THEME
The Epithelial Cell Cytoskeleton and Intracellular Trafficking
V. Polarized compartmentalization of antigen processing and Toll-like receptor signaling in intestinal epithelial cells

Robert M. Hershberg

Corixa Corporation and Infectious Disease Research Institute, Seattle, Washington 98104


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
HLA CLASS II ANTIGEN...
POLARIZED SIGNALING OF TLRS...
REFERENCES

The intestinal epithelial cell (IEC) is exposed at the apical surface to a high concentration of foreign antigen and bacterial products capable of triggering inflammatory responses. Complex intracellular pathways of antigen trafficking and the polarized expression of immunologically active receptors provide additional means to regulate the inflammatory pathways in these cells. In the case of human leukocyte antigen (HLA) class II heterodimers, surface expression is highly restricted to the basolateral surface, and this also appears to be the case for Toll-like receptor 5 (TLR5) on polarized T84 human colon cancer cells. Processing of soluble antigen via HLA class II in IEC can occur following internalization from the apical surface but is highly inefficient. In addition, certain bacteria can facilitate the transport of flagellin (the ligand for TLR5) across an intact epithelium. Disruption of the tight junctions between IECs, allowing direct access of antigen and flagellin to the basolateral surface of the cell, dramatically affects the functional outcome HLA class II and TLR5 pathways.

cell polarity; intestinal epithelium; flagellin; lipopolysaccharide


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
HLA CLASS II ANTIGEN...
POLARIZED SIGNALING OF TLRS...
REFERENCES

THE IMMUNOLOGIC CHALLENGE faced along the mucosal surface of the gastrointestinal (GI) tract is extraordinary. In the colon, for example, the microbial burden is estimated to be 109 organisms present per milliliter of lumenal content. Across the vast majority of the surface area of the GI tract, a single layer of polarized epithelial cells separates this vast amount of antigenic material from the underlying gut-associated lymphoid tissue (GALT). In both anatomical and functional contexts, the structure of the intestinal epithelium poses a formidable barrier. Notably, the tight junctions that connect adjacent epithelial cells preclude passage of even small macromolecules via the paracellular route. In addition, epithelial cells produce copious amounts of mucous, and this, together with the "glycocalyx," limits both particulate antigen and bacterial contact with the apical surface of the cells. Furthermore, epithelial cells secrete antimicrobial peptides and proteins that limit bacterial colonization (4).

In this context, it is not surprising that much of the scientific focus on antigen trafficking and processing in the GI tract has been directed to the follicle-associated epithelial (FAE) structures, and so-called M cells, that overly aggregate lymphoid tissue within the mucosa. These modified epithelial cells have dramatically altered morphology compared with normal epithelium, less mucous associated with the apical surface, and they express various lectins not found on adjacent epithelial cells. Importantly, these cells have the capacity to rapidly "transcytose" particulate (and presumably other) antigens without significant cellular processing for delivery to bone marrow-derived antigen-presenting cells such as dendritic cells in the underlying "dome" structure. Indeed, the FAE and M cells are likely to play a prominent role in the induction of immune responses in the GALT and have been the topic of several recent reviews (23, 33).

Over the past decade, it has become increasingly apparent that "conventional" polarized intestinal epithelial cells [herein referred to as intestinal epithelial cells (IECs)] not only provide a critical barrier function but are also active in the modulation of mucosal immune responses. For example, IECs secrete and respond to a wide array of cytokines, chemokines, and other immunologically active molecules (9). In addition, IECs maintain intimate contact with various populations of interdigitating, bone marrow-derived lymphoid, and myeloid cells including T cells, dendritic cells, and polymorphonuclear lymphocytes (the latter being recruited to sites of inflammation). In this context, IECs also have been demonstrated to express human leukocyte antigen (HLA) class I, HLA class II, and a number of HLA class I-like antigen-presenting molecules, including the functional expression of MICA and MICB (15) and CD1d (34). Further still, IECs express a variety of immunologically relevant molecules on their surface, including several Toll-like receptors (TLRs) that play a critical role in innate immune responses.

