Suppression of interleukin 2 secretion and interleukin 2 receptor expression during tsetse-transmitted trypanosomiasis in cattle

Protozoan co-infections and parasite influence on the efficacy of vaccines against bacterial and viral pathogens

A wide range of protozoan pathogens either transmitted by vectors (Plasmodium, Babesia, Leishmania and Trypanosoma), by contaminated food or water (Entamoeba and Giardia), or by sexual contact (Trichomonas) invade various organs in the body and cause prominent human diseases, such as malaria, babesiosis, leishmaniasis, trypanosomiasis, diarrhea, and trichomoniasis. Humans are frequently exposed to multiple pathogens simultaneously, or sequentially in the high-incidence regions to result in co-infections. Consequently, synergistic or antagonistic pathogenic effects could occur between microbes that also influences overall host responses and severity of diseases. The co-infecting organisms can also follow independent trajectory. In either case, co-infections change host and pathogen metabolic microenvironments, compromise the host immune status, and affect microbial pathogenicity to influence tissue colonization. Immunomodulation by protozoa often adversely affects cellular and humoral immune responses against co-infecting bacterial pathogens and promotes bacterial persistence, and result in more severe disease symptoms. Although co-infections by protozoa and viruses also occur in humans, extensive studies are not yet conducted probably because of limited animal model systems available that can be used for both groups of pathogens. Immunosuppressive effects of protozoan infections can also attenuate vaccines efficacy, weaken immunological memory development, and thus attenuate protection against co-infecting pathogens. Due to increasing occurrence of parasitic infections, roles of acute to chronic protozoan infection on immunological changes need extensive investigations to improve understanding of the mechanistic details of specific immune responses alteration. In fact, this phenomenon should be seriously considered as one cause of breakthrough infections after vaccination against both bacterial and viral pathogens, and for the emergence of drug-resistant bacterial strains. Such studies would facilitate development and implementation of effective vaccination and treatment regimens to prevent or significantly reduce breakthrough infections.

Salivarian Trypanosomes Have Adopted Intricate Host-Pathogen Interaction Mechanisms That Ensure Survival in Plain Sight of the Adaptive Immune System

Salivarian trypanosomes are extracellular parasites affecting humans, livestock and game animals. Trypanosoma brucei rhodesiense and Trypanosoma brucei gambiense are human infective sub-species of T. brucei causing human African trypanosomiasis (HAT—sleeping sickness). The related T. b. brucei parasite lacks the resistance to survive in human serum, and only inflicts animal infections. Animal trypanosomiasis (AT) is not restricted to Africa, but is present on all continents. T. congolense and T. vivax are the most widespread pathogenic trypanosomes in sub-Saharan Africa. Through mechanical transmission, T. vivax has also been introduced into South America. T. evansi is a unique animal trypanosome that is found in vast territories around the world and can cause atypical human trypanosomiasis (aHT). All salivarian trypanosomes are well adapted to survival inside the host’s immune system. This is not a hostile environment for these parasites, but the place where they thrive. Here we provide an overview of the latest insights into the host-parasite interaction and the unique survival strategies that allow trypanosomes to outsmart the immune system. In addition, we review new developments in treatment and diagnosis as well as the issues that have hampered the development of field-applicable anti-trypanosome vaccines for the implementation of sustainable disease control.

To the Skin and Beyond: The Immune Response to African Trypanosomes as They Enter and Exit the Vertebrate Host

