Characteristics of immune response to protozoan infections

Introduction When protozoa enter the blood stream or tissues they can often survive and replicate because they adapt to the resisting natural host defenses. The interaction of immune system with infectious organisms is a dynamic interplay of host mechanisms aimed at eliminating infections and microbial strategies designed to permit survival in the face of powerful effectors mechanisms. Protozoa cause chronic and persistent infections, because natural immunity against them is weak and because protozoa have evolved multiple mechanisms for evading and resisting specific immunity. Natural and specific immune response to protozoa Different protozoa vary greatly in their structural and biochemical properties and stimulate distinct patterns of immune responses and have evolved unique mechanisms for evading specific immunity. Protozoa activate quite distinct specific immune responses, which are different from the responses to fungi, bacteria and viruses. Protozoa may be phagocytozed by macrophages, but many are resistant to phagocytic killing and may even replicate within macrophages. T. brucei gambiense is the best example of protozoa which can induce humoral immune response because of its extra-cellular location. In Leishmania sp. infections, cellular defense mechanisms depend upon CD4+ T-lymphocytes and activate macrophages as effectors cells that are regulated by cytokines of Th1 subset. Plasmodium sp. is a protozoa which show the diversity of defence mechanisms which can be cellular or humoral, depending on Ag and protozoa's location. Immune evasion mechanisms of protozoa Different protozoa have developed remarkably effective ways of resisting specific immunity: a) anatomic sequestration is commonly observed with protozoa Plasmodium and T. gondii; b) some protozoa can become resistant to immune effectors mechanisms: Trypanosoma, Leishmania and T. gondii; c) some protozoa have developed effective mechanisms for varying their surface antigens: Plasmodium and Trypanosoma; d) some protozoa shed their antigen coats, either spontaneously or after binding with specific antibodies: E. histolytica; e) some protozoa alter host immune response by nonspecific and generalized immunosuppression (abnormalities in cytokine production, deficient T cell activation): Trypanosoma, Leishmania, Toxoplasma, Entamoeba. Conclusion Protozoa activate numerous, different immune mechanisms in human body. Evolution, progression and outcome of diseases depend upon these mechanisms. Resent progresses in research have defined and selected Ag as candidates for new vaccines. Better definitions regarding the role of cytokines in protozoaninfections will facilitate rational development of cytokines and cytokine antagonists and their use as immunotherapeutic agents.

Heifers immunized with whole-cell and membrane vaccines against Tritrichomonas foetus and naturally challenged with an infected bull

The performance of a whole-cell vaccine and the other vaccine with cellular membranes of Tritrichomonas foetus applied to heifers naturally challenged by mating with an infected bull was determined. Forty heifers were divided into three groups: a control group (n=16) without immunizing, another group (n=12) immunized with whole cells (10(8)/dose) and a third group (n=12) immunized with cellular membranes (300 micro g of membranes/dose protein). The females were subcutaneously vaccinated at 3-week on two occasions and received a third intravaginal booster dose. After 3 weeks of the last vaccinal doses, the heifers were served by a T. foetus infected bull over 90-day period. The mean duration of infection for membrane-vaccinated heifers was 60 days +/-25, compared with 63 days +/-35.8 of infection for whole-cell-vaccinated heifers and 79 days +/-41.3 for control heifers. Calving rates were 6/12 for membrane-vaccinated heifers, 3/12 for whole-cell-vaccinated animals, and 2/16 for control animals. Fetal mortality rates were 3/12 for membrane-vaccinated animals, 4/12 for those vaccinated with whole cells and 10/16 for control animals. These reproductive parameters were significantly different (P<0.05) between heifers vaccinated with membranes and control heifers. The hemolytic test and enzyme-linked immunoabsorbent assay (ELISA) with T. foetus antigen showed that serum immunoglobulins peaked before and during the breeding period. The heifers vaccinated with membranes developed an important response during the critical period of fetal loss, second and third month of the breeding time, and another month after the same period. The ELISA method was more sensitive and more reliable than the hemolytic test for the evaluation of the systemic immune response in females infected and/or vaccinated with T. foetus.

Acquired cell-mediated protection in rainbow trout, Oncorhynchus mykiss, against the haemoflagellate, Cryptobia salmositica

Disease-free rainbow trout, Oncorhynchus mykiss (Walbaum),were inoculated with either the pathogenic Cryptobia salmositica Katz, 1951 or with the attenuated vaccine strain of the parasite. A number of vaccinated fish were then challenged at 6 weeks post-vaccination with pathogenic C. salmositica. Respiratory burst activity in stimulated macrophages (isolated from infected or vaccinated fish) was demonstrated by detection of released super-oxide anions, and the protein contents in these cells were also determined after the cells were digested. At 5 weeks after infection, the respiratory burst activity of macrophages from infected fish was significantly higher than those in naive and vaccinated fish. In addition, macrophages from vaccinated fish had greater activity than naive fish. Macrophage activity in vaccinated and challenged fish was comparable to that of infected fish. Antibodies were detected (ELISA) at 3 weeks after vaccination and at 5 weeks in infected fish. Complement-fixing antibodies (immune lysis test) were detected in infected fish at 5 weeks post-infection but were not detected in vaccinated fish unless they were challenged with the pathogen, in which case the titre rose rapidly.

Evasion of innate immunity by parasitic protozoa

Parasitic protozoa are a major cause of global infectious disease. These eukaryotic pathogens have evolved with the vertebrate immune system and typically produce long-lasting chronic infections. A critical step in their host interaction is the evasion of innate immune defenses. The ability to avoid attack by humoral effector mechanisms, such as complement lysis, is of particular importance to extracellular parasites, whereas intracellular protozoa must resist killing by lysosomal enzymes and toxic metabolites. They do so by remodeling the phagosomal compartments in which they reside and by interfering with signaling pathways that lead to cellular activation. In addition, there is growing evidence that protozoan pathogens modify the antigen-presenting and immunoregulatory functions of dendritic cells, a process that facilitates their evasion of both innate and adaptive immunity.

