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Bacteria spurned by self-absorbed cells
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© 2005 Nature Publishing Group http://www.nature.com/naturemedicine 18 VOLUME 11 | NUMBER 1 | JANUARY 2005 NATURE MEDICINE NEWS AND VIEWS In response to conditions ranging from starvation to hormone treatment, cells undergo a carefully orchestrated process of self-consumption. During autophagy 1 cytoplasmic organelles and portions of cytoplasm are sequestered in double- membrane bound vacuoles called nascent To dissect the roles of skin epidermis versus T cells in the propagation of the diseased phenotype, the researchers also performed studies involving skin grafting and passive transfer of immune cells. Tape-stripped skin from the transgenic mice that had been allografted to athymic nude mice was insufficient in reconstituting the pso- riatic phenotype; transfer of activated T cells to the dermis, the layer below the epidermis, also was required. The authors next turned to the well- characterized severe combined immunodefi- ciency (SCID)-human skin xenograft model, in which xenotransplanted skin of psoriasis is induced to develop the disease following injection of pathogenic immunocytes. The authors showed that activated Stat3 (PY-Stat3) localized to the nucleus of epidermal keratinocytes following injection of CD4, but not CD8-activated T cells. Lastly, the authors applied a Stat3-specific decoy oligonucleotide—which competes for binding to Stat3 and interferes with its tran- scriptional activity—to diseased skin from the K5.Stat3C transgenic mice. The oligonucleotide markedly improved or resolved the abnormal phenotype (Fig. 1). This result has significant clinical relevance, because decoy oligonucleotides for Stat3 and other molecules such as E2F and NF-κB are showing promise in clinical trials for selected inflammatory diseases, head and neck cancer, and other diseases. This study and others that have attempted to replicate human psoriasis have revealed the com- plexity of cellular communication within skin and this sentinel organ’s ability to orchestrate immune responses and other homeostatic events that normally protect, but occasionally perturb, cutaneous function. The skin arguably can manifest more patterns of inflammation and immunity than any other organ system, and rightfully so. It must protect itself and the individual from a virtually unlimited onslaught of microbial, physical and chemical challenges and environmental insults. Stat3 expression within the epidermis may well serve a crucial role in effectively engaging and coordinating inflammatory and immune responses to various offending agents. Which sets of molecules—cytokines, chemokines and other inflammatory mediators, growth factors, integ- rins and other adhesion proteins—are expressed in response to Stat3 activation in the epidermis remain to be elucidated. Whether there are genetic determinants crucial in this mouse model as there are in humans for the development of psoriasis also are as yet unknown 10 . In the new study, CD4 + T cells from K5.Stat3C mice specifically induce the psoria- sis-like skin lesions when injected into uninvolved K5.Stat3C transgenic skin allografted to nude mice. The ability to definitively characterize the antigenic specificity, if it exists, and genotype the T-cell receptor may aid in further homing in on the identity of the pathogenic T cell in human psoriasis. Another unmet challenge in fully understand- ing psoriasis has been linking the skin disease with the joint inflammation and destruction observed in psoriatic arthritis. No animal model of psoria- sis exhibits both robust skin and joint pathology, except one: mice overexpressing amphiregulin, an epidermal growth factor (EGF)-related growth factor, in the epidermis 11,12 . Interestingly, EGF receptor tyrosine kinase has been reported to induce phosphorylation of Stat3 (ref. 13). The extent and duration of the Stat3 phosphoryla- tion response to amphiregulin versus other EGF- related growth factors, possibly in conjunction with other Stat3-activating cytokines will be relevant, because none of the other EGF-related growth factors have been reported to induce this dramatic psoriatic phenotype in mice. Moreover, it will be interesting to determine whether the Stat3C mouse eventually develops synovial and joint pathology similar to the amphiregulin transgenic mouse and psoriasis. It is now clear that the epidermal keratinocyte and T cell pair up to gener- ate psoriasis. How they interact may be determined by Stat3 or other partners in the Stat3 signaling network. The activity of Stat3 and other modulating factors in the pathway may be the rheostat to control the development and expression of disease. Targeting Stat3 and other critical links in the disease network may improve the outcome of patients afflicted by psoriasis. 1. Nickoloff, B.J. & Nestle, F.O. J. Clin. Invest. 113, 1664– 1675 (2004). 2. Wrone-Smith, T. & Nickoloff, B.J. J. Clin. Invest. 98, 1878–1887 (1996). 3. Boyman, O. et al. J. Exp. Med. 199, 731–736 (2004). 4. Gottlieb, A. & Bos, J. Clin. Immunol. 105, 105–116 (2002). 5. Sano, S. et al. Nat. Med. 10, 43–49 (2004). 6. Levy, D.E. & Darnell, J.E. Jr. Nat. Rev. Mol. Cell. Bio. 3, 651–662 (2002). 