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Finding Leishmania: A Deadly Game of Hide-and-Seek
Cell Host & Microbe
Previews Finding Leishmania: A Deadly Game of Hide-and-Seek Phillip Scott1,* 1Department of Pathobiology, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, PA 19104, USA *Correspondence: [email protected] DOI 10.1016/j.chom.2009.07.002
Leishmaniasis is a chronic infection in which intracellular parasites avoid destruction by the immune system. Using intravital imaging, Filipe-Santos et al. (2009) demonstrate that some parasitized dendritic cells receive much less attention than others during their choreographed dance with T cells, suggesting that these ‘‘wallflowers’’ could allow for parasite survival. The control of intracellular pathogens by cell-mediated immunity is primarily mediated by interferon (IFN)-g-producing T cells, which in turn activate macrophages to enhance microbicidal activity. More than 25 years of in vitro studies have shown how effective this pathway can be at controlling many intracellular pathogens. However, whether this in vitro model recapitulates what is occurring in vivo is unknown. With the development of multiphoton confocal microscopy, it is now possible to image live tissues, allowing us to see how cells of the immune system behave in an in vivo setting. Such imaging has led to a better understanding of how T cells interact with antigen-presenting cells in a variety of tissues (Germain et al., 2008). One of the most interesting aspects of these studies was the finding that naive T cells undergo a choreographed dance when interacting with dendritic cells presenting their cognate antigen. Intravital imaging is being used to define how effector T cells respond during infection with intracellular pathogens, such as Toxoplasma, Listeria, Mycobacterium, and lymphocytic choriomeningitis virus (Aoshi et al., 2008; Egen et al., 2008; Kim et al., 2009; Schaeffer et al., 2009; Wilson et al., 2009). Now, using intravital imaging, Bousso and colleagues (Filipe-Santos et al., 2009) have examined the interactions between T cells, Leishmania major parasites and host cells, and identified roadblocks that may limit the efficacy of the T cell response. Infection of mice with L. major leads to the development of cutaneous lesions that resolve in C57BL/6 mice, but in BALB/c mice results in an uncontrolled and eventually fatal infection. The difference in outcome is directly tied to the development of a strong CD4+ Th1
response. However, even once a Th1 response is established it can take many weeks before C57BL/6 mice resolve their infections—and they never eliminate all of the parasites. Recently, intravital imaging has provided some unexpected views of how a Leishmania infection is initiated. For example, the entry of L. major into dendritic cells is not a passive event on the part of the dendritic cell, as these cells can be seen actively extending dendrites to capture the parasites (Ng et al., 2008). Intravital imaging also helped demonstrate that the large number of neutrophils rapidly invading the infection site following sand fly transmission is crucial for establishing the infection (Peters et al., 2008). Now Bousso and colleagues have used intravital imaging to better understand how effector T cells recognize infected cells within leishmanial lesions. Using a lysozyme reporter mouse (LysEGFP) and DsRed-labeled L. major parasites, Bousso and colleagues (Filipe-Santos et al., 2009) showed that CD11b+ CD11c+Lys-EGFP+MHC+ cells were the primary host cell infected with L. major in cutaneous lesions. While this confirms other recent studies (De Trez et al., 2009), the identification of MHC class II expression on the infected cells reveals the potential for recognition by antigenspecific T cells. The authors then took advantage of a T cell receptor transgenic mouse (WT15) in which T cells recognize the leishmanial antigen, LACK. In this study, LACK-specific T cells were activated in the presence of interleukin-12 to promote a Th1 phenotype, dye-labeled, and transferred into L. major-infected mice and visualized. Surprisingly, T cells appeared to be differentially attracted to their potential partners, with T cells establishing prolonged contacts with some infected cells, while failing to maintain
