Characterization of monoclonal antibodies to the 44-kilodalton major outer membrane protein of the human granulocytic ehrlichiosis agent

Vaccine development: obligate intracellular bacteria new tools, old pathogens: the current state of vaccines against obligate intracellular bacteria

Obligate intracellular bacteria have remained those for which effective vaccines are unavailable, mostly because protection does not solely rely on an antibody response. Effective antibody-based vaccines, however, have been developed against extracellular bacteria pathogens or toxins. Additionally, obligate intracellular bacteria have evolved many mechanisms to subvert the immune response, making vaccine development complex. Much of what we know about protective immunity for these pathogens has been determined using infection-resolved cases and animal models that mimic disease. These studies have laid the groundwork for antigen discovery, which, combined with recent advances in vaccinology, should allow for the development of safe and efficacious vaccines. Successful vaccines against obligate intracellular bacteria should elicit potent T cell memory responses, in addition to humoral responses. Furthermore, they ought to be designed to specifically induce strong cytotoxic CD8+ T cell responses for protective immunity. This review will describe what we know about the potentially protective immune responses to this group of bacteria. Additionally, we will argue that the novel delivery platforms used during the Sars-CoV-2 pandemic should be excellent candidates to produce protective immunity once antigens are discovered. We will then look more specifically into the vaccine development for Rickettsiaceae, Coxiella burnetti, and Anaplasmataceae from infancy until today. We have not included Chlamydia trachomatis in this review because of the many vaccine related reviews that have been written in recent years.

Rab27a via its effector JFC1 localizes to Anaplasma inclusions and promotes Anaplasma proliferation in leukocytes

Anaplasma phagocytophilum is an obligatory intracellular bacterium that causes tick-borne zoonosis called human granulocytic anaplasmosis. Mechanisms by which Anaplasma replicates inside of the membrane-bound compartment called "inclusion" in neutrophils are incompletely understood. A small GTPase Rab27a is found in the secretory granules and multivesicular endosomes. In this study we found Rab27a-containing granules were localized to Anaplasma inclusions in guanine nucleotide-dependent manner, and constitutively active Rab27a enhanced Anaplasma infection and dominant-negative Rab27a inhibited Anaplasma infection. Rab27a effector, JFC1 is known to mediate docking/fusion of Rab27a-bearing granules for exocytosis in leukocytes. shRNA stable knockdown of Rab27a or JFC1 inhibited Anaplasma infection in HL-60 cells. Similar to Rab27a, both endogenous and transfected JFC1 were localized to Anaplasma inclusions by immunostaining or live cell imaging. The JFC1 C2A domain that binds 3'-phosphoinositides, was sufficient and required for JFC1 and Rab27a localization to Anaplasma inclusions which were enriched with phosphatidylinositol 3-phosphate. Nexinhib20, the small molecule inhibitor specific to Rab27a and JFC1 binding, inhibited Anaplasma infection. Taken together, these results imply elevated phosphatidylinositol 3-phosphate in the inclusion membrane recruits JFC1 to mediate Rab27a-bearing granules/vesicles to dock/fuse with Anaplasma inclusions, the lumen of which is topologically equivalent to the exterior of the cell to benefit Anaplasma proliferation.

Anaplasma phagocytophilum Hijacks Flotillin and NPC1 Complex To Acquire Intracellular Cholesterol for Proliferation, Which Can Be Inhibited with Ezetimibe

The intracellular cholesterol transport protein Niemann-Pick type C1 (NPC1) and lipid-raft protein flotillin (FLOT) are required for cholesterol uptake by the obligatory intracellular bacterium Anaplasma phagocytophilum and for infection, and each protein localizes to membrane-bound inclusions containing replicating bacteria. Here, we found striking localization of FLOT2 in NPC1-lined vesicles and a physical interaction between FLOT2 and NPC1. This interaction was cholesterol dependent, as a CRAC (cholesterol recognition/interaction amino acid cholesterol-binding) domain mutant of FLOT2 did not interact with NPC1, and the cholesterol-sequestering agent methyl-β-cyclodextrin reduced the interaction. The stomatin-prohibitin-flotillin-HflC/K domain of FLOT2, FLOT2^1–183, was sufficient for the unique FLOT2 localization and interaction with NPC1. NPC1, FLOT2, and FLOT2^1–183 trafficked to the lumen of Anaplasma inclusions. A loss-of-function mutant, NPC1^P691S (mutation in the sterol-sensing domain), did not colocalize or interact with FLOT2 or with Anaplasma inclusions and inhibited infection. Ezetimibe is a drug that blocks cholesterol absorption in the small intestine by inhibiting plasma membrane Niemann-Pick C1-like 1 interaction with FLOTs. Ezetimibe blocked the interaction between NPC1 and FLOT2 and inhibited Anaplasma infection. Ezetimibe did not directly inhibit Anaplasma proliferation but inhibited host membrane lipid and cholesterol traffic to the bacteria in the inclusion. These data suggest that Anaplasma hijacks NPC1 vesicles containing cholesterol bound to FLOT2 to deliver cholesterol into Anaplasma inclusions to assimilate cholesterol for its proliferation. These results provide insights into mechanisms of intracellular cholesterol transport and a potential approach to inhibit Anaplasma infection by blocking cholesterol delivery into the lumen of bacterial inclusions.

Infection by Anaplasma phagocytophilum Requires Recruitment of Low-Density Lipoprotein Cholesterol by Flotillins

Anaplasma phagocytophilum is an obligatory intracellular bacterium that proliferates in membrane-bound inclusions. A. phagocytophilum is dependent on cholesterol and acquire cholesterol from low-density lipoprotein (LDL) endocytosed by mammalian host cells. The mechanism of cholesterol transport to Anaplasma inclusions, however, is not fully understood. Flotillin-1 (FLOT1) and FLOT2 are cholesterol-associated membrane proteins that form a heterodimer and/or oligomer complex. Here, we found that Anaplasma infection was significantly reduced by small interfering RNA (siRNA) knockdown of FLOT1 or FLOT2. Anaplasma inclusions were encircled with small vesicles containing endogenous FLOT1 or FLOT2 or with ectopically expressed FLOT1-mCherry and FLOT2-green fluorescent protein (FLOT2-GFP). FLOT1- and FLOT2-containing vesicles were enriched with unesterified cholesterol, as indicated by labeling with filipin and aminomethyl coumarin acetic acid-conjugated theonellamide. Localization of FLOT2 to Anaplasma inclusions was dependent on cholesterol, as FLOT2-GFP bearing two mutations in the cholesterol recognition/interaction motif could not target the inclusions. The cholesterol-sequestering agent methyl-β-cyclodextrin abrogated FLOT1 localization to Anaplasma inclusions and cleared infection. FLOT2-GFP also localized to fluorescent 3,3'-dioctadecylindocarbocyanine (DiI)-LDL-containing vesicles, including those surrounding Anaplasma inclusions. FLOT2 siRNA knockdown blocked DiI-LDL trafficking to Anaplasma inclusions and reduced bacteria-associated cholesterol amount, and therefore inhibiting Anaplasma infection. Vesicles containing acid lipase, which hydrolyzes LDL cholesterol esters to free cholesterol, colocalized with FLOT2 and encircled Anaplasma inclusions, while the acid lipase inhibitor orlistat significantly inhibited Anaplasma replication. Together, the data revealed that FLOTs are crucial for Anaplasma replication in host cells, likely by aiding vesicular traffic of LDL-derived free cholesterol to Anaplasma inclusions, and suggest a new way of inhibiting Anaplasma infection.IMPORTANCE Cholesterol is essential for animal cells, but most bacteria do not depend on cholesterol and instead lack cholesterol. However, the intracellular Gram-negative bacterium Anaplasma phagocytophilum that causes human granulocytic anaplasmosis (HGA) is unusual, as it contains significant amount of cholesterol and depends on cholesterol for survival and infection. A. phagocytophilum lacks genes for cholesterol biosynthesis or modification but acquire cholesterol from host cells exclusively from the LDL uptake pathway by a yet-to-be defined mechanism. Here, we uncovered a role of cholesterol-binding proteins FLOT1 and FLOT2 in LDL-derived cholesterol trafficking to Anaplasma inclusions and cholesterol acquisition by Anaplasma species. Importantly, we found that FLOTs localize to A. phagocytophilum-containing inclusions and the compartments containing LDL, and the acid lipase inhibitor orlistat significantly inhibits Anaplasma replication. Our data suggest a fundamental role of FLOTs in intracellular vesicular transport of LDL-derived free cholesterol and may provide insight regarding a new therapeutic target for HGA treatment.

