Opportunistic infections and retrovirus-induced immunodeficiency: studies of acute and chronic infections with Toxoplasma gondii in mice infected with LP-BM5 murine leukemia viruses

Releasing latent Toxoplasma gondii cysts from host cells to the extracellular environment induces excystation

Toxoplasma gondii, an obligate intracellular protozoan parasite, infects a wide variety of mammals and birds. Although T. gondii infects the brain and muscles in its latent cyst form containing bradyzoite stage parasites during chronic infection, when a chronically infected host becomes immunodeficient or is preyed upon by a predator, the latent cyst undergoes excystation. However, it is not yet known how T. gondii recognises the triggers of excystation in the microenvironment surrounding the cyst. In this study, we incubated T. gondii cysts from host cells in several solutions containing a variety of ionic compositions. Excystation occurred in a solution with an ionic composition which mimicked that of the extracellular environment. However, excystation did not occur in a solution that mimicked the intracellular environment. We also found that the specific Na⁺/K⁺ ratio and the presence of Ca²⁺, mimicking the extracellular environment, are required to trigger excystation. To examine whether the stage conversion of bradyzoite to tachyzoite occurs prior to egress, we constructed a gene-modified T. gondii strain expressing a green fluorescent protein specifically in the tachyzoite stage. During the process of cyst reactivation of this strain, green fluorescence was detected prior to excystation. This suggests that stage conversion from bradyzoite to tachyzoite occurs prior to cyst disruption. These results indicate that T. gondii bradyzoites monitor the ionic composition of their surroundings to recognise their expulsion from host cells, to effectively time their excystation and stage conversion.

Implications of infectious agents on results of animal experiments

Report of the Working Group on Hygiene of the Gesellschaft für Versuchstierkunde–Society for Laboratory Animal Science (GV-SOLAS)

GV-SOLAS Working Group on Hygiene: Werner Nicklas (Chairman), Felix R. Homberger, Brunhilde Illgen-Wilcke, Karin Jacobi, Volker Kraft, Ivo Kunstyr, Michael Mähler, Herbert Meyer & Gabi Pohlmeyer-Esch

Diphenyl diselenide and sodium selenite associated with chemotherapy in experimental toxoplasmosis: influence on oxidant/antioxidant biomarkers and cytokine modulation

SUMMARY The aim of this study was to assess the effect of sulfamethoxazole/trimethoprim (ST) supplemented with diphenyl diselenide and sodium selenite in experimental toxoplasmosis, on oxidant/antioxidant biomarkers and cytokine levels. Eighty-four BALB/c mice were divided in seven groups: group A (negative control), and groups B to G (infected). Blood and liver samples were collected on days 4 and 20 post infection (p.i.). Levels of thiobarbituric acid (TBA) reactive substances and advanced oxidation protein products (AOPP) were assessed in liver samples. Both biomarkers were significantly increased in infected groups on day 4 p.i., while they were reduced on day 20 p.i., compared with group A. Glutathione reductase (GR) activity significantly (P<0·01) increased on day 4 p.i., in group G, compared with group A. INF-γ was significantly increased (P<0·001) in both periods, day 4 (groups B, C, F and G) and 20 p.i. (groups C, F and G). IL-10 significantly reduced (P<0·001) on day 4 p.i. in group B; however, in the same period, it was increased (P<0·001) in groups C and G, compared with group A. On day 20 p.i., IL-10 increased (P<0·001) in groups F and G. Therefore, our results highlighted that these forms of selenium, associated with the chemotherapy, were able to reduce lipid peroxidation and protein oxidation, providing a beneficial immunological balance between the production of pro- and anti-inflammatory cytokines.

T-cell reconstitution during murine acquired immunodeficiency syndrome (MAIDS) produces neuroinflammation and mortality in animals harboring opportunistic viral brain infection

BACKGROUND

Highly active antiretroviral therapy (HAART) restores inflammatory immune responses in AIDS patients which may unmask previous subclinical infections or paradoxically exacerbate symptoms of opportunistic infections. In resource-poor settings, 25% of patients receiving HAART may develop CNS-related immune reconstitution inflammatory syndrome (IRIS). Here we describe a reliable mouse model to study underlying immunopathological mechanisms of CNS-IRIS.

