SAAG-4 is a novel mosquito salivary protein that programmes host CD4 T cells to express IL-4

Mosquitoes represent the most important vector for transmitting pathogens that cause human disease. Central to pathogen transmission is the ability to divert the host immune system away from Th1 and towards Th2 responsiveness. Identification of the mosquito factor(s) critical for programming Th2 responsiveness should therefore lead to strategies to neutralize their function and thus prevent disease transmission. In the current study, we used a TCR transgenic adoptive transfer system to screen gene products present in the saliva of the mosquito Aedes aegypti for their ability to programme CD4 T cells to express the signature Th2 cytokine IL-4. The clone SAAG-4 encodes a secreted protein with a predicted size of 20 kDa whose function has previously been uncharacterized. Notably, SAAG-4 reduced host CD4 T cell expression of the signature Th1 cytokine IFN-gamma while simultaneously increasing expression of IL-4. SAAG-4 is therefore the first identified mosquito factor that can programme Th2 effector CD4 T cell differentiation.

A novel sphingomyelinase-like enzyme in Ixodes scapularis tick saliva drives host CD4 T cells to express IL-4

Tick feeding modulates host immune responses. Tick-induced skewing of host CD4(+) T cells towards a Th2 cytokine profile facilitates transmission of tick-borne pathogens that would otherwise be neutralized by Th1 cytokines. Tick-derived factors that drive this Th2 response have not previously been characterized. In the current study, we examined an I. scapularis cDNA library prepared at 18-24 h of feeding and identified and expressed a tick gene with homology to Loxosceles spider venom proteins with sphingomyelinase activity. This I. scapularis sphingomyelinase-like (IsSMase) protein is a Mg(2+)-dependent, neutral (pH 7.4) form of sphingomyelinase. Significantly, in an in vivo TCR transgenic adoptive transfer assay IsSMase programmed host CD4(+) T cells to express the hallmark Th2 effector cytokine IL-4. IsSMase appears to directly programme host CD4 T cell IL-4 expression (as opposed to its metabolic by-products) because induced IL-4 expression was not altered when enzymatic activity was neutralized. TCR transgenic CD4 T cell proliferation (CFSE-dilution) was also significantly increased by IsSMase. Furthermore, a Th2 response is superimposed onto a virally primed Th1 response by IsSMase. Thus, IsSMase is the first identified tick molecule capable of programming host CD4(+) T cells to express IL-4.

Feeding by the tick, Ixodes scapularis, causes CD4+T cells responding to cognate antigen to develop the capacity to express IL-4

Effects of tick feeding on an early antigen-specific T cell response were studied by monitoring a clonotypic population of adoptively transferred T cell receptor (TCR) transgenic CD4 cells responding to a tick-associated antigen. When recipient mice were infested with pathogen-free Ixodes scapularis nymphs several days prior to T cell transfer and intradermal injection of soluble cognate antigen at the feeding site, the clonotypic CD4 cells gained the ability to express the Th2 effector cytokine IL-4. Notably, this effect was not only observed in BALB/c mice predisposed towards developing Th2 responses but also in B10.D2 mice predisposed towards Th1 responsiveness. Furthermore, tick feeding was able to superimpose IL-4 expression potential onto a strong Th1 response (indicated by robust IFN-gamma expression potential) elicited by immunization with a vaccinia virus expressing the cognate antigen. The magnitude to which tick feeding was able to programme IL-4 expression potential in CD4 cells was partially reduced in mice that had been previously exposed to pathogen-free tick nymphs 6 weeks earlier, as well as when the nymphs were infected with Borrelia burgdorferi. Intradermal injection of salivary gland extract programmed IL-4 expression potential similar to that of tick infestation, suggesting that IL-4 programming activity is contained within tick saliva.

Tick genomics: The Ixodes genome project and beyond

Ticks and mites (subphylum Chelicerata; subclass Acari) include important pests of animals and plants worldwide. The Ixodes scapularis (black-legged tick) genome sequencing project marks the beginning of the genomics era for the field of acarology. This project is the first to sequence the genome of a blood-feeding tick vector of human disease and a member of the subphylum Chelicerata. Genome projects for other species of Acari are forthcoming and their genome sequences will likely feature significantly in the future of tick research. Parasitologists interested in advancing the field of tick genomics research will be faced with specific challenges. The development of genetic tools and resources, and the size and repetitive nature of tick genomes are important considerations. Innovative approaches may be required to sequence, assemble, annotate and analyse tick genomes. Overcoming these challenges will enable scientists to investigate the genes and genome organisation of this important group of arthropods and may ultimately lead to new solutions for control of ticks and tick-borne diseases.

Tick immunobiology

Ticks are of vast medical and veterinary public health importance due to direct damage caused by feeding and their roles in transmitting well known and emerging infectious agents. Ticks and tick-borne pathogens stimulate the immune system of the host. Those immune interactions are of importance in tick biology, pathogen transmission and control of ticks and tick-borne diseases. Both innate and specific acquired immune defenses are involved in the responses of vertebrate hosts to infestation. Ticks have evolved countermeasures to circumvent host immune defenses. This review addresses the immunobiology of the tick-host interface from the perspectives of the pharmacology of tick saliva; relationship of tick saliva to pathogen transmission; host immune responses to infestation; tick modulation of host immune defences; and genomic/proteomic strategies for studying tick salivary gland molecules.

Tick modulation of the in-vitro expression of adhesion molecules by skin-derived endothelial cells

As a tick feeds, its saliva induces innate and acquired immune responses in the host, including leucocyte infiltration into the bite site. Tick salivary glands produce molecules, however, that counteract many host defences against blood feeding. The effects of salivary-gland extracts (SGE) of Dermacentor andersoni and Ixodes scapularis on the expression of various adhesion molecules [E-selectin, P-selectin, intercellular adhesion molecule 1 (ICAM-1) and vascular cell adhesion molecule 1 (VCAM-1)] by the sEND.1 cell line (which is based on cells from the subcutaneous tissue of mice) have now been investigated in vitro. The effects were found to differ with the tick species. The SGE of D. andersoni significantly down-regulated the expression of ICAM-1, whereas a similar extract prepared from I. scapularis significantly reduced the expression of P-selectin and VCAM-1. Tick salivary proteins therefore appear to have direct effects on adhesion-molecule expression, in addition to their previously established roles in down-regulating the pro-inflammatory cytokines that activate endothelial cells. It remains unclear exactly how the reduction of adhesion-molecule expression in the host's endothelial cells benefits the feeding tick but it may alter leucocyte migration to the bite site and/or reduce antigen presentation by the endothelial cells. It may also modulate the interactions between the host's leucocytes and any tick-borne pathogens, during initial infection of the endothelium.

