Cell-mediated immunity in patients with amebic abscess of the liver

A Case Series of Amoebic Liver Abscess in Patients With COVID-19 Infection

BACKGROUND

Globally, amoebiasis is the second leading cause of death due to parasitic diseases. Amoebic liver abscess (ALA) as an opportunistic infection in recently recovered COVID 19 disease patients has not been reported earlier.

METHODS

We present here a case series of 13 patients of ALA admitted during the month of May 2021 in BLK hospital, New Delhi, and had recently recovered from COVID 19 disease.

RESULTS

Out of 13, eight patients had moderate, and one had severe COVID 19 disease. All these patients had received steroids for the management of COVID 19 disease. The remaining 4 (30%) patients had mild disease. The mean age of our patients was 50 years (37-81 years). Out of 13 patients, nine (70%) were male, 5 were diabetic, 2 were hypertensive, and none were alcoholic. The most common presentation was fever (11/13) followed by upper abdomen pain (10/13). The mean time of symptom (fever or pain abdomen) onset from COVID 19 positivity date was 19 days (range 15-23 days). A complication the in form of localized intraperitoneal rupture was common in these patients and was seen in more than 50% (7/13) of patients. All the patients were managed with IV Metronidazole and abscess aspiration. All the patients were discharged, and the mean hospital stay was 6.6 days.

CONCLUSION

We report here an increase in the incidence of ALA in recently recovered COVID 19 patients and propose that it is most likely due to alteration in the immune state of these patients. It is important for physicians to investigate for liver abscess in patients who present again with fever and/or pain abdomen within 2 weeks of COVID 19 recovery.

Survival strategies of Entamoeba histolytica: Modulation of cell-mediated immune responses

Tissue invasion and disease associated with the protozoan Entamoeba histolytica has long been connected with suppression of host cellular immunity. Dampening of the host's defences may facilitate survival of amoebae in extraintestinal sites and development of the characteristic amoebic abscesses. In recent years, several studies have begun to clarify, at the cellular level, the specific effects E. histolytica has on immune cell accessory and effector cell functions. Here, Darren Campbell and Kris Chadee discuss the parasite's multiple modulatory effects on macrophages and T cells and how this manipulation of immune defences may enable the parasite to remain viable in the host. They suggest the putative amoebic molecules involved and potential modulation by the cytokines: interleukins IL-4 and IL-10 and transforming growth factor-beta.

How protozoan parasites evade the immune response

Protozoan pathogens such as Plasmodium, Leishmania, Trypanosoma and Entamoeba are responsible for several of the most widespread and lethal human diseases. Their successful survival depends mainly on evading the host immune system by, for example, penetrating and multiplying within cells, varying their surface antigens, eliminating their protein coat, and modulating the host immune response. Immunosuppression is sometimes caused directly by parasite products and sometimes involves antigenic mimicry, which often appears in association with parasitic diseases. However, one of the most sophisticated mechanisms of evasion is the selective activation of a subset of T helper cells.

Emerging and reemerging intestinal protozoa

The intestinal protozoa have gained importance to physicians practicing medicine in the United States, Canada, and Europe during recent years as a result of increasing world travel, the globalization of the world's economy, and the growing number of chronically immunosuppressed people. During the spring of 1996, Cyclospora cayetanensis caused diarrhea in approximately 1500 people exposed to Guatemalan raspberries. This epidemic recurred in 1997, emphasizing the risks of the global economy and food supply on which we depend. In addition to importation of intestinal protozoa from the tropics, AIDS and the increasing use of organ transplants have created a new population of people at risk for chronic infection by ubiquitous protozoa previously not known to cause serious human disease. These infections include cryptosporidiosis, isosporiasis, and microsporidiosis. Finally, Entamoeba histolytica, the etiologic agent of invasive amebiasis, has only recently been recognized to be a distinct species from a nonpathogenic but indistinguishable (by light microscopy) intestinal commensal, Entamoeba dispar. The rapidly changing epidemiology of these intestinal protozoa, as well as new approaches to diagnosis and treatment of these protozoa, are discussed.

