Risk and safety of immunotherapy

The risk and safety of specific immunotherapy (SIT) has been the subject of many well-designed studies since it was introduced. Most authors consider SIT an efficient and safe mode of treatment despite the occurrence of life-threatening anaphylactic reactions. They emphasize the importance of a proper selection of patients as well as the requirement for highly qualified physicians to provide the treatment. Another problem associated with the safety of SIT is the potential enhancement of the synthesis of immune complexes after the treatment. SIT causes a decrease in allergen-specific IgE which is slower in circulating immune complexes than in the serum. No data are available to show that SIT induces immune-complex diseases, in either the immunologic or the clinical sense. As far as anaphylactic reactions are concerned, new alternative methods different from the conventional parenteral mode of delivery seem to be safe. Our increased understanding of the altered functions of the immune system justifies the assumption that the more specific the method of SIT administration, the more effective and safe it is.

Molecular mechanisms of glomerular injury in rat experimental membranous nephropathy (Heymann nephritis)

The molecular pathogenesis of human membranous nephropathy (MN) is unknown, despite the relatively high incidence and severity of this glomerular immune disease. Heymann nephritis (HN) in rats is considered an instructive experimental model of MN. This study summarizes current molecular aspects of two key events common to both MN and HN, i.e., formation of characteristic subepithelial immune deposits in the glomerular basement membrane (GBM), and development of glomerular capillary wall damage resulting in proteinuria. In HN, the antigenic targets of immune deposit-forming antibodies were identified in cell membranes of glomerular epithelial cells as a 515-kd glycoprotein (megalin, or gp330), which is a polyspecific receptor related to the low-density lipoprotein receptor family, and an associated 44-kd protein (receptor associated protein, RAP). One epitope was recently narrowed to 14 amino acids in RAP, and several others on megalin/gp330 are under investigation. Proteinuria requires formation of the complement C5b-9 membrane attack complex, which is presumably triggered by antibodies directed against lipid antigens that associate with immune deposit-forming megalin/gp330 immune complexes. Sublytic C5b-9 attack on glomerular epithelial cells causes upregulation of expression of the NADPH oxidoreductase enzyme complex by glomerular cells, which is translocated to their cell surfaces, similar to activated neutrophil granulocytes in the respiratory burst reaction. Subsequently, reactive oxygen species (ROS) are produced locally, which reach the GBM matrix. Here formation of lipid peroxidation (LPO) adducts is found, preferentially on monomeric and dimerized NCl domains of covalently crosslinked Type IV collagen. These structural changes within the GBM could be of functional relevance because treatment with the potent LPO-antagonist probucol reduces proteinuria by < 80%. Intact or fragmented apoprotein E-containing lipoproteins were identified as potential sources of the polyunsaturated lipids required for the production of LPO adducts. Lipoproteins accumulate within immune deposits and show signs of oxidative damage, similar to oxidized LDL within atherosclerotic lesions. Collectively, the results obtained so far in HN permit the compilation of a sequence of events, linking formation of immune deposits with proteinuria. However, despite this relatively detailed knowledge of pathogenic events in HN, the bridge to human NM remains to be built.

Differential expression of CLIP:MHC class II and conventional endogenous peptide:MHC class II complexes by thymic epithelial cells and peripheral antigen-presenting cells

Major histocompatibility complex (MHC) class II molecules expressed by thymic epithelial cells are involved in positive selection of CD4 T cells, whereas the high-avidity interaction of T cell receptors with the endogenous peptide: MHC class II complexes expressed on bone marrow (BM)-derived antigen-presenting cells (APC) and, to a lesser extent, on thymic epithelial cells mediate negative selection. To understand better the generation of the CD4 T cell repertoire both in the thymus and in the periphery we analyzed relative levels of expression of specific endogenous peptide: MHC class II complexes in thymic epithelial cells (TEC) and peripheral APC. Expression of E alpha52-68: I-A(b) and class II-associated invariant chain peptide (CLIP): I-A(b) complexes in thymic epithelial cells and in the bone-marrow derived splenic APC, i.e. B cells, was studied using YAe and 30-2 monoclonal antibodies which are specific for the corresponding complexes. To distinguish between expression of both complexes in radioresistant thymic epithelial elements and radiation sensitive BM-derived APC, radiation BM chimeras were constructed. Using immunohistochemical and immunochemical approaches we demonstrated that the level of expression of E alpha52-68: I-A(b) complexes in thymic epithelial cells is approximately 5-10% of that seen in splenic cells whereas total class II levels were comparable. In contrast, CLIP: I-A(b) complexes are expressed at substantially higher levels in TEC vs. splenic APC. This result demonstrates quantitative differences in expression of distinct peptide: MHC class II complexes in thymic epithelial cells and peripheral splenic APC.

