Marian R. Neutra

Mucosal vaccines: the promise and the challenge

Most infectious agents enter the body at mucosal surfaces and therefore mucosal immune responses function as a first line of defence. Protective mucosal immune responses are most effectively induced by mucosal immunization through oral, nasal, rectal or vaginal routes, but the vast majority of vaccines in use today are administered by injection. As discussed in this Review, current research is providing new insights into the function of mucosal tissues and the interplay of innate and adaptive immune responses that results in immune protection at mucosal surfaces. These advances promise to accelerate the development and testing of new mucosal vaccines against many human diseases including HIV/AIDS.

Epithelial M Cells: Gateways for Mucosal Infection and Immunization

M cell transport is an important factor in induction of mucosal immune responses and is exploited by pathogenic microorganisms for invasion of the intestinal mucosa. The specific molecular recognition systems and nonspecific adherence mechanisms that determine the efficiency of the M cell transport pathway are largely unknown. Future studies on the interactions of micoorganisms with this highly specialized epithelial cell will enhance our understanding of microbial pathogenesis and will lead to more effective strategies for targeting of vaccines and live microbial vaccine vectors to the mucosal immune system.

Collaboration of epithelial cells with organized mucosal lymphoid tissues

Immune surveillance of mucosal surfaces requires the delivery of intact macromolecules and microorganisms across epithelial barriers to organized mucosal lymphoid tissues. Transport, processing and presentation of foreign antigens, as well as local induction and clonal expansion of antigen-specific effector lymphocytes, involves a close collaboration between organized lymphoid tissues and the specialized follicle-associated epithelium. M cells in the follicle-associated epithelium transport foreign macromolecules and microorganisms to antigen-presenting cells within and under the epithelial barrier. Determination of the earliest cellular interactions that occur in and under the follicle-associated epithelium could greatly facilitate the design of effective mucosal vaccines in the future.

Transport of membrane-bound macromolecules by M cells in follicle-associated epithelium of rabbit Peyer's patch

SummaryM cells in Peyers patch epithelium conduct transepithelial transport of luminal antigens to cells of the mucosal immune system. To determine the distribution of specific lectin-binding sites on luminal membranes of living M cells and to follow the transport route of membranebound molecules, lectin-ferritin conjugates and cationized ferritin were applied to rabbit Peyers patch mucosa in vivo and in vitro. The degree to which binding enhances transport was estimated by comparing quantitatively the transport of an adherent probe, wheat germ agglutinin-ferritin, to that of a nonadherent BSA-colloidal gold probe. When applied to fixed tissue, the lectins tested bound equally well to M cells and columnar absorptive cells. On living mucosa, however, ferritin conjugates of wheat germ agglutinin and Ricinus communis agglutinins I and II bound more avidly to M cells. Absorptive cells conducted little uptake and no detectable transepithelial transport. Lectins on M cell membranes were endocytosed from coated pits, rapidly transported in a complex system of tubulocisternae and vesicles, and remained adherent to M cell basolateral membranes. Cationized ferritin adhered to anionic sites and was similarly transported, but was released as free clusters at M cell basolateral surfaces. When applied simultaneously to Peyers patch mucosa, wheat germ agglutinin-ferritin was transported about 50 times more efficiently than was bovine serum albumin-colloidal gold.

Differential induction of mucosal and systemic antibody responses in women after nasal, rectal, or vaginal immunization: influence of the menstrual cycle.

