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Toll road for Toxoplasma gondii: the mystery continues
Update
TRENDS in Parasitology
Vol.23 No.1
Research Focus
Toll road for Toxoplasma gondii: the mystery continues Imtiaz A. Khan Department of Microbiology, Immunology and Tropical Medicine, George Washington University Medical Center, Ross Hall Suite 736, 2300 Eye Street NW, Washington, DC 20037, USA
Toll-like receptors (TLRs) are considered to be essential for the initiation of immune responses against pathogens. Although myeloid differentiation factor 88 an adaptor molecule for most TLRs, is important for protection against Toxoplasma gondii, the TLR responsible for eliciting an immune response against this obligate intracellular pathogen remains unknown. A recent article reports that mice lacking TLR9 cannot develop severe inflammatory responses to T. gondii infection. The implications of this finding are discussed here.
Innate immunity and Toxoplasma gondii Innate immunity has an important role in restricting the proliferation of invading pathogens until an adaptive immune response is generated. Toll-like receptors (TLRs) comprise a family of pattern recognition receptors that detects conserved molecular products of microorganisms; therefore, TLRs have a key role in the generation of innate immune responses [1]. However, identification of the TLR(s) crucial for elicitation of in vivo immunity against Toxoplasma gondii remains ambiguous. Minns et al. [2] report that TLR9, a ligand for bacterial CpG DNA [3], is important for the induction of the gut immune response against oral T. gondii infection. Mice that lack the gene encoding TLR9 develop a suboptimal dendritic cell (DC) response that compromises the generation of inflammatory CD4+ T cells in the lamina propria, thus preventing the severe gut pathology that is observed in normal wild-type mice. The study by Minns et al. demonstrates, for the first time, the importance of TLR9 in an in vivo T. gondii infection. Toxoplasma gondii, MyD88 and TLRs The importance of TLRs in the generation of an immune response against T. gondii was suggested for the first time by Sher and colleagues [4]. They showed that mice deficient in the gene encoding myeloid differentiation factor 88 (MyD88) cannot produce interleukin (IL)-12 in response to T. gondii infection and are, therefore, extremely susceptible to parasite infection. MyD88 is an adaptor molecule for most of the known TLRs [5]; however, until recently, none of the known TLRs could be conclusively linked to the generation of an immune response against T. gondii. Using an in vitro model, Yarovinsky et al. reported that IL-12 production by murine DCs depends on the interaction Corresponding author: Khan, I.A. ([email protected]) Available online 2 November 2006. www.sciencedirect.com
between T. gondii profilin and TLR11 on DCs [6]. However, enthusiasm for these studies was somewhat dampened because, unlike Myd88 / mice, mice lacking the gene encoding TLR11 exhibit nominal susceptibility to infection. These findings indicate that TLRs other than TLR11 participate in the innate immune mechanism against acute T. gondii infection. Studies with TLR2, which is reported to be important in many bacterial infections [7], have demonstrated that this receptor might also be involved in protective immunity against T. gondii [8,9]. In one study, mice lacking the gene encoding TLR2 could not survive challenge with high doses of T. gondii [8]. However, it was demonstrated in the same study that, at lower infective doses, Tlr2 / mice survive T. gondii infection. Also, it has been reported that, unlike Myd88 / mice, Tlr2 / mice produce the same level of IL-12 as do wild-type mice when infected with a smaller number of T. gondii cysts [10]. Overall, it seems that, although important, TLR2 is not exclusively involved in MyD88-mediated protection against T. gondii. Interestingly, TLR4, which is thought to be an important mediator of innate immunity against other intracellular pathogens [11], could not be assigned a major role in the induction of the innate immune response against T. gondii. Mice lacking TLR4 withstood infection in the same way as their wild-type littermates [8]. The importance of TLR9 Minns et al. [2] report that TLR9 is required for a gut immune response against T. gondii and that the overwhelming gut pathology observed in wild-type mice is absent from Tlr9 / mice. As a result, knockout mice underwent considerably less weight loss and significantly increased length of