Two monoclonal antibodies with defined epitopes of P44 major surface proteins neutralize Anaplasma phagocytophilum by distinct mechanisms

Establishing the intracellular niche of obligate intracellular vacuolar pathogens

Obligate intracellular pathogens occupy one of two niches - free in the host cell cytoplasm or confined in a membrane-bound vacuole. Pathogens occupying membrane-bound vacuoles are sequestered from the innate immune system and have an extra layer of protection from antimicrobial drugs. However, this lifestyle presents several challenges. First, the bacteria must obtain membrane or membrane components to support vacuole expansion and provide space for the increasing bacteria numbers during the log phase of replication. Second, the vacuole microenvironment must be suitable for the unique metabolic needs of the pathogen. Third, as most obligate intracellular bacterial pathogens have undergone genomic reduction and are not capable of full metabolic independence, the bacteria must have mechanisms to obtain essential nutrients and resources from the host cell. Finally, because they are separated from the host cell by the vacuole membrane, the bacteria must possess mechanisms to manipulate the host cell, typically through a specialized secretion system which crosses the vacuole membrane. While there are common themes, each bacterial pathogen utilizes unique approach to establishing and maintaining their intracellular niches. In this review, we focus on the vacuole-bound intracellular niches of Anaplasma phagocytophilum, Ehrlichia chaffeensis, Chlamydia trachomatis, and Coxiella burnetii.

Beyond binding: antibody effector functions in infectious diseases

Antibodies play an essential role in host defence against pathogens by recognizing microorganisms or infected cells. Although preventing pathogen entry is one potential mechanism of protection, antibodies can control and eradicate infections through a variety of other mechanisms. In addition to binding and directly neutralizing pathogens, antibodies drive the clearance of bacteria, viruses, fungi and parasites via their interaction with the innate and adaptive immune systems, leveraging a remarkable diversity of antimicrobial processes locked within our immune system. Specifically, antibodies collaboratively form immune complexes that drive sequestration and uptake of pathogens, clear toxins, eliminate infected cells, increase antigen presentation and regulate inflammation. The diverse effector functions that are deployed by antibodies are dynamically regulated via differential modification of the antibody constant domain, which provides specific instructions to the immune system. Here, we review mechanisms by which antibody effector functions contribute to the balance between microbial clearance and pathology and discuss tractable lessons that may guide rational vaccine and therapeutic design to target gaps in our infectious disease armamentarium.

Functions of Antibodies

Antibodies can impact pathogens in the presence or in the absence of effector cells or effector molecules such as complement, and experiments can often sort out with precision the mechanisms by which an antibody inhibits a pathogen in vitro . In addition, in vivo models, particularly those engineered to knock in or knock out effector cells or effector molecules, are excellent tools for understanding antibody functions. However, it is highly likely that multiple antibody functions occur simultaneously or sequentially in the presence of an infecting organism in vivo . The most critical incentive for measuring antibody functions is to provide a basis for vaccine development and for the development of therapeutic antibodies. In this respect, some functions, such as virus neutralization, serve to inhibit the acquisition of a pathogen or limit its pathogenesis. However, antibodies can also enhance replication or contribute to pathogenesis. This review emphasizes those antibody functions that are potentially beneficial to the host. In addition, this review will focus on the effects of antibodies on organisms themselves, rather than on the toxins the organisms may produce.

Breaking in and grabbing a meal: Anaplasma phagocytophilum cellular invasion, nutrient acquisition, and promising tools for their study

Anaplasma phagocytophilum invades neutrophils to cause the emerging infection, human granulocytic anaplasmosis. Here, we provide a focused review of the A. phagocytophilum invasin-host cell receptor interactions that promote bacterial entry and the degradative and membrane traffic pathways that the organism exploits to route nutrients to the organelle in which it resides. Because its obligatory intracellular nature hinders knock out-complementation approaches, we also discuss the current methods used to study A. phagocytophilum gene function and the potential benefit of applying novel tools that have advanced studies of other obligate intracellular bacterial pathogens.

