Destruction of the multicellular parasite Schistosoma mansoni by T lymphocytes

Innate cell-mediated cytotoxicity of CD8+ T cells against the protozoan parasite Ichthyophthirius multifiliis in the ginbuna crucian carp, Carassius auratus langsdorfii

Cytotoxic T cells are known to have the ability to kill microbe-infected host cells, which makes them essential in the adaptive immunity processes of various vertebrates. In this study, we demonstrated innate cell-mediated cytotoxicity of CD8⁺ T cells against protozoan parasites found in the ginbuna crucian carp. When isolated effector cells such as CD8⁺, CD4⁺ (CD4-1⁺), or CD8^- CD4^- (double-negative, DN), from naïve ginbuna crucian carp were co-incubated with target parasites (Ichthyophthirius multifiliis), CD8⁺ cells from the kidney and gill showed the highest cytotoxic activity. On the other hand, DN cells, which include macrophages and CD4^- CD8^- lymphocytes, showed the lowest cytotoxic activity against I. multifiliis. Additionally, the cytotoxic activity of CD8⁺ cells was found to significantly decrease in the presence of a membrane separating the effector cells from I. multifiliis. Furthermore, the serine protease inhibitor 3,4-dichloroisocoumarin and perforin inhibitor concanamycin A significantly inhibited the cytotoxic activity of CD8⁺ cells. These results demonstrate that CD8⁺ T cells of ginbuna crucian carp can kill extracellular parasites in a contact-dependent manner via serine proteases and perforin. Therefore, we conclude that CD8⁺ T cells play an essential role in anti-parasite innate immunity of teleost fish.

Immune Mechanisms Involved in Schistosoma mansoni-Cathepsin B Vaccine Induced Protection in Mice

A vaccine against schistosomiasis would contribute to a long-lasting decrease in disease spectrum and transmission. Our previous protection studies in mice using Schistosoma mansoni Cathepsin B (Sm-Cathepsin B) resulted in 59 and 60% worm burden reduction with CpG oligodeoxynucleotides and Montanide ISA720 VG as adjuvants, respectively. While both formulations resulted in significant protection in a mouse model of schistosomiasis, the elicited immune responses differed. Therefore, in this study, we aimed to decipher the mechanisms involved in Sm-Cathepsin B vaccine-mediated protection. We performed in vitro killing assays using schistosomula stage parasites as targets for lung-derived leukocytes and serum obtained from mice immunized with Sm-Cathepsin B adjuvanted with either Montanide ISA 720 VG or CpG and from non-vaccinated controls. Lung cells and immune sera from the Sm-Cathepsin B + Montanide group induced the highest killing (63%) suggesting the importance of antibodies in cell-mediated parasite killing. By contrast, incubation with lung cells from Sm-Cathepsin B + CpG immunized animals induced significant parasite killing (53%) independent of the addition of immune serum. Significant parasite killing was also observed in the animals immunized with Sm-Cathepsin B alone (41%). For the Sm-Cathepsin B + Montanide group, the high level killing effect was lost after the depletion of CD4⁺ T cells or natural killer (NK) cells from the lung cell preparation. For the Sm-Cathepsin B + CpG group, high parasite killing was lost after CD8⁺ T cell depletion, and a reduction to 39% was observed upon depletion of NK cells. Finally, the parasite killing in the Sm-Cathepsin B alone group was lost after the depletion of CD4⁺ T cells. Our results demonstrate how the different Sm-Cathepsin B formulations influence the immune mechanisms involved in parasite killing and protection against schistosomiasis.

Lack of a contact requirement for direct antibacterial activity of lymphocyte subpopulations in ginbuna crucian carp

