Characterization of tick salivary gland and saliva alphagalactome reveals candidate alpha-gal syndrome disease biomarkers

BACKGROUND

Ticks are obligate hematophagous arthropods that synthesize the glycan Galα1-3Galβ1-(3)4GlcNAc-R (α-Gal) associated with the alpha-gal syndrome (AGS) or allergy to mammalian meat consumption.

RESEARCH DESIGN AND METHODS

In this study, we used a proteomics approach to characterize tick proteins in salivary glands (sialome SG), secreted saliva (sialome SA) and with α-Gal modification (alphagalactome SG and SA) in model tick species associated with the AGS in the United States (Amblyomma americanum) and Australia (Ixodes holocyclus). Selected proteins reactive to sera (IgE) from patients with AGS were identified to advance in the identification of possible proteins associated with the AGS. For comparative analysis, the α-Gal content was measured in various tick species.

RESULTS

The results confirmed that ticks produce proteins with α-Gal modifications and secreted into saliva during feeding. Proteins identified in tick alphagalactome SA by sera from patients with severe AGS symptomatology may constitute candidate disease biomarkers.

CONCLUSIONS

The results support the presence tick-derived proteins with α-Gal modifications in the saliva with potential implications in AGS and other disorders and protective capacity against tick infestations and pathogen infection. Future research should focus on the characterization of the function of tick glycoproteins with α-Gal in tick biology and AGS.

Demonstration of transplacental transmission of a human isolate of Anaplasma phagocytophilum in an experimentally infected sheep

Anaplasma phagocytophilum, first identified as a pathogen of sheep in Europe, more recently has been recognized as an emerging tick-borne pathogen of humans in the U.S. and Europe. Transmission of A. phagocytophilum is reported to be by ticks, primarily of the genus Ixodes. While mechanical and transplacental transmission of the type genus organism, A. marginale, occur in addition to tick transmission, these modes of transmission have not been considered for A. phagocytophilum. Recently, we developed a sheep model for studying host-tick-pathogen interactions of the human NY-18 A. phagocytophilum isolate. Sheep were susceptible to infection with this human isolate and served as a source of infection for I. scapularis ticks, but they did not display clinical signs of disease, and the pathogen was not apparent in stained blood smears. In the course of these experiments, one sheep unexpectedly gave birth to a lamb 5 weeks after being experimentally infected by inoculation with the pathogen propagated in HL-60 cells. The lamb was depressed and not feeding and was subsequently euthanized 18 h after birth. Tissues were collected at necropsy for microscopic examination and PCR to confirm A. phagocytophilum infection. At necropsy, the stomach contained colostrum, the spleen was moderately enlarged and thickened with conspicuous lymphoid follicles, and mesenteric lymph nodes were mildly enlarged and contained moderate infiltrates of eosinophils and neutrophils. Blood, spleen, heart, skin and cervical and mesenteric lymph nodes tested positive for A. phagocytophilum by PCR, and sequence analysis confirmed that the lamb was infected with the NY-18 isolate. Transplacental transmission should therefore be considered as a means of A. phagocytophilum transmission and may likely contribute to the epidemiology of tick-borne fever in sheep and other mammals, including humans.

Reciprocal regulation of NF-kB (Relish) and Subolesin in the tick vector, Ixodes scapularis

Background

Tick Subolesin and its ortholog in insects and vertebrates, Akirin, have been suggested to play a role in the immune response through regulation of nuclear factor-kappa B (NF-kB)-dependent and independent gene expression via interaction with intermediate proteins that interact with NF-kB and other regulatory proteins, bind DNA or remodel chromatin to regulate gene expression. The objective of this study was to characterize the structure and regulation of subolesin in Ixodes scapularis. I. scapularis is a vector of emerging pathogens such as Borrelia burgdorferi, Anaplasma phagocytophilum and Babesia microti that cause in humans Lyme disease, anaplasmosis and babesiosis, respectively. The genome of I. scapularis was recently sequenced, and this tick serves as a model organism for the study of vector-host-pathogen interactions. However, basic biological questions such as gene organization and regulation are largely unknown in ticks and other arthropod vectors.

Principal Findings

The results presented here provide evidence that subolesin/akirin are evolutionarily conserved at several levels (primary sequence, gene organization and function), thus supporting their crucial biological function in metazoans. These results showed that NF-kB (Relish) is involved in the regulation of subolesin expression in ticks, suggesting that as in other organisms, different NF-kB integral subunits and/or unknown interacting proteins regulate the specificity of the NF-kB-mediated gene expression. These results suggested a regulatory network involving cross-regulation between NF-kB (Relish) and Subolesin and Subolesin auto-regulation with possible implications in tick immune response to bacterial infection.

Significance

These results advance our understanding of gene organization and regulation in I. scapularis and have important implications for arthropod vectors genetics and immunology highlighting the possible role of NF-kB and Subolesin/Akirin in vector-pathogen interactions and for designing new strategies for the control of vector infestations and pathogen transmission.

Sheep experimentally infected with a human isolate of Anaplasma phagocytophilum serve as a host for infection of Ixodes scapularis ticks

Anaplasma phagocytophilum, first identified as a pathogen of ruminants in Europe, has more recently been recognized as an emerging tick-borne pathogen of humans in the U.S. and Europe. A. phagocytophilum is transmitted by Ixodes spp., but the tick developmental cycle and pathogen/vector interactions have not been fully described. In this research, we report on the experimental infection of sheep with the human NY-18 isolate of A. phagocytophilum which then served as a host for infection of I. scapularis nymphs and adults. A. phagocytophilum was propagated in the human promyelocytic cell line, HL-60, and the infected cell cultures were then used to infect sheep by intravenous inoculation. Infections in sheep were confirmed by PCR and an Anaplasma-competitive ELISA. Clinical signs were not apparent in any of the infected sheep, and only limited hematologic and mild serum biochemical abnormalities were identified. While A. phagocytophilum morulae were rarely seen in neutrophils, blood film evaluation revealed prominent large granular lymphocytes, occasional plasma cells, and rare macrophages. Upon necropsy, gross lesions were restricted to the lymphoid system. Mild splenomegaly and lymphadenomegaly with microscopic evidence of lymphoid hyperplasia was observed in all infected sheep. Female I. scapularis that were allowed to feed and acquire infection on each of the 3 experimentally infected sheep became infected with A. phagocytophilum as determined by PCR of guts (80-87%) and salivary glands (67-100%). Female I. scapularis that acquired infection as nymphs on an experimentally infected sheep transmitted A. phagocytophilum to a susceptible sheep, thus confirming transstadial transmission. Sheep proved to be a good host for the production of I. scapularis infected with this human isolate of A. phagocytophilum, which can be used as a model for future studies of the tick/pathogen interface.

Expression of heat shock proteins and subolesin affects stress responses, Anaplasma phagocytophilum infection and questing behaviour in the tick, Ixodes scapularis

We characterized the effects of subolesin and heat shock protein (HSP) expression on Ixodes scapularis Say (Acari: Ixodidae) stress responses to heat shock and feeding, questing behaviour and Anaplasma phagocytophilum (Rickettsiales: Anaplasmataceae) infection. Ticks and cultured tick cells were analysed before and after subolesin, hsp20 and hsp70 gene knock-down by RNA interference. The results of these studies confirm that HSPs are involved in the tick cell response to heat stress and that subolesin and HSPs are both involved in the tick response to blood-feeding stress and A. phagocytophilum infection. Subolesin and hsp20 are involved in the tick protective response to A. phagocytophilum infection and hsp70 expression may be manipulated by the pathogen to increase infectivity. Importantly, these results demonstrate that subolesin, hsp20 and hsp70 expression also affect tick questing behaviour. Overall, this research demonstrates a relationship between hsp and subolesin expression and tick stress responses to heat shock and blood feeding, A. phagocytophilum infection and questing behaviour, thereby extending our understanding of the tick-host-pathogen interface.