Ehrlichia chaffeensis uses its surface protein EtpE to bind GPI-anchored protein DNase X and trigger entry into mammalian cells

Ehrlichia chaffeensis, an obligatory intracellular rickettsial pathogen, enters and replicates in monocytes/macrophages and several non-phagocytic cells. E. chaffeensis entry into mammalian cells is essential not only for causing the emerging zoonosis, human monocytic ehrlichiosis, but also for its survival. It remains unclear if E. chaffeensis has evolved a specific surface protein that functions as an 'invasin' to mediate its entry. We report a novel entry triggering protein of Ehrlichia, EtpE that functions as an invasin. EtpE is an outer membrane protein and an antibody against EtpE (the C-terminal fragment, EtpE-C) greatly inhibited E. chaffeensis binding, entry and infection of both phagocytes and non-phagocytes. EtpE-C-immunization of mice significantly inhibited E. chaffeensis infection. EtpE-C-coated latex beads, used to investigate whether EtpE-C can mediate cell invasion, entered both phagocytes and non-phagocytes and the entry was blocked by compounds that block E. chaffeensis entry. None of these compounds blocked uptake of non-coated beads by phagocytes. Yeast two-hybrid screening revealed that DNase X, a glycosylphosphatidyl inositol-anchored mammalian cell-surface protein binds EtpE-C. This was confirmed by far-Western blotting, affinity pull-down, co-immunoprecipitation, immunofluorescence labeling, and live-cell image analysis. EtpE-C-coated beads entered bone marrow-derived macrophages (BMDMs) from wild-type mice, whereas they neither bound nor entered BMDMs from DNase X(-/-) mice. Antibody against DNase X or DNase X knock-down by small interfering RNA impaired E. chaffeensis binding, entry, and infection. E. chaffeensis entry and infection rates of BMDMs from DNase X(-/-) mice and bacterial load in the peripheral blood in experimentally infected DNase X(-/-) mice, were significantly lower than those from wild-type mice. Thus this obligatory intracellular pathogen evolved a unique protein EtpE that binds DNase X to enter and infect eukaryotic cells. This study is the first to demonstrate the invasin and its mammalian receptor, and their in vivo relevance in any ehrlichial species.

An emerging tick-borne disease of humans is caused by a subset of strains with conserved genome structure

The prevalence of tick-borne diseases is increasing worldwide. One such emerging disease is human anaplasmosis. The causative organism, Anaplasma phagocytophilum, is known to infect multiple animal species and cause human fatalities in the U.S., Europe and Asia. Although long known to infect ruminants, it is unclear why there are increasing numbers of human infections. We analyzed the genome sequences of strains infecting humans, animals and ticks from diverse geographic locations. Despite extensive variability amongst these strains, those infecting humans had conserved genome structure including the pfam01617 superfamily that encodes the major, neutralization-sensitive, surface antigen. These data provide potential targets to identify human-infective strains and have significance for understanding the selective pressures that lead to emergence of disease in new species.

Anaplasma phagocytophilum--a widespread multi-host pathogen with highly adaptive strategies

The bacterium Anaplasma phagocytophilum has for decades been known to cause the disease tick-borne fever (TBF) in domestic ruminants in Ixodes ricinus-infested areas in northern Europe. In recent years, the bacterium has been found associated with Ixodes-tick species more or less worldwide on the northern hemisphere. A. phagocytophilum has a broad host range and may cause severe disease in several mammalian species, including humans. However, the clinical symptoms vary from subclinical to fatal conditions, and considerable underreporting of clinical incidents is suspected in both human and veterinary medicine. Several variants of A. phagocytophilum have been genetically characterized. Identification and stratification into phylogenetic subfamilies has been based on cell culturing, experimental infections, PCR, and sequencing techniques. However, few genome sequences have been completed so far, thus observations on biological, ecological, and pathological differences between genotypes of the bacterium, have yet to be elucidated by molecular and experimental infection studies. The natural transmission cycles of various A. phagocytophilum variants, the involvement of their respective hosts and vectors involved, in particular the zoonotic potential, have to be unraveled. A. phagocytophilum is able to persist between seasons of tick activity in several mammalian species and movement of hosts and infected ticks on migrating animals or birds may spread the bacterium. In the present review, we focus on the ecology and epidemiology of A. phagocytophilum, especially the role of wildlife in contribution to the spread and sustainability of the infection in domestic livestock and humans.