An "immunologically poised" epithelium is a two-edged sword. Although potentially adaptive in the context of infection with a microbial pathogen such as Salmonella or Rotavirus, excessive immunonologic tone at the epithelial interface may contribute to the chronic mucosal inflammation that characterizes both Crohn's disease and ulcerative colitis [collectively, inflammatory bowel disease (IBD)]. Although IBD is very common, occurring in ~1:300 individuals in industrialized countries, the vast majority of people do not develop IBD, even in the face of massive antigenic challenge. A fundamental question in mucosal immunology relates to how the balance between responsiveness and nonresponsiveness at the epithelium is established and maintained. Clearly, the ability of the epithelium to maintain a modulated or dampened tone will depend on both intrinsic (tight junctions, mucous, defensins, regulatory T cell networks, etc.) and extrinsic (probiotic bacteria, helminth infection, etc.) factors.

A fundamental aspect of the regulation of immune responses at epithelial surfaces is likely to relate to the intrinsic polarity of the epithelial cells themselves. As recently reviewed (20), many immunologically relevant molecules are expressed in a strictly polarized manner, localized either to the apical or basolateral surface of the IECs. With the use of two examples of immunologically relevant molecules with demonstrated polarized surface expression, specifically of HLA class II antigens and TLR5, one can see how the topology of IECs provides a basis for unique mechanisms of immune regulation not seen in conventional, nonpolarized antigen-presenting cells, such as dendritic cells and macrophages.


    HLA CLASS II ANTIGEN PROCESSING AND PRESENTATION BY IEC
TOP
ABSTRACT
INTRODUCTION
HLA CLASS II ANTIGEN...
POLARIZED SIGNALING OF TLRS...
REFERENCES

CD4+ T cell responses are critical for both the establishment of oral tolerance and in several experimental models of IBD. Because the expression of major histocompatibility complex (MHC) class II antigens is required on an APC to stimulate CD4+ T cells, it is noteworthy that class II expression has been observed on IECs from human, mouse, and rat, with elevated levels consistently seen in the context of mucosal inflamation (3, 22, 27). Unfortunately, few in vivo data are available regarding specific regional differences (both crypt to villous and small intestine to colon) in the coexpression of class II antigens and other molecules (for example, invariant chain, li, and HLA-DM) essential for efficient class II processing in conventional antigen processing cell (APC). These additional molecules, which faciliate intracellular trafficking and antigenic peptide loading onto newly synthesized class II heterodimers, are typically induced along with class II at the transcriptional level following activation of the APC with interferon-gamma (7). Certainly, important regional differences will emerge with regard to the capacity of IECs to process and present antigen via HLA class II (and likely, HLA class I and class I-like molecules). Despite these anatomical considerations and the challenges associated with the long-term culture of primary intestinal epithelial cells, several groups have demonstrated the capacity of IECs to process and present antigen via HLA class II using a variety of in vitro models (19, 22, 25, 37). To date, the data suggest that IECs can process and present class II peptide epitopes to T cells and that the processing of specific epitopes shows a variable dependence on the expression of II and human HLA class II molecule (HLA-DM) in the IEC.

With the use of distinct in vitro models of polarized IECs transfected with HLA (or MHC) class II molecules, alone or in combination with the directed expression of II and/or HLA-DM, several groups have identified interesting features of class II processing and presentation relevant to the polarized phenotype of these cells (18, 25). Notably, the expression of HLA class II antigens in IECs appears to be restricted to the basolateral membrane. The highly polarized expression of class II molecules has also been reported in Madine-Darby canine kidney (MDCK) cells (36). Hence, antigen presentation in IECs occurs in a highly polarized manner, in which the IEC contacts T cells within the epithelium and, in limited context, T cells within the underlying lamina propria. Similar localization of HLA class I molecules and the beta 2-microglobulin-associated form of CD1d to the basolateral surface of IECs suggest that this highly polarized presentation of antigen by IECs to T cells will be a recurring theme.

