Interaction of African trypanosomes with the immune system

Trypanosoma brucei Co-opts NK Cells to Kill Splenic B2 B Cells

After infection with T. brucei AnTat 1.1, C57BL/6 mice lost splenic B2 B cells and lymphoid follicles, developed poor parasite-specific antibody responses, lost weight, became anemic and died with fulminating parasitemia within 35 days. In contrast, infected C57BL/6 mice lacking the cytotoxic granule pore-forming protein perforin (Prf1-/-) retained splenic B2 B cells and lymphoid follicles, developed high-titer antibody responses against many trypanosome polypeptides, rapidly suppressed parasitemia and did not develop anemia or lose weight for at least 60 days. Several lines of evidence show that T. brucei infection-induced splenic B cell depletion results from natural killer (NK) cell-mediated cytotoxicity: i) B2 B cells were depleted from the spleens of infected intact, T cell deficient (TCR-/-) and FcγRIIIa deficient (CD16-/-) C57BL/6 mice excluding a requirement for T cells, NKT cell, or antibody-dependent cell-mediated cytotoxicity; ii) administration of NK1.1 specific IgG2a (mAb PK136) but not irrelevant IgG2a (myeloma M9144) prevented infection-induced B cell depletion consistent with a requirement for NK cells; iii) splenic NK cells but not T cells or NKT cells degranulated in infected C57BL/6 mice co-incident with B cell depletion evidenced by increased surface expression of CD107a; iv) purified NK cells from naïve C57BL/6 mice killed purified splenic B cells from T. brucei infected but not uninfected mice in vitro indicating acquisition of an NK cell activating phenotype by the post-infection B cells; v) adoptively transferred C57BL/6 NK cells prevented infection-induced B cell population growth in infected Prf1-/- mice consistent with in vivo B cell killing; vi) degranulated NK cells in infected mice had altered gene and differentiation antigen expression and lost cytotoxic activity consistent with functional exhaustion, but increased in number as infection progressed indicating continued generation. We conclude that NK cells in T. brucei infected mice kill B cells, suppress humoral immunity and expedite early mortality.

Intravital imaging of a massive lymphocyte response in the cortical dura of mice after peripheral infection by trypanosomes

Peripheral infection by Trypanosoma brucei, the protozoan responsible for sleeping sickness, activates lymphocytes, and, at later stages, causes meningoencephalitis. We have videoed the cortical meninges and superficial parenchyma of C56BL/6 reporter mice infected with T.b.brucei. By use of a two-photon microscope to image through the thinned skull, the integrity of the tissues was maintained. We observed a 47-fold increase in CD2+ T cells in the meninges by 12 days post infection (dpi). CD11c+ dendritic cells also increased, and extravascular trypanosomes, made visible either by expression of a fluorescent protein, or by intravenous injection of furamidine, appeared. The likelihood that invasion will spread from the meninges to the parenchyma will depend strongly on whether the trypanosomes are below the arachnoid membrane, or above it, in the dura. Making use of optical signals from the skull bone, blood vessels and dural cells, we conclude that up to 40 dpi, the extravascular trypanosomes were essentially confined to the dura, as were the great majority of the T cells. Inhibition of T cell activation by intraperitoneal injection of abatacept reduced the numbers of meningeal T cells at 12 dpi and their mean speed fell from 11.64 ± 0.34 μm/min (mean ± SEM) to 5.2 ± 1.2 μm/min (p = 0.007). The T cells occasionally made contact lasting tens of minutes with dendritic cells, indicative of antigen presentation. The population and motility of the trypanosomes tended to decline after about 30 dpi. We suggest that the lymphocyte infiltration of the meninges may later contribute to encephalitis, but have no evidence that the dural trypanosomes invade the parenchyma.

