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Published online before print May 13, 2005
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* INSERM-U404, CERVI, IFR128 BioSciences Lyon-Gerland, Lyon, France;
EMI-0212 INSERM, Faculté Necker-Enfants Malades, France; and
Institut Cochin, INSERM U-567, UMR CNRS 8104, IFR 116 Universite Paris V, France
1 Correspondence: INSERM-U404, CERVI-IFR128 BioSciences Lyon-Gerland, 21 Avenue Tony Garnier, 69365 Lyon CX 07, France. E-mail: kaiserlian{at}cervi-lyon.inserm;fr
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONCOMMENSAL BACTERIA IN MUCOSAL...DC IN INFECTION, INFLAMMATION,...TR CELLS IN MUCOSAL...NOVEL ROUTES AND ADJUVANTS...MUCOSAL IMMUNITY AGAINST HIV...REFERENCES |
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Key Words: commensal bacteria dendritic cells • regulatory T cells • vaccines • HIV
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONCOMMENSAL BACTERIA IN MUCOSAL...DC IN INFECTION, INFLAMMATION,...TR CELLS IN MUCOSAL...NOVEL ROUTES AND ADJUVANTS...MUCOSAL IMMUNITY AGAINST HIV...REFERENCES |
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COMMENSAL BACTERIA IN MUCOSAL IMMUNITY |
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TOPABSTRACTINTRODUCTIONCOMMENSAL BACTERIA IN MUCOSAL...DC IN INFECTION, INFLAMMATION,...TR CELLS IN MUCOSAL...NOVEL ROUTES AND ADJUVANTS...MUCOSAL IMMUNITY AGAINST HIV...REFERENCES |
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Andrew McPherson (Zurich) studied the gut immune response in specific, pathogen-free mice challenged intragastrically with 109 commensal Enterobacter cloacae. Small numbers of live bacteria can penetrate via Peyer’s patches (PP) and are loaded onto DC, where they are retained for several days. Migration of DC loaded with commensals to mesenteric lymph nodes (MLN) results in selective induction of secretory IgA by T-dependent and T-independent mechanisms, which in turn, limit translocation of commensals across the mucosa. Rapid and efficient killing of commensal bacteria by macrophages in MLN prevents their migration outside the mucosal immune system and preserves systemic ignorance. Accordingly, IgG response to commensals does not occur unless MLN are removed, or bacteria are deliberately injected intravenously (i.v.). This led to proposing that the MLN form an efficient filter limiting the commensal-specific IgA response to the mucosa and ensuring systemic ignorance to the flora [7 8 9 ].
Dana Philpott (Paris, France) gave an overview about innate TLR and NOD receptor families, which are involved in sensoring commensals and pathogenic bacteria [10 ], and discussed the mechanisms at the epithelial interface, which allow the host to discriminate between pathogens and harmless commensal bacteria. TLR recognize pathogen-associated molecular patterns in the extracellular environment, and Nod1 [caspase recruitment domain 4 (CARD4)] and Nod2 (CARD15) are involved in cytosolic surveillance. Differential compartimentalization of these receptors seems to explain that bacteria remaining in the intestinal lumen cannot be recognized by NOD receptors, unless components thereof can translocate cell-wall components inside the cell. Thus, Nod1, constitutively expressed in intestinal epithelial cells and a receptor for a peptidoglycan derivative characteristic of Gram-negative bacteria, is implicated in the detection of invasive Gram-negative bacterial infection such as Shigella flexneri but can also induce an innate immune response to Helicobacter pylori. This bacterium remains extracellular but has a type IV secretion apparatus able to inject the peptidoglycan within the epithelial cells [11 , 12 ].
