Antibody responses of experimentally infected lambs to antigens collected during in vitro maintenance of the adult, metacestode or oncosphere stages of Taenia hydatigena and Taenia ovis with further observations on anti-oncospheral antibodies

The antigenicity and specificity of crude antigens collected during the in vitro maintenance of Taenia hydatigena and T. ovis, excretory/secretory (ES) antigens, were assessed in a peroxidase microenzyme-linked immunosorbent assay (ELISA), using sera from lambs given experimental monospecific infections with T. hydatigena, T. ovis, Echinococcus granulosus or Fasciola hepatica. ES antigens of larval cysts of T. ovis and T. hydatigena were less reactive than those of adult or oncosphere stages. Strong interspecific cros-reactions occurred between all antigen preparations, and these antigens offered no better specificity than crude somatic extracts. IgG1 was the major immunoglobulin detected in sera from lambs experimentally infected with T. ovis or T. hydatigena using antigens prepared from sonicated oncospheres. Discrete peaks of anti-oncospheral antibodies were detected following initial and challenge infections with eggs (whether the homologous or heterologous species), when sera were assayed with a PBS sonicate or an ES antigen from oncospheres. However, when oncospheres solubilised with sodium deoxycholate were used, the antibody response was prolonged and resembled that reported previously when somatic extracts of adult and metacestode stages were used as antigen. The results showed that oncospheres share antigens in common with other life-cycle stages, but also support the notion that they may possess some unique stage-specific antigenic determinants.     

Antibody responses against natural Taenia hydatigena infection in dogs in Kenya

Antibody responses (IgG) against Taenia hydatigena infection in dogs in Kenya were analysed in ELISA using excretory/secretory products of T. hydatigena scoleces derived from goat cysticercus cysts. Helminth infections of individual dogs were confirmed at autopsy. T. hydatigena worms were found in 89.5% of 143 dogs, and positive anti-T. hydatigena antibody levels were detected in 58.7% of infected dogs. Positive antiscolex antibody levels were detected in 40.0% of Turkana dogs uninfected with T. hydatigena, suggesting previous infection. Antibody was not detected in 34.4% of infected dogs. There was no relationship between individual T. hydatigena worm burdens and absorbance values for sera in ELISA. It was not possible to distinguish between sera from T. hydatigena-infected and uninfected dogs.     

Enhanced Expression of Fc Receptors on Neutrophils from Calves with Leukocyte Adhesion Deficiency

The expression of Fc receptors for immunoglobulin G(IgG) and concanavalin A (con A)-binding receptors, luminol-dependent chemiluminescent (LDCL) responses, and the effect of anti-bovine IgG on LDCL responses were evaluated in neutrophils from Holstein calves with leukocyte adhesion deficiency (BLAD). Neutrophils from affected calves showed a 2.1- to 2.5-fold increase in Fc receptor expression compared with those of control calves by flow cytometric analysis. Con A-binding activities of neutrophils from affected calves were similar to those of control calves. Neutrophils from a calf with BLAD, when stimulated with zymosan opsonized with bovine serum (OPZ), heat-aggregated bovine IgG (Agg-bovine IgG), sheep red blood cells (SRBC) sensitized with anti-SRBC antibody (SRBC-anti-SRBC Ab), or con A had LDCL responses of 36 (P<0.05), 77, 126 and 119% of peak LDCL values of controls, respectively. The NBT-reducing value of neutrophils from a calf with BLAD when stimulated with Agg-bovine IgG after pretreatment with anti-bovine IgG was 116.5% of the values of neutrophils from control calves, but the difference was not significant. The LDCL responses of neutrophils from a control calf and a calf with BLAD stimulated with OPZ were inhibited markedly by pre-incubation with anti-bovine IgG antiserum at concentrations ranging from 1.25 to 20 or 40 μg/ml. Although an increase in Fc receptor expression on neutrophils from calves with BLAD was observed, the LDCL responses stimulated with SRBC-anti-SRBC Ab and NBT-reducing activity stimulated with Agg-bovine IgG after pretreatment with anti-bovine IgG did not correlate significantly with the increased Fc receptor expression. These results support that neutrophil functions mediated by the Fc receptors are associated synergistically with the presence of the complement receptor type 3 (CR3)(CD11b/CD18).     

