Intestinal alkaline phosphatase preserves the normal homeostasis of gut microbiota

BACKGROUND AND AIMS

The intestinal microbiota plays a critical role in maintaining human health; however, the mechanisms governing the normal homeostatic number and composition of these microbes are largely unknown. Previously it was shown that intestinal alkaline phosphatase (IAP), a small intestinal brush border enzyme, functions as a gut mucosal defence factor limiting the translocation of gut bacteria to mesenteric lymph nodes. In this study the role of IAP in the preservation of the normal homeostasis of the gut microbiota was investigated.

METHODS

Bacterial culture was performed in aerobic and anaerobic conditions to quantify the number of bacteria in the stools of wild-type (WT) and IAP knockout (IAP-KO) C57BL/6 mice. Terminal restriction fragment length polymorphism, phylogenetic analyses and quantitative real-time PCR of subphylum-specific bacterial 16S rRNA genes were used to determine the compositional profiles of microbiotas. Oral supplementation of calf IAP (cIAP) was used to determine its effects on the recovery of commensal gut microbiota after antibiotic treatment and also on the colonisation of pathogenic bacteria.

RESULTS

IAP-KO mice had dramatically fewer and also different types of aerobic and anaerobic microbes in their stools compared with WT mice. Oral supplementation of IAP favoured the growth of commensal bacteria, enhanced restoration of gut microbiota lost due to antibiotic treatment and inhibited the growth of a pathogenic bacterium (Salmonella typhimurium).

CONCLUSIONS

IAP is involved in the maintenance of normal gut microbial homeostasis and may have therapeutic potential against dysbiosis and pathogenic infections.

Nontyphoidal salmonellosis

Nontyphoidal Salmonella are important foodborne pathogens that cause gastroenteritis, bacteremia, and subsequent focal infection. These hardy bacteria are especially problematic in a wide variety of immunocompromised individuals, including (but not limited to) patients with malignancy, human immunodeficiency virus, or diabetes, and those receiving corticosteroid therapy or treatment with other immunotherapy agents. Endovascular infection and deep bone or visceral abscesses are important complications that may be difficult to treat. The site of infection and the individual's immune status influence treatment choices. The harbingers of resistance of nontyphoidal Salmonella to both fluoroquinolones and third-generation cephalosporins have been reported recently, and such resistance is likely to be a therapeutic problem in the future. The current report presents a brief overview of the problems and trends associated with salmonellosis that are of interest to the infectious diseases clinician.

Pilot study of phoP/phoQ-deleted Salmonella enterica serovar typhimurium expressing Helicobacter pylori urease in adult volunteers

Attenuated Salmonella enterica serovar Typhi has been studied as an oral vaccine vector. Despite success with attenuated S. enterica serovar Typhimurium vectors in animals, early clinical trials of S. enterica serovar Typhi expressing heterologous antigens have shown that few subjects have detectable immune responses to vectored antigens. A previous clinical study of phoP/phoQ-deleted S. enterica serovar Typhi expressing Helicobacter pylori urease from a multicopy plasmid showed that none of eight subjects had detectable immune responses to the vectored antigen. In an attempt to further define the variables important for engendering immune responses to vectored antigens in humans, six volunteers were inoculated with 5 x 10(7) to 8 x 10(7) CFU of phoP/phoQ-deleted S. enterica serovar Typhimurium expressing the same antigen. Two of the six volunteers had fever; none had diarrhea, bacteremia, or other serious side effects. The volunteers were more durably colonized than in previous studies of phoP/phoQ-deleted S. enterica serovar Typhi. Five of the six volunteers seroconverted to S. enterica serovar Typhimurium antigens and had strong evidence of anti-Salmonella mucosal immune responses by enzyme-linked immunospot studies. Three of six (three of five who seroconverted to Salmonella) had immune responses in the most sensitive assay of urease-specific immunoglobulin production by blood mononuclear cells in vitro. One of these had a fourfold or greater increase in end-point immunoglobulin titer in serum versus urease. Attenuated S. enterica serovar Typhimurium appears to be more effective than S. enterica serovar Typhi for engendering immune responses to urease. Data suggest that this may be related to a greater stability of antigen-expressing plasmid in S. enterica serovar Typhimurium and/or prolonged intestinal colonization. Specific factors unique to nontyphoidal salmonellae may also be important for stimulation of the gastrointestinal immune system.