Cellular infiltration at skin lesions and draining lymph nodes of sheep infested with adult Hyalomma anatolicum anatolicum ticks

Immunohistochemical analysis of skin and draining lymph nodes of sheep repeatedly infested with the ixodid tick Hyalomma anatolicum anatolicum were studied for different antigen-presenting cells and lymphocyte subpopulations. Infiltration of neutrophils, macrophages and lymphocytes adjacent to the tick bite site were observed. Skin biopsies showed significant increases in dermal infiltration of CD8+ and gammadelta+ T cells at 72 h and 8 days after both primary and secondary infestation. Infiltrations of MHC-II DR/DQ decreased at 72 h after tick infestation, whereas significant increases were recorded for 8-day skin biopsies. CD1+ cellular infiltrations were observed during secondary infestations at the dermis. Decreased ratios of CD4:CD8 T cells and MHC-II:CD1 antigen-presenting cells were observed in both infestations compared to healthy skin biopsies. Ratios of alphabeta:gammadelta T cells increased gradually during infestation compared to uninfested skin. The regional lymph nodes from tick-infested sheep showed an increased CD8+, gammadelta+ T and CD1+ cellular infiltration compared to control lymph nodes. CD4+ T cells were decreased. There were no significant changes in CD45R+ cellular infiltration either at skin lesions or regional lymph nodes.

Murine extramedullary erythropoiesis induced by tick infestation

Tick saliva contains molecules that modulate the haemostasis, pain/itch responses, wound healing and immune defences of the host. Using BALB/c mice that were each infested with 10 nymphs of Dermacentor andersoni Stiles (Acari: Ixodidae), an attempt has now been made to determine the influence of tick infestation on the expression of leucocyte adhesion molecules in the host. The ticks became fully engorged by the fourth to sixth day of infestation. On the fourth day of infestation, the results of flow cytometry indicated that 2% of the host's splenocytes were expressing high levels of CD49 (alpha4 integrin of VLA-4) and low levels of CD11a (alphaL subunit of the integrin LFA-1). By the eighth day of infestation, 30% of the hosts' splenocytes had this phenotype and were negative for the lineage markers CD3e (T-lymphocytes), DX5 (natural-killer cells of a BALB/c lineage), B220 (B-lymphocytes), CD11b (monocytes/macrophages, granulocytes, natural-killer cells, activated T-lymphocytes, and B-1 cells) and CD11c (myeloid and splenic dendritic cells). Histological examination of the spleens from infested mice revealed disruption of the white-pulp/red-pulp demarcations and the presence of a large number of basophilic normoblasts. The CD11a(lo) population of splenocytes from the tick-infested mice was positive for TER-119 but negative for CD3, B220, CD11b and Gr, confirming that the splenocytes were members of the erythroid lineage. These results indicate that, within 8 days of their initiation, the tick infestations induced extramedullary erythropoiesis in the spleens of their murine hosts.

Modulation of host immunity by haematophagous arthropods

The medical and veterinary public-health importance of haematophagous arthropods is immense and continuing to increase because of the emergence of new vector-borne infectious agents and the resurgence of well known ones. Control of blood-feeding arthropods and the pathogens they transmit is compounded by drug, insecticide and acaricide resistance. Novel control strategies are needed. Immunological control is one very promising approach to these problems. In order to develop anti-arthropod vaccines that block pathogen transmission and establishment, the immunological interactions occurring at the interface of the blood-feeding arthropod and host must be characterized. An important component of these interactions is arthropod modulation of the host's innate and acquired, specific immune defences. This review discusses current knowledge regarding the ability of haematophagous arthropods to alter their hosts' immune defences, the impact of those changes on pathogen transmission, the molecular bases for the immunomodulation, and strategies for identification of the molecules in arthropod saliva that are responsible for the immunomodulation.

Progress toward molecular characterization of ectoparasite modulation of host immunity

Ectoparasitic arthropods and vector-borne infectious agents are global medical and veterinary public health concerns. Economic impact due to direct effects of infestation and disease transmission are significant. These problems are increased by development of arthropod resistance to insecticides/acaricides; drug resistance of vector-borne pathogens; and, lack of effective vaccines to prevent many of these diseases. There is much to be gained from understanding the complex array of immunological interactions occurring at the arthropod-host-pathogen interface. One application of that knowledge is the development of novel vaccines for the control of both ectoparasitic arthropods and the diseases they transmit. We now realize that blood-feeding arthropods are not simply flying or crawling hypodermic needles and syringes. Ectoparasitic arthropods are not passive partners in their relationships with the immune systems of their hosts. These clever invertebrates produce numerous pharmacologically active molecules that help them migrate through tissues of their hosts or to successfully obtain blood meals. Arthropod parasites stimulate a spectrum of host immune responses that could potentially impair development, reduce feeding success, or kill the ectoparasite. Not unexpectedly, arthropods have developed sophisticated arsenals of countermeasures that modulate or deviate host immune responses. Not only does arthropod modulation of host immunity facilitate survival in tissues or increase the likelihood of obtaining a blood meal, but it is increasingly recognized as a critical factor in pathogen transmission. Those countermeasures to host immune defenses are the topics of this review. Emphasis is placed on our current understanding of the molecular bases of those changes; the molecules responsible for host immunomodulation; contemporary approaches for studying these complex relationships; and, the potential for using this information to develop innovative vaccine-based control strategies.

