Humoral and mucosal IgA antibody response to a recombinant 52-kDa cysteine-rich portion of the Entamoeba histolytica galactose-inhibitable lectin correlates with detection of native 170-kDa lectin antigen in serum of patients with amebic colitis

Mucosal delivery of ACNPV baculovirus driving expression of the Gal-lectin LC3 fragment confers protection against amoebic liver abscess in hamster

Mucosal vaccination against amoebiasis using the Gal-lectin of E. histolytica has been proposed as one of the leading strategies for controlling this human disease. However, most mucosal adjuvants used are toxic and the identification of safe delivery systems is necessary. Here, we evaluate the potential of a recombinant Autographa californica baculovirus driving the expression of the LC3 fragment of the Gal-lectin to confer protection against amoebic liver abscess (ALA) in hamsters following oral or nasal immunization. Hamsters immunized by oral route showed complete absence (57.9%) or partial development (21%) of ALA, resulting in some protection in 78.9% of animals when compared with the wild type baculovirus and sham control groups. In contrast, nasal immunization conferred only 21% of protection efficacy. Levels of ALA protection showed lineal correlation with the development of an anti-amoebic cellular immune response evaluated in spleens, but not with the induction of seric IgG anti-amoeba antibodies. These results suggest that baculovirus driving the expression of E. histolytica vaccine candidate antigens is useful for inducing protective cellular and humoral immune responses following oral immunization, and therefore it could be used as a system for mucosal delivery of an anti-amoebic vaccine.

Adherence-inhibitory intestinal immunoglobulin a antibody response in baboons elicited by use of a synthetic intranasal lectin-based amebiasis subunit vaccine

We designed an amebiasis subunit vaccine that is constructed by using four peptide epitopes of the galactose-inhibitable lectin heavy subunit that were recognized by intestinal secretory immunoglobulin A (IgA) antibodies from immune human subjects. These epitopes are contained in the region encompassing amino acids 758 to 1134 of the lectin heavy subunit, designated LC3. Baboons (Papio anubis) are natural hosts for Entamoeba histolytica; naturally infected baboons raised in captivity possess serum IgA antibodies to the same four LC3 epitopes as humans. Uninfected, seronegative baboons received four intranasal immunizations at 7-day intervals with the synthetic peptide vaccine (400, 800, or 1,600 mug per nostril) with cholera toxin (20 mug) as the adjuvant. As determined by an enzyme-linked immunosorbent assay (ELISA), each dose of the peptide vaccine elicited antipeptide serum IgA and IgG and intestinal IgA antibody responses in all six immunized baboons by day 28, 7 days after the last immunization (P, <0.01 for each dose compared to the cholera toxin control). The peptide vaccine elicited serum IgG and intestinal IgA antibodies that recognized purified recombinant LC3 protein (P, <0.008 and 0.02, respectively) and native lectin protein (P < 0.01). In addition, an indirect immunofluorescence assay with whole trophozoites (P < 0.01) and Western blot analysis confirmed that serum IgG antibodies from vaccinated baboons recognized native lectin protein on the surfaces of axenic E. histolytica trophozoites or from solubilized amebae. All four synthetic peptides were immunogenic; the vaccine elicited dose- and time-dependent responses, as determined by ELISA optical density readings indicating the production of serum and intestinal antibodies (P, <0.02 for antipeptide and antilectin antibodies). As a positive control, intranasal immunization with purified recombinant LC3 protein with cholera toxin as the adjuvant elicited a serum anti-LC3 IgA and IgG antibody response (P, 0.05 and <0.0001, respectively); however, no intestinal anti-LC3 IgA antibody response was observed (P = 0.4). Of interest, serum IgA and IgG antibodies elicited by the recombinant LC3 vaccine did not recognize any of the four putatively protective LC3 peptide epitopes. Both serum and fecal antibodies elicited by the peptide vaccine exhibited neutralizing activity, as determined by their dose-dependent inhibition of the galactose-specific adherence of E. histolytica trophozoites to Chinese hamster ovary cells in vitro (P, <0.001 for each group of antibodies compared to the control). In summary, a lectin-based intranasal polylysine-linked synthetic peptide vaccine was effective in eliciting an adherence-inhibitory, intestinal antilectin IgA antibody response in baboons. Future studies with the baboon model will determine vaccine efficacy against asymptomatic E. histolytica intestinal infection.