7. Chan, K.S. et al. Cancer Res. 64, 2382–2389 (2004). 8. Chan, K.S. et al. J. Clin. Invest. 114, 720–728 (2004). 9. Bromberg, J.F. et al. Cell 98, 295–303 (1999). 10. Bowcock, A.M. J. Invest. Dermatol. 122, xv–xvii (2004). 11. Cook, P. W. et al. J. Clin. Invest. 100, 2286–2294 (1997). 12. Cook, P.W. et al. Exp. Dematol. 13, 347–356 (2004). 13. Zhong, Z. et al. Science 264, 95–98 (1994). Bacteria spurned by self-absorbed cells Jean-Pierre Gorvel & Chantal de Chastellier Three studies show that cells are protected against intracellular pathogens by autophagy, a process that degrades cytoplasmic components in bulk. The work may have implications for the development of vaccines against pathogens such as Mycobacterium tuberculosis, Streptococcus pyogenes and Shigella flexneri. The authors are at the Centre d’Immunologie de Marseille Luminy, Marseille, France. e-mail: [email protected] autophagosomes or autophagic vacuoles. Although the origin of autophagosomes is still a matter of debate, a substantial amount of evidence suggests that these membranous structures arise from ribosome-free regions of the endoplasmic reticulum. Upon fusion with lysosomes, these vacuoles acquire hydrolytic enzymes and their contents are degraded 2,3 . Over the past seven years a number of observations have shown that, under certain conditions, several pathogenic bacteria—including Rickettsia conorii 4 , Brucella abortus 5 , Legionella pneumophila 6 , Porphyromonas gingivalis 7 , Listeria mono- cytogenes 8 and Mycobacterium avium—can reside within autophagic compartments inside host cells (Fig. 1). However, the sig- nificance of these observations has been unclear 9 . Three recent studies outline strategies by which infected host cells use autophagy to combat pathogens and examine how pathogens manage to resist the autophagic machinery.
Transcript of Bacteria spurned by self-absorbed cells
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18 VOLUME 11 | NUMBER 1 | JANUARY 2005 NATURE MEDICINE
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In response to conditions ranging from starvation to hormone treatment, cells undergo a carefully orchestrated process of self- consumption. During autophagy1 cytoplasmic organelles and portions of c ytoplasm are sequestered in double- membrane bound vacuoles called nascent
To dissect the roles of skin epidermis versus T cells in the propagation of the diseased phenotype, the researchers also performed studies involving skin grafting and passive transfer of immune cells. Tape-stripped skin from the transgenic mice that had been allografted to athymic nude mice was insufficient in reconstituting the pso-riatic phenotype; transfer of activated T cells to the dermis, the layer below the epidermis, also was required.
The authors next turned to the well- characterized severe combined immunodefi-ciency (SCID)-human skin xenograft model, in which xenotransplanted skin of psoriasis is induced to develop the disease following injection of pathogenic immunocytes. The authors showed that activated Stat3 (PY-Stat3) localized to the nucleus of epidermal keratinocytes following injection of CD4, but not CD8-activated T cells.
Lastly, the authors applied a Stat3- specific decoy oligonucleotide—which competes for binding to Stat3 and interferes with its tran-scriptional activity—to diseased skin from the K5.Stat3C transgenic mice. The oligonucleotide markedly improved or resolved the abnormal phenotype (Fig. 1). This result has significant clinical relevance, because decoy oligonucleotides for Stat3 and other molecules such as E2F and NF-κB are showing promise in clinical trials for selected inflammatory diseases, head and neck cancer, and other diseases.
This study and others that have attempted to replicate human psoriasis have revealed the com-plexity of cellular communication within skin and this sentinel organ’s ability to orchestrate immune responses and other homeostatic events that normally protect, but occasionally
perturb, cutaneous function. The skin arguably can manifest more patterns of inflammation and immunity than any other organ system, and rightfully so. It must protect itself and the individual from a virtually unlimited onslaught of microbial, physical and chemical challenges and environmental insults.
Stat3 expression within the epidermis may well serve a crucial role in effectively engaging and coordinating inflammatory and immune responses to various offending agents. Which sets of molecules—cytokines, chemokines and other inflammatory mediators, growth factors, integ-rins and other adhesion proteins—are expressed in response to Stat3 activation in the epidermis remain to be elucidated. Whether there are genetic determinants crucial in this mouse model as there are in humans for the development of psoriasis also are as yet unknown10.
In the new study, CD4+ T cells from K5.Stat3C mice specifically induce the psoria-sis-like skin lesions when injected into uninvolved K5.Stat3C transgenic skin allografted to nude mice. The ability to definitively characterize the antigenic specificity, if it exists, and genotype the T-cell receptor may aid in further homing in on the identity of the pathogenic T cell in human psoriasis.