African trypanosomes are single-celled extracellular protozoan parasites transmitted by tsetse fly vectors across sub-Saharan Africa, causing serious disease in both humans and animals. Mammalian infections begin when the tsetse fly penetrates the skin in order to take a blood meal, depositing trypanosomes into the dermal layer. Similarly, onward transmission occurs when differentiated and insect pre-adapted forms are ingested by the fly during a blood meal. Between these transmission steps, trypanosomes access the systemic circulation of the vertebrate host via the skin-draining lymph nodes, disseminating into multiple tissues and organs, and establishing chronic, and long-lasting infections. However, most studies of the immunobiology of African trypanosomes have been conducted under experimental conditions that bypass the skin as a route for systemic dissemination (typically via intraperitoneal or intravenous routes). Therefore, the importance of these initial interactions between trypanosomes and the skin at the site of initial infection, and the implications for these processes in infection establishment, have largely been overlooked. Recent studies have also demonstrated active and complex interactions between the mammalian host and trypanosomes in the skin during initial infection and revealed the skin as an overlooked anatomical reservoir for transmission. This highlights the importance of this organ when investigating the biology of trypanosome infections and the associated immune responses at the initial site of infection. Here, we review the mechanisms involved in establishing African trypanosome infections and potential of the skin as a reservoir, the role of innate immune cells in the skin during initial infection, and the subsequent immune interactions as the parasites migrate from the skin. We suggest that a thorough identification of the mechanisms involved in establishing African trypanosome infections in the skin and their progression through the host is essential for the development of novel approaches to interrupt disease transmission and control these important diseases.

Salivarian Trypanosomosis: A Review of Parasites Involved, Their Global Distribution and Their Interaction With the Innate and Adaptive Mammalian Host Immune System

Salivarian trypanosomes are single cell extracellular parasites that cause infections in a wide range of hosts. Most pathogenic infections worldwide are caused by one of four major species of trypanosomes including (i) Trypanosoma brucei and the human infective subspecies T. b. gambiense and T. b. rhodesiense, (ii) Trypanosoma evansi and T. equiperdum, (iii) Trypanosoma congolense and (iv) Trypanosoma vivax. Infections with these parasites are marked by excessive immune dysfunction and immunopathology, both related to prolonged inflammatory host immune responses. Here we review the classification and global distribution of these parasites, highlight the adaptation of human infective trypanosomes that allow them to survive innate defense molecules unique to man, gorilla, and baboon serum and refer to the discovery of sexual reproduction of trypanosomes in the tsetse vector. With respect to the immunology of mammalian host-parasite interactions, the review highlights recent findings with respect to the B cell destruction capacity of trypanosomes and the role of T cells in the governance of infection control. Understanding infection-associated dysfunction and regulation of both these immune compartments is crucial to explain the continued failures of anti-trypanosome vaccine developments as well as the lack of any field-applicable vaccine based anti-trypanosomosis intervention strategy. Finally, the link between infection-associated inflammation and trypanosomosis induced anemia is covered in the context of both livestock and human infections.

Prospects for vaccination against pathogenic African trypanosomes

African trypanosomes cause human and animal African trypanosomiases, which are chronic, debilitating and often fatal diseases of people and livestock in sub-Saharan Africa. The extracellular protozoan parasites are exemplars of antigenic variation. They direct host-protective B-cell and T-cell immune responses towards hypervariable components of their variable surface glycoprotein coat and evade immune elimination by generating new surface coat antigenic variants at a rate that supersedes immune destruction. This results in recurring waves of parasitemia, tissue invasion and escalating immunopathology in trypanosomiasis-susceptible hosts. Here, we discuss the possibility that host control of African trypanosomes might be improved by immunization with conserved VSG peptides and invariant surface glycoproteins. Infection-induced T-cell recall responses to these typically poorly expressed or nonimmunogenic parasite components induce tissue phagocytes to produce microbicidal materials that kill trypanosomes. Preliminary data that support this immune-enhancing vaccine strategy are discussed, as are host and parasite interactions that might downregulate the protective responses. These include infection-induced immunosuppression and increasing virulence of infecting parasites over time.

African Trypanosomes Undermine Humoral Responses and Vaccine Development: Link with Inflammatory Responses?