Cytotoxic T lymphocytes (CTL) recognize and kill cells infected with viruses, intracellular bacteria and tumors with MHC restriction and antigen specificity. In addition to these activities, recent studies in mammals have suggested that CTL can exhibit direct microbicidal activity. In our previous study we documented direct antibacterial activity of CD4(+) T cells and sIgM(+) cells as well as CD8α(+) T cells from immunized fish. However, we also found weak non-specific killing activity of lymphocytes against bacteria. In the present study we further analyzed the weak killing activity of lymphocytes, increasing the effector cell to target bacteria ratio from 10:1 to 10(3):1. Sensitized and non-sensitized effector lymphocytes (CD8α(+), CD4(+) and sIgM(+)) separated by MACS were incubated with target bacteria. CD8α(+) T cells from Edwardsiella tarda-immunized ginbuna crucian carp killed 98%, 100% and 70% of E. tarda, Streptococcus iniae and Escherichia coli, respectively. CD8α(+) T cells from non-immunized fish showed similar but slightly lower killing activity than sensitized cells. CD4(+) and sIgM(+) lymphocytes also showed high killing activity against E. tarda and S. iniae as found for CD8α(+) T cells, although the activity was lower against E. coli. Supernatants from all three types of lymphocytes showed microbicidal activity, although the activity was lower than that evoked by effector lymphocytes. Furthermore, the presence of a membrane between effectors and targets did not affect the killing activity. The present results suggest that both sensitized and non-sensitized lymphocytes non-specifically killed target bacteria without the need of contact. The major difference between the present and previous experiments is the E:T ratio. We suspect that there are two different mechanisms in the direct bacterial killing by lymphocytes in ginbuna.

CD8(+) T cells in leishmania infections: friends or foes?

Host protection against several intracellular pathogens requires the induction of CD8⁺ T cell responses. CD8⁺ T cells are potent effector cells that can produce high amounts of pro-inflammatory cytokines and kill infected target cells efficiently. However, a protective role for CD8⁺ T cells during Leishmania infections is still controversial and largely depends on the infection model. In this review, we discuss the role of CD8⁺ T cells during various types of Leishmania infections, following vaccination, and as potential immunotherapeutic targets.

Direct microbicidal activity of cytotoxic T-lymphocytes

Cytotoxic T-lymphocytes (CTL) are famous for their ability to kill tumor, allogeneic and virus-infected cells. However, an emerging literature has now demonstrated that CTL also possess the ability to directly recognize and kill bacteria, parasites, and fungi. Here, we review past and recent findings demonstrating the direct microbicidal activity of both CD4⁺ and CD8⁺ CTL against various microbial pathogens. Further, this review will outline what is known regarding the mechanisms of direct killing and their underlying signalling pathways.

Perforin-dependent cryptococcal microbicidal activity in NK cells requires PI3K-dependent ERK1/2 signaling

Previously, NK cells have been reported to kill the opportunistic fungal pathogen Cryptococcus neoformans through a perforin-dependent mechanism; however, the receptor and signaling involved are unknown. In this report we sought to identify the signaling pathways activated and required for direct perforin-mediated killing of microbes. In this study, using the NK-like cell line YT and primary peripheral blood NK cells, it is demonstrated that YT cells kill C. neoformans and that the killing is accompanied by the activation of PI3K. We demonstrate that inhibition of either the catalytic subunit (using a pharmacological inhibitor) or the alpha-regulatory subunit (using small interfering RNA knockdown) of PI3K significantly inhibited the killing of C. neoformans. Downstream of PI3K, ERK1/2 was activated in a PI3K-dependent fashion and was required for cryptococcal killing. Furthermore, we demonstrate that perforin release from YT cells can be detected by 4 h after contact of the YT cells with C. neoformans and that the release of perforin is blocked by pharmacological inhibition of either PI3K or ERK1/2. Defective degranulation is rooted in the inability to polarize perforin-containing granules toward the target. Finally, we demonstrate that PI3K-ERK1/2-dependent signaling is activated and required for the killing of C. neoformans by primary NK cells. Taken together, these data identify a conserved PI3K-ERK1/2 pathway that is used by NK cells during the direct killing of C. neoformans and demonstrate that the pathway is essential in the formation and activation of the microbicidal mechanism.

Vaccine immunity to pathogenic fungi overcomes the requirement for CD4 help in exogenous antigen presentation to CD8+ T cells: implications for vaccine development in immune-deficient hosts