Differential Susceptibility of Male Versus Female Laboratory Mice to Anaplasma phagocytophilum Infection

Human granulocytic anaplasmosis (HGA) is a debilitating, non-specific febrile illness caused by the granulocytotropic obligate intracellular bacterium called Anaplasma phagocytophilum. Surveillance studies indicate a higher prevalence of HGA in male versus female patients. Whether this discrepancy correlates with differential susceptibility of males and females to A. phagocytophilum infection is unknown. Laboratory mice have long been used to study granulocytic anaplasmosis. Yet, sex as a biological variable (SABV) in this model has not been evaluated. In this paper, groups of male and female C57Bl/6 mice that had been infected with A. phagocytophilum were assessed for the bacterial DNA load in the peripheral blood, the percentage of neutrophils harboring bacterial inclusions called morulae, and splenomegaly. Infected male mice exhibited as much as a 1.85-fold increase in the number of infected neutrophils, which is up to a 1.88-fold increase in the A. phagocytophilum DNA load, and a significant increase in spleen size when compared to infected female mice. The propensity of male mice to develop a higher level of A. phagocytophilum infection is relevant for studies utilizing the mouse model. This stresses the importance of including SABV and aligns with the observed higher incidence of infection in male versus female patients.

Ehrlichia secretes Etf-1 to induce autophagy and capture nutrients for its growth through RAB5 and class III phosphatidylinositol 3-kinase

Ehrlichia chaffeensis is an obligatory intracellular bacterium that causes a potentially fatal emerging zoonosis, human monocytic ehrlichiosis. E. chaffeensis has a limited capacity for biosynthesis and metabolism and thus depends mostly on host-synthesized nutrients for growth. Although the host cell cytoplasm is rich with these nutrients, as E. chaffeensis is confined within the early endosome-like membrane-bound compartment, only host nutrients that enter the compartment can be used by this bacterium. How this occurs is unknown. We found that ehrlichial replication depended on autophagy induction involving class III phosphatidylinositol 3-kinase (PtdIns3K) activity, BECN1 (Beclin 1), and ATG5 (autophagy-related 5). Ehrlichia acquired host cell preincorporated amino acids in a class III PtdIns3K-dependent manner and ehrlichial growth was enhanced by treatment with rapamycin, an autophagy inducer. Moreover, ATG5 and RAB5A/B/C were routed to ehrlichial inclusions. RAB5A/B/C siRNA knockdown, or overexpression of a RAB5-specific GTPase-activating protein or dominant-negative RAB5A inhibited ehrlichial infection, indicating the critical role of GTP-bound RAB5 during infection. Both native and ectopically expressed ehrlichial type IV secretion effector protein, Etf-1, bound RAB5 and the autophagy-initiating class III PtdIns3K complex, PIK3C3/VPS34, and BECN1, and homed to ehrlichial inclusions. Ectopically expressed Etf-1 activated class III PtdIns3K as in E. chaffeensis infection and induced autophagosome formation, cleared an aggregation-prone mutant huntingtin protein in a class III PtdIns3K-dependent manner, and enhanced ehrlichial proliferation. These data support the notion that E. chaffeensis secretes Etf-1 to induce autophagy to repurpose the host cytoplasm and capture nutrients for its growth through RAB5 and class III PtdIns3K, while avoiding autolysosomal killing.

Immunization, hybridoma generation, and selection for monoclonal antibody production

Monoclonal antibodies (MAbs) produced by a single clone of cells with homogeneous binding specificity for an antigenic determinant have been used in diagnostics and therapeutics. Many new methods have been devised by scientists for making hybridomas and MAbs. The three major steps for producing MAbs are immunization, immortalization, and isolation. Here, we describe technical details of the three important steps for generating mouse hybridomas and MAbs.

Affinity maturation of monoclonal antibodies by multi-site-directed mutagenesis

High-affinity antibodies are crucial for development of monoclonal antibody (MAb)-based therapeutics for human diseases. Many new detailed methods for affinity maturation have been developed to improve MAb qualities by site-directed mutagenesis, chain shuffling, and error-prone PCR. Site-directed mutagenesis on hotspots in variable heavy (VH) complementary-determining region (CDR) 3 is a commonly used method for improving therapeutic potency and efficacy of targeted MAbs. Strategies for affinity maturation via multi-site-directed mutagenesis in VH-CDR3 described here are for valuable technical tool in the armamentarium of immunologists for development of fast-performance MAbs. Our strategy includes (1) selection of targeted MAb, (2) replacement of certain amino acid residues (e.g., negative or neutral charge to positive amino acids) in VH-CDR3, and (3) determination of binding activity to a target antigen.

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.

Subversion of NPC1 pathway of cholesterol transport by Anaplasma phagocytophilum

Intracellular cholesterol amounts, distribution and traffic are tightly regulated to maintain the healthy eukaryotic cell function. However, how intracellular pathogens that require cholesterol, interact with the host cholesterol homeostasis and traffic is not well understood. Anaplasma phagocytophilum is an obligatory intracellular and cholesterol-robbing bacterium, which causes human granulocytic anaplasmosis. Here we found that a subset of cholesterol-binding membrane protein, Niemann-Pick type C1 (NPC1)-bearing vesicles devoid of lysosomal markers were upregulated in HL-60 cells infected with A. phagocytophilum, and trafficked to live bacterial inclusions. The NPC1 localization to A. phagocytophilum inclusions was abolished by low-density lipoprotein (LDL)-derived cholesterol traffic inhibitor U18666A. Studies using NPC1 siRNA and the cell line with cholesterol traffic defect demonstrated that the NPC1 function is required for bacterial cholesterol acquisition and infection. Furthermore, trans-Golgi network-specific soluble N-ethylmaleimide-sensitive factor attachment protein receptors, vesicle-associated membrane protein (VAMP4) and syntaxin 16, which are associated with NPC1 and LDL-derived cholesterol vesicular transport were recruited to A. phagocytophilum inclusions, and VAMP4 was required for bacteria infection. Taken together, A. phagocytophilum is the first example of a pathogen that subverts the NPC1 pathway of intracellular cholesterol transport and homeostasis for bacterial inclusion membrane biogenesis and cholesterol capture.

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.

The prenylation inhibitor manumycin A reduces the viability of Anaplasma phagocytophilum

Anaplasma phagocytophilum is an obligately intracellular bacterium and is the causative agent of human granulocytic anaplasmosis (HGA), an emerging and major tick-borne disease in the USA and other parts of the world. This study showed that the prenylation inhibitor manumycin A effectively blocked A. phagocytophilum infection in host cells (HL-60 or RF/6A cells). A. phagocytophilum infection activated extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase in host cells, and manumycin A treatment reduced ERK activation in A. phagocytophilum-infected host cells. As ERK activation is required for A. phagocytophilum infection, we examined whether manumycin A inhibited the bacteria directly or through host ERK signalling. Treatment of A. phagocytophilum alone with manumycin A significantly reduced the bacterial infectivity of host cells and bacterial viability in the absence of host cells, whereas pre-treatment of host cells did not inhibit bacterial infection in host cells. The inhibitory effect of manumycin A on A. phagocytophilum infection in host cells was achieved even at a concentration 100 times lower than that required for effective inhibition of mammalian cell signalling. These results suggested that manumycin A directly inactivates the bacterium, resulting in reduced infection and ERK1/2 activation. Thus, the manumycin group of drugs may have a therapeutic potential for HGA.