For better or worse: common determinants influencing health and disease in parasitic infections, asthma and reproductive biology

What represents a protective or beneficial immune response in one scenario, may contribute to the pathogenesis of disease in another. This review explores the plasticity of immune responses and the delicate balance between health and disease, using examples from immunoparasitology, allergic lung disease and reproductive biology. Cytokines secreted by lymphocytes and other leukocytes are central to this balance because they regulate both inflammation and adaptive immunity. The type and quantity of cytokines, the timing and location of cytokine release, and coordinated expression with other signals can all contribute in determining the nature of immune responses and, therefore, of disease outcomes. Of necessity, leukocytes control and eliminate infectious agents by interacting with other cells. However, leukocytes also communicate with other cells to maintain homeostasis in healthy organisms. Tissue development, repair, remodelling and immunopathology can be viewed as parts of a continuum and leukocytes are major contributors to all of these processes. The factors, which influence the extent to which an infection will result in host pathology, are multifarious, but include as yet poorly determined elements within the genetic background of the host. Nowhere is this more obvious than in animals chronically infected with parasites. There are parallels between parasite infections and pregnancy, since to survive and develop, the conceptus also must avoid immunological rejection. Therapeutic intervention through manipulation of cytokine profiles may be feasible, but is fraught with risk and should not be undertaken without careful analysis of the possible consequences in a range of genetic backgrounds and with consideration of the diversity of infectious agents which might be encountered.

Stimulation of B lymphocytes, macrophages, and dendritic cells by protozoan DNA

This review summarizes recent work from our laboratory demonstrating the activation of B lymphocytes, macrophages and dendritic cells by DNA from three different protozoan parasites of cattle and humans that is qualitatively similar to the now well-described effects of CpG-containing bacterial DNA. This novel mechanism of protozoan parasite recognition by the innate immune system could facilitate recovery from acute infection or contribute to infection-related pathology.

How protozoan parasites evade the immune response

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

The multitalented pore-forming proteins of intracellular pathogens

Being an intracellular pathogen demands being able to invade a host cell, to circumvent the host immune response and to survive in the intracellular environment. Pore-forming proteins are among the innumerable tools used by intracellular microorganisms to achieve these goals. Remarkably, this seems to be a multipurpose group of proteins that can act in several ways. Making channels may signify entering into host cells, inhibiting phagocytosis, escaping phagosomes or promoting pathogen dissemination. In certain cases, pore-forming proteins are double-edged tools and may benefit the host by eliminating infected cells and/or inducing inflammation.

Vaccinology for control of apicomplexan parasites: a simplified language of immune programming and its use in vaccine design

Most mammalian immune systems and parasites have co-evolved over the millennia, interacting within a common environment and communicating through a common language. This language is comprised of copious dialects in which a variety of host innate and acquired immune pathways actively interact with a multitude of parasite-specific survival strategies. Nonetheless, a simplified language is likely present since the same basic molecular and cellular mechanisms are associated with resistance or susceptibility to parasite infection. Protective immunity against protozoa within the phylum Apicomplexa (e.g. Cryptosporidia, Eimeria, Neospora, Plasmodia and Toxoplasma) is generally CD4+ T cell-dependent and elicited along the IL-12/IFN-gamma/iNOS effector axis. This simplified language can be decoded in part by significant advances in understanding naïve T cell activation, differentiation and generation of immunologic memory. Vaccine adjuvants and new immunisation strategies for generation of more potent immunity can also be viewed through this common language lens. The aim of this paper is to summarise recently published fundamental immunology studies, their relevance through examples in specific coccidian-host immune dialects, and how this simplified language can be used for the more rationale design of parasite vaccine control strategies.

Apoptosis and parasitism: from the parasite to the host immune response

Apoptosis, a form of programmed cell death (PCD), plays a central role in normal tissue development as well as in the pathogenesis of different diseases. PCD is responsible for the non-inflammatory physiological elimination of potentially harmful or unnecessary cells during embryogenesis, and for the proper functioning of continuous cell renewal systems in adult organisms. Maturation of the immune system and the specific immune response are examples of situations where PCD plays important roles. This review discusses the importance of apoptosis in two fundamental elements of a host-parasite interaction: the parasite (Section 1), and the host's immune response (Section 2). Section 1 discusses questions raised by the description of apoptosis in unicellular eukaryotes, such as the evolutionary origin of the molecular components of PCD, its role in the emergence and maintenance of parasitism, and the constraints of a multicellular organization for the proper operation of a cell death programme. The proposal is that PCD can occur in any situation where living cells display features of an organized network which operates through interactions within themselves and/or with elements of their environment. The possibility is also discussed that evolutionary relics of a complete cell death system may operate in unicellular parasites with functions other than inducing cell death. Section 2 reviews data on the mechanisms of host-cell PCD and the consequences of this phenomenon in host defence and pathogenesis. Infectious agents, from viruses to parasites, can either delay or induce apoptosis of different types of host cells. Apoptosis following lymphocyte polyclonal activation and stimulation of peripheral T lymphocytes, as a result of the engagement of specific counter-receptor systems, is of special interest for defining host immunocompetence and mechanisms of immunopathology.

So what if parasites vary?

The British Society for Parasitology Autumn Symposium was held on 14 September 2001 at the Linnean Society of London, UK, only a few yards from the room in which Darwin and Wallace presented their joint papers on organic variation. Fittingly, the symposium--Parasite variation: immunological and ecological significance--considered the consequences of parasite variation.

Antibody responses of cattle immunized with the Tf190 adhesin of Tritrichomonas foetus

The antibody response patterns of cattle after subcutaneous and intranasal immunizations with adhesin Tf190 of Tritrichomonas foetus were investigated. Reactions of antibody from cattle parenterally immunized with Tf190 revealed antigen specificity and Tf190 sensitization in the majority of the animals, as determined by Western blotting. The results also demonstrated strong preimmune immunoglobulin G2 (IgG2) binding to T. foetus antigens not seen in IgG1 profiles. Subcutaneous injections of Tf190 resulted in significant (P < 0.05) increases in serum IgG1 and IgG2 titers over time, as determined by parasite specific enzyme-linked immunosorbent assay. Immune sera also significantly inhibited parasite adhesion to mammalian cell lines compared to the level of inhibition obtained with preimmune sera (P < 0.05). Intranasal immunization with Tf190 failed to produce measurable parasite-specific antibody in serum; however, this immunization route did result in significant (P < 0.05) increases in parasite-specific IgA titers in cervical mucus secretions from immunized animals that were more resistant to intravaginal challenge with T. foetus than controls. These results suggest that systemic immunization with Tf190 results in serum antibody production and antiparasitic adhesin antibodies. Additionally, the results of challenge experiments with intranasally immunized animals suggests that Tf190 primes protective immune responses that lead to lower rates of infection among these animals.