Echinococcus granulosus cysts: Early development in vitro in the presence of serum from infected sheep

When cultured in vitro in the presence of serum from a number of sheep infected with Echinococcus granulosus cysts, varying proportions of oncospheres died within 24 h. Of the survivors, some died during reorganization into cysts; others were able to develop normally but showed evidence of precipitates in the outer layers of the cyst. The lethal effects were removed by heating the serum to 56°C for 30 min and could be restored by the addition of freshly-collected normal sheep serum. In the presence of serum from sheep immunized against E. granulosus, most oncospheres were dead within 24 h, and few or none of the survivors were able to reorganize into cysts.     

Echinococcus granulosus: use of an intermediate host mouse model to evaluate sources of protective antigens and a role for antibody in the immune response.

A Balb/cJ mouse model was used to determine which stage of the E. granulosus life cycle possessed the most potent protective antigens. Mice were immunized with crude extracts of protoscoleces, brood capsules, cyst fluid, adult worm tissue, eggs or oncospheres and then challenged intraperitoneally with 600 activated oncospheres. Sonically disrupted oncospheres induced the highest levels of protection (greater than 90%) at doses greater than or equal to 10(3) oncosphere equivalents per mouse. High levels of protection were maintained when these preparations were solubilized in SDS. Immunization with Taenia ovis or T. hydatigena oncosphere preparations induced a maximum of 62 and 40% cross-protection, respectively. In passive transfer experiments, serum from triple-infected immune donors that were completely resistant to subsequent challenge induced 69% protection in naive recipients (P less than 0.01). Serum from mice that had been immunized with oncosphere sonicates that were shown to be highly immune, failed to induce statistically significant protection in recipients. A sheep trial confirmed the protective ability of prior infections. Immunization of sheep with a SDS solubilized oncosphere preparation produced 91% protection (P less than 0.01).     

Immune complexes and immunoconglutinin interactions associated with altered lymphocyte activity in Plasmodium chabaudi infections

Fresh plasma from rats infected with Plasmodium chabaudi, incubated with splenic lymphocytes from rats immunized 5 days previously with sheep blood cells, suppressed the capacity of the spleen cells to produce antibody against the sheep cells as was indicated by reductions in the numbers of hemolytic Jerne plaques formed by the treated cells. The effect was maximal in plasma of rats drawn on the 7th day of infection at a time the rats experienced a hemolytic crisis. Serologic studies indicated that the active plasma contained elevated titers of antibody against fibrinogen products, antibody against the soluble serum antigens elaborated during blood infections and antibody against the third component of fixed complement (C3) or immunoconglutinin. Titers of lytic complement were reduced and amounts of soluble immune complex precipitated with polyethylene glycol 6000 were elevated. The active plasma may have affected the antibody producing cells by one or both of two mechanisms. Soluble antigen-antibody complexes could have interacted with Fc receptors of activated lymphocytes to alter their function. Alternatively, the complexes may have fixed complement and interacted with receptors for fixed C3 on the lymphocyte membrane. Such cells, being coated with the antigen for immunoconglutinin, could be altered by immunoconglutination. Inasmuch as the immune complexes in the active plasma were generated in vivo, it would seem unlikely that the plasma would contain significant amounts of complex that had not fixed complement. With immunoconglutinin present in the plasma, alteration of the cells by immunoconglutination seems a more likely possibility.     

Fc gamma RI and Fc gamma RII on monocytes and granulocytes are cytotoxic trigger molecules for tumor cells