Growth of Coxiella burnetii in the Ixodes scapularis-derived IDE8 tick cell line

Q fever, a zoonotic disease, is caused by a gram-negative intracellular bacterium, Coxiella burnetii. Although normally transmitted during exposure to infectious aerosols, C. burnetii is also found in arthropod vectors. In the environment, ticks are thought to play a crucial role in bacterial maintenance and transmission by infecting various mammalian species. However, the nature of the pathogen-tick relationship is not well defined. To determine C. burnetii's interactions with a cultured tick cell line, we introduced purified C. burnetii NMII into Ixodes scapularis-derived IDE8 cells and assayed for bacterial presence, replication, gene expression, and subsequent infectivity for mammalian cells. Tick cells were harvested at 24 h, 72 h, 7 days, and 11 days postinfection (PI). C. burnetii uptake and subsequent replication was demonstrated by indirect immunofluorescence assay, electron microscopy, and real-time polymerase chain reaction (PCR). When a genome equivalent multiplicity of infection of 30 was used, 30%-40% of exposed cells were seen to have small, rounded, vacuoles at 72 h PI, whereas at 7 and 11 days PI, 60%-70% of cells contained enlarged vacuoles harboring large numbers of bacteria. Quantitative PCR analysis of total genomic DNA confirmed that C. burnetii genome numbers increased significantly from 24 h to 11 days PI. Expression of C. burnetii type four secretion system homologs at 7 days PI was demonstrated by reverse transcriptase PCR. Finally, indirect immunofluorescence assay demonstrated that C. burnetii propagated within IDE8 cells were infectious for mammalian cells. These studies demonstrate the utility of cultured tick cell lines as a model to investigate C. burnetii's molecular interactions with its arthropod vectors.

Subolesin expression in response to pathogen infection in ticks

BACKGROUND

Ticks (Acari: Ixodidae) are vectors of pathogens worldwide that cause diseases in humans and animals. Ticks and pathogens have co-evolved molecular mechanisms that contribute to their mutual development and survival. Subolesin was discovered as a tick protective antigen and was subsequently shown to be similar in structure and function to akirins, an evolutionarily conserved group of proteins in insects and vertebrates that controls NF-kB-dependent and independent expression of innate immune response genes. The objective of this study was to investigate subolesin expression in several tick species infected with a variety of pathogens and to determine the effect of subolesin gene knockdown on pathogen infection. In the first experiment, subolesin expression was characterized in ticks experimentally infected with the cattle pathogen, Anaplasma marginale. Subolesin expression was then characterized in questing or feeding adult ticks confirmed to be infected with Anaplasma, Ehrlichia, Rickettsia, Babesia or Theileria spp. Finally, the effect of subolesin knockdown by RNA interference (RNAi) on tick infection was analyzed in Dermacentor variabilis males exposed to various pathogens by capillary feeding (CF).

RESULTS

Subolesin expression increased with pathogen infection in the salivary glands but not in the guts of tick vector species infected with A. marginale. When analyzed in whole ticks, subolesin expression varied between tick species and in response to different pathogens. As reported previously, subolesin knockdown in D. variabilis infected with A. marginale and other tick-borne pathogens resulted in lower infection levels, while infection with Francisella tularensis increased in ticks after RNAi. When non-tick-borne pathogens were fed to ticks by CF, subolesin RNAi did not affect or resulted in lower infection levels in ticks. However, subolesin expression was upregulated in D. variabilis exposed to Escherichia coli, suggesting that although this pathogen may induce subolesin expression in ticks, silencing of this molecule reduced bacterial multiplication by a presently unknown mechanism.

CONCLUSIONS

Subolesin expression in infected ticks suggested that subolesin may be functionally important for tick innate immunity to pathogens, as has been reported for the akirins. However, subolesin expression and consequently subolesin-mediated innate immunity varied with the pathogen and tick tissue. Subolesin may plays a role in tick innate immunity in the salivary glands by limiting pathogen infection levels, but activates innate immunity only for some pathogen in the guts and other tissues. In addition, these results provided additional support for the role of subolesin in other molecular pathways including those required for tissue development and function and for pathogen infection and multiplication in ticks. Consequently, RNAi experiments demonstrated that subolesin knockdown in ticks may affect pathogen infection directly by reducing tick innate immunity that results in higher infection levels and indirectly by affecting tissue structure and function and the expression of genes that interfere with pathogen infection and multiplication. The impact of the direct or indirect effects of subolesin knockdown on pathogen infection may depend on several factors including specific tick-pathogen molecular interactions, pathogen life cycle in the tick and unknown mechanisms affected by subolesin function in the control of global gene expression in ticks.

Inoculation of white-tailed deer (Odocoileus virginianus) with Ap-V1 Or NY-18 strains of Anaplasma phagocytophilum and microscopic demonstration of Ap-V1 In Ixodes scapularis adults that acquired infection from deer as nymphs

Four white-tailed deer were inoculated with either the Ap-V1 or NY-18 strain of Anaplasma phagocytophilum. Ixodes scapularis nymphs were then allowed to acquistion feed on the inoculated deer and molt to adults. Only an Ap-V1 infected deer was infected persistently and able to infect nymphal Ixodes scapularis. Molted adult ticks maintained Ap-V1 infection as demonstrated by PCR and microscopy. We report, for the first time, a morphologic description of A. phagocytophilum in I. scapularis.

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.

The natural history of Anaplasma marginale

The intracellular pathogen Anaplasma marginale (Rickettsiales: Anaplasmataceae), described by Sir Arnold Theiler in 1910, is endemic worldwide in tropical and subtropical areas. Infection of cattle with A. marginale causes bovine anaplasmosis, a mild to severe hemolytic disease that results in considerable economic loss to both dairy and beef industries. Transmission of A. marginale to cattle occurs biologically by ticks and mechanically by biting flies and by blood-contaminated fomites. Both male ticks and cattle hosts become persistently infected with A. marginale and serve as reservoirs of infection. While erythrocytes are the major site of infection in cattle, A. marginale undergoes a complex developmental cycle in ticks that begins by infection of gut cells, and transmission to susceptible hosts occurs from salivary glands during feeding. Major surface proteins (MSPs) play a crucial role in the interaction of A. marginale with host cells, and include adhesion proteins and MSPs from multigene families that undergo antigenic change and selection in cattle, thus contributing to maintenance of persistent infections. Many geographic strains of A. marginale have been identified worldwide, which vary in genotype, antigenic composition, morphology and infectivity for ticks. Isolates of A. marginale may be maintained by independent transmission events and a mechanism of infection/exclusion in cattle and ticks. The increasing numbers of A. marginale genotypes identified in some geographic regions most likely resulted from intensive cattle movement. However, concurrent A. marginale strain infections in cattle was reported, but these strains were more distantly related. Phylogenetic studies of selected geographic isolates of A. marginale, using msp4 and msp1alpha, provided information about the biogeography and evolution of A. marginale, and msp1alpha genotypes appear to have evolved under positive selection pressure. Live and killed vaccines have been used for control of anaplasmosis and both types of vaccines have advantages and disadvantages. Vaccines have effectively prevented clinical anaplasmosis in cattle but have failed to block A. marginale infection. Vaccines are needed that can prevent clinical disease and, simultaneously, prevent infection in cattle and ticks, thus eliminating these hosts as reservoirs of infection. Advances in genomics, proteomics, immunology and biochemical and molecular technologies during the last decade have been applied to research on A. marginale and related organisms, and the recent development of a cell culture system for A. marginale has provided a format for studying the pathogen/tick interface. Recent advancements and new research methodologies should provide additional opportunities for development of new strategies for control and prevention of bovine anaplasmosis.