Mucosal immunity to asymptomatic Entamoeba histolytica and Entamoeba dispar infection is associated with a peak intestinal anti-lectin immunoglobulin A antibody response

We monitored 93 subjects cured of amebic liver abscess (ALA) and 963 close associate controls in Durban, South Africa, and determined by enzyme-linked immunosorbent assay that the intestinal immunoglobulin A (IgA) antibody response to the Entamoeba histolytica galactose-inhibitable adherence lectin is most accurately represented by a complex pattern of transitory peaks. One or more intestinal anti-lectin IgA antibody peaks occurred in 85.9% of ALA subjects over 36 months compared to 41.6% of controls (P < 0.0001). ALA subjects exhibited a greater number of anti-lectin IgA antibody peaks (P < 0.0001) than controls. In addition, their peak optical density values were higher (peak numbers 1 to 3, P < 0.003), peaks were of longer duration (for peaks 1 and 2, P </= 0.0054), and there was a shorter time interval between peaks (between 1 and 2 or 2 and 3, P </= 0.0106) than observed for control subjects. A prior E. histolytica infection was associated with the occurrence of an anti-lectin IgA antibody peak (79.1%, P < 0.0001) more so than for Entamoeba dispar infection (57.2%, P < 0.001). The annual number of anti-lectin IgA antibody peaks in ALA subjects was 0.71 per year, compared to just 0.22 in controls (P<0.0001), indicating a higher rate of exposure to the parasite than previously appreciated. Anti-lectin IgA antibody peaks were of higher amplitude following a E. histolytica infection compared to E. dispar (P = 0.01) and, for either, were of greater height in ALA subjects than controls (P < 0.01). ALA subjects demonstrated greater clearance of amebic infection after an anti-lectin IgA antibody peak compared to controls, and only 14.3% remained with a positive culture after the peak, compared to 38.9% in controls (P = 0.035). In summary, this prospective controlled longitudinal study elucidated the dynamic nature of the human intestinal IgA antibody response to E. histolytica and E. dispar infection and revealed that ALA subjects exhibit heightened intestinal anti-lectin IgA antibody peaks that are associated with clearance of E. histolytica and E. dispar infection.

Amoebicidal activity of milk, apo-lactoferrin, sIgA and lysozyme

OBJECTIVES

To identify amoebicidal components in human milk and the effect of iron on the amoebicidal activity.

DESIGN

Investigation in axenic cultures of Entamoeba histolytica trophozoites.

METHODS

Amoebas were treated with 5%-20% of human, bovine and swine milk, with 10% of human milk fractions (i.e., casein, proteins except casein and fat) or with 1 mg/ml of human milk apo-lactoferrin, human secretory immunoglobulin type A (sIgA) and chicken egg-white lysozyme (i.e., purified proteins). Milk proteins were detected using immunoblot. Confocal microscopy was used to define the interaction of milk proteins (100 microM each) and amoebas. Experiments were done at least three times in triplicate, and mean and standard deviations were calculated.

RESULTS

Human and bovine milk were amoebicidal showing a concentration-dependent effect. The amoebicidal effect was increased in the absence of iron. Milk protein fractions, with the exception of casein, were the components responsible for the amoebicidal activity found. Apo-lactoferrin, sIgA and lysozyme were identified in the amoebicidal milk protein fraction. Apo-lactoferrin showed the major amoebicidal effect. These proteins, either alone or in combination, showed a killing effect on the trophozoites. They bound to the amoebic membrane causing cell rounding, lipid disruption and damage.

CONCLUSIONS

Milk proteins such as apo-lactoferrin, sIgA and lysozyme are able to kill Entamoeba histolytica trophozoites. This study confirms the importance of feeding breast milk to newborns.

Amebic liver abscess

Amebic liver abscess is the most common extraintestinal manifestation of infection with Entamoeba histolytica, and it is associated with significant morbidity and mortality. In this article the most recent available information is reviewed relating to epidemiology, pathogenesis, presentation, diagnosis, and treatment. We reviewed thousands of cases of amebic liver abscess in the medical literature and present that information as it pertains to mortality, gender, anatomic location of abscesses, and clinical signs and symptoms.

Identification of the Entamoeba histolytica galactose-inhibitable lectin epitopes recognized by human immunoglobulin A antibodies following cure of amebic liver abscess

