CellMapper: rapid and accurate inference of gene expression in difficult-to-isolate cell types

We present a sensitive approach to predict genes expressed selectively in specific cell types, by searching publicly available expression data for genes with a similar expression profile to known cell-specific markers. Our method, CellMapper, strongly outperforms previous computational algorithms to predict cell type-specific expression, especially for rare and difficult-to-isolate cell types. Furthermore, CellMapper makes accurate predictions for human brain cell types that have never been isolated, and can be rapidly applied to diverse cell types from many tissues. We demonstrate a clinically relevant application to prioritize candidate genes in disease susceptibility loci identified by GWAS.

Effects of flagellin on the functions of follicle-associated epithelium

Bacterial flagellin activates innate immune responses by signaling through Toll-like receptor 5 and is a potential vaccine adjuvant. Mucosal lymphoid follicles, inductive sites for adaptive mucosal immune responses, are covered by a follicle-associated epithelium (FAE) specialized for the uptake of antigens. This study demonstrates that mucosal application of Salmonella dublin flagellin enhanced transepithelial transport of microparticles by the FAE of mouse Peyer's patches in vivo. Flagellin also induced rapid, matrix metalloproteinase-dependent migration of subepithelial dendritic cells (DCs) into the FAE, better positioning DCs for antigen capture. These innate responses to flagellin enhance FAE functions and may promote adaptive immune responses in the mucosa.

Regulation of intestinal goblet cells in situ, in mucosal explants and in the isolated epithelium

Cholinergic secretagogues were previously shown to accelerate mucin secretion from intestinal goblet cells of adult rats and rabbits, both in vitro and in mucosal explants. This rapid secretory response occurs only in crypt cells; surface goblet cells are not affected. Rapid secretion involves the sequential fusion of secretory granule membranes with the plasma membrane and with each other, but does not require granule movement. In unstimulated cells, slow transport of secretory granules towards the luminal cell surface depends on functional microtubules. Goblet cells appear in the rat fetal intestine three to four days before birth but they are insensitive to cholinergic agents in the fetus and neonate. The secretory response of crypt goblet cells to carbachol, both in vivo and in mucosal slices in vitro, is established throughout the intestines only after weaning (20-25 days after birth). To determine whether acetylcholine from nerve endings in the intact mucosa may mediate a mucus secretory response in the absence of exogenous secretagogues, mucosal sheets were mounted in modified Ussing chambers and goblet cell secretion was assessed after electrical field stimulation. Electrical field stimulation elicited mucus secretion from crypt (but not surface) goblet cells. Secretion was inhibited by prior treatment of the mucosa with 500 nM-tetrodotoxin or 100 microM-atropine, but not by 10 microM-atropine. Thus, endogenous nerves may regulate mucus secretion from crypt goblet cells in the intact mucosa. When intact sheets of epithelium were isolated from adult rat ileum and colon, then maintained in vitro and exposed to 20 microM-carbachol, crypt goblet cells released mucin in response to the secretagogue but goblet cells in in portions of the epithelium derived from villi or mucosal surfaces were unresponsive. This suggests that crypt epithelial cells respond directly to cholinergic agents and that they lose this sensitivity as they migrate out of the crypts.

Inhibition of HIV-1 infectivity and epithelial cell transfer by human monoclonal IgG and IgA antibodies carrying the b12 V region

Both IgG and secretory IgA Abs in mucosal secretions have been implicated in blocking the earliest events in HIV-1 transit across epithelial barriers, although the mechanisms by which this occurs remain largely unknown. In this study, we report the production and characterization of a human rIgA(2) mAb that carries the V regions of IgG1 b12, a potent and broadly neutralizing anti-gp120 Ab which has been shown to protect macaques against vaginal simian/HIV challenge. Monomeric, dimeric, polymeric, and secretory IgA(2) derivatives of b12 reacted with gp120 and neutralized CCR5- and CXCR4-tropic strains of HIV-1 in vitro. With respect to the protective effects of these Abs at mucosal surfaces, we demonstrated that IgG1 b12 and IgA(2) b12 inhibited the transfer of cell-free HIV-1 from ME-180 cells, a human cervical epithelial cell line, as well as Caco-2 cells, a human colonic epithelial cell line, to human PBMCs. Inhibition of viral transfer was due to the ability of b12 to block both viral attachment to and uptake by epithelial cells. These data demonstrate that IgG and IgA MAbs directed against a highly conserved epitope on gp120 can interfere with the earliest steps in HIV-1 transmission across mucosal surfaces, and reveal a possible mechanism by which b12 protects the vaginal mucosal against viral challenge in vivo.

TLR2 activation by proteosomes promotes uptake of particulate vaccines at mucosal surfaces

Proteosome-based vaccines have TLR2-based adjuvant activity and show promise for mucosal immunization. We examined the effects of proteosomes on mucosal uptake in Peyer's patches in vivo. Proteosomes accelerated transepithelial transport of microparticles by M cells and induced migration of dendritic cells (DCs) into the follicle-associated epithelium (FAE); both effects were dependent on TLR2. Proteosomes induced the release of the DC-attracting chemokine MIP3alpha from Caco-2 epithelial cells in vitro. In HEK cells, proteosome-mediated MIP3alpha release was dependent on TLR2 expression and matrix metalloproteinase activation. Thus, TLR2 activation by proteosomes may promote mucosal uptake of particulate vaccines, and this may contribute to their adjuvanticity.

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 Pam(3)Cys 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.

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.