Safety and immunogenicity of phoP/phoQ-deleted Salmonella typhi expressing Helicobacter pylori urease in adult volunteers

Salmonella typhi Ty800, deleted for the Salmonella phoP/phoQ virulence regulon has been shown to be a safe and immunogenic single dose oral typhoid fever vaccine in volunteers. This promising vaccine strain was modified to constitutively express a heterologous protein of Gram negative bacterial origin, Helicobacter pylori urease subunits A and B, yielding S. typhi strain Ty1033. Seven volunteers received single oral doses of > or = 10(10) colony forming units of Ty1033; an eighth volunteer received two doses 3 months apart. Side effects were similar to those observed previously in volunteers who received the unmodified vector Ty800. All volunteers had strong mucosal immune responses to vaccination as measured by increases in IgA-secreting cells in peripheral blood directed against S. typhi antigens. Seven of eight volunteers had convincing seroconversion as measured by increases in serum IgG directed against S. typhi flagella and lipopolysaccharide antigens by ELISA. No volunteer had detectable mucosal or humoral immune responses to the urease antigen after immunization with single doses of Ty1033. A subset of three volunteers received an oral booster vaccination consisting of recombinant purified H. pylori urease A/B and E. coli heat labile toxin adjuvant 15 days after immunization with Ty1033. None of three had detectable humoral or mucosal immune responses to urease. Expression of a stable immunogenic protein in an appropriately attenuated S. typhi vector did not engender detectable mucosal or systemic antibody responses; additional work will be needed to define variables important for immunogenicity of heterologous antigens carried by live S. typhi vectors in humans.

Regulation of host immune responses by modification of Salmonella virulence genes

Modifying bacterial virulence genes to probe the nature of host immunity is mostly unexplored. Here we investigate whether host immune responses can be regulated by modification of bacterial virulence genes. In mice, attenuated Salmonella mutant strains with clinical relevance elicited differential host immune responses. Oral administration of a mutant strain with a PhoP-null phenotype promoted potent innate immune responses of macrophages that were sufficient for host defense. In contrast, administration of an Aro- mutant strain elicited stronger specific antibody and T-helper (Th)-cell responses, wherein Th1-type cells were required for clearance. Thus, genetic manipulation of bacteria may be used to broadly alter immune mechanisms that regulate attenuation within the host and to tailor host immunity to specific bacterial pathogens.

Central nervous system infection with Listeria monocytogenes. 33 years' experience at a general hospital and review of 776 episodes from the literature

We reviewed 776 previously reported and 44 new cases of CNS listeriosis outside of pregnancy and the neonatal period, and evaluated the epidemiologic, diagnostic, and therapeutic characteristics of this infection. Among patients with Listeria meningitis/meningoencephalitis, hematologic malignancy and kidney transplantation were the leading predisposing factors, but 36% of patients had no underlying diseases recognized. The infection occurred throughout life, with a higher incidence before the age of 3 and after the age of 45-50 years. Fever, altered sensorium, and headache were the most common symptoms, but 42% of patients had no meningeal signs on admission. Compared with patients with acute meningitis due to other bacterial pathogens, patients with Listeria infection had a significantly lower incidence of meningeal signs, and the CSF profile was significantly less likely to have a high WBC count or a high protein concentration. Gram stain of CSF was negative in two-thirds of cases of CNS listeriosis. One-third of patients had focal neurologic findings, and approximately one-fourth developed seizures over their course. Mortality was 26% overall, and was higher among patients with seizures and those older than 65 years of age. Relapse occurred in 7% of episodes. Ampicillin for a minimum of 15-21 days (with an aminoglycoside for at least the first 7-10 days) remains the treatment of choice. Cerebritis/abscess due to L. monocytogenes, without meningeal involvement, is less common but may be diagnosed by blood cultures and CNS imaging, or by stereotactic biopsy. Longer antibiotic therapy (at least 5-6 weeks) is needed in the presence of localized CNS involvement.

