Shiga toxin-producing Escherichia coli (STEC) is a major cause of severe food-borne disease worldwide, and two Shiga toxins, Stx1 and Stx2, are primarily responsible for the serious disease consequence, hemolytic-uremic syndrome (HUS). Here we report identification of a panel of heavy-chain-only antibody (Ab) VH (VHH) domains that neutralize Stx1 and/or Stx2 in cell-based assays. VHH heterodimer toxin-neutralizing agents containing two linked Stx1-neutralizing VHHs or two Stx2-neutralizing VHHs were generally much more potent at Stx neutralization than a pool of the two-component monomers tested in cell-based assays and in vivo mouse models. We recently reported that clearance of toxins can be promoted by coadministering a VHH-based toxin-neutralizing agent with an antitag monoclonal antibody (MAb), called the “effector Ab,” that indirectly decorates each toxin molecule with four Ab molecules. Decoration occurs because the Ab binds to a common epitopic tag present at two sites on each of the two VHH heterodimer molecules that bind to each toxin molecule. Here we show that coadministration of effector Ab substantially improved the efficacy of Stx toxin-neutralizing agents to prevent death or kidney damage in mice following challenge with Stx1 or Stx2. A single toxin-neutralizing agent consisting of a double-tagged VHH heterotrimer—one Stx1-specific VHH, one Stx2-specific VHH, and one Stx1/Stx2 cross-specific VHH—was effective in preventing all symptoms of intoxication from Stx1 and Stx2 when coadministered with effector Ab. Overall, the availability of simple, defined, recombinant proteins that provide cost-effective protection against HUS opens up new therapeutic approaches to managing disease.
Infection with Shiga toxin- (Stx-) producing E. coli causes life threatening hemolytic uremic syndrome (HUS), a leading cause of acute renal failure in children. Of the two antigenically distinct toxins, Stx1 and Stx2, Stx2 is more firmly linked with the development of HUS. In the present study, selective evolution of ligands by exponential enrichment (SELEX) was used in an attempt to identify RNA aptamers against Stx1 and Stx2. After 5 rounds of selection, significant enrichment of aptamer pool was obtained against Stx2, but not against Stx1, using a RNA aptamer library containing 56 random nucleotides (N56). Characterization of individual aptamer sequences revealed that six unique RNA aptamers (mA/pC, mB/pA, mC, mD, pB, and pD) recognized Stx2 in a filter binding assay. None of these aptamers bound Stx1. Aptamers mA/pC, mB/pA, mC, and mD, but not pB and pD, partially blocked binding of Alexa 488-labeled Stx2 with HeLa cells in a flow cytometry assay. However, none of the aptamers neutralized Stx2-mediated cytotoxicity and death of HeLa cells.
Cryptosporidium hominis and Cryptosporidium
parvum, which infect humans equally, are genetically/antigenically almost identical. It remains unclear, however, whether infection with C. hominis protects against C. parvum. Gnotobiotic piglets were used to investigate cross-protection. After ≥3 days of recovery from C. hominis infection, the piglets were completely protected against subsequent challenge with C. hominis but only partially against challenge with C. parvum, as compared with age-matched control animals challenged with either species. In conclusion, C. hominis–specific immunity was sufficient to completely protect against challenge with the same species but insufficient to provide the same level of protection against C. parvum.
Hemolytic uremic syndrome (HUS) leading to acute kidney failure, is a condition linked to the production of primarily Shiga toxin 2 (Stx2) by some E. coli serotypes. We have previously shown that Stx2 A subunit-specific human monoclonal antibody (HuMAb) 5C12, and B subunit-specific HuMAb 5H8 inhibit cultured cell death, and protect mice and piglets from fatal Stx2-intoxication. We have also shown that 5H8 blocks binding of Stx2 to its cell-surface receptor globotriaosyl ceramide (Gb3), whereas Stx2 when complexed with 5C12 binds Gb3 with higher affinity than Stx2. The mechanism by which 5C12 neutralizes Stx2 in vitro involves trapping of Stx2 in the recycling endosomes and releasing it into the extracellular environment. Because of the clinical implications associated with the formation of Stx2/antibody complexes and the potential for trapping and clearance through a severely damaged kidney associated with HUS, we investigated the likely site(s) of Stx2/antibody localization and clearance in intoxicated mice treated with antibody or placebo.