contact with others. Although this behavior could be due to heterogeneity in the activation status of the T cells, the authors favor the idea that variable levels of peptide-MHC complexes on infected cells contribute to the preferred partner selection by T cells. All infected cells were found to be CD11b+CD11c+ dendritic cells, ruling out the possibility that T cells prefer a unique subset of L. major-infected cells. To confirm that partner selection between WT15 T cells and parasite-infected cells relied on T cell specificity, polyclonal T cells were also transferred into infected mice. In contrast to WT15 cells, few of these T cells interacted with infected cells. Moreover, while polyclonal T cells were evenly distributed throughout the tissue, WT15 cells were preferentially enriched in areas of infected cells (Figure 1). However, there were also areas of the lesion that contained parasites, but no T cells (Figure 1). This latter observation suggests that in addition to limited dance time between the infected cells and T cells, some infected cells may sit out the dance entirely, thus potentially providing safe havens for the parasite within the lesion. Taken together, these findings suggest that the orchestrated dance between T cells and dendritic cells is not uniform and that T cells preferentially select some infected dendritic cells for prolonged interaction, while virtually ignoring others. What is less clear is whether such prolonged interactions between effector T cells and antigen-presenting cells are required for the production of IFN-g, and subsequently for the induction of nitric oxide production required for parasite destruction. A recent study found that only about 25% of infected cells within leishmanial lesions express inducible nitric oxide synthase (iNOS) (De Trez et al., 2009),
Cell Host & Microbe 6, July 23, 2009 ª2009 Elsevier Inc. 3
Cell Host & Microbe
Previews De Trez, C., Magez, S., Akira, S., Ryffel, B., Carlier, Y., and Muraille, E. (2009). PLoS Pathog. 5, e1000494. Egen, J.G., Rothfuchs, A.G., Feng, C.G., Winter, N., Sher, A., and Germain, R.N. (2008). Immunity 28, 271–284. Filipe-Santos, O., Pescher, P., Breart, B., Lippuner, C., Aebischer, T., Glaichenhaus, N., Spa¨th, G., and Bousso, P. (2009). Cell Host Microbe 6, this issue, 23–33. Germain, R.N., Bajenoff, M., Castellino, F., Chieppa, M., Egen, J.G., Huang, A.Y., Ishii, M., Koo, L.Y., and Qi, H. (2008). Immunol. Rev. 221, 163–181. Kim, J.V., Kang, S.S., Dustin, M.L., and McGavern, D.B. (2009). Nature 457, 191–195.
Figure 1. T Cell Interactions with Infected Cells in Leishmanial Lesions Antigen-specific (red) and nonspecific T cells (gray) enter leishmanial lesions from the blood with similar frequency [1], but once in the lesions, behave differently. T cells recognizing Leishmania are found in higher frequency in areas where infected CD11b+, CD11c+ dendritic cells (DCs) are located when compared to nonspecific T cells [2]. Some antigen-specific T cells are in contact with infected DCs, while others only transiently scan the infected cells. Other regions with infected DCs exclude T cells altogether [3].
and it is tempting to speculate that these are the cells that received more attention from the effector Th1 cells. If that is the case, these findings may in part explain why it takes so long after an immune response is initiated to resolve cutaneous lesions. Future studies to define the type of interaction required for a T cell to induce iNOS production by infected dendritic cells may help us to better
understand why L. major is so often associated with chronic infections.
REFERENCES Aoshi, T., Zinselmeyer, B.H., Konjufca, V., Lynch, J.N., Zhang, X., Koide, Y., and Miller, M.J. (2008). Immunity 29, 476–486.
4 Cell Host & Microbe 6, July 23, 2009 ª2009 Elsevier Inc.
Ng, L.G., Hsu, A., Mandell, M.A., Roediger, B., Hoeller, C., Mrass, P., Iparraguirre, A., Cavanagh, L.L., Triccas, J.A., Beverley, S.M., et al. (2008). PLoS Pathog. 4, e1000222. Peters, N.C., Egen, J.G., Secundino, N., Debrabant, A., Kimblin, N., Kamhawi, S., Lawyer, P., Fay, M.P., Germain, R.N., and Sacks, D. (2008). Science 321, 970–974. Schaeffer, M., Han, S.J., Chtanova, T., van Dooren, G.G., Herzmark, P., Chen, Y., Roysam, B., Striepen, B., and Robey, E.A. (2009). J. Immunol. 182, 6379–6393. Wilson, E.H., Harris, T.H., Mrass, P., John, B., Tait, E.D., Wu, G.F., Pepper, M., Wherry, E.J., Dzierzinski, F., Roos, D., et al. (2009). Immunity 30, 300–311.
