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

Trypanosoma brucei brucei causes a rapid and persistent influx of neutrophils in the spleen of infected mice

Trypanosomosis is a chronic parasitic infection, affecting both humans and livestock. A common hallmark of experimental murine infections is the occurrence of inflammation and the associated remodelling of the spleen compartment. The latter involves the depletion of several lymphocyte populations, the induction of T‐cell‐mediated immune suppression, and the activation of monocyte/macrophage cell populations. Here, we show that in experimental T b brucei infections in mice, these changes are accompanied by the alteration of the spleen neutrophil compartment. Indeed, mature neutrophils are rapidly recruited to the spleen, and cell numbers remain elevated during the entire infection. Following the second peak of parasitemia, the neutrophil cell influx coincides with the rapid reduction of splenic marginal zone (MZ)B and follicular (Fo)B cells, as well as CD8⁺ T and NK1.1⁺ cells, the latter encompassing both natural killer (NK) and natural killer T (NKT) cells. This report is the first to show a comprehensive overview of all alterations in spleen cell populations, measured with short intervals throughout the entire course of an experimental T b brucei infection. These data provide new insights into the dynamic interlinked changes in spleen cell numbers associated with trypanosomosis‐associated immunopathology.

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.

Nitric Oxide Protects against Infection-Induced Neuroinflammation by Preserving the Stability of the Blood-Brain Barrier

Nitric oxide (NO) generated by inducible NO synthase (iNOS) is critical for defense against intracellular pathogens but may mediate inflammatory tissue damage. To elucidate the role of iNOS in neuroinflammation, infections with encephalitogenic Trypanosoma brucei parasites were compared in inos^-/- and wild-type mice. Inos^-/- mice showed enhanced brain invasion by parasites and T cells, and elevated protein permeability of cerebral vessels, but similar parasitemia levels. Trypanosome infection stimulated T cell- and TNF-mediated iNOS expression in perivascular macrophages. NO nitrosylated and inactivated pro-inflammatory molecules such as NF-κΒp65, and reduced TNF expression and signalling. iNOS-derived NO hampered both TNF- and T cell-mediated parasite brain invasion. In inos^-/- mice, TNF stimulated MMP, including MMP9 activity that increased cerebral vessel permeability. Thus, iNOS-generated NO by perivascular macrophages, strategically located at sites of leukocyte brain penetration, can serve as a negative feed-back regulator that prevents unlimited influx of inflammatory cells by restoring the integrity of the blood-brain barrier.

Inflammatory responses can lead to harmful effects on the brain during many chronic parasitic infections, including those with African trypanosomes. T. brucei, the causative agent of African trypanosomiasis, that traverse the blood-brain barrier (BBB) to invade the brain, where, together with inflammatory infiltrates, they likely contribute to the neurologic disturbances of the disease. High levels of nitric oxide (NO) released by the inducible NO synthase (iNOS) are critical for defense against parasites, but also mediate inflammatory tissue damage. Using a mouse model of African trypanosomiasis, we uncovered an unexpected role of NO, preserving the integrity of the BBB and limiting the neuroinvasion of leukocytes and parasites, rather than mediating brain damage or killing of trypanosomes. iNOS-derived NO, nitrosylates molecules such as pro-inflammatory transcription factors. iNOS hampered both TNF- and T cell-mediated parasite and leukocyte brain invasion and passage of serum proteins across the BBB. In inos^-/- mice, exacerbated TNF secretion and signalling increased MMP9 activity that mediates cerebral vascular permeability. Thus, NO is crucial for maintenance of the integrity of the cerebral vessels and serves as a feed-back regulator by inhibiting leukocyte brain penetration during T. brucei infection. Consequently, therapies could target iNOS to reduce tissue damage during neuroinflammation.

