Protection against amebic liver abscess formation in the severe combined immunodeficient mouse by human anti-amebic antibodies

Characterization of Entamoeba histolytica intermediate subunit lectin-specific human monoclonal antibodies generated in transgenic mice expressing human immunoglobulin loci

Four fully human monoclonal antibodies (MAbs) to Entamoeba histolytica intermediate subunit lectin (Igl) were prepared in XenoMouse mice, which are transgenic mice expressing human immunoglobulin loci. Examination of the reactivities of these MAbs to recombinant Igl1 and Igl2 of E. histolytica showed that XEhI-20 {immunoglobulin G2(kappa) [IgG2(kappa)]} and XEhI-28 [IgG2(kappa)] were specific to Igl1, XEhI-B5 [IgG2(kappa)] was specific to Igl2, and XEhI-H2 [IgM(kappa)] was reactive with both Igls. Gene analyses revealed that the V(H) and V(L) germ lines were VH3-48 and L2 for XEhI-20, VH3-21 and L2 for XEhI-28, VH3-33 and B3 for XEhI-B5, and VH4-4 and A19 for XEhI-H2, respectively. Flow cytometry analyses showed that the epitopes recognized by all of these MAbs were located on the surfaces of living trophozoites. Confocal microscopy demonstrated that most Igl1 and Igl2 proteins were colocalized on the surface and in the cytoplasm, but different localization patterns in intracellular vacuoles were also present. The preincubation of trophozoites with XEhI-20, XEhI-B5, and XEhI-H2 caused significant inhibition of the adherence of trophozoites to Chinese hamster ovary cells, whereas preincubation with XEhI-28 did not do so. XEhI-20, XEhI-B5, and XEhI-H2 were injected intraperitoneally into hamsters 24 h prior to intrahepatic challenge with E. histolytica trophozoites. One week later, the mean abscess size in groups injected with one of the three MAbs was significantly smaller than that in controls injected with polyclonal IgG or IgM isolated from healthy humans. These results demonstrate that human MAbs to Igls may be applicable for immunoprophylaxis of amebiasis.

Recent Progress in Vaccines for Amebiasis

The persistence of amebiasis as a global health problem, despite the availability of effective treatment, has led to the search for vaccines to prevent this deadly disease. Recent clinical studies suggest that mucosal immunity could provide some protection against recurrent intestinal infection with E. histolytica, but there is contradictory evidence about protective immunity after amebic liver abscess. Progress in vaccine development has been facilitated by new animal models that allow better testing of potential vaccine candidates and by the application of recombinant technology to vaccine design. Oral vaccines utilizing amebic antigens either co-administered with some form of cholera toxin or expressed in attenuated strains of Salmonella or Vibrio cholera have been developed and tested in animals for mucosal immunogenicity. Although there has been significant progress on a number of fronts, there are unanswered questions regarding the effectiveness of immune responses in preventing disease in man and, as yet, no testing of any of these vaccines in humans has been performed.

Distribution of Entamoeba histolytica Gal/GalNAc lectin-specific antibody response in an endemic area

To demonstrate the dynamics of specific antibody isotypes against Entamoeba histolytica Gal/GalNAc adhesin and its correlation, if any, with the development of immunity, we evaluated subjects suffering from a spectrum of amoebic infections ranging from amoebic liver abscess (ALA) to asymptomatic cyst passers. The quantitative analysis of antibody levels was done in the sera by enzyme-linked immunosorbent assay. Gal/GalNAc adhesin-specific immunoglobulin G (IgG) was higher in ALA (and their follow-ups) and intestinal amoebiasis cases as compared with asymptomatic cyst passers (P < 0.05). Among the isotypes of IgG, high levels of IgG1 (60% of the total IgG concentration), suggestive of T-helper 2-type response, was associated with ALA cases. Intestinal amoebiasis cases and cyst passers had high percentage of IgG1 and IgG4 antibodies as compared with per cent IgG2 and IgG3 (of the total IgG), whereas follow-up cases of ALA had predominantly IgG2 and IgG3 isotypes of antibodies. Gal/GalNAc lectin-specific IgM antibodies were maximum in cases of intestinal amoebiasis. ALA cases and their follow-ups had significantly lower levels of Gal/GalNAc-specific IgM levels as compared with cyst passers (P < 0.05). Gal/GalNAc adhesin-specific IgA antibodies were raised maximally in intestinal amoebic infection cases. ALA cases and their 3-month follow-ups had significantly higher concentrations of lectin-specific IgA (P < 0.05) as compared with the healthy subjects.

Expression in fibroblasts and in live animals of Entamoeba histolytica polypeptides EhCP112 and EhADH112

EhCPADH is an immunogenic, heterodimeric protein that is formed by EhCP112 (cysteine protease) and EhADH112 (adhesin), polypeptides involved in Entamoeba histolytica's cytopathic effect, target-cell adherence and phagocytosis. The EhCPADH complex is located in the plasma membrane and cytoplasmic vacuoles. Here, the independent expression of EhCP112 and EhADH112 in fibroblasts and hamsters was analysed. Also investigated was the immunological response in animals independently inoculated with plasmid pcDNA-Ehcp112, which carries the complete cysteine protease-encoding gene, or with plasmid pcDNA-Ehadh112, which carries the C terminus of the adhesin-encoding gene, or with a mixture of both. Both proteins were expressed in the plasma membranes of the transfected fibroblasts. EhCP112 was toxic for the mammalian cells. Proteins were also independently expressed in hamsters after inoculation with the plasmids. Their expression was indirectly evaluated by the presence of antibodies in the inoculated animals. Remarkably, co-immunization of the animals with the two DNA plasmids resulted in an earlier and higher anti-E. histolytica IgG induction than immunization with separate plasmids. In contrast, the cellular immune response was not noticeably improved by the plasmid mixture. Interestingly, protection against liver abscesses was detected only in animals that received the plasmid mixture and no protection was observed in hamsters independently inoculated with plasmid pcDNA-Ehcp112 or pcDNA-Ehadh112.

