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Broadening the Paradigm of Mucosal Dendritic Cell-Mediated Induction of Gut-Homing on T Cells
Gastroenterology 2014;146:854–861
SELECTED SUMMARIES Philip S. Schoenfeld, Section Editor John Y. Kao, Section Editor STAFF OF CONTRIBUTORS Joseph Anderson, White River Junction, VT Darren M. Brenner, Chicago, IL Andrew T. Chan, Boston, MA Francis K. L. Chan, Hong Kong, China Massimo Colombo, Milan, Italy Gregory A. Cote, Indianapolis, IN B. Joseph Elmunzer, Ann Arbor, MI Alex Ford, Leeds, United Kingdom Timothy B. Gardner, Lebanon, NH Lauren B. Gerson, San Francisco, CA
Michelle Kang Kim, New York, NY W. Ray Kim, Rochester, MN Paul Y. Kwo, Indianapolis, IN Edward V. Loftus, Rochester, MN Uma Mahadevan, San Francisco, CA Laurent Peyrin-Biroulet, Vandoeuvre-lès-Nancy, France Mark Pimentel, Los Angeles, CA Jesus Rivera-Nieves, San Diego, CA Joel H. Rubenstein, Ann Arbor, MI
BROADENING THE PARADIGM OF MUCOSAL DENDRITIC CELL-MEDIATED INDUCTION OF GUT-HOMING ON T CELLS Ruane D, Brane L, Reis B, et al. Lung dendritic cells induce migration of protective T cells to the gastrointestinal tract. J Exp Med 2013;210:1871–1888. Homeostasis in the intestine is maintained by dendritic cells (DC), which at the interface between innate and adaptive immunity, educate T cells for both the maintenance of effective immune surveillance and for the prevention of overactive immune responses. In this article, Ruane et al challenge the prevailing paradigm that intestinal DC are exclusively responsible for conferring a gut-homing phenotype to naïve T cells. The investigators provide evidence that extraintestinal mucosal DCs perform a similar function and induce immunity to intestinal pathogens (J Exp Med 2013;210:1871–1888). This study aimed to first assess the relative capacity of DC from various organs to induce a guthoming phenotype. They initially demonstrated that DC from both the lung and the mesenteric lymph nodes induce a4b7 and CCR9 expression in OT-II T cells after ova stimulation. Although this function was preferentially performed by CD103þ DC in the mesenteric lymph nodes, as has been previously described (J Exp Med 2005;202:1063–1073), in the lung it seems that the induction of a gut homing signature can occur in response to either CD11bþ or CD103þ DC, with the latter being less efficient compared with their CD11bþ counterparts (in an retinoic acid [RA]- and transforming growth factor b-dependent manner). Furthermore, the CD11bþ lung DC responsible for inducing this phenotype could be grouped based on their expression of the sialoglycoprotein CD24 (heat-stable protein) and CD64 (FcgR1) almost exclusively into the CD11bþCD24þCD64Neg population of DCs. These cells have been shown recently to be highly proliferative, express DC-associated antigens and a strong capacity to traffic to the lung draining lymph nodes (Immunity 2013;38:970–983). A functional relevance for this aberrant expression of gut-homing molecules was then
Sameer Saini, Ann Arbor, MI Ekihiro Seki, La Jolla, CA Shamita B. Shah, Stanford, CA Pratima Sharma, Ann Arbor, MI Amit Singal, Dallas, TX Jan Tack, Leuven, Belgium Akbar Waljee, Ann Arbor, MI Alastair J. M. Watson, Norwich, United Kingdom
elegantly demonstrated by intranasal sensitization with ova. This drove initial induction of a4b7 on lung and mediastinal T cells, which preceded the detection of increasing frequencies of adoptively transferred cells in intestinal tissues and draining lymph nodes. The kinetics of these 2 events coupled with the degree of proliferation of transferred cells are consistent with the extra-intestinal induction of a4b7 resulting in increased trafficking to the intestine. Blocking T-cell egress with FTY720 (an inhibitor of lymphocyte egress from lymph nodes) lent further credence to the hypothesis that transferred cells are educated by lung DC before their appearance in the intestine, as FTY720 resulted in accumulation of T cells in the mediastinal lymph nodes as seen in the previous experiment. Subsequent recovery of these cells from intestinal tissues was impaired. They then systematically verified the role of CD11bþ lung DC in the induction of gut-homing molecules in response to intranasal challenge by repeating these experiments in diphtheria toxin-sensitive (DTR) CD11c mice to ablate DC subsets. CD11b-DTR mice were used to selectively deplete CD11bþ cells, whereas Langerin-DTR mice preferentially depleted CD103þ DC and the Batf3-/- mouse lacked CD103þ DC. In all cases, strategies that targeted the CD11bþ DC for depletion ablated the capacity for intranasal sensitization to induce a gut-homing phenotype on T cells. By contrast, ablation