Expression of regulatory dendritic cell-related cytokines in cattle experimentally infected with Trypanosoma evansi

Trypanosoma evansi causes wasting disease in many livestock. T. evansi infection gives rise to inflammatory immune responses, which contribute to the development of inflammation-associated tissue injury. We previously reported that regulatory dendritic cells (DCs), which act as potential regulators of inflammation, were activated in infected mice and transfer of regulatory DCs to infected mice prolonged their survival. However, the kinetics of regulatory DCs in cattle, which are natural hosts of T. evansi, remained unclear. In this study, we report that the expressions of CCL8 and IL-10, which promote the development of regulatory DCs, were up-regulated in cattle experimentally infected with T. evansi. This finding is potentially useful for studying the control strategy of T. evansi infection in cattle.

TNF-α and CD8+ T cells mediate the beneficial effects of nitric oxide synthase-2 deficiency in pulmonary paracoccidioidomycosis

BACKGROUND

Nitric oxide (NO), a key antimicrobial molecule, was previously shown to exert a dual role in paracoccidioidomycosis, an endemic fungal infection in Latin America. In the intravenous and peritoneal models of infection, NO production was associated with efficient fungal clearance but also with non-organized granulomatous lesions. Because paracoccidioidomycosis is a pulmonary infection, we aimed to characterize the role of NO in a pulmonary model of infection.

METHODOLOGY/PRINCIPAL FINDINGS

C57Bl/6 wild type (WT) and iNOS(-/-) mice were i.t. infected with 1×10(6) Paracoccidioides brasiliensis yeasts and studied at several post-infection periods. Unexpectedly, at week 2 of infection, iNOS(-/-) mice showed decreased pulmonary fungal burdens associated with an M2-like macrophage profile, which expressed high levels of TGF-β impaired ability of ingesting fungal cells. This early decreased fungal loads were concomitant with increased DTH reactions, enhanced TNF-α synthesis and intense migration of activated macrophages, CD4(+) and CD8(+) T cells into the lungs. By week 10, iNOS(-/-) mice showed increased fungal burdens circumscribed, however, by compact granulomas containing elevated numbers of activated CD4(+) T cells. Importantly, the enhanced immunological reactivity of iNOS(-/-) mice resulted in decreased mortality rates. In both mouse strains, depletion of TNF-α led to non-organized lesions and excessive influx of inflammatory cells into the lungs, but only the iNOS(-/-) mice showed increased mortality rates. In addition, depletion of CD8(+) cells abolished the increased migration of inflammatory cells and decreased the number of TNF-α and IFN-γ CD4(+) and CD8(+) T cells into the lungs of iNOS(-/-) mice.

CONCLUSIONS/SIGNIFICANCE

Our study demonstrated that NO plays a deleterious role in pulmonary paracoccidioidomycosis due to its suppressive action on TNF-α production, T cell immunity and organization of lesions resulting in precocious mortality of mice. It was also revealed that uncontrolled fungal growth can be overcome by an efficient immune response.

Different regulatory mechanisms in protozoan parasitic infections

The immune response to the protozoan pathogens, Leishmania spp., Trypanosoma spp. and Plasmodium spp., has been studied extensively with particular focus on regulation of the immune response by immunological mechanisms. More specifically, in diseases caused by parasites, immunosuppression frequently prevents immunopathology that can injure the host. However, this allows a small number of parasites to evade the immune response and remain in the host after a clinical cure. The consequences can be chronic infections, which establish a zoonotic or anthroponotic reservoir. This review will highlight some of the identified regulatory mechanisms of the immune system that govern immune responses to parasitic diseases, in particular leishmaniasis, trypanosomiasis and malaria, and discuss implications for the development of efficient vaccines against these diseases.

Nitric oxide inhibits the accumulation of CD4+CD44hiTbet+CD69lo T cells in mycobacterial infection