Importantly, data from these same experimental systems indicate that the polarized surface from which antigen is internalized dramatically affects the functional outcome with regard to the generation of T cell epitopes. In our initial studies investigating the processing of tetanus toxoid (TT) in polarized T84 cells transfected with human HLA class II molecule (HLA-DR)B1*0401, we observed that processing of TT (with the readout being stimulation of a TT-specific, HLA-DRB1*0401-restricted T cell hybridoma specific for a single peptide epitope) was more efficient when initiated from the apical surface (18). When these same T84 transfectants were engineered to coexpress II and HLA-DM in addition to HLA-DR, the overall efficiency of processing of TT dramatically increased, and the cells were capable of processing TT from both the apical and basolateral surfaces. With the use of T cell clones as a readout of the generation of specific peptide epitopes, the data suggested that the apical and basolateral processing pathways generated overlapping, but distinct epitopes from TT. With the use of Caco-2 cells transfected with the murine MHC II molecule I-Ak, Lopes and co-workers (25) observed selective processing of hen egg lysozyme only from the basolateral surface. These data are consistent with our recent experience using an additional antigen, human serum albumin, in our T84 system, which we have observed is preferentially processed from the basolateral surface (D. Cho and R. Hershberg, unpublished observations).

What might help explain the apparent discrepancy with regard to polarized class II antigen processing, with some epitopes generated following internalization from the apical surface and others from the basolateral surface? In part, the answers depend on the complex pattern of endosomal trafficking from the respective polarized surfaces of epithelial cells, which has been best studied in the MDCK cell line (for a series of excellent reviews, see Refs. 31, 32). Consider, for instance, the variable fate of a potentially immunogenic protein antigen internalized from the apical surface. After internalization into the apical early endosomal (AEE) compartment(s), the antigen might traffic into a series of late endosomal/lysosomal compartments. Whether this pathway, and the peptide antigens that would be generated due to exposure of the antigen to very low pH and proteases en route, accesses the nascent class II processing pathway in polarized cells is not known. The nascent pathway refers to the newly synthesized class II molecules trafficking, with invariant chain, from the trans-Golgi network into a series of late endosomal/early lysosomal compartments that contain HLA-DM and are often described as MHC class II compartments. We favor the hypothesis, based on the available data and on "teleological" grounds, that most antigens would not intersect the class II pathway and would not be immunogenic following apical endocytosis (Fig. 1).


View larger version (32K):
[in this window]
[in a new window]
 
Fig. 1.   Working model of human leukocyte antigen (HLA) class II antigen processing in polarized intestinal epithelial cells (IECs). With an intact mucosal barrier, soluble antigen is internalized from the apical surface. Most material entering the apical endosomal pathway is degraded and does not access the conventional class II loading compartment [major histocompatibility complex (MHC) class II compartment (MIIc)]. Select antigens with apical receptors may access a transcytotic pathway, which may intersect class II molecules recycled from the basolateral surface. When barrier function is impaired (for example, due to local IFN-gamma production), lumenal antigen gains access to the basolateral surface via paracellular transit, where it is internalized and processed more efficiently via class II. Protein fragments and/or peptides internalized from the basolateral surface may also access internalized/recycled class II (not shown). Antigen presentation via class II is highly restricted due to expression of the heterodimer only on the basolateral surface.

Alternatively, trafficking from the AEE can proceed to a "common endosomal" compartment (40). In this context, "common" implies access to this compartment from a certain fraction of early endosomes derived from the apical and basolateral surface. Again, the potential intersection of the nascent class II pathway with this common endosomal compartment has not been elucidated. It is tempting to speculate, however, that antigen in this compartment may access class II molecules that are "recycled" from the basolateral surface. Bakke and Nordeng (2) demonstrated the recycling of class II molecules in MDCK cells, and we have characterized the recycling of class II molecules in T84 cells (B. Levi and R. Hershberg, unpublished data). This "alternative" class II pathway functions independent of invariant chain and HLA-DM expression and may explain the limited amount of processing of TT seen from the apical surface in our in vitro model. What factors might modulate the fate of antigen trafficking from the apical surface into this pathway? Conceivably, apically expressed molecules such as the polymeric Ig receptor (24) and FcRn (8), which use this pathway for "transcytosis," may function in part to bind certain antigens from the lumen of the intestinal tract and "deliver" them to an alternate class II pathway for limited antigen processing and presentation. In addition, the more efficient internalization of certain antigens via receptor-mediated endocytosis (such as TT via the ganglioside GT-1b) compared with fluid-phase endocytosis may result in a quantitative advantage for an antigen and/or antigen peptide avoiding a purely "degradative" pathway following apical endocytosis. These ideas remain purely speculative at this point.