Pathogenesis of intestinal amebiasis: from molecules to disease

In spite of a wealth of knowledge on the biochemistry and cellular and molecular biology of Entamoeba histolytica, little has been done to apply these advances to our understanding of the lesions observed in patients with intestinal amebiasis. In this review, the pathological and histological findings in acute amebic colitis are related to the molecular mechanisms of E. histolytica pathogenicity described to date. Infection of the human colon by E. histolytica produces focal ulceration of the intestinal mucosa, resulting in dysentery (diarrhea with blood and mucus). Although a complete picture has not yet been achieved, the basic mechanisms involved in the production of focal lytic lesions include complex multifactorial processes in which lectins facilitate adhesion, proteases degrade extracellular matrix components, porins help nourish the parasite and may also kill incoming polymorphonuclear leukocytes and macrophages, and motility is used by the parasite to invade deeper layers of the colon. In addition, E. histolytica has developed mechanisms to modulate the immune response during acute infection. Nevertheless, much still needs to be unraveled to understand how this microscopic parasite has earned its well-deserved histolytic name.

Pathogenesis of intestinal amebiasis: from molecules to disease

In spite of a wealth of knowledge on the biochemistry and cellular and molecular biology of Entamoeba histolytica, little has been done to apply these advances to our understanding of the lesions observed in patients with intestinal amebiasis. In this review, the pathological and histological findings in acute amebic colitis are related to the molecular mechanisms of E. histolytica pathogenicity described to date. Infection of the human colon by E. histolytica produces focal ulceration of the intestinal mucosa, resulting in dysentery (diarrhea with blood and mucus). Although a complete picture has not yet been achieved, the basic mechanisms involved in the production of focal lytic lesions include complex multifactorial processes in which lectins facilitate adhesion, proteases degrade extracellular matrix components, porins help nourish the parasite and may also kill incoming polymorphonuclear leukocytes and macrophages, and motility is used by the parasite to invade deeper layers of the colon. In addition, E. histolytica has developed mechanisms to modulate the immune response during acute infection. Nevertheless, much still needs to be unraveled to understand how this microscopic parasite has earned its well-deserved histolytic name.

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.

Role of prostaglandin E2 on amoebic liver abscess formation in hamsters

Entamoeba histolytica can modulate macrophage functions and cytokine production by a prostaglandin E2 (PGE2) mechanism. To study the participation of PGE2 on amoebic liver abscess formation, we tested the effect of the PG synthesis inhibitor indomethacin (INDO) on abscess development in hamsters infected intrahepatically with E. histolytica trophozoites. Male infected animals had higher levels of plasma PGE2 (5.7 +/- 0.7 pg/ml pre-infection; 26.0 +/- 2.0 pg/ml 7 days postinfection; p < 0.001). INDO prevented this increase, so that infected-treated and control non-infected animals had similar levels of plasma PGE2. INDO reduced liver and abscess weight by 18% and 30% respectively (p < 0.05). Cyclooxygenase (COX) activity determination by thin layer chromatography using (1-14C) arachidonic acid (AA) showed that liver microsomes from infected animals produced more PGE2 than controls. COX activity was considerably inhibited in infected INDO-treated animals. Our data suggest that E. histolytica can stimulate the hepatic production of PGE2 which contributes to pathogenesis of amoebic abscesses through generation and support of the inflammation. The partial effect of INDO treatment suggests that additional factors are involved.

Intestinal amebiasis: cyclic suppression of the immune response

The cellular immune response was evaluated in a C3H/HeJ mouse model of intestinal amebiasis at 5-60 days postinoculation with Entamoeba histolytica. At various intervals, spleen lymphocytes were obtained from infected and noninfected control mice and cultured with concanavalin A (Con A), pokeweed mitogen (PWM), or ameba antigen. E. histolytica infection induced a cyclic depression of DNA synthesis when spleen lymphocytes were stimulated with a T-cell mitogen (Con A), a T- and B-cell (PWM) mitogen, or an ameba antigen. A similar response was observed in the determinations of interleukin-2 in the supernatants of Con A-stimulated spleen cells from infected mice. When spleen cells from E. histolytica-infected mice were stimulated with phorbol myristate acetate plus ionomycin, results indicated a signal-transduction defect. These alterations, observed at the cellular level, might facilitate invasion of the host by the parasite.