Systemic fungal infections with primary and opportunistic pathogens have become increasingly common and represent a growing health menace in patients with AIDS and other immune deficiencies. T lymphocyte immunity, in particular the CD4+ Th 1 cells, is considered the main defense against these pathogens, and their absence is associated with increased susceptibility. It would seem illogical then to propose vaccinating these vulnerable patients against fungal infections. We report here that CD4+ T cells are dispensable for vaccine-induced resistance against experimental fungal pulmonary infections with two agents, Blastomyces dermatitidis an extracellular pathogen, and Histoplasma capsulatum a facultative intracellular pathogen. In the absence of T helper cells, exogenous fungal antigens activated memory CD8+ cells in a major histocompatibility complex class I-restricted manner and CD8+ T cell-derived cytokines tumor necrosis factor alpha, interferon gamma, and granulocyte/macrophage colony-stimulating factor-mediated durable vaccine immunity. CD8+ T cells could also rely on alternate mechanisms for robust vaccine immunity, in the absence of some of these factors. Our results demonstrate an unexpected plasticity of immunity in compromised hosts at both the cellular and molecular level and point to the feasibility of developing vaccines against invasive fungal infections in patients with severe immune deficiencies, including those with few or no CD4+ T cells.

Cytotoxic T cell responses to intracellular pathogens

Host defense against intracellular pathogens is thought to require cytotoxic T cells. Recent studies have investigated the impact of host cell lysis and cytokine production by cytotoxic T lymphocytes on the fate of intracellular pathogens. The identification of two mechanisms of lysis induced by cytotoxic T lymphocytes--the granule exocytosis pathway and the Fas-FasL interaction--have provided new insight into the role of cytotoxic T lymphocyters in immunity to infection.

Direct antimicrobial activity of T cells

T cells are generally thought to contribute to antimicrobial activity either by releasing lymphokines, which recruit and activate other cell types, or by major histocompatibility complex (MHC)-restricted lysis of infected host cells. Recently, it has become apparent that T cells can also mediate antimicrobial activity by direct interaction with microbial targets. Such interactions, which can be either antigen specific or nonspecific, occur in the apparent absence of MHC restriction and do not require the presence of other host cells. Microbial targets recognized by T cells include fungi, parasites and bacteria. Here, Stuart Levitz, Herbert Mathews and Juneann Murphy discuss the direct antimicrobial activity of T cells and speculate on its in vivo relevance.

Nematodes and the spleen: an immunological relationship

Despite being a major organ of the immune system, the spleen's role in resisting, controlling or simply ameliorating nematode infections has been neglected. A review of both filarial and gastrointestinal nematodes suggests that though it is difficult to fully assess or quantify the organ's importance in vivo, the spleen is prominent in acting against nematode parasites in mammals. One manifestation of this is that transfer of lymphocytes from the spleen of immunised individuals can protect recipients against the disease. Expansion of splenic lymphoid tissue also alludes to its activity during nematode infection. There is a considerable need for investigation of the spleen under natural conditions as well as much more rigorously controlled experiments even in mammals besides birds and other vertebrates.

Human immunity to Schistosoma mansoni: observations on mechanisms, and implications for control

This review summarizes the personal experiences of the authors and their colleagues during ten years of field and laboratory studies on human immunity to Schistosoma mansoni infections. There is evidence for the very slow development with age of an acquired resistance to reinfection (demonstrable after chemotherapy of the primary infection) distinguishable from a lack of reinfection due to reduced exposure. The implications of this immunity for the design of chemotherapy programs targeted at infected schoolchildren are discussed. Observational studies on the mechanisms of immunity have demonstrated a marked helminthocidal capacity of eosinophils. Subsequent correlative studies have indicated a role of IgM and IgG2 "blocking" antibodies in maintaining the continued susceptibility of young children, and of IgE antibodies in mediating protection in older individuals. Some problems in studying human immunity, and the implications for vaccine development, are also discussed.

Screening of murine monoclonal antibodies against living schistosomula of Schistosoma mansoni by radioimmunoassay

A radioimmunoassay was developed to screen supernatants of murine monoclonal antibodies against surface antigens of living schistosomula of Schistosoma mansoni. Of 196 clones screened, 10% bound schistosomula. Of these, 74% bound only schistosomula. The remaining molecules also reacted with soluble adult worm antigens and soluble egg antigens as determined by enzyme-linked immunosorbent assay. Immunoblot analysis demonstrated that monoclonal antibody 204-3E4 reacted with a 68 kDa protein, a glycoprotein that induces substantial resistance against S. mansoni infection. Recognition of an 18 kDa antigen by 204-3F1 antibody was stage-specific with the antigen being expressed in cercariae, 3- and 24-h-old parasites but not 4-day, lung stage or adult worms. Monoclonal antibody 204-4E3 reacted with purified S. mansoni paramyosin. These data indicate that radioimmunoassay using living schistosomula is a rapid alternative method to identify murine hybridomas that secrete antibodies which react with surface antigens of S. mansoni.