Another unmet challenge in fully understand-ing psoriasis has been linking the skin disease with the joint inflammation and destruction observed in psoriatic arthritis. No animal model of psoria-sis exhibits both robust skin and joint pathology, except one: mice overexpressing amphiregulin, an epidermal growth factor (EGF)-related growth factor, in the epidermis11,12. Interestingly, EGF receptor tyrosine kinase has been reported
to induce phosphorylation of Stat3 (ref. 13). The extent and duration of the Stat3 phosphoryla-tion response to amphiregulin versus other EGF-related growth factors, possibly in conjunction with other Stat3-activating cytokines will be relevant, because none of the other EGF-related growth factors have been reported to induce this dramatic psoriatic phenotype in mice. Moreover, it will be interesting to determine whether the Stat3C mouse eventually develops synovial and joint pathology similar to the amphiregulin transgenic mouse and psoriasis.
It is now clear that the epidermal keratinocyte and T cell pair up to gener-ate psoriasis. How they interact may be determined by Stat3 or other partners in the Stat3 signaling network. The activity of Stat3 and other modulating factors in the pathway may be the rheostat to control the development and expression of disease. Targeting Stat3 and other critical links in the disease network may improve the outcome of patients afflicted by psoriasis.
1. Nickoloff, B.J. & Nestle, F.O. J. Clin. Invest. 113, 1664–1675 (2004).
2. Wrone-Smith, T. & Nickoloff, B.J. J. Clin. Invest. 98, 1878–1887 (1996).
3. Boyman, O. et al. J. Exp. Med. 199, 731–736 (2004).4. Gottlieb, A. & Bos, J. Clin. Immunol. 105, 105–116
(2002).5. Sano, S. et al. Nat. Med. 10, 43–49 (2004). 6. Levy, D.E. & Darnell, J.E. Jr. Nat. Rev. Mol. Cell. Bio. 3,
651–662 (2002).7. Chan, K.S. et al. Cancer Res. 64, 2382–2389 (2004).8. Chan, K.S. et al. J. Clin. Invest. 114, 720–728 (2004).9. Bromberg, J.F. et al. Cell 98, 295–303 (1999).10. Bowcock, A.M. J. Invest. Dermatol. 122, xv–xvii
(2004).11. Cook, P. W. et al. J. Clin. Invest. 100, 2286–2294
(1997).12. Cook, P.W. et al. Exp. Dematol. 13, 347–356 (2004).13. Zhong, Z. et al. Science 264, 95–98 (1994).
Bacteria spurned by self-absorbed cellsJean-Pierre Gorvel & Chantal de Chastellier
Three studies show that cells are protected against intracellular pathogens by autophagy, a process that degrades cytoplasmic components in bulk. The work may have implications for the development of vaccines against pathogens such as Mycobacterium tuberculosis, Streptococcus pyogenes and Shigella flexneri.
The authors are at the Centre d’Immunologie de
Marseille Luminy, Marseille, France.
e-mail: [email protected]
autophagosomes or autophagic vacuoles. Although the origin of autophagosomes is still a matter of debate, a substantial amount of evidence suggests that these membranous structures arise from ribosome-free regions of the endoplasmic reticulum. Upon fusion with lysosomes, these vacuoles acquire hydrolytic enzymes and their contents are degraded2,3.
Over the past seven years a number of observations have shown that, under certain conditions, several pathogenic
bacteria—including Rickettsia conorii4, Brucella abortus5, Legionella pneumophila6, Porphyromonas gingivalis7, Listeria mono-cytogenes8 and Mycobacterium avium—can reside within autophagic compartments inside host cells (Fig. 1). However, the sig-nificance of these observations has been unclear9.
Three recent studies outline strategies by which infected host cells use autophagy to combat pathogens and examine how pathogens manage to resist the autophagic machinery.
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In the November 5 issue of Science, Nakagawa et al.10 show that the autophagic machinery in nonphagocytic cells destroys pathogenic group A Streptococcus (GAS; Streptococcus pyogenes) in human cells. GAS is among the most common and versatile of human pathogens, causing a wide spec-trum of human diseases, from sore throat and necrotizing fasciitis to streptococcal toxic shock syndrome. After entry into host cells, the release of GAS from early phago-somes into the cytoplasm induces autophagy, which in turn provokes the trapping of GAS inside large autophagosomes. These autophagosomes then fuse with lysosomes, thereby killing GAS inside host cells.
In the December 16 issue of Cell, Gutierrez et al.11 show that Mycobacterium tuberculosis can be eliminated by auto phagy. M. tuber-culosis is a facultative intra cellular and persistent pathogen that infects more than a billion people worldwide. M. tubercu-losis resides in immature phagosomes, plasma membrane–derived vacuoles, thereby avoiding the cytolytic environ-ment of lysosomes in professional phago-cytes12,13. The authors show that starvation conditions or treatment with interferon-γ can trigger autophagy in cells infected with M. tuberculosis.