or deficiency of CD103þ DC had little or no impact on this phenomenon, substantiating the overarching hypothesis that lung CD11bþ DC may drive the imprinting of gut homing on T cells. Finally, the authors established the in vivo relevance of their observations by demonstrating greater immune priming and host protection by intranasal immunization against Salmonella than either through the subcutaneous or oral immunization routes. Comment. The expression of gut-homing molecules on T cells has significant impact on a range of intestinal contexts. The discovery that a4b7 acts as a co-receptor for HIV infection and is coopted by HIV to induce further recruitment of CD4þ T cells to the intestine; improving regulatory T-cell function and promoting immunosuppression are just some of the key mechanisms whereby HIV hijacks the human immune response (Int Immunol 2009;21:283–294). Similarly, the induction of gut-homing molecules a4b7 and
March 2014
chemokine receptor CCR9 are believed to be have central roles in the recruitment and perpetuation of chronic intestinal inflammatory conditions, such as inflammatory bowel diseases (IBD). These molecules have such a critical impact on IBD pathogenesis that strategies to block their function have been pursued as therapeutics in patients with IBD. Therapies blocking integrin beta7 (etrolizumab, AMG181) and MAdCAM-1 (PF-00547659) are currently undergoing large-scale trials (N Engl J Med 2013;369:699–710; N Engl J Med 2013;369:711–721), whereas blockade of CCR9 (vercirnon) and integrin a4b7 (vedolizumab) have completed phase three trials for the treatment of IBD (PLoS One 2013;8:e60094). The study by Ruane et al once and for all defenestrates the hypothesis that intestinal CD103þ DC are the sole cell type capable of inducing a gut-homing phenotype on T cells. It validates previous findings of a study that demonstrated that lung CD11bþ and CD103þ DC express the RA synthetic enzymes (RALDH), required for induction of gut homing (Blood 2010;115:1958–1968). The majority of RA synthesis seems to be split between CD103þ and CD11bþ lung and mediastinal DC, but alveolar macrophages may contribute at significantly lower levels. Studies on the expression of aldehyde dehydrogenases by antigen-presenting cells had also demonstrated that CD11bþ and CD103þ DC express aldehyde dehydrogenase at significantly higher levels than tissueresident macrophages (J Immunol 2011;187:733–747); however, the relative contribution by CD11bþ and CD103þ DC to the induction of gut-homing and/or regulatory phenotypes may have more to do with their relative frequencies, because this will ultimately determine the ratio of DC to naïve T cells in vivo. Of note is that these studies were performed at a higher DC:T-cell ratio than is physiologic, which may impact the potential interpretation of some of these findings. Although bile is a major source of retinoids (Mucosal Immunol 2011;4:438–447) in the gastrointestinal tract, it is less clear what is the source of retinoids in the lung? It seems that lipid interstitial cells within the alveolar wall may represent an initial source of retinoids (Am J Physiol Lung Cell Mol Physiol 2004;286:L249–256), but it remains to be seen if this is adequate to account for the RA-dependent capacity of lung antigen-presenting cells to induce gut homing. A second issue raised by this work is whether or not induction of gut-homing and pro-regulatory signals go hand in hand, because both are potentiated by RA (Immunity 2004;21:527–538; Science 2007;317:256–260). The acute intranasal challenge used in this study provided an immunizing effect against subsequent enteral challenge that was significantly more potent than oral immunization, thereby confirming previous findings that demonstrated that intranasal administration of rotavirus proves more effective than oral immunization in mice (Vaccine 2003;21:3885–3900). This effect occurred in response to a single challenge and was CD11bþ DC-dependent. Repeated challenge with ova has been shown to induce RA conversion enzyme, RALDH2, particularly in CD103þ lung DC leading to generation of regulatory T cells (J Immunol 2013;191:25–29) and a tolerizing immune response. Moreover, a similar induction of
Selected Summaries
855