Production and characterization of chimeric monoclonal antibodies against Burkholderia pseudomallei and B. mallei using the DHFR expression system

Burkholderia pseudomallei (BP) and B. mallei (BM) are closely related gram-negative, facultative anaerobic bacteria which cause life-threatening melioidosis in human and glanders in horse, respectively. Our laboratory has previously generated and characterized more than 100 mouse monoclonal antibodies (MAbs) against BP and BM, according to in vitro and in vivo assay. In this study, 3 MAbs (BP7 10B11, BP7 2C6, and BP1 7F7) were selected to develop into chimeric mouse-human monoclonal antibodies (cMAbs) against BP and/or BM. For the stable production of cMAbs, we constructed 4 major different vector systems with a dihydrofolate reductase (DHFR) amplification marker, and optimized transfection/selection conditions in mammalian host cells with the single-gene and/or double-gene expression system. These 3 cMAbs were stably produced by the DHFR double mutant Chinese hamster ovarian (CHO)-DG44 cells. By ELISA and Western blot analysis using whole bacterial antigens treated by heat (65°C/90 min), sodium periodate, and proteinase K, the cMAb BP7 10B11 (cMAb CK1) reacted with glycoproteins (34, 38, 48 kDa in BP; 28, 38, 48 kDa in BM). The cMAb BP7 2C6 (cMAb CK2) recognized surface-capsule antigens with molecular sizes of 38 to 52 kDa, and 200 kDa in BM. The cMAb CK2 was weakly reactive to 14∼28, 200 kDa antigens in BP. The cMAb BP1 7F7 (cMAb CK3) reacted with lipopolysaccharides (38∼52 kDa in BP; 38∼60 kDa in B. thailandensis). Western blot results with the outer surface antigens of the 3 Burkholderia species were consistent with results with the whole Burkholderia cell antigens, suggesting that these immunodominant antigens reacting with the 3 cMAbs were primarily present on the outer surface of the Burkholderia species. These 3 cMAbs would be useful for analyzing the role of the major outer surface antigens in Burkholderia infection.

Cloning of the major outer membrane protein expression locus in Anaplasma platys and seroreactivity of a species-specific antigen

Anaplasma platys infects peripheral blood platelets and causes infectious cyclic thrombocytopenia in canines. The genes, proteins, and antigens of A. platys are largely unknown, and an antigen for serodiagnosis of A. platys has not yet been identified. In this study, we cloned the A. platys major outer membrane protein cluster, including the P44/Msp2 expression locus (p44ES/msp2ES) and outer membrane protein (OMP), using DNA isolated from the blood of four naturally infected dogs from Venezuela and Taiwan, Republic of China. A. platys p44ES is located within a 4-kb genomic region downstream from a putative transcriptional regulator, tr1, and a homolog of the Anaplasma phagocytophilum, identified here as A. platys omp-1X. The predicted molecular masses of the four mature A. platys P44ES proteins ranged from 43.3 to 43.5 kDa. Comparative analyses of the deduced amino acid sequences of Tr1, OMP-1X, and P44/Msp2 proteins from A. platys with those from A. phagocytophilum showed sequence identities of 86.4% for Tr1, 45.9% to 46.3% for OMP-1X, and 55.0% to 56.9% for P44/Msp2. Comparison between A. platys and Anaplasma marginale proteins showed sequence identities of 73.1% for Tr1/Tr, 39.8% for OMP-1X/OMP1, and 41.5% to 42.1% for P44/Msp2. A synthetic OMP-1X peptide was shown to react with A. platys-positive sera but not with A. platys-negative sera or A. phagocytophilum-positive sera. Together, determination of the genomic locus of A. platys outer membrane proteins not only contributes to the fundamental understanding of this enigmatic pathogen but also helps in developing A. platys-specific PCR and serodiagnosis.

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.

Anaplasma phagocytophilum Ats-1 is imported into host cell mitochondria and interferes with apoptosis induction

Anaplasma phagocytophilum, the causative agent of human granulocytic anaplasmosis, infects human neutrophils and inhibits mitochondria-mediated apoptosis. Bacterial factors involved in this process are unknown. In the present study, we screened a genomic DNA library of A. phagocytophilum for effectors of the type IV secretion system by a bacterial two-hybrid system, using A. phagocytophilum VirD4 as bait. A hypothetical protein was identified as a putative effector, hereby named Anaplasmatranslocated substrate 1 (Ats-1). Using triple immunofluorescence labeling and Western blot analysis of infected cells, including human neutrophils, we determined that Ats-1 is abundantly expressed by A. phagocytophilum, translocated across the inclusion membrane, localized in the host cell mitochondria, and cleaved. Ectopically expressed Ats-1 targeted mitochondria in an N-terminal 17 residue-dependent manner, localized in matrix or at the inner membrane, and was cleaved as native protein, which required residues 55–57. In vitro-translated Ats-1 was imported in a receptor-dependent manner into isolated mitochondria. Ats-1 inhibited etoposide-induced cytochrome c release from mitochondria, PARP cleavage, and apoptosis in mammalian cells, as well as Bax-induced yeast apoptosis. Ats-1(55–57) had significantly reduced anti-apoptotic activity. Bax redistribution was inhibited in both etoposide-induced and Bax-induced apoptosis by Ats-1. Taken together, Ats-1 is the first example of a bacterial protein that traverses five membranes and prevents apoptosis at the mitochondria.

Anaplasma phagocytophilum is the pathogen that causes human granulocytic anaplasmosis, an emerging infectious disease. As an obligate intracellular organism, this bacterium cannot reproduce outside of eukaryotic cells due to the loss of many genes that are present in free-living bacteria. Paradoxically, it specifically infects short-lived white blood cells that play critical roles in anti-microbial defense, by subverting a number of host innate immune responses including programmed cell death (apoptosis). A. phagocytophilum factors that are involved in this process are largely unknown. In this study, we first searched A. phagocytophilum proteins that are secreted by its specialized secretion system into eukaryotic cells. We found a protein of unknown function, here named Ats-1, which is abundantly produced by A. phagocytophilum and traverses five membranes to enter the mitochondria of human cells. Our further study showed that Ats-1 reduces the sensitivity of mitochondria to respond to apoptosis-inducing factors, leading to the inhibition of host cell apoptosis. Thus, present findings identified a bacterial protein that allows infected white blood cells to live longer to support bacterial growth. The absence of similarity of the sequence or the mode of action to any other known cell death suppressor suggests that Ats-1 defines a previously undescribed class of anti-apoptotic protein. This protein and the mechanism thereof may provide insight regarding a new therapeutic target for treatment of human granulocytic anaplasmosis.

Cholesterol-dependent anaplasma phagocytophilum exploits the low-density lipoprotein uptake pathway

In eukaryotes, intracellular cholesterol homeostasis and trafficking are tightly regulated. Certain bacteria, such as Anaplasma phagocytophilum, also require cholesterol; it is unknown, however, how this cholesterol-dependent obligatory intracellular bacterium of granulocytes interacts with the host cell cholesterol regulatory pathway to acquire cholesterol. Here, we report that total host cell cholesterol increased >2-fold during A. phagocytophilum infection in a human promyelocytic leukemia cell line. Cellular free cholesterol was enriched in A. phagocytophilum inclusions as detected by filipin staining. We determined that A. phagocytophilum requires cholesterol derived from low-density lipoprotein (LDL), because its replication was significantly inhibited by depleting the growth medium of cholesterol-containing lipoproteins, by blocking LDL uptake with a monoclonal antibody against LDL receptor (LDLR), or by treating the host cells with inhibitors that block LDL-derived cholesterol egress from late endosomes or lysosomes. However, de novo cholesterol biosynthesis is not required, since inhibition of the biosynthesis pathway did not inhibit A. phagocytophilum infection. The uptake of fluorescence-labeled LDL was enhanced in infected cells, and LDLR expression was up-regulated at both the mRNA and protein levels. A. phagocytophilum infection stabilized LDLR mRNA through the 3′ UTR region, but not through activation of the sterol regulatory element binding proteins. Extracellular signal–regulated kinase (ERK) was up-regulated by A. phagocytophilum infection, and inhibition of its upstream kinase, MEK, by a specific inhibitor or siRNA knockdown, reduced A. phagocytophilum infection. Up-regulation of LDLR mRNA by A. phagocytophilum was also inhibited by the MEK inhibitor; however, it was unclear whether ERK activation is required for LDLR mRNA up-regulation by A. phagocytophilum. These data reveal that A. phagocytophilum exploits the host LDL uptake pathway and LDLR mRNA regulatory system to accumulate cholesterol in inclusions to facilitate its replication.

