prokaryotes; therefore gut flora are notthe source. A dietary source can bequestioned, as it is unlikely that theHsp110 would be recovered in a struc-turally intact form in the large intes-tine. And so the most likely source isthe gut epithelium itself. Yet, Hsp110does not possess a signal peptide and socannot be secreted from IECs via theclassical secretory pathway. Non-canonical secretion, as seen, for exam-ple, with FGF-2, is a possibility (14).However, this release pathway, and theanalogous pathway in yeast, appear totransport very few proteins. Perhaps,then, Hsp110 is released into the intes-tinal lumen as a consequence of epithe-lial renewal, particularly in response toIEL activation (15). If this is the case,though, it might be expected that otherabundant cytosolic chaperone proteins,such as Hsp90 and Hsp70, would berecovered in the intestinal contents aswell. Nonetheless, perhaps inappropri-ately elevated levels of Hsp110 in theintestinal lumen serve as an immuno-logical “trigger” leading to aberrantinduction of CD1d expression, a subse-quent activation of NK T cells, and

IL-13–elicited destruction of IECs (Fig-ure 1) (16). Whatever the mechanism,one is left to ponder the fascinating mys-tery of why IECs might (uniquely?)release Hsp110 into the gut lumen.Insights into the precise cellular source ofthe gut-lumen Hsp110 and the mecha-nism of its release will provide all-impor-tant clues to this new riddle in the ever-expanding world of Hsp110 function.

1. Fiocchi, C. 1998. Inflammatory bowel disease: eti-ology and pathogenesis. Gastroenterology.115:182–205.

2. Blumberg, R.S. 2001. Characterization of CD1d inmucosal immune function: an immunotherapeutictarget for inflammatory bowel disease. Keio J. Med.50:39–44.

3. Mizoguchi, A., Mizoguchi, E., Takedatsu, H., Blum-berg, R.S., and Bhan, A.K. 2002. Chronic intestinalinflammatory condition generates IL-10-producingregulatory B cell subset characterized by CD1dupregulation. Immunity. 16:219–230.

4. Page, M.J., et al. 2000. Cd1d-restricted cellular lysisby peripheral blood lymphocytes: relevance to theinflammatory bowel diseases. J. Surg. Res.92:214–221.

5. Bendelac, A., et al. 1995. CD1 recognition by mouseNK1+ T lymphocytes. Science. 12:863–865.

6. Mendiratta, S.K., et al. 1997. CD1d1 mutant miceare deficient in natural T cells that promptly pro-duce IL-4. Immunity. 6:467–477.

7. Colgan, S.P., Hershberg, R.M., Furuta, G.T., andBlumberg, R.S. 1999. Ligation of intestinal epithe-lial CD1d induces bioactive IL-10: critical role of thecytoplasmic tail in autocrine signaling. Proc. Natl.

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physiological basis and clinical applications. InMucosal immunology. P.L. Ogra et al., editors. Acade-mic Press Inc. San Diego, California, USA. 587–618.

9. Colgan, S.P., et al. 2003. Intestinal heat shock pro-tein 110 regulates expression of CD1d on intestin-al epithelial cells. J. Clin. Invest. 112:745–754.doi:10.1172/JCI200317241.

10. Lee-Yoon, D., Easton, D., Murawski, M., Burd, R.,and Subjeck, J.R. 1995. Identification of a majorsubfamily of large hsp70-like proteins through thecloning of the mammalian 110-kDa heat shock pro-tein. J. Biol. Chem. 270:15725–15733.

11. Easton, D.P., Kaneko, Y., and Subjeck, J.R. 2000. Thehsp110 and Grp1 70 stress proteins: newly recog-nized relatives of the Hsp70s. Cell Stress Chaperones.5:276–290.

12. Anderson, K.M., and Srivastava, P.K. 2000. Heat,heat shock, heat shock proteins and death: a centrallink in innate and adaptive immune responses.Immunol. Lett. 74:35–39.

13. Srivastava, P.K., and Amato, R.J. 2001. Heatshock proteins: the ‘Swiss Army Knife’ vaccinesagainst cancers and infectious agents. Vaccine.19:2590–2597.