African trypanosomosis is a debilitating disease of great medical and socioeconomical importance. It is caused by strictly extracellular protozoan parasites capable of infecting all vertebrate classes including human, livestock, and game animals. To survive within their mammalian host, trypanosomes have evolved efficient immune escape mechanisms and manipulate the entire host immune response, including the humoral response. This report provides an overview of how trypanosomes initially trigger and subsequently undermine the development of an effective host antibody response. Indeed, results available to date obtained in both natural and experimental infection models show that trypanosomes impair homeostatic B-cell lymphopoiesis, B-cell maturation and survival and B-cell memory development. Data on B-cell dysfunctioning in correlation with parasite virulence and trypanosome-mediated inflammation will be discussed, as well as the impact of trypanosomosis on heterologous vaccine efficacy and diagnosis. Therefore, new strategies aiming at enhancing vaccination efficacy could benefit from a combination of (i) early parasite diagnosis, (ii) anti-trypanosome (drugs) treatment, and (iii) anti-inflammatory treatment that collectively might allow B-cell recovery and improve vaccination.

Low-dose intradermal infection with trypanosoma congolense leads to expansion of regulatory T cells and enhanced susceptibility to reinfection

BALB/c mice are highly susceptible to experimental intraperitoneal Trypanosoma congolense infection. However, a recent report showed that these mice are relatively resistant to primary intradermal low-dose infection. Paradoxically, repeated low-dose intradermal infections predispose mice to enhanced susceptibility to an otherwise noninfectious dose challenge. Here, we explored the mechanisms responsible for this low-dose-induced susceptibility to subsequent low-dose challenge infection. We found that akin to intraperitoneal infection, low-dose intradermal infection led to production of interleukin-10 (IL-10), IL-6, IL-12, tumor necrosis factor alpha (TNF-α), transforming growth factor β (TGF-β), and gamma interferon (IFN-γ) by spleen and draining lymph node cells. Interestingly, despite the absence of parasitemia, low-dose intradermal infection led to expansion of CD4+ CD25+ Foxp3+ cells (T regulatory cells [Tregs]) in both the spleens and lymph nodes draining the infection site. Depletion of Tregs by anti-CD25 monoclonal antibody (MAb) treatment during primary infection or before challenge infection following repeated low-dose infection completely abolished the low-dose-induced enhanced susceptibility. In addition, Treg depletion was associated with dramatic reduction in serum levels of TGF-β and IL-10. Collectively, these findings show that low-dose intradermal infection leads to rapid expansion of Tregs, and these cells mediate enhanced susceptibility to subsequent infection.

Secondary bacterial infection in plasma endotoxin levels and the acute-phase response of mice infected with Trypanosoma brucei brucei

Murine Trypanosoma brucei brucei infection leads to elevated plasma endotoxin-like activity levels not related to parasitaemia levels accompanied by the development of acute-phase response and increased plasma levels of serum amyloid P (SAP) and haptoglobin (Hp). To determine the source of the endotoxin-like activity and role of secondary bacterial infection in the pathogenesis of trypanosomosis, infected mice were treated with the antibiotic ciprofloxacin. Plasma endotoxin-like activity levels, irrespective of treatment, were elevated three- to fourfold, beginning 7 days after infection. Plasma protein concentrations increased markedly following infection from 7 days after infection (DAI). Peak Hp and SAP concentrations in ciprofloxacin-treated and -untreated infected mice were attained 7 and 14 DAI, respectively. Thereafter, both protein levels gradually declined until the end of the experiment, but Hp levels for non-treated mice declined up to 21 DAI and thereafter significantly increased on 28 and 35 DAI. Whole-trypanosome lysate and the membrane-enriched fraction demonstrated endotoxin-like activity, with the former having higher levels. The results suggest that the endotoxin-like activity in trypanosome fractions and plasma of infected mice is due to the trypanosome. Further elevation of haptoglobin during the late stages of infection in non-treated mice suggests the involvement of secondary bacterial infection.