As part of an effort to define the cytotoxic trigger molecules on human myeloid cells, the ability of the different Fc receptors for IgG (Fc gamma R) to mediate killing of tumor cell lines by monocytes and granulocytes was examined. This was accomplished by studying cytolysis of hybridoma cell (HC) targets bearing surface antibody directed toward the different Fc gamma R. The HC line, HC IV.3A, which bears Ig directed to the low affinity Fc gamma R (Fc gamma RII) on monocytes and neutrophils was lysed by human monocytes. The extent of lysis of HC IV.3A was approximately equal to that of anti-Fc gamma RI (the high affinity Fc gamma R on human monocytes) bearing HC lines (HC 32.2A and HC 62A) and was not augmented by treatment of the monocytes with interferon-gamma (IFN-gamma). In contrast, neutrophils lysed HC IV.3A and HC 32.2A only after activation with IFN-gamma. Since Fc gamma RI is not detectable on untreated neutrophils and is induced by IFN-gamma on these cells, lysis of HC 32.2A by IFN-gamma-activated neutrophils correlated with receptor induction. On the other hand, Fc gamma RII was present at equal levels on untreated and IFN-gamma-treated neutrophils, but only IFN-gamma-treated neutrophils mediated cytotoxicity via Fc gamma RII. In this case, enhanced killing appeared to be due to events other than an increase in Fc gamma RII number. Neither untreated nor IFN-gamma-treated neutrophils mediated the lysis of the anti-Fc gamma RIII bearing HC 3G8A. Thus, binding to the tumor target via this Fc receptor does not lead to lysis and may initiate signals distinct from those triggered through Fc gamma RI or Fc gamma RII. Surprisingly, HC bearing high amounts of mouse IgG1 antibody of irrelevant specificity were also lysed by monocytes. This lysis was blocked by soluble IV.3 antibody and thus appeared to be due to binding of the Fc portion of the surface Ig to Fc gamma RII on monocytes. Furthermore, monocytes from donors with a form of Fc gamma RII incapable of binding aggregated mouse IgG1 did not lyse these HC, but displayed normal lysis of HC IV.3, demonstrating that this structurally different Fc gamma RII remained a functional trigger molecule. Overall, these studies have demonstrated the specificity of Fc receptors in triggering monocyte- and granulocyte-mediated antibody-dependent tumor cell killing and have begun to dissect functional similarities and differences among the three defined Fc gamma R on human myeloid cells.     

Influence of tumor necrosis factor-alpha on the ability of monocytes and lymphocytes to destroy intraerythrocytic Plasmodium falciparum in vitro

It has been shown that administration of TNF-alpha causes an increase of survival of plasmodium-infected mice. However, this anti-parasitic effect cannot be reproduced in vitro upon direct incubation of the cytokine with the parasite. This suggests that TNF-alpha may act through modulation of some plasmodicidal mechanism not yet clarified. We evaluated the effect of exogenous TNF-alpha on the phagocytosis of Plasmodium falciparum-infected erythrocytes by monocytes and its influence on the ability of monocytes and lymphocytes to inhibit parasite growth. The capacity of endogenous TNF-alpha to influence the ability of monocytes to inhibit the parasite was also verified. We found that addition of 33 ng TNF-alpha/mL to cultures of human monocytes and P. falciparum-infected erythrocytes increased the phagocytic index from 3.8 to 7.8 in the presence of serum containing P. falciparum antibody. TNF-alpha increased the capacity of monocyte plus lymphocyte to inhibit parasite growth by about 3 times at 0.5 and 5 ng/mL. Sera from severely ill P. falciparum-infected individuals inhibited the parasite growth, but addition of anti-TNF-alpha antibody was unable to modify this inhibition. These data show that TNF-alpha can increase the phagocytic capacity. This was probably due to an increased expression of Fc receptors on monocytes or to the modulation of Fc receptor signaling pathways by signals originating from the binding of TNF-alpha to its receptors. TNF-alpha also acted on lymphocytes plus monocytes by increasing the inhibition of P. falciparum by a mechanism not related to phagocytosis. These findings suggest that TNF-alpha has a pleiotropic anti-malaria effect and that this protective effect depends on the interplay of different factors, such as monocytes/macrophages, lymphocytes, and antibodies, in addition to other cells and molecules.     

Monoclonal antibody to human IgG Fc receptors. Cross-linking of receptors induces lysosomal enzyme release and superoxide generation by neutrophils

The monoclonal antibody KuFc79 binds to a determinant on the Fc receptors (Fc gamma R) of human leukocytes. We examined the biologic effects of the interaction of this antibody with Fc gamma R on human neutrophils (PMNL). The univalent Fab fragment of KuFc79 inhibits the formation of rosettes with IgG-sensitized sheep erythrocytes by as much as 91.7%. In other experiments in which PMNL were washed after exposure to Fab of KuFc79, phagocytosis of IgG-sensitized sheep erythrocytes was inhibited by 36%. Fab fragments of other mouse IgG2b monoclonal proteins did not have these effects. When PMNL are exposed to coverslips coated with univalent Fab fragments of this antibody, the Fc gamma R are removed from the surface of the PMNL. Under these conditions, rosetting could be inhibited by 85.4%. We examined cross-linking of receptor bound monoclonal antibody or its Fab fragment by either Protein A or F(ab')2 of an anti-mouse Ig. As much as 31.7% of beta-glucuronidase, a marker for lysosomal enzymes, is specifically released by cross-linking the Fc gamma R on PMNL. The generation of O2- is also induced by specifically cross-linking Fc gamma R with Fab and anti-Fab. The data constitute the first formal demonstration that cross-linking of Fc gamma R on PMNL leads to enzyme release and superoxide generation.     