African trypanosomes are motile parasites that cause sleeping sickness. They multiply first in the blood then cause death mainly by effects on the brain: immune system cells, including T cells and dendritic cells, play major roles in this. Thinking we might see the attack on the brain, we infected mice with trypanosomes and used a two-photon microscope, which allowed us to image the superficial brain and the delicate tissue between the skull and the brain called the meninges without making a hole in the skull. The mice (which were anesthetized) had been genetically modified so that T cells and dendritic cells were fluorescent, as were the trypanosomes. We did not notice much happening in the brain itself, but in the meninges, in a compartment called the dura, huge numbers of T cells and dendritic cells appeared. Trypanosomes also moved from the blood into this compartment. Since T cells, dendritic cells and trypanosomes had not been videoed in the meninges before, we began by observing them carefully: their numbers, their movements and their interactions. The accumulation of lymphocytes is a sign of meningitis, a feature of infection by a wide range of pathogens and our results suggest interesting future work.

Type I IFNs play a role in early resistance, but subsequent susceptibility, to the African trypanosomes

Macrophages express a spectrum of proinflammatory and regulatory mediators during African trypanosomiasis. Microarray analyses revealed similar profiles of induced genes in macrophages stimulated with the trypanosome soluble variant surface glycoprotein in vitro and in macrophages taken from infected mice. Genes associated with the acute phase response and with type I IFN responses were prominent components of the macrophage activation profiles expressed within 72 h in vitro and in vivo. Thus, induction of proinflammatory gene expression is a characteristic of early trypanosome infection that is driven primarily by soluble variant surface glycoprotein exposure, and it may be that IFN-alpha/beta plays a central role in regulation of early resistance to trypanosomes. To test this hypothesis, we assessed parameters of infection in mouse strains with genetic alterations in the IFN-alpha/beta response pathway. We found that Ifnar1(-/-) mice, which lack the receptor for type I IFNs, exhibited delayed control of parasite burden during the first week of infection and died earlier than did wild-type controls. However, infection of Ubp43(-/-) mice, which are hyperresponsive to type I IFNs, did not exhibit enhanced resistance to trypanosomes. Instead, these animals also failed to control parasite burden and were more susceptible than wild-type animals. Additionally, the Ubp43(-/-) mice exhibited a significant defect in IFN-gamma production, which is definitively linked to host resistance in trypanosomiasis. These results show that type I IFNs play a role in early control of parasites in infected mice but may contribute to down-regulation of IFN-gamma production and subsequent loss of host resistance later in infection.

CpG oligodeoxynucleotide treatment enhances innate resistance and acquired immunity to African trypanosomes

Relative resistance to African trypanosomiasis is based on the development of a type I cytokine response, which is partially dependent on innate immune responses generated through MyD88 and Toll-like receptor 9 (TLR9). Therefore, we asked whether enhancement of the immune response by artificial stimulation with CpG oligodeoxynucleotide (ODN), a TLR9 agonist, would result in enhanced protection against trypanosomes. In susceptible BALB/c mice, relative resistance to infection was significantly enhanced by CpG ODN treatment and was associated with decreased parasite burden, increased cytokine production, and elevated parasite-specific B- and T-cell responses. In relatively resistant C57BL/6 mice, survival was not enhanced but early parasitemia levels were reduced 100-fold and the majority of the parasites were nondividing, short stumpy (SS) forms. CpG ODN treatment of lymphocyte-deficient C57BL/6-scid and BALB/cByJ-scid mice also enhanced survival and reduced parasitemia, indicating that innate resistance to trypanosome infection can be enhanced. In C57BL/6-scid and BALB/cByJ-scid mice, the parasites were also predominantly SS forms during the outgrowth of parasitemia. However, the effect of CpG ODN treatment on parasite morphology was not as marked in gamma interferon (IFN-gamma)-knockout mice, suggesting that downstream effects of IFN-gamma production may play a discrete role in parasite cell differentiation. Overall, these studies provide the first evidence that enhancement of resistance to African trypanosomes can be induced in susceptible animals in a TLR9-dependent manner and that CpG ODN treatment may influence the developmental life cycle of the parasites.