Mice were injected with radiolabeled Stx2 (125I-Stx2) 4 hours after administration of 5C12, 5H8, or phosphate buffered saline (PBS) and the sites of localization of labeled Stx2, were investigated 3, 24 and 48 hours later. The liver recorded statistically much higher concentrations of labeled Stx2 for groups receiving 5C12 and 5H8 antibodies after 3, 24 and 48 hours, as compared with the PBS group. In contrast, highest levels of labeled Stx2 were detected in the kidneys of the PBS group at all 3 sampling times. Mice receiving either of the two HuMAbs were fully protected against the lethal effect of Stx2, as compared with the fatal outcome of the control group.
The results suggest that HuMAbs 5C12 and 5H8 promoted hepatic accumulation and presumably clearance of toxin/antibody complexes, significantly diverting Stx2 localization in the kidneys, the target of Stx2 and the cause of HUS. This is in contrast to the fatal outcome of the control group receiving PBS. The results also confirm earlier observations that both HuMAbs are highly and equally protective against Stx2 intoxication in mice.
Shiga toxin; Radiolabel; Antibody; Toxin elimination; Toxin concentration; Pharmacokinetic; Human monoclonal antibody
Shiga toxin 2 (Stx2) producing Escherichia coli (STEC) strains are more frequently isolated from hemolytic-uremic syndrome cases than strains that produce Stx1 and Stx2, and rarely the strains that produce only Stx1. Studies have implicated Stx2 as the sole contributor to acute kidney failure and other systemic complications in humans, and our study adds further support to this assumption since Stx2- and Stx1-specific antibodies protected 100% and 0% of piglets, respectively, against an oral challenge with a Stx1- and Stx2-producing STEC strain. We conclude that Stx2-specific antibody is sufficient to protect piglets, and possibly humans, against STEC producing both toxins.
Shiga toxin; Stx; antibody; enterohemorrhagic; piglet; hemolytic uremic syndrome; kidney failure; E. coli
The genus Cryptosporidium includes several species of intestinal protozoan parasites which multiply in intestinal epithelial cells. The impact of this infection on the transcriptome of cultured host cells was investigated using DNA microarray hybridizations. The expression of 14 genes found to be consistently up- or down-regulated in infected cell monolayers was validated with RT PCR. Using immunofluorescence we examined the expression of Protease Activated Receptor-2, which is encoded by one of the up-regulated genes. In infected cells this receptor localized to the host cell membrane which covers intracellular trophozoites and meronts. This observation indicates that the composition of the host cell membrane is affected by the developing trophozoite, a phenomenon which has not been described previously.
Cryptosporidium parvum; human ileocecal epithelial cells; HCT-8; Protein Activated Receptor-2
5C12 HuMAb is a human monoclonal antibody against the A subunit of Shiga toxin 2 (Stx2). We have previously shown that 5C12 HuMAb effectively neutralizes the cytotoxic effects of this toxin by redirecting its transport within the cell and also by neutralizing the toxin's ability to inhibit protein synthesis. The 5C12 HuMAb and its recombinant IgG1 version protect mice at a dose of 0.6 μg against a lethal challenge of Stx2. The contribution of the Fc region to this observed neutralization activity of the 5C12 antibody against Stx2 was investigated in this study. Using recombinant DNA technology, 5C12 isotype variants (IgG1, IgG2, IgG3, and IgG4) and antibody fragments [Fab, F(ab′)2] were expressed in Chinese hamster ovary cells and evaluated in vitro and in vivo. All four 5C12 isotype variants showed protection in vitro, with the IgG3 and IgG4 variants showing the highest protection in vivo. The Fab and F(ab′)2 fragments also showed protection in vitro but no protection in the mouse toxicity model. Similar results were obtained for a second HuMAb (5H8) against the B subunit of Stx2. The data suggest the importance of the Fc region for neutralization activity, but it is not clear if this is related to the stability of the full-length antibody or if the Fc region is required for effective elimination of the toxin from the body.
Shiga toxin 2 (Stx2), one of two Stx liberated by Stx-producing Escherichia coli, is composed of an A subunit monomer and a B subunit pentamer, and is directly linked with hemolytic uremic syndrome in children. The pentameric B subunit binds to its cell surface receptor Gb3 for toxin internalization, and the A subunit follows intracellular retrograde transport to the cytosol where its RNA N-glycosidase activity (RNA-NGA) shuts down the protein synthesis, and leads to cell death. The present study investigated the ability of 19 Stx2 A subunit-specific human monoclonal antibodies (HuMAbs) to neutralize the RNA-NGA, and the association this neutralizing activity with protection of HeLa cells and mice against Stx2-induced death.