CRP-mediated activation of complement in vivo: assessment by measuring circulating complement-C-reactive protein complexes

The in vivo function of C-reactive protein (CRP) is unknown. Among the in vitro functions assigned to CRP is the ability to activate complement via the classical pathway. To date, there is no evidence supporting that CRP exerts this function in vivo. We here show a novel approach to assess CRP-mediated complement activation in vivo, which is based on the property that activated complement factors C3 and C4 fix to CRP during complement activation induced by this acute phase protein. We developed specific ELISAs for complexes between CRP and C4b, C4d, C3b, or C3d. We established that in vitro complement-CRP complexes were formed only during CRP-dependent activation, and not during activation by other activators, even in the presence of high CRP levels. Circulating levels of complement-CRP complexes were undetectable in normal donors, but significantly increased in nine patients following implantation of a renal allograft. Importantly, levels of complement-CRP complexes did not change in these patients upon a bolus infusion of mAb OKT3, which induces activation of the classical complement pathway, demonstrating in vivo that complement-CRP complexes are not formed during CRP-independent activation of complement, even when CRP is elevated. We conclude that measurement of complement-CRP complexes provides a suitable tool to study CRP-mediated activation of complement in vivo. Furthermore, increased levels of these complexes occur in clinical samples, indicating that CRP may induce activation of complement in vivo.

The Heymann nephritis antigenic complex: megalin (gp330) and RAP

Heymann nephritis (HN) has been extensively studied as a model of human membranous nephropathy since it was first described by Heymann in 1959. HN was induced in active form by the immunization of rats with antigens derived from the proximal tubule brush border, resulting in subepithelial glomerular immune deposits. HN was also induced passively by the injection of antibrush border antibodies into normal rats. A breakthrough in the understanding of the pathogenesis of HN was made in the 1970s, when it was established that the disease was due to the binding of circulating antibodies to glomerular components. This in turn led to a search to identify the endogenous antigen(s). In 1982, gp330 (now called megalin), a glycoprotein located in clathrin-coated pits of glomerular and proximal tubular epithelia, was identified as a target antigen. In 1990, a second protein (44 kd), now known as RAP (for receptor associated protein), that binds to megalin was also shown to be a target antigen. Both molecules have been cloned and sequenced, and their role in normal epithelial cells has been explored. It has come to light that megalin (gp330) is a member of the low-density lipoprotein receptor gene family and functions as a multiligand receptor for the uptake of a variety of macromolecules (plasminogen, protease: protease inhibitor complexes, apolipoprotein E-enriched very low-density lipoproteins, lactoferrin, among others). RAP associates with megalin and appears to function as a chaperone assisting in the folding of megalin in the endoplasmic reticulum and its transport to the cell surface. This review considers what is now known about the structure, function, and trafficking of megalin and RAP and the role of these two molecules in the pathogenesis of HN.

Recombinant single-chain antibody peptide conjugates expressed in Escherichia coli for the rapid diagnosis of HIV

Recombinant single chain Fv (scFv) antibody fragments can form the basis of a rapid, whole-blood diagnostic assay. The scFv described in this study is derived from a monoclonal antibody which has a high affinity for glycophorin A, an abundant glycoprotein on the human red blood cell membrane surface. The prototype reagent built around the scFv was designed to detect, in whole blood samples, the presence of antibodies that have arisen through infection with a foreign organism such as human immunodeficiency virus. The scFv was composed of the antibody heavy-chain variable domain (Vh) joined by a 15 residue linker -(GGGGS)3- to the light-chain variable domain (V1) terminated by either a C-terminal octapeptide tail (FLAG) or a 35 amino acid segment from the gp41 surface glycoprotein of HIV-1. Constructs were cloned into a Escherichia coli expression vector, pHFA, and expressed in a soluble form into culture supernatant. The product retained anti-glycophorin activity which could be detected directly in culture supernatants by ELISA. Furthermore, the scFv-epitope fusion functioned efficiently in the whole blood agglutination assay and was able to distinguish between HIV-1 positive and negative sera.