As noted above, several groups have consistently seen that multiple protein antigens can be processed and presented via class II when the protein antigen is internalized from the basolateral surface. These data, together with the work of Bakke and Nordeng (2) evaluating the compartmentalization of class II in MDCK, suggest a functional intersection between the nascent class II pathway and the basolateral endosomal system in polarized IECs, in a manner analogous to class II antigen processing and presentation in conventional APC such as dendritic cells. One intriguing possibility is that the distinct pattern of antigen trafficking from the respective polarized surfaces (i.e., via the apical or basolateral endosomal compartments) allows selective exposure to specific proteases en route to a class II loading compartment. To date, the exact nature of the class II compartment(s) and the proteases involved in class II processing in polarized IEC remain poorly defined. An additional consideration is that antigen internalized from the basolateral surface could also access class II recycled from the basolateral membrane. In this manner, processing of antigen and loading of peptide onto class II would occur in less acidified, early basolateral endosomes before transit back to the basolateral surface. This pathway may be particularly relevant to class II processing by IEC, especially when the class II expression may be limited (in the absence of extensive inflammation) and when the antigen itself may be denatured or fragmented following "preprocessing" during its transit in the lumen of the GI tract.

In this context, it is important to consider how an intact protein or peptide antigen gains access to the basolateral surface of an intact epithelium when tight junctions preclude paracellular trafficking of even small molecules. The answer lies in the dynamic nature of the tight junction itself and the fact that it is regulated by a variety of mediators including proinflammatory cytokines such as interferon-gamma , which dramatically increases paracellular permeability (26). We still favor the hypothesis that with the exception of a limited amount of class II antigen processing that occurs from the apical surface, most of the antigen processing function of IECs occurs in the context of impaired barrier function (i.e., inflammation). Under "normal" circumstances, the sequestration of antigen, and/or antigenic peptides, from the class II molecules and class II processing pathway across the polarized cell offers an additional layer of "protection" from an unnecessary inflammatory response from the intestinal epithelium. It is not unreasonable to suggest that under pathological circumstances, when the balance shifts toward uncontrolled inflammation (for reasons that remain obscure), the IEC functions as an accessory APC, stimulating mucosal CD4+ T cell responses. Conceivably, "pathogenic epitopes" within an antigen (that normally elicits no significant response when processed apically) may be unmasked via internalization and trafficking from the basolateral surface and presented to underlying CD4+ T cells.

There is a large number of class II positive cells in the intestinal mucosa other than IECs. As a result, it cannot be overemphasized that there are no clear in vivo data to support a role for IECs in stimulating mucosal CD4+ T cell responses. To specifically address this issue, we have recently generated several lines of genetically engineered mice in which class II expression is restricted to the intestinal epithelium (L. Maggio-Price, A. Burich, and R. Hershberg, unpublished observations). Experiments directed toward unraveling the role of IECs in stimulating mucosal CD4+ T cells using this newly established mouse model are ongoing.


    POLARIZED SIGNALING OF TLRS IN IECS
TOP
ABSTRACT
INTRODUCTION
HLA CLASS II ANTIGEN...
POLARIZED SIGNALING OF TLRS...
REFERENCES

Systemic infection with pathogenic microorganisms typically results in B cell (i.e., antibody mediated) and/or T cell (cytotoxic and/or "helper") immune responses with sustained and/or repeated immunologic challenge. It has been recognized for decades that cells other than B and T cells are capable of responding to microbial infection. More recently, a paradigm is emerging in which these more proximal "innate" immune responses provide both early protection and signals required to initiate the "acquired" B and T cell responses that appear later in the course of infection. A crucial component of innate immunity appears to be mediated by TLRs, transmembrane receptors homologous to the Drosophila melanogaster protein Toll, which functions to protect the fly against fungal and other microbial pathogens (reviewed in Ref. 38).

To date, the genes encoding 10 distinct members of the TLR family (TLR1-10) have been identified in humans. Consistent with their role in protective immunity against microbial pathogens, each TLR appears to respond (alone or in combination) to specific microbial structures that represent conserved patterns (so-called pathogen-associated molecular patterns) between multiple species within a given group of organisms. In the cases of TLR2 (lipoproteins, peptidoglycan, etc.), TLR3 (poly I:C), TLR4 [lipopolysaccharide (LPS)], TLR5 (flagellin), TLR7 (nucleotide analogs such as imiquamod), and TLR9 (CpG motifs in bacterial DNA), the ligand(s) responsible for stimulating the receptor have been identified. Although differences in signaling pathways between the different TLRs are emerging, the basic premise to date has been that activation of TLRs stimulates the induction of proinflammatory (and other) genes via the activation of NF-kappa B pathways (30).