tolerance has been described for lung alveolar macrophages in response to intranasal vaccination with ovalbumin (Am J Respir Cell Mol Biol 2013;48:773–780). Taken together, these 2 studies suggest that timing is critical. Although acute antigen exposure of lung DC results in induction of guthoming receptors to prime the enteral immune system to subsequent challenge, chronic exposure induces a distinct subset of DC that drive tolerogenic T-cell responses. The decision to drive a gut-homing or a regulatory phenotype may therefore pivot on the simultaneous presence of transforming growth factor-b. As the story of gut homing continues to evolve, several aspects of the plot have been made clear. First, the dogma of CD103þ intestinal DC as sole inductors of gut-homing needs revision. Second, the determinants of induction of immunogenic or tolerogenic immune responses remain incompletely understood and warrant further investigation, both for the treatment of immune-mediated processes and to improve vaccination strategies. What the big picture now clearly suggests is that there seems to be much more cross-talk between mucosal compartments than previously anticipated. We might now envision a world in which challenges to the mucosal immune system via a pulmonary route might serve as an environmental trigger for IBD, whereas chronic challenges (eg, pulmonary stages of intestinal helminthes) might exert a protective role. Although the study by Ruane et al describes a role for extra-intestinal induction of gut homing, can the same phenomenon be recapitulated in humans and what relevance does it have for human health? Previous studies by Schlitzer et al (Immunity 2013;38:970–983) have clearly demonstrated that the same population of CD24þ lung DC are conserved between humans and mice. Similarly, the capacity to induce gut homing of human T cells has been demonstrated to a limited degree in humanized mice, lending further credence to the feasibility of this mechanism being conserved across species (Blood 2012;120:2610–2619). Clinically, this phenomenon may already be undergoing testing indirectly. A HIV vaccine named Vacc-4x was originally administered alongside granulocyte-macrophage colony-stimulating factor by intradermal injection to target skin DC with considerable benefit to HIV-positive patients (APMIS 2012120:204–209). This group is now testing a nasally delivered Vacc-4x alternative. If the mouse data hold true, this nasal approach may offer even better efficacy than its intradermal counterpart and may even receive additional benefits from the induction of a4b7, a known HIV co-receptor (Nat Immunol 2008;9:301–309). COLM B. COLLINS Department of Pediatrics University of Colorado School of Medicine Denver, Colorado JESÚS RIVERA-NIEVES Inflammatory Bowel Disease Center Division of Gastroenterology UCSD San Diego, California
SELECTED SUMMARIES Philip S. Schoenfeld, Section Editor John Y. Kao, Section Editor STAFF OF CONTRIBUTORS Joseph Anderson, White River Junction, VT Darren M. Brenner, Chicago, IL Andrew T. Chan, Boston, MA Francis K. L. Chan, Hong Kong, China Massimo Colombo, Milan, Italy Gregory A. Cote, Indianapolis, IN B. Joseph Elmunzer, Ann Arbor, MI Alex Ford, Leeds, United Kingdom Timothy B. Gardner, Lebanon, NH Lauren B. Gerson, San Francisco, CA
Michelle Kang Kim, New York, NY W. Ray Kim, Rochester, MN Paul Y. Kwo, Indianapolis, IN Edward V. Loftus, Rochester, MN Uma Mahadevan, San Francisco, CA Laurent Peyrin-Biroulet, Vandoeuvre-lès-Nancy, France Mark Pimentel, Los Angeles, CA Jesus Rivera-Nieves, San Diego, CA Joel H. Rubenstein, Ann Arbor, MI
BROADENING THE PARADIGM OF MUCOSAL DENDRITIC CELL-MEDIATED INDUCTION OF GUT-HOMING ON T CELLS Ruane D, Brane L, Reis B, et al. Lung dendritic cells induce migration of protective T cells to the gastrointestinal tract. J Exp Med 2013;210:1871–1888. Homeostasis in the intestine is maintained by dendritic cells (DC), which at the interface between innate and adaptive immunity, educate T cells for both the maintenance of effective immune surveillance and for the prevention of overactive immune responses. In this article, Ruane et al challenge the prevailing paradigm that intestinal DC are exclusively responsible for conferring a gut-homing phenotype to naïve T cells. The investigators provide evidence that extraintestinal mucosal DCs perform a similar function and induce immunity to intestinal pathogens (J Exp Med 2013;210:1871–1888). This study aimed to first assess the relative capacity of DC from various organs to induce a guthoming phenotype. They initially demonstrated that DC from both the lung and the mesenteric lymph nodes induce a4b7 and CCR9 expression in OT-II T cells after ova stimulation. Although this function was preferentially performed by CD103þ DC in the mesenteric lymph nodes, as has been previously described (J Exp Med 2005;202:1063–1073), in the lung it seems that the induction of a gut homing signature can occur in response to either CD11bþ or CD103þ DC, with the latter being less efficient compared with their CD11bþ counterparts (in an retinoic acid [RA]- and transforming growth factor b-dependent manner). Furthermore, the CD11bþ lung DC responsible for inducing this phenotype could be grouped based on their expression of the sialoglycoprotein CD24 (heat-stable protein) and CD64 (FcgR1) almost exclusively into the CD11bþCD24þCD64Neg population of DCs. These cells have been shown recently to be highly proliferative, express DC-associated antigens and a strong capacity to traffic to the lung draining lymph nodes (Immunity 2013;38:970–983). A functional relevance for this aberrant expression of gut-homing molecules was then