Induction of antigen-specific human cytotoxic T cells by Toxoplasma gondii

To further the understanding of the role of T cells in immunity to the parasite Toxoplasma gondii, antigen-specific T cell clones were generated using peripheral blood mononuclear cells from seropositive individuals. Whole parasites were used to stimulate a proliferative expansion of antigen-reactive cells, followed by limiting dilution cloning in the presence of irradiated, autologous PBMC and rIL-2. Three parasite antigen-specific T cell clones expressing the CD3+ phenotype were selected for further characterization. Phenotypic analysis with monoclonal antibodies revealed two clones reactive with CD8 (RTg1 and RTg3) while the other (RTg2) phenotyped as CD4+, CD8-. When tested in a proliferation assay using a panel of different T. gondii proteins, clone RTg1 reacted with a single large protein (Mr greater than 180,000) as well as smaller components (less than 12,000), clone RTg2 reacted with a protein of Mr = 28,000 and clone RTg3 reacted with a protein of 116,000 plus smaller components (less than 12,000). Only the 28,000 = Mr antigen recognized by RTg2 was reactive on Western blot with autologous donor antisera. All three clones produced IFN-gamma and IL-2 in varying amounts upon antigenic stimulation in the presence of irradiated APC. Moreover, one clone RTg1, exhibited direct parasite cytotoxicity, inhibiting extracellular T. gondii by greater than 70% when incubated at an effector/target ratio of 40:1. This clone was alpha, beta TCR heterodimer positive and exerted its cytotoxic parasiticidal activity in the apparent absence of MHC restriction. The results provide evidence for the existence of circulating antigen-specific cytotoxic T cells in normal humans who are toxoplasma antibody seropositive.

Immunity to schistosomes: progress toward vaccine

Among the major parasitic infections, schistosomiasis may be the most promising candidate for human vaccination. Information about mechanisms of immunity, gained mainly from experimental models but likely to be relevant to human infection, indicates a dynamic balance between protective and regulatory (blocking) mechanisms. Besides cell-mediated responses leading to macrophage activation, antibody-dependent cell-mediated cytotoxicity systems involving precise antibody isotypes and nonlymphoid cells (mononuclear phagocytes, eosinophils, and platelets) appear to be essential effectors of immune attack. The slow development of immunity in humans seems related to the production of antibodies that cross-react with schistosomulum surface antigen and block the binding of antibodies of the effector isotype. Schistosomes that survive in the bloodstream and produce chronic infections may evade the immune system as a result of intrinsic changes in membrane susceptibility and of transient expression of target antigens; at other stages of the parasite life cycle, cross-reactive molecules may be secreted that play an essential role in the induction of immunity. Several schistosome proteins have been characterized as candidates for vaccination. Among these, an antigen of 28 kilodaltons has been cloned and shown to be immunogenic in humans and protective in mice, rats, and baboons.

The interaction of human T-lymphocytes and Entamoeba histolytica: killing of virulent amoebae by lectin-dependent lymphocytes

Clinical and experimental studies indicate that following invasive disease due to Entamoeba histolytica, development of human cell-mediated immune mechanisms may provide protective immunity. Activated, human monocyte-derived macrophages in vitro can kill virulent axaenic amoebic trophozoites. This study describes the interaction of lectin-stimulated T-lymphocytes and E. histolytica trophozoites (virulent strain HM1-IMSS). Amoebae progressively killed unstimulated nonimmune T-lymphocytes over 18 h incubation with no effect on amoebic viability. T-lymphocytes, stimulated with phytohaemagglutinin (PHA), were progressively cytotoxic for virulent HMI amoebae over 18 h incubation, but were also reduced in viability themselves. Lymphocyte cytotoxicity for amoebae was absent if PHA was removed before or added only during the assay. PHA-stimulated T-lymphocytes killed amoebae at cell ratios of lymphocytes to amoebae as low as 50:1 and cytotoxicity was antibody-independent. PHA-stimulated T-lymphocytes, depleted of T8-bearing cells by complement-mediated lysis, were unable to kill amoebae. Adherence of PHA-stimulated T-lymphocytes to amoebae was greater than with unstimulated T-lymphocytes. Inhibition of the amoebic adherence lectin with N-acetyl-D-galactosamine decreased lymphocyte-amoebic adherence and resulted in increased lymphocyte amoebicidal activity and lymphocyte survival. Suspension of amoebae with or without adherent PHA-stimulated T-lymphocytes in a 10% dextran solution indicated that cytotoxicity was contact dependent. In summary, PHA-stimulated T-lymphocytes of the T8-phenotype can kill virulent axaenic E. histolytica trophozoites through a contact-dependent, antibody-independent mechanism.