Silencing of genes involved in Anaplasma marginale-tick interactions affects the pathogen developmental cycle in Dermacentor variabilis

BACKGROUND

The cattle pathogen, Anaplasma marginale, undergoes a developmental cycle in ticks that begins in gut cells. Transmission to cattle occurs from salivary glands during a second tick feeding. At each site of development two forms of A. marginale (reticulated and dense) occur within a parasitophorous vacuole in the host cell cytoplasm. However, the role of tick genes in pathogen development is unknown. Four genes, found in previous studies to be differentially expressed in Dermacentor variabilis ticks in response to infection with A. marginale, were silenced by RNA interference (RNAi) to determine the effect of silencing on the A. marginale developmental cycle. These four genes encoded for putative glutathione S-transferase (GST), salivary selenoprotein M (SelM), H+ transporting lysosomal vacuolar proton pump (vATPase) and subolesin.

RESULTS

The impact of gene knockdown on A. marginale tick infections, both after acquiring infection and after a second transmission feeding, was determined and studied by light microscopy. Silencing of these genes had a different impact on A. marginale development in different tick tissues by affecting infection levels, the densities of colonies containing reticulated or dense forms and tissue morphology. Salivary gland infections were not seen in any of the gene-silenced ticks, raising the question of whether these ticks were able to transmit the pathogen.

CONCLUSION

The results of this RNAi and light microscopic analyses of tick tissues infected with A. marginale after the silencing of genes functionally important for pathogen development suggest a role for these molecules during pathogen life cycle in ticks.

Defining the role of subolesin in tick cell culture by use of RNA interference

Development of tick vaccines provides new opportunities for control of tick infestations and tick-borne diseases. Recently, the tick-protective protein, subolesin, was identified in a cell line derived from Ixodes scapularis by expression library immunization and a mouse model of tick infestations. While subolesin was conserved among ixodid tick species, the biological function of this gene is unknown. Subolesin expression in ticks was silenced by RNA interference (RNAi) to provide information on the gene's function, and silencing of subolesin profoundly impacted tick survival, feeding, and reproduction. In this research we used RNAi in the IDE8 tick cell line to further study the role of subolesin in development of cultured tick cells. The cells were incubated with subolesin double-stranded (ds)RNA and cell growth was monitored. Incorporation of dsRNA by tick cells was monitored with Cy3-labeled dsRNA. After 72 h cells were harvested for cell counts, morphology, and for confirmation of gene silencing by reverse transcriptase-PCR. While the expression of subolesin in treated cells was reduced 80 +/- 9% by RNAi as compared with mock-treated cells, cell growth did not appear to be affected over the 72-h period. This is the first report of the use of RNAi in tick cell culture. RNAi is a powerful tool for studying tick gene function and will likely contribute to our understanding of the role that tick genes play in cell development and infection with pathogens.

Differential expression of the tick protective antigen subolesin in anaplasma marginale- and A. phagocytophilum-infected host cells

Subolesin was recently shown in vaccine and RNA interference (RNAi) studies to protect against tick infestations and to affect tick feeding, reproduction, and development as well as infection of host cells by Anaplasma marginale and A. phagocytophilum. Recent experiments provided evidence that infection of both tick and vertebrate host cells with these two pathogens modified gene expression. We therefore hypothesized that infection of host cells with A. marginale and A. phagocytophilum affects expression of subolesin. Subolesin mRNA levels were determined by real-time reverse transcriptase (RT)-PCR in uninfected and A. marginale-infected Dermacentor variabilis guts and salivary glands and IDE8-cultured tick cells and in uninfected and A. phagocytophilum-infected Ixodes scapularis nymphs, ISE6-cultured tick cells, and the human cell line HL-60. In addition, the effect of subolesin on Anaplasma spp. infection/multiplication was characterized by RNAi in tick tissues and/or cultured tick and human cells. These experiments presented evidence of differential expression of subolesin in A. marginale- and A. phagocytophilum-infected cells. Subolesin was differentially expressed in A. marginale-infected ticks in a tissue-specific manner in which mRNA levels increased in response to A. marginale infection in tick salivary gland cells but not in the gut cells. Subolesin knockdown by RNAi reduced Anaplasma infection/multiplication only in cells in which infection increased subolesin expression, i.e., in A. marginale-infected D. variabilis salivary glands and IDE8 cells. The results reported herein further support the role of subolesin in Anaplasma-host interactions and suggest a putative role of subolesin in vaccines for the control of pathogen infection/multiplication in ticks.

Transmission of Cytauxzoon felis to a domestic cat by Amblyomma americanum

Cytauxzoon felis was transmitted to a domestic cat by Amblyomma americanum. The infection was produced by the bite of A. americanum adults that were acquisition fed as nymphs on a domestic cat that naturally survived infection of C. felis. Fever, inappetence, depression, and lethargy were first noted 11 days post-infestation (dpi). Pale mucus membranes, splenomegaly, icterus, and dyspnea were also observed during the course of the disease. The body temperature of the experimentally infected C. felis cat was subnormal from 16 dpi until 24 dpi when it returned to within normal limits. All clinical signs of cytauxzoonsis began to resolve by 23 dpi when the cat became subclinically infected with C. felis. The cat developed a marked, regenerative anemia beginning by 13 dpi and reached a nadir at 20 dpi before recovering. A moderate neutrophilia and marked lymphocytosis also developed between 18 and 26 dpi. Schizonts of C. felis were observed in spleen aspirates of the infected cat at 15 dpi. DNA of C. felis was amplified by real-time PCR starting 17 dpi and piroplasms of C. felis were first noted by light microscopy 18 dpi. Dermacentor variabilis, Ixodes scapularis, and Rhipicephalus sanguineus were also tested in a similar manner at the same time but did not transmit C. felis. Prior to the present study, only D. variabilis had been shown experimentally to transmit infection of C. felis. This is the first report of C. felis being transmitted by A. americanum. The transmission of C. felis infection from one domestic cat to another indicates that domestic cats subclinically infected with C. felis may be a reservoir of infection for naive domestic cats.

Evidence of the role of tick subolesin in gene expression

Background

Subolesin is an evolutionary conserved protein that was discovered recently in Ixodes scapularis as a tick protective antigen and has a role in tick blood digestion, reproduction and development. In other organisms, subolesin orthologs may be involved in the control of developmental processes. Because of the profound effect of subolesin knockdown in ticks and other organisms, we hypothesized that subolesin plays a role in gene expression, and therefore affects multiple cellular processes. The objective of this study was to provide evidence for the role of subolesin in gene expression.

