemly repeated peptides in the amino-terminal region, while the remainder of the protein is highly conserved among isolates. The number of repeats varies among geographic isolates of A. marginale but is constant within an isolate and has been used as a stable genetic marker of isolate identity. Because the sequence of the tandem repeats is the most variable part of the protein among isolates, this region of the protein is most likely to be involved in adhesion to host cells, a prerequisite to infection. The purpose of this study was to characterize the organization and function of the MSP1a tandem repeats of A. marginale in adhesion to host cells. We demonstrated by use of recombinant mutant proteins that the tandemly repeated region of MSP1a was necessary and sufficient to mediate adhesion of MSP1a to tick cells and bovine erythrocytes. Synthetic peptides representing the predominant sequences of individual repeats were tested for their adhesive capacity for tick cell extract (TCE). Peptides containing acidic amino acids D or E at position 20 bound to TCE, while peptides with a G as the 20th amino acid were not adhesive to TCE. Antibodies produced in rabbits against a synthetic repeat peptide neutralized A. marginale infection of cultured tick cells, and the neutralization observed was similar to that effected by antibodies produced against the whole MSP1a recombinant protein. Analysis of tandemly repeated MSP1a peptides of several geographic isolates of A. marginale revealed a complex relationship between the msp1alpha genotype and the tick-transmissible phenotype of the isolate and suggested that both the sequence and conformation of the repeated peptides influenced the adhesive properties of MSP1a. These studies demonstrated that the tandemly repeated region of the protein mediates the adhesive function of MSP1a.

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.

Human ehrlichioses

Human ehrlichioses represent one of the best examples of newly emergent infectious diseases in which the classic triad of host, infectious agent, and environment are intertwined closely. These pathogens have existed for eons on the planet, and some were described as veterinary pathogens decades ago. Because of dramatic increases of deer and small mammal populations in certain areas and the subsequent increased populations of particular blood-feeding ticks, the risk of developing these diseases is higher than before. Increasing human populations in suburban areas and increased immunosuppressed populations (transplant patients, human immunodeficiency virus patients, and cancer survivors) also have increased risk of developing severe forms of these diseases.

CD4(+) T lymphocytes from calves immunized with Anaplasma marginale major surface protein 1 (MSP1), a heteromeric complex of MSP1a and MSP1b, preferentially recognize the MSP1a carboxyl terminus that is conserved among strains

Native major surface protein 1 (MSP1) of the ehrlichial pathogen Anaplasma marginale induces protective immunity in calves challenged with homologous and heterologous strains. MSP1 is a heteromeric complex of a single MSP1a protein covalently associated with MSP1b polypeptides, of which at least two (designated MSP1F1 and MSP1F3) in the Florida strain are expressed. Immunization with recombinant MSP1a and MSP1b alone or in combination fails to provide protection. The protective immunity in calves immunized with native MSP1 is associated with the development of opsonizing and neutralizing antibodies, but CD4(+) T-lymphocyte responses have not been evaluated. CD4(+) T lymphocytes participate in protective immunity to ehrlichial pathogens through production of gamma interferon (IFN-gamma), which promotes switching to high-affinity immunoglobulin G (IgG) and activation of phagocytic cells to produce nitric oxide. Thus, an effective vaccine for A. marginale and related organisms should contain both T- and B-lymphocyte epitopes that induce a strong memory response that can be recalled upon challenge with homologous and heterologous strains. This study was designed to determine the relative contributions of MSP1a and MSP1b proteins, which contain both variant and conserved amino acid sequences, in stimulating memory CD4(+) T-lymphocyte responses in calves immunized with native MSP1. Peripheral blood mononuclear cells and CD4(+) T-cell lines from MSP1-immunized calves proliferated vigorously in response to the immunizing strain (Florida) and heterologous strains of A. marginale. The conserved MSP1-specific response was preferentially directed to the carboxyl-terminal region of MSP1a, which stimulated high levels of IFN-gamma production by CD4(+) T cells. In contrast, there was either weak or no recognition of MSP1b proteins. Paradoxically, all calves developed high titers of IgG antibodies to both MSP1a and MSP1b polypeptides. These findings suggest that in calves immunized with MSP1 heteromeric complex, MSP1a-specific T lymphocytes may provide help to MSP1b-specific B lymphocytes. The data provide a basis for determining whether selected MSP1a CD4(+) T-lymphocyte epitopes and selected MSP1a and MSP1b B-lymphocyte epitopes presented on the same molecule can stimulate a protective immune response.

Molecular cloning and characterization of the 120-kilodalton protein gene of Ehrlichia canis and application of the recombinant 120-kilodalton protein for serodiagnosis of canine ehrlichiosis

The 120-kDa outer membrane protein (p120) is a potential adhesin of Ehrlichia chaffeensis, and recombinant p120 is very useful for serodiagnosis of human monocytotropic ehrlichiosis. The analogous gene of p120 in Ehrlichia canis was cloned, sequenced, and expressed. Like the E. chaffeensis p120, the E. canis p120 contains tandem repeat units. However, neither the repeat number nor the amino acid sequences in the repeats are identical in the two Ehrlichia species. The repeat units are hydrophilic and by probability analysis are predicted to be surface exposed in both species. The repeat regions of the p120s of the two species have common amino acid sequences that are predicted to be surface exposed. The overall amino acid sequence of the E. canis p120 is 30% homologous to that of E. chaffeensis p120. Protein immunoblotting demonstrated that the recombinant E. canis p120 reacted with convalescent sera from dogs with canine ehrlichiosis. These results indicate that the recombinant p120 is a potential antigen for the serodiagnosis of canine ehrlichiosis.

Glycosylation of homologous immunodominant proteins of Ehrlichia chaffeensis and Ehrlichia canis

The glycoprotein genes of Ehrlichia chaffeensis (1,644 bp) and Ehrlichia canis (2,064 bp) encode proteins of 548 to 688 amino acids with predicted molecular masses of only 61 and 73 kDa but with electrophoretic mobilities of 120 kDa (P120) and 140 kDa (P140), respectively. The 120-kDa protein gene of E. chaffeensis contains four identical 240-bp tandem repeat units, and the 140-kDa protein gene of E. canis has 14 nearly identical, tandemly arranged 108-bp repeat units. Conserved serine-rich motifs identified in the repeat units of P120 and P140 were also found in the repeat units of the human granulocytotropic ehrlichiosis agent 130-kDa protein and of the fimbria-associated adhesin protein Fap1 of Streptococcus parasanguis. Nearly the entire (99%) E. chaffeensis P120 gene (1,616 bp), the 14-repeat region (78%) of the E. canis P140 gene (1,620 bp), and a 2-repeat region from the E. chaffeensis P120 gene (520 bp) were expressed in Escherichia coli. The recombinant proteins exhibited molecular masses ranging from 1.6 to 2 times larger than those predicted by the amino acid sequences. Antibodies against the recombinant proteins reacted with E. chaffeensis P120 and E. canis P140, respectively. Carbohydrate was detected on the E. chaffeensis and E. canis recombinant proteins, including the two-repeat polypeptide region of E. chaffeensis P120. A carbohydrate compositional analysis identified glucose, galactose, and xylose on the recombinant proteins. The presence of only one site for N-linked (Asn-Xaa-Ser/Thr) glycosylation, a lack of effect of N-glycosidase F, the presence of 70 and 126 Ser/Thr glycosylation sites in the repeat regions of P120 and P140, respectively, and a high molar ratio of carbohydrate to protein suggest that the glycans may be O linked.