Previews Finding Leishmania: A Deadly Game of Hide-and-Seek Phillip Scott1,* 1Department of Pathobiology, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, PA 19104, USA *Correspondence: [email protected] DOI 10.1016/j.chom.2009.07.002
Leishmaniasis is a chronic infection in which intracellular parasites avoid destruction by the immune system. Using intravital imaging, Filipe-Santos et al. (2009) demonstrate that some parasitized dendritic cells receive much less attention than others during their choreographed dance with T cells, suggesting that these ‘‘wallflowers’’ could allow for parasite survival. The control of intracellular pathogens by cell-mediated immunity is primarily mediated by interferon (IFN)-g-producing T cells, which in turn activate macrophages to enhance microbicidal activity. More than 25 years of in vitro studies have shown how effective this pathway can be at controlling many intracellular pathogens. However, whether this in vitro model recapitulates what is occurring in vivo is unknown. With the development of multiphoton confocal microscopy, it is now possible to image live tissues, allowing us to see how cells of the immune system behave in an in vivo setting. Such imaging has led to a better understanding of how T cells interact with antigen-presenting cells in a variety of tissues (Germain et al., 2008). One of the most interesting aspects of these studies was the finding that naive T cells undergo a choreographed dance when interacting with dendritic cells presenting their cognate antigen. Intravital imaging is being used to define how effector T cells respond during infection with intracellular pathogens, such as Toxoplasma, Listeria, Mycobacterium, and lymphocytic choriomeningitis virus (Aoshi et al., 2008; Egen et al., 2008; Kim et al., 2009; Schaeffer et al., 2009; Wilson et al., 2009). Now, using intravital imaging, Bousso and colleagues (Filipe-Santos et al., 2009) have examined the interactions between T cells, Leishmania major parasites and host cells, and identified roadblocks that may limit the efficacy of the T cell response. Infection of mice with L. major leads to the development of cutaneous lesions that resolve in C57BL/6 mice, but in BALB/c mice results in an uncontrolled and eventually fatal infection. The difference in outcome is directly tied to the development of a strong CD4+ Th1
response. However, even once a Th1 response is established it can take many weeks before C57BL/6 mice resolve their infections—and they never eliminate all of the parasites. Recently, intravital imaging has provided some unexpected views of how a Leishmania infection is initiated. For example, the entry of L. major into dendritic cells is not a passive event on the part of the dendritic cell, as these cells can be seen actively extending dendrites to capture the parasites (Ng et al., 2008). Intravital imaging also helped demonstrate that the large number of neutrophils rapidly invading the infection site following sand fly transmission is crucial for establishing the infection (Peters et al., 2008). Now Bousso and colleagues have used intravital imaging to better understand how effector T cells recognize infected cells within leishmanial lesions. Using a lysozyme reporter mouse (LysEGFP) and DsRed-labeled L. major parasites, Bousso and colleagues (Filipe-Santos et al., 2009) showed that CD11b+ CD11c+Lys-EGFP+MHC+ cells were the primary host cell infected with L. major in cutaneous lesions. While this confirms other recent studies (De Trez et al., 2009), the identification of MHC class II expression on the infected cells reveals the potential for recognition by antigenspecific T cells. The authors then took advantage of a T cell receptor transgenic mouse (WT15) in which T cells recognize the leishmanial antigen, LACK. In this study, LACK-specific T cells were activated in the presence of interleukin-12 to promote a Th1 phenotype, dye-labeled, and transferred into L. major-infected mice and visualized. Surprisingly, T cells appeared to be differentially attracted to their potential partners, with T cells establishing prolonged contacts with some infected cells, while failing to maintain