survival compared with parental wildtype mice. A hyperinflammatory response mediated by T. gondii in the gut of wild-type C57BL/6 mice has been reported by many investigators [12,13]. Although interferon-g (IFN-g)-producing CD4+ T cells of the lamina propria are major contributors to this overwhelming immune response, innate immune cells such as neutrophils or natural killer cells also have a role [14,15]. Minns et al. [2] attribute the subdued inflammatory condition in the gut of infected TLR9-deficient mice to the reduced DC population in the mesenteric lymph nodes (MLNs) of these animals. The authors demonstrate that the most noticeable difference among DCs was in the subpopulation that expresses the CD11clow and CD8int receptors. Based on the observation that this subpopulation of DCs
2
Update
TRENDS in Parasitology Vol.23 No.1
showed increased levels of expression of co-stimulatory molecules (CD80 and CD86), they interpret that these cells have an important role in the generation of IFN-gsecreting CD4+ T cells, which migrate to the lamina propria after being primed. This T-cell subset causes severe gut pathology that leads to wasting and, ultimately, host death. Tlr9 / mice have a defect in the generation or migration of the CD11clow CD8int DCs whereby these cells cannot prime naı¨ve CD4+ T cells. As a result, they are not involved in the gut pathology but do have a reduced ability to clear pathogens, which leads to a larger parasite burden in tissues. Although direct activation of TLR9 by T. gondii was not demonstrated, the study by Minns et al. is a ‘first’ because: (i) the role of this receptor in the immune response against T. gondii had not been reported previously; and (ii) experiments carried out with other TLRs have used mostly an intraperitoneal (i.p.) route of infection. Therefore, this is the first study to address the importance of TLRs in the induction of mucosal immunity against T. gondii. Although Minns et al. do not establish whether immunodeficiency in Tlr9 / mice leads to death, the fact that hyperinflammation in these animals is prevented is, in itself, an important advancement in the field. It will be interesting to determine whether knockout mice from genetic backgrounds that do not develop gut pathology (e.g. BALB/c or CBA) have increased susceptibility to T. gondii infection because it has been reported recently that Tlr9 / mice do not exhibit increased mortality following systemic T. gondii infection [16]. Role of other TLRs Although Tlr9 / mice have a reduced ability to clear parasites, unlike Myd88 / mice they do not exhibit a substantially increased susceptibility to T. gondii infection [4] or a complete reversal of cytokine profile from T helper (Th)1 cells to Th2 cells [17]. None of the Th2 cytokines (IL4, IL-10 and IL-13) was detected in the study carried out on Tlr9 / mice. Although the observations made with Tlr9 / mice shed light on the involvement of this TLR in natural T. gondii infection, the picture regarding the involvement of a particular receptor is unclear. Results obtained using Myd88 / mice have not been matched in mice deficient in any of the TLRs studied. As mentioned by Minns et al., synergism between two or more known TLRs in the induction of an innate immune response against T. gondii remains a possibility. However, the involvement of other unknown TLRs cannot be excluded. Also, it is noteworthy that Minns et al. [2] used an oral (natural) route of infection, whereas most of the studies with other TLRs were carried out with i.p. infection. Differences in the outcome of immunity to pathogens based on the route of infection have been reported [18]; therefore, it might be worthwhile to compare the immune response to T. gondii in various Tlr knockouts using the oral versus i.p. route of infection. TLR9 and the DC response during Toxoplasma gondii infection Another important finding by Minns et al. [2] is a difference in DC populations between wild-type and www.sciencedirect.com