TLRs are widely expressed, most notably by bone marrow-derived cells, such as dendritic cells and monocytes/macrophages. As noted, IECs are exposed to extremely high concentrations of both intact bacteria and bacterial products and are poised to provide a "first-line" innate immune response to microbial invasion. In this context, it is perhaps not surprising that several TLRs appear to be expressed in IECs. In a manner similar to the expression of HLA class II on IECs, the expression of TLRs at the epithelial surface poses a significant risk in the development of exaggerated or chronic inflammatory responses within the intestinal mucosa. In a sea of intact bacteria and bacterial products, one must ask the question: how do IECs expressing TLRs regulate their ability to respond (or not respond)? The answer, in part, lies in the intrinsic polarity of IECs themselves.

The best example of the polarized regulation of TLR activation in IECs is with TLR5. McDermott et al. (28) reported that flagellin, the monomeric subunit of bacterial flagella, is capable of stimulating monocytes to secrete a variety of proinflammatory cytokines. The molecular mechanism associated with this activity was clarified when Hayashi et al. (17) identified flagellin as a ligand for TLR5. Concurrently, Gewirtz, Madara, and co-workers were investigating a so-called "proinflammatory factor" (PIF) derived from Salmonella-infected cells that stimulated the expression of interleukin-8 when applied to the basolateral but not the apical surface of polarized T84. PIF was biochemically purified and determined to be flagellin itself (12). Furthermore, the highly polarized expression of TLR5 on the basolateral surface of the T84 colon carcinoma cell line used in the study provided the molecular basis for the polarized activation observed (11). It remains to be determined whether the highly restricted pattern of basolateral TLR5 will be observed in other polarized cell lines and/or tissue sections from normal or inflamed intestine. Although the current data suggest that flagellin from a variety of pathogenic and nonpathogenic bacterial species appear capable of stimulating IECs to similar degrees, it is conceivable and (given the diversity of the intestinal microflora and the polymorphic nature of flagellin itself) perhaps likely that individual flagellins will emerge with varying capacity to activate cells via TLR5. There is certainly precedent for this concept with LPS signaling via TLR4 (16).

A model is emerging in which only flagellin capable of crossing the intestinal barrier would be capable of provoking an inflammatory response (Fig. 2). Just as an intact epithelial barrier greatly restricts the amount of lumenal antigen that can be internalized from the basolateral surface, access of flagellin to the basolateral surface via the paracellular route would be severely limited under normal (i.e., noninflamed) conditions. In the context of mucosal inflammation or other alterations in barrier function, flagellin would gain access to the basolateral surface of IECs via permeable tight junctions and would be able to activate basolateral TLR5 (and TLR5 expressed on monocytes/dendritic cells in the lamina propria). This would have the effect of promoting (or exacerbating) an inflammatory response in the mucosa. Interestingly, it appears that certain flagellated bacteria are capable of "translocating" flagellin across polarized IECs and are still capable of stimulating TLR5. Although it is clear that purified flagellin (i.e., in the absence of intact bacteria) cannot use the trancytotic pathway for delivery to the basolateral surface (or basolateral endosomes that might contain functional, internalized TLR5), details regarding the precise compartmentalization and intracellular trafficking of flagellin within polarized IECs remain an area of active investigation. It is tempting to speculate that the trafficking of flagellin may intersect class II or other antigen-processing pathways. Consistent with this hypothesis is the observation that CD4+ T cell responses against flagellin appear to be important in a mouse model of Salmonella infection (29). Whether class II positive IECs or other APCs in the lamina propria (or draining lymph nodes) are required to process epitopes from flagellin is currently not known.


View larger version (27K):
[in this window]
[in a new window]
 
Fig. 2.   Working model of Toll-like receptor (TLR) compartmentalization and signaling in polarized IECs. Intact bacteria and free lipopolysaccharide (LPS) and flagellin contact the apical surface of polarized IECs. Invasive bacteria are internalized and release LPS (and flagellin) into intracellular, vesicular compartments. In 1 model, TLR4 is expressed in the Golgi apparatus and responds to internalized LPS. TLR5 expression is restricted to the basolateral surface and responds to flagellin that is transcytosed and/or accessible to the basolateral surface in the context of permeable tight junctions (for example, inflammation). On the basis of studies from Gerwitz et al. (11, 12) and Hornef et al. (21).