Animals lacking the inducible nitric oxide synthase gene (nos2(-/-)) are less susceptible to Mycobacterium avium strain 25291 and lack nitric oxide-mediated immunomodulation of CD4(+) T cells. Here we show that the absence of nos2 results in increased accumulation of neutrophils and both CD4(+) and CD8(+) T cells within the M. avium containing granuloma. Examination of the T-cell phenotype in M. avium infected mice demonstrated that CD4(+)CD44(hi) effector T cells expressing the Th1 transcriptional regulator T-bet (T-bet(+)) were specifically reduced by the presence of nitric oxide. Importantly, the T-bet(+) effector population could be separated into CD69(hi) and CD69(lo) populations, with the CD69(lo) population only able to accumulate during chronic infection within infected nos2(-/-) mice. Transcriptomic comparison between CD4(+)CD44(hi)CD69(hi) and CD4(+)CD44(hi)CD69(lo) populations revealed that CD4(+)CD44(hi)CD69(lo) cells had higher expression of the integrin itgb1/itga4 (VLA-4, CD49d/CD29). Inhibition of Nos2 activity allowed increased accumulation of the CD4(+) CD44(hi)T-bet(+)CD69(lo) population in WT mice as well as increased expression of VLA-4. These data support the hypothesis that effector T cells in mycobacterial granulomata are not a uniform effector population but exist in distinct subsets with differential susceptibility to the regulatory effects of nitric oxide.

Nitric oxide synthase activity has limited influence on the control of Coccidioides infection in mice

The functions of inducible nitric oxide synthase (iNOS) activity in protection against microbial insults are still controversial. In this study, we explored the role of iNOS in protection against Coccidioides infection in mice. We observed that wild type (WT) and iNOS(-/-) mice showed similar percent survival and fungal burden in their lungs at days 7 and 11 after intranasal challenge with Coccidioides. Vaccinated WT and iNOS(-/-) mice revealed comparable fungal burden in their lungs and spleen at 7 and 11 days postchallenge. However, at 11 days the non-vaccinated, iNOS-deficient mice had significantly higher fungal burden in their spleen compared to WT mice. Additionally, higher numbers of lung-infiltrated neutrophils, macrophages and dendritic cells were observed in WT mice at day 11 postchallenge compared to iNOS(-/-) mice. Moreover, no difference in numbers of T, B, NK or regulatory T cells, or concentrations of selected cytokines and chemokines were detected in lungs of both mouse strains at 7 and 11 days postchallenge. Although iNOS-derived NO production appears to influence the inflammatory response and dissemination of the fungal pathogen, our results suggest that iNOS activity does not play a significant role in the control of coccidioidal infection in mice and that other, still undefined mechanisms of host protection are involved.

Developmental regulation and extracellular release of a VSG expression-site-associated gene product from Trypanosoma brucei bloodstream forms

Trypanosomes evade host immunity by exchanging variant surface glycoprotein (VSG) coats. VSG genes are transcribed from telomeric expression sites, which contain a diverse family of expression-site-associated genes (ESAGs). We have discovered that the mRNAs for one ESAG family, ESAG9, are strongly developmentally regulated, being enriched in stumpy forms, a life-cycle stage in the mammalian bloodstream that is important for the maintenance of chronic parasite infections and for tsetse transmission. ESAG9 gene sequences are highly diverse in the genome and encode proteins with weak similarity to the massively diverse MASP proteins in Trypanosoma cruzi. We demonstrate that ESAG9 proteins are modified by N-glycosylation and can be shed to the external milieu, this being dependent upon coexpression with at least one other family member. The expression profile and extracellular release of ESAG9 proteins represents a novel and unexpected aspect of the transmission biology of trypanosomes in their mammalian host. We suggest that these molecules might interact with the external environment, with possible implications for infection chronicity or parasite transmission.

Role of IL-23 in mobilization of immunoregulatory nitric oxide- or superoxide-producing Gr-1+ cells from bone marrow

Spleens of mice injected with heat-killed Mycobacterium tuberculosis increase their Gr-1+ cell content and develop a system of interactive Ly-6G+ and Ly-6G-Gr-1+ populations or "Greg" subsets, which, upon stimulation by activated T cells, produce immunoregulatory superoxide (O2(-)) and nitric oxide (NO), respectively. The balance between immunosuppressive NO and its antagonist O2(-) regulates T cell expansion, similar to regulation of vasodilation. Reduction of NO levels by O2(-) is required for efficient T cell expansion and development of autoimmunity. We studied the source of Gr-1+ cells in bone marrow (BM), where their levels were higher than in spleen, with both Greg subsets expressing strong activity. In the spleens of primed IL-23-/- mice, Ly-6G+ cells remained at naïve levels and produced no O2(-). The complementary Ly-6G(-)Gr-1+ splenocytes and their suppressive activity were partially reduced. Surprisingly, Gr-1+ cell levels in BM of IL-23-/- mice were increased, as were their O2(-) and NO production. Transfer of primed BM cells partially restored regulatory function in the spleen of IL-23-/- recipients. The results suggest that IL-23 is involved in mobilization of O2(-)- and NO-producing Gr-1+ cells from BM, which may contribute to its widely studied role in (auto)immunity.