Protective immunoglobulin A and G antibodies bind to overlapping intersubunit epitopes in the head domain of type 1 reovirus adhesin sigma1

Nonfusogenic mammalian orthoreovirus (reovirus) is an enteric pathogen of mice and a useful model for studies of how an enteric virus crosses the mucosal barrier of its host and is subject to control by the mucosal immune system. We recently generated and characterized a new murine immunoglobulin A (IgA)-class monoclonal antibody (MAb), 1E1, that binds to the adhesin fiber, sigma1, of reovirus type 1 Lang (T1L) and thereby neutralizes the infectivity of that strain in cell culture. 1E1 is produced in hybridoma cultures as a mixture of monomers, dimers, and higher polymers and is protective against peroral challenges with T1L either when the MAb is passively administered or when it is secreted into the intestines of mice bearing subcutaneous hybridoma tumors. In the present study, selection and analysis of mutants resistant to neutralization by 1E1 identified the region of T1L sigma1 to which the MAb binds. The region bound by a previously characterized type 1 sigma1-specific neutralizing IgG MAb, 5C6, was identified in the same way. Each of the 15 mutants isolated and analyzed was found to be much less sensitive to neutralization by either 1E1 or 5C6, suggesting the two MAbs bind to largely overlapping regions of sigma1. The tested mutants retained the capacity to recognize specific glycoconjugate receptors on rabbit M cells and cultured epithelial cells, even though viral binding to epithelial cells was inhibited by both MAbs. S1 sequence determinations for 12 of the mutants identified sigma1 mutations at four positions between residues 415 and 447, which contribute to forming the receptor-binding head domain. When aligned with the sigma1 sequence of reovirus type 3 Dearing (T3D) and mapped onto the previously reported crystal structure of the T3D sigma1 trimer, the four positions cluster on the side of the sigma1 head, across the interface between two subunits. Three such interface-spanning epitopes are thus present per sigma1 trimer and require the intact quaternary structure of the head domain for MAb binding. Identification of these intersubunit epitopes on sigma1 opens the way for further studies of the mechanisms of antibody-based neutralization and protection with type 1 reoviruses.

Transepithelial transport and mucosal defence I: the role of M cells

How do cells of the immune system encounter the majority of antigens that enter the body through the gut and airways? The epithelia lining these systems contain a remarkable cell type, the M cell, that delivers antigens across the epithelium to lymphocytes and macrophages. In this article, Marian Neutra and Jean-Pierre Kraehenbuhl describe the structure of the M cell, its function in promoting the immune response and its exploitation by invading pathogens. In the next issue of Trends in Cell Biology, these authors will review the other immunological function of epithelia, secretion of polymeric IgA.

Secretory immunoglobulin A antibodies against the sigma1 outer capsid protein of reovirus type 1 Lang prevent infection of mouse Peyer's patches

Reovirus type 1 Lang (T1L) adheres to M cells in the follicle-associated epithelium of mouse intestine and exploits the transport activity of M cells to enter and infect the Peyer's patch mucosa. Adult mice that have previously cleared a reovirus T1L infection have virus-specific immunoglobulin G (IgG) in serum and IgA in secretions and are protected against reinfection. Our aim in this study was to determine whether secretory IgA is sufficient for protection of Peyer's patches against oral reovirus challenge and, if so, against which reovirus antigen(s) the IgA may be directed. Monoclonal antibodies (MAbs) of the IgA isotype, directed against the sigma1 protein of reovirus T1L, the viral adhesin, were produced and tested along with other, existing IgA and IgG MAbs against reovirus T1L outer capsid proteins. Anti-sigma1 IgA and IgG MAbs neutralized reovirus T1L in L cell plaque reduction assays and inhibited T1L adherence to L cells and Caco-2(BBe) intestinal epithelial cells in vitro, but MAbs against other proteins did not. Passive oral administration of anti-sigma1 IgA and IgG MAbs prevented Peyer's patch infection in adult mice, but other MAbs did not. When anti-sigma1 IgA and IgG MAbs were produced in mice from hybridoma backpack tumors, however, the IgA prevented Peyer's patch infection, but the IgG did not. The results provide evidence that neutralizing IgA antibodies specific for the sigma1 protein are protective in vitro and in vivo and that the presence of these antibodies in intestinal secretions is sufficient for protection against entry of reovirus T1L into Peyer's patches.

The viral sigma1 protein and glycoconjugates containing alpha2-3-linked sialic acid are involved in type 1 reovirus adherence to M cell apical surfaces

Type 1 reoviruses invade the intestinal mucosa of mice by adhering selectively to M cells in the follicle-associated epithelium and then exploiting M cell transport activity. The purpose of this study was to identify the apical cell membrane component and viral protein that mediate the M cell adherence of these viruses. Virions and infectious subviral particles of reovirus type 1 Lang (T1L) adhered to rabbit M cells in Peyer's patch mucosal explants and to tissue sections in an overlay assay. Viral adherence was abolished by pretreatment of sections with periodate and in the presence of excess sialic acid or lectins MAL-I and MAL-II (which recognize complex oligosaccharides containing sialic acid linked alpha2-3 to galactose). The binding of T1L particles to polarized human intestinal (Caco-2(BBe)) cell monolayers was correlated with the presence of MAL-I and MAL-II binding sites, blocked by excess MAL-I and -II, and abolished by neuraminidase treatment. Other type 1 reovirus isolates exhibited MAL-II-sensitive binding to rabbit M cells and polarized Caco-2(BBe) cells, but type 2 or type 3 isolates including type 3 Dearing (T3D) did not. In assays using T1L-T3D reassortants and recoated viral cores containing T1L, T3D, or no sigma1 protein, MAL-II-sensitive binding to rabbit M cells and polarized Caco-2(BBe) cells was consistently associated with the T1L sigma1. MAL-II-recognized oligosaccharide epitopes are not restricted to M cells in vivo, but MAL-II immobilized on virus-sized microparticles bound only to the follicle-associated epithelium and M cells. The results suggest that selective binding of type 1 reoviruses to M cells in vivo involves interaction of the type 1 sigma1 protein with glycoconjugates containing alpha2-3-linked sialic acid that are accessible to viral particles only on M cell apical surfaces.