The HuMAbs that were stronger inhibitors of RNA-NGA were also better at neutralizing Stx2 mediated HeLa cell death, and those that were weaker inhibitors of RNA-NGA activity were also weaker in protecting HeLa cells. These results suggest that the ability of an A subunit-specific antibody to block the RNA-NGA of the toxin is directly related to its ability to neutralize Stx2-mediated HeLa cell death. However, with the exception of the best RNA-NGA blocking antibodies 5C12 and 2F10, the efficacies of antibody neutralization of RNA-NGA of Stx2 did not correlate with their in vivo protective efficacies. The HuMAb 6C3, which neutralized RNA N-glycosidase activity of Stx2 less effectively than the HuMAbs 6D8 and 6B7, protected 100% of the mice against Stx2 challenge at 50 μg/mouse dose. In contrast, the HuMAbs 6D8 and 6B7, which neutralized RNA N-glycosidase activity of Stx2 more effectively than 6C3, protected 20% and 0% mice at that dose, respectively.
The neutralization efficiency of the RNA-NGA of Stx2 by A subunit-specific antibodies correlate strongly with their abilities to protect HeLa cells against Stx2-mediated toxicity but only the strongest RNA-NGA-neutralizing antibodies correlate very well with both protecting HeLa cells and mice against Stx2 challenge.
Infection of children with Shiga toxin (Stx)-producing Escherichia coli (STEC) is the leading cause of hemolytic-uremic syndrome (HUS). Stx2, one of two toxins liberated by the bacteria, is directly linked with HUS. We have previously shown that Stx2-specific human monoclonal antibodies (HuMAbs) protect mice and piglets from fatal systemic complications of Stx2. The present study investigates the mechanisms by which our most efficacious A- and B-subunit-specific HuMAbs neutralize the cytotoxic effects of Stx2 in vitro. Whereas the B-subunit-specific HuMAb 5H8 blocked binding of Stx2 to its receptor on the cell surface, the A-subunit-specific HuMAb 5C12 did not interfere with the toxin-receptor binding. Further investigations revealed that 5C12 did not block endocytosis of Stx2 by HeLa cells as both Stx2 and 5C12 colocalized with early endosomes. However, 5C12 blocked the retrograde transport of the toxin into the Golgi and the endoplasmic reticulum, preventing the toxin from entering the cytosol where the toxin exerts its cytotoxic effect. The endocytosed 5C12/Stx2 complexes appear to be rapidly transported to the plasma membrane and/or to the slow recycling perinuclear compartments, followed by their slow recycling to the plasma membrane, and release into the extracellular environment.
Enterocytozoon bieneusi, a microsporidian, is clinically one of the most significant opportunistic causes of diarrhea and wasting associated with profound human immunodeficiencies. The lack of an animal model for E. bieneusi hinders serious investigations and limits the availability of spores to individuals with severe human immunodeficiency virus/AIDS disease who are infected with E. bieneusi. The development of procedures for purification and concentration of spores from stools of infected humans has led to the production of immune reagents and provided a source of spores to conduct research, including attempts to develop and serially propagate E. bieneusi in rodent models. We have evaluated and successfully infected six different immunodeficient and/or immunosuppressed rodent models and have demonstrated persistent infections lasting at least 18 weeks in SCID mice and in nude rats. To enhance the intensity and duration of infection in these two models, animals were given anti-gamma interferon monoclonal antibody injections at regular intervals. Of the six models evaluated, nude rats and gerbils immunosuppressed with dexamethasone excreted the highest number of spores and for longer time periods. Four different E. bieneusi isolates were equally infectious, and one of them was serially propagated in nude rats six times over a period of 10 months. Typically, rats challenged orally with 104 spores yielded 2 × 107 to 6.3 × 107 spores per single fecal sample when the level of spores was measured 2 weeks later. Rodent models and a nonhuman source of fresh spores will considerably enhance future investigations on this important opportunistic pathogen, including the screening and evaluation of urgently needed chemotherapeutic agents.