Production and characterization of monoclonal antibodies against a highly immunogenic fraction of Entamoeba histolytica (NIH:200) and their application in the detection of current amoebic infection

Six monoclonal antibodies (MAbs) were produced against a highly immunogenic fraction derived by the chromatographic separation of the soluble preparation of axenic Entamoeba histolytica (strain NIH:200) trophozoites. Isotype characterization of the six MAbs revealed that four belonged to the IgM class and one each to the IgG1 and the IgG2a subclasses. The immunoreactivity patterns and the specificity of the MAbs with homologous and heterologous antigens were analyzed by the enzyme-linked immunotransfer blot technique and by the enzyme-linked immunosorbent assay. The MAbs reacted intensely with isolates of E. histolytica (strain NIH:200 as well as a local isolate MX1) but showed no reactivity with Entamoeba coli, Iodamoeba butschlii, Endolimax nana, Entamoeba hartmanni, free-living amoeba (Acanthamoeba harticolus) and other enteric parasites. Using the IgG1 MAb as a detecting antibody, a polyclonal-monoclonal antibody-based enzyme-linked immunosorbent assay was developed for the detection of E. histolytica antigens in stool samples of infected patients. The detection limit of the assay was 8 ng of amoebic antigen. This test was found to be specific and sensitive and yielded 100% positive results in cases with amoebiasis but did not react with controls included in the evaluation. The MAb-based enzyme-linked immunosorbent assay developed in this study will be an important test for the diagnosis of E. histolytica in the feces of infected humans; however, the limitation of the test is the inability to discriminate the pathogenic status of the amoeba detected in the stool.

Expression of a recombinant Entamoeba histolytica antigen in a Salmonella typhimurium vaccine strain

The expression of a major surface antigen of the intestinal protozoal parasite Entamoeba histolytica in an attenuated Salmonella typhimurium vaccine strain is described. A polymerase chain reaction fragment derived from cDNA encoding the serine-rich Entamoeba histolytica protein, SREHP, was introduced into S. typhimurium chi 3987 (delta cya delta crp delta asd) using a plasmid expression vector (pYA292) containing the aspartate semialdehyde (asd) gene. S. typhimurium expressing recombinant SREHP as a SREHP/maltose binding protein fusion protein was administered orally to mice and gerbils (an important animal model for E. histolytica infection) and was recovered from splenic tissue in both species. Our study demonstrates the feasibility of expressing recombinant amoebic proteins in attenuated S. typhimurium strains, and shows that vaccine strains of S. typhimurium can successfully infect the gerbil, a widely used model for amoebic liver abscess and intestinal amoebiasis.

Down-regulation of murine lymphocyte responsiveness to mitogens after treatment with antigens of Entamoeba histolytica

Injection of mice with pathogenic Entamoeba histolytica (strain HM1-IMSS) antigens resulted in a decreased capacity of splenocytes to respond to mitogen-induced blastogenesis following a challenge with concanavalin A (Con A), phytohemagglutinin (PHA), and lipopolysaccharide (LPS), whereas no inhibition was observed in mice that had previously been injected with equivalent amounts of non-pathogenic E. histolytica-like Laredo antigens. Depletion of adherent cells in the splenocyte preparation indicated that these cells were not a major contributor to the observed immunosuppression. Quantification of splenic T-lymphocyte subsets demonstrated a significant decrease in Thy-1+ and Lyt-1+ cells, but Lyt-2+ cells were not affected. Splenocytes treated with pathogenic amoebic antigens in vitro affected the capacity of these cells to respond optimally to Con A- and LPS-induced blastogenesis but not to that induced by PHA. These findings demonstrate that amoebic antigens affect lymphocyte function and may be important co-factors in the immunoregulation and pathogenesis of amoebiasis.

Fatal amoebic liver abscess: an autopsy study

An autopsy study of 20 cases is presented. The mean age was 32 years (range: 14-45). Four cases (20%) presented with jaundice. Another four (20%) had continuous fever with abdominal pain. Six had loose motions with blood and mucus in the stools. The mean duration of illness was 7.2 days. All cases were thin and emaciated. Liver was grossly enlarged (mean weight: 2680 g). The abscess was single in all cases except one. The right lobe was involved in 15 cases, the left in four and both in one. The average size of abscess was 13 cm. The abscess had ruptured in the abdomen in 3 patients, for which laparotomy was performed. Pulmonary involvement was seen in 3 cases. Colon showed ulceration in half the patients. In 2 cases perforation was also present. No cerebral involvement was present. A number of interesting features emerged from this study. Although amoebic liver abscess is a common disease, its diagnosis can be missed due to unusual presentation. In the present study, diagnosis of viral hepatitis, carcinoma lung, bacillary dysentery and enteric fever with perforation were made, which probably contributed materially to fatal outcome.