METHODS

Utilizing our HSV brain infection model and mice with MAIDS, we investigated the effect of immune reconstitution on MAIDS mice harboring opportunistic viral brain infection. Using multi-color flow cytometry, we quantitatively measured the cellular infiltrate and microglial activation.

RESULTS

Infection with the LP-BM5 retroviral mixture was found to confer susceptibility to herpes simplex virus (HSV)-1 brain infection to normally-resistant C57BL/6 mice. Increased susceptibility to brain infection was due to severe immunodeficiency at 8 wks p.i. and a marked increase in programmed death-1 (PD-1) expression on CD4+ and CD8+ T-cells. Both T-cell loss and opportunistic brain infection were associated with high level PD-1 expression because PD-1-knockout mice infected with LP-BM5 did not exhibit lymphopenia and retained resistance to HSV-1. In addition, HSV-infection of MAIDS mice stimulated peripheral immune cell infiltration into the brain and its ensuing microglial activation. Interestingly, while opportunistic herpes virus brain infection of C57BL/6 MAIDS mice was not itself lethal, when T-cell immunity was reconstituted through adoptive transfer of virus-specific CD3+ T-cells, it resulted in significant mortality among recipients. This immune reconstitution-induced mortality was associated with exacerbated neuroinflammation, as determined by MHC class II expression on resident microglia and elevated levels of Th1 cytokines in the brain.

CONCLUSIONS

Taken together, these results indicate development of an immune reconstitution disease within the central nervous system (CNS-IRD). Experimental immune reconstitution disease of the CNS using T-cell repopulation of lymphopenic murine hosts harboring opportunistic brain infections may help elucidate neuroimmunoregulatory networks that produce CNS-IRIS in patients initiating HAART.

Regulation of CD8+ T cell responses to infection with parasitic protozoa

There are over 10,000 species of parasitic protozoa, a subset of which can cause considerable disease in humans. Here we examine in detail the complex immune response generated during infection with a subset of these parasites: Trypanosoma cruzi, Leishmania sp., Toxoplasma gondii, and Plasmodium sp. While these particular species perhaps represent the most studied parasites in terms of understanding how T cells function during infection, it is clear that the lessons learned from this body of work are also relevant to the other protozoa known to induce a CD8(+) T cell response. This review will highlight some of the key studies that established that CD8(+) T cells play a major role in protective immunity to protozoa, the factors that promote the generation as well as maintenance of the CD8(+) T cell response during these infections, and draw attention to some of the gaps in our knowledge. Moreover, the development of new tools, including MHC-Class I tetramer reagents and the use of TCR transgenic mice or genetically modified parasites, has provided a better appreciation of how parasite specific CD8(+) T cell responses are initiated and new insights into their phenotypic plasticity.

Citrobacter-induced colitis in mice with murine acquired immunodeficiency syndrome

Over the period of a year, colitis was observed in 44 mice raised in a conventional nonspecific pathogen-free colony, 41 of these having concomitant retrovirus-induced murine acquired immunodeficiency syndrome (MAIDS). The lesions varied from bacterial colonization to hyperplasia of colonic mucosa to severe, often fatal, ulceration. Citrobacter rodentium was isolated from the colon and/or liver of 2 mice with colitis. When C57BL/6 mice with or without MAIDS were given graded doses of the bacterium, only those with MAIDS developed colitis, and C rodentium was reisolated from their livers. Thus, mice with MAIDS can develop severe disease following opportunistic infection with an environmental contaminant of the colony that is nonpathogenic for normal adult mice.