Nitric oxide and the immune response

During the past two decades, nitric oxide (NO) has been recognized as one of the most versatile players in the immune system. It is involved in the pathogenesis and control of infectious diseases, tumors, autoimmune processes and chronic degenerative diseases. Because of its variety of reaction partners (DNA, proteins, low-molecular weight thiols, prosthetic groups, reactive oxygen intermediates), its widespread production (by three different NO synthases (NOS) and the fact that its activity is strongly influenced by its concentration, NO continues to surprise and perplex immunologists. Today, there is no simple, uniform picture of the function of NO in the immune system. Protective and toxic effects of NO are frequently seen in parallel. Its striking inter- and intracellular signaling capacity makes it extremely difficult to predict the effect of NOS inhibitors and NO donors, which still hampers therapeutic applications.

Molecular machinations: chemokine signals in host-pathogen interactions

Chemokines and their G-protein-coupled receptors represent an ancient and complex system of cellular communication participating in growth, development, homeostasis and immunity. Chemokine production has been detected in virtually every microbial infection examined; however, the precise role of chemokines is still far from clear. In most cases they appear to promote host resistance by mobilizing leukocytes and activating immune functions that kill, expel, or sequester pathogens. In other cases, the chemokine system has been pirated by pathogens, especially protozoa and viruses, which have exploited host chemokine receptors as modes of cellular invasion or developed chemokine mimics and binding proteins that act as antagonists or inappropriate agonists. Understanding microbial mechanisms of chemokine evasion will potentially lead to novel antimicrobial and anti-inflammatory therapeutic agents.

Antigen-specific T-cell responses in cattle immunized with antigens of Tritrichomonas foetus

The cellular immune responses of cattle immunized with antigens of Tritrichomonas foetus were investigated. Subcutaneous injections of antigen preparations primed bovine peripheral blood mononuclear cells (PBMC) by 30 days of immunization as demonstrated by antigen-specific proliferation and by cytokine production upon antigen challenge of PBMC. Antigen-specific T-cells derived from PBMC responded by production of interferon (IFN)-gamma message detected by reverse transcriptase polymerase chain reaction, secreted IFN-gamma detected by enzyme-linked immunosorbent assay, and intracellular IFN-gamma detected by flow cytometry. Phenotypic analysis of PBMC responding in vitro to parasite antigen demonstrated a shift from a mixed CD4+, CD8+, gammadelta+, to predominantly CD4+, CD8-, gammadelta- phenotype in the Tf190-primed PBMC. In conclusion, systemic immunization of cattle with parasite antigen results in priming of bovine T-cells that are antigen specific and can produce an anamnestic IFN-gamma response to subsequent stimulation with antigens of T. foetus.

Inhibition of apoptosis by intracellular protozoan parasites

Protozoan parasites which reside inside a host cell avoid direct destruction by the immune system of the host. The infected cell, however, still has the capacity to counteract the invasive pathogen by initiating its own death, a process which is called programmed cell death or apoptosis. Apoptotic cells are recognised and phagocytosed by macrophages and the parasite is potentially eliminated together with the infected cell. This potent defence mechanism of the host cell puts strong selective pressure on the parasites which have, in turn, evolved strategies to modulate the apoptotic program of the host cell to their favour. Within the last decade, the existence of cellular signalling pathways which inhibit the apoptotic machinery has been demonstrated. It is not surprising that intracellular pathogens subvert these pathways to ensure their own survival in the infected cell. Molecular mechanisms which interfere with apoptotic pathways have been studied extensively for viruses and parasitic bacteria, but protozoan parasites have come into focus only recently. Intracellular protozoan parasites which have been reported to inhibit the apoptotic program of the host cell, are Toxoplasma gondii, Trypanosoma cruzi, Leishmania sp., Theileria sp., Cryptosporidium parvum, and the microsporidian Nosema algerae. Although these parasites differ in their mechanism of host cell entry and in their final intracellular localisation, they might activate similar pathways in their host cells to inhibit apoptosis. In this respect, two families of molecules, which are known for their capacity to interrupt the apoptotic program, are currently discussed in the literature. First, the expression of heat shock proteins is often induced upon parasite infection and can directly interfere with molecules of the cellular death machinery. Secondly, a more indirect effect is attributed to the parasite-dependent activation of NF-kappaB, a transcription factor that regulates the transcription of anti-apoptotic molecules.

Concomitant infections, parasites and immune responses

Concomitant infections are common in nature and often involve parasites. A number of examples of the interactions between protozoa and viruses, protozoa and bacteria, protozoa and other protozoa, protozoa and helminths, helminths and viruses, helminths and bacteria, and helminths and other helminths are described. In mixed infections the burden of one or both the infectious agents may be increased, one or both may be suppressed or one may be increased and the other suppressed. It is now possible to explain many of these interactions in terms of the effects parasites have on the immune system, particularly parasite-induced immunodepression, and the effects of cytokines controlling polarization to the Th1 or Th2 arms of the immune response. In addition, parasites may be affected, directly or indirectly, by cytokines and other immune effector molecules and parasites may themselves produce factors that affect the cells of the immune system. Parasites are, therefore, affected when they themselves, or other organisms, interact with the immune response and, in particular, the cytokine network. The importance of such interactions is discussed in relation to clinical disease and the development and use of vaccines.