Sameer Saini, Ann Arbor, MI Ekihiro Seki, La Jolla, CA Shamita B. Shah, Stanford, CA Pratima Sharma, Ann Arbor, MI Amit Singal, Dallas, TX Jan Tack, Leuven, Belgium Akbar Waljee, Ann Arbor, MI Alastair J. M. Watson, Norwich, United Kingdom
elegantly demonstrated by intranasal sensitization with ova. This drove initial induction of a4b7 on lung and mediastinal T cells, which preceded the detection of increasing frequencies of adoptively transferred cells in intestinal tissues and draining lymph nodes. The kinetics of these 2 events coupled with the degree of proliferation of transferred cells are consistent with the extra-intestinal induction of a4b7 resulting in increased trafficking to the intestine. Blocking T-cell egress with FTY720 (an inhibitor of lymphocyte egress from lymph nodes) lent further credence to the hypothesis that transferred cells are educated by lung DC before their appearance in the intestine, as FTY720 resulted in accumulation of T cells in the mediastinal lymph nodes as seen in the previous experiment. Subsequent recovery of these cells from intestinal tissues was impaired. They then systematically verified the role of CD11bþ lung DC in the induction of gut-homing molecules in response to intranasal challenge by repeating these experiments in diphtheria toxin-sensitive (DTR) CD11c mice to ablate DC subsets. CD11b-DTR mice were used to selectively deplete CD11bþ cells, whereas Langerin-DTR mice preferentially depleted CD103þ DC and the Batf3-/- mouse lacked CD103þ DC. In all cases, strategies that targeted the CD11bþ DC for depletion ablated the capacity for intranasal sensitization to induce a gut-homing phenotype on T cells. By contrast, ablation or deficiency of CD103þ DC had little or no impact on this phenomenon, substantiating the overarching hypothesis that lung CD11bþ DC may drive the imprinting of gut homing on T cells. Finally, the authors established the in vivo relevance of their observations by demonstrating greater immune priming and host protection by intranasal immunization against Salmonella than either through the subcutaneous or oral immunization routes. Comment. The expression of gut-homing molecules on T cells has significant impact on a range of intestinal contexts. The discovery that a4b7 acts as a co-receptor for HIV infection and is coopted by HIV to induce further recruitment of CD4þ T cells to the intestine; improving regulatory T-cell function and promoting immunosuppression are just some of the key mechanisms whereby HIV hijacks the human immune response (Int Immunol 2009;21:283–294). Similarly, the induction of gut-homing molecules a4b7 and
March 2014
chemokine receptor CCR9 are believed to be have central roles in the recruitment and perpetuation of chronic intestinal inflammatory conditions, such as inflammatory bowel diseases (IBD). These molecules have such a critical impact on IBD pathogenesis that strategies to block their function have been pursued as therapeutics in patients with IBD. Therapies blocking integrin beta7 (etrolizumab, AMG181) and MAdCAM-1 (PF-00547659) are currently undergoing large-scale trials (N Engl J Med 2013;369:699–710; N Engl J Med 2013;369:711–721), whereas blockade of CCR9 (vercirnon) and integrin a4b7 (vedolizumab) have completed phase three trials for the treatment of IBD (PLoS One 2013;8:e60094). The study by Ruane et al once and for all defenestrates the hypothesis that intestinal CD103þ DC are the sole cell type capable of inducing a gut-homing phenotype on T cells. It validates previous findings of a study that demonstrated that lung CD11bþ and CD103þ DC express the RA synthetic enzymes (RALDH), required for induction of gut homing (Blood 2010;115:1958–1968). The majority of RA synthesis seems to be split between CD103þ and CD11bþ lung and mediastinal DC, but alveolar macrophages may contribute at significantly lower levels. Studies on the expression of aldehyde dehydrogenases by antigen-presenting cells had also demonstrated that CD11bþ and CD103þ DC express aldehyde dehydrogenase at significantly higher levels than tissueresident macrophages (J Immunol 