The growing literature on the functional expression of TLR4 in polarized IECs is less clear than with TLR5 but is certainly provocative. Cellular responsiveness to LPS requires the expression of TLR4 in combination with MD-2 (35) and, to a variable extent, CD-14 (39). Although current data suggest that MD-2 and CD-14 are not abundantly expressed in IECs under most conditions (1), several groups have reported expression of TLR4 in IECs (5, 21) and functional responses in these cells to LPS (6, 21). Given the controversy regarding the surface expression of TLR4 on IECs despite their ability to respond to LPS, a recent report from Hornef et al. (21) is particularly interesting. With the use of a murine small intestinal cell line (m-ICc12), these investigators observed the intracellular localization of TLR4 in the Golgi apparatus of these cells in the absence of surface expression (21), in marked contrast to the abundant surface expression seen in monocytes and dendritic cells. Interestingly, the intracellular TLR4 colocalized with internalized LPS. These observations are consistent with those of Garcia-del Portillo et al. (10) showing the release of LPS into vesicular structures within infected epithelial cells. Although the molecular events associated with the cytoplasmic TLR4 pathway remain to be elucidated, these observations provide another example of IECs exploiting polarized compartmentalization to separate inflammatory ligands present in excess at the apical surface from receptors that remain capable of triggering inflammatory responses.

In summary, polarized compartmentalization is important in antigen processing and in TLR responsiveness by IECs. In the case of HLA class II pathways, antigen processing via the apical surface is limited, except when facilitatated by specific receptors (such as the polymeric immunoglobulin receptor), which direct antigens toward a transcytotic endosomal pathway. In addition, antigen presentation via class II is highly restricted to the basolateral surface due to polarized surface expression of the class II heterodimer itself. In the case of TLR signaling, receptors are compartmentalized in a manner that separates them from the abundant ligands at the apical surface. In both situations, alterations in barrier function result in paracellular transport of antigen and/or TLR ligands that greatly enhance their proinflammatory capacity via enhanced basolateral uptake and processing or access to basolateral receptors. Clearly, the polarized phenotype of IECs offers a crucial means to regulate its potentially proinflammatory responses to foreign antigens and bacterial products.


    ACKNOWLEDGEMENTS

This work was supported by a grant from the National Institute of Diabetes and Digestive and Kidney Diseases (R01 DK-56204).


    FOOTNOTES

Address for reprint requests and other correspondence: R. Hershberg, Corixa Corp., 1124 Columbia St., Seattle, WA 98104 (E-mail. hershberg{at}corixa.com).

10.1152/ajpgi.00208.2002


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
HLA CLASS II ANTIGEN...
POLARIZED SIGNALING OF TLRS...
REFERENCES

1.   Abreu, MT, Vora P, Faure E, Thomas LS, Arnold ET, and Arditi M. Decreased expression of Toll-like receptor-4 and MD-2 correlates with intestinal epithelial cell protection against dysregulated proinflammatory gene expression in response to bacterial lipopolysaccharide. J Immunol 167: 1609-1616, 2001[Abstract/Free Full Text].

2.   Bakke, O, and Nordeng TW. Intracellular traffic to compartments for MHC class II peptide loading: signals for endosomal and polarized sorting. Immunol Rev 172: 171-87, 1999[ISI][Medline].

3.   Bland, PW, and Whiting CV. Induction of MHC class II gene products in rat intestinal epithelium during graft-versus-host disease and effects on the immune function of the epithelium. Immunology 75: 366-371, 1992[ISI][Medline].

4.   Boman, HG. Innate immunity and the normal microflora. Immunol Rev 173: 5-16, 2000[ISI][Medline].

5.   Cario, E, Brown D, McKee M, Lynch-Devaney K, Gerken G, and Podolsky DK. Commensal-associated molecular patterns induce selective Toll-like receptor-trafficking from apical membrane to cytoplasmic compartments in polarized intestinal epithelium. Am J Pathol 160: 165-173, 2002[Abstract/Free Full Text].