The role of mucosal immunity in prevention of HIV transmission

Vaccines designed to prevent mucosal transmission of HIV should establish multiple immune effectors in vaccine recipients, including antibodies which are capable of blocking HIV entry at mucosal epithelial barriers and of preventing initial infection of target cells in the mucosa. Immunological analyses of HIV-resistant humans and data obtained in nonhuman primate vaccine studies indicate that both secretory and serum antibodies may play an important role in protection against mucosal transmission of HIV or SIV, whereas cytotoxic T cells are required for clearance of mucosal infection and prevention of systemic spread. This review summarizes the roles of IgA and IgG antibodies in preventing mucosal infection by other viral and bacterial pathogens, and then discusses the various mechanisms by which antibodies might contribute to protection against HIV at mucosal surfaces. These include prevention of mucosal contact, blocking attachment of virus or infected cells to epithelial cells, interception of virus during transepithelial transport, neutralization of virus in the mucosa, and elimination of locally infected cells through antibody-dependent cell-mediated cytotoxic reactions. The regional nature of mucosal immune responses is reviewed in light of its relevance to HIV vaccine development. We conclude that mucosal immunization should be considered a component of vaccine strategies against HIV.

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

Intestinal M cells deliver macromolecules, particles, and pathogens into the subepithelial dome (SED) region of Peyer's 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 Peyer's 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.

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 Calpha1 and Calpha2 are required for IgA adherence to mouse Peyer's patch M cells. This distinguishes the M cell IgA receptor from CD89 (FcalphaI), which binds domains Calpha2-Calpha3. 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.

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 (R(imm)) route, nasal immunization (N(imm)) route, or vaginal immunization route during either the follicular (V-FP(imm)) or luteal (V-LP(imm)) menstrual cycle phase. N(imm) 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 R(imm) or vaginal immunization. Concentrations of Ab induced in sera and secretions were measured by ELISA. None of these routes produced durable salivary Ab responses. N(imm) induced greatest levels of CTB-specific IgG in sera. R(imm) failed to generate CTB-specific IgA in genital tract secretions. N(imm), V-FP(imm), and V-LP(imm) all produced cervical CTB-specific IgA responses comparable in magnitude and frequency. However, only V-FP(imm) induced cervical IgA2-restricted Ab to the bacterial LPS vaccine component. V-FP(imm), but not V-LP(imm), also induced CTB-specific IgA in rectal secretions. N(imm) was superior to V-FP(imm) 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 R(imm) women. These data suggest that in women, N(imm) 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-FP(imm) or R(imm) will likely require administration of comparably high nasal vaccine dosages.

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 (R(imm)) route, nasal immunization (N(imm)) route, or vaginal immunization route during either the follicular (V-FP(imm)) or luteal (V-LP(imm)) menstrual cycle phase. N(imm) 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 R(imm) or vaginal immunization. Concentrations of Ab induced in sera and secretions were measured by ELISA. None of these routes produced durable salivary Ab responses. N(imm) induced greatest levels of CTB-specific IgG in sera. R(imm) failed to generate CTB-specific IgA in genital tract secretions. N(imm), V-FP(imm), and V-LP(imm) all produced cervical CTB-specific IgA responses comparable in magnitude and frequency. However, only V-FP(imm) induced cervical IgA2-restricted Ab to the bacterial LPS vaccine component. V-FP(imm), but not V-LP(imm), also induced CTB-specific IgA in rectal secretions. N(imm) was superior to V-FP(imm) 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 R(imm) women. These data suggest that in women, N(imm) 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-FP(imm) or R(imm) will likely require administration of comparably high nasal vaccine dosages.

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 (R(imm)) route, nasal immunization (N(imm)) route, or vaginal immunization route during either the follicular (V-FP(imm)) or luteal (V-LP(imm)) menstrual cycle phase. N(imm) 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 R(imm) or vaginal immunization. Concentrations of Ab induced in sera and secretions were measured by ELISA. None of these routes produced durable salivary Ab responses. N(imm) induced greatest levels of CTB-specific IgG in sera. R(imm) failed to generate CTB-specific IgA in genital tract secretions. N(imm), V-FP(imm), and V-LP(imm) all produced cervical CTB-specific IgA responses comparable in magnitude and frequency. However, only V-FP(imm) induced cervical IgA2-restricted Ab to the bacterial LPS vaccine component. V-FP(imm), but not V-LP(imm), also induced CTB-specific IgA in rectal secretions. N(imm) was superior to V-FP(imm) 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 R(imm) women. These data suggest that in women, N(imm) 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-FP(imm) or R(imm) will likely require administration of comparably high nasal vaccine dosages.

Antigen delivery to mucosa-associated lymphoid tissues using liposomes as a carrier

Oral vaccination requires an antigen delivery vehicle to protect the antigen and to enhance translocation of the antigen to the mucosa-associated lymphoid tissue. A variety of antigen delivery vehicles including liposomes have been studied for mucosal immunization. The advantages of liposome formulations are their particulate form and the ability to accommodate immunomodulators and targeting molecules in the same package. Many conventional liposomes are variably unstable in acids, pancreatic juice and bile. Nevertheless, carefully designed liposomes have demonstrated an impressive efficacy in inducing mucosal IgA responses, compared to free antigens and other delivery vehicles. However, liposomes as an oral vaccine vehicle are not yet optimized. To design liposomes that are stable in the harsh intestinal environment and are efficiently taken up by the M cells remains a challenge. This review summarizes recent research efforts using liposomes as an antigen carrier for oral vaccines with practical attention to liposome designs and interaction with the M cells.