Regulation of OspE-related, OspF-related, and Elp lipoproteins of Borrelia burgdorferi strain 297 by mammalian host-specific signals

In previous studies we have characterized the cp32/18 loci in Borrelia burgdorferi 297 which encode OspE and OspF orthologs and a third group of lipoproteins which possess OspE/F-like leader peptides (Elps). To further these studies, we have comprehensively analyzed their patterns of expression throughout the borrelial enzootic cycle. Serial dilution reverse transcription-PCR analysis indicated that although a shift in temperature from 23 to 37 degrees C induced transcription for all nine genes analyzed, this effect was often markedly enhanced in mammalian host-adapted organisms cultivated within dialysis membrane chambers (DMCs) implanted within the peritoneal cavities of rats. Indirect immunofluorescence assays performed on temperature-shifted, in vitro-cultivated spirochetes and organisms in the midguts of unfed and fed ticks revealed distinct expression profiles for many of the OspE-related, OspF-related, and Elp proteins. Other than BbK2.10 and ElpA1, all were expressed by temperature-shifted organisms, while only OspE, ElpB1, OspF, and BbK2.11 were expressed in the midguts of fed ticks. Additionally, although mRNA was detected for all nine lipoprotein-encoding genes, two of these proteins (BbK2.10 and ElpA1) were not expressed by spirochetes cultivated in vitro, within DMCs, or by spirochetes within tick midguts. However, the observation that B. burgdorferi-infected mice generated specific antibodies against BbK2.10 and ElpA1 indicated that these antigens are expressed only in the mammalian host and that a form of posttranscriptional regulation is involved. Analysis of the upstream regions of these genes revealed several differences between their promoter regions, the majority of which were found in the -10 and -35 hexamers and the spacer regions between them. Also, rather than undergoing simultaneous upregulation during tick feeding, these genes and the corresponding lipoproteins appear to be subject to progressive recruitment or enhancement of expression as B. burgdorferi is transmitted from its tick vector to the mammalian host. These findings underscore the potential relevance of these molecules to the pathogenic events of early Lyme disease.

Dermacentor andersoni: effects of repeated infestations on lymphocyte proliferation, cytokine production, and adhesion-molecule expression by BALB/c mice

The effects of repeated infestations with Dermacentor andersoni nymphs on the lymphocyte functions of BALB/c mice were investigated. The in-vitro proliferation responses to concanavalin-A or salivary-gland molecules, the production of cytokines, and the expression of two adhesion molecules-leucocyte function-associated antigen-1 (LFA-1) and very late activation-4 (VLA-4)-were all studied. In addition, the ability of salivary-gland extract or saliva from D. andersoni to modulate expression of lymphocyte adhesion molecules in vitro was determined. The proliferative responses of T-lymphocytes to concanavalin-A were significantly suppressed after first and second infestations, and significant increases in lymphocyte proliferation in the presence of tick salivary-gland antigen were observed in infested mice. After two infestations, production of interleukin-2 was significantly decreased but that of interferon-gamma remained unchanged. Production of interleukin-4 and interleukin-10 was significantly enhanced in infested mice after both the first and second infestations. Expression of LFA-1 and VLA-4 by lymphocytes from infested mice was suppressed. Furthermore, both a salivary-gland extract and the saliva of D. andersoni reduced the in-vitro expression of both of these adhesion molecules by lymphocytes from tick-naive mice.

Influence of repeated infestations with pathogen-free Ixodes scapularis (Acari: Ixodidae) on in vitro lymphocyte proliferation responses of C3H/HeN mice

In the United States, Ixodes scapularis Say has been implicated as the vector of at least three human pathogens. Tick induced modulation of host immunity is increasingly recognized as an important factor in successful transmission or establishment of tick-borne pathogens. This study was conducted to determine the effects of repeated infestations with pathogen-free I. scapularis nymphs on in vitro proliferative responses of splenic lymphocytes from C3H/HeN mice. Lymphocytes from repeatedly infested and uninfested mice were exposed to concanavalin A (Con A), Escherichia coli Castellini & Chalmers lipopolysaccharide (LPS), or I. scapularis salivary gland soluble proteins (SGSP), to determine if lymphocyte responses differed between tick-exposed and nonexposed mice. Female C3H/HeN mice were infested one to four times with pathogen-free I. scapularis nymphs, with a 14-d tick-free period between each exposure. After each infestation, tick biology parameters were measured and lymphocyte proliferative responses assessed. Acquired resistance to I. scapularis was not evident in mice subjected to tick feeding. Significant differences in the responses of lymphocytes exposed to I. scapularis SGSP were observed between infested and noninfested mice. In contrast, few differences between infested and noninfested mice were evident for lymphocytes exposed to Con A or LPS. Our results suggest that repeated exposure to I. scapularis nymphs does not affect Con A or LPS-induced proliferation of splenic lymphocytes, but significantly effects lymphocyte responses to tick salivary gland antigens.

Interdependence of environmental factors influencing reciprocal patterns of gene expression in virulent Borrelia burgdorferi

The paradigm for differential antigen expression in Borrelia burgdorferi, the agent of Lyme disease, is the reciprocal expression of its outer surface (lipo)proteins (Osp) A and C; as B. burgdorferi transitions from its arthropod vector into mammalian tissue, ospC is upregulated, and ospA is downregulated. In the current study, using B. burgdorferi cultivated under varying conditions in BSK-H medium, we found that a decrease in pH, in conjunction with increases in temperature (e.g. 34 degrees C or 37 degrees C) and cell density, acted interdependently for the reciprocal expression of ospC and ospA. The lower pH (6.8), which induced the reciprocal expression of ospC and ospA in BSK-H medium, correlated with a drop in pH from 7.4 to 6.8 of tick midgut contents during tick feeding. In addition to ospC and ospA, other genes were found to be regulated in reciprocal fashion. Such genes were either ospC-like (e.g. ospF, mlp-8 and rpoS) (group I) or ospA-like (lp6.6 and p22) (group II); changes in expression occurred at the mRNA level. That the expression of rpoS, encoding a putative stress-related alternative sigma factor (sigma(s)), was ospC-like suggested that the expression of some of the group I genes may be controlled through sigma(s). The combined results prompt a model that allows for predicting the regulation of other B. burgdorferi genes that may be involved in spirochaete transmission, virulence or mammalian host immune responses.

Decorin-binding protein A (DbpA) of Borrelia burgdorferi is not protective when immunized mice are challenged via tick infestation and correlates with the lack of DbpA expression by B. burgdorferi in ticks

Previous studies showed that decorin-binding protein A (DbpA) of Borrelia burgdorferi was a protective immunogen in the murine model of Lyme borreliosis when mice were challenged (needle inoculated) intradermally with in vitro-cultivated spirochetes. In the present study, DbpA-immunized C3H/HeJ mice were not protected from infection when infested with Ixodes scapularis nymphs harboring virulent B. burgdorferi 297. This lack of protection correlated with the failure to detect DbpA on B. burgdorferi in ticks, suggesting that DbpA is not available as a target for bactericidal antibodies in serum when B. burgdorferi-infected ticks take their blood meal from an immunized host. The failure of DbpA immunization to protect tick-challenged mice contradicts the results of earlier needle inoculation vaccination experiments and suggests that DbpA may not be suitable as a Lyme disease vaccine.