2011;187:733–747); however, the relative contribution by CD11bþ and CD103þ DC to the induction of gut-homing and/or regulatory phenotypes may have more to do with their relative frequencies, because this will ultimately determine the ratio of DC to naïve T cells in vivo. Of note is that these studies were performed at a higher DC:T-cell ratio than is physiologic, which may impact the potential interpretation of some of these findings. Although bile is a major source of retinoids (Mucosal Immunol 2011;4:438–447) in the gastrointestinal tract, it is less clear what is the source of retinoids in the lung? It seems that lipid interstitial cells within the alveolar wall may represent an initial source of retinoids (Am J Physiol Lung Cell Mol Physiol 2004;286:L249–256), but it remains to be seen if this is adequate to account for the RA-dependent capacity of lung antigen-presenting cells to induce gut homing. A second issue raised by this work is whether or not induction of gut-homing and pro-regulatory signals go hand in hand, because both are potentiated by RA (Immunity 2004;21:527–538; Science 2007;317:256–260). The acute intranasal challenge used in this study provided an immunizing effect against subsequent enteral challenge that was significantly more potent than oral immunization, thereby confirming previous findings that demonstrated that intranasal administration of rotavirus proves more effective than oral immunization in mice (Vaccine 2003;21:3885–3900). This effect occurred in response to a single challenge and was CD11bþ DC-dependent. Repeated challenge with ova has been shown to induce RA conversion enzyme, RALDH2, particularly in CD103þ lung DC leading to generation of regulatory T cells (J Immunol 2013;191:25–29) and a tolerizing immune response. Moreover, a similar induction of
Selected Summaries
855
tolerance has been described for lung alveolar macrophages in response to intranasal vaccination with ovalbumin (Am J Respir Cell Mol Biol 2013;48:773–780). Taken together, these 2 studies suggest that timing is critical. Although acute antigen exposure of lung DC results in induction of guthoming receptors to prime the enteral immune system to subsequent challenge, chronic exposure induces a distinct subset of DC that drive tolerogenic T-cell responses. The decision to drive a gut-homing or a regulatory phenotype may therefore pivot on the simultaneous presence of transforming growth factor-b. As the story of gut homing continues to evolve, several aspects of the plot have been made clear. First, the dogma of CD103þ intestinal DC as sole inductors of gut-homing needs revision. Second, the determinants of induction of immunogenic or tolerogenic immune responses remain incompletely understood and warrant further investigation, both for the treatment of immune-mediated processes and to improve vaccination strategies. What the big picture now clearly suggests is that there seems to be much more cross-talk between mucosal compartments than previously anticipated. We might now envision a world in which challenges to the mucosal immune system via a pulmonary route might serve as an environmental trigger for IBD, whereas chronic challenges (eg, pulmonary stages of intestinal helminthes) might exert a protective role. Although the study by Ruane et al describes a role for extra-intestinal induction of gut homing, can the same phenomenon be recapitulated in humans and what relevance does it have for human health? Previous studies by Schlitzer et al (Immunity 2013;38:970–983) have clearly demonstrated that the same population of CD24þ lung DC are conserved between humans and mice. Similarly, the capacity to induce gut homing of human T cells has been demonstrated to a limited degree in humanized mice, lending further credence to the feasibility of this mechanism being conserved across species (Blood 2012;120:2610–2619). Clinically, this phenomenon may already be undergoing testing indirectly. A HIV vaccine named Vacc-4x was originally administered alongside granulocyte-macrophage colony-stimulating factor by intradermal injection to target skin DC with considerable benefit to HIV-positive patients (APMIS 2012120:204–209). This group is now testing a nasally delivered Vacc-4x alternative. If the mouse data hold true, this nasal approach may offer even better efficacy than its intradermal counterpart and may even receive additional benefits from the induction of a4b7, a known HIV co-receptor (Nat Immunol 2008;9:301–309). COLM B. COLLINS Department of Pediatrics University of Colorado School of Medicine Denver, Colorado JESÚS RIVERA-NIEVES Inflammatory Bowel Disease Center Division of Gastroenterology UCSD San Diego, California