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.

Role of immunoglobulin A in protection against reovirus entry into Murine Peyer's patches

Reovirus type 1 Lang (T1L) infects the mouse intestinal mucosa by adhering specifically to epithelial M cells and exploiting M-cell transport to enter the Peyer's patches. Oral inoculation of adult mice has been shown to elicit cellular and humoral immune responses that clear the infection within 10 days. This study was designed to determine whether adult mice that have cleared a primary infection are protected against viral entry upon oral rechallenge and, if so, whether antireovirus secretory immunoglobulin A (S-IgA) is a necessary component of protection. Adult BALB/c mice that were orally inoculated on day 0 with reovirus T1L produced antiviral S-IgA in feces and IgG in serum directed primarily against the reovirus sigma1 attachment protein. Eight hours after oral reovirus challenge on day 21, the Peyer's patches of previously exposed mice contained no detectable virus whereas Peyer's patches of naive controls contained up to 2,300 PFU of reovirus/mg of tissue. Orally inoculated IgA knockout (IgA(-/-)) mice cleared the initial infection as effectively as wild-type mice and produced higher levels of reovirus-specific serum IgG and secretory IgM than C57BL/6 wild-type mice. When IgA(-/-) mice were rechallenged on day 21, however, their Peyer's patches became infected. These results indicate that intestinal S-IgA is an essential component of immune protection against reovirus entry into Peyer's patch mucosa.

Immunization of mice with recombinant gp41 in a systemic prime/mucosal boost protocol induces HIV-1-specific serum IgG and secretory IgA antibodies

We tested the immunogenicity in mice of a recombinant fusion protein (gp41HA) consisting of the ectodomain of the HIV-1(IIIB) envelope glycoprotein gp41 fused to a fragment of the influenza virus HA2 hemagglutinin protein. An intraperitoneal prime followed by intranasal or intragastric boosts with gp41HA induced high concentrations of serum IgG antibodies and fecal IgA antibodies that reacted with gp41 in HIV-1(IIIB) viral lysate and were cross-reactive with gp41 in HIV-1(MN) lysate. By indirect immunofluorescence, serum IgG and fecal IgA from immunized mice were also shown to recognize gp41 in acetone-fixed human peripheral blood mononuclear cells infected with either syncytium-inducing (SI) or non-syncytium-inducing (NSI) North American HIV-1 field isolates, but not uninfected cells. Thus, this recombinant antigen may be useful in prime/boost immunization protocols designed to induce systemic and mucosal antibodies that recognize multiple primary HIV-1 isolates.

Epithelial M cells: differentiation and function

M cells are distinctive epithelial cells that occur only in the follicle-associated epithelia that overlie organized mucosa-associated lymphoid tissues. They are structurally and functionally specialized for transepithelial transport, delivering foreign antigens and microorganisms to organized lymphoid tissues within the mucosae of the small and large intestines, tonsils and adenoids, and airways. M cell transport is a double-edged sword: Certain pathogens exploit the features of M cells that are intended to promote uptake for the purpose of immunological sampling. Eludication of the molecular architecture of M cell apical surfaces is important for understanding the strategies that pathogens use to exploit this pathway and for utilizing M cell transport for delivery of vaccines to the mucosal immune system. This article reviews the functional and biochemical features that distinguish M cells from other intestinal cell types. In addition it synthesizes the available information on development and differentiation of organized lymphoid tissues and the specialized epithelium associated with these immune inductive sites.

Effective induction of simian immunodeficiency virus-specific systemic and mucosal immune responses in primates by vaccination with proviral DNA producing intact but noninfectious virions

We report a pilot evaluation of a DNA vaccine producing genetically inactivated simian immunodeficiency virus (SIV) particles in primates, with a focus on eliciting mucosal immunity. Our results demonstrate that DNA vaccines can be used to stimulate strong virus-specific mucosal immune responses in primates. The levels of immunoglobulin A (IgA) detected in rectal secretions of macaques that received the DNA vaccine intradermally and at the rectal mucosa were the most striking of all measured immune responses and were higher than usually achieved through natural infection. However, cytotoxic T lymphocyte responses were generally low and sporadically present in different animals. Upon rectal challenge with cloned SIVmac239, resistance to infection was observed, but some animals with high SIV-specific IgA levels in rectal secretions became infected. Our results suggest that high levels of IgA alone are not sufficient to prevent the establishment of chronic infection, although mucosal IgA responses may have a role in reducing the infectivity of the initial viral inoculum.

Differential effects of simian immunodeficiency virus infection on immune inductive and effector sites in the rectal mucosa of rhesus macaques

The rectal mucosa, a region involved in human immunodeficiency virus/simian immunodeficiency virus (SIV) infection and transmission, contains immune inductive sites, rectal lymphoid nodules (RLN), and effector sites, the lamina propria (LP). This study was designed to evaluate cell populations involved in rectal mucosal immune function in both RLN and LP, by immunocytochemical analysis of rectal mucosa from 11 SIV-infected (2 to 21 months postinfection) and five naive rhesus macaques. In the rectum, as previously observed in other intestinal regions, CD4(+) cells were dramatically reduced in the LP of SIV-infected macaques, but high numbers of CD4(+) cells remained in RLN indicating maintenance of T cell help in inductive sites. Cells expressing the mucosal homing receptor alpha4beta7 were dramatically decreased in the RLN and LP of most SIV-infected macaques. The RLN of both naive and SIV-infected macaques contained high numbers of CD68 + MHC-II+ macrophages and cells expressing the co-stimulatory molecules B7-2 and CD40, as well as IgM + MHCII+ and IgM + CD40+ B cells, indicating maintenance of antigen presentation capacity. The LP of all three macaques SIV-infected for 2 months contained many B7-2+ cells, suggesting increased activation of antigen-presenting cells. LP of SIV-infected rectal mucosa contained increased numbers of IgM+ cells, confirming previous observations in small intestine and colon. The data suggest that antigen-presentation capacity is maintained in inductive sites of SIV-infected rectal mucosa, but immune effector functions may be altered.