Suppression of T-lymphocyte responses to Entamoeba histolytica antigen by immune sera

Lymphocytes from patients cured of amebic liver abscesses proliferate and produce gamma interferon upon incubation with soluble Entamoeba histolytica antigen: however, amebic liver abscesses exhibit a relentless progression without treatment. To determine whether suppressive factors are present in sera, we studied T-lymphocyte responses to total soluble E. histolytica antigen by using cells from five patients treated for amebic liver abscesses in the presence of 15 different immune sera and 10 control sera. In the presence of immune sera, E. histolytica antigen-induced lymphocyte proliferation decreased by 63% and production of gamma interferon was reduced by 93.2% (P less than 0.01). Immune sera had no effect on the mitogenic responses of patient lymphocytes to phytohemagglutinin or on the proliferative responses of control lymphocytes to phytohemagglutinin or tetanus toxoid. The suppressive activity of immune sera diminished as the time between therapy for amebic liver abscesses and serum collection increased (P less than 0.05). Suppressive activity did not correlate with the titers of serum anti-amebic antibody and was not affected when serum was absorbed with viable amebic trophozoites. In conclusion, soluble factors present in the sera of amebic liver abscess patients suppressed in vitro lymphocyte responses to E. histolytica antigen and may have contributed to the lack of development of effective in vivo cell-mediated immune responses following the onset of amebic liver abscesses.

Interaction of rheumatoid factor and Entamoeba histolytica

The amoebae's cytotoxicity test and the amoebae's lysis test were used to show possible interactions between rheumatoid factor (RF) and Entamoeba histolytica. Amoebae's cytotoxic activity (ACA) was inhibited by affinity chromatography purified antiamoebae rabbit IgG (RIgG). Enhanced inhibition could be demonstrated with RIgG plus RF. But the same marked inhibition of ACA could be seen when replacing RF by heat inactivated normal human serum as a control. About 50% amoebae's lysis occurred when amoebae were brought together with native normal human serum (NNHS) as a source of complement. Amoebae's lysis increased to 60% when incubated with NHS plus human antiamoebae antibodies. No further augmentation could be obtained by the addition of RF. Using RIgG instead of human antibodies the lysis rate did not increase. Incubation of amoebae, NNHS, RIgG and RF even reduced amoebae's lysis. RF neither has an effect on ACA nor on complement mediated AL in vitro.

Kinetics of the immune responses during the course of hepatic amoebic infection--an experimental study

Following intrahepatic inoculation of axenic Entamoeba histolytica (strain NIH:200V), all hamsters developed amoebic abscesses in the liver by 7 d after inoculation. All the animals cleared the infection spontaneously by 28 d after inoculation. The establishment of amoebae leading to acute liver abscess was accompanied by significantly reduced levels of antibodies to the plasma membrane (PM) protein of E. histolytica, reduced direct macrophage cytotoxicity, and reduced anti-PM antibody-mediated macrophage-dependent cytotoxicity to amoebic trophozoites. This phase of infection was also accompanied by a significant reduction in cellular sensitization to PM proteins and to phytohaemagglutinin. Reduction in size of the liver abscesses and clearance of infection were accompanied by significant development of anti-PM antibodies, cellular sensitization to the PM proteins, enhanced direct macrophage cytotoxicity, and anti-PM antibody-mediated macrophage-dependent cytotoxicity to trophozoites. The data suggest that the amoebic disease process is modulated by immune responses to the PM protein of E. histolytica.

Delayed-type hypersensitivity to Entamoeba histolytica in mice and hamsters: a comparison

Delayed-type hypersensitivity (DTH) to live or fixed Entamoeba histolytica was induced and compared in mice and hamsters. Peak reactions were obtained 24 h post-challenge. In mice, challenge with 10(5) amoebae produced maximal, specific footpad swelling; in hamsters, 5 x 10(4) were required. Elicitation of DTH in mice was strongest 1 week after induction and remained comparatively high for 8 weeks. In hamsters, elicitability declined after 1-2 weeks. Cyclophosphamide increased footpad reactions in mice and hamsters when given 1 day prior to induction but not prior to challenge. Reactions were usually somewhat (but not significantly) stronger in mice than in hamsters, which was also evident from adoptive transfer experiments. Thus, differences in cell-mediated immunity as expressed by DTH in mice and hamsters do not explain the differential susceptibility of these animals to infection with this parasite. In hamsters, multiple footpad injections of live or fixed amoebae lowered the percentage of subsequent liver infections after i.p. injection of virulent amoebae.