Contributions of experimental mouse models to the understanding of African trypanosomiasis

African trypanosomiasis is the collective name for a wide variety of trypanosome infections that affect humans and livestock. In recent years, experimental mice infection models have provided new insights into both human and animal trypanosomiasis. Mouse models seem to be a valuable and versatile tool in trypanosomiasis-associated pathology and immunology research and highlight the variety shown by African trypanosomiases. Indeed, inbred mouse strains have enabled the study of genetic determinants of susceptibility and of the roles of anti-parasite antibodies, inflammatory mediators and anti-inflammatory mediators for each trypanosome species. Remarkable advances relating to the encephalitic stage of sleeping sickness have also been achieved thanks to murine models. The different contributions of murine models to the African trypanosomiases knowledge are presented here. Future search directions are finally proposed, with respect to mouse model opportunities and limitations.

Genetic manipulation of African trypanosomes as a tool to dissect the immunobiology of infection

The variant surface glycoprotein (VSG) coat of African trypanosomes exhibits immunobiological functions distinct from its prominent role as a variant surface antigen. In order to address questions regarding immune stealth effects of VSG switch-variant coats, and the innate immune system activating effects of shed VSG substituents, several groups have genetically modified the ability of trypanosomes to express or release VSG during infection of the mammalian host. The role of mosaic surface coats expressed by VSG switch-variants (VSG double-expressors) in escaping early immune detection, and the role of VSG glycosylphosphatidylinositol (GPI) anchor substituents in regulating host immunity have been revealed, respectively, by stable co-expression of an exogenous VSG gene in trypanosomes expressing an endogenous VSG gene, and by knocking out the genetic locus for GPI-phospholipase C (PLC) that releases VSG from the membrane. Both approaches to genetic modification of African trypanosomes have suggested interesting and unexpected immunobiological effects associated with surface coat molecules.

Mycobacteria-induced Gr-1+ subsets from distinct myeloid lineages have opposite effects on T cell expansion

Similar to the regulation of vasodilation, the balance between NO and superoxide (O2-) regulates expansion of activated T cells in mice. Reduction of suppressive NO levels by O2- is essential for T cell expansion and development of autoimmunity. In mice primed with heat-killed Mycobacterium, a splenocyte population positive for Gr-1 (Ly-6G/C) is the exclusive source of both immunoregulatory free radicals. Distinct Gr-1+ cell subpopulations were separated according to Ly-6G expression. In culture with activated T cells, predominantly monocytic Ly-6G- Gr-1+ cells produced T cell-inhibitory NO but no O2-. However, mostly granulocytic Ly-6G+ cells produced O2- simultaneously but had no measurable effect on proliferation. Recombination of the two purified Gr-1+ subpopulations restored controlled regulation of T cell proliferation through NO and O2- interaction. Coculture of p47phox-/- and inducible NO synthase-/- Gr-1+ cells confirmed this intercellular interaction. These data suggest that bacterial products induce development of distinct Gr-1+ myeloid lineages, which upon stimulation by activated T cells, interact via their respective free radical products to modulate T cell expansion.

Regulation of innate and acquired immunity in African trypanosomiasis

African trypanosomes are well known for their ability to avoid immune elimination by switching the immunodominant variant surface glycoprotein (VSG) coat during infection. However, antigenic variation is only one of several means by which trypanosomes manipulate the immune system of their hosts. In this article, the role of parasite factors such as GPI anchor residues of the shed VSG molecule and the release of CpG DNA, in addition to host factors such as IFN-gamma, in regulating key aspects of innate and acquired immunity during infection is examined. The biological relevance of these immunoregulatory events is discussed in the context of host and parasite survival.