Modified wick method using Weck-Cel sponges for collection of human rectal secretions and analysis of mucosal HIV antibody

Weck-Cel sponges were examined for suitability as an absorbent material for nontraumatic collection of rectal secretions in humans. Sponges were tested in vitro and determined by quantitative enzyme-linked immunosorbent assay (ELISA) to be capable of releasing 100% of absorbed albumin and all immunoglobulin subtypes after treatment with detergent-supplemented buffer. Protein composition in rectal secretions collected from normal women with dry sponges (DS) or with sponges previously softened by moistening with saline (MS) was subsequently compared. DS secretions showed evidence of contamination with blood and interstitial fluid-derived albumin, immunoglobulin G (IgG), and monomeric IgA. MS secretions appeared to represent local mucosal secretions more accurately because they contained negligible blood, a greater percentage of secretory IgA within the total IgA, and both lower albumin/IgG ratios and more dramatic alterations in IgG subclass distribution compared with corresponding serum. Anti-HIV IgG, IgM, IgA, and antibodies with secretory component could be demonstrated by ELISA in rectal secretions collected with moist sponges from 8 of 8, 1 of 8, 5 of 8, and 3 of 8 HIV-infected women, respectively. The data show that Weck-Cel sponges, if premoistened, can be used to collect rectal fluids nontraumatically and to obtain quantitative information about concentrations of immunoglobulins and specific antibodies on rectal mucosal surfaces.

Molecular cloning and expression of a gene encoding Cryptosporidium parvum glycoproteins gp40 and gp15

Cryptosporidium parvum is a significant cause of diarrheal disease worldwide. The specific molecules that mediate C. 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 Cpgp40/15 revealed 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 of Cpgp40/15 in the C. parvum genome, and this gene does not contain introns. Our data indicate that the two Cpgp40/15-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.

Accessibility of glycolipid and oligosaccharide epitopes on rabbit villus and follicle-associated epithelium

The initial step in many mucosal infections is pathogen attachment to glycoconjugates on the apical surfaces of intestinal epithelial cells. We examined the ability of virus-sized (120-nm) and bacterium-sized (1-microm) particles to adhere to specific glycolipids and protein-linked oligosaccharides on the apical surfaces of rabbit Peyer's patch villus enterocytes, follicle-associated enterocytes, and M cells. Particles coated with the B subunit of cholera toxin, which binds the ubiquitous glycolipid GM1, were unable to adhere to enterocytes or M cells. This confirms that both the filamentous brush border glycocalyx on enterocytes and the thin glycoprotein coat on M cells can function as size-selective barriers. Oligosaccharides containing terminal beta(1,4)-linked galactose were accessible to soluble lectin Ricinus communis type I on all epithelial cells but were not accessible to lectin immobilized on beads. Oligosaccharides containing alpha(2, 3)-linked sialic acid were recognized on all epithelial cells by soluble Maackia amurensis lectin II (Mal II). Mal II coated 120-nm (but not 1-microm) particles adhered to follicle-associated enterocytes and M cells but not to villus enterocytes. The differences in receptor availability observed may explain in part the selective attachment of viruses and bacteria to specific cell types in the intestinal mucosa.

Immunization of mice with peptomers covalently coupled to aluminum oxide nanoparticles

Subunit vaccines generally require adjuvants to elicit immune responses, but adjuvants may alter the conformation of critical epitopes and reduce vaccine efficacy. We therefore tested an immunization strategy in which antigen is covalently coupled to aluminum oxide nanoparticles using a method that favors preservation of the native conformation. The test antigen consisted of "peptomers" (head-to-tail-linked peptide homopolymers) derived from the 4th conserved region (C4) of HIV-1 gp120 which is believed to be in an alpha-helical conformation prior to binding to CD4. Immune responses in mice to peptomer-nanoparticle conjugates were compared to responses elicited by free C4 peptide and C4 peptomers, with and without the hydrophilic adjuvant muramyl dipeptide (MDP). Highest peptomer-specific serum antibody responses were induced by peptomer-particles without MDP. Serum antibodies induced by peptomer-particles also showed highest reactivity towards recombinant, glycosylated gp120 and HIV-1 infected T cells. The results suggest that this novel vaccine approach could be useful for induction of immune responses against conformation-sensitive viral antigens without the need for additional adjuvants.

The composition and function of M cell apical membranes: implications for microbial pathogenesis

M cells, an epithelial cell phenotype that occurs only over organized mucosal lymphoid follicles, deliver samples of foreign material by transepithelial transport from the lumen to organized lymphoid tissues within the mucosa of the small and large intestines. The apical membranes of M cells in the intestine are designed to facilitate adherence and uptake of antigens and microorganisms, a prerequisite for immunological sampling. The molecular features of M cell apical surfaces that promote adherence and transport are crucial for understanding the strategies that pathogens use to exploit this pathway.