Immunology and immunopathology of African trypanosomiasis

Major modifications of immune system have been observed in African trypanosomiasis. These immune reactions do not lead to protection and are also involved in immunopathology disorders. The major surface component (variable surface glycoprotein,VSG) is associated with escape to immune reactions, cytokine network dysfunctions and autoantibody production. Most of our knowledge result from experimental trypanosomiasis. Innate resistance elements have been characterised. In infected mice, VSG preferentially stimulates a Th 1-cell subset. A response of <FONT FACE=Symbol>gd</FONT> and CD8 T cells to trypanosome antigens was observed in trypanotolerant cattle. An increase in CD5 B cells, responsible for most serum IgM and production of autoantibodies has been noted in infected cattle. Macrophages play important roles in trypanosomiasis, in synergy with antibodies (phagocytosis) and by secreting various molecules (radicals, cytokines, prostaglandins,...). Trypanosomes are highly sensitive to TNF-alpha, reactive oxygen and nitrogen intermediates. TNF-alpha is also involved in cachexia. IFN-gamma acts as a parasite growth factor. These various elements contribute to immunosuppression. Trypanosomes have learnt to use immune mechanisms to its own profit. Recent data show the importance of alternative macrophage activation, including arginase induction. L-ornithine produced by host arginase is essential to parasite growth. All these data reflect the deep insight into the immune system realised by trypanosomes and might suggest interference therapeutic approaches.

The dynamics of antigenic variation and growth of African trypanosomes

Antigenic variation in African trypanosomes, which is a simple strategy for survival in the immune host, is rendered complex by its magnitude. For protection from nonspecific immunity and escape from specific immunity, each trypanosome is covered by a replaceable surface coat composed of the variant surface glycoprotein (VSG), which specifies the variable antigen type (VAT) of the trypanosome. Antigenic variation is the process by which the trypanosome switches from one coat to another. Here, David Barry and Michael Turner consider this phenomenon within the context of the course of trypanosome infection.

Interferon-gamma and interleukin-12 genes are preferentially expressed during early experimental African trypanosomiasis and suppressed by denervation of the spleen

The cross talk between the central nervous system (CNS) and the immune system includes among others, the modulation of immune responses by the autonomic nervous system. Here, we investigated the effects of a splenic denervation on cytokine induction in early experimental African trypanosomiasis. Profiles of the cytokine mRNA expression for interleukin (IL)-4, interleukin (IL)-6, interleukin (IL)-10, interleukin (IL)-12, tumour necrosis factor (TNF)-alpha, tumour necrosis factor (TNF)-beta, transforming growth factor (TGF)-beta and interferon (IFN)-gamma were examined at 4 h, 8 h and 12 h postinfection (p.i.), and in noninfected controls. Only IFN-gamma and IL-12 were significantly expressed over noninfected controls. Already at 4 h p.i. both cytokines were expressed and showed more increased levels at 12 h. Sympathetic denervation of the spleen markedly reduced the mRNA expression for both IFN-gamma and IL-12. Con A was used as a positive control and showed an enhanced mRNA expression, which was also suppressed by a splenic denervation. To demonstrate that the mRNA expression had resulted in a cytokine production, we looked for the protein level of IFN-gamma at 4 h p.i. by immunohistochemistry and found increased levels of IFN-gamma, which was also inhibited by the denervation. Sham-operated animals exhibited similar responses as the nondenervated controls. Our data present for the first time very early kinetics for a cytokine gene expression during an experimental African trypanosomiasis. Furthermore, the data suggest a regulatory role for the autonomic nervous system on cytokine responses at both the mRNA and the protein levels.

Retrograde axonal cytokine transport: a pathway for immunostimulation in the brain inducing hypoxia and sudden infant death?

The etiology and pathogenesis of sudden infant death syndrome (SIDS) remain unknown. A hypothesis for SIDS should explain three characteristic findings: (a) an age distribution peaking at 2-4 months; (b) frequent association with respiratory tract infections; and (c) occurrence during sleep. The diagnosis of SIDS is applied when death cannot be explained, and this syndrome therefore probably includes various underlying causes. Based on recent observations, however, we suggest a pathogenic pathway that might be common to most SIDS victims.