Schistosomiasis mansoni: evidence for a milder response in germfree mice

Germfree (GF and conventional (CV) mice were infected intraperitoneally with GF cercariae of Schistosoma mansoni and kept for six weeks. Twenty four hours before killing, they were injected with [³H]-thymidine. Schistosoma worms, harvested after perfusion of portal system, were counted as well as eggs from liver and intestines. Liver was also used for DNA, protein, and collagen determinations. [³H] -Thymidine incorporation and collagen determinations were used to establish the indices given by the difference between their contents in infected and control animals and expressed per thousand eggs in liver. The recovery of worms in GF mice was around twice as much as in CV ones, and the total number of eggs was higher in the liver of GF animals. No hypertrophy of liver cells was observed by the ratio protein/DNA, but [³H]-thymidine incorporation into DNA was higher than in controls in both GF and CV infected animals. The [³H]-thymidine and collagen indices were lower in GF animals which indicate a more discrete cellular replication and smaller collagen content in relation to the number of eggs present in livers of these mice. It was concluded that the disease seems to be less severe in GF animals.

Human schistosomiasis: deficiency of large granular lymphocytes and indomethacin-sensitive suppression of natural killing

Twenty-eight children infected with Schistosoma haematobium and S. mansoni were tested for natural killer (NK) cell activity in vitro using the myeloid/erythroid cell line K562 as target. In addition, the frequency of large granular lymphocytes (LGL) and the number of HNK-1+ lymphocytes were examined in peripheral blood. NK cell activity was found to be markedly reduced in most patients when compared with a group of healthy Caucasian individuals (P less than 0.005). Moreover, the impairment of NK activity clearly correlated with the intensity of infection, which was quantified by parasite ova excretion in stool and urine. Within the lymphocyte compartment the percentages of cells with the NK phenotype (HNK-1+) were found to be normal, although the majority of patients exhibited decreased numbers of LGL (P less than 0.005). The absolute and relative frequencies of LGL and HNK-1+ lymphocytes by no means correlated with the parasite load. In vitro results suggest an at least partly prostaglandin-mediated and interferon-resistant functional defect of NK cells.

Relationship between intensity of infection and immunomodulation in human schistosomiasis. II. NK cell activity and in vitro lymphocyte proliferation

Natural killer (NK) cell activity against K-562 targets and lymphoproliferative responses to Con A, interleukin-2 (IL-2) and Con A + IL-2 were examined in a group of 41 Sudanese children suffering from schistosomiasis mansoni and haematobium. The results were correlated to the intensity of infection as determined by enumeration of parasite ova in urine and stool. NK cell activity measured at three effector to target cell ratios was significantly depressed in the patient group as compared to a German control group. Impairment of NK cell activity showed a direct relationship with the patients' parasite load. Furthermore lymphoproliferation to Con A, IL-2 and Con A + IL-2 was depressed in the group of patients. Interestingly the costimulation effect of IL-2 expressed as coefficient of delta ct/min(Con A + IL-2)/delta ct/minCon A correlated significantly to the intensity of infection suggesting that lymphocytes from heavily infected patients were defective in producing appropriate amounts of IL-2 in response to Con A. Our findings support the concept that heavy infections with S. mansoni and/or S. haematobium induce a peculiar dichotomy of cellular and humoral immune parameters. Whereas T cell-dependent cellular immune responsiveness and NK cell function decrease with increasing worm burden specific IgE and IgG antibody responses increase.