Acute phase response in mice experimentally infected with Trypanosoma congolense: a molecular gauge of parasite-host interaction

Mice infected with Trypanosoma congolense developed a severe anaemia 1 week after infection, which persisted till treatment with diminazine aceturate when the packed cell volume (PCV) recovered to pre-infection levels. This was accompanied by a marked increase in the plasma levels of the acute phase proteins (APP), serum amyloid P-component (SAP) and haptoglobin (Hp). The initial peak levels of Hp and SAP were attained 7 and 12 days post-infection (DPI), respectively. Thereafter SAP levels decreased significantly to near pre-infection levels, but later increased even after treatment to give a second peak 34 DPI after which there was a decline till the study was terminated. The Hp levels on the other hand decreased to an intermediate level after the initial peak increasing to a second peak 22 DPI. Thereafter Hp decreased significantly following diminazine aceturate treatment to reach pre-infection levels within 5 days post-treatment. This indicates that T. congolense-infected mice develop severe anaemia accompanied by an acute phase response leading to an increase in SAP and Hp but that following treatment divergent responses occurred indicating differences in the pathways for stimulation of the APP. Haptoglobin was shown to be an earlier indicator of infection and a better marker in monitoring the response to treatment.

Cytokine mRNA profiling of peripheral blood mononuclear cells from trypanotolerant and trypanosusceptible cattle infected with Trypanosoma congolense

To examine differences in cytokine profiles that may confer tolerance/susceptibility to bovine African trypanosomiasis, N'Dama (trypanotolerant, n = 8) and Boran (trypanosusceptible, n = 8) cattle were experimentally challenged with Trypanosoma congolense. Blood samples were collected over a 34-day period, and RNA was extracted from peripheral blood mononuclear cells. The expression levels of a panel of 14 cytokines were profiled over the time course of infection and between breeds. Messenger RNA (mRNA) transcript levels for the IL2, IL8, and IL1RN genes were significantly downregulated across the time course of infection in both breeds. There was an early increase in transcripts for genes encoding proinflammatory mediators (IFNG, IL1A, TNF, and IL12) in N'Dama by 14 days postinfection (dpi) compared with preinfection levels that was not detected in the susceptible Boran breed. By the time of peak parasitemia, a type 2 helper T cells (T(H)2)-like cytokine environment was prevalent that was particularly evident in the Boran. Increases in transcripts for the IL6 (29 and 34 dpi) and IL10 (21, 25, and 29 dpi) genes were detected that were higher in the Boran compared with N'Dama. These findings highlight the implications for using murine models to study the bovine immune response to trypanosomiasis, where in some cases cytokine expression patterns differ. Overall, these data suggest that the trypanotolerant N'Dama are more capable of responding very early in infection with proinflammatory and T(H)1 type cytokines than the trypanosusceptible Boran and may explain why N'Dama control parasitemia more efficiently than Boran during the early stages of infection.

Comparison of cytokine plasma levels in human African trypanosomiasis

BACKGROUND

Immunological studies suggest that human African trypanosomiasis (HAT) is associated with inflammatory responses. A better understanding of the complex cytokine interactions regulating HAT infections is essential to elucidate the mechanisms of generalized immunosuppression.

METHOD

We determined levels of interleukin (IL)-2, IL-4, IL-6, IL-10, tumour necrosis factor (TNF)-alpha and interferon (IFN)-gamma protein levels in plasma samples from three groups of individuals from the Democratic Republic of Congo: (i) HAT cases; (ii) seropositive individuals for whom parasite detection was negative and (ii) controls.

RESULTS

Plasma levels of six cytokines were significantly higher in HAT cases than in both controls (P<0.003) and seropositive individuals (P<0.016). IL-2 and IL-10 concentrations were significantly lower (P<0.02) in the seropositive group than in the control one.

CONCLUSION

Human African trypanosomiasis leads to the development of strong cytokine responses, indicating the potential involvement of IL-2 and IL-10 in the phenomenon of seropositivity without parasitological confirmation. This strongly suggests the involvement of immunity in this particular aspect of HAT epidemiology.