From trypanosomes to the nervous system, from molecules to behavior: a survey, on the occasion of the 90th anniversary of Castellani's discovery of the parasites in sleeping sickness

The observation of Trypanosomes in patients affected by sleeping sickness has been reported in 1903 by Aldo Castellani. On the occasion of the 90th anniversary of this discovery, we here present the findings recently obtained in and experimental model of African trypanosomiasis in the rat. The molecular and cellular mechanisms of the interplay between the parasite and the host have been largely clarified: a bidirectional signalling occurs between trypanosomes and CD8+ T cells of the host animal. This new pathogenetic mechanism of infection involves a lymphocyte triggering factor released by the parasite and interferon-gamma. A recently isolated neuronal interferon-gamma could also play a role in the disease. The selective induction of major histocompatibility antigens class I has revealed the involvement of the hypothalamic paraventricular and supraoptic nuclei in trypanosomiasis. Finally, studies based on the expression of the immediate early gene c-fos have pointed out during the infection a selective dysregulation of the suprachiasmatic nucleus of the hypothalamus, that plays the role of biological clock. The latter finding could account for the disruption of endogenous rhythms in sleeping sickness.

Mechanisms of development of immunosuppression during Trypanosoma infections

Acute infection with Trypanosoma cruzi or its African relatives, including T. brucei rhodesiense, T. b. gambiense, T. b. brucei and T. congolense, is frequently accompanied by manifestations of immunological dysfunction. Initially investigators catalogued the ensuing immunologic alterations and identified a number of modifications in lymphoid or accessory cell properties. More recently, the emphasis has switched towards the molecular underpinnings of immunosuppression in these infections. In this article, Marcelo Sztein and Felipe Kierszenboum focus on recent progress made in the quest to delineate the mechanisms behind altered lymphocyte functions in tryponosomal infections, point out particular and common features of immunosuppression induced by T. cruzi and African trypanosomes, and outline possible directions for future research.

Hypogonadism induced by African trypanosomes in humans and animals

1. The clinical syndromes of the African trypanosomiasis (also called sleeping sickness in humans) are defined. The basic knowledge on the hypothalamo-anterior pituitary-gonad axis functions is briefly recalled. 2. Hypogonadism caused by the trypanosomes both in men and in women as well as in male and female animals are extensively reviewed in publications over the last two decades as well as on our own very recent works which provided new insights on the physiopathology of the gonadal disorders.

Nitric oxide-mediated cytostatic activity on Trypanosoma brucei gambiense and Trypanosoma brucei brucei

Macrophages collected from BCG-infected mice or exposed in vitro to interferon-gamma plus lipopolysaccharide developed a cytostatic activity on Trypanosoma brucei gambiense and Trypanosoma brucei brucei. This trypanostatic activity of activated macrophages was inhibited by addition of N-monomethyl-L-arginine, an inhibitor of the L-arginine-nitric oxide (NO) metabolic pathway, indicating a role for NO as the effector molecule. Contrary to trypanosomes treated with N2gas, trypanosomes treated with NO gas did not proliferate in vitro on normal macrophages. Compared to mice infected with control parasites, mice infected with NO-treated parasites had decreased parasitemias in the first days postinfection and had a prolonged survival. Addition of excess iron reversed the trypanostatic effect of both activated macrophages and NO gas. These data show that activated macrophages exert an antimicrobial effect on T.b. gambiense and T.b. brucei through the L-arginine-NO metabolic pathway. In trypanosomes, NO could trigger iron loss from critical targets involved in parasite division. The participation of this effector mechanism among the other immune elements involved in the control of African trypanosomes (antibodies, complement, phagocytic events) remains to be defined.