Immunity to Entamoeba species intestinal infection is associated with the presence of intestinal IgA antibodies against the parasite's galactose-inhibitable adherence lectin. We determined the epitope specificity of serum and intestinal antilectin IgA antibodies by enzyme-linked immunosorbent assay using overlapping fragments of a recombinant portion of the lectin heavy subunit, designated LC3. These findings were correlated with the effects of epitope-specific murine antilectin immunoglobulin A (IgA) monoclonal antibodies (MAbs) on amebic in vitro galactose-specific adherence. LC3 is a highly antigenic and immunogenic cysteine-rich protein (amino acids [aa] 758 to 1150) that includes the lectin's carbohydrate binding domain. The study subjects, from Durban, South Africa, were recently cured of amebic liver abscess (ALA) with or without concurrent Entamoeba histolytica intestinal infection or were infection free 1 year after cure. We also studied seropositive subjects that were infected with E. histolytica, disease free, and asymptomatic. Serum anti-LC3 IgA antibodies from all study groups exclusively recognized the third (aa 868 to 944) and the seventh (aa 1114 to 1134) LC3 epitopes regardless of clinical status; epitope 6 (aa 1070 to 1114) was also recognized by serum anti-LC3 IgG antibodies. However, IgG antibody recognition of epitope 6 but not 3 or 7 was lost 1 year following cure of ALA. We produced 14 murine anti-LC3 IgA MAbs which collectively recognized five of the seven LC3 epitopes. The majority of the murine MAbs recognized the first epitope (aa 758 to 826), which was not recognized by human IgA antibodies. Interestingly, adherence of E. histolytica trophozoites to CHO cells was inhibited by MAbs against epitopes 1, 3, 4 (aa 944 to 987), and 6 (P < 0.01). The LC3 epitopes recognized by human IgA antibodies (3 and 7) were further characterized by use of overlapping synthetic peptides. We identified four peptides (aa 891 to 903, 918 to 936, 1114 to 1134, and 1128 to 1150) that in linear or cyclized form were recognized by pooled intestinal IgA antibodies and serum IgG antibodies from subjects with ALA and asymptomatic, seropositive infected subjects. This study identifies the lectin epitopes to be studied in an amebiasis subunit vaccine designed to elicit mucosal immunity mimicking that of humans cured of ALA.

Intestinal antilectin immunoglobulin A antibody response and immunity to Entamoeba dispar infection following cure of amebic liver abscess

We followed 93 subjects with amebic liver abscess (ALA) and 963 close associate controls at 3-month intervals for 36 months to characterize intestinal and humoral antibody responses to the amebic galactose-inhibitable lectin and to determine whether immunity developed to Entamoeba histolytica or Entamoeba dispar infection following cure of ALA. We found that ALA subjects had a higher prevalence and level of intestinal antilectin immunoglobulin A (IgA) and serum anti-LC3 (cysteine-rich recombinant lectin protein) IgA and IgG antibodies, P < 0.01 and P < 0.05, respectively, compared to controls. The intestinal antilectin IgA antibody response was sustained over a longer time period in ALA subjects (71.8% remained positive at 18 months and 52.6% at 36 months, P < 0.001 compared to 17.6% and 10.3% of controls, respectively). ALA subjects were highly immune to E. dispar infection throughout the study (0% infected at 6 and 36 months, compared to 6.5% and 4.9% of control subjects, respectively, P < 0.05). Upon entry into the study, 6.3% of ALA subjects were infected with E. histolytica; the incidence of new E. histolytica infections in controls (as determined by culture) was too low (1.4%) to determine whether ALA subjects exhibited immunity to new infections. We found that stool cultures every 3 months markedly underestimated the occurrence of new E. histolytica infections, as 15.3% of controls seroconverted after 12 months of follow-up. Unfortunately, under the field conditions present in Durban, South Africa, enzyme-linked immunosorbent assay for detection of lectin antigen in stool yielded unreliable results. In summary, subjects cured of ALA exhibited sustained mucosal IgA antibody responses to the amebic galactose-inhibitable lectin and a high level of immunity to E. dispar infection. Determination of immunity to E. histolytica following cure of ALA will require the use of more sensitive and reliable diagnostic methods.

Diagnosis of amoebic colitis by antigen capture ELISA in patients presenting with acute diarrhoea in Cairo, Egypt

We studied 84 consecutive patients presenting with acute diarrhoea (less than 1 week in duration) at an outpatient tropical medicine clinic in Cairo, Egypt. The diagnosis of amoebic colitis was established by the presence of Entamoeba histolytica galactose-inhibitable lectin antigen and the presence of occult blood in stool. Controls were 182 healthy regional people and 64 patients complaining of prolonged diarrhoea lasting more than 1 week. Entamoeba histolytica infection was found more frequently in patients with acute diarrhoea (57.1%) than in healthy controls (21.4%) or patients with prolonged diarrhoea (25%) (P < 0.001). There was a higher prevalence of Entamoeba dispar infection in the two control groups (24.2 and 20.3%, respectively, P=0.004 and 0.061) compared with those with acute diarrhoea (8.3%). Of the 84 patients with acute diarrhoea 32 had amoebic colitis (38%), and of these, 31 (97%) had at least one positive assay for serum amoebic antibodies (P < 0.001 compared with control groups). In summary, as determined by antigen-detection enzyme-linked immunosorbent assay, there is an unexpectedly high prevalence of amoebic colitis among patients presenting with acute diarrhoea to a tropical disease clinic in Cairo, Egypt.