Sex-associated hormones and immunity to protozoan parasites

Numerous epidemiological and clinical studies have noted differences in the incidence and severity of parasitic diseases between males and females. Although in some instances this may be due to gender-associated differences in behavior, there is overwhelming evidence that sex-associated hormones can also modulate immune responses and consequently directly influence the outcome of parasitic infection. Animal models of disease can often recreate the gender-dependent differences observed in humans, and the role of sex-associated hormones can be confirmed by experimentally altering their levels. Under normal circumstances, levels of sex hormones not only differ between males and females but vary according to age. Furthermore, not only are females of reproductive age subject to the regular hormonal cycles which control ovulation, they are also exposed to dramatically altered levels during pregnancy. It is thus not surprising that the severity of many diseases, including those caused by parasites, has been shown to be affected by one or more of these circumstances. In addition, infection with many pathogens has been shown to have an adverse influence on pregnancy. In this article we review the impact of sex-associated hormones on the immune system and the development and maintenance of immunity to the intracellular protozoan parasites Toxoplasma gondii, Plasmodium spp., and Leishmania spp.

Il-12: keeping cell-mediated immunity alive

Interleukin (IL)-12 has dominated the field of cell-mediated immunity since its discovery more than 10 years ago, and clearly plays an essential role in the development of Th1 cells under a variety of conditions. Recent studies now indicate that the importance of IL-12 is not limited to initiating an immune response, but may contribute to maintaining immunity. Thus, Th1 responses rapidly wane in the absence of IL-12, leading to a loss in protective immunity against intracellular pathogens, such as Leishmania and Toxoplasma. Determining how IL-12 maintains Th1 cells, and consequently cell-mediated immunity, will provide new insights useful in controlling the immune response, and may thus influence the design of new vaccines and immunotherapies.

The other side of the coin: the protective role of the TH2 cytokines

Although T(H)2 cytokine involvement in allergy makes these cytokines attractive therapeutic targets, they protect against ectoparasites and gastrointestinal worms and suppress inflammation induced by T(H)1 cytokines. T(H)2 cytokines induce mastocytosis, eosinophilia, IgE synthesis, and mucus production. Each element of this response protects against some worms; however, different worms are protected against by different elements of the total response. The induction of the entire response by most parasitic worms suggests that it is safer for the immune system to make a stereotyped worm-protective response than to attempt to match a more specific response to a particular worm. In contrast, the reciprocal antagonism between T(H)1 and T(H)2 cytokines suggests that it is safer for the immune system to limit immunopathology by suppressing inflammatory effector mechanisms not required for host protection against a particular pathogen class than to make an all-purpose inflammatory response. This, in turn, implies that innate immunity can distinguish different classes of parasites (eg, worms vs protozoa) but has limited ability to distinguish individual parasites within a class (eg, different worms). Although these considerations suggest that T(H)2 cytokine antagonists may increase the risk and severity of worm infections and T(H)1 cytokine-mediated inflammatory disorders, such therapy should be relatively safe if it is restricted to areas in which worm infections are rare and commonsense precautions are taken to minimize the risk of inducing T(H)1 cytokine-related inflammatory disease.

Cryptobiosis and its control in North American fishes

Cryptobiosis is caused by the haemoflagellates Cryptobia bullocki and Cryptobia salmositica. These parasites infect food fishes (e.g. flounders, salmon) on both the Atlantic and Pacific coasts of North America and clinical signs of the disease include anaemia, and abdominal distention with ascites. The virulent factor in salmonid cryptobiosis, caused by C. salmositica, is a secretory metalloprotease (200 kDa). Fish mortality may be up to 100% in the absence of treatment, consequently strategies have been developed to protect them from disease/mortality. A single dose of a live vaccine protects fish for at least 2 years, and it is via the production of complement-fixing antibodies, enhanced phagocytosis and cell-mediated cytotoxicity. Inhibition of the parasite's cysteine protease by a monoclonal antibody reduces multiplication, infectivity and survival of the parasite. Consequently, the recombinant cysteine protease (49 kDa) of the parasite will be tested as a potential vaccine. The trypanocidal drug, isometamidium chloride (1.0 mg/kg), is effective (therapeutic and prophylactic) against C. salmositica in chinook salmon. Its efficacy is significantly enhanced if it is conjugated either to a monoclonal antibody or to polyclonal antibodies from immune fish. Selective breeding of Cryptobia-resistant brook charr (innate resistance to infection) is possible, and the resistant factor(s) is controlled by a dominant Mendelian locus. In these resistant charr the parasite is lysed via the alternate pathway of complement activation (innate immunity to infection). There are also Cryptobia-tolerant charr, fish that are susceptible to infection but have no clinical disease (innate resistance to disease). In these fish, one of the natural anti-proteases, alpha2-macroglobulin, neutralises the metalloprotease secreted by C. salmositica. Production of transgenic Cryptobia-tolerant salmon is an option to vaccination and or chemotherapy. Also, transgenic pathogen-tolerant animals may be an alternate strategy against other pathogens where the disease mechanism is similar to cryptobiosis.

Experimental pathogenicity of a presumed monoxenous trypanosomatid isolated from humans in a murine model

Two strains of a presumed lower trypanosomatid isolated from immunocompetent and HIV-infected humans in French West Indies were investigated in vitro and in vivo in a murine experimental model. The ability of parasites to grow in vitro in bone marrow-derived macrophages and their virulence in vivo were assessed. For in vivo infection, two groups of BALB/c mice were inoculated either by the subcutaneous or intravenous route with 10(7) promastigotes at day 0. Infection was monitored by measuring parasite load in liver, spleen, foot pad, popliteal, and mesenteric lymph nodes and brain from day 7 to day 150 post-infection using a microtitration technique. Parasites multiplied in mouse macrophages in vitro. In vivo, both strains proved infective to mice and capable of visceralization and dissemination in the popliteal and mesenteric lymph nodes, liver, spleen, and even brain. Both strains elicited a strong humoral response against trypanosomatid antigen in mice, which cross-reacted with Leishmania antigen. Contrasting with the straightforward dissemination of parasites, the infection was strikingly well tolerated by the murine host with no clinical signs and minimal tissue changes around parasitized macrophage infiltrates.