Xenogenic humoral responses to islets transplanted in biohybrid diffusion chambers

It has been hypothesized that chronic antigen leakage from the hybrid artificial pancreas could stimulate a host humoral response. Such antibodies could be induced by antigens shed from the islet cell surface, or by proteins secreted by live cells or liberated after cell death. To determine if this humoral response occurs, porcine (n = 15) or canine (n = 7) islets were seeded (2-5 x 10(4) equivalent islet number, density 30 islets/mm3) into diffusion chambers fabricated from permselective acrylic membranes (nominal M(r) exclusion of 80,000). The chambers were implanted intraperitoneally into streptozotocin-induced diabetic rats. Sera were collected at various intervals (0-12 weeks) and tested against isolated canine and porcine islets, for tissue specificity and interspecies cross-reactivity by fluorescence immunocytochemistry. No immunofluorescence (or only weak background staining) was obtained when islets were exposed to horse sera, or to sera obtained before to xenodevice implantation. Within 2-6 weeks, however, the postimplantation sera showed strong immunoreactivity. The antibodies were found to be reactive to multiple tissues, and to possess little or no interspecies cross-reactivity. The appearance of these xenoantibodies coincided with the appearance of circulating soluble immune complexes. However, none of the respiratory, cutaneous, or gastrointestinal manifestations that are characteristic of an anaphylactic reaction, or of the diseases of immediate-type hypersensitivity, were observed, even after intraperitoneal injection of additional naked islet tissue. Renal glomeruli did not stain for IgG or C3 in islet recipients. These results suggest that islet cell antigens crossed the membrane and stimulated antibody formation in the host, although they did not appear to cause renal or immune complex disease during the course of this study.

The effects of histidine residue modification on the immune precipitating ability of rabbit IgG

Treatment of anti-ovalbumin rabbit IgG with diethylpyrocarbonate (DEPC) at concentrations up to 100 microM led to a progressive decrease in the rates of formation of insoluble immune complexes, without affecting the final extent of immune complex formation. DEPC concentrations approximately 10-fold higher were needed to give comparable decreases in the rates of immune complex formation by F(ab')2. Treatment of DEPC-treated IgG with hydroxylamine led to substantial restoration of the rates of formation of insoluble immune complexes. Carbethoxylation of two histidine residues per IgG molecule had little effect on rates of formation of insoluble immune complexes, but these rates were markedly decreased in samples of IgG with four to five histidines per molecule modified. There were parallel decreases in the protein A-binding activity and in the rates of formation of insoluble immune complexes in IgG treated with increasing concentrations of DEPC. The presence of complement protein C1q restored the rates of formation of insoluble immune complexes of DEPC-treated IgG.

Evaluation of in vivo immune complex formation and complement activation in patients receiving intravenous streptokinase

The usefulness of several different methods for detecting immune complex formation and complement activation in the circulation were applied to samples from patients receiving intravenous Streptokinase therapy for myocardial infarction. Streptokinase is a foreign antigen and can cause immune reactions. We collected samples from 13 patients, before Streptokinase administration (baseline), at the end of infusion (1 h), 12 h later and on day 7. We measured IgG containing immune complexes (IgG-IC), free C3d and antibodies to Streptokinase by ELISA, and CR1, C3d and C4d on erythrocytes by flow cytometric assay. Antibodies to Streptokinase are common, as all but two of the patients had measurable antibody levels. During Streptokinase treatment there was a drop in antibody levels, most prominent in those patients who had high baseline levels. At the same time increased levels of free C3d and erythrocyte-bound C3d were observed. After 12 h free C3d was usually back to baseline level, but C3d on erythrocytes was still raised. These data indicate the formation of Streptokinase immune complexes in patients with high Streptokinase antibody levels, and show that these complexes are cleared rapidly from the circulation, leaving more persistent signs of complement activation. We conclude that free C3d is a good indicator of ongoing complement activation, whereas C3d on erythrocytes indicates that complement activation has recently taken place.

Polyclonal activation of B-lymphocytes and induction of autoimmunity in retrovirus infected NMRI mice

Polyclonal activation of B-lymphocytes accompanies many retroviral infections. Friend derived murine immunosuppressive virus (Fd-MIV) is a non-defective murine retrovirus which was isolated from T-helper cells from mice infected with the acute transforming retrovirus Friend leukaemia complex (FLC). In contrast to FLC, Fd-MIV does not induce acute transformation of lymphoid and erythroid cells but causes immunosuppression and lymphadenopathy in adult NMRI mice. The effect of Fd-MIV infection on B-lymphocytes was studied. Fd-MIV infection led to a persistent hypergammaglobulinemy with a significant increase in the level of circulating IgG, IgM and immune complexes. In the spleen and lymph nodes, B-lymphocyte proliferation was found. Parallel to the development of hypergammaglobulinemy, autoantibodies to a variety of nuclear and other autoantigens was detected. In conclusion, the Fd-MIV infection leads to a B-lymphocyte dysfunction that has many parallels with AIDS. Furthermore, the Fd-MIV infection seems to represent an example of an autoimmune condition caused by an exogenous infectious agent.