An alternative strategy developed by the host to avoid inappropriate triggering of intestinal inflammation in response to the microbiota might be to activate inhibitory pathways, exerting a strict retrocontrol. Pierre Desreumaux (Lille, France) showed that peroxisome proliferator-activated receptor (PPAR)-
is one good candidate for this function. In animal models of colitis, intestinal inflammation is triggered by commensal bacteria and dampened by synthetic PPAR- agonists, an effect ascribed to inhibition of nuclear factor-
B and mitogen-activated protein kinase signaling pathways in intestinal epithelial cell lines [13
14
15
]. As strong expression of PPAR- is induced via the TLR-4 pathway in normal, colonic epithelial cells upon bacterial colonization, activation of PPAR- might be one important retrocontrol mechanism to prevent inappropriate, proinflammatory responses to the commensal flora [16
]. Along these lines, patients with ulcerative colitis have an abnormally low expression of PPAR- in their colonic epithelium, which may contribute to disease [16
]. Furthermore, that 5-aminosalicic acid (5-ASA), commonly used as a therapy in ulcerative colitis, is a potent agonist of PPAR- provides a rationale for the therapeutic use of 5-ASA and supports the value of PPAR- as a therapeutic target in chronic intestinal inflammatory diseases.
Several presentations from posters provided complementary insights into the interactions between the host and its microflora. Markus Heimesaat (Oliver Liesenfeld, Berlin, Germany) described the deleterious impact of the flora on the severe ileitis induced by peroral infection with Toxoplasma gondii. Frank Ruemmele (Paris, France) observed that the Nod2 ligand, muramyl-dipeptide (a peptidoglycan derivative common to Gram-negative and -positive bacteria), can gain access into colonic epithelial cell lines and triggers a proinflammatory response without being injected intracellularly. They suggested that activation of Nod2 might induce the release of bactericidal peptides, able in turn to enhance the epithelial barrier and to avoid further proinflammatory contacts. Jean-Christophe Bambou (N. Cerf-Bensussan, Paris, France) showed that the TLR-5 pathway can be activated from the apical side of enterocytes ex vivo in normal murine ileum by intraluminal flagellin derived from a commensal Escherichia coli strain [17 ].
A large number of reports documented the anti-inflammatory and antiallergic properties of probiotics, in vitro and in vivo. Meng-Tsung Tien (Philippe Sansonetti, Paris, France) observed that pretreatment of intestinal epithelial cells with Lactobacillus casei prevented up-regulation of proinflammatory genes in response to bacterial or cytokine stimulation as a result of the blockade of proteasome and inhibitory B degradation, as previously observed with nonvirulent salmonella strains. Sandrine Menard (Martine Heyman, Paris, France) showed that Bifidobacterium breve releases a low molecular compound able to cross the epithelial layer, which inhibits lipopolysaccharide (LPS)-induced secretion of tumor necrosis factor 
(TNF-) by monocytes [18
]. In vivo studies documented the ability of probiotics to modulate T cell-mediated allergic responses. Ludivine Chapat (Dominique Kaiserlian, Lyon, France) demonstrated the in vivo anti-inflammatory effect of L. casei oral regimen in a mouse model of T cell-mediated, allergic contact dermatitis. Daily oral administration of the probiotic reduced hapten-specific skin inflammation by decreasing the priming and/or expansion of specific interferon- (IFN-)-producing CD8+ effector cells [19
]. This effect was dependent on the presence of CD4+ T cells and was associated with enhanced production of interleukin (IL)-10 by CD4+CD25+ TR cells, suggesting that L. casei may be acting by triggering the function of TR cells. Probiotic lactic acid bacteria may also induce immune deviation of allergic responses, as reported by Catherine Daniel (Annick Mercenier, Lille, France) in a mouse model of T helper cell type 2 (Th2) allergy to birch pollen, in which recombinant lactic acid bacteria expressing the major allergen Bet v 1 induce a specific Th1-biased, humoral response. Finally, Yan Zizka (Ludmilla Prokesova, Prague, Czeck Republic) reported that colonization of human newborns from allergic mothers with a probiotic strain of E. coli reduced the level of cytokines [IFN-, IL-13, and transforming growth factor-ß (TGF-ß)] detected in the feces to the levels observed in infants born from nonallergic mothers. The mechanism of action of probiotics and their potential use in humans for maintaining health or treating chronic allergic and inflammatory disease by modulating the mucosal immune system are new, challenging areas of research.