Maintenance of the cholesterol amount and transport within cells are essential for healthy human cell functions. Most bacteria do not need cholesterol, but certain bacteria that infect human cells are dependent on host cell cholesterol for their infection. How infected human cells deal with these cholesterol-robbing bacteria, and in turn how these bacteria hijack host cholesterol, are intriguing questions. Anaplasma phagocytophilum is a bacterium that lives inside white blood cells, and causes the disease human granulocytic anaplasmosis (HGA). A. phagocytophilum needs host cholesterol to live. Here, we discovered that A. phagocytophilum infection increases the amount of cholesterol in host cells and sequesters the majority of cholesterol in A. phagocytophilum inclusions inside host cells. Human cells acquire cholesterol from two sources: receptor-mediated endocytosis of cholesterol-containing low-density lipoprotein (LDL) from the circulating blood, and synthesis of cholesterol inside the cells. Since A. phagocytophilum depends on cholesterol derived from LDL, it coaxes the host cell to take up more LDL by increasing LDL receptor, through inhibition of LDL receptor mRNA degradation. A. phagocytophilum infection may serve as a model to improve our understanding of the cellular cholesterol regulation in white blood cells, and may provide insight regarding new therapeutic target for treatment of HGA.

Subversion of cellular autophagy by Anaplasma phagocytophilum

Anaplasma phagocytophilum, the causative agent of human granulocytic anaplasmosis, is an obligatory intracellular pathogen. After entry into host cells, the bacterium is diverted from the endosomal pathway and replicates in a membrane-bound compartment devoid of endosomal or lysosomal markers. Here, we show that several hallmarks of early autophagosomes can be identified in A. phagocytophilum replicative inclusions, including a double-lipid bilayer membrane and colocalization with GFP-tagged LC3 and Beclin 1, the human homologues of Saccharomyces cerevisiae autophagy-related proteins Atg8 and Atg6 respectively. While the membrane-associated form of LC3, LC3-II, increased during A. phagocytophilum infection, A. phagocytophilum-containing inclusions enveloped with punctate GFP-LC3 did not colocalize with a lysosomal marker. Stimulation of autophagy by rapamycin favoured A. phagocytophilum infection. Inhibition of the autophagosomal pathway by 3-methyladenine did not inhibit A. phagocytophilum internalization, but reversibly arrested its growth. Although autophagy is considered part of the innate immune system that clears a variety of intracellular pathogens, our study implies that A. phagocytophilum subverts this system to establish itself in an early autophagosome-like compartment segregated from lysosomes to facilitate its proliferation.

Outer membrane protein sequence variation in lambs experimentally infected with Anaplasma phagocytophilum

Anaplasma phagocytophilum has long been known to cause tick-borne fever in ruminants and has been identified more recently as the causative agent of the emerging disease human granulocytic anaplasmosis. The related organism Anaplasma marginale uses gene conversion of the expression site for two major outer membrane proteins (OMPs) to generate extensive sequence and antigenic variation in these OMPs. This is thought to present a continuously varying repertoire of epitopes to the mammalian host and allow disease persistence. Recent genomic and structural data on human strains of A. phagocytophilum, together with animal studies in model systems, have implicated an orthologous OMP of A. phagocytophilum in a similar mechanism of variation. However, to date there has been little investigation of the mechanisms of antigenic variation or disease persistence in hosts naturally infected with field strains of A. phagocytophilum. Approximately 300,000 lambs in Norway suffer severe disease caused by A. phagocytophilum annually. We show here the persistent and cyclic nature of infection in these animals that is accompanied by loosely programmed sequence variation of the major OMP expression site in each rickettsemic peak. These data will allow analysis of interactions between A. phagocytophilum and the host immune system in naturally occurring persistent infections and provide an important comparison with enduring infections of cattle caused by A. marginale.

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.

Anaplasma phagocytophilum AnkA secreted by type IV secretion system is tyrosine phosphorylated by Abl-1 to facilitate infection

Anaplasma phagocytophilum, the agent of human granulocytic anaplasmosis, is an obligate intracellular bacterium of granulocytes. A. phagocytophilum specifically induces tyrosine phosphorylation of a 160 kDa protein (P160) in host cells. However, identity of P160, kinases involved, and effects of tyrosine phosphorylation on bacterial infection remain largely unknown. Here, we demonstrated through proteomic analysis that P160, an abundant and rapidly tyrosine-phosphorylated protein throughout infection, was AnkA of bacterial origin. Differential centrifugation and confocal microscopy revealed that AnkA was rarely retained within A. phagocytophilum or its inclusion, but localized mainly in the cytoplasm of infected cells. Using Cre recombinase reporter assay of Agrobacterium tumefaciens, we proved that AnkA could be secreted by VirB/D4-dependent type IV secretion (T4S) system. Yeast two-hybrid and coimmunoprecipitation analyses demonstrated that AnkA could bind to Abl-interactor 1 (Abi-1), an adaptor protein that interacts with Abl-1 tyrosine kinase, thus mediating AnkA phosphorylation. AnkA and Abl-1 were critical for bacterial infection, as infection was inhibited upon host cytoplasmic delivery of anti-AnkA antibody, Abl-1 knockdown with targeted siRNA, or treatment with a specific pharmacological inhibitor of Abl-1. These data establish AnkA as the first proven T4S substrate in members of obligate intracellular alpha-proteobacteria; furthermore, it demonstrated that AnkA plays an important role in facilitating intracellular infection by activating Abl-1 signalling pathway, and suggest a novel approach to treatment of human granulocytic anaplasmosis through inhibition of host cell signalling pathways.

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

Anaplasma phagocytophilum is an obligatory intracellular bacterium that causes human granulocytic anaplasmosis. The polymorphic 44-kDa major outer membrane proteins of A. phagocytophilum are dominant antigens recognized by patients and infected animals. However, the ability of anti-P44 antibody to neutralize the infection has been unclear due to a mixture of P44 proteins with diverse hypervariable region amino acid sequences expressed by a given bacterial population and lack of epitope-defined antibodies. Monoclonal antibodies (MAbs) 5C11 and 3E65 are directed to different domains of P44 proteins, the N-terminal conserved region and P44-18 central hypervariable region, respectively. Passive immunization with either MAb 5C11 or 3E65 partially protects mice from infection with A. phagocytophilum. In the present study, we demonstrated that the two monoclonal antibodies recognize bacterial surface-exposed epitopes of naturally folded P44 proteins and mapped these epitopes to specific peptide sequences. The two MAbs almost completely blocked the infection of the A. phagocytophilum population that predominantly expressed P44-18 in HL-60 cells by distinct mechanisms: MAb 5C11 blocked the binding, but MAb 3E65 did not block binding or internalization. Instead, MAb 3E65 inhibited internalized A. phagocytophilum to develop into microcolonies called morulae. Some plasma from experimentally infected horses and mice reacted with these two epitopes. Taken together, these data indicate the presence of at least two distinct bacterial surface-exposed neutralization epitopes in P44 proteins. The results indicate that antibodies directed to certain epitopes of P44 proteins have a critical role in inhibiting A. phagocytophilum infection of host cells.

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.

Establishment of cloned Anaplasma phagocytophilum and analysis of p44 gene conversion within an infected horse and infected SCID mice

Diverse p44 alleles at the p44 expression locus (p44Es) encoding surface-exposed major membrane proteins, P44s, of Anaplasma phagocytophilum were hypothesized to be garnered by recombination to enact antigenic variation. However, this hypothesis has not been proven so far, due to inability to clone this obligate intragranulocytic rickettsia. To define the p44E recombination, we developed a novel method to clone A. phagocytophilum. This isogenic cloned population containing a defined p44E was used to infect a naive horse and severe combined immunodeficiency (SCID) mice. During a 58-day infection period in the blood of the horse, p44E conversion was evident in a total of 11 new p44Es, 48% (115/242) of the sequenced p44E population. During a 50-day infection period in the blood of SCID mice, p44E conversion was manifested in a total of 13 new p44Es, 42% (192/460) of the p44E population. Thus, similar levels of p44E convertants were detected in either the presence or absence of an acquired immune system, suggesting that T- and B-cell immune pressure was not essential for recombination and/or selection of the p44E variants. Analysis of sequentially changed p44Es revealed that the entire central hypervariable region of donor p44 pseudogenes or of donor full-length p44s replaced the same region of the resident p44E as a cassette. Putative recombination points were detected within p44 conserved regions flanking the central hypervariable region by the TOPALi analysis. Our results unambiguously demonstrated p44E recombination. The cloning method developed would facilitate precise analysis of the recombination process and the extent of diversity which the recombination creates in the antigenic repertoire.