Monoclonal antibodies to the spike protein of feline infectious peritonitis virus mediate antibody-dependent enhancement of infection of feline macrophages.

Antibody-dependent enhancement of virus infection is a process whereby virus-antibody complexes initiate infection of cells via Fc receptor-mediated endocytosis. We sought to investigate antibody-dependent enhancement of feline infectious peritonitis virus infection of primary feline peritoneal macrophages in vitro. Enhancement of infection was assessed, after indirect immunofluorescent-antibody labelling of infected cells, by determining the ratio between the number of cells infected in the presence and absence of virus-specific antibody. Infection enhancement was initially demonstrated by using heat-inactivated, virus-specific feline antiserum. Functional compatibility between murine immunoglobulin molecules and feline Fc receptors was demonstrated by using murine anti-sheep erythrocyte serum and an antibody-coated sheep erythrocyte phagocytosis assay. Thirty-seven murine monoclonal antibodies specific for the nucleocapsid, membrane, or spike proteins of feline infectious peritonitis virus or transmissible gastroenteritis virus were assayed for their ability to enhance the infectivity of feline infectious peritonitis virus. Infection enhancement was mediated by a subset of spike protein-specific monoclonal antibodies. A distinct correlation was seen between the ability of a monoclonal antibody to cause virus neutralization in a routine cell culture neutralization assay and its ability to mediate infection enhancement of macrophages. Infection enhancement was shown to be Fc receptor mediated by blockade of antibody-Fc receptor interaction using staphylococcal protein A. Our results are consistent with the hypothesis that antibody-dependent enhancement of feline infectious peritonitis virus infectivity is mediated by antibody directed against specific sites on the spike protein.     

Involvement of host Fc receptors in antibody-mediated cutaneous basophil hypersensitivity reactions.

Cutaneous basophil hypersensitivity (CBH) reactions are heterogeneous delayed time course basophil-rich responses that can be mediated by either T cells, B cells, or serum antibodies. The current study examined the mechanism by which antibodies mediate CBH in guinea pigs. Fc competition experiments were constructed by passively transferring mixtures of anti-KLH serum and normal heterologous gamma-globulins. It was found that rabbit IgG and its isolated and purified Fc fragment [but not the (Fab')2 fragment] inhibited the ability of guinea pig immune serum to transfer CBH. Concurrent inhibition of transferred KLH-specific CBH and systemic passive cutaneous anaphylaxis (PCA) reactions by rabbit IgG or its Fc fragment, and not by sheep or bovine gamma-globulins, indicated that Fc receptors on cutaneous mast cells were probably involved in both CBH and PCA. It was also found that the basophil aspect of delayed cutaneous responses elicited by PHA was inhibited by Fc competition maneuvers. This could mean that some forms of apparently T cell-mediated CBH may be T cell dependent, but via secretion of molecules that bind to Fc receptors, as seems required in antibody-mediated CBH.     

Production and characterization of monoclonal antibodies against excretory/secretory products of adult Echinococcus granulosus, and their application to coproantigen detection

Two IgM murine monoclonal antibodies (MAbs), EgC1 and EgC3, were produced against the excretory/secretory (E/S) products of Echinococcus granulosus adult worms. Immunoblotting revealed that both predominantly recognized a 50 kDa antigen in the somatic extract and an 85 kDa component in the E/S products. Immunolocalization showed that both MAbs reacted with the tegument of the parasite, and additionally EgC3 reacted with parenchyma and the tegument lining the external surface of the reproductive organs. A coproantigen capture ELISA was developed using a rabbit polyclonal antibody against E/S products from adult tapeworms as catching antibodies, and each one of MAbs as detecting antibody. The assays detected seven out of eight (EgC1), and eight out of eight (EgC3) experimentally infected dogs (worm burdens ranging from 61 to 57,500), using heat-treated samples obtained at prepatent period, and none (n=8) of helminth-free samples. Time course analysis showed that, after a 12-25 days lag, coproantigen levels rose above cut off O.D. values and typically peaked around 30 days post-infection (DPI) at the end of the experiment. One dog experimentally infected with Taenia hydatigena metacestodes was slightly detected as positive at different time points after 30 DPI. Both MAbs showed a similar pattern of recognition, but T. hydatigena antigens were undetectable for a longer period, and reached lower O.D. values with EgC1. Interestingly, fecal samples from two experimentally infected dogs with Echinococcus multilocularis were not recognized by the EgC1 assay, suggesting a potential value as species-specific diagnostic tool.     