Mechanisms of obligatory intracellular infection with Anaplasma phagocytophilum

Anaplasma phagocytophilum persists in nature by cycling between mammals and ticks. Human infection by the bite of an infected tick leads to a potentially fatal emerging disease called human granulocytic anaplasmosis. A. phagocytophilum is an obligatory intracellular bacterium that replicates inside mammalian granulocytes and the salivary gland and midgut cells of ticks. A. phagocytophilum evolved the remarkable ability to hijack the regulatory system of host cells. A. phagocytophilum alters vesicular traffic to create an intracellular membrane-bound compartment that allows replication in seclusion from lysosomes. The bacterium downregulates or actively inhibits a number of innate immune responses of mammalian host cells, and it upregulates cellular cholesterol uptake to acquire cholesterol for survival. It also upregulates several genes critical for the infection of ticks, and it prolongs tick survival at freezing temperatures. Several host factors that exacerbate infection have been identified, including interleukin-8 (IL-8) and cholesterol. Host factors that overcome infection include IL-12 and gamma interferon (IFN-γ). Two bacterial type IV secretion effectors and several bacterial proteins that associate with inclusion membranes have been identified. An understanding of the molecular mechanisms underlying A. phagocytophilum infection will foster the development of creative ideas to prevent or treat this emerging tick-borne disease.

Proteomic analysis of Neorickettsia sennetsu surface-exposed proteins and porin activity of the major surface protein P51

Neorickettsia sennetsu is an obligate intracellular bacterium of monocytes and macrophages and is the etiologic agent of human Sennetsu neorickettsiosis. Neorickettsia proteins expressed in mammalian host cells, including the surface proteins of Neorickettsia spp., have not been defined. In this paper, we isolated surface-exposed proteins from N. sennetsu by biotin surface labeling followed by streptavidin-affinity chromatography. Forty-two of the total of 936 (4.5%) N. sennetsu open reading frames (ORFs) were detected by liquid chromatography-tandem mass spectrometry (LC/MS/MS), including six hypothetical proteins. Among the major proteins identified were the two major β-barrel proteins: the 51-kDa antigen (P51) and Neorickettsia surface protein 3 (Nsp3). Immunofluorescence labeling not only confirmed surface exposure of these proteins but also showed rosary-like circumferential labeling with anti-P51 for the majority of bacteria and polar to diffuse punctate labeling with anti-Nsp3 for a minority of bacteria. We found that the isolated outer membrane of N. sennetsu had porin activity, as measured by a proteoliposome swelling assay. This activity allowed the diffusion of L-glutamine, the monosaccharides arabinose and glucose, and the tetrasaccharide stachyose, which could be inhibited with anti-P51 antibody. We purified native P51 and Nsp3 under nondenaturing conditions. When reconstituted into proteoliposomes, purified P51, but not Nsp3, exhibited prominent porin activity. This the first proteomic study of a Neorickettsia sp. showing new sets of proteins evolved as major surface proteins for Neorickettsia and the first identification of a porin for the genus Neorickettsia.