Influence of soluble proteins from the salivary glands of ixodid ticks on the in-vitro proliferative responses of lymphocytes from BALB/c and C3H/HeN mice

In the U.S.A., Borrelia burgdorferi, the causative agent of Lyme borreliosis, is transmitted to humans by the ticks Ixodes scapularis and I. pacificus. Tick modulation of host immunity is an important factor in tick transmission of such pathogens. The proliferative responses of lymphocytes from BALB/c and C3H/HeN mice exposed to the salivary-gland soluble proteins (SGSP) of I. scapularis, I. pacificus or Dermacentor andersoni were therefore compared in vitro. This produced the present report, the first to describe the effects of I. pacificus SGSP on the proliferative responses of a host's lymphocytes in vitro. The effects of four concentrations of SGSP from each tick species were evaluated with unstimulated, and concanavalin-A-stimulated lymphocytes of each mouse strain. The responses of lymphocytes from both mouse strains were significantly effected when exposed to SGSP derived from each tick species. Responses of the unstimulated lymphocytes to SGSP indicated that the proteins from I. pacificus suppressed in-vitro lymphocyte proliferation to a greater degree than those from the other species investigated. For the concanavalin-A stimulated cells, however, suppression of the proliferative responses was greatest for cells exposed to I. scapularis SGSP.

Isolation and molecular cloning of a secreted immunosuppressant protein from Dermacentor andersoni salivary gland

A 36-kDa immunosuppressant protein (Da-p36) was isolated from salivary glands of feeding female ixodid ticks Dermacentor andersoni, using its affinity for UltraLink Biosupport Medium (Pierce, Rockford, Illinois)/protein complexes. Using a nested set of forward degenerate oligonucleotide primers corresponding to Da-p36 N-terminal amino acids, a cDNA encoding the immunosuppressant protein was isolated by 3' rapid amplification of cDNA ends. The resulting 772-base pair cDNA encodes a novel protein with predicted molecular weight of 24.9 kDa. Sequence analysis revealed the presence of 5 potential glycosylation sites and 1 myristylation site. Immunoblot analyses showed native Da-p36 is present in salivary glands and saliva from both male and female D. andersoni but not in salivary glands or saliva from Amblyomma americanum or Ixodes scapularis. Reverse transcription polymerase chain reaction and immunoblot analyses showed that Da-p36 expression is temporally regulated in salivary glands with maximum mRNA levels preceding maximum Da-p36 accumulation that occurred at day 6 of feeding. The levels of Da-p36 mRNA and protein were greatly reduced in salivary glands from near-replete females removed from sheep after 8 days of feeding. These data are consistent with a role of Da-p36 in immunosuppression during feeding.

Cytokine responses of C3H/HeN mice infested with Ixodes scapularis or Ixodes pacificus nymphs

Lyme borreliosis, caused by Borrelia burgdorferi, is transmitted by Ixodes scapularis in the eastern and midwestern United States and by Ixodes pacificus in the far-Western United States. Studies have shown that infestation with I. scapularis nymphs modulates host cytokine production; however, the influence of I. pacificus infestation on host cytokines remains uninvestigated. This study demonstrated how repeated infestations with pathogen-free I. scapularis or I. pacificus nymphs affects the production of the macrophage cytokines IL-1beta, IL-6 and tumour necrosis factor-alpha and the T lymphocyte cytokines IL-2, IL-4, IL-5, IL-6, IL-10 and IFN-gamma by C3H/HeN mice. Female mice were infested once or twice with pathogen-free I. scapularis or I. pacificus nymphs, with a 14-day tick-free period between exposures. After each infestation, tick biology parameters were assessed and macrophage and T lymphocyte cytokine production measured by antigen capture ELISA. Acquired resistance to tick feeding did not develop after infestation with either tick species. Differences in cytokine production were observed between infested and noninfested mice, and between mice infested with either I. scapularis or I. pacificus nymphs. Infestations polarized cytokine production towards a Th2 cytokine profile, with suppression of pro-inflammatory Th1 cytokines. This pattern of cytokine production is more pronounced for I. pacificus infested mice.

Identification, characterization, and expression of three new members of the Borrelia burgdorferi Mlp (2.9) lipoprotein gene family

We previously reported on the existence of a family of lipoprotein genes, designated 2.9 lipoprotein genes, encoded in at least seven versions on the circular (supercoiled) cp32 and cp18 plasmids of Borrelia burgdorferi 297. A distinguishing feature of the 2.9 lipoproteins were highly similar signal sequences but variable mature polypeptides that segregated into two antigenic classes. Further screenings of B. burgdorferi 297 genomic libraries led to the identification of three additional 2.9 lipoprotein genes, renamed herein mlp, for multicopy lipoprotein genes. Computer analyses and immunoblotting revealed that Mlp-9 segregated with the antigenic class I lipoproteins, whereas Mlp-8 and Mlp-10 were members of class II. Northern blotting showed that all three of the mlp genes were expressed when B. burgdorferi was cultivated in vitro at 34 degrees C, although mlp-9 and mlp-10 transcripts were expressed at very low levels. Additional combined immunoblotting and comparative reverse transcription-PCR analyses performed on borreliae cultivated in vitro at 23, 34, or 37 degrees C indicated that although Mlp-8 was substantially more abundant than Mlp-9 or Mlp-10, all three of the mlp genes were upregulated during B. burgdorferi replication at 37 degrees C. Expression of the same three lipoproteins was further enhanced upon growth of the spirochetes within dialysis membrane chambers (DMCs) implanted intraperitoneally in rats (i.e., spirochetes in a mammalian host-adapted state), suggesting that temperature alone did not account for maximal upregulation of the mlp genes. That certain mlp genes are likely expressed during the growth of B. burgdorferi in mammalian tissues was supported by findings of antibodies against all three Mlp lipoproteins in mice after challenge with Ixodes scapularis nymphs harboring B. burgdorferi 297. The combined data suggest that as opposed to being differentially expressed in any reciprocal fashion (e.g., OspA/OspC), at least three mlp genes are simultaneously upregulated by temperature (37 degrees C) and some other mammalian host factor(s). The findings have importance not only for understanding alternative modes of differential antigen expression by B. burgdorferi but also for assessing whether one or more of the Mlp lipoproteins represent new candidate vaccinogens for Lyme disease.

