Mucosal vaccines for biodefense.

Bioterrorism is the deliberate release of biological toxins, pathogenic viruses, bacteria, parasites, or other infectious agents into the public sphere with the objective of causing panic, illness, and/or death on a local, regional, or possibly national scale. The list of potential biological agents compiled by the Centers for Disease Control and Prevention is long and diverse. However, a trait common to virtually all the potential bioterrorism agents is the fact that they are likely to be disseminated by either aerosol or in food/water supplies with the intention of targeting the mucosal surfaces of the respiratory or gastrointestinal tracts, respectively. In some instances, inhalation or ingestion would mimic the natural route by which humans are exposed to these agents. In other instances, (e.g., the inhalation of a toxin is normally associated with food borne illness), it would represent an unnatural route of exposure. For most potential bioterrorism agents, the respiratory or gastrointestinal mucosa may simply serve as a route of entry by which they gain access to the systemic compartment where intoxication/replication occurs. For others, however, the respiratory or gastrointestinal mucosa is the primary tissue associated with pathogenesis, and therefore, the tissue for which countermeasures must be developed.

Secretory IgA's complex roles in immunity and mucosal homeostasis in the gut.

Secretory IgA (SIgA) serves as the first line of defense in protecting the intestinal epithelium from enteric toxins and pathogenic microorganisms. Through a process known as immune exclusion, SIgA promotes the clearance of antigens and pathogenic microorganisms from the intestinal lumen by blocking their access to epithelial receptors, entrapping them in mucus, and facilitating their removal by peristaltic and mucociliary activities. In addition, SIgA functions in mucosal immunity and intestinal homeostasis through mechanisms that have only recently been revealed. In just the past several years, SIgA has been identified as having the capacity to directly quench bacterial virulence factors, influence composition of the intestinal microbiota by Fab-dependent and Fab-independent mechanisms, promote retro-transport of antigens across the intestinal epithelium to dendritic cell subsets in gut-associated lymphoid tissue, and, finally, to downregulate proinflammatory responses normally associated with the uptake of highly pathogenic bacteria and potentially allergenic antigens. This review summarizes the intrinsic biological activities now associated with SIgA and their relationships with immunity and intestinal homeostasis.

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.

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.

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.

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.

The nadB gene of Salmonella typhimurium complements the nicotinic acid auxotrophy of Shigella flexneri.

Shigella species are characteristically nicotinic acid (NA) auxotrophs. The invasive S. flexneri strain M90T, transformed with the multicopy plasmid pZT349 encoding the nadB gene of Salmonella typhimurium, can grow in minimal glucose medium without exogenous NA, whereas, M90T containing the control vector, pUC18 does not, suggesting that this species lacks L-aspartic acid oxidase, the first enzyme in the de novo NAD biosynthetic pathway. The estimated growth rate of strain M90T (pZT349) in HeLa cells was identical to that of M90T (pUC18), indicating the available intracellular concentration of NA is not limiting for bacterial growth.

Host recognition by the VirA, VirG two-component regulatory proteins of agrobacterium tumefaciens.

Agrobacterium tumefaciens contains about 25 vir genes localized on a 200-kb tumour-inducing (Ti) plasmid that direct a conjugation-like transfer of tumorigenic DNA from the bacterium to the nuclei of infected plant cells. These genes are strongly and coordinately induced during infection in response to three different classes of stimuli which are thought to be key chemical features of a typical wound site. These stimuli are (i) guaiacol and syringol derivatives such as acetosyringone, (ii) sugars such as glucose and glucuronic acid, and (iii) acidic pH. The sensing of these compounds is carried out by the VirA, VirG and ChvE proteins. VirA is a four-domain histidine protein kinase, while VirG is a transcriptional activator which is activated by VirA-mediated phosphorylation. ChvE is a chromosomally encoded periplasmic sugar binding protein which is required for sensing sugars but dispensable for sensing the other two stimuli. Here we will review the nature of these chemical stimuli, the structure and function of the three regulatory proteins, their similarity to sensors found in human and animal pathogens, the factors influencing their pool size, and their role in the host range of different strains of A. tumefaciens.