Cyclic di-GMP signaling regulates invasion by Ehrlichia chaffeensis of human monocytes

Cyclic di-GMP (c-di-GMP) is a bacterial second messenger produced by GGDEF domain-containing proteins. The genome of Ehrlichia chaffeensis, an obligatory intracellular bacterium that causes human monocytic ehrlichiosis, encodes a single protein that contains a GGDEF domain, called PleD. In this study, we investigated the effects of c-di-GMP signaling on E. chaffeensis infection of the human monocytic cell line THP-1. Recombinant E. chaffeensis PleD showed diguanylate cyclase activity as it generated c-di-GMP in vitro. Because c-di-GMP is not cell permeable, the c-di-GMP hydrophobic analog 2'-O-di(tert-butyldimethylsilyl)-c-di-GMP (CDGA) was used to examine intracellular c-di-GMP signaling. CDGA activity was first tested with Salmonella enterica serovar Typhimurium. CDGA inhibited well-defined c-di-GMP-regulated phenomena, including cellulose synthesis, clumping, and upregulation of csgD and adrA mRNA, indicating that CDGA acts as an antagonist in c-di-GMP signaling. [(32)P]c-di-GMP bound several E. chaffeensis native proteins and two E. chaffeensis recombinant I-site proteins, and this binding was blocked by CDGA. Although pretreatment of E. chaffeensis with CDGA did not reduce bacterial binding to THP-1 cells, bacterial internalization was reduced. CDGA facilitated protease-dependent degradation of particular, but not all, bacterial surface-exposed proteins, including TRP120, which is associated with bacterial internalization. Indeed, the serine protease HtrA was detected on the surface of E. chaffeensis, and TRP120 was degraded by treatment of E. chaffeensis with recombinant E. chaffeensis HtrA. Furthermore, anti-HtrA inhibited CDGA-induced TRP120 degradation. Our results suggest that E. chaffeensis invasion is regulated by c-di-GMP signaling, which stabilizes some bacterial surface-exposed proteins against proteases.

Anaplasma phagocytophilum and Ehrlichia chaffeensis: subversive manipulators of host cells

Anaplasma spp. and Ehrlichia spp. cause several emerging human infectious diseases. Anaplasma phagocytophilum and Ehrlichia chaffeensis are transmitted between mammals by blood-sucking ticks and replicate inside mammalian white blood cells and tick salivary-gland and midgut cells. Adaptation to a life in eukaryotic cells and transmission between hosts has been assisted by the deletion of many genes that are present in the genomes of free-living bacteria (including genes required for the biosynthesis of lipopolysaccharide and peptidoglycan), by the acquisition of a cholesterol uptake pathway and by the expansion of the repertoire of genes encoding the outer-membrane porins and type IV secretion system. Here, I review the specialized properties and other adaptations of these intracellular bacteria.

Functional genomics and evolution of tick-Anaplasma interactions and vaccine development

The genus Anaplasma (Rickettsiales: Anaplasmataceae) includes several tick-transmitted pathogens that impact veterinary and human health. Tick-borne pathogens cycle between tick vectors and vertebrate hosts and their interaction is mediated by molecular mechanisms at the tick-pathogen interface. These mechanisms have evolved characteristics that involve traits from both the tick vector and the pathogen to insure their mutual survival. Herein, we review the information obtained from functional genomics and genetic studies to characterize the tick-Anaplasma interface and evolution of A. marginale and A. phagocytophilum. Anaplasma and tick genes and proteins involved in tick-pathogen interactions were characterized. The results of these studies demonstrated that common and Anaplasma species-specific molecular mechanism occur by which pathogen and tick cell gene expression mediates or limits Anaplasma developmental cycle and trafficking through ticks. These results have advanced our understanding of the biology of tick-Anaplasma interactions and have opened new avenues for the development of improved methods for the control of tick infestations and the transmission of tick-borne pathogens.

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.