Previous work showed that a member of the 47-kilodalton (p47) guanosine triphosphatase family, LRG-47, controls M. tuberculosis replication14. Gutierrez et al.11 show that LRG-47 may participate in the biogenesis of autophagosomes. By fusing with lysosomes, the M. tuberculo-sis- containing autophagosomes acquire lysosomal markers and become acidified, and this leads to the suppression of M. tuberculosis intracellular survival.
In the third study, Ogawa et al.15 examine how a bacterium can fight back against autophagy. Shigella flexneri is an invasive pathogen of the gut that causes shigellosis, an acute infectious colitis that is a major pediatric issue worldwide, and has the highest mortality rate among the diarrheal diseases. Shigella is a Gram-negative bacterium that is able to export bacterial effectors into cells through a type III secretion system. Several such effectors promote survival of pathogenic Shigella by circumventing the host cell’s physiological functions16.
In the 2 December online issue of Science15, the authors find that one of these effectors, IcsB, prevents recognition of S. flexneri by the autophagic machinery. They show that in the absence of IcsB, S. flexneri is trapped within autophagosomes and bacterial survival is affected. The authors also show that the
S. flexneri VirG (IcsA) protein, required for S. flexneri actin-based motility, is the target for recognition of the autophagy pathway. This recognition occurs through the interaction of VirG with the host protein Atg5, an autophagy marker protein required for elongation of autophagic surrounding membranes.
As the three studies make clear, autophagy has been underestimated as an innate immunity pathway. Autophagy has been neglected in the past because of a lack of specific markers, a situation that is now changing. Thanks mainly to yeast genetics, several proteins have been found to be specific for this pathway, therefore allowing cell biology studies. Although it is important to unambiguously discriminate between markers associated with autophagic compartments and with organelles closely apposed to pathogen- containing vacuoles, the use of such markers has opened up new avenues in cell biology and immunity.
The work of Guttierez et al.11 shows for the first time that interferon-γ treatment in macrophages induces autophagy in a manner independent of reactive nitrogen and oxygen—in contrast with previous experiments in which efficient killing of Mycobacterium was shown to require production of nitric oxide. The new findings also suggest that there may still be unknown means to stimulate autophagy for eliminating dangerous intracellular pathogens such as Brucella, Chlamydia and Rickettsia.
It is interesting from an evolutionary point of view that pathogens such as S. flexneri are also able to detect autophagy as a possible source of danger and interfere with the autophagic pathway. This finding is potentially clinically relevant because the discovery of pathogen molecules that inhibit autophagy-mediated recognition may lead to the generation of new vaccines. Identifying and characterizing such inhibitors will allow the construction of live, attenuated bacterial mutants suceptible to autophagy—so mim-icking the bacterial vulnerability expected with such a vaccine.
Degradation of such mutants through the autophagic pathway might elicit strong CD4 and CD8 immune responses. These responses might be expected because autophagy is known to connect the exo-cytic and endocytic pathways2,3, thereby bringing antigenic peptides to MHC class I and MHC class II molecules for adaptive immune responses.
We must, however, keep in mind that induction of autophagy is not always a benefit for host cells. A recent study by Hernandez et al.17 proposes that Salmonella enterica kills
macrophages through SipB, a type III secreted effector that disrupts mitochondria—thereby inducing autophagy and cell death. Efforts to find new targets to induce autophagy could be promising as long as they focus on events that prevent pathogen survival.
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(2001).13. Thilo, L. & de Chastellier, C. in J. P. Gorvel (ed.)
Intracellular Pathogens in Membrane Interactions and Vacuole Biogenesis 153–169. (Kluwer Academic/Plenum Publishers, New York, 2004).
14. MacMicking, J.D., Taylor, G.A. & McKinney, J.D. Science 302, 654–659 (2003).
15. Ogawa, M. et al. Science published online 2 Dec 2004 (doi: 10.1126/science.1106036).
16. Sansonetti, P.J. Nat. Rev. Immunol. 4, 953–964 (2004).
17. Hernandez, L.D., Pypaert, M., Flavell, R.A. & Galán, J.E. J. Cell Biol. 163, 1123–1131 (2003).
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Figure 1 Electron micrograph showing Mycobacterium avium in a late autophagosome. Because of fusion with lysosomes, as shown by the dense contents of this compartment, the remnants of the inner membrane (IM) of the autophagosome and of portions of the cells’ cytoplasm (Cyt) are visible. The phagosome membrane around the three bacteria is also undergoing degradation. Bar, 0.5 µm