Cell-mediated damage to helminths

Although it is difficult to draw any sweeping conclusions that would be applicable to all helminth infections, the main features that are emphasized in this review may be summarized briefly. Pathogenic helminths, although extremely diverse in structure and behaviour, have one common feature, namely that they present to the host's defenses large, non-phagocytosable surfaces. Because of this, they are susceptible to a range of effector mechanisms differing either quantitatively or qualitatively from those that are active against other parasites or against normal or abnormal host cells. As an extreme example, the various types of cytotoxic lymphocyte, with one interesting exception, are inactive against helminths. Instead, helminth infections are characterized by high IgE responses and increased numbers of circulating eosinophils. Such eosinophils are activated, and show a marked capacity to kill a variety of target helminths in vitro. Further activation may occur in response to mast cell mediators released as a result of IgE-dependent degranulation; and IgE, as well as IgG and complement, can mediate eosinophil attachment and killing. It may therefore be suggested that the eosinophil/IgE/mast cell axis represents a powerful host defense against helminth infections. IgE can also mediate macrophage-dependent killing of several helminths, a process which involves a functional change in the macrophage, resembling activation. Although eosinophil-mediated and IgE-dependent macrophage-mediated effects are particularly potent, other effector cells are not excluded: in certain circumstances, neutrophils and conventionally activated macrophages may be equally or more effective. Neutrophils appear to act solely by oxidative killing mechanisms, whereas degranulation and the release of toxic granule contents is equally or more important in eosinophil-mediated damage. Different stages of different helminths vary in their degree of susceptibility to different mechanisms. Eosinophils appear to be somewhat less active than neutrophils against ensheathed nematodes, whereas trematodes and exsheathed nematodes are highly susceptible to eosinophil attack. In many experimental helminth infections, studies in vivo suggest a role for antibody-dependent cell-mediated immune effector mechanisms. The identity of the effector cell is difficult to establish because of a lack of techniques for specific manipulation of individual cell types, but histological studies frequently point to a strong eosinophil or macrophage involvement. The development and analysis of in vitro assays allows the study of immune effector mechanisms in man.(ABSTRACT TRUNCATED AT 400 WORDS)

Host granulomatous response in schistosomiasis mansoni. Antibody and cell-mediated damage of parasite eggs in vitro

In chronic schistosomiasis mansoni the major pathologic lesions are granulomas surrounding eggs deposited in host tissues. Parasite ova release antigenic material that sensitize the host, resulting in the development of delayed-type hypersensitivity granulomas. The objectives of the present study were to assess the ability of components of the host granulomatous response to induce biochemical and biologic alterations in eggs in vitro, and to correlate these with the capacity of ova to induce granulomas in vivo. An assay of egg tricarboxylic acid cycle activity was developed by use of 2-[14C]acetate as substrate and measurement of accumulation of released 14CO2. Addition of human granulocytes (96% neutrophils, 4% eosinophils) to eggs (cell/egg ratio 1,000:1) and heat-inactivated normal human serum reduced predicted egg 14CO2 generation by 15.6 +/- 3.0%. This effect was greater in the presence of sera of subjects with schistosomiasis (25.6 +/- 2.8% reduction) or when complement was present (24.4 +/- 4.0%). Autologous eosinophils and neutrophils were equally effective in decreasing egg 2-[14C]acetate metabolism (25.6 and 21.4% reductions, respectively). Since the biological role of schistosome eggs relates to their ability to hatch and produce miracidia, we evaluated the effect of granulocytes and sera on this function. The hatching rate of eggs incubated with normal serum was 52.8 +/- 3.3 miracidia/100 eggs; this value decreased to 37.0 +/- 2.6 when granulocytes were added (P less than 0.01). Granulocytes plus antibody- or complement-containing sera led to hatching rates of 23 and 20 miracidia/100 eggs. When ova were pre-incubated with granulocytes and various sera and injected into mice, the areas of egg-induced pulmonary granulomas measured 8 d later were reduced 32 to 45% as compared with lesions elicited by parasite eggs not exposed to granulocytes. Exposure of antigen-coated Sepharose beads to granulocytes and immune serum before injection into mice also led to a reduction in granuloma formation as compared with beads pre-incubated with serum alone. These data indicate that granulocytes in conjunction with antibodies and complement inflict biologically relevant toxic effects on eggs that are manifest in vivo by a decreased ability to elicit granulomas.