MAIDS resistance-associated gene expression patterns in secondary lymphoid organs

Murine acquired immunodeficiency syndrome (MAIDS) is caused by exposure to murine leukemia virus and serves as a model to study human AIDS. In MAIDS-susceptible C57BL/6 mice, virus exposure leads to progressive immune deficiency, while resistant strains such as BALB/c recover from infection and develop protective immunity. The goal of this study was to identify early gene expression patterns that may be important in establishing this strain-specific differential response. Total RNA was isolated from spleens and pooled lymph nodes of both mouse strains at 3 and 7 days post virus infection. The complementary DNA generated from this RNA was hybridized to mouse oligonucleotide DNA microarrays using a strategy that controlled for inherent variability and highlighted only virus-induced changes. Fluorescent intensities were normalized and analyzed for statistically significant differential expression between strains across both time points and lymphoid organs. The majority of the resistance-associated genes was identified at day 3 post-infection and demonstrated the highest fold differences between strains, while more susceptibility-associated sequences were seen at 7 days post-infection. Among the most highly differentially expressed sequences seen at the earlier time point were genes related to protein metabolism, especially serine proteases. Differential patterns of chemokine-related genes were observed at the later time point. The overall pattern of expression suggests strain-specific differences in proteases and chemokines within secondary lymphoid organs shortly after infection influence the likelihood of disease progression.

Detection of the initial site of Toxoplasma gondii reactivation in brain tissue

Detection of the initial site of Toxoplasma gondii reactivation in brain tissue is difficult because the number of latent cysts is small and reactivation is a transient event. To detect the early stage of reactivation in mouse brain tissue, we constructed a cyst-forming strain of T. gondii in the tachyzoite stage, specifically expressing red fluorescence. The PLK strain of T. gondii was stably transfected with a red fluorescent protein gene, DsRed Express, under the control of a tachyzoite-specific SAG-1 promoter and the resulting parasite was designated as PLK/RED. Tachyzoites of PLK/RED growing in Vero cells showed red fluorescence. When C57BL/6J mice were i.p. infected with tachyzoites of PLK/RED, red fluorescent tachyzoites were detected in their brains at the fourth day p.i. However, red fluorescent tachyzoites were not detected in BALB/c mice latently infected with PLK/RED, although non-fluorescent cysts were detected in their brains. After treatment of latently infected mice with dexamethasone for 1 month, the mice showed neurological symptoms. In mice with symptoms, red fluorescent tachyzoites were again detected in their brains and in other organs. To detect the initial site of reactivation, BALB/c mice latently infected with the strain were treated with dexamethasone for 3 weeks, and brains were excised before any symptoms appeared. Excised brains were examined for red fluorescence-positive sites. By a histological study of red fluorescent-positive sites, we detected a cyst containing red fluorescent zoites, which still had a PAS stain-positive cyst wall. A few red fluorescent zoites breaking away from the cyst were also observed. The stage-specific expression of fluorescent protein facilitates detection of a rare transient event and makes it possible to detect the initial site of reactivation.

A critical role for IL-10 in limiting inflammation during toxoplasmic encephalitis

IL-10 plays a vital role in controlling the inflammatory response during acute Toxoplasma gondii infection, however the production of IL-10 during the chronic phase of toxoplasmosis has been associated with parasite persistence. To address this paradox, the production and effect of IL-10 in the brain during toxoplasmic encephalitis (TE) was investigated. Analysis of brain mononuclear cells (BMNC) from chronically infected mice revealed that infiltrating macrophages and CD4(+) T cells were the major sources of IL-10. Endogenous levels of IL-10 inhibited the production of IL-12, IFN-gamma, TNF-alpha, and IL-6 from both hematopoetic and non-hematopoetic cells in the brain, as well as anti-microbial activity of astrocytes. Furthermore, IL-10-/- mice that progressed to the chronic phase of infection had equivalent parasite burden to WT mice but developed a lethal inflammatory response within the brain characterized by increased numbers of CD4(+) T cells and macrophages, and elevated production of inflammatory cytokines. Finally, partial depletion of CD4(+) T cells decreased the severity of the inflammation in the brain and allowed IL-10-/- mice to survive infection. Together these results point to a vital role for IL-10 in the control of CD4(+) T cell mediated inflammation in the brain during TE.