Environmental, genetic and immunological factors in human resistance to Schistosoma mansoni

The design of programs for the control of endemies requires the knowledge of the principal factors that determine parasite transmission and infection levels in exposed populations. In the studies summarized in this article, the role of environmental and host specific factors in the infection by S. mansoni have been evaluated. It is shown that a limited number of factors actually influences infection intensity: water contacts, age, and sex were shown to account for 20 to 25% of infection variance, while 35 to 40% of it was accounted for by the effect of a major codominant gene. A remarkable fact is the important weighting (around 55% of the variance) of factors (the major gene and age) that influence human capacities of resistance. This observation strongly supports control measures aimed at increasing human resistance, such as vaccination. The effect of age on the development of resistance has now been observed in several studies on S. mansoni or S. haematobium. It is, therefore, a constant finding in schistosomiasis infections that resistance develops extremely slowly requiring a long period of exposure to the parasite and repeated infections. These studies provide strong incentives to increase efforts in the evaluation of the immune response of subjects living in endemic areas. Such evaluations are necessary to define vaccine and vaccination programs, and they are also urgently needed to evaluate the effects of chemotherapy on the development of immunity in children and adolescents, as well as on the persistence of protective immunity in adults. Immunological studies begin to provide a clearer picture of the role of acquired immunity in human protection against S. mansoni. It is increasingly clear that the slow development of resistance in children, as well as its alteration in certain age groups, are related to the maturation of parasite specific immunity and its alteration by specific immune factors. Thus, the development of resistance is associated with the maturation of IgE-dependent immunity, whereas blocking Ab may interfere in children and adolescents with the expression of full resistance. This finding raises the question as to whether a vaccine could include major allergens without triggering the well-known deleterious side effects associated with hypersensitivity reactions. The absence of such reactions in subjects with high parasite-specific IgE levels who are exposed to daily infections suggests that this may be feasible.

Immunopathology of experimental African sleeping sickness: detection of cytokine mRNA in the brains of Trypanosoma brucei brucei-infected mice

The immunopathology of Trypanosoma brucei brucei in the central nervous system was studied by using an experimental model of chronic meningoencephalitis in outbred CD-1 mice. Mice infected with T. b. brucei were treated with a subcurative dose of the trypanocidal compound diaminazine aceturate. These mice relapsed and were again drug treated. The brains were examined histologically and by immunocytochemistry to identify activated astrocytes. The polymerase chain reaction was used to detect cytokine RNA transcripts. The infected and treated animals developed severe chronic meningoencephalitis characterized by large numbers of inflammatory cells and widespread astrocyte proliferation. In uninfected controls, only interleukin 1 and beta-actin RNA transcripts were detected while transcripts for beta-actin, tumor necrosis factor alpha, macrophage inflammatory protein 1 and interleukins 1 and 4 were demonstrated in the brains of infected animals. Gamma interferon and interleukin 6 were also detected in a few of the infected animals, but interleukin 2 was not found in any of these animals.

Gamma interferon expression and major histocompatibility complex induction during measles and vesicular stomatitis virus infections of the brain

Lymphocytic interferon gamma (IFN-gamma) production and major histocompatibility complex (MHC) antigen induction were studied in experimental measles and vesicular stomatitis virus infections in the brain. Fifteen-day-old Sprague-Dawley rats injected intracerebrally with the HNT strain of measles virus showed already within 1 day after infection an increased number of cells producing IFN-gamma in the spleen, cervical lymph nodes and leptomeninges. These rats recovered after a transient neuronal infection in the brain. Rats infected intracerebrally with vesicular stomatitis virus, on the other hand, all succumbed after 2 days and showed no IFN-gamma production in lymphoid cells. Immunohistochemically MHC class I antigen appeared in infected and uninfected cells in the brain during replication of both viruses. A role for the recently discovered nerve fibres with IFN-gamma-like immunoreactivity, which are normally present in the brain, in the MHC antigen induction is discussed.