The use of excretory and secretory antigens of the scolex of Taenia ovis for the serodiagnosis of infection in dogs

The excretory and secretory antigens from the evaginated scoleces of Taenia ovis were collected for 3 days in vitro, and used in an ELISA test to detect antibodies to T. ovis in the serum of dogs. When tested on sequentially collected sera, diagnostic ELISA values could be detected in many dogs 4 wk after infection, and remained for an average of a further 4 wk after worms were removed from dogs with an anthelmintic. Using an ELISA discriminant value that eliminated all false positives from 70 uninfected laboratory dog sera and from 57 uninfected farm dog sera, 54/62 true positives were found in sera from dogs infected with various numbers of T. ovis for various intervals. Sera from dogs infected with T. hydatigena gave 11/15 false positive reactions, whereas sera from 15 dogs infected with Echinococcus granulosus or 7 dogs infected with T. pisiformis were all negative. For T. ovis the test had a high repeatability, was not greatly influenced by the number of worms carried by the dog and higher titres were correlated with long-standing infections. Approximately 1,000 scoleces could be recovered from each experimentally infected sheep. Using the ELISA test with undiluted antigen and serum diluted 1:40, approximately 10 sera could be tested in duplicate with the excretions and secretions from each T. ovis scolex.     

Macrophage Fc receptors control infectivity and neutralization of canine distemper virus-antibody complexes.

Dogs that are persistently infected or that become moribund after exposure to canine distemper virus (CDV) have antibody that neutralized CDV when tested in dog lung macrophage cultures but failed to neutralize CDV when tested in epithelial, fibroblastic, or lymphatic cells. The antibody attached to protein A and was found in the immunoglobulin G fraction. The antibody bound complement and lysed CDV-infected target cells. The neutralizing activity in macrophages could be abolished (i) by pepsin digestion and removal of Fc portions from the antibody, (ii) by blocking the Fc receptors of macrophages with heat-treated normal dog serum, and (iii) by binding of protein A to Fc portions of the antibody. It was concluded that attachment of the CDV-antibody complex to Fc receptors of macrophages was essential for virus neutralization. If this attachment was hindered, the CDV-antibody complex became infectious for macrophages. In contrast, serum from recovering dogs neutralized CDV when tested in epithelial, fibroblastic, or lymphatic cells as well as in macrophages.     

Development of Neospora caninum cultured with human serum in vitro and in vivo

Because there has been no report of symptomatic Neospora caninum infection in humans, we examined the effect of human serum on the parasite's growth in either a bovine angioendothelial cell or Caco-2 cell culture in vitro and in immunocompromised mice in vivo. There was no difference in intracellular parasite numbers between cells incubated with human serum at 24 hr after challenge and those incubated with fetal bovine serum (FBS), which has no titer for the anti-N. caninum agglutination antibody test. Serum of sheep infected with N. caninum, which has the anti-N. caninum antibody, reduced the numbers of the intracellular parasite significantly. We also showed that there was no inhibitory effect on the intracellular multiplication of the parasite in cells incubated with human serum through incorporation of 3H-uracil. CB-17 scid mice administered human serum daily and challenged with N. caninum died on day 20 or 22 after challenge, when large numbers of parasite clusters were found in the brain, oviduct, adrenal gland, lung, stomach, spleen, skeletal muscle, pancreas, and mesenteric lymph nodes. Scid mice administered FBS survived until the end of the experiment. These results suggest that adult human serum may have no inhibitory effect on the development of N. caninum in vitro and in vivo.     

Echinococcus multilocularis: inhibition of murine neutrophil and macrophage chemotaxis

Resident peritoneal neutrophils and macrophages from mice infected with 50 +/- 5 cysts of Echinococcus multilocularis were collected at 2, 4, 6, 8, 10, and 16 weeks postinfection. Their ability to respond and migrate to purified parasite larval antigens or endotoxin-activated mouse serum (EAMS) in comparison to normal peritoneal cells from uninfected mice was tested in vitro using Boyden chambers. Early in the infection, both cell types responded to the specific and nonspecific chemoattractants as the control group. However, at 8 and 10 weeks postinfection, the neutrophils and macrophages lost their response to parasite antigens but retained their ability to migrate to EAMS. Toward the 12th and 16th week postinfection, both cell types lost their ability to migrate to the specific as well as the nonspecific factors. The data presented suggest that the cellular mechanisms of recognition and chemotaxis in mice infected with alveolar hydatid cysts are impaired.     