Bile salts such as sodium taurocholate (NaTC) are routinely used to induce the excystation of Cryptosporidium oocysts. Here we show that NaTC significantly enhanced the invasion of several cultured cell lines by freshly excysted Cryptosporidium parvum and Cryptosporidium hominis sporozoites. A variety of purified bile salts or total bile from bovine also enhanced the invasion of cultured cells by C. parvum. Further studies demonstrated that NaTC increased protein secretion and gliding motility of sporozoites, the key processes for successful invasion. These observations may lead to improved Cryptosporidium infectivity of cultured cells and help future studies on the host-parasite interaction.
Enterocytozoon bieneusi is clinically the most significant among the microsporidia causing chronic diarrhea, wasting, and cholangitis in individuals with human immunodeficiency virus/AIDS. Microscopy with either calcofluor or modified trichrome stains is the standard diagnostic test for microsporidiosis and does not allow species identification. Detection of E. bieneusi infection based on PCR is limited to a few reference laboratories, and thus it is not the standard diagnostic assay. We have recently reported the development and characterization of a panel of monoclonal antibodies against E. bieneusi, and in this publication we evaluated the specificity and sensitivity of an immunofluorescence assay (IFA), compared with PCR, in simian immunodeficiency virus-infected macaques. The IFA, which correlated with the primary PCR method, with a detection limit of 1.5 ×105 spores per gram of feces, will simplify considerably the detection of E. bieneusi spores in clinical and environmental specimens and in laboratory and epidemiological investigations.
Enterocytozoon bieneusi is clinically the most significant among the microsporidia infecting humans, causing chronic diarrhea, wasting, and cholangitis in individuals with human immunodeficiency virus/AIDS. The lack of immune reagents is largely due to the absence of methods for laboratory propagation of E. bieneusi. We recently described a procedure for the concentration and purification of spores from diarrheic stool of infected humans. Purified spores were used to immunize mice for production and screening of monoclonal antibodies (MAbs) against E. bieneusi. The eight immunoglobulin M MAbs generated and fully characterized did not cross-react with other human microsporidia or with other microorganisms normally present in stool. One of the MAbs, 2G4, reacted with E. bieneusi spores in stools from monkeys and humans, without background fluorescence, which makes it an ideal diagnostic reagent. It also recognizes intracellular stages of the parasite and will be suitable for determining tissue distribution of E. bieneusi in infected hosts. At least two immunodominant antigens of E. bieneusi of 33,000 and 35,000 Da exist, which were recognized by rabbit and mouse antisera. The availability of MAbs against E. bieneusi will simplify considerably the diagnosis of this infection in humans and will provide tools for epidemiologic investigations regarding the true prevalence of the infection in various human and mammalian populations and the environmental sources of infection.
Infection of children with Shiga toxin (Stx)-producing Escherichia coli (STEC) can lead to hemolytic-uremic syndrome (HUS) in 5 to 10% of patients. Stx2, one of two toxins liberated by the bacterium, is directly linked with HUS. We have previously shown that Stx-specific human monoclonal antibodies protect STEC-infected animals from fatal systemic complications. The present study defines the protective antibody dose in relation to the time of treatment after the onset of diarrhea in infected gnotobiotic piglets. Using the mouse toxicity model, we selected 5C12, an antibody specific for the A subunit, as the most effective Stx2 antibody for further characterization in the piglet model in which piglets developed diarrhea 16 to 40 h after bacterial challenge, followed by fatal neurological symptoms at 48 to 96 h. Seven groups of piglets received doses of 5C12 ranging from 6.0 mg/kg to 0.05 mg/kg of body weight, administered parenterally 48 h after bacterial challenge. The minimum fully protective antibody dose was 0.4 mg/kg, and the corresponding serum antibody concentration in these piglets was 0.7 μg (±0.5)/ml, measured 7 to 14 days after administration. Of 40 infected animals which received Stx2 antibody treatment of ≥0.4 mg/kg, 34 (85%) survived, while only 1 (2.5%) of 39 placebo-treated animals survived. We conclude that the administration of the Stx2-specific antibody was protective against fatal systemic complications even when it was administered well after the onset of diarrhea. These findings suggest that children treated with this antibody, even after the onset of bloody diarrhea, may be equally protected against the risk of developing HUS.