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DC IN INFECTION, INFLAMMATION, AND TOLERANCE |
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TOPABSTRACTINTRODUCTIONCOMMENSAL BACTERIA IN MUCOSAL...DC IN INFECTION, INFLAMMATION,...TR CELLS IN MUCOSAL...NOVEL ROUTES AND ADJUVANTS...MUCOSAL IMMUNITY AGAINST HIV...REFERENCES |
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Paola Ricciardi-Castagnoli (Milan, Italy) provided an interpretation of DC plasticity at the genomic level by developing tightly controlled, functional genomic analysis of more than 600 DC genes to monitor the most relevant kinetic changes of their expression after interaction with bacteria, helminthes, or parasites [20 ]. These studies revealed that sensoring of most micro-organisms by DC resulted in the rapid production of IL-2 by DC, suggesting a primary role of DC in activation of innate natural killer (NK) cell responses but also of acquired CD8+ T cell immunity [21 , 22 ].
Mucosal DC play a key role for induction of adaptive mucosal immune responses but also contribute to local and systemic tolerance to dietary or respiratory antigens. Mucosal DC, localized in organized lymphoid organs, such as PP and MLN draining the intestine or mediastinal lymph nodes draining the lung, are responsible for induction of protective immune responses. However, the function of DC, diffusely distributed in mucosal tissues such as the lamina propria (LP) of the intestine or lung or forming a network of epithelial Langerhans-like cells in the trachea, bronchus, as well the buccal mucosa, vagina, and rectum, is less clear. Allan Mowat (Glasgow, UK) provided an overview of the different subsets of intestinal DC with particular emphasis on the phenotype and function of LP CD11c+ DC (LP-DC). Mouse LP-DC comprise primarily CD11b+CD8– DC and two minor subsets, including a CD11clo subset reminiscent of pDC and CD11b–CD8– DC.
LP-DC were responsible for in vivo uptake of orally administered ovalbumin (OVA). LP-DC were able to present in vivo-loaded OVA to specific T cell receptor (TcR) transgenic T cells in vitro and in vivo and induced partial suppression of an OVA-specific DTH response. Although LP-DC constitutively produced IL-10 and type I IFN (but not IL-12), they did up-regulate accessory molecules in response to LPS, indicating that they can respond partially to proinflammatory signals and thus are not inherently tolerogenic.
Studies in recent years have documented that DC play an important role in the control allergic and inflammatory diseases. Pat Holt (Perth, Australia) demonstrated that DC could regulate the effector function of pathogenic T cells in the upper respiratory tract during the late-phase response in asthma. Studies in aerosol-challenged, sensitized rats revealed that DC are recruited rapidly into the tracheal epithelium and cluster in situ with memory T cells. This resulted in functional DC maturation and concomitant T cell activation, leading to enhanced airway hyper-reactivity, which terminated upon migration of airway mucosa DC to draining lymph nodes [23 ]. It is interesting that upon repeated allergen exposure, CD4+CD25+ T cells appeared after the late-phase response and blocked DC activation in the airway mucosa, suggesting that CD4+CD25+ TR cells require persistent exposure to the aeroallergen to dampen the late-phase response in asthma.
Although numerous studies have emphasized the tolerogenic potential of DC, there are, as yet, only limited studies about the DC subsets involved in mucosal tolerance. Anne Goubier (D. Kaiserlian, Lyon, France) documented the in vivo suppressive function of liver pDC in models of oral and i.v. tolerance. These studies revealed that the liver is highly enriched in CD11C+ DC, among which functional type I IFN-producing pDC constituted the major lymphoid DC subset. Liver pDC were the only subset of liver DC with intrinsic, suppressive properties in vivo, inasmuch as only transfer of pDC (but not of myeloid DC, CD8+DC, or NK/DC) at the time of skin sensitization with antigen completely prevented in vivo priming of specific CD8+ T cell effectors mediating contact hypersensitivity (CHS). Moreover, in vivo antibody depletion of pDC abrogated hapten-specific CD8+ T cell tolerance induced by i.v. or intragastric administration of the antigen before skin sensitization. Thus, liver pDC are the main subset of liver DC endowed with intrinsic, tolerogenic properties and may contribute to orally induced tolerance by inhibiting priming of CD8+ effector T cell responses in lymphoid organs.