Ixodes scapularis: effects of repeated infestations with pathogen-free nymphs on macrophage and T lymphocyte cytokine responses of BALB/c and C3H/HeN mice

Schoeler, G. B., Manweiler, S. A., and Wikel, S. K. 1999. Ixodes scapularis: Effects of repeated infestations with pathogen-free nymphs on macrophage and T lymphocyte cytokine responses of BALB/c and C3H/HeN mice. Experimental Parasitology 92, 239-248. Ixodes scapularis is the principal vector in the United States of Borrelia burgdorferi, the causative agent of Lyme borreliosis, the human granulocytic ehrichiosis agent, and Babesia microti. Infestation with I. scapularis nymphs has previously been shown to modulate host T lymphocyte cytokine production. Tick-induced host immunomodulation is increasingly recognized as a contributing factor in successful transmission and/or establishment of tick-borne pathogens. This study was conducted to determine the effects of repeated infestations with pathogen-free I. scapularis nymphs on the production of the macrophage cytokines interleukin (IL)-1beta and tumor necrosis factor-alpha and the T lymphocyte cytokines IL-2, IL-4, IL-10, and interferon-gamma in both BALB/c and C3H/HeN mice. The pattern of T lymphocyte cytokine production was evaluated to determine if repeated tick infestation polarizes the immune response toward a Th-1 or Th-2 cytokine profile. Female BALB/c and C3H/HeN mice were infested one to four times with pathogen-free I. scapularis nymphs, with a 14-day tick-free period between each exposure. After each infestation, tick biology parameters were measured and macrophage and T lymphocyte cytokine production was assessed. Elaboration of T lymphocyte and macrophage cytokines was quantitated by antigen capture enzyme-linked immunosorbent assay. Acquired resistance to I. scapularis feeding was not developed by either mouse strain. Significant differences in cytokine production were observed between infested and noninfested mice, as well as between the two mouse strains, following tick infestation. Infestation of both strains with pathogen-free I. scapularis results in a polarization of the host immune response toward a Th-2, anti-inflammatory pattern, with a corresponding suppression of Th-1 responses.

Tick modulation of host immunity: an important factor in pathogen transmission

Immunological interactions at the tick host interface involve innate and specific acquired host immune defenses and immunomodulatory countermeasures by the tick. Tick feeding stimulates host immune response pathways involving antigen-presenting cells, cytokines, B-cells, T-cells, circulating and homocytotropic antibodies, granulocytes, and an array of biologically active molecules. In response to host immune defenses, tick-mediated host immunosuppressive countermeasures inhibit: host antibody responses; complement activation; T-cell proliferation; and cytokine elaboration by macrophages and Th1-lymphocytes. Immunosuppressive proteins identified in tick salivary glands and saliva have been partially characterised. Tick-induced host immunosuppression facilitates blood meal acquisition and is an important factor in the transmission/establishment of the tick-borne disease-causing agent, Borrelia burgdorferi. A novel strategy for control of tick-borne pathogens is proposed.

Characterization of an immunosuppressant protein from Dermacentor andersoni (Acari: Ixodidae) salivary glands

A 36-kDa soluble protein was found in the salivary glands of female Dermacentor andersoni (Stiles) ticks that suppressed the in vitro proliferative response of murine splenocytes to concanavalin A (Con A). Incubating the purified protein with splenocytes reduced the incorporation of thymidine into the DNA of proliferating T-lymphocytes by more than 90% compared with cells exposed to Con A and buffer alone. The N-terminal amino acid sequence of the immunosuppressant protein was determined to be NH2-Leu-His-Lys-Ala(Asp)-Lys-Ile-Val-Lys-Leu-Thr -Glu-Glu-Ala -Arg-Lys-Tyr-Val-Gly-Arg-Xxx-Xxx-Thr-Thr-Ala-Leu-Gly-. Although the sequence exhibited a modest degree of similarity with a segment of immunoglobulin-binding protein found in several species of mammals, the mode of action of the immunosuppressant protein is unknown. This protein may play an important role in suppressing the host's acquisition of resistance to ticks.

Tick-host immunology: Significant advances and challenging opportunities

Immunological interactions at the tick-host interface involve innate and acquired host defenses against infestation and immunomodulatory countermeasures by the tick. The cellular and molecular immunological bases of these host-parasite relationships are being defined. Acquired resistance to tick infestation involves humoral and cellular immunoregulatory and effector pathways. Ticks respond by suppressing antibody production, complement, and cytokine elaboration by both antigen-presenting cells and specific T-cell subsets. Tick-borne disease-causing agents probably exploit tick suppression of host defenses during transmission and initiation of infection. Because of the public health importance of ticks and Pick-borne diseases, it is crucial that we understand these interactions and exploit them in novel immunological control strategies. Here, Stephen Wikel and Douglas Bergman discuss recent advances in understanding tick-host immunology and propose future studies.

Immunology of interactions between ticks and hosts

Infestation with ixodid tick stimulates the immune regulatory and effector pathways of the hosts involving antigen presenting cells, T-lymphocytes, B-lymphocytes, basophils, mast cells, eosinophils and a variety of bioactive molecules like cytokines, antibodies and complement. Tick-mediated immunosuppression has been investigated using cells derived from infested animals and by exposing cells from uninfected animals to tick salivary gland molecules. Tick-induced suppression of host immune defences is characterized by reduced ability of lymphocytes from infested animals to proliferate in vitro in the presence of concanavalin A (Con A), diminished primary antibody responses to T-cell dependent antigen, and decreased elaboration of macrophage (IL-1 and TNF-alpha) and Th1-lymphocyte cytokines (IFN-gamma), whereas Th2 cytokines production (IL-4, IL-5 and IL-10) is enhanced. It is known that IL-10 inhibits Th1 cell development and also reduces the in vitro T-lymphocyte proliferative response to Con A stimulation. Proteins which inhibited T-lymphocyte in vitro responsiveness to Con A were also isolated from tick salivary glands.