Results

Two subolesin-interacting proteins were identified and characterized by yeast two-hybrid screen, co-affinity purification and RNA interference (RNAi). The effect of subolesin knockdown on the tick gene expression pattern was characterized by microarray analysis and demonstrated that subolesin RNAi affects the expression of genes involved in multiple cellular pathways. The analysis of subolesin and interacting protein sequences identified regulatory motifs and predicted the presence of conserved protein kinase C (PKC) phosphorylation sites.

Conclusion

Collectively, these results provide evidence that subolesin plays a role in gene expression in ticks.

Advances toward understanding the molecular biology of the Anaplasma-tick interface

The genus Anaplasma includes a diverse group of tick-borne pathogens found exclusively within membrane-bound vacuoles in host cells. While A. marginale, A. centrale and A. ovis, vectored by Dermacentor and Rhipicephalus ticks, are host-specific for ruminants, A. phagocytophilum, vectored by Ixodes spp., infects a wide range of hosts. In ticks Anaplasma undergoes a developmental cycle that is coordinated with the tick feeding cycle. Although research at the tick/Anaplasma interface is in its infancy, recent studies have provided evidence that Anaplasma infection and transmission is mediated by a molecular mechanism involving both tick cell and pathogen genes. Application of a growing array of molecular approaches, such as RNA interference, genomics and proteomics, are rapidly expanding our knowledge of the tick/pathogen interface. Targeting key tick cell molecules required for pathogen development in vaccine strategies may compromise the vector capacity of ticks for Anaplasma, thus reducing transmission and infection of vertebrates. Collectively, this information will likely lead to the development of dual target vaccines designed to protect vertebrates against tick infestations and prevent the transmission of pathogens.

Targeting the tick-pathogen interface for novel control strategies

Ticks are ectoparasites of wild and domestic animals and humans that most notably impact global health by transmitting disease-causing pathogens. While information on the molecular interactions between ticks and pathogens that facilitate pathogen infection, development and transmission is limited, a comprehensive understanding of the tick-pathogen interface would be fundamental toward development of new and novel measures for control of both tick infestations and tick-borne pathogens. Recently, vaccine studies using key tick antigens and characterization of tick gene function by RNA interference (RNAi) have provided new information on genes that impact the tick-pathogen interface. In this review we summarize current research and prospects of tick vaccines and genetic manipulation of ticks targeted to the tick-pathogen interface. The knowledge gained from these collective studies will be fundamental toward understanding of tick-pathogen interactions and for formulation of control methods targeted at both ticks and tick-borne pathogens. Use of these molecular approaches will likely contribute to control measures that will notably reduce tick populations and tick-borne diseases in the future.

Targeting the tick/pathogen interface for developing new anaplasmosis vaccine strategies

Bovine anaplasmosis is a tick-borne hemolytic disease of cattle that occurs worldwide caused by the intraerythrocytic rickettsiae Anaplasma marginale. Control measures, including use of acaricides, administration of antibiotics and vaccines, have varied with geographic location. Our research is focused on the tick-pathogen interface for development of new vaccine strategies with the goal of reducing anaplasmosis, tick infestations and the vectorial capacity of ticks. Toward this approach, we have targeted (1) development of an A. marginale cell culture system to provide a non-bovine antigen source, (2) characterization of an A. marginale adhesion protein, and (3) identification of key tick protective antigens for reduction of tick infestations. A cell culture system for propagation of A. marginale was developed and provided a non-bovine source of A. marginale vaccine antigen. The A. marginale adhesion protein, MSP1a, was characterized and use of recombinant MSP1a in vaccine formulations reduced clinical anaplasmosis and infection levels in ticks that acquired infection on immunized cattle. Most recently, we identified a tick-protective antigen, subolesin, that reduced tick infestations, as well as the vectorial capacity of ticks for acquisition and transmission of A marginale. This integrated approach to vaccine development shows promise for developing new strategies for control of bovine anaplasmosis.

Tick vaccines and the transmission of tick-borne pathogens

Ticks transmit pathogens that cause diseases which greatly impact both human and animal health. Vaccines developed against Boophilus spp. using Bm86 and Bm95 tick gut antigens demonstrated the feasibility of using vaccines for control of tick infestations. These vaccines also reduced transmission of tick-borne pathogens by decreasing exposure of susceptible hosts to ticks. The recently discovered tick antigens, 64P putative cement protein and subolesin involved in the regulation of tick feeding and reproduction, were also shown to reduce tick infestations. These antigens, together with the TROSPA receptor for Burrelia burgdorferi OspA were effective against tick-borne pathogens by reducing the infection levels in ticks and/or the transmission of the pathogen. Development of a vaccine targeted at both the tick vector and pathogen would contribute greatly to the control of tick infestations and the transmission of tick-borne diseases. These results have demonstrated that tick vaccines can be developed for control tick infestations and show promise for the prevention of the transmission of tick-borne pathogens.

Sp110 transcription is induced and required by Anaplasma phagocytophilum for infection of human promyelocytic cells

Background

The tick-borne intracellular pathogen, Anaplasma phagocytophilum (Rickettsiales: Anaplasmataceae) causes human granulocytic anaplasmosis after infection of polymorphonuclear leucocytes. The human Sp110 gene is a member of the nuclear body (NB) components that functions as a nuclear hormone receptor transcriptional coactivator and plays an important role in immunoprotective mechanisms against pathogens in humans. In this research, we hypothesized that Sp110 may be involved in the infection of human promyelocytic HL-60 cells with A. phagocytophilum.

Methods

The human Sp110 and A. phagocytophilum msp4 mRNA levels were evaluated by real-time RT-PCR in infected human HL-60 cells sampled at 0, 12, 24, 48, 72 and 96 hours post-infection. The effect of Sp110 expression on A. phagocytophilum infection was determined by RNA interference (RNAi). The expression of Sp110 was silenced in HL-60 cells by RNAi using pre-designed siRNAs using the Nucleofector 96-well shuttle system (Amaxa Biosystems, Gaithersburg, MD, USA). The A. phagocytophilum infection levels were evaluated in HL-60 cells after RNAi by real-time PCR of msp4 and normalizing against human Alu sequences.

Results

While Sp110 mRNA levels increased concurrently with A. phagocytophilum infections in HL-60 cells, the silencing of Sp110 expression by RNA interference resulted in decreased infection levels.

Conclusion

These results demonstrated that Sp110 expression is required for A. phagocytophilum infection and multiplication in HL-60 cells, and suggest a previously undescribed mechanism by which A. phagocytophilum modulates Sp110 mRNA levels to facilitate establishment of infection of human HL-60 cells.

Tick cell lines: tools for tick and tick-borne disease research

Over 40 cell lines are currently available from 13 ixodid and one argasid tick species. The successful isolation and propagation of several economically important tick-borne pathogens in tick cell lines has created a useful model to study interactions between tick cells and these viral and bacterial disease agents. Tick cell lines have already proved to be a useful tool in helping to define the complex nature of the host-vector-pathogen relationship. With the availability of genomics tools, tick cell lines will become increasingly important as a complement to tick and tick-borne disease research in vivo once genetic transformation and gene silencing using RNA interference become routine.