Selected oral presentation from posters highlighted the in vivo dynamics and function of mucosal tissue DC. To investigate the relative contribution of M cells and LP-DC in pathogen uptake, Alexander Eberhard (Steffan Jung, Weizmann Institute, Israel, and CNRS-BioMerieux, Lyon, France) performed in situ imaging of a ligated intestinal loop using CD11c-diphtheria toxin receptor and CXC 3 chemokine receptor 1 (CX3CR1)+/green fluorescent protein transgenic mice. Upon pathogen exposure, LP-DC formed transepithelial extensions, which were dependent on CX3CR1 expression. Lack of LP-DC or defect in CX3CR1 increased loading of pathogens via M cells of gut villi. Two presentations focused on the role of pathogens on DC. Fabienne Anjuere (Cecil Czerkinsky, Nice, France) described in vivo mobilization and maturation of intestinal DC subsets induced by oral administration of cholera toxin (CT), which induced in 2 h the appearance of CD8-DC in the LP close to the epithelium, followed at 24 h by a dramatic increase in CD8int and a moderate increase in CD8– DC subsets in MLN. Both DC subsets exhibited enhanced expression of costimulatory molecules and enhanced priming of Th1 and Th2 responses against a coadministered antigen [24 ]. The ability of CT to promote DC mobilization into the LP and their subsequent migration into MLN may explain the exceptional adjuvant properties of CT. Malin Sundquist (Marie-Jo Wick, Göteborg, Sweden) reported that DC maturation induced by oral Salmonella infection is not limited to the DC that get infected and that in vivo signaling through TNF receptor 1 is involved in DC maturation in MLN.
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TR CELLS IN MUCOSAL INFLAMMATION |
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TOPABSTRACTINTRODUCTIONCOMMENSAL BACTERIA IN MUCOSAL...DC IN INFECTION, INFLAMMATION,...TR CELLS IN MUCOSAL...NOVEL ROUTES AND ADJUVANTS...MUCOSAL IMMUNITY AGAINST HIV...REFERENCES |
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Fiona Powrie (Oxford, UK) presented recent updates about the therapeutic potential of CD4+CD25+ TR in chronic intestinal inflammation, as assessed in models of T cell transfer into immunodeficient mice [25 ]. CD4+CD25+ TR can prevent or cure established colitis by homing to MLN and colon and inhibiting effector T cell proliferation at these sites [26 ]. The therapeutic potential of TR requires IL-10, mainly produced by CD4+CD25+ cells in the LP (but not the MLN), which inhibits activation of the innate immune system. In addition, TGF-ß plays a key role in the functioning of CD4+CD25+ TR by acting directly on effector T cells to inhibit their differentiation and effector function. It is important that CD4+CD25+ TR can also control T cell-independent colitis induced by Helicobacter hepaticus infection in immune-deficient mice [27 ]. CD4+CD25+ TR accumulated in MLN and colon of colitic mice and inhibit sustained rather than initial activation of the innate immune system, indicating that TR require the chronic inflammatory signals that they regulate to perform suppression.
Antigen-specific Tr1 clones have been described following repeated in vitro stimulation with APC and antigen in the presence of IL-10 and have also been reported to regulate colitis in the severe combined immunodeficiency (SCID) transfer model. Hervé Groux (Nice, France) highlighted the relative contribution of constitutive CD4+CD25+ TR and Tr1 cells in control of colitis. He identified a tolerogenic CD11clowCD45RB+ DC subset, distinct from pDC, which can expand with IL-10, is absent in SCID mice, but can differentiate in vivo only after CD4+CD25+ TR transfer. This tolerogenic DC subset promotes in vivo differentiation of IL-10-secreting Tr1 cells expressing CD18+ and CD49b+, which are able to cure colitis in the SCID transfer model [28 , 29 ]. Thus, cure of colitis results from the capacity of transferred CD4+CD25+ TR to generate tolerogenic DC able to induce the differentiation of Tr1 cells. Along these lines, Edward Lavelle (Kevin Mills, Dublin) reported that bacterial toxins such as CT, heat-labile enterotoxins (LT), and adenylate cyclase from Bordetella pertussis, known as effective mucosal adjuvants, induce in vivo generation of Tr1 cells specific for bystander antigens by promoting maturation of IL-10-producing DC [30 ].