Infestation with pathogen-free nymphs of the tick Ixodes scapularis induces host resistance to transmission of Borrelia burgdorferi by ticks

Female BALB/c mice were infested four times with pathogen-free Ixodes scapularis nymphs prior to infestation with nymphs infected with Borrelia burgdorferi B31. Each infestation was separated by a 14-day tick-free period. Mean weights of fed ticks and percentage reaching repletion did not indicate development of acquired resistance. Only 16.7% of mice repeatedly infested with pathogen-free ticks prior to infected I. scapularis nymph challenge became positive for B. burgdorferi. One hundred percent of control mice infested only with infected ticks were culture positive for B. burgdorferi.

Host immunity to ticks

The tick-host-pathogen interface is characterized by complex immunological interactions. Tick feeding induces host immune regulatory and effector pathways involving antibodies, complement, antigen-presenting cells, T lymphocytes, and other bioactive molecules. Acquired resistance impairs tick engorgement, ova production, and viability. Tick countermeasures to host defenses reduce T-lymphocyte proliferation, elaboration of the TH1 cytokines interleukin-2 and interferon-gamma, production of macrophage cytokines interleukin-1 and tumor necrosis factor, and antibody responses. The dynamic balance between acquired resistance and tick modulation of host immunity affects engorgement and pathogen transmission. A thorough understanding of acquired immunity to ticks is essential for rational development of antitick vaccines.

Dermacentor andersoni: salivary gland proteins suppressing T-lymphocyte responses to concanavalin A in vitro

Salivary glands obtained from feeding adult female Dermacentor andersoni (Acari:Ixodidae) were fractionated using differential centrifugation, detergents, centrifugal concentrators incorporating filter membranes with various molecular weight cutoffs, and preparative SDS-PAGE. A lymphocyte proliferation assay was used to evaluate the effects of salivary gland fractions on ConA-induced blastogenesis of normal murine splenocytes. Lipid, soluble, and detergent-soluble fractions were found to significantly suppress ConA-induced proliferation of splenocytes. Fractions containing soluble proteins suppressed splenocyte proliferation by ca. 26%. Suppressant activity in these fractions was due to components with molecular weights greater than 30 kDa. This suppression of splenocyte proliferation occurred with as little as 0.25 microgram protein per well. Salivary gland preparative SDS-PAGE fractions containing one or more soluble polypeptides or proteins with molecular weights in the range 36 to 43 kDa significantly suppressed murine splenocyte responses to ConA in vitro.

Effects of Dermacentor andersoni (Acari: Ixodidae) salivary gland extracts on Bos indicus and B. taurus lymphocytes and macrophages: in vitro cytokine elaboration and lymphocyte blastogenesis

Cattle and laboratory animal species-acquired resistance to tick infestation has an immunological basis involving antigen presenting cells, B-lymphocytes, T-lymphocytes, and cytokines. Tick infestation has been shown to impair guinea pig antibody responses to a thymic-dependent antigen and in vitro responsiveness of lymphocytes to T-cell mitogens. Tick salivary gland extracts inhibited in vitro proliferative responses of normal murine lymphocytes to the T-cell mitogen concanavalin A (Con A) and enhanced reactivity of normal B-lymphocytes to the mitogen E. coli lipopolysaccharide (LPS). Salivary gland extracts collected daily during engorgement were shown to inhibit normal murine macrophage elaboration of interleukin-1 (IL-1) and tumor necrosis factor alpha (TNF) as well as murine T-lymphocyte production of interleukin-2 (IL-2) and interferon-gamma (IFN-G). Peripheral blood mononuclear cells collected from purebred Bos indicus and B. taurus were significantly inhibited in their in vitro responses to Con A by salivary gland extracts prepared daily from female Dermacentor andersoni stiles during the course of engorgement. Percentage of suppression of Con A responsiveness was similar for both B. indicus and B. taurus cells. The overall responsiveness of B. indicus derived T cells is significantly greater than that of similar cells from B. taurus, when mean counts per minute of methyl-tritiated-thymidine incorporation were compared for both groups. Cells of B. indicus origin were 34.5% more reactive. In vitro responsiveness of the same cell populations to LPS were significantly enhanced by the presence of tick salivary gland extracts. B. indicus lymphocyte reactivity to LPS was significantly greater (42.9%) than that of similar B. taurus cells in the absence of salivary gland extracts. B. indicus and B. taurus macrophage elaboration of IL-1 were suppressed in a similar manner by tick salivary gland extracts prepared on days 5-9 of engorgement. B. indicus macrophages produced more IL-1 than similar cells of B. taurus origin either in the presence (45.6%) or absence (43.0%) of LPS. Macrophages derived from both genetic backgrounds were significantly suppressed in their LPS induced production of TNF in the presence of tick salivary gland extracts collected on days 0-9 of engorgement. B. indicus might be able to develop more vigorous immune responses to foreign immunogens presented to the animal during tick feeding.

Tick-induced modulation of the host immune response

The tick-host-pathogen interface is characterised by complex immunological interactions. Host immune responses to tick infestation and infection with tick-borne pathogens involve cytokines, antibodies, complement and T lymphocyte regulatory and effector pathways. A successful host-parasite relationship is a balance between limiting the parasite by host defenses and the ability of the parasite to modulate, evade or restrict the host response. Hosts acquire immunological based resistance to tick infestation, which reduces engorgement, production of ova and viability. Salivary glands of ixodid ticks produce a complex array of immunogens and pharmacologically active molecules. Tick salivary gland derived material can modulate host cytokine, antibody and cell mediated immune responses. Both immunoregulatory and immune effector pathways of the host are suppressed. Tick feeding impairs the ability to develop a primary immune response to a thymic dependent immunogen. Lymphocytes obtained from tick infested hosts are reduced in their ability to proliferate in vitro to T lymphocyte mitogens, while responses to B lymphocyte polyclonal activators are unaltered. Normal macrophages and lymphocytes were exposed to female tick salivary gland extracts prepared daily during the course of engorgement. All extracts reduced lymphocyte responses to T cell mitogens and enhanced in vitro proliferation in the presence of a B lymphocyte mitogen. Macrophage elaboration of tumour necrosis factor alpha and interleukin-1 are significantly reduced in a differential manner. Production of interleukin-2 and interferon-gamma by T lymphocytes is reduced. Tick modulation of the host immune response could enhance the ability of the arthropod to obtain a blood meal and facilitate pathogen transmission to an immunocompetent host.