Interactions of viruses and microparticles with apical plasma membranes of M cells: implications for human immunodeficiency virus transmission

In the rectal mucosa, specialized M cells of the lymphoid follicle-associated epithelium conduct vesicular transport of antigens from the mucosal surface into organized mucosal lymphoid tissues, where immune responses are induced. Bacteria and viruses may exploit this mechanism to initiate mucosal or systemic infection. Viral pathogens, including reovirus, poliovirus, and possibly human immunodeficiency virus (HIV), can enter the intestinal or rectal mucosa by adhering to apical membranes of M cells, but the membrane components involved in these interactions are unknown. Glycoprotein coats on the apical surfaces of epithelial cells act as diffusion barriers that limit access of particles and microorganisms to membrane glycolipids and to certain oligosaccharide epitopes of enterocytes but allow selective adherence to M cells. The accessibility of membrane glycolipids of M cells, along with their active endocytic activity, could promote entry of HIV into the rectal mucosa.

Mucosal vaccination strategies for women

Women were immunized orally, rectally, or vaginally with a recombinant cholera toxin B-containing vaccine to determine which of these mucosal immunization routes generate the greatest levels of antibody in the female genital tract and rectum. ELISA was used to measure concentrations of cholera toxin B-specific IgA and IgG antibody in serum and secretions before and after three immunizations. Each immunization route similarly increased specific IgG in serum and specific IgA in saliva. Only the vaginal route increased IgA antibodies in genital tract secretions and could be shown to induce a local IgG response. However, vaginal immunization failed to produce antibody in the rectum. In a similar fashion, rectal immunization elicited highest concentrations of locally derived IgA and IgG antibody in the rectum but was ineffective for generating antibody in the genital tract. The data suggest that local immunization may induce the greatest immune responses in the female genital tract and rectum of humans.

Human intestinal M cells display the sialyl Lewis A antigen

The biochemical features that distinguish human M cells from other intestinal epithelial cell types are important for understanding microbial pathogenesis and for targeting vaccines to the mucosal immune system. We applied a large panel of carbohydrate-specific monoclonal antibodies and lectins to Peyer's patch and cecum biopsy specimens from three normal individuals and a patient with inflammatory bowel disease. The results show that human M-cell glycosylation patterns are distinct from those of other species examined and that human M cells preferentially display the sialyl Lewis A antigen. This carbohydrate epitope is also present in a small subpopulation of enterocytes in the follicle-associated epithelium and in goblet cell mucins.

Current concepts in mucosal immunity. V Role of M cells in transepithelial transport of antigens and pathogens to the mucosal immune system

Specialized epithelial M cells, a phenotype that occurs only in the epithelium over organized lymphoid follicles, deliver samples of foreign material by transepithelial transport from the lumen to organized lymphoid tissues within the mucosa of the small and large intestines. Mounting evidence indicates that a complex interplay of mucosal lymphoid cells and luminal microorganisms with epithelial cells underlies differentiation of the M cell phenotype. The cellular and molecular features of M cells that promote adherence and transport of antigens and microorganisms are crucial for the design of mucosal vaccines and for understanding the strategies that pathogens use to exploit this pathway.

Review article: Intestinal epithelia and barrier functions

The mucosal epithelia of the digestive tract acts as a selective barrier, permeable to ions, small molecules and macromolecules. These epithelial cells aid the digestion of food and absorption of nutrients. They contribute to the protection against pathogens and undergo continuous cell renewal which facilitates the elimination of damaged cells. Both innate and adaptive defence mechanisms protect the gastrointestinal-mucosal surfaces against pathogens. Interaction of microorganisms with epithelial cells triggers a host response by activating specific transcription factors which control the expression of chemokines and cytokines. This host response is characterized by the recruitment of macrophages and neutrophils at the site of infection. Disruption of epithelial signalling pathways that recruit migratory immune cells results in a chronic inflammatory response. The adaptive defence mechanism relies on the collaboration of epithelial cells (resident sampling system) with antigen-presenting and lymphoid cells (migratory sampling system); in order to obtain samples of foreign antigen, these samples must be transported across the barriers without affecting the integrity of the barrier. These sampling systems are regulated by both environmental and host factors. Fates of the antigen may differ depending on the way in which they cross the epithelial barrier, i.e. via interaction with motile dendritic cells or epithelial M cells in the follicle-associated epithelium.

Peptomer aluminum oxide nanoparticle conjugates as systemic and mucosal vaccine candidates: synthesis and characterization of a conjugate derived from the C4 domain of HIV-1MN gp120

Peptomers are polymers composed of peptides that are specifically cross-linked in a head-to-tail fashion. Recently, a peptomer composed of an amphipathic peptide from the C4 domain of HIV-1MN gp120 was shown to display a prominent alpha-helical conformation that, as an immunogen, elicited rabbit antibodies recognizing native and recombinant gp120 [Robey et al. (1995) J. Biol. Chem. 270, 23918-23921]. For the present study, we synthesized a conjugate composed of the C4 peptomer covalently linked to calcinated aluminum oxide nanoparticles. The nanoparticles were first reacted with (3-aminopropy])-triethoxysilane to provide an amine load of 15.9 mmol of R-NH2/g of solid. The amine-modified aluminum oxide nanoparticles then were reacted with N-acetylhomocysteine thiolactone at pH 10 to place a reactive thiol on the nanoparticles. A bromoacetylated C4 peptomer, modified at the epsilon-amines of lysine residues, then was reacted with the thiolated nanoparticles to give the peptomer covalently linked to aluminum oxide via a thioether bond. The peptomer load was determined to be 16 mg of peptomer/g of particles, a 55% theoretical yield. Particle shape and size of the peptomer-conjugated alumina were analyzed by electron microscopy and displayed a mean maximum diameter of 355 nm and a mean minimum diameter of 113 nm, well within the desired size range of 300 nm believed to be optimal for mucosal immunization purposes. Experimentally determined values of mean particle diameters, specific surface area, and specific peptomer load provided the information necessary to calculate the mean antigen load, which was determined to be 53000 +/- 42000 peptomer epitopes per particle. Peptomer-alumina conjugates, such as that described here, could form the basis of a new class of biomaterial that combines a chemically defined organic immunogen with a nontoxic chemically defined inorganic adjuvant.