Studies of hyperalgesia induced by Trypanosoma brucei brucei infection in rats

Sprague-Dawley rats were infected by intraperitoneal injections of a strain of Trypanosoma brucei brucei. This parasite spreads early to areas in the nervous system which lack a so-called blood-brain or blood-nerve barrier including spinal ganglia. Signs of sensory disturbance in the form of hyperalgesia were observed already seven days post infection, using a hot plate analgesimeter. The sensory disturbances progressed and the rats started to lose weight about 3 weeks post infection, and died about one week later. Immunohistochemistry showed no reduction in density of calcitonin gene-related peptide or substance P immunoreactive fibres in the skin or spinal cord. It is suggested that the sensory disturbances are mediated via release of kinin or interleukin-1 which are known as potent hyperalgesic agents. The hot plate analgesimeter is a simple and reliable method and may be useful in experimental trypanosomiasis to test drugs developed to combat this disease.

The somnogenic T lymphocyte suppressor prostaglandin D2 is selectively elevated in cerebrospinal fluid of advanced sleeping sickness patients

To help to elucidate the changes induced by Trypanosoma brucei gambiense in the central nervous system (CNS) in advanced sleeping sickness patients, levels of interleukin-1 (IL-1) and prostaglandins D2 (PGD2) and E2 (PGE2) were measured by radioimmunoassay in the cerebrospinal fluid (CSF) from 24 patients diagnosed on the criteria of CSF protein, leucocyte count and parasite presence as having CNS (i.e. late stage) involvement, and from 12 patients without CNS involvement. PGD2 concentrations were selectively and markedly elevated in the late stage patients. The increased PGD2 may in part account for the increased somnolence and the immunosuppression within the CNS. Measurement of PGD2 levels in CSF may be a useful criterion for CNS involvement.

Depletion of CD8+ T cells suppresses growth of Trypanosoma brucei brucei and interferon-gamma) production in infected rats

Sprague-Dawley rats infected with Trypanosoma brucei brucei showed a strong and rapid induction of splenocyte IFN-gamma (within 12 h post-infection) as measured by a single cell assay for IFN-gamma secretion. Depletion of CD8+ cells in infected rats abrogated the IFN-gamma production, suppressed parasite growth and increased survival of the animals. Induction of MHC class I antigens in the paraventricular and supra-optic hypothalamic nuclei caused by the trypanosome infection was also inhibited by the CD8+ cell depletion. It is suggested that the CD8+ cells are involved directly or indirectly in growth regulation of the parasite and that IFN-gamma induced by the parasite may be one of the factors that trigger MHC expression and immunosuppression.

Early major histocompatibility complex (MHC) class I antigen induction in hypothalamic supraoptic and paraventricular nuclei in trypanosome-infected rats

Sprague-Dawley rats were injected intraperitoneally with a suspension of Trypanosoma brucei brucei. An early induction of major histocompatibility complex (MHC) class I antigens as well as an infiltration of macrophage-like cells and cytotoxic T-cells was detected with immunohistochemical techniques in circumventricular organs, such as the median eminence, neurohypophysis, subfornical organ, pineal gland and area postrema. These areas, which lack a blood-brain barrier, correspond to those showing early invasion of trypanosomes. In addition, there was a marked induction of MHC class I in neurons in two hypothalamic nuclei, the paraventricular and supraoptic nuclei. Neurons in these two nuclei are located behind the blood-brain barrier, but project to the neurohypophysis and to the median eminence, thereby exposing their axon terminals to factors circulating in the blood or released locally from invading trypanosomes or from macrophages or cytotoxic T-cells. It is suggested that the alteration in the nerve cell bodies in the hypothalamic nuclei is caused by retrograde axonal signals from these target areas.

High frequency of antigenic variation in Trypanosoma brucei rhodesiense infections