Acquired resistance of guinea pigs to Dermacentor andersoni mediated by humoral factors

Humoral and cell-mediated immune resistance to tick infestation has been documented in many host-parasite relationships. This study examines passive transfer of resistance to Dermacentor andersoni expressed by recipients of serum pools derived from guinea pigs that had acquired resistance through experimental infestation. Recipients of sera from high titer adult-infested animals expressed resistance evidenced by reduced engorgement weights, histological changes at tick attachment sites, and tick antigen-reactive cytophilic antibody. Recipients of lower titer sera from nymph-infested guinea pigs did not show significant reductions in engorgement weights but did exhibit inflammatory reactions at tick attachment sites similar to those of high titer sera recipients. This study, together with previous findings, suggests that immature instars of D. andersoni may not be as effective as adult ticks in stimulating sufficient humoral responses to convey biological resistance to naive recipients, and it underscores the influence the route of serum administration can have on passive transfer studies.

Host immune response to northern fowl mite: immunoblot and lectin blot identification of mite antigens

White leghorn hens were experimentally infested with northern fowl mites (Ornithonyssus sylviarum) and antibody responses to mite immunogens were monitored over 12 weeks. Mite burdens increased during the early phase of infestation and declined over the latter weeks of the study. Antigen was prepared from homogenized whole mites, which were then sonicated and extracted with non-ionic detergent. Antigen extract was fractionated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and antibody-reactive polypeptides were identified by immunoblotting. At the start of infestation, hens had natural, pre-existing antibodies that reacted with several mite-extract components. Individual hens had different natural antibody reactivities; however, all birds had immunoglobulins reactive with extract polypeptides of 117,000, 77,000 and 36,000 molecular weight. A variety of mite extract components reacted with hen antibodies generated in response to experimental infestation. The number of antibody-reactive mite polypeptides increased through week 8 of infestation and then decreased by week 12. Fifteen polypeptides of northern fowl mite extract were reactive with antibodies developed by the majority of infested birds. These commonly reactive polypeptides had molecular weights ranging from 40,000 to 160,000. Glycoconjugates of fractionated mite extract were identified by blotting with lectins that have different carbohydrate binding specificities. Also identified were lectins that bound extract components with the same molecular weights as those moieties complexed by immunoglobulins of infested birds.

Immunological control of hematophagous arthropod vectors: utilization of novel antigens

Immunological control of ixodid ticks requires development of a vaccine that stimulates an effective anti-tick response. However, it is important to avoid induction of intense host skin reactivity to tick feeding. Salivary-gland-derived molecules are introduced into the host during ixodid engorgement; therefore, use of these moieties might not be an optimal immunization strategy. Antigens not normally involved in acquired resistance can be used to induce anti-tick immunity. These "novel" antigens, obtained from tick gut absorptive surface, are not introduced into the host during tick feeding, but are exposed to host-immune effector elements in the blood meal, resulting in ixodid rejection, prevention of ova production and death. Anti-tick immunity is induced with microgram quantities of this ixodid gut antigen preparation. Sera obtained from immunized animals were used to identify antibody-reactive components of the resistance-inducing extract. Tick gut absorptive surface antigen glycoconjugates were identified by lectin blotting, using a series of probes with different carbohydrate specificities. The lectins, peanut and wheat germ agglutinin, and immunized host antibodies bound extract components with similar molecular weights. Solubilization and fractionization of tick gut proteins with the non-ionic detergent Triton X-114 is described.

Host immune responsiveness to the chigger, Eutrombicula cinnabaris

Chigger infestation is often associated with severe cutaneous reactions. Mice were given four infestations with the pest chigger Eutrombicula cinnabaris, and each exposure was separated by a 14-day mite-free period. Mean duration of engorgement was nine to ten days for a first exposure and four to five days for a fourth exposure. An initial exposure did not elicit macroscopic changes at chigger attachment sites, while all third and fourth exposure animals had marked reactions consisting of erythema, epidermal thickening and serous exudation. Approximately 20% of second exposure animals had macroscopic changes at chigger feeding sites, but these reactions were much less intense than the responses of third and fourth infestation hosts. Third and fourth exposure animals had infiltrates of lymphocytes, eosinophils, basophils and neutrophils at attachment sites, with eosinophil influx the most intense. Cutaneous reactivity to chigger feeding was adoptively transferred with lymphocytes from fourth exposure animals. Passive transfer of serum from fourth infestation donors resulted in heightened reactivity to a challenge infestation. Skin testing, after a fourth infestation, with an extract of whole E. cinnabaris larvae provided evidence for Arthus and delayed type hypersensitivity responses to chigger antigens. Chigger-reactive homocytotropic antibody was not detected by skin testing and active cutaneous anaphylaxis.

Ixodid-host immune interaction. Identification and characterization of relevant antigens and tick-induced host immunosuppression

Ixodid ticks are the most important vectors of pathogens to domestic and wild animals. It is established that cattle and laboratory animal species acquire resistance to tick infestation; acquired resistance has an immunological basis consisting of cell-mediated, antibody-mediated and complement-dependent effector mechanisms. Even though acquired resistance to tick feeding is expressed, host immune competence is possibly impaired during the course of tick feeding. Ixodid-induced transient immunosuppression could possibly facilitate the transmission of vector-borne pathogens and/or enhance tick feeding capabilities in the presence of a host immune response to the hematophagous arthropod. Tick tissue extracts have been used to artificially induce resistance to ixodid feeding, and this has become an area of increasing interest as a possible strategy for tick control. It is essential to have defined antigenic molecules for analysis of host responses to infestation, characterization of immunopathologic processes and for vaccine development. This report focuses on attempts to identify, characterize and isolate tick immunogens. Protein immunoblotting, utilizing sera from animals of different genetic composition and infestation patterns, was used to detect a number of tick polypeptides which are reactive with sera of infested hosts. It is clear that infestation with one ixodid species stimulates antibodies reactive with molecules derived from the sensitizing species and/or tick species in the same genus or different genera. This approach is used to identify molecules that are good candidates for use in immunization studies and for analysis of mechanisms involved in acquisition and expression of resistance to tick feeding.