6.   Cario, E, Rosenberg IM, Brandwein SL, Beck PL, Reinecker HC, and Podolsky DK. Lipopolysaccharide activates distinct signaling pathways in intestinal epithelial cell lines expressing Toll-like receptors. J Immunol 164: 966-972, 2000[Abstract/Free Full Text].

7.   Chang, CH, and Flavell RA. Class II transactivator regulates the expression of multiple genes involved in antigen presentation. J Exp Med 181: 765-767, 1995[Abstract].

8.   Dickinson, BL, Wu Z, Ahouse JC, Zhu X, Simister NE, Blumberg RS, and Lencer WI. Bidirectional FcRn-dependent IgG transport in a polarized human intestinal epithelial cell line. J Clin Invest 104: 903-911, 1999[Abstract/Free Full Text].

9.   Eckmann, L, and Kagnoff MF. Cytokines in host defense against Salmonella. Microbes Infect 3: 1191-1200, 2001[ISI][Medline].

10.   Garcia-del Portillo, F, Stein MA, and Finlay BB. Release of lipopolysaccharide from intracellular compartments containing Salmonella typhimurium to vesicles of the host epithelial cell. Infect Immun 65: 24-34, 1997[Abstract].

11.   Gerwirtz, AT, Navas TA, Lyons S, Godowski PJ, and Madara JL. Cutting edge: bacterial flagellin activates basolaterally expressed TLR5 to induce epithelial proinflammatory gene expression. J Immunol 167: 1882-1885, 2001[Abstract/Free Full Text].

12.   Gerwirtz, AT, Simon PO, Jr, Schmitt CK, Taylor LJ, Hagedorn CH, O'Brien AD, Neish AS, and Madara JL. Salmonella typhimurium translocates flagellin across intestinal epithelia, inducing a proinflammatory response. J Clin Invest 107: 99-109, 2001[Abstract/Free Full Text].

15.   Groh, V, Bauer S, and Spies T. Recognition of stress-induced MHC molecules by intestinal epithelial gammadelta T cells. Science 279: 1737-1740, 1998[Abstract/Free Full Text].

16.   Hajjar, AM, Ernst RK, Tsai JH, Wilson CB, and Miller SI. Human Toll-like receptor 4 recognizes host-specific LPS modifications. Nat Immun 3: 354-359, 2002[ISI].

17.   Hayashi, F, Smith KD, Ozinsky A, Hawn TR, Yi EC, Goodlett DR, Eng JK, Akira S, Underhill DM, and Aderem A. The innate immune response to bacterial flagellin is mediated by Toll-like receptor 5. Nature 410: 1099-1103, 2001[ISI][Medline].

18.   Hershberg, RM, Cho DH, Youakim A, Bradley MB, Lee JS, Framson PE, and Nepom GT. Highly polarized HLA class II antigen processing and presentation by human intestinal epithelial cells. J Clin Invest 102: 792-803, 1998[Abstract/Free Full Text].

19.   Hershberg, RM, Framson PE, Cho DH, Lee LY, Kovats S, Beitz J, Blum JS, and Nepom GT. Intestinal epithelial cells utilize two distinct pathways for HLA class II antigen processin. J Clin Invest 100: 204-215, 1997[Abstract/Free Full Text].

20.   Hershberg, RM, and Mayer LF. Antigen processing and presentation by intestinal epithelial cells - polarity and complexity. Immunol Today 21: 123-128, 2000[ISI][Medline].

21.   Hornef, MW, Frisan T, Vandewalle A, Normark S, and Richter-Dahlfors A. Toll-like receptor 4 resides in the Golgi apparatus and colocalizes with internalized lipopolysaccharide in intestinal epithelial cells. J Exp Med 195: 559-570, 2002[Abstract/Free Full Text].

22.   Kaiserlian, D, Vidal K, and Revillard JP. Murine enterocytes can be present soluble antigen to specific class II-restricted CD4+ T cells. J Immunol 19: 1513-1516, 1989.

23.   Kraehenbuhl, JP, and Neutra MR. Epithelial M cells: differentiation and function. Annu Rev Cell Dev Biol 16: 301-332, 2000[ISI][Medline].