Coincidence of deficient delayed hypersensitivity and intestinal protozoa in homosexual men

Stool samples of 134 subjectively and apparently healthy male homosexual volunteers were studied, and in 86 men (64.2%) 190 parasite infestations, up to 6 per host, were identified. The most frequent species were Endolimax nana (40), Entamoeba coli (38), Entamoeba histolytica (34), Entamoeba hartmanni (32), and Iodamoeba bütschlii (22); Cryptosporidium was not found. All 11 isolates of Entamoeba histolytica analysed belonged to a noninvasive zymodeme: 10 were of zymodeme I and I of zymodeme III. In 172 healthy control adults 6 parasite infestations were found. In the homosexual men, a negative tuberculin test was more frequent in the presence than in the absence of parasites. Entamoeba histolytica (p = 0.005) Entamoeba coli (p = 0.013) and Iodamoeba bütschlii (p = 0.004) were associated with the anergy.

In vitro and in vivo studies of macrophage functions in amebiasis

Experimental intrahepatic inoculation of the gerbil with Entamoeba histolytica trophozoites was used as a model of liver amebiasis to study the cellular immune response elicited by the parasite. It was shown that abscess-derived macrophages (5 to 20 days old) were deficient in their capacity to develop a respiratory burst, to secrete and express membrane-bound interleukin-1-like activity, and to kill E. histolytica trophozoites as well as to respond to lymphokines in vitro. However, macrophages isolated from the spleen and peritoneal cavities from the same infected animals were not significantly down regulated in these functions. Splenocytes from infected gerbils were shown to develop a strong responsiveness to amebic antigen, whereas their response to concanavalin A was suppressed. Crude E. histolytica extracts or conditioned medium down regulated murine BALB/c macrophage accessory and effector cell functions in vitro in a manner similar to abscess-derived macrophages, whereas crude extracts of the nonvirulent E. histolytica-like Laredo strain did not. Our results indicate that intrinsic or secreted products or both from E. histolytica are actively regulating macrophage functions at the abscess site and can possibly mediate other immunoregulatory mechanisms at distant targets.

Entamoeba histolytica antigens as possible vaccinogens? A short review

A few key papers which have recently been published on the characterization of amoeba antigens are reviewed. Immunofluorescence tests and immunoelectron microscopy have demonstrated the localization of certain surface antigens on axenically cultured trophozoites. Most of the surface antigens have largely been shown to elicit a humoral response. The elicitation of cellular response has not been well illustrated. The localization of a large number of antigens in cytoplasmic vacuoles and plasma membrane indicates that a greater stimulus to the host would be provided by intracellular antigens than by those located on the surface of amoeba trophozoites. In a few inoculation studies, amoeba antigens, in combination with several adjuvants, have been successfully employed for inducing protective immunity in various animal model systems. These and other results clearly demonstrate that amoeba antigens are fully capable of generating humoral and as well as CMI responses. A combination of these two effector limbs of immunity can be fully exploited through effective use of future vaccines.

Isolation of the galactose-binding lectin that mediates the in vitro adherence of Entamoeba histolytica

Entamoeba histolytica adheres to human colonic mucus, colonic epithelial cells, and other target cells via a galactose (Gal) or N-acetyl-D-galactosamine (GalNAc) inhibitable surface lectin. Blockade of this adherence lectin with Gal or GalNAc in vitro prevents amebic killing of target cells. We have identified and purified the adherence lectin by two methods: affinity columns derivatized with galactose monomers or galactose terminal glycoproteins, and affinity columns and immunoblots prepared with monoclonal antibodies that inhibit amebic adherence. By both methods the adherence lectin was identified as a 170-kD secreted and membrane-bound amebic protein. The surface location of the lectin was confirmed by indirect immunofluorescence. Purified lectin competitively inhibited amebic adherence to target cells by binding to receptors on the target Chinese hamster ovary cells in a Gal-inhibitable manner.

Immunological studies in amebiasis: isotypic characterization of specific antibodies by enzyme-linked immunofiltration assay

Fifty sera from 17 cases of invasive amebiasis (15 hepatic localizations, 1 ameboma, 1 diffused colic localization) were studied by enzyme-linked immunofiltration assay (ELIFA). IgM and IgE anti-Entamoeba histolytica were detected in 10 and 15, respectively, of the 17 patients studied. IgM and IgE antibodies were found simultaneously in 10 cases; these 2 isotypes were only recognized twice in the same serum by the same antigenic components. During post-therapy monitoring, in less than 90 days IgE or IgM-Ab regressed completely in half the cases. If they appeared or reappeared after the third month, prognosis was bad. In addition to its value for diagnosis and prognosis, the ELIFA allowed us to detect the functional antigenic components revealed more particularly by some IgG, IgM, IgE, or IgA antibodies.