Tlr9 / mice in the MLNs of infected animals. This difference was more pronounced in the CD11clow CD8int subpopulation of cells. The authors suggest that these cells could be important for priming inflammatory CD4+ T cells during T. gondii infection in the gut. Although Minns et al. do not provide direct evidence to prove their point, the fact that Tlr9 / mice also have reduced numbers of IFN-g-producing CD4+ T cells strengthens their hypothesis. These findings raise a question about the involvement of TLR9 in the DC response to T. gondii infection in nonmucosal immune compartments such as the spleen. It is possible that the TLRs involved in the generation of peripheral immunity are different from those that initiate the mucosal response against the parasite. The TLR (or TLRs) responsible for orchestrating immunity against T. gondii remains to be identified and further investigation is needed. Nonetheless, Minns et al. [2] have made an important contribution to the field of innate immunity to T. gondii, and observations made in this study can be extended to other pathogens or inflammatory diseases. References 1 Iwasaki, A. and Medzhitov, R. (2004) Toll-like receptor control of the adaptive immune responses. Nat. Immunol. 5, 987–995 2 Minns, L.A. et al. (2006) TLR9 is required for the gut-associated lymphoid tissue response following oral infection of Toxoplasma gondii. J. Immunol. 176, 7589–7597 3 Rutz, M. et al. (2004) Toll-like receptor 9 binds single-stranded CpG-DNA in a sequence- and pH-dependent manner. Eur. J. Immunol. 34, 2541–2550 4 Scanga, C.A. et al. (2002) Cutting edge: MyD88 is required for resistance to Toxoplasma gondii infection and regulates parasite-induced IL-12 production by dendritic cells. J. Immunol. 168, 5997–6001 5 Medzhitov, R. et al. (1998) MyD88 is an adaptor protein in the hToll/IL-1 receptor family signaling pathways. Mol. Cell 2, 253– 258 6 Yarovinsky, F. et al. (2005) TLR11 activation of dendritic cells by a protozoan profilin-like protein. Science 308, 1626–1629 7 Wetzler, L.M. (2003) The role of Toll-like receptor 2 in microbial disease and immunity. Vaccine 21 (Suppl. 2), S55–S60 8 Mun, H.S. et al. (2003) TLR2 as an essential molecule for protective immunity against Toxoplasma gondii infection. Int. Immunol. 15, 1081–1087 9 Del Rio, L. et al. (2004) Toxoplasma gondii triggers myeloid differentiation factor 88-dependent IL-12 and chemokine ligand 2 (monocyte chemoattractant protein 1) responses using distinct parasite molecules and host receptors. J. Immunol. 172, 6954– 6960 10 Yarovinsky, F. and Sher, A. (2006) Toll-like receptor recognition of Toxoplasma gondii. Int. J. Parasitol. 36, 255–259 11 Schnare, M. et al. (2006) Toll-like receptors: sentinels of host defence against bacterial infection. Int. Arch. Allergy Immunol. 139, 75–85 12 Liesenfeld, O. et al. (1996) Association of CD4+ T cell-dependent, interferon-g-mediated necrosis of the small intestine with genetic susceptibility of mice to peroral infection with Toxoplasma gondii. J. Exp. Med. 184, 597–607 13 Khan, I.A. et al. (1997) A dichotomous role for nitric oxide during acute Toxoplasma gondii infection in mice. Proc. Natl. Acad. Sci. U. S. A. 94, 13955–13960 14 Khan, I.A. et al. (2001) Mice lacking the chemokine receptor CCR1 show increased susceptibility to Toxoplasma gondii infection. J. Immunol. 166, 1930–1937 15 Khan, I.A. et al. (2006) CCR5 is essential for NK cell trafficking and host survival following Toxoplasma gondii infection. PLoS Pathog. 2, e49
Update
TRENDS in Parasitology
16 Hitziger, N. et al. (2005) Dissemination of Toxoplasma gondii to immunoprivileged organs and role of Toll/interleukin-1 receptor signalling for host resistance assessed by in vivo bioluminescence imaging. Cell. Microbiol. 7, 837–848 17 Jankovic, D. et al. (2002) In the absence of IL-12, CD4+ T cell responses to intracellular pathogens fail to default to a Th2 pattern and are host protective in an IL-10 / setting. Immunity 16, 429–439
Vol.23 No.1
3
18 Combe, C.L. et al. (2006) Lack of IL-15 results in the suboptimal priming of CD4+ T cell response against an intracellular parasite. Proc. Natl. Acad. Sci. U. S. A. 103, 6635– 6640
1471-4922/$ – see front matter ß 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.pt.2006.10.001
Anaplasma phagocytophilum subverts tick salivary gland proteins Janet Foley and Nathan Nieto School of Veterinary Medicine, Department of Medicine and Epidemiology, University of California, Davis, CA 95616, USA
Anaplasma phagocytophilum is a bacterium that is transmitted by Ixodes spp. ticks, in which it resides in salivary glands. Ticks inoculate the pathogen into hosts together with an array of salivary molecules that reduce host anti-tick inflammation. Sukumaran et al. recently showed that A. phagocytophilum uses a tick salivary protein, Salp16, to enhance its uptake from the host and into the salivary gland. Occupation and exploitation of tick salivary glands have implications for the maintenance and detection of A. phagocytophilum in its vector and early pathogen interactions with its hosts.