RNA interference for the study and genetic manipulation of ticks

Ticks are ectoparasites of wild and domestic animals, and humans. A more comprehensive understanding of tick function and the tick-pathogen interface is needed to formulate improved tick-control methods. RNA interference (RNAi) is the most widely used gene-silencing technique in ticks where the use of other methods of genetic manipulations has been limited. In the short time that RNAi has been available, it has proved to be a valuable tool for studying tick gene function, the characterization of the tick-pathogen interface, and the screening and characterization of tick protective antigens. This review considers the applications of RNAi to tick research and the potential of this technique for tick functional studies, and to elucidate the tick-pathogen and tick-host interface. It is probable that the knowledge gained from this experimental approach will contribute to development of vaccines to control tick infestations and the transmission of tick-borne pathogens.

Applications of a cell culture system for studying the interaction ofAnaplasmamarginale with tick cells

A cell culture system for the tick-borne rickettsiaAnaplasma marginaleoffers new opportunities for research on this economically important pathogen of cattle.A. marginalemultiplies in membrane-bound inclusions in host cells. Whereas erythrocytes appear to be the only site of infection in cattle,A. marginaleundergoes a complex developmental cycle in ticks and transmission occurs via the salivary glands during feeding. We recently developed a cell culture system forA. marginaleusing a cell line derived from embryos ofIxodes scapularis. Here we review the use of this cell culture system for studying the interaction ofA. marginalewith tick cells. Several assays were developed using theA. marginale/tick cell system. An adhesion assay was developed for the identification of proteins required byA. marginalefor adhesion to tick cells. The effect of antibodies against selected major surface proteins in inhibitingA. marginaleinfection was tested in an assay that allowed further confirmation of the role of surface proteins in the infection of tick cells. A drug screening assay forA. marginalewas developed and provides a method of initial drug selection without the use of cattle. The culture system was used to test for enhancing effects of tick saliva and saliva components onA. marginaleinfection. The tick cell culture system has proved to be a good model for studyingA. marginale–tick interactions. Information gained from these studies may be applicable to other closely related tick-borne pathogens that have been propagated in the same tick cell line.

Experimental infection of C3H/HeJ mice with the NY18 isolate of Anaplasma phagocytophilum

Human granulocytic anaplasmosis (HGA), an emerging disease of public health concern in many areas of the world, is caused by Anaplasma phagocytophilum. Small animal models of A phagocytophilum in laboratory mice have been developed and used to study the pathogenesis of HGA. In this study, we characterized the pathologic changes in acute infection of C3H/HeJ mice experimentally infected with the NY18 isolate of A phagocytophilum. Although no clinical signs were noted, acute infection was associated with gross splenomegaly, microscopic inflammatory lesions in the lung and liver, hyperplastic lesions on the spleen, and clinical pathology abnormalities including neutropenia and monocytosis. This study emphasizes the use of well-defined animal models as a valuable tool for the study of A phagocytophilum infections.

Evolution and function of tandem repeats in the major surface protein 1a of the ehrlichial pathogenAnaplasma marginale

The major surface protein (MSP) 1a of the ehrlichial cattle pathogenAnaplasma marginale, encoded by the single-copy genemsp1α, has been shown to have a neutralization-sensitive epitope and to be an adhesin for bovine erythrocytes and tick cells.msp1αhas been found to be a stable genetic marker for the identification of geographic isolates ofA. marginalethroughout development in acutely and persistently infected cattle and in ticks. The molecular weight of MSP1a varies among geographic isolates ofA. marginalebecause of a varying number of tandemly repeated peptides of 28–29 amino acids. Variation in the sequence of the tandem repeats occurs within and among isolates, and may have resulted from evolutionary pressures exerted by ligand–receptor and host–parasite interactions. These repeated sequences include markers for tick transmissibility that may be important in the identification of ehrlichial pathogens because they may influence control strategies and the design of subunit vaccines.

Prevalence and Genetic Diversity of Anaplasma marginale Strains in Cattle in South Africa

Bovine anaplasmosis, caused by the tick-borne rickettsia Anaplasma marginale, is endemic in South Africa and results in considerable economic loss to the cattle industry. This study was designed to characterize strains of A. marginale at the molecular level from cattle raised in communal and commercial farms in the north-eastern and south-western regions of the Free State Province, South Africa, that varied in rainfall and vegetation. Seroprevalence to A. marginale was determined in 755 cattle by an Anaplasma spp. competitive enzyme-linked immunosorbent assay and ranged from 44% to 98% and was similar in both regions. While Anaplasma centrale was not targeted in this study, A. marginale infections were identified by species-specific msp1alpha polymerase chain reaction in 129 of 215 of the samples studied. Similar genetic diversity of A. marginale strains was found in both the north-eastern and south-western regions. The sequences of 29 A. marginalemsp1alpha amplicons from South African strains revealed considerable genetic diversity providing 14 new repeat sequences. However, 42% of MSP1a repeat sequences were not unique to this region. These results indicated the presence of common genotypes between South African, American and European strains of A. marginale. Cattle movement between different parts of South Africa was suggested by the presence of identical A. marginale MSP1a genotypes in north-eastern and south-western regions of the Free State Province. Control strategies for anaplasmosis in South Africa should therefore be designed to be protective against genetically heterogeneous strains of A. marginale.

Anaplasmosis: focusing on host-vector-pathogen interactions for vaccine development

Anaplasma marginale and A. phagocytophylum are intracellular rickettsiae that cause bovine anaplasmosis and human granulocytic anaplasmosis, respectively. The ultimate vaccine for the control of anaplasmosis would be one that reduces infection and transmission of the pathogen by ticks. Effective vaccines for control of anaplasmosis are not available despite attempts using different approaches, such as attenuated strains, infected erythrocyte and tick cell-derived purified antigens, and recombinant pathogen and tick-derived proteins. Three lines of functional analyses were conducted by our laboratory to characterize host-tick-Anaplasma interactions to discover potential vaccine candidate antigens to control tick infestations and the infection and transmission of Anaplasma spp.: (1) characterization of A. marginale adhesins involved in infection and transmission of the pathogen, (2) global expression analysis of genes differentially expressed in HL-60 human promyelocytic cells in response to infection with A. phagocytophilum, and (3) identification and characterization of tick-protective antigens by expression library immunization (ELI) and analysis of expressed sequence tags (EST) in a mouse model of tick infestations and by RNA interference in ticks. These experiments have resulted in the characterization of the A. marginale MSP1a as an adhesin for bovine erythrocytes and tick cells, providing support for its use as candidate vaccine antigen for the control of bovine . Microarray analysis of genes differentially expressed in human cells infected with A. phagocytophilum identified key molecules involved in pathogen infection and multiplication. The screening for tick-protective antigens resulted in vaccine candidates reducing tick infestation, molting, and oviposition and affecting Anaplasma infection levels in ticks.

In vitro cultivation of a south African isolate of an Anaplasma sp. in tick cell cultures

This paper describes the first successful in vitro cultivation of a South African isolate of an Anaplasma sp., initially thought to be Anaplasma marginale, in the continuous tick cell line IDE8. Blood from a bovine naturally infected with A. marginale kept on the farm Kaalplaas (28 degrees 08' E, 25 degrees 38' S) was collected, frozen, thawed and used as inoculum on confluent IDE8 cell cultures. Twenty days after culture initiation small intracellular colonies were detected in a Cytospin smear prepared from culture supernatant. Cultures were passaged on Day 34. Attempts to infect IRE/CTVM18 cell cultures with the Kaalplaas isolate derived from IDE8 cultures failed, whereas a reference stock of A. marginale from Israel infected IRE/CTVM18 tick cell cultures. Attempts to infect various mammalian cell lines (BA 886, SBE 189, Vero, L 929, MDBK) and bovine erythrocytes, kept under various atmospheric conditions, with tick cell-derived Anaplasma sp. or the Israeli strain of A. marginale failed. Molecular characterization revealed that the blood inoculum used to initiate the culture contained both A. marginale and Anaplasma sp. (Omatienne) whereas the organisms from established cultures were only Anaplasma sp. (Omatjenne).