Immunosuppression and splenomegaly in Entamoeba histolytica infection in mice

Intestinal amoebiasis caused by Entamoeba histolytica trophozoites in mice is accompanied by a depression in the ability of this host to develop an immune response to sheep red blood cells. The number of splenic plaque-forming cells was reduced in mice inoculated intracecally with 2.5 x 10(5) trophozoites at 15, 25, 40, 65 and 75 days after infection when compared with non-infected mice. It was found that there was no significant difference between the spleen weight of the infected and non-infected control animals at 5 and 10 days following infection. However, a significant increase in spleen weight was observed by 15 days of infection and the spleens remained enlarged until termination of the experiment at 75 days. Thus, there was an inverse correlation between the PFC response and the spleen weight of infected animals.

In vitro and in vivo interaction between trophozoites of Entamoeba histolytica and gerbil lymphoid cells

The in vitro and in vivo antiamoebic cytotoxic effects of peritoneal exudate cells and mesenteric lymph node lymphocytes of gerbils with cecal amoebiasis or those immunized with amoebic extract were investigated. A differential effect of the lymphoid cells against trophozoites of nonpathogenic and pathogenic strains of Entamoeba histolytica was observed. Nonpathogenic amoebae were more susceptible to killing by lymphoid cells than pathogenic amoebae in vitro and in vivo in infected or immunized animals. These data suggest that during the course of cecal amoebiasis in gerbils, a differential stimulation or depletion of cytotoxic cells in the lymphoreticular tissues occurs, resulting in an impaired cell-mediated immune response.

Interaction of human leukocytes and Entamoeba histolytica. Killing of virulent amebae by the activated macrophage

Capable effector mechanisms in the human immune response against the cytolytic, protozoan parasite Entamoeba histolytica have not been described. To identify a competent human effector cell, we studied the in vitro interactions of normal human polymorphonuclear neutrophils, peripheral blood mononuclear cells (PBMC), monocytes (MC), and MC-derived macrophages with virulent axenic amebae (strain HMI-IMSS). Amebae killed neutrophils, PBMC, MC, and MC-derived macrophages (P less than 0.001), without loss of parasite viability. The addition of heat-inactivated immune serum did not enable leukocytes to kill amebae, nor did it protect these host cells from amebae. MC-derived macrophages, activated with lymphokine elicited by the mitogens conconavalin A, phytohemagglutinin, or an amebic soluble protein preparation (strain HK9), killed 55% of amebae by 3 h in a trypan blue exclusion assay (P less than 0.001); during this time, 40% of the activated macrophages died. Lysis of amebae was confirmed using 111Indium oxine radiolabeled parasites and was antibody independent. Macrophage death appeared to be due to the deleterious effect of lysed amebae rather than the contact-dependent effector mechanisms of E. histolytica. Adherence between activated macrophages and amebae was greater than that between other leukocytes and amebae (P less than 0.001). Microscopic observations, kinetic analysis of the killing of amebae by activated macrophages, and suspension of amebae with adherent activated macrophages in a 10% dextran solution indicated that contact by activated macrophages was necessary to initiate the killing of amebae. Catalase but not superoxide dismutase inhibited the amebicidal capacity of activated macrophages (P less than 0.001). However, activated macrophages from an individual with chronic granulomatous disease were able to kill amebae, but not as effectively as normal cells (P less than 0.01). In summary, activated MC-derived macrophages killed virulent E. histolytica trophozoites through a contact-dependent, antibody-independent mechanism involving oxidative-dependent and -independent processes.

Vaccination of rabbits against Entamoeba histolytica with aqueous suspensions of trehalose-dimycolate as the adjuvant

Rabbits were immunized with soluble Entamoeba histolytica antigen with an aqueous suspension of trehalose-6,6'-dimycolate used as the adjuvant. Induction of protective immunity in the immunized animals was demonstrated by enhanced humoral and cell-mediated immune responses and 100% survival after challenge. Administration of soluble antigen only failed to induce a similar degree of protective immunity. Trehalose-6,6'-dimycolate alone produced only a slight increase in nonspecific resistance.