Comparison of the oral, rectal, and vaginal immunization routes for induction of antibodies in rectal and genital tract secretions of women

To determine which mucosal immunization routes may be optimal for induction of antibodies in the rectum and female genital tract, groups of women were immunized a total of three times either orally, rectally, or vaginally with a cholera vaccine containing killed Vibrio cholerae cells and the recombinant cholera toxin B (CTB) subunit. Systemic and mucosal antibody responses were assessed at 2-week intervals by quantitation of CTB-specific antibodies in serum and in secretions collected directly from mucosal surfaces of the oral cavity, rectum, cervix, and vagina with absorbent wicks. The three immunization routes increased levels of specific immunoglobulin G (IgG) in serum and specific IgA in saliva to similar extents. Rectal immunization was superior to other routes for inducing high levels of specific IgA and IgG in rectal secretions but was least effective for generating antibodies in female genital tract secretions. Only vaginal immunization significantly increased both specific IgA and specific IgG in both the cervix and the vagina. In addition, local production of CTB-specific IgG in the genital tract could be demonstrated only in vaginally immunized women. Vaginal immunization did not generate antibodies in the rectum, however. Thus, generation of optimal immune responses to sexually transmitted organisms in both the rectal and the genital mucosae of women may require local immunization at both of these sites.

Comparative analysis of methods for collection and measurement of immunoglobulins in cervical and vaginal secretions of women

The purpose of this study was to systematically compare 3 collection methods, Sno-strips, wicks and cervical-vaginal lavage, for analysis of immunoglobulin concentrations in female genital secretions. In each of 8 women, absorbent wicks and Sno-strips were applied at 4 locations: the lateral wall of the vagina; the posterior vaginal fornix; the surface of the exocervix; and the endocervical canal. Cervical-vaginal lavage was then performed in 4 women with 5 ml PBS. Immunoglobulin and protein concentrations in lavage samples were generally over 100 times lower than in the secretions captured directly from mucosal surfaces with either Sno-strips or wicks. Capture of undiluted secretions with either wicks or Sno-strips allowed calculation of actual immunoglobulin concentrations at specific mucosal sites: for example, median IgA levels were consistently highest in the endocervix and lowest in the vagina. Such information may be crucial in evaluating the correlates of protective immunity against micro-organisms that infect or invade discrete regions of the genital mucosa.

Targeting of mucosal vaccines to Peyer's patch M cells

Transepithelial transport of antigens and pathogens is the first step in the induction of a mucosal immune response. In the intestine, the delivery of antigens across the epithelial barrier to the underlying lymphoid tissue is accomplished by M cells, a specialized epithelial cell type that occurs only in the lymphoid follicle-associated epithelium. Selective and efficient transport of antigen by M cells is considered an essential requirement for effective mucosal vaccines. Therefore, particulate antigen formulations are currently being developed to take advantage of the capacity of M cells to endocytose particles. Based on pathogens that exploit the M cell as an invasion route into the body, live mucosal vaccines have been designed using genetically-engineered, attenuated strains of pathogens such as poliovirus and Salmonella. In an alternative approach, antigens are coupled to or encapsulated in particulate synthetic carriers. To enhance binding and uptake of such nonviable vectors, ligands are being attached which direct the vaccine particle to receptors on the M cell surface.

Distribution of monoclonal antibodies in intestinal and urogenital secretions of mice bearing hybridoma 'backpack' tumours

Mice bearing IgA hybridoma 'backpack' tumours have been used to demonstrate that secretion of a single monoclonal IgA can protect against mucosal infection, but the relevance of this model to normal IgA protection is not clear. The authors analysed the distribution of specific monoclonal and total antibodies in bile, local intestinal secretions, cervical-vaginal secretions, urine and serum of mice bearing anti-cholera toxin (CT) IgA and IgG backpack tumours, with and without bile duct ligation. Backpack tumours resulted in high levels of both anti-CT and total IgA or IgG in serum, and IgA (but not IgG) in bile. Secretions recovered by absorbent filter 'wicks' from mucosal surfaces throughout the intestines of backpack tumour mice contained significant concentrations of monoclonal anti-CT IgA, but total IgA levels were as in normal mice. Neither monoclonal nor total IgA levels on mucosal surfaces were altered by bile duct ligation. Furthermore, anti-CT monoclonal IgA levels in local intestinal secretions of backpack tumour mice were comparable to specific polyclonal IgA levels previously elicited by mucosal immunization with CT. Thus, IgA-mediated protection against enteric challenge in the backpack tumour model may be a valid predictor of protection provided by natural mucosal immunization in vivo. High monoclonal IgA levels in bile, urine and the female genital tract, however, may not reflect the situation in normal immunized mice.