Innate immunity to amebic liver abscess is dependent on gamma interferon and nitric oxide in a murine model of disease

Evidence from in vitro studies suggests that gamma interferon (IFN-gamma) and nitric oxide (NO) are important in host defense against the protozoan parasite Entamoeba histolytica. We used SCID mice with targeted disruption of the IFN-gamma receptor gene and mice with targeted disruption of the gene encoding inducible NO synthase to show that IFN-gamma plays a role in the innate immunity to amebic liver abscess seen in SCID mice while NO is required for control of amebic liver abscess in immunocompetent mice.

Preparation of recombinant human monoclonal antibody Fab fragments specific for Entamoeba histolytica

Genes coding for human antibody Fab fragments specific for Entamoeba histolytica were cloned and expressed in Escherichia coli. Lymphocytes were separated from the peripheral blood of a patient with an amebic liver abscess. Poly(A)+ RNA was isolated from the lymphocytes, and then genes coding for the light chain and Fd region of the heavy chain were amplified by a reverse transcriptase PCR. The amplified DNA fragments were ligated with a plasmid vector and were introduced into Escherichia coli. Three thousand colonies were screened for the production of antibodies to E. histolytica HM-1:IMSS by an indirect fluorescence-antibody (IFA) test. Lysates from five Escherichia coli clones were positive. Analysis of the DNA sequences of the five clones showed that three of the five heavy-chain sequences and four of the five light-chain sequences differed from each other. When the reactivities of the Escherichia coli lysates to nine reference strains of E. histolytica were examined by the IFA test, three Fab fragments with different DNA sequences were found to react with all nine strains and another Fab fragment was found to react with seven strains. None of the four human monoclonal antibody Fab fragments reacted with Entamoeba dispar reference strains or with other enteric protozoan parasites. These results indicate that the bacterial expression system reported here is effective for the production of human monoclonal antibodies specific for E. histolytica. The recombinant human monoclonal antibody Fab fragments may be applicable for distinguishing E. histolytica from E. dispar and for use in the serodiagnosis of amebiasis.

Progress towards development of a vaccine for amebiasis

The application of molecular biologic techniques over the past decade has seen a tremendous growth in our knowledge of the biology of Entamoeba histolytica, the causative agent of amebic dysentery and amebic liver abscess. This approach has also led to the identification and structural characterization of three amebic antigens, the serine-rich Entamoeba histolytica protein (SREHP), the 170-kDa subunit of the Gal/GalNAc binding lectin, and the 29-kDa cysteine-rich protein, which all show promise as recombinant antigen-based vaccines to prevent amebiasis. In recent studies, an immunogenic dodecapeptide derived from the SREHP molecule has been genetically fused to the B subunit of cholera toxin, to create a recombinant protein capable of inducing both antiamebic and anti-cholera toxin antibodies when administered by the oral route. Continued progress in this area will bring us closer to the goal of a cost-effective oral combination "enteric pathogen" vaccine, capable of inducing protective mucosal immune responses to several clinically important enteric diseases, including amebiasis.

Oral immunization with attenuated vaccine strains of Vibrio cholerae expressing a dodecapeptide repeat of the serine-rich Entamoeba histolytica protein fused to the cholera toxin B subunit induces systemic and mucosal antiamebic and anti-V. cholerae antibody responses in mice

Entamoeba histolytica is a significant cause of morbidity and mortality worldwide. The serine-rich E. histolytica protein (SREHP) is a surface-expressed trophozoite protein that includes multiple hydrophilic tandem repeats. A purified fusion protein between the dodecapeptide repeat of SREHP and cholera toxin B subunit (CTB) has previously been shown to be immunogenic in mice after oral inoculation when cholera toxin is coadministered as an immunoadjuvant. We engineered a live attenuated El Tor Vibrio cholerae vaccine strain, Peru2, to express the SREHP-12-CTB fusion protein to the supernatant from either a plasmid [Peru2 (pETR5.1)] or from a chromosomal insertion (ETR3). Vector strains were administered orally to germfree mice that were subsequently housed under nongermfree conditions; mice received one (day 0) or two (days 0 and 14) inoculations. No immunoadjuvant or cholera holotoxin was administered. Mice that received two inoculations of Peru2(pETR5.1) had the most pronounced antiamebic systemic and mucosal immunologic responses. Less marked, but significant, anti-SREHP serum immunoglobulin G antibody responses were also induced in mice that received either one or two oral inoculations of strain ETR3. Anti-V. cholerae responses were also induced, as measured by the induction of serum vibriocidal antibodies and by serum and mucosal anti-CTB antibody responses. These results suggest that V. cholerae vector strains can be successful delivery vehicles for the SREHP-12-CTB fusion protein, to induce mucosal and systemic antiamebic and anti-V. cholerae immune responses. The magnitude of these responses is proportional to the amount of SREHP-12-CTB produced by the vector strain.