Emerging anaplasmosis and tick-transmitted pathogens Human granulocytic anaplasmosis is an emerging disease caused by Anaplasma phagocytophilum, with a course of infection that can range from asymptomatic to fatal [1]. A. phagocytophilum is widely distributed geographically [2] and infects a range of host species including rodents, which serve as reservoirs. The bacterium targets host granulocytes, where it survives in intracellular vacuoles, inhibiting phagosome–lysosome fusion and diminishing host-cell apoptosis. Ticks of the subgroup Ixodes ricinus are infected as larvae or nymphs from reservoirs such as rodents, maintain infection transstadially and then transmit infection as nymphs or adults, sometimes to incidental hosts such as humans, dogs and horses (Table 1). During feeding, Ixodes spp. ticks secrete bioactive salivary molecules into the lesion to promote host bleeding and reduce anti-tick inflammation [3]. Saliva can contain complement-, cytokine- and antibody-inhibitors, histamine-binding proteins, leukocyte modulators and antihemostatics [4]. In this context, the activity of an intradermally delivered pathogen is limited to one of three outcomes: (i) the pathogen can be ingested and remain in the gut; (ii) the pathogen can reside in the gut until feeding begins and then migrate to the salivary gland, as for Corresponding author: Foley, J. ([email protected]) Available online 7 November 2006. www.sciencedirect.com
Borrelia burgdorferi; or (iii) the pathogen can translocate rapidly to the salivary gland and reside there through the molt. This third outcome is proposed as the norm for A. phagocytophilum, which has important implications for pathogenesis in both tick and host. Residence of A. phagocytophilum in tick salivary glands Little is known about the behavior of this pathogen in the tick: for example, how long it remains in the host-cell vacuole after being ingested, when it enters the salivary gland or how it protects itself during tick ingestion and transmission to the host. A recent paper by Sukumaran et al. [5] evaluated A. phagocytophilum–tick interactions by focusing on upregulation of tick salivary proteins during infection. Other studies show low prevalence or low titers of A. phagocytophilum in the salivary glands of unfed ticks and that prefeeding of ticks induces apparent replication of the bacteria to greater, more detectable levels. Before feeding, the salivary glands from only a single unfed Ixodes scapularis nymph from a group of experimentally infected ticks were infected, whereas after prefeeding, at least 35% of glands from this group of ticks were PCR-positive for A. phagocytophilum and infective on mouse inoculation [6]. No test-positive gut tissue was detected, although not all gut samples were analyzed. In a Japanese study, 32% of salivary gland samples from questing Ixodes persulcatus and Ixodes ovatus were PCR-positive [7]. A. phagocytophilum was also present in Ixodes ricinus salivary glands, but only in small proportions from 0.5 to 2.0%, depending on technique, prefeeding and tick stage [8]. These data reveal considerable geographical (or at least betweenstudy) variability in rates of A. phagocytophilum in ticks; the effect of prefeeding indicates that there is a low load of A. phagocytophilum in the salivary tissue of questing ticks. A. phagocytophilum exploits endogenous tick salivary proteins Residence within the salivary gland provides the opportunity for the bacterium to exploit tick salivary proteins. Sukumaran et al. [5] showed that the tick protein
TRENDS in Parasitology
Vol.23 No.1
Research Focus
Toll road for Toxoplasma gondii: the mystery continues Imtiaz A. Khan Department of Microbiology, Immunology and Tropical Medicine, George Washington University Medical Center, Ross Hall Suite 736, 2300 Eye Street NW, Washington, DC 20037, USA
Toll-like receptors (TLRs) are considered to be essential for the initiation of immune responses against pathogens. Although myeloid differentiation factor 88 an adaptor molecule for most TLRs, is important for protection against Toxoplasma gondii, the TLR responsible for eliciting an immune response against this obligate intracellular pathogen remains unknown. A recent article reports that mice lacking TLR9 cannot develop severe inflammatory responses to T. gondii infection. The implications of this finding are discussed here.