Infections by protozoa in immunocompromised hosts

Protozoa are among the most important pathogens that can cause infection in immunocompromised patients. They infect particularly individuals with impaired cell immunity, such as those with hematologic neoplasias, those submitted to transplant of solid organs, those under high-dose corticosteroid therapy, and carriers of the human immunodeficiency virus. Among the protozoa that most commonly cause disease in immunocompromised individuals are Toxoplasma gondii, Trypanosoma cruzi, Cryptosporidium parvum, Isospora belli, Cyclospora cayetanensis and microsporidia; the former two cause severe encephalitis and myocarditis, and the others cause gastrointestinal infections. Early diagnosis and prompt institution of specific therapy for each of these organisms are basic measures to decrease morbidity and mortality associated with these infections.

Degradation of bovine complement C3 by trichomonad extracellular proteinase

Bovine trichomoniasis is a local infection of the reproductive tract making interaction with mucosal host defenses crucial. Since the parasite is susceptible to killing by bovine complement, we investigated the role of the third component of complement (C3) in host parasite interactions. Bovine C3 was purified by anionic and cationic exchange chromatography. The purified protein was characterized by immunoreactivity, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), and peptide sequencing of the amino terminus of the beta chain. When purified bovine C3 was incubated for varying time periods with trichomonad extracellular proteinases, SDS-PAGE gels revealed digestion of the alpha chain to small fragments. Such degradation in vivo would prevent formation of C3b and completion of the complement cascade, resulting in evasion of killing. To evaluate the relevance of this data, we determined whether C3 was present in bovine genital secretions. With a quantitative ELISA assay, C3 could be demonstrated in both uterine and vaginal washes. To our knowledge, this is the first demonstration of bovine C3 in genital secretions. The C3 concentration increased significantly in vaginal secretions by 8 and 10 weeks in heifers infected with Tritrichomonas foetus. An increase was also seen in uterine secretions of infected heifers, but sample numbers were insufficient for statistical analysis. Transcription of the major extracellular cysteine proteinase (TFCP8) was demonstrated in T. foetus cells from uterine secretions of infected heifers by RT-PCR and Southern blotting. The results indicate that C3 may be important in genital defense and that trichomonad extracellular proteinases may play a role in evasion of complement-mediated killing.

The apical organelles of malaria merozoites: host cell selection, invasion, host immunity and immune evasion

Malaria is caused by protozoan parasites belonging to the phylum Apicomplexa. These obligate intracellular parasites depend on the successful invasion of an appropriate host cell for their survival. This article is a broad overview of the molecular strategies employed by the merozoite, an invasive form of the malaria parasite, to successfully invade a suitable red blood cell.

Signaling of immune system cells by glycosylphosphatidylinositol (GPI) anchor and related structures derived from parasitic protozoa

Glycosylphosphatidylinositol (GPI) anchor and glycoinositolphospholipid (GIPL) are abundant molecules present in the membrane of parasitic protozoa that are common etiologic agents of medical and veterinary diseases. Recent studies have documented the immunostimulatory/regulatory activity of protozoan-derived GPI-anchors and related structures. Among the bioactivity displayed by the protozoan-derived GPI-anchor is the ability to elicit the synthesis of pro-inflammatory cytokines as well as nitric oxide by host macrophages. In contrast, at high concentrations GIPL and lipophosphoglycan (LPG) derived from protozoan parasites suppress several functions of the host immune system. Additionally, the protozoan-derived GPI-anchor and GIPL have been shown to serve as targets for both specific B and NK-T lymphocyte responses. This information extends our knowledge about parasite molecules that stimulate/regulate the host immune system during protozoan infection. The identification of receptor(s) and signaling pathways triggered by these GPI-related glycolipids may provide new insights for the development of therapies that inhibit detrimental immune responses or potentiate beneficial immune responses observed during infection with protozoan parasites.

The divergent role of tumor necrosis factor receptors in infectious diseases

Tumor necrosis factor (TNF) receptor types 1 and 2 are broadly expressed by most cell types and are activated by binding of either TNF or lymphotoxin-beta. TNF receptor-mediated immune reactions are critically important in the pathogenesis and control of a variety of infections caused by bacteria, viruses, protozoa, and fungi. This review summarizes recent findings on the role of TNF receptors in infectious diseases and discusses the divergent functions of these receptors in immune responses.

CD8+ T cell effector mechanisms in resistance to infection

Based on T cell subset depletion studies and the analysis of gene knockout mice, it is evident that CD8(+) T cells contribute to resistance against intracellular infections with certain viral, protozoan, and bacterial pathogens. Although they are known primarily for their capacity to kill infected cells, CD8(+) T cells elaborate a variety of effector mechanisms with the potential to defend against infection. Microbes use multiple strategies to cause infection, and the nature of the pathogenhost interaction may determine which CD8(+) T cell effector mechanisms are required for immunity. In this review, we summarize our current understanding of the effector functions used by CD8(+) T cells in resistance to pathogens. Analyses of mice deficient in perforin and/or Fas demonstrate that cytolysis is critical for immunity against some, but not all, infections and also reveal the contribution of cytolysis to the pathogenesis of disease. The role of CD8(+) T cell-derived cytokines in resistance to infection has been analyzed by systemic treatment with neutralizing antibodies and cytokine gene knockout mice. These studies are complicated by the fact that few, if any, cytokines are uniquely produced by CD8(+) T cells. Thus, the requirement for CD8(+) T cell- derived cytokines in resistance against most pathogens remains to be defined. Finally, recent studies of human CD8(+) T cells reveal the potential for novel effector mechanisms in resistance to infection.

Tritrichomonas foetus extracellular cysteine proteinase cleavage of bovine IgG2 allotypes

Bovine trichomoniasis is a sexually transmitted disease associated with reproductive failure. Systemic immunization results in protective IgG antibodies in uterine and vaginal secretions. Because bovine IgG2 is a better opsonin than IgG1, it is potentially important in defense. Yet, Tritrichomonas foetus extracellular cysteine proteinase (TFECP) cleaves bovine IgG2, evading protective IgG2 responses. Variations in resistance of the 2 IgG2 allotypes to digestion may explain inherited differences in protection. To address this hypothesis, TFECP was incubated with both IgG2 allotypes at different concentrations and times. The digestion products were separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis, stained, and quantitated by image analysis. IgG2a was digested faster by TFECP than IgG2b. Differences in the sizes and numbers of digestion products were observed, but the presence of bands the size of Fc and Fd fragments indicated that both allotypes were cleaved at the hinge. Cysteine in the digestion mixture reduced the antibody molecules and increased the rate of digestion, but IgG2a was still more susceptible to cleavage than IgG2b in the absence of cysteine. Thus, not only reduced H chains can be cleaved by cysteine proteinase secreted by T. foetus but also intact functional antibody molecules. Because parasites may evade protective antibody responses by cleaving IgG2, animals with the more resistant IgG2b allotype may be better protected by immunization than animals with the more readily digested IgG2a allotype.