[Anti-influenzal antibodies inducing an immune response to the influenza virus]

The injection of anti-influenza antibodies into rabbits induces a specific immune response, including the production of anti-idiotypic antibodies and then the production of immune complexes and anti-influenza antibodies. The injection of antibodies induces the development of immunological memory; as a result, the animals are primed to respond to the injection of influenza vaccine.

Polyclonal amanitin-specific antibodies: production and cytoprotective properties in vitro

The amanitins found in several mushroom species are responsible for many deaths every year. Based on its successful application to cardiac glycoside overdose, immunotherapy could be applicable to amanitin toxicity. Therefore, we produced polyclonal amanitin antibodies by immunizing rabbits with a novel conjugate of alpha-amanitin. Purified antibodies had an average association constant for alpha-amanitin of 1.3 x 10(9) M-1. A partially protective effect of the antibodies against amanitin toxicity in vitro in Chang cells was evident at a molar ratio of antibody binding sites to alpha-amanitin of 4:1. Together with reported studies in vivo, these investigations indicate the potential of immunotherapy for amanitin poisoning.

Humoral immune response during filarial fever in Bancroftian filariasis

Humoral immune responses against filarial parasitic infection were studied in 62 cases of acute filarial disease presenting with filarial fever with adenolymphangitis, in a community where Bancroftian filariasis was endemic, during and about one month before and after the febrile episode. Their total leucocyte and differential peripheral blood cell counts and anti-streptolysin O titre were determined and compared. Polymorphonuclear cellular responses and anti-streptolysin O titre did not show any significant alteration during and after fever. Three of 53 previously amicrofilaraemic subjects (9 of whom were initially microfilaraemic) had microfilaria in their circulation during fever, with a significant increase in their geometric mean microfilaria count. Titres of specific immunoglobulin (Ig) G and IgG4 antibody to Wuchereria bancrofti microfilarial excretory/secretory antigens (measured by enzyme-linked immunosorbent assay) decreased significantly during the fever and the lower levels were maintained one month after fever. The mean circulating immune complex level increased significantly during fever, and a significant percentage of cases demonstrated circulating filarial antigen during fever, which declined after one month, suggesting the release of filarial antigen into the circulation during fever which bound to antibodies to form immune complexes. These observations do not support the suggestion that bacterial infection is the aetiology of filarial fever. It is postulated that antigens released from parasites into the circulation during parturition by adult worms may evoke an allergic response in the host, causing periodic febrile episodes.

Immunological abnormalities in HIV-free haemophiliacs

Since some haemophiliacs manifest profound immunodeficiency with no evidence of human immunodeficiency virus type 1 (HIV) infection, we measured the circulating immune complex (CIC) level in sera obtained from haemophiliacs and addressed the question of whether viral infection is associated directly or indirectly with enhanced CIC production. While more than 90% of HIV-positive individuals had a high level of CIC, around 60% of seronegative ones also showed CIC levels comparable to those of seropositive patients. These sera activated fresh complement in vitro. The patients infected with either HIV or Hepatitis C virus (HCV) or both showed higher frequency and concentration of serum CIC than those free of either pathogens. It is worth noting, however, that 64% of patients with no evidence of infection with HIV or HCV produced significant amounts of CIC. Among the infectious viruses examined, parvovirus is considered as one of the pathogens associated with CIC synthesis, since all the haemophiliacs including the HIV-free patients who had been supplied with heated coagulation factors for several years from birth carried antibodies to parvovirus B19. Strikingly, 60% of the children in this category were positive for CIC, suggesting the possible contribution of parvovirus infection to CIC formation.

Suppression of ADCC by immune complexes formed in vitro in Mycobacterium leprae-infected mice

Antibody-dependent cellular cytotoxicity (ADCC) was assessed in mice infected experimentally with Mycobacterium leprae and injected simultaneously with in vitro-formed immune complexes (IC). Significant decrease in the ADCC function was observed in animals given IC at zero day (0dIC) and 3 months (3mIC) post inoculation with M. leprae, when ADCC activity was assessed at 3, 6 and 9 months period. From the data obtained we believe that ADCC is suppressed by IC formed in vitro.