contact with others. Although this behavior could be due to heterogeneity in the activation status of the T cells, the authors favor the idea that variable levels of peptide-MHC complexes on infected cells contribute to the preferred partner selection by T cells. All infected cells were found to be CD11b+CD11c+ dendritic cells, ruling out the possibility that T cells prefer a unique subset of L. major-infected cells. To confirm that partner selection between WT15 T cells and parasite-infected cells relied on T cell specificity, polyclonal T cells were also transferred into infected mice. In contrast to WT15 cells, few of these T cells interacted with infected cells. Moreover, while polyclonal T cells were evenly distributed throughout the tissue, WT15 cells were preferentially enriched in areas of infected cells (Figure 1). However, there were also areas of the lesion that contained parasites, but no T cells (Figure 1). This latter observation suggests that in addition to limited dance time between the infected cells and T cells, some infected cells may sit out the dance entirely, thus potentially providing safe havens for the parasite within the lesion. Taken together, these findings suggest that the orchestrated dance between T cells and dendritic cells is not uniform and that T cells preferentially select some infected dendritic cells for prolonged interaction, while virtually ignoring others. What is less clear is whether such prolonged interactions between effector T cells and antigen-presenting cells are required for the production of IFN-g, and subsequently for the induction of nitric oxide production required for parasite destruction. A recent study found that only about 25% of infected cells within leishmanial lesions express inducible nitric oxide synthase (iNOS) (De Trez et al., 2009),
Cell Host & Microbe 6, July 23, 2009 ª2009 Elsevier Inc. 3
Cell Host & Microbe
Previews De Trez, C., Magez, S., Akira, S., Ryffel, B., Carlier, Y., and Muraille, E. (2009). PLoS Pathog. 5, e1000494. Egen, J.G., Rothfuchs, A.G., Feng, C.G., Winter, N., Sher, A., and Germain, R.N. (2008). Immunity 28, 271–284. Filipe-Santos, O., Pescher, P., Breart, B., Lippuner, C., Aebischer, T., Glaichenhaus, N., Spa¨th, G., and Bousso, P. (2009). Cell Host Microbe 6, this issue, 23–33. Germain, R.N., Bajenoff, M., Castellino, F., Chieppa, M., Egen, J.G., Huang, A.Y., Ishii, M., Koo, L.Y., and Qi, H. (2008). Immunol. Rev. 221, 163–181. Kim, J.V., Kang, S.S., Dustin, M.L., and McGavern, D.B. (2009). Nature 457, 191–195.
Figure 1. T Cell Interactions with Infected Cells in Leishmanial Lesions Antigen-specific (red) and nonspecific T cells (gray) enter leishmanial lesions from the blood with similar frequency [1], but once in the lesions, behave differently. T cells recognizing Leishmania are found in higher frequency in areas where infected CD11b+, CD11c+ dendritic cells (DCs) are located when compared to nonspecific T cells [2]. Some antigen-specific T cells are in contact with infected DCs, while others only transiently scan the infected cells. Other regions with infected DCs exclude T cells altogether [3].
and it is tempting to speculate that these are the cells that received more attention from the effector Th1 cells. If that is the case, these findings may in part explain why it takes so long after an immune response is initiated to resolve cutaneous lesions. Future studies to define the type of interaction required for a T cell to induce iNOS production by infected dendritic cells may help us to better
understand why L. major is so often associated with chronic infections.
REFERENCES Aoshi, T., Zinselmeyer, B.H., Konjufca, V., Lynch, J.N., Zhang, X., Koide, Y., and Miller, M.J. (2008). Immunity 29, 476–486.
4 Cell Host & Microbe 6, July 23, 2009 ª2009 Elsevier Inc.
Ng, L.G., Hsu, A., Mandell, M.A., Roediger, B., Hoeller, C., Mrass, P., Iparraguirre, A., Cavanagh, L.L., Triccas, J.A., Beverley, S.M., et al. (2008). PLoS Pathog. 4, e1000222. Peters, N.C., Egen, J.G., Secundino, N., Debrabant, A., Kimblin, N., Kamhawi, S., Lawyer, P., Fay, M.P., Germain, R.N., and Sacks, D. (2008). Science 321, 970–974. Schaeffer, M., Han, S.J., Chtanova, T., van Dooren, G.G., Herzmark, P., Chen, Y., Roysam, B., Striepen, B., and Robey, E.A. (2009). J. Immunol. 182, 6379–6393. Wilson, E.H., Harris, T.H., Mrass, P., John, B., Tait, E.D., Wu, G.F., Pepper, M., Wherry, E.J., Dzierzinski, F., Roos, D., et al. (2009). Immunity 30, 300–311.