24.   Lamm, ME. Current Concepts in Mucosal Immunity. IV. How epithelial transport of IgA antibodies relates to host defense. Am J Physiol Gastrointest Liver Physiol 274: G614-G617, 1998[Abstract/Free Full Text].

25.   Lopes, LM, HE, Anstee Q, O'Neil D, Katz DR, and Chain BM. Vectorial function of major histocompatibility complex class II in a human intestinal cell line. Immunology 98: 16-26, 1999[ISI][Medline].

26.   Madara, JL, and Stafford J. Interferon-gamma directly affects barrier function of cultured intestinal epithelial monolayers. J Clin Invest 83: 724-727, 1989[ISI][Medline].

27.   Mayer, L, Eisenhardt D, Salomon P, Bauer W, Plous R, and Piccinini L. Expression of class II molecules on intestinal epithelial cells in humans. Differences between normal and inflammatory bowel disease. Gastroenterology 100: 3-12, 1991[ISI][Medline].

28.   McDermott, PF, Ciacci-Woolwine F, Snipes JA, and Mizel SB. High-affinity interaction between gram-negative flagellin and a cell surface polypeptide results in human monocyte activation. Infect Immun 68: 5525-5529, 2000[Abstract/Free Full Text].

29.   McSorley, SJ, Cookson BT, and Jenkins MK. Characterization of CD4+ T cell responses during natural infection with Salmonella typhimurium. J Immunol 164: 986-993, 2000[Abstract/Free Full Text].

30.   Medzhitov, R. Toll-like receptors and innate immunity. Nature Rev Immunol 1: 135-145, 2001[Medline].

31.   Mostov, KE, and Altschuler Y. Membrane traffic in polarized epithelial cells. Curr Opin Cell Biol 12: 483-490, 2000[ISI][Medline].

32.   Nelson, WJ, and Yeaman C. Protein trafficking in the exocytic pathway of polarized epithelial cells. Trends Cell Biol 11: 483-486, 2001[ISI][Medline].

33.   Neutra, MR, and Kraehenbuhl JP. Collaboration of epithelial cells with organized mucosal lymphoid tissues. Nat Immun 2: 1004-1009, 2001[ISI].

34.   Saubermann, LJ, BP, De Jong YP, Pitman RS, Ryan MS, Kim HS, Exley M, Snapper S, Balk SP, Hagen SJ, Kanauchi O, Motoki K, Sakai T, Terhorst C, Koezuka Y, Podolsky DK, and Blumberg RS. Activation of natural killer T cells by alpha-galactosylceramide in the presence of CD1d provides protection against colitis in mice. Gastroenterology 119: 119-128, 2000[ISI][Medline].

35.   Shimazu, R, Akashi S, Ogata H, Nagai Y, Fukudome K, Miyake K, and Kimoto M. MD-2, a molecule that confers lipopolysaccharide responsiveness on Toll-like receptor 4. J Exp Med 189: 1777-1782, 1999[Abstract/Free Full Text].

36.   Simonsen, A, Bremnes B, Roe M, Prydz K, and Bakke O. Sorting of MHC class II molecules and the associated invariant chain (li) in polarized MDCK cells. J Cell Sci 110: 597-609, 1997[Abstract/Free Full Text].

37.   Telega, GW, Baumgart DC, and Carding SR. Uptake and presentation of antigen to T cells by primary colonic epithelial cells in normal and diseased states. Gastroenterology 119: 1548-1559, 2000[ISI][Medline].

38.   Tzou, P, De Gregorio E, and Lemaitre B. How Drosophila combats microbial infection: a model to study innate immunity and host-pathogen interactions. Curr Opin Microbiol 5: 102-110, 2002[ISI][Medline].

39.   Ulevitch, RJ, and Tobias PS. Recognition of gram-negative bacteria and endotoxin by the innate immune system. Curr Opin Immunol 11: 19-22, 1999[ISI][Medline].

40.   Wang, E, Aroeti B, Chapin SJ, Mostov KE, and Dunn KW. Apical and basolateral endocytic pathways of MDCK cells meet in acidic common endosomes distinct from a nearly-neutral apical recycling endosome. Traffic 1: 480-493, 2000[ISI][Medline].


Am J Physiol Gastrointest Liver Physiol 283(4):G833-G839
0193-1857/02 $5.00 Copyright © 2002 the American Physiological Society