Mouse strain susceptibility to trypanosome infection: an arginase-dependent effect

We previously reported that macrophage arginase inhibits NO-dependent trypanosome killing in vitro and in vivo. BALB/c and C57BL/6 mice are known to be susceptible and resistant to trypanosome infection, respectively. Hence, we assessed the expression and the role of inducible NO synthase (iNOS) and arginase in these two mouse strains infected with Trypanosoma brucei brucei. Arginase I and arginase II mRNA expression was higher in macrophages from infected BALB/c compared with those from C57BL/6 mice, whereas iNOS mRNA was up-regulated at the same level in both phenotypes. Similarly, arginase activity was more important in macrophages from infected BALB/c vs infected C57BL/6 mice. Moreover, increase of arginase I and arginase II mRNA levels and of macrophage arginase activity was directly induced by trypanosomes, with a higher level in BALB/c compared with C57BL/6 mice. Neither iNOS expression nor NO production was stimulated by trypanosomes in vitro. The high level of arginase activity in T. brucei brucei-infected BALB/c macrophages strongly inhibited macrophage NO production, which in turn resulted in less trypanosome killing compared with C57BL/6 macrophages. NO generation and parasite killing were restored to the same level in BALB/c and C57BL/6 macrophages when arginase was specifically inhibited with N(omega)-hydroxy-nor-L-arginine. In conclusion, host arginase represents a marker of resistance/susceptibility to trypanosome infections.

Inducible nitric oxide synthase regulates production of isoprostanes in vivo during chlamydial genital infection in mice

Urinary nitrite and F(2)-isoprostanes, an index of oxidant stress, were elevated during chlamydial genital infection of mice. Enhancement of urinary nitrite and F(2)-isoprostanes was observed in phagocyte oxidase-deficient mice. Inhibition of inducible nitric oxide synthase reduced isoprostane excretion. We conclude that nitrogen radicals induce F(2)-isoprostane production and excretion during murine chlamydial genital infection.

Enhanced endothelial nitric oxide-synthase activity in mice infected with Trypanosoma brucei

Infection of humans with Trypanosoma brucei causes sleeping sickness, which is invariably fatal if left untreated. The course of infection is characterised, among others, by multiple organ damage including cardiovascular dysfunctions such as hypotension and breakdown of the blood-brain barrier. The latter eventually leads to the parasite invasion into central nervous system and ultimately to the death of the patient. Nitric oxide (NO) synthesised from L-arginine via endothelial NO-synthase (eNOS) is involved in the control of vascular tone and permeability. The present study explores the effect of T. brucei infection on the endothelium-dependent in vitro vasomotor response of isolated mouse aortas. Aorta rings were suspended in organ chambers for isometric tension recording. The endothelium-dependent NO-mediated relaxation in response to acetylcholine (10(-9) to 10(-5) M) was markedly enhanced in the infected mice compared to controls (P<0.05), whereas the endothelium-independent vasodilation to an exogenous NO-donor, sodium nitroprusside, was comparable in both groups. Norepinephrine-stimulated contraction was also comparable in the absence or presence of the NO-synthase inhibitor N(omega)-Nitro-L-arginine methyl ester (L-NAME; 10(-4)M) in both groups. The enhanced endothelium-dependent relaxation in the infected mice correlated well with a 3.5-fold increase in eNOS protein level in these aortas as compared to those of control mice (P=0.05). Thus, T. brucei infection enhances eNOS protein expression in the endothelium, causing a pronounced vasodilation. Overproduction of NO in trypanosomiasis may be involved in the observed generalised hypotension and in an increased vascular permeability that facilitates T. brucei invasion into surrounding tissues and its penetration into the central nervous system in later phases of infection.