Oral tolerance is a physiological mechanism that controls untoward T cell-mediated hypersensitivity and autoimmune reactions to self or dietary antigens and was shown to involve anergy/deletion of effector T cells and TR cells, including TGF-ß-producing Th3 T cell clones. Although studies revealed that antigen feeding in OVA-specific TcR transgenic mice increases the number and suppressive function of CD4+CD25+ transgenic T cells, the role of naturally occurring CD4+CD25+ cells in tolerance induction in normal host remained to be clarified. Bertrand Dubois (D. Kaiserlian, Lyon, France) highlighted this issue in a well-characterized model of hapten-specific skin inflammation (i.e., CHS), mediated by CD8+ CTL effectors, independently of CD4 help. In this model, hapten gavage, prior to skin immunization, completely prevents CHS by inhibiting priming of specific CD8+ effectors in skin draining lymph nodes. That naturally occurring CD4+CD25+ TR are the main subset of regulatory cells responsible for induction of oral tolerance was demonstrated by in vivo antibody depletion and transfer experiments. Oral tolerance required environmental IL-10 in the host (and not IL-10 produced by CD4+CD25+ TR) and major histocompatibility complex class II molecules [31 ] and proceeds in two distinct, temporal phases. Gavage results in rapid uptake and presentation of the hapten to naive CD8+ T cells in gut-draining and nondraining secondary lymphoid organs, leading in a few days to functional inactivation of a hapten-specific CD8+ T cell pool in MLN and spleen, independently of CD4+CD25+ TR. It is interesting that gavage enhances the suppressive function of CD4+CD25+ TR, which completely suppresses priming/differentiation of residual CD8+ T cells into specific CD8+ CHS effectors during skin sensitization.
Formal proof of the pivotal role of CD4+CD25+TR in oral tolerance in human was provided by Per Brandtzaeg (Institute of Pathology, Oslo) in clinical studies of children with outgrown cow’s milk allergy (tolerant children), compared with those with clinically active allergy. Cow’s milk allergy in children is of short duration, and a proportion of children become tolerant with age. After a 2-month milk-free diet period, allergic children who became tolerant to cow’s milk challenge exhibited a diminished in vitro proliferative response to cow’s milk proteins as compared with children with active disease, as a result of a higher percentage of functional CD4+CD25+ TR [32 ]. It is interesting that milk reintroduction led to activation of CD4+CD25+ TR and to increased production of TGF-ß by peripheral blood cells, although it is still unclear whether CD4+CD25+ TR required TGF-ß for their functioning. This study demonstrates that acquisition of oral tolerance to dietary protein is associated with development of CD4+CD25+ TR, which requires exposure to the allergen to control specific T cells.
NK-T cells represent a distinct population of T cells that can act as regulatory cells in autoimmune and infectious processes. However, whether and how they could contribute to control inflammation in the gut have not been addressed. Dominique Buzzoni-Gatel (Pasteur Institute, Paris) showed that NK-T cells are attracted in the gut LP after oral infection with the parasite T. gondi and that CD1d-restricted NK-T cells contribute to the lethal ileitis as a result of overproduction of Th1 cytokines. However, when activated in vivo by -galactosylceramide (-GalCer), NK-T cells convert into Th2 cells and induce a dramatic influx of Foxp3+ CD4+CD25+ TR in the gut LP, which contributes to protection against infection. This indicates that NK-T cells can play a crucial role in initiation of inflammation and in its control via activation of CD4+CD25+ TR cells.