14. Wakisaka, N., Murono, S., Yoshizaki, T.,Furukawa, M., and Pagano, J.S. 2002. Epstein-Barrvirus latent membrane protein 1 induces andcauses release of fibroblast growth factor-2. Can-cer Res. 62:6337–6344.

15. Guy-Grand, D., DiSanto, J.P., Henchoz, P., Malassis-Seris, M., and Vassalli, P. 1998. Small bowelenteropathy: role of intraepithelial lymphocytes andof cytokines (IL-12, IFN-gamma, TNF) in the induc-tion of epithelial cell death and renewal. Eur. J.Immunol. 28:730–744.

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648 The Journal of Clinical Investigation | September 2003 | Volume 112 | Number 5

Dendritic cells and the intestinal bacterial flora: a role for localized mucosal immune responses

Holm H. Uhlig and Fiona Powrie

Sir William Dunn School of Pathology, University of Oxford, Oxford, United Kingdom

Mammals coexist in an overall symbiotic relationship with a complexarray of commensal bacterial flora that colonizes the gastrointestinaltract. These intestinal bacteria interface with cells of the mucosal immunesystem, including DCs (see the related article beginning on page 693).Here we discuss mechanisms of interaction between intestinal bacteriaand DCs and the role of localized gastrointestinal immune responses.

J. Clin. Invest. 112:648–651 (2003). doi:10.1172/JCI200319545.

Address correspondence to: F. Powrie, Sir William Dunn School of Pathology,University of Oxford, South Parks Road,Oxford OX1 3RE, United Kingdom. Phone: 44-1865-285494; Fax: 44-1865-275591; E-mail: fiona.powrie@pathology.ox.ac.uk.Conflict of interest: The authors havedeclared that no conflict of interest exists.Nonstandard abbreviations used:gastrointestinal tract (GIT); inflammatorybowel disease (IBD); nucleotide-bindingoligomerization domain 2 (NOD2).

Inflammatory bowel disease is associated with a dysregulatedimmune response to intestinalbacterial floraThrough processes of evolutionaryand individual adaptation, mammalscoexist with an estimated 300 to 500different species of commensal bacte-ria that colonize the gastrointestinaltract (GIT) in an overall symbiotic rela-

tionship (1, 2). The presence of intes-tinal bacteria plays an important rolein host metabolism, the developmentof the intestinal epithelium, and theintestinal immune system, and it alsoprotects the host against rapid colo-nization by intestinal pathogens (1, 2).To allow sufficient defense againstpotential pathogens but restrict theimmune response to nonpathogenicresident commensal bacteria, themucosal immune system needs to betightly regulated.

In human inflammatory bowel dis-ease (IBD), which encompasses Crohndisease and ulcerative colitis, it isthought that a dysregulated T cellresponse to the intestinal bacterialmicroflora leads to chronic intestinalinflammation (2, 3). Although increas-ing evidence suggests that the intestin-al flora is involved in the pathogenesisof human IBD, to date no specific bac-terial pathogen has been identified. Itseems more likely that different bacte-ria are involved in the initiation of thepathogenic immune response (4, 5).

Recently it was found that a subset ofpatients with Crohn disease haveloss-of-function mutations in thegene that encodes the nucleotide-binding oligomerization domain 2(NOD2) protein (6). NOD2 is a patho-gen-recognition receptor that recog-nizes muramyl dipeptide derivedfrom bacterial peptidoglycans (7).Although it is not understood howmutations in the NOD2 pathway giverise to IBD, these findings suggestthat alterations in the recognition ofintestinal bacteria can contribute tochronic inflammation.

Studies of IBD models in rodentsprovide compelling evidence that the

bacterial flora plays a key role in thepathogenesis of the disease, as chronicintestinal inflammation fails to devel-op under germfree conditions (8).However, precisely how commensalbacteria in the intestine interface local-ly with cells of the immune system toinitiate and perpetuate intestinalinflammation remains unclear.