A cholera vaccine containing killed vibrios and cholera toxin B subunit (CTB) was used to compare mucosal immunization routes for induction of systemic and mucosal Ab. Four groups of women were given three monthly immunizations by the rectal immunization (Rimm) route, nasal immunization (Nimm) route, or vaginal immunization route during either the follicular (V-FPimm) or luteal (V-LPimm) menstrual cycle phase. Nimm was performed with 10-fold less vaccine to determine if administration of less Ag by this route can, as in rodents, produce mucosal Ab responses comparable to those induced by higher dose Rimm or vaginal immunization. Concentrations of Ab induced in sera and secretions were measured by ELISA. None of these routes produced durable salivary Ab responses. Nimm induced greatest levels of CTB-specific IgG in sera. Rimm failed to generate CTB-specific IgA in genital tract secretions. Nimm, V-FPimm, and V-LPimm all produced cervical CTB-specific IgA responses comparable in magnitude and frequency. However, only V-FPimm induced cervical IgA2-restricted Ab to the bacterial LPS vaccine component. V-FPimm, but not V-LPimm, also induced CTB-specific IgA in rectal secretions. Nimm was superior to V-FPimm for producing rectal CTB-specific IgA, but the greatest amounts of CTB-specific IgA and LPS-specific IgA, IgG, and IgM Ab were found in rectal secretions of Rimm women. These data suggest that in women, Nimm alone could induce specific Ab in serum, the genital tract, and rectum. However, induction of genital tract and rectal Ab responses of the magnitude generated by local V-FPimm or Rimm will likely require administration of comparably high nasal vaccine dosages.

Selective adherence of IgA to murine Peyer's patch M cells: Evidence for a novel IgA receptor

M cells represent the primary route by which mucosal Ags are transported across the intestinal epithelium and delivered to underlying gut-associated lymphoid tissues. In rodents and rabbits, Peyer’s patch M cells selectively bind and endocytose secretory IgA (SIgA) Abs. Neither the nature of the M cell IgR nor the domains of SIgA involved in this interaction are known. Using a mouse ligated ileal loop assay, we found that monoclonal IgA Abs with or without secretory component, but not IgG or IgM Abs, bound to the apical surfaces of Peyer’s patch M cells, indicating that the receptor is specific for the IgA isotype. Human serum IgA and colostral SIgA also bound to mouse M cells. The asialoglycoprotein receptor or other lectin-like receptors were not detected on the apical surfaces of M cells. We used recombinant human IgA1 and human IgA2 Abs and domain swapped IgA/IgG chimeras to determine that both domains Cα1 and Cα2 are required for IgA adherence to mouse Peyer’s patch M cells. This distinguishes the M cell IgA receptor from CD89 (FcαI), which binds domains Cα2-Cα3. Finally, we observed by immunofluorescence microscopy that some M cells in the human ileum are coated with IgA. Together these data suggest that mouse, and possibly human, M cells express an IgA-specific receptor on their apical surfaces that mediates the transepithelial transport of SIgA from the intestinal lumen to underlying gut-associated organized lymphoid tissues.

Cholera toxin induces migration of dendritic cells from the subepithelial dome region to T- and B-cell areas of Peyer's patches

ABSTRACT Intestinal M cells deliver macromolecules, particles, and pathogens into the subepithelial dome (SED) region of Peyers patch mucosa, an area rich in dendritic cells (DCs). We tested whether uptake of the mucosal adjuvant cholera toxin (CT) or live Salmonella bacteria can induce DC migration within Peyers patches. Virus-sized, fluorescent polystyrene microparticles were efficiently transported by M cells and ingested by CD11c+, CD11b−, and CD8a− DCs in the SED region. DCs loaded with microparticles remained in the SED for up to 14 days. CT (but not the CT B subunit) and live attenuated Salmonella enterica serovar Typhimurium bacteria induced migration of the microparticle-loaded DCs from the SED region into underlying B-cell follicles and adjacent parafollicular T-cell zones. Our data provide the first demonstration that DCs move in response to enterotoxin adjuvants and live bacteria that enter the mucosa via M cells.