Implications of early apoptosis of infected cells as an important host defense

Apoptosis is widely recognized as being a host defense against viral infections, since viruses require live cells. There has been increasing acceptance of the view that apoptosis is also a defense against other intracellular pathogens and even against pathogens that adhere to host cells. An implication of apoptosis being a host defense is a need to reassess to what extent the cell death at infection sites may constitute a protective host response. A concept stressed here is that infected cells are a hazard to other cells and to the individual, so the benefits of early apoptosis are emphasized. Therefore, promoting the survival of infected cells, even though still functional, may carry risks. A further consideration is the possibility that the apoptotic stimulus of nutrient restriction may be acting in infection-induced anorexia to promote apoptosis of infected cells, thereby serving as a non-specific host defense.

IFN-gamma-independent IgG2a production in mice infected with viruses and parasites

After infection with some viruses and intracellular parasites, antibody production is restricted to IgG2a. We first observed that, whereas live viruses such as lactate dehydrogenase-elevating virus (LDV) or mouse adenovirus induced mostly an IgG2a response, a large proportion of antibodies produced against killed viruses were IgG1. This IgG1 antiviral response was suppressed when live virions were added to inactivated viral particles. These results indicate that the IgG2a preponderance is related to the infectious process itself rather than to the type of antigen involved. Since IFN-gamma is known to stimulate IgG2a production by activated B lymphocytes and to be secreted after infection, we examined the role of this cytokine in the antibody isotypic distribution caused by LDV. Most IgG2a responses were relatively unaffected in mice deficient for the IFN-gamma receptor or treated with anti-IFN-gamma antibody. A similar IFN-gamma-independent IgG2a secretion was observed after infection with the parasites Toxoplasma gondii and Trypanosoma cruzi. However, the IFN-gamma-independent IgG2a production triggered by infection still required the presence of functional T(h) lymphocytes. Therefore, signal(s) other than IFN-gamma secretion may explain the T(h)-dependent isotypic bias in antibody secretion triggered by viruses and parasites.

[The antimicrobial activity of nitric oxide and its role in the infectious process]

In this article information on an important role of nitric oxide (NO) in inhibiting the growth of a number of pathogenic microorganisms, including intracellular parasites, and their elimination from the host body is presented. Differences between the mechanisms of the production of NO and free-radical compounds having antimicrobial action are given. The regulation of the activity of constitutive NO-synthase and inducible NO-synthase and the relationship between the latter and the phagocytic activity and production of anti-inflammatory cytokines are described. An important role of NO in the development of the nonspecific resistance of the body is mentioned.

Demonstration of Tritrichomonas foetus in the external genitalia and of specific antibodies in preputial secretions of naturally infected bulls

Portions of penis and prepuce were collected from 24 bulls with current or recent Tritrichomonas foetus infection. Epididymides were collected from seven of the bulls, and seminal vesicles and prostate were collected from four. Following immunohistochemical staining with two monoclonal antibodies (34.7C4.4 and TF1.15) prepared against T. foetus surface antigens, trichomonads were identified in sections from 15 of the bulls. Organisms were most often located in penile crypts in the midshaft and caudal regions and less often in preputial crypts. Trichomonads were not observed in sections from other genitalia or in subepithelial tissue. T. foetus antigen, however, was present in the cytoplasm of some epithelial cells and the cytoplasm of some mononuclear cells in subepithelial lymphoid aggregates and follicles. Preputial smegma was collected from 16 T. foetus-infected bulls and from 16 control bulls with negative T. foetus cultures. Preputial antibody levels to TF1.17, a surface antigen of T. foetus, were determined by an enzyme-linked immunosorbent assay. Preputial secretions from infected bulls contained specific antibody of each isotype and subisotype tested. IgG1 responses were the greatest, IgM and IgA responses were approximately equal, and IgG2 responses were low. Each isotype and subisotype response in infected bulls was significantly greater than that in the controls. These results confirm previous speculation concerning anatomical sites of infection and suggest that parasite antigen can be taken up and processed locally, resulting in deposition of specific IgG1, IgG2, IgA, and IgM antibodies in the preputial cavity.

The role of dendritic cells in the induction and regulation of immunity to microbial infection

Dendritic cells (DCs) play a critical role in the initiation and regulation of immune responses. Recent advances have begun to uncover the nature and diversity of DC-pathogen interactions and the modulation of DC function by microbial stimuli. Antigen pulsed DCs have also been shown in several infection models to induce high levels of protective immunity and to display immunotherapeutic potential. The study of the function of DCs in the response to infection is thus an exciting and rapidly expanding field with important implications for both fundamental and clinical immunology.

Characterization of antigenic proteins from Tritrichomonas foetus recognized by antibodies in rabbit serum, bovine serum and bovine cervicovaginal mucus

Tritrichomonas foetus is an obligate parasite of the bovine urogenital tract and is recognized as 1 of the more common infectious agents causing decreased reproductive efficiency in beef cattle. Infections result in reproductive failure and produce considerable economic loss. Vaccination of heifers with vaccines containing T. foetus induces elevated serological responses to many T. foetus antigens, decreases the rate and/or length of infection with T. foetus, and decreases fetal loss caused by infection. Because T. foetus infections are usually limited to lumen and mucosal surfaces of the reproductive tract, it has been assumed that protection from infection and abortion is partially mediated by immunoglobulins in the uterus and vagina. The objective of this study was to identify and characterize specific antigens of T. foetus that show promise for use in a recombinant vaccine that will generate a protective mucosal immune response in cattle. Surface proteins were identified by using polyclonal rabbit anti-trichomonal sera eluted from paraformaldehyde-fixed cells. Analyses of these proteins, utilizing mucosal antibodies from vaccinated and convalescent cows, have identified proteins involved in generating a local immune response. Western immunoblot analysis indicates that these proteins are well conserved and are excellent candidates for incorporation into a recombinant vaccine.