The role of complement-derived chemotactic factors in lung injury induced by preformed immune complexes

Our previous studies have suggested a role for complement fragments presumably activated by immune complexes in patients with hypersensitivity pneumonitis. The present study has shown that circulating complement depletion by cobra venom factor resulted in the reduction in severity of immune-complex-mediated pulmonary inflammation. The activity of chemotactic factors for neutrophils generated in bronchoalveolar lavage fluids in complement-depleted animals was significantly diminished to 61.2% compared to the undepleted animals. In addition, reduced activity of chemotactic factors resulted in a marked reduction of accumulation of neutrophils in bronchoalveolar lavage fluids indicating that chemotactic factors play an important role in the sequestration of neutrophils on the alveolar side of the lung. In conclusion, chemotactic factors in bronchoalveolar lavage fluids which preceded the accumulation of polymorphonuclear cells are partially derived from complement.

Markers of inflammatory activation: upregulation of complement receptors CR1 and CR3 on synovial fluid neutrophils from patients with inflammatory joint disease

Expression of the C3 receptors CR1 and CR3 was investigated on neutrophils from paired peripheral blood and synovial fluid samples from 34 patients with inflammatory joint disease (21 patients with rheumatoid arthritis (RA) and 13 patients with other articular diseases (OAD)). Using monoclonal antibodies (anti-CD35, anti-CD11b) and immunofluorescence flow cytometric analyses the percentages of positively labeled cells and the relative fluorescence intensities (as a measure of receptor number) were determined. CR1 and CR3 were found to be present on the majority (> 85%) of circulating neutrophils from normal subjects, RA and OAD patients, and on synovial fluid neutrophils from both patient groups. A strong correlation between neutrophil CR1 and CR3 expression was observed in peripheral blood samples from normal subjects (r = 0.81; P = 0.001), RA (r = 0.79; P = 0.001), and OAD patients (r = 0.83; P = 0.001); in each case the levels of CR3 expression were approximately twice those recorded for CR1. Both CR1 and CR3 expression was upregulated on synovial fluid neutrophils compared with that observed on the corresponding peripheral blood cells. Mean percentage increases observed were: RA patients: CR1, 16.5% (P < 0.001) and CR3, 28.7% (P < 0.001); and OAD patients: CR1, 4.1% and CR3, 26.9% (P = 0.001). Correlation of serum and synovial fluid IL-6, IL-8, and immune complex levels with neutrophil CR1 and CR3 expression failed to demonstrate any significant relationship between the concentrations of these soluble factors and receptor expression. Upregulation of CR1 and CR3 receptors, reflecting neutrophil activation within the inflamed joint, is a consistent finding in patients with inflammatory arthropathies.

Production of rheumatoid factor in adoptively immune guinea-pigs after challenge with Treponema pallidum

Guinea-pigs of inbred strains 2 and C4D were infused with various concentrations (1 x 10(8) to 4 x 10(8) of syngeneic nylon wool-purified Treponema pallidum-immune T lymphocytes (TPI-T) and challenged 24 hr later with virulent T. pallidum (10(8) organisms). The degree of protection depended on the number of infused T cells and was associated with an accelerated production of IgM rheumatoid factor (RF). Fully protected animals (4 x 10(8) TPI-T) did not produce treponemal antibodies or circulating immune complexes (CIC) but produced IgM RF detectable 10 days after infection. Partially protected animals (< or = 2 x 10(8) TPI-T) produced, 30 days post-infection, relatively low levels of treponemal antibodies but high levels of CIC and RF. Control animals infused with 2 x 10(8) TPI-T lymphocytes but not infected with T. pallidum, when monitored for a period of 6 weeks, did not produce treponemal antibodies, CIC, or RF, excluding the possibility that IgM RF could be generated by the donor's B cells contaminating (circa 3%) the TPI-T lymphocytes. Moreover, unprotected syngeneic control animals infused, prior to infection, with T. phagedenis biotype Reiter-immune T cells or with T. pallidum-free testicular inflammatory fluid-immune T cells responded with increasing levels of treponemal antibodies; only a few animals produced RF and CIC 5 months after infection similarly to control guinea-pigs infected only. The production of RF in partially protected animals responding to infection with treponemal antibodies and CIC was apparently associated with the presence of the CIC; but the mechanism of RF production in fully protected animals in which no antibodies or CIC were detected is currently unknown.