Differential expression and glycosylation of anaplasma phagocytophilum major surface protein 2 paralogs during cultivation in sialyl Lewis x-deficient host cells

Many microbial pathogens alter expression and/or posttranslational modifications of their surface proteins in response to dynamics within their host microenvironments to retain optimal interactions with their host cells and/or to evade the humoral immune response. Anaplasma phagocytophilum is an intragranulocytic bacterium that utilizes sialyl Lewis x (sLe(x))-modified P-selectin glycoprotein ligand 1 as a receptor for infecting myeloid cells. Bacterial populations that do not rely on this receptor can be obtained through cultivation in sLe(x)-defective cell lines. A. phagocytophilum major surface protein 2 [Msp2(P44)] is encoded by members of a paralogous gene family and is speculated to play roles in host adaptation. We assessed the complement of Msp2(P44) paralogs expressed by A. phagocytophilum during infection of sLe(x)-competent HL-60 cells and two HL-60 cell lines defective for sLe(x) expression. Multiple Msp2(P44) and N-terminally truncated 25- to 27-kDa isoforms having various isoelectric points and electrophoretic mobilities were expressed in each cell line. The complement of expressed msp2(p44) paralogs and the glycosyl residues modifying Msp2(P44) varied considerably among bacterial populations recovered from sLe(x)-competent and -deficient host cells. Thus, loss of host cell sLe(x) expression coincided with both differential expression and glycosylation of A. phagocytophilum Msp2(P44). This reinforces the hypothesis that this bacterium is able to generate a large variety of surface-exposed molecules that could provide great antigenic diversity and result in multiple binding properties.

The Anaplasma phagocytophilum PleC histidine kinase and PleD diguanylate cyclase two-component system and role of cyclic Di-GMP in host cell infection

Anaplasma phagocytophilum, the etiologic agent of human granulocytic anaplasmosis (HGA), has genes predicted to encode three sensor kinases, one of which is annotated PleC, and three response regulators, one of which is PleD. Prior to this study, the roles of PleC and PleD in the obligatory intracellular parasitism of A. phagocytophilum and their biochemical activities were unknown. The present study illustrates the relevance of these factors by demonstrating that both pleC and pleD were expressed in an HGA patient. During A. phagocytophilum development in human promyelocytic HL-60 cells, PleC and PleD were synchronously upregulated at the exponential growth stage and downregulated prior to extracellular release. A recombinant PleC kinase domain (rPleCHKD) has histidine kinase activity; no activity was observed when the conserved site of phosphorylation was replaced with alanine. A recombinant PleD (rPleD) has autokinase activity using phosphorylated rPleCHKD as the phosphoryl donor but not with two other recombinant histidine kinases. rPleCHKD could not serve as the phosphoryl donor for a mutant rPleD (with a conserved aspartic acid, the site of phosphorylation, replaced by alanine) or two other A. phagocytophilum recombinant response regulators. rPleD had diguanylate cyclase activity to generate cyclic (c) di-GMP from GTP in vitro. UV cross-linking of A. phagocytophilum lysate with c-di-[(32)P]GMP detected an approximately 47-kDa endogenous protein, presumably c-di-GMP downstream receptor. A new hydrophobic c-di-GMP derivative, 2'-O-di(tert-butyldimethylsilyl)-c-di-GMP, inhibited A. phagocytophilum infection in HL-60 cells. Our results suggest that the two-component PleC-PleD system is a diguanylate cyclase and that a c-di-GMP-receptor complex regulates A. phagocytophilum intracellular infection.

Anaplasma phagocytophilum MSP2(P44)-18 predominates and is modified into multiple isoforms in human myeloid cells