Molecular Cloning and Expression of a Gene Encoding Cryptosporidium parvum Glycoproteins gp40 and gp15

ABSTRACT Cryptosporidium parvum is a significant cause of diarrheal disease worldwide. The specific molecules that mediateC. parvum-host cell interactions and the molecular mechanisms involved in the pathogenesis of cryptosporidiosis are unknown. In this study we have shown that gp40, a mucin-like glycoprotein, is localized to the surface and apical region of invasive stages of the parasite and is shed from its surface. gp40-specific antibodies neutralize infection in vitro, and native gp40 binds specifically to host cells, implicating this glycoprotein in C. parvum attachment to and invasion of host cells. We have cloned and sequenced a gene designated Cpgp40/15 that encodes gp40 as well as gp15, an antigenically distinct, surface glycoprotein also implicated in C. parvum-host cell interactions. Analysis of the deduced amino acid sequence of the 981-bp Cpgp40/15revealed the presence of an N-terminal signal peptide, a polyserine domain, multiple predicted O-glycosylation sites, a single potential N-glycosylation site, and a hydrophobic region at the C terminus, a finding consistent with what is required for the addition of a GPI anchor. There is a single copy ofCpgp40/15 in the C. parvum genome, and this gene does not contain introns. Our data indicate that the twoCpgp40/15-encoded proteins, gp40 and gp15, are products of proteolytic cleavage of a 49-kDa precursor protein which is expressed in intracellular stages of the parasite. The surface localization of gp40 and gp15 and their involvement in the host-parasite interaction suggest that either or both of these glycoproteins may serve as effective targets for specific preventive or therapeutic measures for cryptosporidiosis.

TLRs Regulate the Gatekeeping Functions of the Intestinal Follicle-Associated Epithelium

Initiation of adaptive mucosal immunity occurs in organized mucosal lymphoid tissues such as Peyer’s patches of the small intestine. Mucosal lymphoid follicles are covered by a specialized follicle-associated epithelium (FAE) that contains M cells, which mediate uptake and transepithelial transport of luminal Ags. FAE cells also produce chemokines that attract Ag-presenting dendritic cells (DCs). TLRs link innate and adaptive immunity, but their possible role in regulating FAE functions is unknown. We show that TLR2 is expressed in both FAE and villus epithelium, but TLR2 activation by peptidoglycan or Pam3Cys injected into the intestinal lumen of mice resulted in receptor redistribution in the FAE only. TLR2 activation enhanced transepithelial transport of microparticles by M cells in a dose-dependent manner. Furthermore, TLR2 activation induced the matrix metalloproteinase-dependent migration of subepithelial DCs into the FAE, but not into villus epithelium of wild-type and TLR4-deficient mice. These responses were not observed in TLR2-deficient mice. Thus, the FAE of Peyer’s patches responds to TLR2 ligands in a manner that is distinct from the villus epithelium. Intraluminal LPS, a TLR4 ligand, also enhanced microparticle uptake by the FAE and induced DC migration into the FAE, suggesting that other TLRs may modulate FAE functions. We conclude that TLR-mediated signals regulate the gatekeeping functions of the FAE to promote Ag capture by DCs in organized mucosal lymphoid tissues.

How the gut senses its content

for a human adult. These surfaces are covered by thin epi- thelial layers that are protected from potentially harmful microorganisms by innate and adaptive defence mecha- nisms. Intestinal mucosal surfaces are in continuous contact with a heterogeneous population of microorgan- isms of the endogenous flora (up to 10 11-12 per g of lumenal material in the colon) and are exposed to food and microbes. A major role for the mucosal epithelia is in barrier function, essential for preventing colonization or invasion of the host by foreign microorganisms. Epithelial tissues also provide the mucosal immune system with a continuous stream of information about the external envi- ronment. Depending on the nature and the dose of the antigens transported from the gut lumen into mucosal lym- phoid tissues, strong immune responses or unresponsive- ness can be induced. Immune responses participate in the elimination of pathogenic microorganisms, while immune tolerance prevents harmful reactions against the gut flora and food antigens. The type of immune response triggered by environmental antigens appears to depend, in part, on initial recognition by the innate immune system. Recent data underscore the importance of innate immunity in sensing the microbial environment and the role of the epithelium in releasing signals that allow recruitment of pro-inflammatory leucocytes, immune cells, or both. We shall review these various aspects of mucosal immunity, with special emphasis on the cross talk that takes place between the microflora, the epithelium and the immune cells in the gut.