Reduction of tick infections with Anaplasma marginale and A. phagocytophilum by targeting the tick protective antigen subolesin

Subolesin was recently shown by both gene silencing and immunization with the recombinant protein to protect against tick infestations, and to cause reduced tick survival and degeneration of gut and salivary gland tissues. In this research, we extended these studies by testing whether targeting subolesin by RNAi or vaccination interfered with the ability of ticks to become infected with two tick-borne pathogens, Anaplasma marginale which causes bovine anaplasmosis and Anaplasma phagocytophilum, the causative agent of human granulocytin anaplasmosis. For the A. marginale studies, Dermacentor variabilis males were injected with subolesin dsRNA or saline and then were allowed to feed on cattle with ascending rickettsemias, while for the A. phagocytophilum studies, mice were immunized with the recombinant subolesin protein, infected with the pathogen and then infested with larval Ixodes scapularis. Tick infections were determined by quantitative polymerase chain reaction of gut and salivary gland tissues. In both experimental approaches, tick infections were significantly reduced. These results suggest that subolesin appears to be a candidate vaccine antigen that may contribute to control of multiple tick species and the reduction of tick-borne pathogens.

Autocidal control of ticks by silencing of a single gene by RNA interference

Ticks impact human and animal health worldwide and new control methods are needed to circumvent drawbacks of tick control by acaricide application including selection of drug resistant ticks and environmental pollution. Using RNA interference we silenced the expression of a single gene, subolesin, and produced ticks with diminished reproductive performance and prevented successful mating and production of viable offspring. We propose a sterile acarine technique (SAT) for reduction of tick populations by release of subolesin-silenced ticks. Conservation of subolesin among tick species suggests that SAT may be useful for control of many medically and economically important tick species.

The tick protective antigen, 4D8, is a conserved protein involved in modulation of tick blood ingestion and reproduction☆

The gene that encodes the tick protective antigen, 4D8, was cloned from 10 species belonging to 6 genera, and the nucleotide and amino acid sequences were analyzed. 4D8 nucleotide and protein sequences were conserved among these tick species with identity/similarity between 65-98 and 60-98%, respectively. The function of 4D8 was characterized by RNA interference (RNAi) in five tick species. After the ticks were allowed to feed, degeneration of gut, salivary glands and reproductive tissues was observed, and tick survival, weight and oviposition were significantly reduced. 4D8 RNAi effected >90% reduction in oviposition in all tick species tested. Because of the critical role that 4D8 plays during tick feeding and oviposition, which ultimately results in the reduction of tick progeny, we proposed the generic name "subolesin" (Latin, suboles: offspring, progeny) for tick 4D8 proteins and subA for the subolesin-encoding gene.

Synergistic effect of silencing the expression of tick protective antigens 4D8 and Rs86 in Rhipicephalus sanguineus by RNA interference

Tick proteins have been shown to be useful for the development of vaccines which reduce tick infestations. Potential tick protective antigens have been identified and characterized, in part, by use of RNA interference (RNAi). RNAi allows for analysis of gene function by characterizing the impact of loss of gene expression on tick physiology. Herein, we used RNAi in Rhipicephalus sanguineus to evaluate gene functions of two tick protective antigens, 4D8 and Rs86, the homologue of Bm86, on tick infestation, feeding and oviposition. Silencing of 4D8 alone resulted in decreased tick attachment, survival, feeding and oviposition. Although the effect of Rs86 RNAi was less pronounced, silencing of this gene also reduced tick weight and oviposition. Most notably, simultaneous silencing of 4D8 and Rs86 by RNAi resulted in a synergistic effect in which tick survival, attachment, feeding, weight and oviposition were profoundly reduced. Microscopic evaluation of tick tissues revealed that guts from dual injected ticks were distended with epithelial cells sparsely distributed along the basement membrane. These results demonstrated the synergistic effect of the silencing expression of two tick protective genes. Inclusion of multiple tick protective antigens may, therefore, enhance the efficacy of tick vaccines.

Vaccination with recombinant tick antigens for the control of Ixodes scapularis adult infestations

Antigens protective against Ixodes scapularis infestations were identified by cDNA expression library immunization (ELI) and analysis of expressed sequenced tags (EST). Three cDNAs protective against larval tick infestations, 4F8, with homology to a nucleotidase, and 4D8 and 4E6 of unknown function, were characterized and obtained as recombinant proteins for immunization studies. Vaccination trials with recombinant proteins demonstrated an effect of these antigens against I. scapularis larvae in a mouse model. Herein, we evaluated the effect of recombinant antigens on I. scapularis adult infestations on immunized sheep. Vaccination with recombinant 4D8, 4F8, 4E6 and the combination of all three antigens reduced adult tick infestations by 58, 12, 20, and 16%, respectively, when compared to the control group but was statistically significant for 4D8 and 4F8 only. Oviposition was reduced by 22-49% in all groups immunized with recombinant tick antigens (P<0.05). The overall efficacy of vaccine formulations considering the effect on tick infestations and oviposition averaged 33-71%. These antigens, and especially 4D8, appear to be good candidates for continued development of a vaccine for control of tick infestations.

Genetic diversity of anaplasma species major surface proteins and implications for anaplasmosis serodiagnosis and vaccine development

The genus Anaplasma (Rickettsiales: Anaplasmataceae) includes several pathogens of veterinary and human medical importance. An understanding of the diversity of Anaplasma major surface proteins (MSPs), including those MSPs that modulate infection, development of persistent infections, and transmission of pathogens by ticks, is derived in part, by characterization and phylogenetic analyses of geographic strains. Information concerning the genetic diversity of Anaplasma spp. MSPs will likely influence the development of serodiagnostic assays and vaccine strategies for the control of anaplasmosis.

Characterization of three Ixodes scapularis cDNAs protective against tick infestations

cDNA expression library immunization (ELI) and analysis of expressed sequenced tags (EST) in a mouse model of tick infestations was used to identified cDNA clones that affected I. scapularis. Three protective antigens against larval tick infestations, 4F8, with homology to a nucleotidase, and 4D8 and 4E6 of unknown function, were selected for further characterization. All three antigens were expressed in all I. scapularis stages and localized in adult tick tissues. 4D8 was shown to be conserved in six other tick species. Based on immunization trials with synthetic polypeptides against larvae and nymphs and on artificial feeding experiments of adults, these antigens, especially 4D8, appear to be good candidates for continued development of a vaccine for control of tick infestations and may be useful in a formulation to target multiple species of ticks.