Transcriptional regulation of Salmonella virulence: a PhoQ periplasmic domain mutation results in increased net phosphotransfer to PhoP

A mutation in the phoP/phoQ locus (pho-24) that results in unregulated expression of PhoP-activated genes (phenotype PhoP constitutive [PhoP(c)]) was mapped to phoQ. Change of a Thr to Ile at position 48 of PhoQ was responsible for the PhoP(c) phenotype (attenuation of mouse virulence, defects in epithelial cell invasion, and macrophage spacious phagosome formation). PhoP phosphorylation by membrane extracts required PhoQ, and PhoQ Ile-48-containing membranes demonstrated increased net phosphorylation of PhoP.

phoP/phoQ-deleted Salmonella typhi (Ty800) is a safe and immunogenic single-dose typhoid fever vaccine in volunteers

The phoP/phoQ virulence regulatory genes of Salmonella typhi Ty2 were deleted, and the resultant strain (Ty800) was tested as a live attenuated typhoid fever vaccine in human volunteers. Groups of 2 or 3 subjects received single oral doses of 10(7), 10(8), 10(9), or 10(10) cfu. Two volunteers who received the largest dose had self-limited side effects; no bacteremias were detected. Ten of 11 subjects had evidence of intestinal immune responses to the vaccine as measured by increases in S. typhi lipopolysaccharide-specific IgA-secreting cells in peripheral blood samples. Humoral immune responses were measured and compared with those of control vaccinees who received 4 oral doses of S. typhi Ty21a. In the most sensitive assays, 9 of 11 volunteers and 5 of 8 Ty21a control vaccinees had evidence of IgG directed against S. typhi antigens. Ty800 is safe, and single oral doses are highly immunogenic in humans.

Evaluation of a phoP/phoQ-deleted, aroA-deleted live oral Salmonella typhi vaccine strain in human volunteers

Improved live oral typhoid fever vaccines may be engineered by deletion of Salmonella-specific virulence genes in Salmonella typhi. Ty445, an aroA-deleted S. typhi Ty2 strain also deleted for the phoP/phoQ Salmonella typhimurium virulence regulatory locus, was tested in human volunteers. Volunteers received escalating single doses of the vaccine; subsequently 14 individuals received two doses of 10(10) c.f.u. without significant side-effects. Control vaccinees received four doses of the live oral typhoid vaccine Ty21a. Of controls, 5/8 seroconverted as measured by increases in serum IgG directed against S. typhi O antigen or whole bacterial antigens in ELISAs. Only 2/14 volunteers receiving the experimental vaccine Ty445 seroconverted. Although a delta aroA delta phoP/phoQ S. typhi strain is overattenuated for use as a typhoid fever vaccine, our data demonstrate that the deletion of the phoP/phoQ locus in S. typhi significantly attenuates this human pathogen.

Macrophage-inducible expression of a model antigen in Salmonella typhimurium enhances immunogenicity

Attenuated Salmonella are useful oral vaccine vectors capable of carrying multiple heterologous antigen genes, but optimal expression of foreign antigens has not yet been achieved. We hypothesized that Salmonella phoP-activated genes, which are transcriptionally activated within antigen-processing macrophages, could prove useful for delivery of heterologous antigens to the immune system. We have created a suicide vector that allows the stable chromosomal insertion of heterologous antigen genes within the phoP-activated gene C (pagC) of Salmonella and permits the expression of heterologous antigens as fusion proteins between the first 84 amino acids of PagC and the chosen antigen. The Escherichia coli phoA gene encoding alkaline phosphatase was cloned into this vector; the resultant plasmid was used to construct Salmonella typhimurium strains that express PagC-alkaline phosphatase fusion proteins from a single chromosomal gene copy. Such strains were administered orally and i.p. as vaccines to BALB/c mice and compared with control strains expressing alkaline phosphatase constitutively. After 3 weeks, mouse sera were analyzed for IgG responses to S. typhimurium lipopolysaccharide and alkaline phosphatase. Remarkably, though all mice had comparable antibody responses to lipopolysaccharide, only mice immunized with strains bearing phoP-activated fusion genes had antibody responses to the heterologous antigen. We conclude that expression of a heterologous antigen from an S. typhimurium in vivo-induced promoter that is activated within macrophages markedly enhances the immunogenicity of a model antigen expressed from a single chromosomal gene copy.