Innate immunity and Toxoplasma gondii Innate immunity has an important role in restricting the proliferation of invading pathogens until an adaptive immune response is generated. Toll-like receptors (TLRs) comprise a family of pattern recognition receptors that detects conserved molecular products of microorganisms; therefore, TLRs have a key role in the generation of innate immune responses [1]. However, identification of the TLR(s) crucial for elicitation of in vivo immunity against Toxoplasma gondii remains ambiguous. Minns et al. [2] report that TLR9, a ligand for bacterial CpG DNA [3], is important for the induction of the gut immune response against oral T. gondii infection. Mice that lack the gene encoding TLR9 develop a suboptimal dendritic cell (DC) response that compromises the generation of inflammatory CD4+ T cells in the lamina propria, thus preventing the severe gut pathology that is observed in normal wild-type mice. The study by Minns et al. demonstrates, for the first time, the importance of TLR9 in an in vivo T. gondii infection. Toxoplasma gondii, MyD88 and TLRs The importance of TLRs in the generation of an immune response against T. gondii was suggested for the first time by Sher and colleagues [4]. They showed that mice deficient in the gene encoding myeloid differentiation factor 88 (MyD88) cannot produce interleukin (IL)-12 in response to T. gondii infection and are, therefore, extremely susceptible to parasite infection. MyD88 is an adaptor molecule for most of the known TLRs [5]; however, until recently, none of the known TLRs could be conclusively linked to the generation of an immune response against T. gondii. Using an in vitro model, Yarovinsky et al. reported that IL-12 production by murine DCs depends on the interaction Corresponding author: Khan, I.A. ([email protected]) Available online 2 November 2006. www.sciencedirect.com
between T. gondii profilin and TLR11 on DCs [6]. However, enthusiasm for these studies was somewhat dampened because, unlike Myd88 / mice, mice lacking the gene encoding TLR11 exhibit nominal susceptibility to infection. These findings indicate that TLRs other than TLR11 participate in the innate immune mechanism against acute T. gondii infection. Studies with TLR2, which is reported to be important in many bacterial infections [7], have demonstrated that this receptor might also be involved in protective immunity against T. gondii [8,9]. In one study, mice lacking the gene encoding TLR2 could not survive challenge with high doses of T. gondii [8]. However, it was demonstrated in the same study that, at lower infective doses, Tlr2 / mice survive T. gondii infection. Also, it has been reported that, unlike Myd88 / mice, Tlr2 / mice produce the same level of IL-12 as do wild-type mice when infected with a smaller number of T. gondii cysts [10]. Overall, it seems that, although important, TLR2 is not exclusively involved in MyD88-mediated protection against T. gondii. Interestingly, TLR4, which is thought to be an important mediator of innate immunity against other intracellular pathogens [11], could not be assigned a major role in the induction of the innate immune response against T. gondii. Mice lacking TLR4 withstood infection in the same way as their wild-type littermates [8]. The importance of TLR9 Minns et al. [2] report that TLR9 is required for a gut immune response against T. gondii and that the overwhelming gut pathology observed in wild-type mice is absent from Tlr9 / mice. As a result, knockout mice underwent considerably less weight loss and significantly increased length of survival compared with parental wildtype mice. A hyperinflammatory response mediated by T. gondii in the gut of wild-type C57BL/6 mice has been reported by many investigators [12,13]. Although interferon-g (IFN-g)-producing CD4+ T cells of the lamina propria are major contributors to this overwhelming immune response, innate immune cells such as neutrophils or natural killer cells also have a role [14,15]. Minns et al. [2] attribute the subdued inflammatory condition in the gut of infected TLR9-deficient mice to the reduced DC population in the mesenteric lymph nodes (MLNs) of these animals. The authors demonstrate that the most noticeable difference among DCs was in the subpopulation that expresses the CD11clow and CD8int receptors. Based on the observation that this subpopulation of DCs