Rapid sequential changeover of expressed p44 genes during the acute phase of Anaplasma phagocytophilum infection in horses

Anaplasma phagocytophilum immunodominant polymorphic major surface protein P44s have been hypothesized to go through antigenic variation, but the within-host dynamics of p44 expression has not been demonstrated. In the present study we investigated the composition and changes of p44 transcripts in the blood during the acute phase of well-defined laboratory A. phagocytophilum infections in naive equine hosts. Three traveling waves of sequential population changeovers of the p44 transcript species were observed within a single peak of rickettsemia of less than 1 month. During the logarithmic increase, the rapid switch-off of the initial dominant transcript p44-18 occurred regardless of whether the bacterium was transmitted by ticks or by intravenous inoculation. Each of the subsequently dominant p44 transcript species was phylogenetically dissimilar from p44-18. Development of antibody to the hypervariable region of P44-18 during the rickettsemia suggests the suppression of dominance of immuno-cross-reactive p44 populations. When A. phagocytophilum was preincubated with plasma from the infected horse and then coincubated with HL-60 cells, the dominance of the p44-18 transcript was rapidly suppressed in vitro and most of the newly emerged p44 transcript species were previously undetected in this horse. This work provides experimental evidence of within-host p44 antigenic variation. Results suggest that the rapid and synchronized switch of expression is an intrinsic property of p44s reinitiated after transmission to naive mammalian hosts and shaped upon exposure to immune plasma.

Polymorphism and transcription at the p44-1/p44-18 genomic locus in Anaplasma phagocytophilum strains from diverse geographic regions

A polymorphic multigene family (p44) of Anaplasma phagocytophilum encodes the immunodominant 44-kDa major outer membrane proteins. With p44-specific PCR and gene-specific probes, p44-1 was found in all human isolates from New York State but not in isolates from Minnesota, whereas p44-18 and two other p44 species were found in isolates from both regions. We therefore sequenced the genomic locus corresponding to the p44-1/p44-18 tandem locus of A. phagocytophilum HZ in 14 other geographically divergent strains from various hosts. The locus was found in all 14 strains, and p44-18 was conserved among all 13 United States isolates studied. In all nine northeastern strains, p44-1 was conserved. However, in three of the Minnesota strains and in one California strain, p44-1 was replaced at this genomic locus by the novel gene p44-61 (p44-61/18), whose hypervariable region (hv) was a chimera of p44-20hv and p44-23hv. The conserved base sequence within the hv region linked the two segments. In contrast, in the Old Sourhope strain isolated from sheep in the United Kingdom, only a single and distinct p44, p44-OS, was found in this locus. This suggests different rates of evolution of p44-1 and p44-18 at this locus and conservation of the locus within strains isolated from the same geographic region. Locus-specific reverse transcription-PCR revealed expression of p44-1 by New York and p44-61 by Minnesota strains at this locus. These p44 loci provide insight into the molecular evolution and functional divergence of p44 paralogs and may serve as markers for typing strains from different geographic regions.

Sequence analysis of p44 homologs expressed by Anaplasma phagocytophilum in infected ticks feeding on naive hosts and in mice infected by tick attachment

The 44-kDa immunodominant outer membrane proteins (P44 proteins) of Anaplasma phagocytophilum are encoded by the p44 polymorphic multigene family. The present study examined p44 expression and analyzed the cDNA sequences of various p44 transcripts from the spleens and blood of mice infected by the bites of ticks infected with the A. phagocytophilum NTN-1 strain or of naturally infected nymphal ticks and in the salivary glands and midgut tissues of these ticks. A total of 300 p44 cDNAs were subjected to sequence analysis. Of these, 40 distinct p44 species were found, and all of these had orthologs in the A. phagocytophilum HZ strain genome that shared 95 to 100% base sequence identity. The number of unique p44 species expressed in mouse blood was greater than that for mouse spleens. Higher numbers of different p44 transcripts were also expressed in the salivary glands of ticks than in the midgut tissues. Variations in the sequences of the same p44 cDNA species within a single A. phagocytophilum strain and among different strains were concentrated in the conserved regions flanking the central hypervariable region of p44 genes. No mosaic sequences derived from two or more p44 species were found within the p44 hypervariable region. The conservation of the hypervariable region of each p44 cDNA species of A. phagocytophilum in naturally infected ticks and in different geographic isolates suggests that each A. phagocytophilum genome carries a set of p44 paralogs to be expressed. Thus, a large but restricted repertoire of p44 hypervariable sequences exists in A. phagocytophilum strains in the Northeastern United States.

Mechanisms of variable p44 expression by Anaplasma phagocytophilum

The human intragranulocytic bacterium Anaplasma phagocytophilum promotes variation of P44s, which are surface-exposed proteins encoded by a p44 multigene family. In the present study, the specific p44 gene expression loci in four strains of A. phagocytophilum were identified and it was determined that each consisted of four tandem genes, tr1, omp-1X, omp-1N, and p44. A putative sigma(70)-type promoter was found upstream of tr1. The p44 genes include a central hypervariable region flanked by conserved regions. The hypervariable region sequence in the p44 expression locus was duplicated and, regardless of the expression status, conserved at another locus in both low- and high-passage cell cultures of strain NY-37. No significant differences in the hypervariable region were found when we compared p44 sequences, at the level of cDNA, within the expression locus and within other loci in the genomes of strains NY-37 and HZ. Similarly, in cDNA isolated from patients and from assorted cultures of strains NY-31, NY-36, and NY-37, hypervariable regions of 450 deduced amino acid sequences of various p44s within each strain were found to be identical, as were those of p44 sequences in the genome of strain HZ. These data suggest that variations in p44 sequences at the level of the p44 expression locus occur through unidirectional conversion of the entire (nonsegmental) p44 hypervariable region including flanking regions with a corresponding sequence copied from one of the conserved donor p44 genomic loci. The data suggest that the P44 antigenic repertoire within the hypervariable region is restricted.

Differences in clinical manifestations and hematological and serological responses after experimental infection with genetic variants of Anaplasma phagocytophilum in sheep

Five-month-old lambs were experimentally infected with two 16S rRNA genetic variants of Anaplasma phagocytophilum, variants 1 (GenBank accession no. M73220) and 2 (GenBank accession no. AF336220). Additional sequencing of the groESL heat shock operon gene indicated that these variants differ in three nucleotides at positions 782, 824, and 890. The variants were obtained by blood sampling of A. phagocytophilum-infected lambs from one sheep flock in Norway and were stored at -70 degrees C with 10% dimethyl sulfoxide as a cryoprotectant before being inoculated intravenously into susceptible lambs. The infectious blood contained, per ml, approximately 0.5 x 10(6) neutrophils infected with either of the variants. Six weeks after the primary inoculation, the lambs were challenged with the same infectious dose of the heterologous variant. The results of the study indicate a marked difference in clinical manifestation, neutropenia, antibody response, and cross-protection after experimental infection with the two variants of A. phagocytophilum.

Expression of multiple outer membrane protein sequence variants from a single genomic locus of Anaplasma phagocytophilum

Anaplasma phagocytophilum is the causative agent of an emerging tick-borne zoonosis in the United States and Europe. The organism causes a febrile illness accompanied by other nonspecific symptoms and can be fatal, especially if treatment is delayed. Persistence of A. phagocytophilum within mammalian reservoir hosts is important for ensuring continued disease transmission. In the related organism Anaplasma marginale, persistence is associated with antigenic variation of the immunoprotective outer membrane protein MSP2. Extensive diversity of MSP2 is achieved by combinatorial gene conversion of a genomic expression site by truncated pseudogenes. The major outer membrane protein of A. phagocytophilum, MSP2(P44), is homologous to MSP2 of A. marginale, has a similar organization of conserved and variable regions, and is also encoded by a multigene family containing some truncated gene copies. This suggests that the two organisms could use similar mechanisms to generate diversity in outer membrane proteins from their small genomes. We define here a genomic expression site for MSP2(P44) in A. phagocytophilum. As in A. marginale, the msp2(p44) gene in this expression site is polymorphic in all populations of organisms we have examined, whether organisms are obtained from in vitro culture in human HL-60 cells, from culture in the tick cell line ISE6, or from infected human blood. Changes in culture conditions were found to favor the growth and predominance of certain msp2(p44) variants. Insertions, deletions, and substitutions in the region of the genomic expression site encoding the central hypervariable region matched sequence polymorphisms in msp2(p44) mRNA. These data suggest that, similarly to A. marginale, A. phagocytophilum uses combinatorial mechanisms to generate a large array of outer membrane protein variants. Such gene polymorphism has profound implications for the design of vaccines, diagnostic tests, and therapy.