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NOVEL ROUTES AND ADJUVANTS FOR MUCOSAL VACCINES |
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TOPABSTRACTINTRODUCTIONCOMMENSAL BACTERIA IN MUCOSAL...DC IN INFECTION, INFLAMMATION,...TR CELLS IN MUCOSAL...NOVEL ROUTES AND ADJUVANTS...MUCOSAL IMMUNITY AGAINST HIV...REFERENCES |
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Jan Holmgren (Göteborg, Sweden) gave a thorough overview about the use of CT and the closely related E. coli LT, as well as their nontoxic B-subunit derivatives (CT-B and LT-B) as mucosal immunogens and adjuvants. The already licensed oral cholera vaccine Dukoral® (composed of inactivated whole-cell Vibrio cholerae bacteria and CT-B) has a safe and long-term, protective efficacy in children and adults of endemic and nonendemic countries. It also provides cross-protection against diarrhea caused by LT-producing enterotoxigenic E. coli, a pathogen for which no vaccine is currently available. Preclinical studies showed that vaccination with antigen-CT-pulsed DC drives tumor-specific CD8+ CTL, which eliminate established tumors, indicating that CT may be used as a combined carrier-delivery system and adjuvant for DC vaccination for therapeutic, anti-tumor vaccination [33 ]. Conversely to CT, which breaks oral tolerance, CT-B acts as an antigen carrier, promoting peripheral tolerance to appropriate antigen when administered via mucosal routes. The proof of principle about the use of CT-B for mucosal immunotherapy was provided in a phase I/II clinical trial in patients with Behcet’s disease, who develop autoimmune uveitis as a result of abnormal immune response against the 60-kd heat-shock protein (HSP60). Patients were treated by repeated oral administration of CT-B coupled to the immunodominant HSP60 peptide. Clinical remission was associated with immune unresponsiveness to HSP60, determined by an abrogation of HSP60-specific CD4+ T cell proliferation, down-regulation of Th1 cytokine and chemokine production, and decrease in CC chemokine receptor (CCR)7+ T cells, and these parameters were increased in patients with a relapse in uveitis [34 ]. Thus, oral tolerization by peptide CT-B is a novel, therapeutic vaccination strategy that may be applicable to other human autoimmune diseases, in which specific autoantigens are identified.
Ali Harandi (Göteborg, Sweden) described a novel adjuvant effect of CT-B, relying on activation of innate immunity. Initial in vivo mouse studies revealed that administration of CpG oligodeoxynucleotide (CpG-ODN) alone by vaginal, rectal, or gastrointestinal routes provided nonspecific protection against genital herpes, via stimulation of local production of macrophage-inflammatory protein-1 (MIP-1), MIP-1ß, regulated on activation, normal T expressed and secreted, and IFN-inducible protein 10, as well as of IL-12 and IL-18 [35
, 36
]. It is remarkable that mucosal administration of CpG coupled to CT-B (CTB-CpG) increased the local production of these factors, even across the species specificity of CpG oligonucleotides and could drive a Th1-biased, humoral response against coadministered tetanus toxoid. The molecular mechanism of such CT-B-linked CpG-ODN adjuvant effect is critically dependent on TLR-9 and myeloid differentiation primary-response protein 88 (MyD88). Thus, CTB-CpG represents a powerful adjuvant delivery system suitable for systemic and mucosal immune vaccination.
Transcutaneous vaccination is a novel route of vaccination that has yielded promising results and safety with several vaccine antigens in healthy human volunteers. F. Anjuere (C. Czerkinsky, Nice, France) documented that transcutaneous immunization (TCI) using CT-B (but not CT) as adjuvant promotes a Th1-biased immune response to a coadministered protein antigen. It is notable that preclinical data showed that TCI with CT-B and coadministered OVA is able to suppress pre-established anti-OVA IgE responses, suggesting a medical potential of this immunotherapeutic approach in anti-infectious vaccination and type I allergy suppression/desensitization [37 ]. The divergent effect of CT and CT-B administered by different routes appears to be a result of their respective effect on local resident DC. Namely, although CT and CT-B induce recruitment of DC into skin after TCI, they differ in the ability to induce DC maturation when given by certain mucosal versus transcutaneous routes [24 ]. It is most likely that these differences result from the variable outcome of the local epithelial microenvironment that differs between tissues covered with monostratified and pluristratified epithelia.
A key issue for development of protective mucosal vaccine is to ensure homing of lymphocytes to mucosal organs, a process dictated by two major chemokines thymus-expressed chemokine (TECK) [CC chemokine ligand 25 (CCL25)] and mucosal-associated epithelial cells (MEC) (CCL28). Marianne Quiding-Järbrink (Göteborg, Sweden) verified this hypothesis in humans by testing in vitro chemotaxis to TECK and MEC of circulating B cells from volunteers immunized with an inactivated oral cholera vaccine combined with a subcutaneous (s.c.) recall vaccination against tetanus toxoid. Most cholera-specific, IgA-producing cells migrate in response to TECK (and to a lesser extent, to MEC), and tetanus toxoid-specific, IgG-producing cells failed to do so. This indicates that TECK could promote selective mucosal homing of antibody-producing cells but that this occurred only during a narrow time-slot in the immune response. Finally, Johanna Nyström (Göteborg, Sweden) reported promising preclinical data showing that protection against H. pylori could be induced by oral (but not systemic) therapeutic vaccination using H. pylori lysate mixed with CT, resulting in decreased stomach bacterial load and increased specific T and B cell-mediated mucosal responses.