How do protozoan parasites survive inside macrophages?

During infections with intracellular microbes, macrophages have two roles. On the one hand, they are important effector cells for the control and killing of intracellular bacteria and protozoan parasites by oxidative and non-oxidative mechanisms. On the other hand, macrophages may also serve as long-term host cells that facilitate the replication and survival of the pathogens, for example, by protecting them against toxic components of the extracellular milieu. In this review, Christian Bogdan and Martin Röllinghoff summarize some of the more recently discovered mechanisms by which intracellular protozoan parasites, such as Leishmania spp, Trypanosoma cruzi and Toxoplasma gondii, manage to exploit macrophages as safe target cells.

Development of a natural model of cutaneous leishmaniasis: powerful effects of vector saliva and saliva preexposure on the long-term outcome of Leishmania major infection in the mouse ear dermis

We have developed a model of cutaneous leishmaniasis due to Leishmania major that seeks to mimic the natural conditions of infection. 1,000 metacyclic promastigotes were coinoculated with a salivary gland sonicate (SGS) obtained from a natural vector, Phlebotomus papatasii, into the ear dermis of naive mice or of mice preexposed to SGS. The studies reveal a dramatic exacerbating effect of SGS on lesion development in the dermal site, and a complete abrogation of this effect in mice preexposed to salivary components. In both BALB/c and C57Bl/6 (B/6) mice, the dermal lesions appeared earlier, were more destructive, and contained greater numbers of parasites after infection in the presence of SGS. Furthermore, coinoculation of SGS converted B/6 mice into a nonhealing phenotype. No effect of SGS was seen in either IL-4- deficient or in SCID mice. Disease exacerbation in both BALB/c and B/6 mice was associated with an early (6 h) increase in the frequency of epidermal cells producing type 2 cytokines. SGS did not elicit type 2 cytokines in the epidermis of mice previously injected with SGS. These mice made antisaliva antibodies that were able to neutralize the ability of SGS to enhance infection and to elicit IL-4 and IL-5 responses in the epidermis. These results are the first to suggest that for individuals at risk of vector-borne infections, history of exposure to vector saliva might influence the outcome of exposure to transmitted parasites.

DNA vaccines

DNA vaccination against infectious diseases has created a new field of applied molecular immunology. cDNAs for 'protective' protein epitopes can be inserted into vectors containing strong mammalian promoters for high expression. Here we discuss the mechanisms of DNA vaccination and the successful and sometimes unsuccessful applications of DNA vaccination to protect animals against many different viral, bacterial mycoplasmal, protozoal, and worm infections or infestations. DNA immunization has been used to prevent or inhibit tumor development and to inhibit IgE responses by diverting the immune response from Th2 to Th1 helper cell dominance. Advantages and disadvantages of a variety of routes of administration and methods of immunization discussed include the use of the 'gene gun', the delivery of genes by aerosols, and deliberate induction of injury to muscles prior to injection of DNA to enhance gene expression. Vaccination performed using DNA without knowing beforehand the protective epitopes, using 'expression library immunization', is discussed. While this field is bound to expand rapidly for future clinical applications, we try to point out potential pitfalls as well as advantages of this relatively new technology.

Acquired immunity to amyloodiniosis is associated with an antibody response

The dinoflagellate Amyloodinium ocellatum, which causes amyloodiniosis or 'marine velvet disease', is one of the most serious ectoparasitic diseases plaguing warmwater marine fish culture worldwide. We report that tomato clownfish Amphiprion frenatus develop strong immunity to Amyloodinium ocellatum infection following repeated nonlethal challenges and that specific antibodies are associated with this response. Reaction of immune fish antisera against dinospore and trophont-derived antigens in Western blots indicated both shared and stage-specific antibody-antigen reactions. A mannan-binding-protein affinity column was used to isolate IgM-like antibody from A. frenatus serum. The reduced Ig consisted of one 70 kD heavy chain and one 32 kD light chain with an estimated molecular weight of 816 kD for the native molecule. Immunoglobulin (Ig) isolated from immune but not non-immune fish serum significantly inhibited parasite infectivity in vitro. An enzyme-linked immunosorbent assay (ELISA) was developed using polyclonal rabbit antibody produced against affinity-purified A. frenatus Ig. Anti-Amyloodinium serum antibody was not always detectable in immune fish, although serum antibody titers in immune fish increased after repeated exposure to the parasite. These results suggest that there may be a localized antibody response in skin/gill epithelial tissue, although antibody was rarely detected in skin mucus.

Experimental infection of inbred mouse strains with Spironucleus muris

Four inbred mouse strains: BALB/c ByJ, 129/J, C3H/HeJ, and DBA/lJ, differing in major histocompatibility type, were orally inoculated with 2 x 10(5) infectious cysts of Spironucleus muris. Fecal samples were collected for fecal cyst output prior to infection, and on days 2, 6, 7, 8, 9, 11, 13, 15, and 17 after infection. Following necropsy, formalin-fixed intestinal sections were examined for the presence of trophozoites. On post-inoculation days 6 and 8, mice of the 129/J strain shed significantly (p<0.05) fewer cysts than other strains. This pilot study suggests that major histocompatibility haplotype may influence susceptibility of inbred mouse strains to S. muris.

Cross-reaction of an anti-Cryptosporidium monoclonal antibody with sporocysts of Monocystis species

The non-specific cross-reaction of a fluorescently labelled anti-Cryptosporidium monoclonal antibody was observed microscopically when testing faecal specimens from small mammals. The reactive particles were identified as sporocysts of the Gregarine family Monocystidae, and indicate that considerable care should be taken so that false positives are not recorded.