The contribution of antibody-mediated cytotoxicity and immune-complex formation to tubulointerstitial disease in passive Heymann nephritis

Passive Heymann nephritis (PHN), an experimental model of membranous nephropathy, is produced by Fx1A antiserum, which also reacts with antigens on the brush border (gp 330) and basolateral membrane (gp 90) of proximal tubules. We examined tubulointerstitial disease in PHN, identifying two distinct processes occurring on the luminal and basolateral membranes, respectively. Injected antibody bound diffusely to the tubular brush border from Day 1 to Day 7, followed by sloughing of microvilli and tubular-cell regeneration. Fine granular deposits of Fx1A antibody were present along the basolateral cell membrane by Day 1. These deposits rearranged in situ, enlarged, and became more focally distributed along tubular basement membranes (TBM). Interstitial inflammation, dominated by macrophages (Ia+, ED-1+) in association with a smaller number of T-cytotoxic cells (OX19+, OX8+) began by Day 3, reached peak intensity and persisted throughout the autologous phase (to Day 21). The distribution of focal clusters of interstitial macrophages predominately in association with TBM-immune deposits was demonstrated. Complement depletion prevented proteinuria but TBM deposits developed and the interstitial inflammation was unchanged. All aspects of the tubulointerstitial disease were amplified by a second injection of Fx1A antiserum. In vitro, Fx1A antibody bound to the surface of isolated proximal tubular epithelial cells and redistributed to form clusters of immune aggregates. Anti-Fx1A-induced cytotoxicity of tubular cells was demonstrated by prelabeling cells with 2'-7'-bis(carboxyethyl)-5(6)-carboxyfluorescein. The degree of cytotoxicity was dependent on complement concentration and the duration of incubation at 37 degrees C. PHN induced by Fx1A antiserum causes tubular-cell injury following interactions with brush-border antigens and TBM immune-complex disease associated with interstitial inflammation. These findings may be relevant to the acute and chronic interstitial disease of human membranous nephropathy.

Evaluation of chemotherapy in experimental toxocarosis by determination of specific immune complexes

Parasitism by the larval phase of Toxocara canis is a chronic process in which the larvae survive in the tissues, resulting in the constant stimulation of the immune system. As a result, the detection of specific antibodies may not reflect the active state of the parasite. We have studied the dynamics of the production of specific immune complexes by ELISA with the monoclonal antibody TC-1 in rabbits inoculated with single and multiple doses of T. canis eggs. We also compared this with the production of specific antibodies and their possible modification after treatment with mebendazole. The specific antibodies against excretory-secretory antigen were detected with peaks at 10 and 12 weeks depending on the dose and remained positive during the entire experiment (62 weeks). Treatment caused an increase in the level of detectable antibodies dropping to similar levels to the controls. Specific immune complexes were detected only in multiple doses, and were then positive during the entire experiment. From the beginning of treatment the values of immune complexes fell quickly, remaining at undetectable levels during the rest of the experiment. For this reason the detection of specific immune complexes is a valid technique for monitoring the efficiency of treatment.

Echinostoma caproni in mice: ultrastructural studies on the formation of immune complexes on the surface of an intestinal trematode

The binding of mouse antibodies to the surface antigens of juvenile and 7 and 28 day old Echinostoma caproni was examined by transmission electron microscopy of thin sections of parasites, which were treated with antibodies in a double sandwich technique with ferritin-conjugated antibody. The surface of freshly recovered mature adult parasites was covered with an irregular but often rather intensive mouse antibody containing matrix, which probably represents a layer of mouse antibody/parasite antigen complexes. The complexes were lost after in vitro culturing of the parasites for 24 h, but incubation of the in vitro-maintained antibody-negative adult parasites with immune mouse serum led to reformation of a similar but less intensive cover with immune complexes. Juvenile and young stages of E. caproni, which had never been exposed to host antibodies, obtained a layer of immune complexes on their surface after incubation with immune mouse serum in vitro. In both young and mature parasites, the antibody-antigen complexes were observed to be rather loosely attached to the outer surface of the parasites, where the antigens probably constitute a part of the irregular glycocalyx of the organisms. It may also be that the antigens are present as isolated excretion along the surface of the parasites. Several sections indicated that the parasite surface antigens may be present in the tegument in vesicles which fuse with the outer membrane of the parasite whereby their contents are released to the exterior.