Besides CT, LT, and their derivative gold standard mucosal adjuvant, two novel adjuvants promoting Th2-biased or Th1/Tc1-biased immune responses are on the pipeline. Jean-Claude Sirard (Lille, France) highlighted the exquisite Th2-promoting efficacy of bacterial flagellin, the TLR-5 ligand in mice, previously shown to up-regulate CCL20 gene expression by epithelial cell lines and to promote CCL20-specific chemotaxis of immature DC in vitro [38
]. Flagellin, delivered via the nasal route, induces CCL20 production in the respiratory tract and promoted MyD88-dependent enhancement of mucosal and systemic antibody production responses to a coadministered, inert protein The mucosal instillation of flagellin is also associated with development of Th2 responses, as shown for s.c. administration [39
]. Thus, epithelial signaling via specific TLR by stimulating DC recruitment into mucosal tissues may be a prerequisite for instruction of the mucosal immune system. Nathalie Etchart (D. Kaiserlian, Lyon, France) reported about the CD8+ T cell adjuvant property of a recombinant measles virus nucleoprotein (NP). This large molecular weight protein forms self-aggregates and has the unique property to induce cross-priming of IFN--producing CD8+ CTL against a soluble protein coadministered via pluristratified epithelia, i.e., the buccal mucosa and the skin. Similarly to flagellin, NP enhances local CCL20 production, resulting in rapid CCR6-mediated recruitment of DC at the site of immunization. Functional in vivo studies revealed that the adjuvant effect of NP is strictly dependent on DC recruitment via CCR6/CCL20. This unique adjuvant property of NP to trigger DC recruitment concomitantly in epithelia and functional CTL against an inocuous protein may be exploited for development of anti-infectious and anti-tumor vaccines.
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MUCOSAL IMMUNITY AGAINST HIV AND VACCINATION STRATEGIES |
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TOPABSTRACTINTRODUCTIONCOMMENSAL BACTERIA IN MUCOSAL...DC IN INFECTION, INFLAMMATION,...TR CELLS IN MUCOSAL...NOVEL ROUTES AND ADJUVANTS...MUCOSAL IMMUNITY AGAINST HIV...REFERENCES |
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Robin Shattock (London, UK) found that HIV transmitted by cells emigrating from human cervical explants was contained mostly in human leukocyte antigen-DR+ DC and to a lesser degree, in T cells. It is important that simultaneous blockade of mannose C-type lectin receptors, such as DC-specific intercellular adhesion molecule-grabbing nonintegrin) or the HIV envelope itself (with monoclonal antibody b12), and the CD4 receptor (CD4-IgG2 fusion protein) was able to inhibit localized infection and virus dissemination [43 ], thus identifying key targets for developing topical microbicides. Several compounds, including polyanions, which bind to positively charged gp120 residues, and more specific molecules preventing direct viral infection or even targeting reverse transcriptase are currently being evaluated as microbicides in clinical trials. Florian Hladik (Julie McElrath, Seattle, WA) found that conjugates of DC and CCR5+ T cells emigrating from the human vaginal mucosa support replication of R5 and X4-tropic HIV-1 strains. Further investigations were focused on the interactions of HIV-1 with Langerhans and T cells embedded in the outer epithelial layer of the mucosa. Using epithelial sheets separated from the vaginal stroma by ex vivo suction blistering, it was found that HIV-1 rapidly binds to and enters Langerhans and T cells, suggesting a parallel rather than a sequential infection of these two cell types. Complementary to these ex vivo models, in vitro reconstructed vaginal mucosa developed by Marielle Bousbacher (Jenny Valladeau and Colette Dezutter, Lyon, France) should provide an accurate model to dissect the cellular and molecular mechanism of mucosal HIV pathogenesis and to screen local microbicides.