Anaplasma phagocytophilum is the etiologic agent of human granulocytic anaplasmosis. MSP2(P44), the bacterium's major surface protein, is encoded by a paralogous gene family and has been implicated in a variety of pathobiological processes, including antigenic variation, host adaptation, adhesion, porin activity, and structural integrity. The consensus among several studies performed at the DNA and RNA levels is that a heterogeneous mix of a limited number of msp2(p44) transcripts is expressed by A. phagocytophilum during in vitro cultivation. Such analyses have yet to be extended to the protein level. In this study, we used proteomic and molecular approaches to determine that MSP2(P44)-18 is the predominant if not the only paralog expressed and is modified into multiple 42- to 44-kDa isoforms by A. phagocytophilum strain HGE1 during infection of HL-60 cells. The msp2(p44) expression profile was homogeneous for msp2(p44)-18. Thus, MSP2(P44)-18 may have a fitness advantage in HL-60 cell culture in the absence of selective immune pressure. Several novel 22- to 27-kDa MSP2 isoforms lacking most of the N-terminal conserved region were also identified. A. phagocytophilum MSP2(P44) orthologs expressed by other pathogens in the family Anaplasmataceae are glycosylated. Gas chromatography revealed that recombinant MSP2(P44)-18 is modified by glucose, galactose, xylose, mannose, and trace amounts of other glycosyl residues. These data are the first to confirm differential modification of any A. phagocytophilum MSP2(P44) paralog and the first to provide evidence for expression of truncated versions of such proteins.

Anaplasma phagocytophilum p44 mRNA expression is differentially regulated in mammalian and tick host cells: involvement of the DNA binding protein ApxR

The natural life cycle of Anaplasma phagocytophilum, an obligatory intracellular bacterium that causes human granulocytic anaplasmosis, consists of alternate infection of two distinct hosts, ticks and mammals, in which bacterial surface proteins are expected to have a critical role. The present study investigated regulation of A. phagocytophilum p44 genes, which encode the P44 major surface proteins. Quantitative real-time reverse transcription-PCR analysis revealed that the amount of p44 mRNA obtained from spleens of A. phagocytophilum-infected SCID mice was approximately 10-fold greater than the amount obtained from salivary glands of A. phagocytophilum-infected Ixodes scapularis nymphs. Similarly, the amount of p44 mRNA obtained from A. phagocytophilum-infected HL-60 cells per bacterium was significantly greater than the amount obtained from infected ISE6 tick cells. The relative amount of p44 mRNA was approximately threefold higher in A. phagocytophilum-infected HL-60 cells cultured at 37 degrees C than in A. phagocytophilum-infected HL-60 cells cultured at 28 degrees C. Although there are more than 100 p44 paralogs, we observed expression mainly from the p44 expression locus (p44E) in various host environments. Interestingly, transcription of the A. phagocytophilum gene encoding the DNA binding protein ApxR was also significantly greater in A. phagocytophilum-infected HL-60 cells than in infected ISE6 tick cells. Gel mobility shift and DNase I protection assays revealed recombinant ApxR binding to the promoter regions of p44E and apxR. ApxR also transactivated the p44E and apxR promoter regions in a lacZ reporter assay. These results indicate that p44 genes and apxR are specifically up-regulated in the mammalian host environment and suggest that ApxR not only is positively autoregulated but also acts as a transcriptional regulator of p44E.

Identification of novel surface proteins of Anaplasma phagocytophilum by affinity purification and proteomics

Anaplasma phagocytophilum is the etiologic agent of human granulocytic anaplasmosis (HGA), one of the major tick-borne zoonoses in the United States. The surface of A. phagocytophilum plays a crucial role in subverting the hostile host cell environment. However, except for the P44/Msp2 outer membrane protein family, the surface components of A. phagocytophilum are largely unknown. To identify the major surface proteins of A. phagocytophilum, a membrane-impermeable, cleavable biotin reagent, sulfosuccinimidyl-2-[biotinamido]ethyl-1,3-dithiopropionate (Sulfo-NHS-SS-Biotin), was used to label intact bacteria. The biotinylated bacterial surface proteins were isolated by streptavidin agarose affinity purification and then separated by electrophoresis, followed by capillary liquid chromatography-nanospray tandem mass spectrometry analysis. Among the major proteins captured by affinity purification were five A. phagocytophilum proteins, Omp85, hypothetical proteins APH_0404 (designated Asp62) and APH_0405 (designated Asp55), P44 family proteins, and Omp-1A. The surface exposure of Asp62 and Asp55 was verified by immunofluorescence microscopy. Recombinant Asp62 and Asp55 proteins were recognized by an HGA patient serum. Anti-Asp62 and anti-Asp55 peptide sera partially neutralized A. phagocytophilum infection of HL-60 cells in vitro. We found that the Asp62 and Asp55 genes were cotranscribed and conserved among members of the family Anaplasmataceae. With the exception of P44-18, all of the proteins were newly revealed major surface-exposed proteins whose study should facilitate understanding the interaction between A. phagocytophilum and the host. These proteins may serve as targets for development of chemotherapy, diagnostics, and vaccines.