A pathogen-specific epitope inserted into recombinant secretory immunoglobulin A is immunogenic by the oral route

Oral administration of rabbit secretory IgA (sIgA) to adult BALB/c mice induced IgA+, IgM+, and IgG+ lymphoblasts in the Peyer's patches, whose fusion with myeloma cells resulted in hybridomas producing IgA, IgM, and IgG1 antibodies to the secretory component (SC). This suggests that SC could serve as a vector to target protective epitopes into mucosal lymphoid tissue and elicit an immune response. We tested this concept by inserting a Shigella flexneri invasin B epitope into SC, which, following reassociation with IgA, was delivered orally to mice. To identify potential insertion sites at the surface of SC, we constructed a molecular model of the first and second Ig-like domains of rabbit SC. A surface epitope recognized by an SC-specific antibody was mapped to the loop connecting the E and F beta strands of domain I. This 8-amino acid sequence was replaced by a 9-amino acid linear epitope from S. flexneri invasin B. We found that cellular trafficking of recombinant SC produced in mammalian CV-1 cells was drastically altered and resulted in a 50-fold lower rate of secretion. However, purification of chimeric SC could be achieved by Ni2+-chelate affinity chromatoraphy. Both wild-type and chimeric SC bound to dimeric IgA, but not to monomeric IgA. Reconstituted sIgA carrying the invasin B epitope within the SC moiety triggers the appearance of seric and salivary invasin B-specific antibodies. Thus, neo-antigenized sIgA can serve as a mucosal vaccine delivery system inducing systemic and mucosal immune responses.

Role of the glycocalyx in regulating access of microparticles to apical plasma membranes of intestinal epithelial cells: implications for microbial attachment and oral vaccine targeting

Transepithelial transport of antigens and pathogens across the epithelial barrier by M cells may be a prerequisite for induction of mucosal immunity in the intestine. Efficient transport of antigens and pathogens requires adherence to M cell apical surfaces. Coupling of antigen-containing particles to the pentameric binding subunit of cholera toxin (CTB) has been proposed as a means for increasing antigen uptake because the CTB receptor, ganglioside GM1, is a glycolipid present in apical membranes of all intestinal epithelial cells. To test the accessibility of enterocyte and M cell membrane glycolipids to ligands in the size ranges of viruses, bacteria, and particulate mucosal vaccines, we analyzed binding of CTB probes of different sizes to rabbit Peyer's patch epithelium. Soluble CTB-fluorescein isothiocyanate (diameter 6.4 nm) bound to apical membranes of all epithelial cells. CTB coupled to 14 nm colloidal gold (final diameter, 28.8 nm) failed to adhere to enterocytes but did adhere to M cells. CTB-coated, fluorescent microparticles (final diameter, 1.13 microns) failed to adhere to enterocytes or M cells in vivo or to well-differentiated Caco-2 intestinal epithelial cells in vitro. However, these particles bound specifically to GM1 on BALB/c 3T3 fibroblasts in vitro and to undifferentiated Caco-2 cells that lacked brush borders and glycocalyx. Measurements of glycocalyx thickness by electron microscopy suggested that a relatively thin (20 nm) glycocalyx was sufficient to prevent access of 1-micron microparticles to glycolipid receptors. Thus, the barrier function of the intestinal epithelial cell glycocalyx may be important in limiting microbial adherence to membrane glycolipids, and in CTB-mediated targeting of vaccines to M cells and the mucosal immune system.

Lack of association of secretory component with IgA in J chain-deficient mice

J chain has been proposed to play a role in the mucosal transport of polymeric Igs (pIg) by the polymeric Ig receptor (pIgR). We have previously reported the generation of J chain-deficient mice. These mice exhibited elevated serum IgA and depleted biliary and fecal IgA levels compared with wild-type mice. We report here that, unlike the IgA levels in bile and feces, IgA levels in local mucosal and glandular secretions were not depressed in J chain-deficient mice. Breast milk, intestinal mucosal surface, and nasal wash IgA levels in the mutant mice were similar to wild-type mice while bronchoalveolar lavage IgA levels were higher in the J chain-deficient animals. Western blot analysis with an Ab to secretory component (SC), the portion of the pIgR that remains bound to pig in secretions, and immunoprecipitation with Abs directed against IgA showed that secreted IgA was associated with SC in wild-type but not J chain-deficient mice. The IgA in wild-type secretions was polymeric while the secretions of J chain-deficient mice contained IgA monomers and other nonpolymeric IgA forms. Thus, J chain is not essential for IgA transport by intestinal, mammary, or respiratory epithelia but is necessary for the stable association of pIgA with SC. Further, we suggest that J chain-deficient IgA is transported into secretions by a different mechanism than wild-type IgA.

Lack of association of secretory component with IgA in J chain-deficient mice

J chain has been proposed to play a role in the mucosal transport of polymeric Igs (pIg) by the polymeric Ig receptor (pIgR). We have previously reported the generation of J chain-deficient mice. These mice exhibited elevated serum IgA and depleted biliary and fecal IgA levels compared with wild-type mice. We report here that, unlike the IgA levels in bile and feces, IgA levels in local mucosal and glandular secretions were not depressed in J chain-deficient mice. Breast milk, intestinal mucosal surface, and nasal wash IgA levels in the mutant mice were similar to wild-type mice while bronchoalveolar lavage IgA levels were higher in the J chain-deficient animals. Western blot analysis with an Ab to secretory component (SC), the portion of the pIgR that remains bound to pig in secretions, and immunoprecipitation with Abs directed against IgA showed that secreted IgA was associated with SC in wild-type but not J chain-deficient mice. The IgA in wild-type secretions was polymeric while the secretions of J chain-deficient mice contained IgA monomers and other nonpolymeric IgA forms. Thus, J chain is not essential for IgA transport by intestinal, mammary, or respiratory epithelia but is necessary for the stable association of pIgA with SC. Further, we suggest that J chain-deficient IgA is transported into secretions by a different mechanism than wild-type IgA.