Immune factors influencing the course of infection with Neospora caninum in the murine host

This paper investigates the role of specific immune factors on the course of infection in genetic knockout (gko) mice infected with 3 different strains of Neospora caninum. Survival time and parasite persistence were examined in interferon-gamma (IFN-gamma), tumor necrosis factor receptor-2 (TNFR2), interleukin 10 (IL-10), beta 2 microglobulin (beta2M), and inducible nitric oxide synthase (iNOS2) gko or wild-type (wt) mice following infection with either pathogenic (NC-1 or NC-2) or attenuated (NCts-8) N. caninum strains. Infection with NC-1 was 100% lethal in IFN-gamma gko mice, as evidenced by mean survival times of 10-13 days, depending on the challenge dose used. TNFR2 and beta2M gko mice infected with NC-1 or NC-2 strain demonstrated partial susceptibility to disease, as evidenced by histopathology and survival curves. TNFR2 or beta2M gko mice were not susceptible to infection with NCts-8, on the basis of absence of pathology and lack of mortality. Lack of mortality and minimal histopathology scores demonstrated that NC-1, NC-2, and NCts-8 infections were avirulent in IL-10 and iNOS2 gko mice. Adoptive transfer of immune cells from NCts-8 vaccinated normal syngeneic mice into IFN-gamma gko mice significantly (P < 0.05) prolonged mean survival times at all 3 challenge doses of NC-1 but failed to protect against mortality. Interestingly, there was a notable change in the tissue tropism of tachyzoites from the lung and brain in immunocompetent wt, TNFR2 gko, IL-10 gko, beta2M gko, and iNOS2 gko mice to the liver and spleen in IFN-gamma gko mice when challenged with N. caninum. On the basis of these results in gko mice, IFN-gamma is a critical cytokine in the host response against acute neosporosis.

Serum xanthine oxidase: origin, regulation, and contribution to control of trypanosome parasitemia

African trypanosomiasis is caused by Salivarian trypanosomes, tsetse fly-transmitted protozoa that inhabit the blood plasma, lymph and interstitial fluids, and, in the case of Trypanosoma brucei species, also the cerebrospinal fluid of mammal hosts. Trypanosomiasis in people and domestic animals manifests as recurring waves of parasites in the blood and is typically fatal. In contrast, trypanosomiasis in Cape buffaloes, which are naturally selected to resist the disease, is characterized by the development of only one or a few waves of parasitemia, after which the infection becomes cryptic, being maintained by the presence of 1-20 mammal-infective organisms/ml of blood. The control of the acute phase of parasitemia in Cape buffaloes correlates with a decline in blood catalase activity and the generation of trypanocidal H(2)O(2) in serum during the catabolism of endogenous purine by xanthine oxidase. Here we review features of this response, and of trypanosome metabolism, that facilitate H(2)O(2)-mediated killing of the parasites with minimal damage to the host. We also discuss the origin and regulation of serum xanthine oxidase and catalase, and show how recovery of serum catalase in infected Cape buffaloes precludes a role for H(2)O(2) in the long-term, stable suppression of trypanosome parasitemia.

Role for inducible nitric oxide synthase in protection from chronic Chlamydia trachomatis urogenital disease in mice and its regulation by oxygen free radicals

It has been previously reported that although inducible nitric oxide synthase (iNOS) gene knockout (NOS2(-/-)) mice resolve Chlamydia trachomatis genital infection, the production of reactive nitrogen species (RNS) via iNOS protects a significant proportion of mice from hydrosalpinx formation and infertility. We now report that higher in vivo RNS production correlates with mouse strain-related innate resistance to hydrosalpinx formation. We also show that mice with a deletion of a key component of phagocyte NADPH oxidase (p47(phox-/-)) resolve infection, produce greater amounts of RNS in vivo, and sustain lower rates of hydrosalpinx formation than both wild-type (WT) NOS2(+/+) and NOS2(-/-) controls. When we induced an in vivo chemical block in iNOS activity in p47(phox-/-) mice using N(G)-monomethyl-L-arginine (L-NMMA), a large proportion of these mice eventually succumbed to opportunistic infections, but not before they resolved their chlamydial infections. Interestingly, when compared to WT and untreated p47(phox-/-) controls, L-NMMA-treated p47(phox-/-) mice resolved their infections more rapidly. However, L-NMMA-treated p47(phox-/-) mice lost resistance to chronic chlamydial disease, as evidenced by an increased rate of hydrosalpinx formation that was comparable to that for NOS2(-/-) mice. We conclude that phagocyte oxidase-derived reactive oxygen species (ROS) regulate RNS during chlamydial urogenital infection in the mouse. We further conclude that while neither phagocyte oxidase-derived ROS nor iNOS-derived RNS are essential for resolution of infection, RNS protect from chronic chlamydial disease in this model.