Diagnosis of invasive amebiasis by enzyme-linked immunosorbent assay of saliva to detect amebic lectin antigen and anti-lectin immunoglobulin G antibodies

Saliva from subjects with amebic liver abscess (ALA), acute amebic colitis, asymptomatic infection with Entamoeba histolytica or Entamoeba dispar, and uninfected controls was tested by enzyme-linked immunosorbent assay (ELISA) for the presence of E. histolytica galactose-inhibitable lectin antigen and salivary immunoglobulin (IgG) antibodies to a recombinant cysteine-rich lectin-derived protein (LC3). Salivary lectin antigen was found in 65.8% of subjects with acute colitis, compared to 22.2% of those convalescent from ALA, 10.0% with asymptomatic E. histolytica infection, 9.8% with E. dispar infection, and 2.6% of controls (subjects from the United States and study patients with nonamebic diarrhea) (P < 0.001 for each compared to values for subjects with colitis). Salivary anti-LC3 IgG antibodies were found in 92% of ALA patients regardless of duration of illness and in 83.3% of colitis patients who were symptomatic for at least 7 days (P < 0.001 compared to other study groups). Serum anti-LC3 IgG antibodies were detected in 56.3% of subjects with acute colitis, 100% of subjects with ALA or prolonged colitis, 45% of subjects with asymptomatic E. histolytica infection, 32.3% of subjects with E. dispar infection, and 23.4% of diarrhea controls. In comparison to ELISA for serum anti-LC3 IgG antibodies, the salivary lectin antigen assay is a more sensitive and specific test for acute amebic colitis. Detection of salivary anti-LC3 IgG antibodies by ELISA is an effective means for the diagnosis of ALA and prolonged cases of amebic colitis.

Cysteine proteinases and the pathogenesis of amebiasis

Amebiasis is a major cause of morbidity and mortality throughout the tropical world. Entamoeba histolytica is now recognized as a separate species from the morphologically identical E. dispar, which cannot invade. Cysteine proteinases are a key virulence factor of E. histolytica and play a role in intestinal invasion by degrading the extracellular matrix and circumventing the host immune response through cleavage of secretory immunoglobulin A (sIgA), IgG, and activation of complement. Cysteine proteinases are encoded by at least seven genes, several of which are found in E. histolytica but not E. dispar. A number of new animal models, including the formation of liver abscesses in SCID mice and intestinal infection in human intestinal xenografts, have proven useful to confirm the critical role of cysteine proteinases in invasion. Detailed structural analysis of cysteine proteinases should provide further insights into their biochemical function and may facilitate the design of specific inhibitors which could be used as potential chemotherapeutic agents in the future.

Cysteine proteinases and the pathogenesis of amebiasis

Amebiasis is a major cause of morbidity and mortality throughout the tropical world. Entamoeba histolytica is now recognized as a separate species from the morphologically identical E. dispar, which cannot invade. Cysteine proteinases are a key virulence factor of E. histolytica and play a role in intestinal invasion by degrading the extracellular matrix and circumventing the host immune response through cleavage of secretory immunoglobulin A (sIgA), IgG, and activation of complement. Cysteine proteinases are encoded by at least seven genes, several of which are found in E. histolytica but not E. dispar. A number of new animal models, including the formation of liver abscesses in SCID mice and intestinal infection in human intestinal xenografts, have proven useful to confirm the critical role of cysteine proteinases in invasion. Detailed structural analysis of cysteine proteinases should provide further insights into their biochemical function and may facilitate the design of specific inhibitors which could be used as potential chemotherapeutic agents in the future.

Host-pathogen interaction in amebiasis and progress in vaccine development

Entamoeba histolytica, the causative organism of invasive intestinal and extraintestinal amebiasis, infects approximately 50 million people each year, causing an estimated 40 to 100 thousand deaths annually. Because amebae only infect humans and some higher non-human primates, an anti-amebic vaccine could theoretically eradicate the organism. Uncontrolled epidemiologic studies indicate that acquired immunity to amebic infection probably occurs and that such a vaccine might be feasible. Application of molecular biologic techniques has led to rapid progress towards understanding how Entamoeba histolytica causes disease, and to the identification of several amebic proteins associated with virulence. These proteins are now being evaluated as potential vaccine components. Parenteral and oral vaccine preparations containing recombinant amebic proteins have been effective in preventing disease in a gerbil model of amebic liver abscess. Although systemic and mucosal cellular and humoral immunity both appear to play a role in protection against Entamoeba histolytica, the relative importance of each in the human immune response remains unknown. No animal model of intestinal amebiasis currently exists, moreover, so it has been impossible to evaluate protection against colonization and colitis. Further investigation of the fundamental mechanisms by which Entamoeba histolytica causes disease and of the human immune response to amebic infection is necessary to assess the true feasibility of an anti-amebic vaccine.

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.