The chromosomal response regulatory gene chvI of Agrobacterium tumefaciens complements an Escherichia coli phoB mutation and is required for virulence.

In an effort to identify the Agrobacterium tumefaciens phosphate regulatory gene(s), we isolated a clone from an A. tumefaciens cosmid library that restored regulated alkaline phosphatase activity to an Escherichia coli phoB mutant. The gene that complemented phoB was localized by subcloning and deletion analysis, and the DNA sequence was determined. An open reading frame, denoted chvI, was identified that encoded a predicted protein with amino acid similarity to the family of bacterial response regulators and 35% identify to PhoB. Surprisingly, an A. tumefaciens chvI mutant showed normal induction of phosphatase activity and normal virG expression when grown in phosphate-limiting media. However, this mutant was unable to grow in media containing tryptone, peptone, or Casamino Acids and was also more sensitive than the wild type to acidic extracellular pH. This mutant was avirulent on Kalanchoeë diagremontiana and was severely attenuated in vir gene expression. The pH-inducible expression of virG was also abolished. Growth of the chvI mutant was inhibited by K. diagremontiana wound sap, suggesting that avirulence may be due, in part, to the inability of this mutant to survive the plant wound environment.

The Agrobacterium tumefaciens vir gene transcriptional activator virG is transcriptionally induced by acid pH and other stress stimuli.

A set of Agrobacterium tumefaciens operons required for pathogenesis is coordinately induced during plant infection by the VirA and VirG proteins. The intracellular concentration of VirG increases in response to acidic media, and this response was proposed to be regulated at the level of transcription at a promoter (P2) that resembles the Escherichia coli heat shock promoters. To test this hypothesis, we first constructed a virG-lacZ transcriptional fusion. A strain containing this fusion had higher levels of beta-galactosidase activity in acidic media than in media at neutral pH. Second, primer extension analysis of virG indicated that acidic media stimulated the transcription of this promoter. To determine whether P2 is a member of a heat shock-like regulon in A. tumefaciens, five agents that induce E. coli heat shock genes were tested for their abilities to induce a P2-lacZ fusion in A. tumefaciens. P2 was most strongly induced by low pH, was moderately stimulated by CdCl2 or mitomycin C, and was slightly induced by P2 as measured by beta-galactosidase activity and primer extension analysis. Induction by these treatments did not require any Ti plasmid-encoded function or the chromosomally encoded RecA protein. We also pulse-labeled cellular proteins after a shift to low pH and detected several proteins whose synthesis was induced by these conditions. We conclude that P2 is primarily induced by acid pH and secondarily by certain other stimuli, each of which is stressful to cell growth. This stress induction is at least partly independent of the heat shock and SOS responses.

Characterization of the Agrobacterium tumefaciens heat shock response: evidence for a sigma 32-like sigma factor.

We have characterized the heat shock response of Agrobacterium tumefaciens and compared it with the well-characterized Escherichia coli heat shock response. Four major heat shock proteins with apparent molecular masses of 98, 75, 65, and 20 kDa were identified by pulse-labelling cultures after temperature upshift. The three largest proteins comigrated with proteins that were antigenically related to the E. coli heat shock proteins sigma 70, DnaK, and GroEL, respectively. The heat shock proteins were also strongly induced by ethanol and cadmium chloride and were mildly induced by mitomycin C. To determine whether the A. tumefaciens heat shock regulatory system was similar to that of E. coli, we introduced the E. coli dnaK gene into A. tumefaciens. The E. coli DnK protein was expressed in A. tumefaciens, and its synthesis was induced after heat shock. Primer extension analysis of the E. coli dnaK gene in A. tumefaciens indicated that transcription initiated from one or possibly both of the E. coli heat shock promoters. We conclude that A. tumefaciens has a heat shock response similar to that of E. coli, in that (i) similar proteins are induced by heat shock, (ii) synthesis of these proteins is induced in response to similar stimuli, and (iii) A. tumefaciens can recognize an E. coli heat shock promoter, suggesting that A. tumefaciens has a sigma factor similar to sigma 32.