2
Update
TRENDS in Parasitology Vol.23 No.1
showed increased levels of expression of co-stimulatory molecules (CD80 and CD86), they interpret that these cells have an important role in the generation of IFN-gsecreting CD4+ T cells, which migrate to the lamina propria after being primed. This T-cell subset causes severe gut pathology that leads to wasting and, ultimately, host death. Tlr9 / mice have a defect in the generation or migration of the CD11clow CD8int DCs whereby these cells cannot prime naı¨ve CD4+ T cells. As a result, they are not involved in the gut pathology but do have a reduced ability to clear pathogens, which leads to a larger parasite burden in tissues. Although direct activation of TLR9 by T. gondii was not demonstrated, the study by Minns et al. is a ‘first’ because: (i) the role of this receptor in the immune response against T. gondii had not been reported previously; and (ii) experiments carried out with other TLRs have used mostly an intraperitoneal (i.p.) route of infection. Therefore, this is the first study to address the importance of TLRs in the induction of mucosal immunity against T. gondii. Although Minns et al. do not establish whether immunodeficiency in Tlr9 / mice leads to death, the fact that hyperinflammation in these animals is prevented is, in itself, an important advancement in the field. It will be interesting to determine whether knockout mice from genetic backgrounds that do not develop gut pathology (e.g. BALB/c or CBA) have increased susceptibility to T. gondii infection because it has been reported recently that Tlr9 / mice do not exhibit increased mortality following systemic T. gondii infection [16]. Role of other TLRs Although Tlr9 / mice have a reduced ability to clear parasites, unlike Myd88 / mice they do not exhibit a substantially increased susceptibility to T. gondii infection [4] or a complete reversal of cytokine profile from T helper (Th)1 cells to Th2 cells [17]. None of the Th2 cytokines (IL4, IL-10 and IL-13) was detected in the study carried out on Tlr9 / mice. Although the observations made with Tlr9 / mice shed light on the involvement of this TLR in natural T. gondii infection, the picture regarding the involvement of a particular receptor is unclear. Results obtained using Myd88 / mice have not been matched in mice deficient in any of the TLRs studied. As mentioned by Minns et al., synergism between two or more known TLRs in the induction of an innate immune response against T. gondii remains a possibility. However, the involvement of other unknown TLRs cannot be excluded. Also, it is noteworthy that Minns et al. [2] used an oral (natural) route of infection, whereas most of the studies with other TLRs were carried out with i.p. infection. Differences in the outcome of immunity to pathogens based on the route of infection have been reported [18]; therefore, it might be worthwhile to compare the immune response to T. gondii in various Tlr knockouts using the oral versus i.p. route of infection. TLR9 and the DC response during Toxoplasma gondii infection Another important finding by Minns et al. [2] is a difference in DC populations between wild-type and www.sciencedirect.com
Tlr9 / mice in the MLNs of infected animals. This difference was more pronounced in the CD11clow CD8int subpopulation of cells. The authors suggest that these cells could be important for priming inflammatory CD4+ T cells during T. gondii infection in the gut. Although Minns et al. do not provide direct evidence to prove their point, the fact that Tlr9 / mice also have reduced numbers of IFN-g-producing CD4+ T cells strengthens their hypothesis. These findings raise a question about the involvement of TLR9 in the DC response to T. gondii infection in nonmucosal immune compartments such as the spleen. It is possible that the TLRs involved in the generation of peripheral immunity are different from those that initiate the mucosal response against the parasite. The TLR (or TLRs) responsible for orchestrating immunity against T. gondii remains to be identified and further investigation is needed. Nonetheless, Minns et al. [2] have made an important contribution to the field of innate immunity to T. gondii, and observations made in this study can be extended to other pathogens or inflammatory diseases. References 1 Iwasaki, A. and Medzhitov, R. (2004) Toll-like receptor control of the adaptive immune responses. Nat. Immunol. 