Dot-ELISA assessment of guinea pig antibody responses to repeated Dermacentor andersoni infestations

Acquired resistance to ixodid tick infestation is expressed by cattle and laboratory animals. Humoral factors appear to be involved in host acquired resistance to tick bite; however, specific immune responses have yet to be fully characterized. This study examined tick resistance expressed by Hartley guinea pigs upon repeated infestation with Dermacentor andersoni, and describes longitudinal development of antigen specific immunoglobulin over approximately 180 days. Guinea pigs were infested either 4 times with D. andersoni adults, or twice with nymphs. Both infestation groups, adults and nymphs, demonstrated a significant level of resistance to re-infestation, following initial exposure. Dot enzyme-linked immunosorbent assay (Dot-ELISA) was employed to detect antibody reactive with D. andersoni salivary gland antigens (SGA). Animals infested with adults had antibody that increased at a relatively constant rate until the fourth infestation, which was differentiated by a sharp increase in titer, that was maintained for approximately 2 wk. Guinea pigs that received nymph infestations had SGA-specific antibody; however, titers were lower than those in the adult infestation group. Antibody levels continued to increase approximately 80 days beyond the final (second) infestation for this group. A direct correlation between resistance and antibody titer was not evident, since resistance was relatively stable after the second infestation in both infestation groups, and tick-specific immunoglobulin levels continually increased.

Resistance to ixodid tick infestation induced by administration of tick-tissue culture cells

Primary tissue culture cells of developing larvae of Amblyomma americanum were administered to guinea-pigs never previously exposed to ixodid ticks. Guinea-pigs were given 1 X 10(6) primary culture cells on Days 0, 7 and 21 by subcutaneous injection and challenged with male and female A. americanum on Day 35. A significant degree of induced host tick resistance was expressed by reduced engorgement weight of females, reduced oviposition by those females which did obtain a blood meal, and by death of ticks at the attachment site. Resistance induced by A. americanum primary culture cells stimulated a significant degree of resistance to infestation with Dermacentor andersoni adults.

Effects of tick infestation on the plaque-forming cell response to a thymic dependent antigen

Strain-2 guinea-pigs were given two five-day infestations with Dermacentor andersoni larvae. Each exposure consisted of 100 larvae, and the first and second infestations were separated by a seven-day tick-free period. Tick-exposed animals were given an intravenous injection with sheep red blood cells (SRBC) at selected times during and after infestation: (a) the last, fifth day, of a first exposure, (b) the second day of a second infestation, (c) the fifth day of a second infestation and (d) four days after termination of the second infestation. Ability of these animals to respond immunologically to the SRBC injection was assessed by the direct haemolytic plaque-forming cell assay, a very sensitive test used to determine the number of spleen cells producing IgM to SRBC target cells. Strain-2 animals given SRBC at the end of an initial infestation, or during a second tick exposure, produced significantly fewer direct haemolytic plaque-forming cells than did uninfested controls given a similar SRBC immunization regimen. Spleen cells of animals administered SRBC on the fourth day after termination of a second infestation displayed a haemolytic plaque-forming cell response which did not differ significantly from that of uninfested controls.

Acquisition and expression of resistance by Bos indicus and Bos indicus X Bos taurus calves to Amblyomma americanum infestation

Purebred and crossbred Bos indicus calves were infested 1, 2, or 3 times with 10 female and 5 male Amblyomma americanum. Resistance was acquired by both the purebred and the crossbred calves after 1 infestation and resulted in statistically significant decreases in the percentages of females that engorged, the mean weights of engorged females, and the mean weights of egg masses. Comparisons between breeds of the percent of female ticks that engorged during the first and second infestations indicate that purebred B. indicus expressed a stronger acquired resistance to A. americanum more readily than did crossbred animals. However, calves of both genetic compositions displayed similar levels of resistance during a third exposure. All tick-exposed and control animals were skin tested with salivary gland extracts of A. americanum, A. cajennense and Dermacentor andersoni. Control, uninfested calves, did not display significant cutaneous reactivity to these extracts. All calves that had been infested had immediate, 30-min, 5-hr and delayed, 24-hr, skin reactions to Amblyomma species antigens. Reactions to D. andersoni salivary antigens in tests of both purebred and crossbred calves with acquired resistance to A. americanum suggest that Amblyomma species salivary gland antigens might have cross reactive moieties with a salivary extract prepared from D. andersoni. Peripheral blood lymphocyte in vitro responsiveness to Amblyomma species antigens was detected in purebred calves after a first, second, and third infestation, indicating the presence of cells of the immune system capable of recognizing and undergoing blast transformation in response to tick salivary components.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunomodulation of host responses to ectoparasite infestation--an overview

Immune reactivity stimulated by ectoparasites and other arthropods has received increased attention in recent years. It is clear that a broad range of host responses are elicited, and immune effector mechanisms appear to be active in limiting infestation with blood-feeding arthropods. Host immune responsiveness appears to be altered as a consequence of infestation by certain arthropods, and the impact of these alterations on vector-borne pathogen transmission remains to be determined. Successful techniques have been developed by a number of investigators for the artificial induction of resistance to blood-feeding arthropods. These areas are examined in this paper.

The induction of host resistance to tick infestation with a salivary gland antigen

Salivary gland antigen, SGA, derived from partially engorged female Dermacentor andersoni was shown to be capable of inducing resistance to tick infestation in guinea pigs never previously exposed to ticks. Immunization regimens involved the administration of SGA by different routes and with or without the use of adjuvants. Induced resistance was expressed by significantly fewer larvae engorging, and the weight of larvae which did engorge was reduced. Salivary gland antigen has been shown to interact with the immune effector elements of animals which acquired and expressed tick resistance due to infestation. These findings demonstrated the potential for an immunologic approach to tick control.

Resistance to tick-borne Francisella tularensis by tick-sensitized rabbits: allergic klendusity

Mammals become hypersensitive to ticks that feed upon them. That hypersensitivity was thought responsible for an observation that a large number of Francisella tularensis-infected Dermacentor variabilis failed to infect a rabbit previously exposed to ticks of that species. In a series of tests of that hypothesis, rabbits sensitized to ticks were often significantly more resistant than control animals to tick-borne tularemia. The conditions that determine the klendusity are thought to be variable and complex but the phenomenon must be of importance in the epidemiology of some arthropod-borne agents.