The effect of Trypanosoma evansi infection on pig performance and vaccination against classical swine fever

Although Trypanosoma evansi is not considered as an important pathogen in pigs, it may interfere with other pathogens or vaccinations by its immunosuppressive nature. In order to determine whether T. evansi alters pig performance and induces immunosuppression in pigs, induction of immune responses by vaccination against classical swine fever (CSF) and by immunization with a control antigen, human serum albumin (HSA), was assessed in T. evansi-infected and non-infected animals. Although T. evansi infection did not have a significant influence on growth performance, feed conversion or PCV, antibody responses against both the test antigen HSA and the CSF vaccine were significantly reduced in T. evansi-infected animals as compared to uninfected animals. Moreover, the reduced response against the CSF vaccine appears to be accompanied by a less well-developed protection against CSF with higher fever responses and leukopenia. This immunosuppression might explain the accounts of poor protection of CSF-vaccinated pigs reported in T. evansi-endemic areas of Vietnam, and suggests that prior treatments with trypanocidal drugs to improve the efficacy of CSF vaccination, may be justified.

Antigen-presenting cell function during Plasmodium yoelii infection

Antigen-presenting cells (APC) play a key role in orchestrating immune responses. T-cell proliferative responses are inhibited during the erythrocyte stages of malaria infection, and a number of studies have suggested that APC are responsible for this phenomenon. In the present studies we examine individual components of the T-cell-activating function of APC: expression of costimulatory and major histocompatibility complex (MHC) class II proteins, the ability to process and present antigen to T cells, and the ability to support cytokine production. We find that during the acute phases of Plasmodium yoelii erythrocyte stage infection, APC upregulate the expression of class II MHC and CD80, maintain expression of CD86, process and present antigen, and support gamma interferon production. However the CD11b(+) subpopulation produces a soluble factor or factors that specifically inhibit interleukin-2 (IL-2) production by responding CD4 T cells. This factor is distinct from prostaglandin E(2), NO, or transforming growth factor beta. The data suggest that IL-2 suppression observed during malaria infection is not due to functional defects of APC but is triggered by production of a factor(s) that actively suppresses production of IL-2 by T cells.

The influence of T. evansi infection on the immuno-responsiveness of experimentally infected water buffaloes

In order to define the immuno-suppressive capacity of Trypanosoma evansi infections in buffaloes on the induction of immune responses against heterologous antigens, infected and non-infected buffaloes were vaccinated against Pasteurella multocida (haemorrhagic septicemia) and were simultaneously immunised with a control antigen, human serum albumin (HSA). Antibody responses against HSA were significantly reduced in T. evansi-infected animals, but no conclusive data were obtained on the antibody responses against P. multocida. Conversely, the local inflammatory response at the site of Pasteurella vaccination, as measured by increase in size, was significantly reduced in T. evansi-infected animals. These results indicate that the inductive capacity to mount humoral and cell-mediated immune responses against heterologous antigens is suppressed in T. evansi-infected animals. Consequently, T. evansi infection might interfere with the development of protective immunity upon heterologous vaccinations and could explain the poor protection of Pasteurella-vaccinated buffaloes in T. evansi-endemic areas of Vietnam.

Susceptibility and resistance to Trypanosoma congolense infections

We have put emphasis on recent findings in experimental Trypanosoma congolense infections in highly susceptible BALB/c and relatively resistant C57Bl/6 mice. Based on various analyses, it has been shown that a major difference in resistance to T. congolense infections is expressed early in infection at the macrophage level. A novel plastic-adherent Thy1.2(+) suppressor lymphocyte, which in absolute synergy with a Thy 1.2(-) cell exerts its suppression via interleukin-10 and interferon-gamma opens up an exciting new field of research.

T-Cell responses during Trypanosoma brucei infections in mice deficient in inducible nitric oxide synthase

We have investigated the possibility that nitric oxide (NO) synthesis may affect the course of a trypanosome infection via T-cell responses using mice deficient in inducible NO synthase (iNOS). Parasitemia levels increased at the same rate in both iNOS-deficient homozygous and control heterozygous mice, and peak parasitemia values were the same in both groups. However, the heterozygous mice maintained higher parasitemia levels after the peak of an infection than the homozygous mice due to a decrease in the rate of clearance of parasites. In iNOS-deficient mice there was an increase in the numbers of total CD4(+) cells and activated (interleukin-2 receptor-expressing) CD4(+) cells in infected mice compared with the numbers in uninfected mice. Spleen cells from infected iNOS-deficient mice displayed increased proliferative responses and gamma interferon secretion when stimulated in vitro than those of control mice. These data suggest that NO production depresses T-helper 1-like responses generated during Trypanosoma brucei infections, thus promoting the survival of the parasite.