Enterocytozoon bieneusi spores derived from rhesus macaque feces were purified by serial salt-Percoll-sucrose-iodixanol centrifugation, resulting in two bands with different specific densities of 95.6% and 99.5% purity and with a recovery efficiency of 10.8%. An ultrastructural examination revealed typical E. bieneusi spores. Twenty-six stable hybridomas were derived from BALB/c mice immunized with spores and were cloned twice by limiting dilution or growth on semisolid medium. Four monoclonal antibodies (MAbs), reacting exclusively with spores, were further characterized. These MAbs specifically reacted with spores present in stools of humans and macaques, as visualized by immunofluorescence, and with spore walls, as visualized by immunoelectron microscopy. A blocking enzyme-linked immunosorbent assay and Western blotting revealed that the epitope recognized by 8E2 was different from those recognized by 7G2, 7H2, and 12G8, which identified the same 40-kDa protein. These MAbs will be valuable tools for diagnostics, for epidemiological investigations, for host-pathogen interaction studies, and for comparative genomics and proteomics.
Enterocytozoon bieneusi is clinically the most significant of the microsporidia in humans, causing chronic diarrhea wasting and cholangitis in individuals with human immunodeficiency virus infection and AIDS. Little progress on this infection has been made because of the inability to propagate E. bieneusi in vitro and in vivo, which limits the source of parasite spores to the stools of infected human patients. Given the size and shape of the E. bieneusi spores (1.1 to 1.6 by 0.7 to 1.0 μm) and the lack of specific immune reagents, the identification and purification of large quantities of spores from feces are technically challenging. Consequently, diagnosis relies entirely on PCR, a labor-intensive approach that requires highly skilled personnel. We describe a method for the purification of E. bieneusi spores from human stools and the production of rabbit-specific antisera. Spores were purified by a combination of isopycnic Percoll gradient centrifugation and continuous sucrose gradient centrifugation. Specific polyclonal antibodies raised in mice and rabbits reacted by indirect immunofluorescence with E. bieneusi but not with Encephalitozoon spp., Candida albicans, Staphylococcus aureus, Escherichia coli, or other forms present in human stools.
Hemolytic uremic syndrome (HUS) is a disease that can lead to acute renal failure and often to other serious sequelae, including death. The majority of cases are attributed to infections with Escherichia coli, serotype O157:H7 strains in particular, which cause bloody diarrhea and liberate one or two toxins known as Shiga toxins 1 and 2. These toxins are thought to directly be responsible for the manifestations of HUS. Currently, supportive nonspecific treatment is the only available option for the management of individuals presenting with HUS. The benefit of antimicrobial therapy remains uncertain because of several reports which claim that such intervention can in fact exacerbate the syndrome. There have been only a few specific therapies directed against neutralizing the activities of these toxins, but none so far has been shown to be effective. This article reviews the literature on the mechanism of action of these toxins and the clinical manifestations and current management and treatment of HUS. The major focus of the article, however, is the development and rationale for using neutralizing human antibodies to combat this toxin-induced disease. Several groups are currently pursuing this approach with either humanized, chimeric, or human antitoxin antibodies produced in transgenic mice. They are at different phases of development, ranging from preclinical evaluation to human clinical trials. The information available from preclinical studies indicates that neutralizing specific antibodies directed against the A subunit of the toxin can be highly protective. Such antibodies, even when administered well after exposure to bacterial infection and onset of diarrhea, can prevent the occurrence of systemic complications.
Shiga toxin-producing Escherichia coli (STEC) strains are responsible for causing hemolytic-uremic syndrome (HUS), and systemic administration of Shiga toxin (Stx)-specific human monoclonal antibodies (HuMAbs) is considered a promising approach for prevention or treatment of the disease in children. The goal of the present study was to investigate the ability of Stx2-specific HuMAbs to protect against infections with STEC strains that produce Stx2 variants. Dose-response studies on five HuMAbs, using the mouse toxicity model, revealed that only the three directed against the A subunit were protective against Stx2 variants, and 5C12 was the most effective among the three tested. Two HuMAbs directed against the B subunit, while highly effective against Stx2, were ineffective against Stx2 variants. In a streptomycin-treated mouse model, parenteral administration of 5C12 significantly protected mice up to 48 h after oral bacterial challenge. We conclude that 5C12, reactive against the Stx2 A subunit, is an excellent candidate for immunotherapy against HUS and that antibodies directed against the A subunit of Stx2 have broad-spectrum activity that includes Stx2 variants, compared with those directed against the B subunit.