Capillary tube feeding system for studying tick-pathogen interactions of Dermacentor variabilis (Acari: Ixodidae) and Anaplasma marginale (Rickettsiales: Anaplasmataceae)

A capillary tube feeding (CTF) system was adapted for studying the interaction between Dermacentor variabilis (Say) and the rickettsial cattle pathogen Anaplasma marginale Theiler. A. marginale undergoes a complex developmental cycle in ticks that begins in midguts and ends by transmission from salivary glands. In this CTF system, male D. variabilis were fed A. marginale-infected blood or cultured tick cells. Ticks that fed on highly rickettsemic calves developed midgut and salivary gland infections as detected by PCR, whereas ticks that were fed from capillary tubes on the same blood developed only midgut infections. An unexpected result of capillary tube feeding was that antibodies against the A. marginale adhesin, major surface protein la, enhanced midgut infections and caused cell culture-derived A. marginale to infect midguts. Another unexpected result was the infection of the midguts of the nonvector tick Amblyomma americanum (L.), after capillary tube feeding on infected bovine blood. The gut cell response of ticks to A. marginale, as determined from SDS-polyacrylamide gel electrophoresis protein profiles, did not differ when ticks were fed infected or uninfected cells from capillary tubes. Selected protein bands, as identified by tryptic digestion-mass spectrometry, contained mostly proteins of bovine origin, including bovine albumin, undigested alpha- and beta-chain hemoglobin and hemoglobin fragments. Although infection of ticks by A. marginale CTF system was not the same as infection by feeding on cattle, the results obtained demonstrated the potential use of this system for identifying aspects of pathogen-vector interactions that are not readily recognized in naturally feeding ticks.

Gene expression profiling of human promyelocytic cells in response to infection with Anaplasma phagocytophilum

Anaplasma phagocytophilum (Rickettsiales: Anaplasmataceae) causes human, equine and canine granulocytic anaplasmosis and tick-borne fever of ruminants. The rickettsia parasitizes granulocytes and bone marrow progenitor cells, and can be propagated in human promyelocytic and tick cell lines. In this study, microarrays of synthetic polynucleotides of 21,329 human genes were used to identify genes that are differentially expressed in HL-60 human promyelocytic cells in response to infection with A. phagocytophilum. Semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) of selected genes confirmed the results of the microarray analysis. Six genes in the A. phagocytophilum-infected cells were found to be upregulated greater than 30-fold, while expression of downregulated genes most often did not change more than sixfold. Genes that were found to be differentially regulated in infected cells were those essential for cellular mechanisms including growth and differentiation, cell transport, signalling and communication and protective response against infection, some of which are most likely necessary for infection and multiplication of A. phagocytophilum in host cells. The differentially regulated genes described herein provide new information on the gene expression profiles in A. phagocytophilum-infected HL-60 cells, thus expanding in a global manner the existing information on the response of mammalian cells to A. phagocytophilum infection.

Anaplasma marginale(Rickettsiales: Anaplasmataceae): recent advances in defining host–pathogen adaptations of a tick-borne rickettsia

The tick-borne intracellular pathogenAnaplasma marginale(Rickettsiales: Anaplasmataceae) develops persistent infections in cattle and tick hosts. While erythrocytes appear to be the only site of infection in cattle,A. marginaleundergoes a complex developmental cycle in ticks and transmission occurs via salivary glands during feeding. Many geographic isolates occur that vary in genotype, antigenic composition, morphology and infectivity for ticks. In this chapter we review recent research on the host–vector–pathogen interactions ofA. marginale. Major surface proteins (MSPs) play a crucial role in the interaction ofA. marginalewith host cells. The MSP1a protein, which is an adhesin for bovine erythrocytes and tick cells, is differentially regulated and affects infection and transmission ofA. marginalebyDermacentorspp. ticks. MSP2 undergoes antigenic variation and selection in cattle and ticks, and contributes to the maintenance of persistent infections. Phylogenetic studies ofA. marginalegeographic isolates usingmsp4andmsp1α provide information about the biogeography and evolution ofA. marginale:msp1α genotypes evolve under positive selection pressure. Isolates ofA. marginaleare maintained by independent transmission events and a mechanism of infection exclusion in cattle and ticks allows for only the infection of one isolate per animal. Prospects for development of control strategies by use of pathogen and tick-derived antigens are discussed. TheA. marginale/vector/host studies described herein could serve as a model for research on other tick-borne rickettsiae.

RNA interference screening in ticks for identification of protective antigens

Ticks are ectoparasites of wild and domestic animals and humans, and are considered to be the most important arthropod vector of pathogens in North America. Development of vaccines directed against tick proteins may effect reduction of tick infestations and transmission of tick-borne pathogens. The limiting step for the development of tick vaccines has been the identification of tick protective antigens. Reverse vaccinology approaches aimed at reducing animal experimentation while allowing for the rapid screening of pools of potential tick vaccine candidates would greatly facilitate progress towards the development of tick vaccines. Herein, we describe the screening of Ixodes scapularis cDNAs for identification of tick protective antigens using RNA interference (RNAi). The results of the RNAi screening were similar to those obtained previously using expression library immunization and demonstrated that RNAi could serve as a more rapid and cost-effective tool for vaccine antigen discovery in ticks and in other nonmodel organisms.

Dedication: Conrad Yunker

This volume of the proceedings of STVM-03 is dedicated to Dr. Connie Yunker for his many contributions to tropical veterinary medicine and for being a good colleague and friend.

Glycosylation of Anaplasma marginale major surface protein 1a and its putative role in adhesion to tick cells

Anaplasma marginale, the causative agent of bovine anaplasmosis, is a tick-borne rickettsial pathogen of cattle that multiplies in erythrocytes and tick cells. Major surface protein 1a (MSP1a) and MSP1b form the MSP1 complex of A. marginale, which is involved in adhesion of the pathogen to host cells. In this study we tested the hypothesis that MSP1a and MSP1b were glycosylated, because the observed molecular weights of both proteins were greater than the deduced molecular masses. We further hypothesized that the glycosylation of MSP1a plays a role in adhesion of A. marginale to tick cells. Native and Escherichia coli-derived recombinant MSP1a and MSP1b proteins were shown by gas chromatography to be glycosylated and to contain neutral sugars. Glycosylation of MSP1a appeared to be mainly O-linked to Ser/Thr residues in the N-terminal repeated peptides. Glycosylation may play a role in adhesion of A. marginale to tick cells because chemical deglycosylation of MSP1a significantly reduced its adhesive properties. Although the MSP1a polypeptide backbone alone was adherent to tick cell extract, the glycans in the N-terminal repeats appeared to enhance binding and may cooperatively interact with one or more surface molecules on host cells. These results demonstrated that MSP1a and MSP1b are glycosylated and suggest that the glycosylation of MSP1a plays a role in the adhesion of A. marginale to tick cells.