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.

Porin activity of Anaplasma phagocytophilum outer membrane fraction and purified P44

Anaplasma phagocytophilum, an obligatory intracellular bacterium that causes human granulocytic anaplasmosis, has significantly less coding capacity for biosynthesis and central intermediary metabolism than do free-living bacteria. Thus, A. phagocytophilum needs to usurp and acquire various compounds from its host. Here we demonstrate that the isolated outer membrane of A. phagocytophilum has porin activity, as measured by a liposome swelling assay. The activity allows the diffusion of L-glutamine, the monosaccharides arabinose and glucose, the disaccharide sucrose, and even the tetrasaccharide stachyose, and this diffusion could be inhibited with an anti-P44 monoclonal antibody. P44s are the most abundant outer membrane proteins and neutralizing targets of A. phagocytophilum. The P44 protein demonstrates characteristics consistent with porins of gram-negative bacteria, including detergent solubility, heat modifiability, a predicted structure of amphipathic and antiparallel beta-strands, an abundance of polar residues, and a C-terminal phenylalanine. We purified native P44s under two different nondenaturing conditions. When reconstituted into proteoliposomes, both purified P44s exhibited porin activity. P44s are encoded by approximately 100 p44 paralogs and go through extensive antigenic variation. The 16-transmembrane-domain beta-strands consist of conserved P44 N- and C-terminal regions. By looping out the hypervariable region, the porin structure is conserved among diverse P44 proteins yet enables antigenic variation for immunoevasion. The tricarboxylic acid (TCA) cycle of A. phagocytophilum is incomplete and requires the exogenous acquisition of L-glutamine or L-glutamate for function. Efficient diffusion of L-glutamine across the outer membrane suggests that the porin feeds the Anaplasma TCA cycle and that the relatively large pore size provides Anaplasma with the necessary metabolic intermediates from the host cytoplasm.

Characterization of a sialic acid- and P-selectin glycoprotein ligand-1-independent adhesin activity in the granulocytotropic bacterium Anaplasma phagocytophilum

Anaplasma phagocytophilum, the aetiologic agent of human granulocytic anaplasmosis, is an obligate intracellular bacterium that colonizes neutrophils and neutrophil precursors. The granulocytotropic bacterium uses multiple adhesins that cooperatively bind to the N-terminal region of P-selectin glycoprotein ligand-1 (PSGL-1) and to sialyl Lewis x (sLe(x)) expressed on myeloid cell surfaces. Recognition of sLe(x) occurs through interactions with alpha2,3-sialic acid and alpha1,3-fucose. It is unknown whether other bacteria-host cell interactions are involved. In this study, we have enriched for A. phagocytophilum organisms that do not rely on sialic acid for cellular adhesion and entry by maintaining strain NCH-1 in HL-60 cells that are severely undersialylated. The selected bacteria, termed NCH-1A, also exhibit lessened dependencies on PSGL-1 and alpha1,3-fucose. Optimal adhesion and invasion by NCH-1A require interactions with the known determinants (sialic acid, PSGL-1 and alpha1,3-fucose), but none of them is absolutely necessary. NCH-1A binding to sLe(x)-modified PSGL-1 requires recognition of the known determinants in the same manners as other A. phagocytophilum strains. These data suggest that A. phagocytophilum expresses a separate adhesin from those targeting sialic acid, alpha1,3-fucose and the N-terminal region of PSGL-1. We propose that NCH-1A upregulates expression of this adhesin.

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

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