Hypodermin A and Inhibition of Lymphocyte Proliferation

Infection with Hypoderma lineatum is accompanied by modulation of the host's immune system. Here, Nathalie Moiré describes some in vitro studies examining how the parasitic enzyme hypodermin A interferes with lymphocyte proliferation.

Immunoregulation in experimental murine Trypanosoma congolense infection: anti-IL-10 antibodies reverse trypanosome-mediated suppression of lymphocyte proliferation in vitro and moderately prolong the lifespan of genetically susceptible BALB/c mice

We infected highly susceptible BALB/c and relatively resistant C57BL/6 mice with cloned Trypanosoma congolense and followed the effects of these infections on the circulating parasite numbers, mouse mortality and cytokine expression. C57BL/6 mice controlled their parasitaemia and survived for up to 163 +/- 12 days, while BALB/c mice could not control their parasitaemia and succumbed to the infection within 8.4 +/- 0.5 days. Susceptible BALB/c mice had dramatically higher plasma levels of IL-10 than the resistant C57BL/6 mice from day 7 forward. This was preceded by an earlier and higher level induction of splenic IL-10 messenger RNA (mRNA) expression in the infected BALB/c mice. There was a strong negative correlation between the splenocyte proliferative responses to Concanavalin-A (Con-A) and their production of IL-10 in these infected BALB/c mice. Co-treatment of the Con-A-stimulated spleen cell cultures with monoclonal anti-IL-10 antibodies, but not isotype-matched control antibodies, could completely reverse this suppression of the splenocyte proliferative response. Finally, in three experiments, anti-IL-10 antibody treatment in vivo reduced the peak circulating parasitaemia of infected BALB/c mice by 43% and increased their median survival periods by 38% relative to isotype-matched control antibody-treated mice.

CD5+ B lymphocytes are the main source of antibodies reactive with non-parasite antigens in Trypanosoma congolense-infected cattle

Mice infected with African trypanosomes produce exceptionally large amounts of serum IgM, a major part of which binds to non-trypanosome antigens such as trinitrophenol and single-strand DNA. In this paper, we describe that in cattle infected with Trypanosoma congolense and T. vivax, similar antibodies are found, although they bind mainly to protein antigens, such as beta-galactosidase, ovalbumin and ferritin. The parasite non-specific IgM antibodies appear around the same time as the parasite-specific antibodies, but their origin and function are not clear. We tested the hypothesis that CD5+ B cells (or B-1 cells), which increase during trypanosome infections in cattle, are responsible for production of antibodies to non-trypanosome antigens. Splenic CD5+ and CD5- B cells from infected cattle were sorted and tested in a single cell blot assay. The numbers of immunoglobulin-secreting cells were similar in both B-cell populations. However, antibodies with reactivity for non-trypanosome antigens were significantly more prevalent in the CD5+ B-cell fraction and were exclusively IgM. The preference for production of these antibodies by CD5+ B cells and the expansion of this subpopulation during infections in cattle, strongly suggest that CD5+ B cells are the main source of trypanosome non-specific antibodies. We propose that these antibodies are natural, polyreactive antibodies that are predominantly secreted by CD5+ B cells. Since B-1 cells are up-regulated in many states of immune insufficiency, the immunosuppression associated with trypanosome infections may be responsible for the increase of this subset and the concomitant increase in trypanosome non-specific antibodies.