Intestinal bacterial microflorainduces localized IL-23 secretion by DCsThe proinflammatory cytokine IL-12has been implicated in the pathogene-sis of IBD in a number of mouse mod-els and also in the immunopathogen-

esis of Crohn disease (3, 8). In order tostudy IL-12 production in situ underphysiological conditions, Becker andcoworkers (9) developed a transgenicmouse expressing a reporter geneunder the control of the IL-12p40 sub-unit promoter (see their article in thisissue of the JCI). Somewhat unexpect-edly, they found that p40 transcriptionis restricted primarily to the terminalileum, where it is produced by a subsetof DCs. Recently it has become clearthat p40 is a component not only ofIL-12 (p40/p35) but also of the Th1-inducing cytokine IL-23 (p40/p19)(10). Strikingly, the elevated IL-12p40in the ileum, observed by Becker et al.,

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Figure 1How do intestinal DCs sense the bacterial flora? Current theories of how DCs come into contact with the intestinal bacterial flora can be divid-ed into those that propose an active sampling of bacteria by the host (I and II) and those that involve DCs acting as antigen-sampling cellswhen bacteria have already crossed the epithelial barrier (III and IV). M cells as specialized epithelial cells can mediate the uptake of bacteriatoward DCs in the intestinal lymphoid tissue (I) (13). This mechanism can be exploited as an entry site by pathogens. In addition, it has beenshown that DCs can reach through the basal membrane and the epithelial layer toward the lumen via dendrites (II) (21). In this case, DCsexpress the tight junction proteins occludin, claudin 1, and zonula occludens 1, by which they can keep the barrier integrity intact (21). DCsmay also sample translocated bacteria that reach the lamina propria because of a low degree of physiological leakiness of the epithelium (III),and they might contribute to the clearance of pathogenic bacteria that reach the lamina propria via invasion and/or tissue damage (IV). Beckeret al. describe a population of DCs in the crypt lamina propria of the terminal ileum that produce IL-23 and contain bacteria (9). Whetherthese DCs actively sample bacteria from the crypt lumen or respond to invasive bacteria is currently not understood. After antigen encounter,DCs travel in local lymphoid structures such as Peyer’s patches (PP) and toward the draining mesenteric lymph nodes (MLN) to initiate ormaintain T and B cell immune responses.

was accompanied by p19 but not p35mRNA and increased IL-23 proteincomplex formation. IL-23 was pro-duced in response to the bacterialflora, as it was not detectable in theileum of mice housed under germfreeconditions. Indeed, using an elegantcombination of immunohistology and16S-rRNA fluorescence in situ hybrid-ization, the authors were able to showthat some of the DCs with active p40transcription contained intracellularnondegraded bacteria.

Restriction of DC IL-23 productionto the terminal ileum and not the

proximal small intestine or the colonsuggests a specific activation state ofCD11c+ cells in the former site. A keyquestion is whether this represents theconsequence of a hitherto unappreci-ated active sampling of commensalbacteria by DCs in the ileum, or is theresult of selective colonization andinvasion of a specific pathogen in theterminal ileum that leads to a localhost immune response involving IL-23 secretion. The finding that bac-teria-induced DC activation resultedin IL-23 as opposed to IL-12 produc-tion raises the possibility that IL-23 is

an important mediator in intestinalinflammation, as has recently beenreported in a model of cerebral inflam-mation (11). IL-23 may also play animportant role in host defense againstinvading bacteria as a consequence ofits ability to maintain Th1 cell memo-ry responses by acting directly onCD4+ memory lymphocytes (10). Fur-thermore, IL-23 can activate DCs andmacrophages via autocrine mecha-nisms (12). However, understanding ofthe biology of IL-23 is at an early stage,and further work is required to eluci-date its role in host defense and intes-tinal inflammation.

The results of the study reported byBecker et al. (9) raise the question ofhow intestinal DCs come into contactwith bacterial antigens. Several routeshave been proposed, and these mayvary depending on the nature of thebacterium (Figure 1) (13). Obviously, inorder to prevent systemic infection,there is a clear need for the immunesystem to respond to bacteria thattranslocate from the lumen toward thelamina propria. Evidence also suggeststhat there are more active modes ofantigen sampling involving M cells orintra- and transepithelial DCs. Howev-er, the reason why the immune systembothers to actively sample intestinalantigens from the lumen is less clear.Such a mechanism might aid sustainedantigen delivery for T cell–dependentand T cell–independent IgA produc-tion (14). Alternatively, the active sam-pling of low amounts of intestinal anti-gens may be involved in inducing andmaintaining T cell tolerance towardthe intestinal bacterial flora.