Macrophage apoptosis in microbial infections

Upon infection with a pathogen, eukaryotic cells can undergo programmed cell death as an ultimate response. Therefore, modulation of apoptosis is often a prerequisite to establish a host-pathogen relationship. Some pathogens kill macrophages by inducing apoptosis and thus overcome the microbicidal arsenal of the phagocyte. Apoptotic macrophages, on the other hand, can elicit an inflammation by secretion of proinflammatory cytokines. Shigella flexneri, the aetiological agent of bacillary dysentery, induces apoptosis in macrophages which, in agony, specifically release mature interleukin-1 beta (IL-1 beta). This cytokine attracts neutrophils (PMN) to the site of infection resulting in the massive colonic inflammation characteristic of bacillary dysentery. Shigellosis represents a paradigm of a proinflammatory apoptosis in a bacterial infection. The molecular link between apoptosis and inflammation is interleukin-1 beta converting enzyme (ICE) which is activated during macrophage apoptosis and binds to IpaB, a secreted Shigella protein.

Genital and systemic immune responses in a murine model of Tritrichomonas foetus infection

A reliable laboratory animal model would be useful for the study of immune responses to trichomoniasis, a sexually transmitted disease of human beings and cattle. Murine models are available, but pretreatment with estrogen is used, which may influence immune responses. To evaluate whether vaginal trichomoniasis could be established in nonestrogenized mice and to define the immune responses associated with the infection, CD1 and BALB/c mice were studied with or without estrogen treatment prior to inoculation with Tritrichomonas foetus. Tritrichomonas Foetus was cultured from the vagina and uterus of both estrogen-treated and untreated control mice for up to 26 wk. The infection was sustained better in BALB/c than in CD1 mice, suggesting that the former strain was most susceptible. In CD1 mice, infection was sustained less well in estrogen-treated than in untreated control mice, but there was no difference between treatment groups of BALB/c mice. IgA and IgG antibodies in vaginal secretions, uterine secretions, and serum specific for a surface antigen of T. foetus (TF1.17) were measured by enzyme-linked immunosorbent assay. In infected CD1 mice, vaginal IgA and IgG antibodies were detected by 8 wk postinoculation (PI). In infected BALB/c mice, vaginal IgA and IgG antibodies were detected by 12 wk PI. Uterine IgG responses predominated over IgA in estrogen-treated and untreated CD1 and BALB/c mice. There were high levels of IgG, but relatively no IgA in the sera of CD1 and BALB/c mice. Overall, the highest IgA response was in the vaginal secretions of infected CD1 mice, and some animals of this strain cleared the infection. These results show that a chronic trichomonad infection was established in mice without prior treatment with estrogen. The infection was associated with antibody responses in reproductive secretions and serum. This animal model will be useful in studying immunization to protect against trichomoniasis in mice not immunocompromised by estrogen.

The role of four anopheline species (Diptera: Culicidae) in malaria transmission in coastal Tanzania

Malaria is holoendemic in coastal Tanzania with Anopheles funestus and members of the A. gambiae complex being mainly responsible for transmission. Over a 4 months' sampling period 2222 anopheline mosquitoes were collected using light-traps and indoor resting catches, of which 58.6% were A. gambiae, 7.6% A. arabiensis, 6.9% A. merus and 26.9% A. funestus. Plasmodium falciparum circumsporozoite antigen (CSA) rates were: A. funestus 6.05% (n = 479), A. gambiae 8.4% (n = 1042), A. arabiensis 7.3% (n = 136) and A. merus 9.8% (n = 122). The P. malariae CSA rate for all anophelines was 0.07% (n = 1862). Estimated sporozoite densities were less than 2000 for at least 50% of all the positive mosquitoes. Along the coast the abundance of A. merus (41.3%) and A. gambiae (46.1%) was similar, and their CSA rates were comparable (11.6% and 12.5%, respectively) and higher than those for A. arabiensis (7.7%) and A. funestus (4.6%). These results indicate that A. merus plays an unexpectedly important role in malaria transmission in coastal Tanzania.

Female reproductive tract immunity in bovine trichomoniasis

PROBLEM

Mechanisms of protective immunity in the female reproductive tract are poorly understood. For sexually transmitted diseases, bovine trichomoniasis is a useful model because it resembles human trichomoniasis to some extent, and antibodies play an important role in protection against these extracellular parasites. Protective efficacy was compared in animals with genital responses of predominantly immunoglobulin G (IgG) or predominantly IgA antibodies to a purified surface antigen of Tritrichomonas foetus.

METHOD OF STUDY

Immunization of mice by various routes with immunoaffinity-purified T. foetus surface antigen (TF1.17) or killed cells was used to define the best routes and antigen combinations to give predominantly IgG or IgA antibodies to TF1.17 antigens in genital secretions. Cattle were then immunized either subcutaneously (SC) two times with TF1.17 antigen and once SC with killed T. foetus or twice SC with TF1.17 antigen and once intravaginally with killed T. foetus. All immunizations were in Quil A adjuvant. Controls were not immunized. Animals were challenged intravaginally with 10(6) T. foetus 3 weeks after the third immunization. Vaginal mucus was collected weekly for culture and antibody assays. Serum was collected weekly, and uterine secretions were collected at 10 weeks post-challenge. Tissues were fixed at 10 weeks also.

RESULTS

Murine studies showed systemic priming with vaginal boosting gave the highest genital IgA responses. In cattle, systemic immunization (group S) induced high IgG1 antibody levels in vaginal secretions. Systemic priming with vaginal boosting (group S/V) primed for an anamnestic vaginal IgA response after challenge with T. foetus. Cattle with predominantly IgG or predominantly IgA responses in vaginal secretions either did not become infected or cleared infection faster than controls. Uterine IgA responses at 10 weeks were highest in the vaginally boosted group, but other responses were not different from the controls at this time point. Microscopic examination of genital tissues showed subepithelial infiltration of mononuclear cells in all groups. Lymphoid aggregates or nodules were detected in vaginal sections in cattle of groups S/V and C as well as in uterine sections of all animals in all three groups.

CONCLUSIONS

Both IgG and IgA antibodies to T. foetus superficial antigen were associated with protection. The timing of the response was related to the time of clearance. Lymphoid organization in the vagina and uterine tissues suggested development of mucosal inductive sites.