Cloning and characterization of an Ehrlichia canis gene encoding a protein localized to the morula membrane

A gene encoding a 23.5-kDa ehrlichial morula membrane protein designated MmpA was cloned by screening an Ehrlichia canis expression library with convalescent dog sera, which resulted in three positive clones. Sequence analysis of the insert DNAs from all three clones indicated an open reading frame with a size of 666 bp that encodes MmpA. The structural analysis of MmpA indicated that it is a transmembrane protein with extreme hydrophobicity. Southern blot analysis of the HindIII-digested chromosomal DNA demonstrated the presence of a single copy of the mmpA gene in E. canis and Ehrlichia chaffeensis but not in the human granulocytic ehrlichiosis agent. The mmpA gene was amplified, cloned, and expressed as a fusion protein. Polyclonal antibodies to the recombinant protein (rMmpA) were raised in rabbits. Western blot analysis of E. canis and E. chaffeensis lysates with the anti-rMmpA serum resulted in the presence of an MmpA band only in E. canis, not in E. chaffeenesis. Sera from dogs which were either naturally or experimentally infected with E. canis recognized the recombinant protein. Double immunofluorescence confocal microscopy studies demonstrated that MmpA was localized mainly on the morula membrane of E. canis. Since the morula membrane is the interface between the ehrlichial growing environment and the host cytoplasm, MmpA may play a role in bacterium-host cell interactions.

Differential expression of the p44 gene family in the agent of human granulocytic ehrlichiosis

Using reverse transcription-PCR targeting of the p44 genes of the agent of human granulocytic ehrlichiosis (HGE) with primers flanking the hypervariable region, we show differential expression in a murine model of HGE infection and during tick transmission. The p44 genes were differentially expressed in salivary glands of infected nymphal ticks removed during transmission feeding but not in nonfeeding infected ticks. Similarly, the p44 genes were differentially expressed in infected C3H mice, in SCID mice, and in cultured HGE bacteria. Thus, differential p44 expression exists in vivo and in vitro and could provide a basis for antigenic variation.

Cytokine gene expression by peripheral blood leukocytes in horses experimentally infected with Anaplasma phagocytophila

Human granulocytic ehrlichiosis (HGE), a tick-borne zoonosis, is caused by an obligatory intragranulocytic bacterium, the HGE agent, a strain of Anaplasma phagocytophila. The equine model of HGE is considered valuable in understanding pathogenic and immune mechanisms of HGE. In the present study, cytokine mRNA expression by peripheral blood leukocytes (PBLs) in horses was examined during the course of infection by intravenous inoculation of A. phagocytophila or by allowing feeding by infected ticks. The p44 genes encoding the major outer membrane protein P44s of A. phagocytophila were detected by PCR in PBLs of all four horses from 4 to 20 days postexposure. During the 20-day infection period, interleukin-1beta (IL-1beta) and tumor necrosis factor alpha (TNF-alpha) mRNA expression was upregulated in PBLs of all four horses, and IL-8 mRNA expression was upregulated in three horses. Gamma interferon, IL-10, and IL-12 p35 mRNAs were weakly expressed in only one horse each. IL-2, IL-4, IL-6, and IL-12 p40 mRNA expression, however, could not be detected in the PBLs of any of the four horses. These results suggest that IL-1beta, TNF-alpha, and IL-8 generation during A. phagocytophila infection has a primary role in HGE pathogenesis and immunomodulation.

Identification of Anaplasma phagocytophila (formerly Ehrlichia phagocytophila) variants in blood from sheep in Norway

A total of 41 blood samples were collected from 40 Anaplasma phagocytophila-infected sheep in 11 sheep flocks from four different counties of southern Norway. The presence and nature of the Anaplasma species were identified by microscopic detection of morulae, PCR, reverse line blot hybridization, and 16S rRNA gene sequencing. A. phagocytophila was identified in all of the samples, and sequencing of the 16S rRNA gene revealed the presence of four variants of A. phagocytophila. Two of these variants have been described before, but two were newly identified 16S rRNA variants of this species. A. phagocytophila variant 1 was found in nine flocks, A. phagocytophila variant 2 was found in four flocks, the A. phagocytophila prototype was found in two flocks, and A. phagocytophila variant 5 was found in one flock. In two flocks, some sheep were infected with A. phagocytophila variant 1, whereas others were infected with A. phagocytophila variant 2, and in three animals a double infection with two variants was registered. Analyses of the blood samples revealed that blood from sheep infected with A. phagocytophila variant 2 contained nearly twice as many neutrophils and eight times as many Anaplasma-infected neutrophils as blood from sheep infected with the A. phagocytophila variant 1. Furthermore, only 43% of the A. phagocytophila variant 2-infected sheep displayed antibody responses in an immune fluorescence assay, whereas 93% of the sheep with the A. phagocytophila variant 1-infected sheep were seropositive.

Analysis of sequences and loci of p44 homologs expressed by Anaplasma phagocytophila in acutely infected patients

Anaplasma phagocytophila is an obligatory intragranulocytic bacterium that causes human granulocytic ehrlichiosis. Immunodominant 44-kDa outer membrane proteins of A. phagocytophila are encoded by a p44 multigene family. In the present study, expression profiles of p44 genes in the blood of acutely infected patients in the year 2000 were characterized. A single p44 gene was predominantly expressed in peripheral blood leukocytes from one patient, while up to 17 different p44 genes were transcribed without a single majority in the other two patients. The cDNA sequences of the central hypervariable region of several p44 genes were identical among the isolates from the three patients and a 1995 A. phagocytophila isolate. A. phagocytophila was isolated by cell culture from all of the three 2000 patients. Genomic Southern blot analysis of the three 2000 and two 1995 A. phagocytophila isolates with probes specific to the most dominant p44 transcript in each patient showed that the p44 loci in the A. phagocytophila genome were conserved. Analysis of the predicted amino acid sequences of 43 different p44 genes including 19 new sequences found in the present study, revealed that five amino acids were absolutely conserved. The hypervariable region was subdivided into five domains, including three extremely hypervariable central domains. These results suggest that variations in the sequences of p44 are not random but are restricted. Furthermore, several p44 genes are not hypermutatable in nature, based on the conservation of gene sequences and loci among isolates obtained 5 years apart.

Transcript heterogeneity of the p44 multigene family in a human granulocytic ehrlichiosis agent transmitted by ticks

Human granulocytic ehrlichiosis (HGE) is an emerging tick-borne zoonosis caused by a strain of Anaplasma phagocytophila called the HGE agent, an obligatory intracellular bacterium. The agent expresses immunodominant 44-kDa outer membrane proteins (P44s) encoded by a multigene family. The present study established an experimental process for transmission of the HGE agent from infected mice (a reservoir model) to nymphal Ixodes scapularis ticks (a biological vector) and subsequently to horses (a patient model) by the adult infected ticks. Overall, a total of 20 different p44 transcripts were detected in the mammals, ticks, and cell cultures. Among them, a transcript from a p44-18 gene was major at acute stage in mice and horses but minor in ticks. Both mRNA and protein produced from the p44-18 gene were detected in the HGE agent cultivated in HL-60 cells at 37 degrees C, but their expression levels decreased in the organisms cultivated at 24 degrees C, suggesting that temperature is one of the factors that influence the expression of members of the p44 multigene family. Several additional p44 transcripts that were not detected in the mammals at the acute stage of infection were detected in ticks. Phylogenetic analysis of the 20 different p44 transcripts revealed that the major transcripts found in mammals and ticks were distinct, suggesting a difference in surface properties between populations of the HGE agent in different host environments. The present study provides new information for understanding the role of the p44 multigene family in transmission of the HGE agent between mammals and ticks.

Exploitation of interleukin-8-induced neutrophil chemotaxis by the agent of human granulocytic ehrlichiosis

The agent of human granulocytic ehrlichiosis (HGE) is an obligate intracellular bacterium with a tropism for neutrophils; however, the mechanisms of bacterial dissemination are not yet understood. Interleukin-8 (IL-8) is a chemokine that induces neutrophil migration to sites of infection for host defense against pathogens. We now show that HGE bacteria, and the HGE-44 protein, induce IL-8 secretion in a promyelocytic (HL-60) cell line that has been differentiated along the neutrophil lineage with retinoic acid and in neutrophils. Infected HL-60 cells also demonstrate upregulation of CXCR2, an IL-8 receptor, but not CXCR1. Human neutrophils migrate towards Ehrlichia sp.-infected cells in a chemotaxis chamber assay, and this movement can be blocked with antibodies to IL-8. Finally, immunocompetent and severe combined immunodeficient mice administered CXCR2 antisera, and CXCR2(-/-) mice that lack the human IL-8 receptor homologue, are much less susceptible to granulocytic ehrlichiosis than are control animals. These results demonstrate that HGE bacteria induce IL-8 production by host cells and, paradoxically, appear to exploit this chemokine to enhance infection.