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.

Nitric oxide in parasitic infections

Nitric oxide (NO) is implicated as an integral component of the host armament against invading parasites. Strongest evidence has come from laboratory models of protozoan infections. During malaria, toxoplasmosis and leishmaniasis, to name just a few, the preferential production of pro-inflammatory cytokines predisposes to the increased synthesis of NO, which mediates host protection through either direct parasite killing or by limiting parasite growth. More recently, evidence has been put forward for a beneficial role of NO during helminthic infections. In the case of Schistosomiasis mansoni, for example, NO plays a role in regulation of egg-induced inflammation, preventing hepatocyte death and widespread tissue damage. In spite of these findings, rather than being the ultimate panacea, NO production requires tight control to limit cytotoxic damage to the host's own cells. Unregulated production may lead to a variety of damaging effects including alterations to normal neurological functions during cerebral malaria and intestinal pathology during trichinosis. In this review, I will summarize the role of NO during a number of parasitic infections, drawing on specific examples of disease caused by protozoan and metazoan parasites.

Effects of the depletion of CD8(+) T cells and monocytes on the proliferative responses of peripheral blood leucocytes from Trypanosoma evansi-infected sheep

Sheep peripheral blood mononuclear cells and those depleted of CD8(+) T cells and/or monocytes were stimulated with polyclonal mitogens and specific antigens, and analysed by means of cell proliferation assay procedure to examine whether these cell populations are involved in Trypanosoma evansi-induced immunosuppression. The removal of CD8(+) T cells failed to normalize the proliferative responses of peripheral blood mononuclear cells from infected sheep to concanavalin A stimulation while the depletion of monocytes resulted in full and enhanced response, showing that macrophages are mainly responsible for the suppression. Although the depletion of CD8(+) T cells, monocytes or both restored the responses of the cells to lipopolysaccharide stimulation, the responsiveness of the undepleted cells to this mitogen was significantly higher from day 24 post infection (p<0.01). The results were discussed in relation to current known mechanisms of depressed lymphocyte proliferation in tsetse-transmitted African trypanosome infections.

Antimicrobial actions of the NADPH phagocyte oxidase and inducible nitric oxide synthase in experimental salmonellosis. II. Effects on microbial proliferation and host survival in vivo

The roles of the NADPH phagocyte oxidase (phox) and inducible nitric oxide synthase (iNOS) in host resistance to virulent Salmonella typhimurium were investigated in gp91phox(-/)-, iNOS(-/)-, and congenic wild-type mice. Although both gp91phox(-/)- and iNOS(-/)- mice demonstrated increased susceptibility to infection with S. typhimurium compared with wild-type mice, the kinetics of bacterial replication were dramatically different in the gp91phox(-/)- and iNOS(-/)- mouse strains. Greater bacterial numbers were present in the spleens and livers of gp91phox(-/)- mice compared with C57BL/6 controls as early as day 1 of infection, and all of the gp91phox(-/)- mice succumbed to infection within 5 d. In contrast, an increased bacterial burden was detected within reticuloendothelial organs of iNOS(-/)- mice only beyond the first week of infection. Influx of inflammatory CD11b(+) cells, granuloma formation, and serum interferon gamma levels were unimpaired in iNOS(-/)- mice, but the iNOS-deficient granulomas were unable to limit bacterial replication. The NADPH phagocye oxidase and iNOS are both required for host resistance to wild-type Salmonella, but appear to operate principally at different stages of infection.