Nitric oxide synthesis is depressed in Bos indicus cattle infected with Trypanosoma congolense and Trypanosoma vivax and does not mediate T-cell suppression

Infection with African trypanosomes causes the diseases sleeping sickness in humans and nagana in cattle in sub-Saharan Africa. Suppression of cellular immune responses is a feature of trypanosomiasis in bovine, human, and murine hosts. Some aspects of immunosuppression in the murine model are mediated by nitric oxide (NO) produced by gamma interferon (IFN-gamma)-activated macrophages. We have investigated whether a similar mechanism is responsible for T-cell unresponsiveness in bovine trypanosomiasis. Bovine monocytes and macrophages from uninfected cattle and activated in vitro with IFN-gamma produced NO; however, this response was down-regulated in infected cattle. Similarly, the expression of inducible NO synthase messenger RNA was depressed in macrophages of infected cattle. Proliferation of mononuclear cells of trypanosome-infected cattle cultured with mitogen or trypanosome antigens was unchanged by the addition of an NO synthase inhibitor. Lymphocytes of infected cattle secreted interleukins with T-cell growth factor activity after in vitro activation with mitogens but not after activation with trypanosome antigens. Although lymph node cells secreted IFN-gamma after in vitro activation, ex vivo expression of mRNA was depressed. In contrast, the level of expression of interleukin 10 mRNA was higher during infection. We conclude that NO is not involved in the loss of T-cell proliferative function associated with trypanosomiasis in cattle and that, in contrast to the mouse model, the capacity of monocytes and macrophages to produce NO is actually down-regulated in infected cattle.

Changes in T-cell subpopulations in mice during prolonged experimental secondary infection with Echinococcus granulosus

Balb/c mice were infected intraperitoneally with protoscoleces of Echinococcus granulosus. After 15 months of infection, and by means of flow cytometry, the expression of T-cell markers CD3, CD4, and CD8 on T cells from peripheral blood, spleen, and thymus was analyzed and compared with that of age-matched controls. Infected mice had higher percentages of CD3+, and CD4+ cells in peripheral blood, and higher percentages of CD8+ cells in the spleen, when compared with control mice. CD4+ and CD8+ cells in peripheral blood and CD8+ cells in thymus also showed higher percentages of expression of interleukin-2 receptor. The results infer a role for interleukin-2 in experimental secondary echinococcosis.

T cells and macrophages in Trypanosoma brucei-related glomerulopathy

In a previous study, susceptibility for Trypanosoma brucei-related glomerulopathy in mice was shown to be dependent on non-major histocompatibility complex genes. Glomerular disease in this model could not be explained by the production of autoantibodies alone. In order to analyze which part of the defense system, in addition to the B-cell compartment, is involved in the development of this infection-related glomerular disease, groups of athymic (BALB/c rnu/rnu), splenectomized, or macrophage-depleted BALB/c mice were inoculated with T. brucei parasites. Polyclonal B-cell activation, invariably observed in infected BALB/c mice, was absent in BALB/c rnu/rnu mice. Glomerular disease in athymic mice, however, as defined by albuminuria and deposition of immune complexes, was not different from that seen in euthymic infected BALB/c mice. Splenectomy prior to inoculation of parasites led to a decreased incidence of albuminuria in 40% of the animals, whereas splenectomy 21 days after inoculation reduced albuminuria significantly, suggesting a role for spleen cells in the induction of glomerular disease. After macrophage depletion with liposome-encapsulated dichlorodimethylene-diphosphonate, infected BALB/c mice developed significantly higher albuminuria levels for a period up to 2 weeks after depletion. Therefore, it was concluded that the development of T. brucei-related glomerular disease is independent of thymus-matured T cells, while the involvement of macrophages in the development of proteinuria is inhibitory rather than disease inducing. Spleen cells other than thymus-dependent T cells, B cells, and macrophages should be investigated for their role in the pathogenesis of this glomerulopathy.