Lymphocyte subpopulations transformation studies in an experimental model of intestinal and hepatic amoebiasis

The lymphocyte subpopulations in peripheral blood and blast transformation were studied on days 0, 3, 7, 11, 15, 20 and 35 after infection in progesterone-treated guinea-pigs which had been experimentally infected with Entamoeba histolytica by intracaecal inoculation. In animals with intestinal infection, lowered T cell numbers and blast transformation were seen during the acute phase with a subsequent tendency to recover. In guinea-pigs with hepatic amoebiasis, both these parameters were significantly depressed throughout the period of study with no tendency to recovery. In progesterone-treated and sham-operated animals, a less marked depression of T cells, was observed. The B cell population did not vary greatly in either group. This study has thus shown that in hepatic amoebiasis there is highly significant depression in both number and function of T cells in comparison to intestinal amoebiasis.

Cell-mediated immunity in hepatic amoebiasis

Cell-mediated immunity (CMI) was measured by blastic transformation of peripheral blood lymphocytes in 26 patients with amoebic liver abscess (ALA) and matched control subjects with no demonstrable clinical amoebiasis. During active disease, the mean mitogenic response, measured by the stimulation index of the patients' lymphocytes to Entamoeba histolytica antigen, was increased (mean +/- SD: 25.98 +/- 46.62 compared with 11.27 +/- 21.39), whereas that to phytohaemagglutinin (PHA) was reduced (54.80 +/- 56.26 compared with 111.70 +/- 70.61). Both these results were statistically significant (P less than 0.01); they do not, however, appear to be due to a quantitative defect in T-cell numbers, as both total peripheral lymphocytes (3332 +/- 1450 cells/microliters compared with 2447 +/- 531 cells/microliters) and T-cells (2652 +/- 1128 cells/microliters compared with 1908 +/- 386 cells/microliters) were significantly elevated in the patients (P less than 0.01). When these tests were repeated two months later in 12 cured patients they were found to be comparable to those of the control subjects. The results indicate that there is a transient cellular sensitization to amoebic antigen together with an increase in T-lymphocytes and an impairment of the mitogenic response of lymphocytes to PHA. The significance of these findings is discussed.

Increased macrophage migration inhibition factor production in hamsters sensitized by amoebic antigen and glucan

Well defined cell-mediated immune responses were detectable following experimental immunization of hamsters with Entamoeba histolytica antigen, using glucan as an adjuvant. Peritoneal cells from amoeba antigen-glucan sensitized animals, upon incubation with specific antigen in vitro, were found to release into the supernatant a macrophage migration inhibition factor (MIF). Such supernatant fluids inhibited the migration of macrophages from non-sensitized hamsters. The production of MIF was found to be greatly increased if glucan is added to amoeba antigen when sensitizing animals. The optimal concentration for maximum inhibition was recorded at 10(-8) dilution of the supernatant.

Anti-Ia reactivity in sera from subjects with Entamoeba histolytica infection

Sera from 15 patients with Entamoeba histolytica infection were tested for anti T-cell antibodies by assessing cross-reacting specificities with the antigens defined by an anti-Ia hybridoma antibody. T cells prepared by sheep erythrocyte rosetting were preincubated with the test sera and then with the anti-Ia antibody. Binding of the specific monoclonal antibody was assessed by rosetting with ox erythrocytes conjugated with goat anti-mouse IgG. Eight sera from amoebic patients were found to block the binding of monoclonal mouse hybridoma anti Ia-antibody to T cells. Blocking of anti-Ia binding was not due to Fc IgG receptor binding by immune complexes nor was it HLA-DR restricted. T cells pre-treatment with the amoebic sera positive for anti-Ia activity showed reduced activity when tested in the autologous mixed lymphocyte reaction (AMLR). The results of our study seem to suggest the existence of specific anomalies of immunoregulation during E. histolytica infection which may play a role in inducing immune disregulation in vivo.

Passive transfer of immunity against hepatic amoebiasis in the hamster by cells

The role of passive cell-mediated transfer of immunity in hepatic amoebiasis in hamsters was studied. The transfer of peritoneal cells from hamsters vaccinated against or protected from hepatic amoebiasis and from those with hepatic amoebiasis, as well as of spleen cells from vaccinated or protected, but not from infected hamsters, conferred immunity against hepatic amoebiasis in recipient normal hamsters. Treatment of the spleen cells from protected hamsters with anti T-cell serum abolished their ability to transfer immunity. It appears that the effector mechanism in this system is T-cell dependent.