Rates at which Trypanosoma brucei rhodesiense trypanosomes switch from expression of one variable antigen type (VAT) to that of another have been determined in cloned populations that have been recently tsetse-fly transmitted. Switching rates have been determined between several, specific pairs of VATs in each population. High rates of switching were observed in 2 cloned trypanosome lines, each derived from a separate cyclical transmission of the same parental stock and each expressing a different major VAT. Five estimates of the switching rate between one particular pair of VATs were consistently high (approximately 1 x 10(-3) switches/cell/generation). These high switching rates were similar both in bloodstream populations of mice and in populations confined to subcutaneously implanted growth chambers in mice, thus indicating that the interaction of the bloodstream population with other trypanosome populations in the lymphatics or extravascular sites in systemic infections did not influence the estimates of the rate of switching. Fourteen estimates were made of VAT-specific switching rates in bloodstream infections involving 8 combinations from among 6 VATs. Switching rate estimates were VAT-specific and showed considerable variation between different combinations of VATs--from 1.9 x 10(-6) to 6.9 x 10(-3) switches/cell/generation. The rates of switching to different metacyclic-VATs were, however, very similar. Summation of between 3 and 5 VAT-specific switching rate values in each of 4 experiments conducted in bloodstream infections has provided minimum estimates of the overall rate of antigenic variation: 2.0-9.3 x 10(-3) switches/cell/generation. These values are between 20 and 66,000-fold higher than previously published estimates.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurobiology of sleeping sickness

The advanced stages of sleeping sickness are correlated with a spread of trypanosomes into the central nervous system (CNS), producing a disseminated encephalitis. Inflammatory reactions extend along the blood vessels causing perivascular cuffing, which consists of in filtrations and proliferations of lymphocytes and also increased numbers of astrocytes and microglia. Progress in our understanding of the functions of astrocytes suggests that they are efficient antigen-presenting cells, initiating and regulating the intracerebral inflammatory response and limiting parasite spread to the perivascular spaces.

Spread of Trypanosoma brucei to the nervous system: early attack on circumventricular organs and sensory ganglia

The distribution of Trypanosoma brucei brucei in the nervous system of experimentally infected Sprague-Dawley rats and BALB/c and deer mice was examined with immunohistochemical techniques. The trypanosomes showed an early invasion in areas lacking a so-called blood-brain or blood-nerve barrier, i.e., in sensory ganglia and circumventricular organs including the area postrema, pineal gland, and median eminence. This distribution of trypanosomes may relate to the origin of cardinal symptoms of the disease, e.g., sensory disturbances, nausea, disturbed circadian rhythm, and neuroendocrinological dysfunctions. Trypanosome infections in rodents may provide a model for studies of how an infectious agent or factors released by the immune response may relatively selectively interfere with these functionally defined regions of the nervous system.

Trypanosoma brucei: susceptibility to hydrogen peroxide and related products of activated macrophages

The susceptibility of procyclic, trypsinized, or bloodstream forms of Trypanosoma brucei to lysis by hydrogen peroxide and by activated mouse macrophages was investigated in vitro using the release of biosynthetically labeled proteins as an assay. Uncoated parasites were more resistant than coated bloodstream forms in both cases. Macrophage trypanolysis upon triggering with phorbol myristate acetate occurred extracellularly and seemed to depend on the release of hydrogen peroxide, as it was prevented by catalase. However, when presented trypanosomes in presence of fresh immune serum which was itself lytic, macrophages did not show any additional lytic effect, and phagocytosis was related to already damaged parasites.

The alternative pathway of complement in sheep during the course of infection with Trypanosoma congolense and after Berenil treatment

Experimental T. congolense infections in sheep resulted in a striking decrease in parameters of the alternative complement pathway (ACP), ie. factor B, C3 and haemolytic complement activity (HA) initiated via the ACP. The levels of factor B, C3 and HA declined before, during and after the first wave of parasitemia which reached a peak at day 8. Levels of 20 to 25% of normal values (factor B) and 20% (C3, HA) persisted throughout the course of the infection. After Berenil treatment, when no parasites were detected in blood, their serum levels remained low. The returned to normal values about 8 (factor B) to 20 days (C3, HA) after trypanocidal treatment. Serum concentrations of factor B were significantly elevated in some, but not all sheep 6 days after infection with T. congolense. The sheep were tested for their potential state of immuno-modulation by immunization with Brucella abortus 4 days after trypanocidal treatment. In contrast to other sheep, the sheep which had shown early elevated serum factor B levels were found to express immune enhancement. It is suggested that there might be a positive correlation between the degree of enhanced serum levels of factor B at the early stage of infection and enhanced immune responsiveness.