Modulation of the phenotype and function of bovine afferent lymph cells during infection with Trypanosoma congolense

Alterations in the phenotype and function of cells isolated from bovine afferent lymph were studied following tsetse-transmitted Trypanosoma congolense infection. Little alteration was observed in the output of the CD2+ T cells in the lymph, and within this population the CD4:CD8 ratio remained relatively constant. By contrast, a marked decrease was observed in the output of gamma delta T cells over the first 7 days following infection. The number of B cells increased between 2 and 6 days post-infection, and thereafter returned to pre-infection values. Little change was observed within the afferent lymph veiled cell population. Examination of activation markers on the lymphocyte fraction of afferent lymph revealed a decrease in the number of cells expressing the Interleukin-2 receptor alpha-chain from Day 5 post-infection. At this time the expression of ACT 1, another early activation marker, was seen to increase. Afferent lymph cells collected pre-infection and on the first 4 days post-infection proliferated in response to stimulation with Concanavalin A in vitro. This response to mitogenic stimulation was completely abrogated from day five post-infection. However, these cells were not capable of suppressing the capacity of normal peripheral blood mononuclear cells to respond to mitogenic stimulus in co-culture assays. These studies suggest that although a degree of lymphocyte activation occurs in the afferent lymph following tsetse-transmitted infection with T. congolense, this may be sub-optimal owing to the immunosuppression which appears to operate at the level of the skin and the lymph nodes.

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.

Immunosuppression in trypanotolerant N'Dama cattle following Trypanosoma congolense infection

Tsetse-transmitted Trypanosoma congolense infection causes an impairment of in vitro T cell proliferative responses in Boran (Bos indicus) cattle. To assess the importance of this phenomenon as it may relate to the ability of trypanotolerant cattle to control infection with trypanosomes, T cell proliferative responses to mitogenic stimulus with Concanavalin A were measured in N'Dama (Bos taurus) cattle throughout infection. The responses of peripheral blood mononuclear cells from Boran and N'Dama cattle were similar. Depressed proliferative responses were observed with cells of both breeds at 12 days post infection, after which the responses returned to levels similar to those recorded pre-infection. Immunosuppression was also studied in the lymph nodes of a major histocompatibility complex (MHC)-matched pair of N'Dama cattle. Lymph node cells from the infected animal failed to respond to mitogenic stimulus. Co-culture experiments in which the cells from this node were mixed with either lymph node cells or peripheral blood mononuclear cells from the non-infected MHC-compatible animal revealed the presence of suppressor cells, acting in a prostaglandin-independent manner, capable of arresting mitogen-induced T cell proliferation.

Secretion of co-stimulatory cytokines by monocytes and macrophages during infection with Trypanosoma (Nannomonas) congolense in susceptible and tolerant cattle

Bovine macrophages and monocytes were cultured in vitro and analyzed for their capacity to secrete co-stimulatory cytokines. To this end, the culture medium was titrated on suboptimally stimulated murine thymocytes. A low residual release by normal monocytes was noted which usually remained below the detection limit of the assay. These cells could be induced to secrete high titres following activation with bacterial lipopolysaccharide. When harvested from animals infected with Trypanosoma congolense, the cells released high titres spontaneously. This increase in co-stimulatory cytokine secretion was noted in both peripheral blood monocytes and splenic macrophages and was amplified by addition of indomethacin. The activation was transient, and the titres had dropped to pre-infection values at the end of the experiment. At that time, the monocytes were, however, still able to respond to external stimuli. Addition of neutralizing anti-transforming growth factor beta antibodies did not influence the thymocyte co-stimulatory activity of the supernatants. High levels of co-stimulatory cytokine secretion were noted with monocytes from both the susceptible Boran breed and the tolerant N'Dama breed. Early in infection, at Day 10 post infection, the production by the N'Dama monocytes was 16 times higher than the production by the Boran monocytes. Later in the infection, the titres were similar in both breeds.

Suppression of T-Cell Responsiveness during Tsetse-Transmitted Trypanosomiasis in Cattle

In the present study, we demonstrate that lymph node cells from cattle infected with T. congolense through tsetse fly challenge were unable to proliferate in vitro following activation with the T-cell mitogen Concanavalin A. This was associated with a simultaneous suppression of interleukin 2 (IL-2) production and interleukin 2 receptor (IL-2R) expression. However, the capacity of the cells to secrete interferon gamma following the mitogenic activation was not affected by the infection.