Coinfection with Borrelia burgdorferi and the agent of human granulocytic ehrlichiosis alters murine immune responses, pathogen burden, and severity of Lyme arthritis

Lyme disease and human granulocytic ehrlichiosis (HGE) are tick-borne illnesses caused by Borrelia burgdorferi and the agent of HGE, respectively. We investigated the influence of dual infection with B. burgdorferi and the HGE agent on the course of murine Lyme arthritis and granulocytic ehrlichiosis. Coinfection resulted in increased levels of both pathogens and more severe Lyme arthritis compared with those in mice infected with B. burgdorferi alone. The increase in bacterial burden during dual infection was associated with enhanced acquisition of both organisms by larval ticks that were allowed to engorge upon infected mice. Coinfection also resulted in diminished interleukin-12 (IL-12), gamma interferon (IFN-gamma), and tumor necrosis factor alpha levels and elevated IL-6 levels in murine sera. During dual infection, IFN-gamma receptor expression on macrophages was also reduced, implying a decrease in phagocyte activation. These results suggest that coinfection of mice with B. burgdorferi and the HGE agent modulates host immune responses, resulting in increased bacterial burden, Lyme arthritis, and pathogen transmission to the vector.

Analysis of transcriptionally active gene clusters of major outer membrane protein multigene family in Ehrlichia canis and E. chaffeensis

Ehrlichia canis and E. chaffeensis are tick-borne obligatory intramonocytic ehrlichiae that cause febrile systemic illness in humans and dogs, respectively. The current study analyzed the pleomorphic multigene family encoding approximately 30-kDa major outer membrane proteins (OMPs) of E. canis and E. chaffeensis. Upstream from secA and downstream of hypothetical transcriptional regulator, 22 paralogs of the omp gene family were found to be tandemly arranged except for one or two genes with opposite orientations in a 28- and a 27-kb locus in the E. canis and E. chaffeensis genomes, respectively. Each locus consisted of three highly repetitive regions with four nonrepetitive intervening regions. E. canis, in addition, had a 6.9-kb locus which contained a repeat of three tandem paralogs in the 28-kb locus. These total 47 paralogous and orthologous genes encoded OMPs of approximately 30 to 35 kDa consisting of several hypervariable regions alternating with conserved regions. In the 5'-end half of the 27-kb locus or the 28-kb locus of each Ehrlichia species, 14 paralogs were linked by short intergenic spaces ranging from -8 bp (overlapped) to 27 bp, and 8 remaining paralogs in the 3'-end half were connected by longer intergenic spaces ranging from 213 to 632 bp. All 22 paralogs, five unknown genes, and secA in the omp cluster in E. canis were transcriptionally active in the monocyte culture, and the paralogs with short intergenic spaces were cotranscribed with their adjacent genes, including the respective intergenic spaces at both the 5' and the 3' sides. Although omp genes are diverse, our results suggest that the gene organization of the clusters and the gene locus are conserved between two species of Ehrlichia to maintain a unique transcriptional mechanism for adaptation to environmental changes common to them.

Sequence analysis of the ank gene of granulocytic ehrlichiae

The ank gene of the agent of human granulocytic ehrlichiosis (HGE) codes for a protein with a predicted molecular size of 131.2 kDa that is recognized by serum from both dogs and humans infected with granulocytic ehrlichiae. As part of an effort to assess the phylogenetic relatedness of granulocytic ehrlichiae from different geographic regions and in different host species, the ank gene was PCR amplified and sequenced from a variety of sources. These included 10 blood specimens from patients with confirmed human granulocytic ehrlichiosis (three from New York, four from Wisconsin, two from Slovenia, and one from Sweden). Also examined was a canine granulocytic ehrlichia sample obtained from Minnesota, Ehrlichia equi from California, Ehrlichia phagocytophila from Sweden, and the granulocytic ehrlichia isolate USG3. The sequences showed a high level of homology (>95.5% identity), with the lowest homology occurring between a New York HGE agent and the Swedish E. phagocytophila. Several 3-bp deletions and a variable number of 51- and 81-bp direct repeats were noted. Although the North American HGE sequences showed the highest conservation (>98.1% identity), phylogenetic analyses indicated that these samples represent two separate clades, one including the three New York HGE samples and the USG3 strain and another with the Wisconsin HGE and Minnesota canine sequences. Two of the New York samples and the USG3 strain showed 100% identity over the entire 3,696-bp product. Likewise, three of the Wisconsin human samples and the Minnesota dog sample were identical (3,693 bp). Whereas phylogenetic analysis showed that the E. equi sequence was most closely related to the Upper Midwest samples, analysis of the repeat structures showed it to be more similar to the European samples. Overall, the genetic analysis based on the ank gene showed that the granulocytic ehrlichiae are closely related, appear to infect multiple species, and can be grouped into at least three different clades, two North American and one European.

Comparison of two recombinant major outer membrane proteins of the human granulocytic ehrlichiosis agent for use in an enzyme-linked immunosorbent assay

Enzyme-linked immunosorbent assay (ELISA) for human granulocytic ehrlichiosis (HGE) using two different recombinant P44 proteins (rP44 and rP44-2hv) of the HGE agent as antigens was evaluated. Sera from a total of 72 healthy humans both from regions where HGE is nonendemic and regions where HGE is endemic were used as negative controls to determine the cutoff value for ELISA. Sera from a total of 14 patients (nine from whom the HGE agent was isolated and five who were HGE-PCR positive) were used as positive controls. One hundred nine sera from 72 patients in an area where HGE is endemic who were suspected of having HGE were examined by ELISA and indirect immunofluorescence assay (IFA). All IFA-negative sera were negative by both ELISAs. Of 39 sera that were IFA positive, 35 and 27 were positive by ELISA using rP44 and rP44-2hv, respectively, indicating that the use of rP44 is more sensitive. Western blot analysis of the four rP44-ELISA-negative IFA-positive sera using whole HGE agent as antigen suggests that these four sera were false IFA positive. There was no difference in results with or without the preabsorption of sera with Escherichia coli or with or without the cleavage of the fused protein derived from the vector. There was a significant positive correlation between IFA titers and optical densities of ELISAs. Four Ehrlichia chaffeensis-positive and 10 Borrelia burgdorferi-positive sera were negative by ELISA. However, two Babesia microti-positive sera showed strong cross-reactivity to the fused vector protein, which was eliminated after cleavage of the protein. Thus, ELISA using rP44 nonfusion protein would provide a simple, specific, and objective HGE serologic test which can be easily automated.

Expression of interleukin-1beta, tumor necrosis factor alpha, and interleukin-6 in human peripheral blood leukocytes exposed to human granulocytic ehrlichiosis agent or recombinant major surface protein P44

Human granulocytic ehrlichiosis (HGE) is an emerging febrile systemic disease caused by the HGE agent, an obligatory intracellular bacterium of granulocytes. The pathogenicity- and immunity-related mechanisms of HGE are unknown. In this study, several cytokines generated in human peripheral blood leukocytes (PBLs) incubated with the HGE agent or a recombinant 44-kDa major surface protein (rP44) of the HGE agent were examined by reverse transcription-PCR and a capture enzyme-linked immunosorbent assay. The HGE agent induced expression of interleukin-1beta (IL-1beta), tumor necrosis factor alpha (TNF-alpha), and IL-6 mRNAs and proteins in PBLs in a dose-dependent manner to levels as high as those resulting from Escherichia coli lipopolysaccharide stimulation. The kinetics of induction of these three cytokines in PBLs by rP44 and by the HGE agent were similar. Proteinase K treatment of the HGE agent or rP44 eliminated the ability to induce these three cytokines. Induction of these cytokine mRNAs was not dependent on superoxide generation. These results suggest that P44 proteins have a major role in inducing the production of proinflammatory cytokines by PBLs. Expression of IL-8, IL-10, gamma interferon, transforming growth factor beta, and IL-2 mRNAs in response to the HGE agent was not remarkable. Among PBLs, neutrophils and lymphocytes expressed IL-1beta mRNA but not TNF-alpha or IL-6 mRNA in response to the HGE agent, whereas monocytes expressed all three of these cytokine mRNAs. These observations suggest that induction of proinflammatory-cytokine gene expression by the major outer membrane protein of the HGE agent in monocytes, which are not the primary host cells of the HGE agent, contributes to HGE pathogenesis and immunomodulation.