Adhesion of outer membrane proteins containing tandem repeats of Anaplasma and Ehrlichia species (Rickettsiales: Anaplasmataceae) to tick cells

Infection of cells by tick-borne rickettsiae appears to be mediated by outer membrane proteins that allow pathogens to adhere to host cells. Major surface protein (MSP) 1a of Anaplasma marginale, the type species for the genus Anaplasma, was shown previously to be an adhesin for tick cells. The A. marginale MSP1a has a variable number of tandem 28 or 29 amino acid repeats located in the amino terminal region of the protein that contains an adhesion domain that is necessary and sufficient for infection of tick cells. The MSP1a studies demonstrated the importance of combining structural and functional characteristics for identification of adhesive proteins. In the present study other outer membrane proteins containing tandem repeats were selected from organisms of the family Anaplasmataceae and studied for their adhesive properties to tick cells. The adhesive properties and protein characteristics were then analyzed in order to provide a predictor of the adhesion function of proteins identified from genome sequences. Proteins selected included the A. marginale MSP1a, A. phagocytophilum 100 and 130 kDa, Ehrlichia chaffeensis 120 kDa, E. canis 140 kDa and E. ruminantium "mucin", which were all cloned and expressed in Escherichia coli and then tested as adhesins for cultured IDE8 cells. Of the proteins studied, the A. marginale MSP1a and the E. ruminantium "mucin" were found to be adhesins for tick cells. Although all of these recombinant outer membrane proteins were glycosylated, the A. marginale MSP1a and E. ruminantium "mucin" adhesins shared a common feature of having a high Ser/Thr content in the tandem repeats. The results reported herein provide new information on the role of E. ruminantium "mucin" as an adhesin for tick cells and also suggest a role of glycans in adhesin molecules.

Differential expression of the msp1alpha gene of Anaplasma marginale occurs in bovine erythrocytes and tick cells

Major surface proteins (MSP) 1a and 1b of the tick-borne pathogen Anaplasma marginale (Rickettsiales: Anaplasmataceae) are conserved on A. marginale derived from bovine erythrocytes and tick cells. MSP1a and MSP1b form the MSP1 complex and are adhesins involved in infection of host cells. While both MSP1a and MSP1b are adhesins for bovine erythrocytes, only MSP1a is an adhesin for cultured and native tick cells. These studies were initiated because antibody responses to MSP1a and MSP1b differed in cattle immunized with killed A. marginale derived from bovine erythrocytes or cultured tick cells. A strong antibody response to MSP1a was observed in cattle immunized with erythrocyte-derived A. marginale, whereas cattle immunized with tick cell culture-derived A. marginale produced antibodies preferentially to MSP1b. The molecular basis of this differential antibody response was then studied using Western blot, confocal microscopy and reverse transcriptase (RT)-PCR. Whereas expression of MSP1b by A. marginale derived from both bovine and tick host cells was similar at the protein and RNA levels, expression of MSP1a by A. marginale in these cells differed. Low levels of MSP1a were observed in cultured tick cells and tick salivary glands, but high expression of MSP1a occurred on A. marginale derived from bovine erythrocytes. The analysis of the expression of the msp1alpha gene by RT-PCR suggests that the differential expression of MSP1a is regulated at the transcriptional level and may influence the infectivity of A. marginale for host cells. Variation in the expression of MSP1a may also contribute to phenotypic and antigenic changes in the pathogen.

Mapping of B-cell epitopes in the N-terminal repeated peptides of Anaplasma marginale major surface protein 1a and characterization of the humoral immune response of cattle immunized with recombinant and whole organism antigens

Major surface protein (MSP) 1a of the genus type species Anaplasma marginale (Rickettsiales: Anaplasmataceae) together with MSP1b forms the MSP1 complex. MSP1a has been shown to be involved in adhesion, infection and tick transmission of A. marginale, as well as to contribute to protective immunity in cattle. A differential antibody response to MSP1a and MSP1b was observed in cattle immunized with A. marginale derived from bovine erythrocytes (anti-MSP1a response) or cultured tick cells (anti-MSP1b response). In this study, we further characterized the MSP1a antibody response of cattle using several immunogens, including recombinant MSP1a (rMSP1a) protein, erythrocyte- or tick cell culture-derived A. marginale, or a combination of tick cell culture-derived A. marginale and rMSP1a. The MSP1a antibody response to all these immunogens was directed primarily against the N-terminal region of MSP1a that contains tandemly repeated peptides, whereas low antibody levels were detected against the C-terminal portion. Linear B-cell epitopes of MSP1a were mapped using synthetic peptides representing the entire sequence of the protein that were prepared by SPOT synthesis technology. Only two peptides in the N-terminal repeats were recognized by sera from immunized cattle. These peptides shared the sequence SSAGGQQQESS, which is likely to contain the linear B-cell epitope that was recognized by the pools of bovine sera. The average differential of antibody titers against MSP1a minus those against MSP1b correlated with lower percent reductions in PCV. A preferential antibody response to MSP1a was observed in cattle immunized with erythrocyte-derived, cell culture-derived plus rMSP1a or rMSP1a alone, and the percent reduction PCV was significantly lower in these cattle as compared with the other immunization groups. These results provide insight into the bovine antibody response against A. marginale and the role of MSP1a in protection of cattle against A. marginale infection.

Identification of protective antigens for the control of Ixodes scapularis infestations using cDNA expression library immunization

Identification of antigens that induce an immune response against tick infestations is required for the development of vaccines against these economically important ectoparasites. In order to identify protective antigens, we constructed a cDNA expression library from a continuous Ixodes scapularis cell line (IDE8) that was initially derived from tick embryos. cDNA clones were subjected to several rounds of screening in which mice were immunized with individual pools and then challenge-exposed by allowing I. scapularis larvae to feed on the immunized and control mice. Immunity against tick infestation was determined by the reduction in the ability of the larvae to feed to repletion and molt to the nymphal stage. Individual clones in pools that induced immunity to larval infestations were partially sequenced and grouped according to their putative protein function by comparison with sequence databases. The screening identified several individual antigens that induced a protective immune response against I. scapularis infestations. Our studies demonstrated for the first time that cDNA expression library immunization (ELI) combined with sequence analysis is a powerful and efficient tool for identification of candidate antigens for use in vaccines against ticks.

Antibodies to Anaplasma marginale major surface proteins 1a and 1b inhibit infectivity for cultured tick cells

Major surface protein 1 (MSP1) of the cattle pathogen Anaplasma marginale (Rickettsiales: Anaplasmataceae) is a complex of two proteins, MSP1a and MSP1b. Previous studies demonstrated that MSP1a and MSP1b are adhesins for bovine erythrocytes, while only MSP1a proved to be an adhesin for tick cells. In this study, a tick cell culture system for propagation of A. marginale was used to develop an infection inhibition assay for testing the ability of antisera to block infection of A. marginale for cultured tick cells. A. marginale derived from cell culture was incubated with various antisera prior to inoculation onto cell monolayers. The monolayers were harvested 7 days post-inoculation and A. marginale in the cultures was quantified using an antigen detection ELISA. Antisera tested in the infection inhibition assay were derived from persistently infected cattle, from cattle immunized with A. marginale purified from bovine erythrocytes, and from rabbits and cattle that were immunized with the recombinant MSP1a, MSP1b and MSP1 complex. Antibodies from cattle persistently infected with A. marginale, cattle immunized with A. marginale from bovine erythrocytes or cattle immunized with the recombinant MSP1 complex did not inhibit the infectivity of A. marginale for tick cells. Antiserum from rabbits immunized with MSP1a and MSP1b (individually or combined) reduced infection of both the Virginia and Oklahoma isolates of A. marginale for tick cells by 25-70%. Likewise, antisera from cattle immunized with recombinant MSP1a or MSP1b inhibited infection of tick cells by 26-37%. These results further confirm the role of MSP1 complex proteins in infection of tick cells. Lack of inhibition of infection by antisera from naturally infected cattle or cattle immunized with whole organisms suggests that the bovine immune response is not directed toward blocking infection of A. marginale for tick cells and may contribute to the continued infectivity of the pathogen for ticks.

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

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