Interferon gamma eliminates responding CD4 T cells during mycobacterial infection by inducing apoptosis of activated CD4 T cells

In Mycobacterium bovis Bacille Calmette-Guérin (BCG)-infected wild-type mice, there was a large expansion of an activated (CD44(hi)) splenic CD4 T cell population followed by a rapid contraction of this population to normal numbers. Contraction of the activated CD4 T cell population in wild-type mice was associated with increased apoptosis of activated CD4 T cells. In BCG-infected interferon (IFN)-gamma knockout (KO) mice, the activated CD4 T cell population did not undergo apoptosis. These mice accumulated large numbers of CD4(+)CD44(hi) T cells that were responsive to mycobacterial antigens. Addition of IFN-gamma to cultured splenocytes from BCG-infected IFN-gamma KO mice induced apoptosis of activated CD4 T cells. IFN-gamma-mediated apoptosis was abolished by depleting adherent cells or Mac-1(+) spleen cells or by inhibiting nitric oxide synthase. Thus, IFN-gamma is essential to a regulatory mechanism that eliminates activated CD4 T cells and maintains CD4 T cell homeostasis during an immune response.

Trypanosoma cruzi: the effect of nitric oxide synthesis inhibition on the CD4 T cell response to the trans-sialidase superfamily

During Trypanosoma cruzi infection the trans-sialidase superfamily stimulates the development of a large population of CD4 T lymphocytes that produces IFNgamma. These CD4 T cells fail to proliferate when stimulated in vitro. Why they fail to proliferate remains unclear. Nitric oxide is a critical component of the host immune response against T. cruzi, and to determine if NO inhibits trans-sialidase superfamily-specific proliferative responses, mice were fed either N(G)-nitro-L-arginine methylester (L-NAME), an inhibitor of inducible nitric oxide synthase (iNOS), or N(G)-nitro-D-arginine methyl ester (D-NAME), an inactive analog of L-NAME. The L-NAME-fed mice had increased parasitemia and mortality compared to the D-NAME-fed mice. Following stimulation with a T. cruzi trans-sialidase superfamily protein, splenocytes from both groups of mice failed to proliferate but continued to make similar amounts of IFNgamma, suggesting that the development of the trans-sialidase superfamily-specific CD4 response was not affected by iNOS inhibition. In addition, IL-2 receptor (IL-2R) expression was increased on T cells isolated from L-NAME-fed mice. These data suggest that during T. cruzi infection NO causes downregulation of IL-2R expression, but does not cause inhibition of trans-sialidase superfamily-specific CD4 T cell proliferation. Rather, the trans-sialidase superfamily proliferation may be inhibited by epitope variation.

Reactive oxygen and reactive nitrogen intermediates in innate and specific immunity

Nitric oxide, nitric oxide derivatives and reactive oxygen intermediates are toxic molecules of the immune system which contribute to the control of microbial pathogens and tumors. There is recent evidence for additional functions of these oxygen metabolites in innate and adaptive immunity; these functions include the modulation of the cytokine response of lymphocytes and the regulation of immune cell apoptosis, as well as immunodeviating effects. Components of several signal transduction pathways have been identified as intracellular targets for reactive nitrogen and oxygen intermediates.

Antigenic variation in Trypanosoma brucei infections: an holistic view

Trypanosoma brucei parasites undergo clonal phenotypic (antigenic) variation to promote their transmission between mammals and tsetse-fly vectors. This process is classically considered to be a mechanism for evading humoral immune responses, but such an explanation cannot account for the high rate of switching between variable antigens or for their hierarchical (i.e. non-random) expression. I suggest that these anomalies can be explained by a new model: that antigenic variation has evolved as a bifunctional, rather than as a unifunctional, strategy that not only evades humoral immune responses but also enables competition between parasite strains in concomitantly infected hosts. This competition causes a depression of cellular responses. My proposal gives rise to a number of testable predictions. First, low numbers of trypanosomes should express some variable antigen types (VATs) in infections several weeks before these VATs are detectable. Second, as an infection progresses, the number of VATs expressed simultaneously in the population should decrease. Third, immunisation to generate a T helper 1 response against those VATs that are expressed most frequently should lower parasitaemias and reduce virulence.