Amblyomma americanum: requirement for host Fc receptors in antibody-mediated acquired immune resistance to ticks

Guinea pig recipients of anti-tick immune serum or immune peritoneal exudate cells expressed 25 and 30% tick rejection, respectively, when challenged with Amblyomma americanum larval ticks. Previous studies have shown that IgG1 antibodies are responsible for the ability of immune serum to transfer resistance to ticks and to mediate the accompanying, and required, cutaneous basophil response. Since IgG1 antibodies induce mast cell-mediated passive cutaneous anaphylaxis and cutaneous basophil responses by interaction with cell surface Fc receptors, we investigated whether host Fc receptors were involved in the mechanism of antibody-mediated immune resistance to ticks. Recipients of immune serum pretreated intravenously with rabbit IgG failed to express resistance when challenged. In contrast, recipients of immune peritoneal exudate cells similarly pretreated expressed normal resistance. Sheep IgG had no inhibitory effect on the transfer of resistance by either immune serum or peritoneal exudate cells. Furthermore, recipients of immune serum pretreated with the Fc fragment from papain digestion of rabbit IgG failed to express resistance when challenged with ticks. Rabbit Fab and sheep Fc and Fab had no effect on the transfer of resistance by immune serum. Purity of rabbit Fc preparations was verified by the ability to inhibit mast cell-mediated passive cutaneous anaphylaxis due to high-titered IgG1 antiovalbumin antibodies. Rabbit Fab and sheep Fc fractions did not inhibit passive cutaneous anaphylaxis reactions. These findings suggest that immunoglobulin Fc receptors on host cells, such as mast cells and basophils, are required for antibody-mediated immune rejection of ticks from guinea pigs.     

Trypanosoma musculi co-express several receptors binding rodent IgM, IgE, and IgG subclasses

The present work demonstrates the expression of receptors for the Fc portion of rodent Ig by the murine parasite Trypanosoma musculi. By using a rosette assay adapted to the parasite morphology and by flow cytometry analysis, three distinct receptors were identified. A receptor binding rabbit or rat polyclonal IgG and mouse monoclonal IgG1, IgG2a, and IgG2b was found on parasites purified from the blood and the peritoneal cavity of infected mice and on parasites maintained in culture conditions. This IgG receptor was degraded by pepsin. A separate receptor, binding only mouse monoclonal IgG3 was observed on cultured parasites. A receptor binding rabbit, rat, and mouse IgM was found on cultured and peritoneal parasites, but not on blood parasites. This receptor did not bind IgG or IgA but it bound mouse and rat IgE as well as IgM. It was degraded by trypsin. IgG and IgM/IgE receptors were co-expressed on single parasites. They were not of host origin but synthesized by trypanosomes as shown by reexpression in vitro after proteolytic degradation. Their expression was variable with the development of trypanosomes both in vitro and in vivo.     

Discovery and characterization of an antibody, anti-egressin, that is able to inhibit Trypanosoma cruzi egress in vitro

Trypanosoma cruzi, a protozoan parasite, is the etiologic agent of Chagas disease. The disease is characterized by acute and chronic phases, with high and low parasitemia, respectively. A strong immune activation is necessary for the host to enter the chronic phase; however, immune mechanisms participating in the reduction of parasites between the acute and chronic phases of the disease have been very difficult to elucidate. We report here the discovery of anti-egressin, an antibody present in serum from chronically infected BALB/c mice that is able to inhibit parasite egress from infected BALB/c fibroblast cultures in vitro. The antibody is very concentrated in serum from these mice; chronic serum may be diluted 1:20 while still maintaining functional activity. Isotype analysis of anti-egressin has suggested it to be IgG2a. Further analysis revealed that anti-egressin was specific for a component expressed on the surface of infected host cells. The specificity of anti-egressin toward the extracellular portion of infected host cells was demonstrated both by using a quantitative assay measuring released trypomastigotes and through immunocytochemical staining. The novel role of anti-egressin in the inhibition of parasite egress from infected host cells has not been described in the literature to date. We believe that anti-egressin plays an important role in achieving the low parasitemia characteristic of chronic Chagas disease.