5, 987–995 2 Minns, L.A. et al. (2006) TLR9 is required for the gut-associated lymphoid tissue response following oral infection of Toxoplasma gondii. J. Immunol. 176, 7589–7597 3 Rutz, M. et al. (2004) Toll-like receptor 9 binds single-stranded CpG-DNA in a sequence- and pH-dependent manner. Eur. J. Immunol. 34, 2541–2550 4 Scanga, C.A. et al. (2002) Cutting edge: MyD88 is required for resistance to Toxoplasma gondii infection and regulates parasite-induced IL-12 production by dendritic cells. J. Immunol. 168, 5997–6001 5 Medzhitov, R. et al. (1998) MyD88 is an adaptor protein in the hToll/IL-1 receptor family signaling pathways. Mol. Cell 2, 253– 258 6 Yarovinsky, F. et al. (2005) TLR11 activation of dendritic cells by a protozoan profilin-like protein. Science 308, 1626–1629 7 Wetzler, L.M. (2003) The role of Toll-like receptor 2 in microbial disease and immunity. Vaccine 21 (Suppl. 2), S55–S60 8 Mun, H.S. et al. (2003) TLR2 as an essential molecule for protective immunity against Toxoplasma gondii infection. Int. Immunol. 15, 1081–1087 9 Del Rio, L. et al. (2004) Toxoplasma gondii triggers myeloid differentiation factor 88-dependent IL-12 and chemokine ligand 2 (monocyte chemoattractant protein 1) responses using distinct parasite molecules and host receptors. J. Immunol. 172, 6954– 6960 10 Yarovinsky, F. and Sher, A. (2006) Toll-like receptor recognition of Toxoplasma gondii. Int. J. Parasitol. 36, 255–259 11 Schnare, M. et al. (2006) Toll-like receptors: sentinels of host defence against bacterial infection. Int. Arch. Allergy Immunol. 139, 75–85 12 Liesenfeld, O. et al. (1996) Association of CD4+ T cell-dependent, interferon-g-mediated necrosis of the small intestine with genetic susceptibility of mice to peroral infection with Toxoplasma gondii. J. Exp. Med. 184, 597–607 13 Khan, I.A. et al. (1997) A dichotomous role for nitric oxide during acute Toxoplasma gondii infection in mice. Proc. Natl. Acad. Sci. U. S. A. 94, 13955–13960 14 Khan, I.A. et al. (2001) Mice lacking the chemokine receptor CCR1 show increased susceptibility to Toxoplasma gondii infection. J. Immunol. 166, 1930–1937 15 Khan, I.A. et al. (2006) CCR5 is essential for NK cell trafficking and host survival following Toxoplasma gondii infection. PLoS Pathog. 2, e49
Update
TRENDS in Parasitology
16 Hitziger, N. et al. (2005) Dissemination of Toxoplasma gondii to immunoprivileged organs and role of Toll/interleukin-1 receptor signalling for host resistance assessed by in vivo bioluminescence imaging. Cell. Microbiol. 7, 837–848 17 Jankovic, D. et al. (2002) In the absence of IL-12, CD4+ T cell responses to intracellular pathogens fail to default to a Th2 pattern and are host protective in an IL-10 / setting. Immunity 16, 429–439
Vol.23 No.1
3
18 Combe, C.L. et al. (2006) Lack of IL-15 results in the suboptimal priming of CD4+ T cell response against an intracellular parasite. Proc. Natl. Acad. Sci. U. S. A. 103, 6635– 6640
1471-4922/$ – see front matter ß 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.pt.2006.10.001
Anaplasma phagocytophilum subverts tick salivary gland proteins Janet Foley and Nathan Nieto School of Veterinary Medicine, Department of Medicine and Epidemiology, University of California, Davis, CA 95616, USA
Anaplasma phagocytophilum is a bacterium that is transmitted by Ixodes spp. ticks, in which it resides in salivary glands. Ticks inoculate the pathogen into hosts together with an array of salivary molecules that reduce host anti-tick inflammation. Sukumaran et al. recently showed that A. phagocytophilum uses a tick salivary protein, Salp16, to enhance its uptake from the host and into the salivary gland. Occupation and exploitation of tick salivary glands have implications for the maintenance and detection of A. phagocytophilum in its vector and early pathogen interactions with its hosts.