Innervation of periosteum and bone by sympathetic vasoactive intestinal peptide-containing nerve fibers

Neural control of bone metabolism and growth has been suggested, although the identity of participating neurons and neurotransmitters effecting this control has not been established. Immunohistochemical studies demonstrated a system of vasoactive intestinal peptide (VIP)-immunoreactive nerve fibers that innervate periosteum and bone in several mammalian species. Thoracic sympathetic chain ganglionectomy resulted in an ipsilateral loss of VIP-immunoreactive fibers in the periosteum of ribs, whereas dorsal root ganglionectomy had no effect. Injection of fast blue into rib periosteum labeled a population of VIP-immunoreactive sympathetic postganglionic neurons. Thus, postganglionic sympathetic neurons may provide an important means by which VIP regulates bone mineralization.

Functional receptors for vasoactive intestinal peptide on human osteosarcoma cells

We have previously reported that vasoactive intestinal peptide (VIP) stimulates bone resorption in organ culture via a cAMP-dependent mechanism. Here we describe functional receptors for VIP on a clonal line of human osteosarcoma cells, SaOs-2. SaOs-2 cells respond to VIP with an increase in cAMP. The effect was rapid (2 min) and dose dependent from 0.15-15 nM VIP, with half-maximal stimulation at 1.4 nM. SaOs-2 cells produce prostaglandin E2 (PGE2) and respond to exogenous PGE2 with increases in cAMP approximately one third as great as those induced by VIP. However, the VIP-stimulated increases in cAMP occurred without detectable increases in PGE2 production, and increases in cAMP were unaffected by the cyclooxygenase inhibitor indomethacin. SaOs-2 cells pretreated with VIP for 24 h were significantly less responsive to a second acute challenge with VIP, but retained their ability to respond to PGE2. Similarly, pretreatment with PGE2 induced homologous desensitization to PGE2, but had no effect on the VIP-stimulated increase in cAMP. These patterns of response paralleled those previously described in whole bone in organ culture. Binding studies with [125I]VIP demonstrated specific, saturable, high affinity receptors for VIP on SaOs-2 cells. Scatchard analysis of [125I]VIP binding at 37 C resulted in a curvilinear plot. Analysis based upon the assumption of two independent binding sites gave Kd values of 0.44 and 17 nM for high and low affinity binding sites, respectively. The numbers of high and low affinity sites per cell were determined to be 8,500 and 57,000, respectively. Binding of [125I]VIP was partially inhibited by two related peptides, secretin and PHI-27, but not by PTH, calcitonin or a variety of unrelated peptides. We conclude that the action of VIP on human SaOs-2 cells is similar to that observed in intact mouse calvaria, and that these cells provide a good model for the study of the initial steps of VIP action in bone.

Platelet-derived growth factor stimulates bone resorption via a prostaglandin-mediated mechanism

Platelet-derived growth factor (PDGF) stimulated up to 15-fold the production of prostaglandin E2 (PGE2) and bone resorption in neonatal mouse calvaria in organ culture. The action of PDGF on bone resorption occurred at low concentrations of the protein (ED50 = 10 ng/ml). All concentrations of PDGF which stimulated resorption also enhanced the production of PGE2 by bone; concentrations of PDGF which did not stimulate resorption did not enhance PGE2 production. PDGF-induced formation of PGE2 and bone resorption were inhibited completely by indomethacin (100 ng/ml) and hydrocortisone (1 microgram/ml). Indomethacin did not inhibit the bone resorption-stimulating activity of exogenous PGE2. In the continued presence of a maximum concentration of PDGF (100 ng/ml), an increase in bone resorption, as measured by an increase in medium calcium, was detected at 16 h (P less than 0.01), but not at 12 h; however, an increase in PGE2 production occurred within the first 2 h of treatment. A similar lag period for the onset of bone resorption was seen after the addition of exogenous PGE2 to the culture medium. On the other hand, exposure of bones to PDGF (50 ng/ml) for as brief a period as 5-15 min, followed by washout of PDGF, triggered bone resorption over the subsequent 48 h. PDGF increased cAMP production by bone within 30 min, and this effect of PDGF was blocked completely by indomethacin while the action of exogenous PGE2 on the production of cAMP was not blocked by indomethacin. The action of a low concentration of PDGF (1 ng/ml), which did not stimulate bone resorption alone, was potentiated by the phosphodiesterase inhibitor isobutylmethylxanthine (4 microM). We conclude that low concentrations of PDGF stimulate bone resorption via the enhanced local production of PGE2.