Immunosuppressive effect of Entamoeba histolytica extract on hamsters

The immune response to sheep red blood cells (SRBC) in mice and hamsters injected with an extract of entamoeba histolytica was studied. Both the primary and secondary immune response, measured by anti-SRBC antibody titers, were unaltered in the mouse, while a significant depression of the primary, but not the secondary, response was observed in the hamster. The effect was greatest when the amebic extract (AE) and SRBC were injected on the same day. The number of anti-SRBC rosettes formed in the spleen cells of hamsters treated with both AE and SRBC on day 0 was measured from days 1-16. The response peaked on day 13, while cells from animals injected with SRBC alone gave a maximal response on day 5. The formation of anti-SRBC rosettes in T-lymphocyte-enriched spleen cells treated with anti-gamma globulin serum and complement was almost abolished for the duration of the experiment. It is suggested that the mechanism responsible for this immunosuppressive phenomenon could involve early interference in the afferent limb of the immune response.

In vitro amoebicidal activity of immune cells

The amoebicidal activity of peripheral blood lymphocytes and spleen and peritoneal cells from hamsters vaccinated against or protected from hepatic amoebiasis and from those with hepatic amoebiasis was investigated. Peripheral blood lymphocytes and peritoneal and spleen cells from vaccinated or protected animals can kill trophozoites of Entamoeba histolytica in vitro. In contrast, spleen cells from infected hamsters showed no significant cytotoxic effect on the parasite. These data suggest that cellular immunity plays an important role in host defense against hepatic amoebiasis.

Virulence of Entamoeba histolytica in rat and its comparison with the serological responses of the amoebic patients

The virulence of 70 isolates of Entamoeba histolytica was studied in the Wistar strain of albino rat by intracaecal inoculation of amoebae. The results were evaluated by the Neal scoring system, histopathological grading of ulcers and virulence indices. It was found that isolates from acute amoebic cases could infect and produce ulcers in significantly more animals than could the isolates from asymptomatic cyst passers. The virulence indices of isolates from acute cases were also significantly higher as compared to those from asymptomatic cyst passers. However, there was no significant difference between virulence indices of isolates from acute cases and non-dysenteric amoebic colitis. This study also showed that Neal scores failed to correlate with the type of pathology produced by the amoebae in the caecum. It is felt that virulence indices which depend upon the histopathological lesions should also be taken into consideration. The serological response of the patients tended to correlate with the virulence indices of isolates but only minimally. The results thus contradict a common belief amongst clinicians that higher levels of anti-amoebic antibodies reflect severe disease, as compared to low levels of antiamoebic antibodies.

Protective effects of crude and chromatographic fractions of axenic Entamoeba histolytica in guinea-pigs

Axenic amoebic antigen was chromatographed on Sephadex G-200. Two peaks divided into three fractions were obtained which were concentrated by lyophilization. Guinea-pigs were given a series of subcutaneous injections of crude Entamoeba histolytica antigen and its chromatographic fractions mixed with Freund's complete adjuvant. After three immunizing doses, the animals were challenged with intracaecal injection of a virulent isolate of amoebae (Strain No. H-25) and were killed seven days later. The antibody response was studied by indirect haemagglutination (IHA) and counter-current immunoelectrophoresis (CIEP) before intracaecal inoculation and at the time of killing. Fraction I, which had the highest molecular weight, stimulated high levels of antibodies in guinea-pigs and showed 92% protection. Only one animal of 12 inoculated had gross lesions and these were very mild. The caecal contents of three of the 11 animals without gross lesions were positive for amoebae but no tissue invasion was observed. Partial protection was observed with other antigens--30% with whole amoebae, 18% with Fraction II and 30% with Fraction III. In most there were multiple ulcers of the deep penetrating type and amoebae either penetrating and invading the muscular layer or the lytic process extending up to the muscular layer. In one animal, immunized with whole amoebic antigen, multiple hepatic lesions were also observed and trophozoites of E. histolytica were demonstrated histologically.

Experimental amoebic infection in guinea-pigs immunized with low grade amoebic infection

Guinea-pigs were immunzied with a low grade amoebic infection, either intracaecally or intramesenterically. The immunized animals were challenged with a virulent strain of Entamoeba histolytica. Some of the animals, immunized via the intracaecal route were challenged repeatedly. 10 to 30% of immunized animals developed caecal lesions compared with 92.8% of non-immunized animals. The percentage of protection in animals challenged repeatedly was greater than in those challenged once. Neither immunized nor non-immunized guinea-pigs developed hepatic abscesses. It appears that prior low grade infection could lead to resistance to subsequent amoebic challenge.