Non-specific potentiation of T- and B-lymphocyte proliferation at the early stage of infection by Schistosoma mansoni: role of factors secreted by the larvae

The response of rat lymphocytes to schistosomula released products (SRP) was examined. SRP non-specifically activated lymphocytes by potentiating their proliferative response to PHA, Con A or LPS. The parasite factor involved was dialysable and heat stable. The addition of SRP to cultures containing nylon-wool non-adherent lymph node cells resulted in a significant enhancement of cell proliferation. The effect of SRP on athymic nude (Nu/Nu) and litter mate (Nu/+) control rat cells indicated an effect on the proliferation of both B and T lymphocytes. SRP acted in a dose-dependent manner and its action was observed as early as the beginning of cell division. This corresponds to the in vivo situation, since at the early stage of infection increased proliferative responses of the lymph node cells to mitogens were observed. The adjuvant effect of SRP could partly explain the regulation of the cellular immune response observed during S. mansoni infection by the parasite itself and could represent one of the mechanisms involved in immunity to reinfection that is under the control of the parasite.

Immunopathology of parasitic diseases: a conceptual approach

These studies would indicate tremendous variations in the clinical manifestations of parasitic disease, resulting from characteristics of the parasite, the host, and their interaction. They further suggest that the conceptual mechanistic model described in the introduction is highly applicable. Previous evidence to substantiate the validity of such a model in schistosomiasis, a variety of protozoan diseases, and leprosy has already been presented (Phillips and Fox, 1982). This report would appear to lend additional credence to the postulates and suggests that upon scrutiny, the model represents a reasonable explanation for a wide variety of clinical manifestations of a parasitic disease. In addition, it may provide a working hypothesis for the interpretation of the immunopathology found in other diseases such as filariasis. Figure 3 compares and contrasts schistosomiasis and filariasis within the context of this hypothesis. Immunopathology results from the relative balance of host-parasite immunogenic factors and modulatory specific and nonspecific factors. The resultant immunopathology results from a number of immunologic mechanisms, but for the sake of comparison can be placed in certain analogous groups. Clearly, although a number of experimental questions still exist, vis-à-vis these analogies, it would appear that they are reasonable comparisons. It is hoped that such a conceptual approach might provide a useful framework for an understanding of the spectrum of immunopathology resulting from parasitic disease. These concepts might possibly lead to the eventual control of immunopathology.

Complement-dependent killing of Nippostrongylus brasiliensis infective larvae by rat alveolar macrophages

Histopathological studies have provided circumstantial evidence that helminth parasite destruction occurs in the lung; however controlled in vitro studies on the helminthocidal activity of lung cells have not been reported. This study presents evidence that Nippostrongylus brasiliensis infection in the rat induces alterations in broncho-alveolar lavage (BAL) cell numbers, differential counts, and in vitro helminthocidal activity. Normal, uninfected rats yielded 3.3 +/- 0.6 X 10(6) BAL cells/rat, consisting predominantly of alveolar macrophages (greater than 90%). However on days 2-8 post-infection there was a 1.5-2.4-fold increase in BAL cell numbers with a significant neutrophilia on day 2 and a significant increase in the absolute number of all cell types on day 8. On day 32 post-infection, BAL cell numbers had returned to control levels. Normal BAL cells neither adhered to nor killed N. brasiliensis infective larvae (L3) in the presence of rat complement. By contrast BAL cells recovered from infected rats on days, 2, 8 or 32 post-infection (D2, D8 and D32 BAL cells, respectively) adhered under similar conditions. However, only D8 and D32 BAL cells killed L3. This complement-dependent killing correlated with significantly increased numbers of C3 receptor bearing alveolar macrophages in D8 and D32 BAL cells. Complement-dependent alveolar macrophage helminthocidal activity may therefore play an important role in lung resistance against resident or migrating helminths.

A new function for platelets: IgE-dependent killing of schistosomes

Several killing mechanisms against schistosomes have been described in vitro, involving cellular and humoral factors. Neutrophils, eosinophils--with an accessory role for mast cells--monocytes and macrophages have been shown to exhibit cytotoxic properties against Schistosoma mansoni larvae, in association with antibodies of various isotypes or with complement (reviewed in ref. 1). Lymphocyte participation in effector functions is mediated mainly through lymphokines inducing cytotoxic macrophages, and, in certain cases, directly by T cells. The experiments reported here show that platelets, taken from rats after specific periods of infection with S. mansoni, were able to kill schistosomula, and that normal human or rat platelets acquired toxic properties towards the same target in the presence of serum from infected individuals. The humoral factor involved in this process was shown to be IgE, and evidence was obtained of a Fc receptor for IgE on human and rat platelets. The passive transfer of immune platelets to normal rats conferred a high degree of protection towards a challenge infection by the parasite.