Localized mucosal immuneresponses as a result of differentialcolonization and pathogenicityIt is tempting to speculate, as do Beck-er et al. (9), that the distinct elevation ofIL-23 expression in the ileum reflects anincreased susceptibility to inflamma-tion in the terminal ileum, a site that isaffected in patients with Crohn disease(3). However, this simple model with adominant IL-23 response in the termi-nal ileum is difficult to reconcile withthe finding that most mouse models ofintestinal inflammation express domi-nant pathology in the cecum and colon,but not in the terminal ileum (8).

650 The Journal of Clinical Investigation | September 2003 | Volume 112 | Number 5

Figure 2Specialized commensal bacteria occupy niches in different compartments of the GIT. Thestructure of the murine intestine in different parts of the GIT most certainly reflects its domi-nant purpose of digesting and absorbing nutrients. However, the presence and usage of vari-ous immune-response mechanisms also mirror the interplay with the respective resident intes-tinal bacteria in compartments that these bacteria are adapted to colonize as a permanent ortemporary ecological niche. The immune response, in return, can shape the commensal bac-terial flora (15). Whereas the total bacterial load increases toward the lower GIT, distinct bac-teria are adapted to predominantly colonize niches in the small intestine or cecum (refs. 1, 14;and M. Strus et al., unpublished observations). Original magnification of the micrographs,×200. spp., species.

The GIT provides distinct niches forcolonization of commensal bacteria,as indicated by qualitative and quanti-tative differences in the bacterial florathroughout the GIT (Figure 2) (1, 15).The anatomical architecture in differ-ent parts of the GIT reflects its func-tional role in digestion and also inhost defense (16). Thus, there are cleardifferences not only in the epithelialintestinal architecture, but also in thedensity of cells that are closely associ-ated with the innate and adaptiveimmune response, such as goblet-pro-ducing cells, Paneth cells, DCs, and Band T lymphocytes. Furthermore,regional differences are found in thepresence and density of mucosal lym-phoid structures such as Peyer’s orcecal patches, or isolated lymphoidfollicles (16). It is likely that the lym-phoid tissue throughout the GIT playsan important role in localizedimmune responses to bacteria thatpopulate the respective compartment.Indeed, the development of the local-ized mucosa-associated immune sys-tem is only in part genetically deter-mined; it is also functionallydependent on the presence of the bac-terial microflora (17). In addition, dif-ferential complementary determiningregion-3 T cell receptor usage amongT cells in different regions of the colonprovides further support for localizedT cell immune responses within thelarge intestine (18). Interestingly, inHLA-B27 transgenic rats, the bacterialload in the cecum determines theseverity of mucosal inflammation, andthe development of colitis in T cellreceptor-α mutant mice can be pre-vented by removal of parts of thececum. These results suggest that themanipulation of bacteria and/or cor-responding mucosa-associated lym-phoid tissue in specific intestinal com-partments can influence the inductionof intestinal immunopathology (19,20). Localized innate and adaptive

mucosal immune responses may pro-vide an efficient response to therespective flora but restrict extensiveimmunopathology. In this regard, theIL-12p40 promoter transgenic miceproduced by Becker and coworkers (9)will be an excellent tool to study theinteraction between particular bacte-ria and the host immune system andhow this influences the localization ofthe immune response.

A dilemma for those using modelsystems in mice to study interactionsbetween the intestinal bacterial floraand the immune system is that,despite increasing control of hostgenetics, afforded by the use of inbredmice with defined genetic alterations,our knowledge about the compositionand function of the correspondingintestinal bacterial flora is limited tojust a few bacteria. Because of the greatvariety of housing conditions, theremay be large variations within the bac-terial flora that might influence theactivation state of the immune systemand the interpretation of experimentsin different laboratories. The furtheridentification of distinct intestinalcommensal and pathogenic bacteriaand their ability to colonize and invadedistinct compartments of the GIT willafford a better understanding of theinterplay among different bacteria andbetween bacteria and the host in phys-iology and disease.

AcknowledgmentsHolm H. Uhlig is supported by theEuropean Society of Clinical Micro-biology and Infectious Diseases andthe Deutsche Forschungsgemein-schaft. Fiona Powrie is supported bythe Wellcome Trust.

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