Lessons regarding protective mechanisms against AIDS can be drawn from the immune response of HIV-exposed, yet seronegative subjects (ESN). Indeed, some ESN sex workers seroconverted and became infected after having reduced exposure or stopping risk behavior for several months, indicating that continuous exposure to the virus is necessary to maintain mucosal protection. Furthermore, lack of HIV immune memory indicates that innate immune mechanisms play a crucial role in the resistant status of these individuals. Kristina Broliden (Stockholm, Sweden) identified virus-neutralizing IgA in the serum and mucosal secretions from most ESN sex workers in Kenya. These antibodies neutralized primary HIV-1 isolates when peripheral blood mononuclear cells were used as target cells, could induce cross-clade neutralization, and inhibited virus transcytosis. Studies of discordant couples including ESN and of long-term, nonprogressor, seropositive individuals by Lucia Lopalco (Milan, Italy) [44 , 45 ] revealed the presence of CCR5-specific mucosal and systemic IgA antibodies, able to neutralize different HIV-1 clades by down-regulating CCR5 and blocking virus transcytosis specifically in epithelial cells. Morgane Bomsel (Paris, France) screened a phage-display Fab IgA library from mucosal B cells isolated from Cambodian ESN. The library was screened for Fabs specific for oligomeric HIV-1 gp41 and a derived peptide P1 (651–685), the epithelial receptor GalCer-binding site that also includes neutralizing epitopes ELDKWA and NWFDIT. Novel, recombinant Fab-blocking HIV-1 transcytosis and CD4+ T cell infection by primary isolates were selected. These IgA Fabs might thus be exploited as microbicides.
Kenneth Rosenthal (Hamilton, Canada) presented HIV vaccines with the dual capacity to trigger innate and acquired immunity in mice. He reported that mice, immunized intranasally with gp120-depleted, inactivated HIV-1 and CpG to stimulate TLR-9, develop HIV-specific CD4+ and CD8+ T cell responses in the genital tract and are protected against a cross-clade vaginal challenge. Intranasal immunization using a nonreplicating Herpes simplex virus type 2 (HSV-2) gB protein with CpG induces similar protection against intravaginal infection with HSV-2. Local activation of TLRs by CpG or double-stranded RNA in the vaginal mucosa of female mice causes epithelial thickening and prevents HSV-2 replication in the genital mucosa, illustrating the potential of innate immune mechanisms in protection against sexually transmitted virus [46 47 48 ]. The possibility to induce specific systemic and genital CD8+ CTL by the vaginal route was illustrated by Carmelo Luci (F. Anjuère and C. Czerkinsky, Nice, France) using OVA conjugated to the CT-B subunit or coadministered with whole CT. Whereas the holotoxin promoted local and disseminated CTL responses, CT-B only evoked regional T cell responses in the draining genital lymph nodes.
As a multicentric human trial on mucosal HIV vaccine has been launched by the ANRS, Karine Petitprez, Laurent Belec, Dominique Salmon, and Gilles Pialoux (Paris, France) have developed standardized assays to monitor the level and functional activity of antibodies against HIV gp160 in the cervico-vaginal and other mucosal compartments of HIV-1-infected women. Progress has been made in the development of ex vivo human models of mucosal infection and transmission of HIV as well as of in vivo animal studies using HIV isolated from infected human tissues. Elucidating the protective innate and acquired immune mechanisms that are active in mucosal tissues will shed light on immune correlates of resistance to HIV infection. Thus, candidate vaccines able to trigger innate and adaptive immunity combined with microbicides appear as the rationale to prevent sexual HIV transmission.
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ACKNOWLEDGEMENTS |
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Received January 28, 2005; revised April 4, 2005; accepted April 5, 2005.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONCOMMENSAL BACTERIA IN MUCOSAL...DC IN INFECTION, INFLAMMATION,...TR CELLS IN MUCOSAL...NOVEL ROUTES AND ADJUVANTS...MUCOSAL IMMUNITY AGAINST HIV...REFERENCES |
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-helix of gp41 AIDS 16,1731-1741[CrossRef][Medline]
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| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