Gamma interferon dominates the murine cytokine response to the agent of human granulocytic ehrlichiosis and helps to control the degree of early rickettsemia

The cytokine response to the agent of human granulocytic ehrlichiosis (HGE) was assessed in a murine infection model and the role of gamma interferon (IFN-gamma), a cytokine that is crucial for host defenses against intracellular pathogens, was investigated by using IFN-gamma-deficient mice. The agent of HGE (aoHGE) is an obligate intracellular bacterium that survives within neutrophils: morulae (vacuoles containing HGE organisms) are evident in polymorphonuclear leukocytes of experimentally infected immunocompetent mice for 1 to 2 weeks. We now show that IFN-gamma levels increase during early infection of C3H/HeN or C57BL/6 mice with HGE bacteria. Moreover, in response to aoHGE extracts or concanavalin A, splenocytes from ehrlichia-infected mice produced more IFN-gamma and less interleukin-4 than controls, suggesting that aoHGE partially skewed the immune response towards a Th1 phenotype. Absolute concentration of morulae containing neutrophils in blood was 122 +/- 22 cells/microliter on day 8. The bacterial DNA burden was also highest on day 8 and then declined after IFN-gamma levels peaked. In contrast, IFN-gamma-deficient mice had a markedly elevated HGE bacteria burden with morulae concentration of 282 +/- 48 cells/microliter on day 5 (P = 0.004) and 242 +/- 63 cells/microliter on day 8 (P = 0.005). Rickettsemia resolved in immunocompetent and IFN-gamma deficient mice after 2 weeks, while both the immunocompetent and the IFN-gamma-deficient mice had increased serum antibodies against aoHGE antigens at this time point. These data demonstrate that the HGE agent elicits a prominent IFN-gamma response in mice and that IFN-gamma is important in controlling the degree of rickettsemia during the early phase of infection, while IFN-gamma independent mechanisms play a role at later time points.

Intracellular infection by the human granulocytic ehrlichiosis agent inhibits human neutrophil apoptosis

In patients with human granulocytic ehrlichiosis (HGE), the HGE agent has been seen only in the peripheral blood granulocytes, which have a life span too short for ehrlichial proliferation. To determine if the HGE agent delays the apoptosis of human peripheral blood neutrophils for its advantage, peripheral blood granulocytes consisting mostly of neutrophils were incubated with freshly freed host cell-free HGE agent in vitro. The HGE agent induced a significant delay in morphological apoptosis and the cytoplasmic appearance of histone-associated DNA fragments in the granulocytes. This antiapoptotic effect was dose dependent. Although much weaker than the HGE agent freshly freed from the host cells, noninfectious purified HGE agent stored frozen and thawed also had antiapoptotic effect, which was lost with proteinase K treatment but not with periodate treatment. Treatment of neutrophils with a transglutaminase inhibitor, monodansylcadaverine, blocked the antiapoptotic effect of the HGE agent. Addition of oxytetracycline, however, did not prevent or reverse the antiapoptotic effect of the HGE agent. These results suggest that binding of a protein component(s) of the HGE agent to neutrophils and subsequent cross-linking and/or internalization of the receptor and ehrlichiae are required for antiapoptotic signaling, but ehrlichial protein synthesis and/or proliferation is not required. MG-132, a proteasome inhibitor, and cycloheximide accelerated the apoptosis of neutrophils and overrode the antiapoptotic effect of the HGE agent. Studies with specific inhibitors suggest that protein kinase A, NF-kappaB, and interleukin 1beta are not involved in the antiapoptotic mechanism of the HGE agent.

Antigenic variation in the persistence and transmission of the ehrlichia Anaplasma marginale

Tick-borne transmission of ehrlichial pathogens requires rickettsemic reservoir hosts to maintain a population of infected vectors. Persistence in their respective mammalian hosts appears to be a common feature of the tick-transmitted ehrlichiae. How infection persists in immunocompetent hosts is unknown. In this review, we describe studies on Anaplasma marginale, an ehrlichial pathogen of cattle, that support antigenic variation as a primary mechanism of persistence.

Quantification of the human granulocytic ehrlichiosis agent based on analysis of rRNA isolated from control and infected HL-60 cells

Human granulocytic ehrlichiosis (HGE) is an emerging vector-borne disease caused by an Ehrlichia species similar or identical to E. equi and E. phagocytophila. Previous studies have shown that the pathogen can be cultivated in vitro in permissive cells such as human promyelocytic HL-60 leukemia cells. The mechanism(s) of its infection and propagation in target cells, however, is not well understood, due in part to lack of a method capable of quantitatively determining the amount of the infectious agent. Although several assays currently exist for the HGE agent, they are mostly qualitative and have a number of limitations. In this report, size differences between prokaryotic and eukaryotic rRNAs are utilized to quantitatively assay the HGE agent in HL-60 cells. By comparing the integrated intensity of agarose gel resolved HGE-specific rRNA in host cells, with identically prepared and analyzed rRNA isolated from known quantities of E. coli (JM 109), it is possible to calculate the E. coli-equivalence of the HGE agent present in HL-60 cells according to the equation: Y (E. coli, in viable cells x 10(8)) = -2.573 + 0.11X (% infection by the HGE agent in HL-60 cells). The method described is reproducible, sensitive, and is not limited by availability of antisera. Furthermore, since the assay has no designer primer and repeated amplification requirements, it can be easily disseminated to and standardized in other laboratories.

Multiple p44 genes encoding major outer membrane proteins are expressed in the human granulocytic ehrlichiosis agent

Human granulocytic ehrlichiosis (HGE) is caused by infection with an obligatory intracellular bacterium, the HGE agent. We previously cloned a gene encoding HGE agent 44-kDa major outer membrane protein and designated it p44. In this study, we (i) identified five different mRNAs that are transcribed from p44-homologous genes in the HGE agent cultivated in HL-60 cells; (ii) cloned genes corresponding to the mRNAs from the genomic DNA of the HGE agent; (iii) showed that the genes being expressed were not clustered in the HGE agent genome; (iv) estimated that a minimum copy number of the p44-homologous genes in the genome is 18; (v) detected two different P44-homologous proteins expressed by the HGE agent; and (vi) demonstrated existence of antibodies specific to the two proteins in sera from patients with HGE. These findings showed that p44 multigenes have several active expression sites and the expression is regulated at transcriptional level, suggesting a potentially unique mechanism for generating the diversity in major antigenic outer membrane proteins of the HGE agent. Characterization of p44-homologous genes expressed by the HGE agent in a tissue culture would assist in understanding a role of the p44 multigene family in pathogenesis and immune response in HGE.

Changes in expression of the 44-kilodalton outer surface membrane antigen (p44 kD) for monitoring progression of infection and antimicrobial susceptibility of the human granulocytic ehrlichiosis (HGE) agent in HL-60 cells

Changes in human granulocytic ehrlichiosis (HGE)-specific major outer membrane protein (p44 kD) were assayed by Western blot analysis in HL-60 cells in vitro infected by the HGE agent. Time course study demonstrated that the expression of p44 preceded the rise in cell infection as determined by the presence of intracellular morulae. To test whether the expression of p44 may be suitable for evaluating the effects of antibiotics in vitro, three recent isolates of the HGE agent were exposed to doxycycline and ampicillin during culture with HL-60 cells. Loss of infection concurrent with disappearance of the 44 kD protein was found with doxycycline treatment. In contrast, ampicillin treatment had no discernible effects on infection or 44 kD expression. There was excellent agreement between infection, as measured by morulae, and 44 kD expression (coefficient of correlation r = 0.97, p < 0.01). Following treatment with doxycycline, the 44 kD protein disappeared with an estimated t1/2 of approximately 24-30 h, which was considerably shorter than a t1/2 of >60 h calculated for loss of morulae. Measurement of p44 expression may be a more rapid and simple assay to determine antibiotic susceptibility of the HGE agent in cell culture. Furthermore, it may be used to indicate the presence of infection before morulae are apparent.