Emerging anaplasmosis and tick-transmitted pathogens Human granulocytic anaplasmosis is an emerging disease caused by Anaplasma phagocytophilum, with a course of infection that can range from asymptomatic to fatal [1]. A. phagocytophilum is widely distributed geographically [2] and infects a range of host species including rodents, which serve as reservoirs. The bacterium targets host granulocytes, where it survives in intracellular vacuoles, inhibiting phagosome–lysosome fusion and diminishing host-cell apoptosis. Ticks of the subgroup Ixodes ricinus are infected as larvae or nymphs from reservoirs such as rodents, maintain infection transstadially and then transmit infection as nymphs or adults, sometimes to incidental hosts such as humans, dogs and horses (Table 1). During feeding, Ixodes spp. ticks secrete bioactive salivary molecules into the lesion to promote host bleeding and reduce anti-tick inflammation [3]. Saliva can contain complement-, cytokine- and antibody-inhibitors, histamine-binding proteins, leukocyte modulators and antihemostatics [4]. In this context, the activity of an intradermally delivered pathogen is limited to one of three outcomes: (i) the pathogen can be ingested and remain in the gut; (ii) the pathogen can reside in the gut until feeding begins and then migrate to the salivary gland, as for Corresponding author: Foley, J. ([email protected]) Available online 7 November 2006. www.sciencedirect.com
Borrelia burgdorferi; or (iii) the pathogen can translocate rapidly to the salivary gland and reside there through the molt. This third outcome is proposed as the norm for A. phagocytophilum, which has important implications for pathogenesis in both tick and host. Residence of A. phagocytophilum in tick salivary glands Little is known about the behavior of this pathogen in the tick: for example, how long it remains in the host-cell vacuole after being ingested, when it enters the salivary gland or how it protects itself during tick ingestion and transmission to the host. A recent paper by Sukumaran et al. [5] evaluated A. phagocytophilum–tick interactions by focusing on upregulation of tick salivary proteins during infection. Other studies show low prevalence or low titers of A. phagocytophilum in the salivary glands of unfed ticks and that prefeeding of ticks induces apparent replication of the bacteria to greater, more detectable levels. Before feeding, the salivary glands from only a single unfed Ixodes scapularis nymph from a group of experimentally infected ticks were infected, whereas after prefeeding, at least 35% of glands from this group of ticks were PCR-positive for A. phagocytophilum and infective on mouse inoculation [6]. No test-positive gut tissue was detected, although not all gut samples were analyzed. In a Japanese study, 32% of salivary gland samples from questing Ixodes persulcatus and Ixodes ovatus were PCR-positive [7]. A. phagocytophilum was also present in Ixodes ricinus salivary glands, but only in small proportions from 0.5 to 2.0%, depending on technique, prefeeding and tick stage [8]. These data reveal considerable geographical (or at least betweenstudy) variability in rates of A. phagocytophilum in ticks; the effect of prefeeding indicates that there is a low load of A. phagocytophilum in the salivary tissue of questing ticks. A. phagocytophilum exploits endogenous tick salivary proteins Residence within the salivary gland provides the opportunity for the bacterium to exploit tick salivary proteins. Sukumaran et al. [5] showed that the tick protein