Infection with Toxoplasma gondii increases atherosclerotic lesion in ApoE-deficient mice

Toxoplasma gondii is an intracellular protozoan that elicits a potent inflammatory response during the acute phase of infection. Herein, we evaluate whether T. gondii infection alters the natural course of aortic lesions. ApoE knockout mice were infected with T. gondii, and at 5 weeks of infection, serum, feces, and liver cholesterol; aortic lesion size, cellularity, and inflammatory cytokines; and levels of serum nitrite and gamma interferon (IFN-gamma) were analyzed. Our results showed that serum cholesterol and atherogenic lipoproteins were reduced after T. gondii infection. The reduction of serum levels of total cholesterol and atherogenic lipoproteins was associated with increases in the aortic lesion area, numbers of inflammatory cells, and expression of monocyte chemoattractant protein 1 and inducible nitric oxide synthase mRNA in the site of lesions as well as elevated concentrations of IFN-gamma and nitrite in sera of T. gondii-infected animals. These results suggest that infection with T. gondii accelerates atherosclerotic development by stimulating the proinflammatory response and oxidative stress, thereby increasing the area of aortic lesion.

The induction and kinetics of antigen-specific CD8 T cells are defined by the stage specificity and compartmentalization of the antigen in murine toxoplasmosis

Toxoplasma gondii forms different life stages, fast-replicating tachyzoites and slow-growing bradyzoites, in mammalian hosts. CD8 T cells are of crucial importance in toxoplasmosis, but it is unknown which parasite stage is recognized by CD8 T cells. To analyze stage-specific CD8 T cell responses, we generated various recombinant Toxoplasma gondii expressing the heterologous Ag beta-galactosidase (beta-gal) and studied whether 1) secreted or cytoplasmic Ags and 2) tachyzoites or bradyzoites, which persist intracerebrally, induce CD8 T cells. We monitored the frequencies and kinetics of beta-gal-specific CD8 T cells in infected mice by MHC class I tetramer staining. Upon oral infection of B6C (H-2(bxd)) mice, only beta-gal-secreting tachyzoites induced beta-gal-specific CD8 T cells. However, upon secondary infection of mice that had received a primary infection with tachyzoites secreting beta-gal, beta-gal-secreting tachyzoites and bradyzoites transiently increased the frequency of intracerebral beta-gal-specific CD8 T cells. Frequencies of splenic and cerebral beta-gal-specific CD8 T cells peaked at day 23 after infection, thereafter persisting at high levels in the brain but declining in the spleen. Splenic and cerebral beta-gal-specific CD8 T cells produced IFN-gamma and were cytolytic upon specific restimulation. Thus, compartmentalization and stage specificity of an Ag determine the induction of CD8 T cells in toxoplasmosis.

Control of latent Mycobacterium tuberculosis infection is dependent on CD8 T cells

It is estimated that one-third of the world's population is infected with Mycobacterium tuberculosis, but that only 10% of infected people break down with the disease. In the remaining 90% the infection remains clinically latent. In the present study, the immune mechanisms controlling the latent phase of tuberculosis infection were evaluated in a mouse model of latency and reactivation. Mice aerosol-infected with M. tuberculosis were treated with anti-mycobacterial drugs resulting in very low, stable bacterial numbers (<500 CFU in the spleen and lung) for 10-12 weeks followed by reactivation of the disease with increasing bacterial numbers. During latency, pathological changes in the lung had almost completely resolved and lymphocyte number and turnover were at the pre-infection level. The CD4 subset was highly active during the acute phase of infection and could be detected by intracellular staining for IFN-gamma as well as after antigen-specific stimulation with mycobacterial antigens. The CD8 subset was not involved in the acute stage of infection, but this subset was active and produced IFN-gamma during the latent phase of infection. In vivo depletion of T cell subsets supported these findings with a 6-7-fold increase in bacterial numbers in the lung following anti-CD4 treatment during the acute phase, while anti-CD8 treatment did not have an effect. The opposite was found during the latent phase where anti-CD8 treatment as well as anti-IFN-gamma treatment both resulted in a 10-fold increase in bacterial numbers in the lung, while anti-CD4 treatment induced only a modest change.

Immune CD8(+) T cells prevent reactivation of Toxoplasma gondii infection in the immunocompromised host

Toxoplasma gondii remains a serious cause of morbidity and mortality in individuals that are immunosuppressed, patients with AIDS in particular. The cellular immune response, especially by gamma interferon (IFN-gamma)-producing CD8(+) T cells, is an essential component of protective immunity against the parasite. In the present study the role of CD8(+) T cells during the reactivation of Toxoplasma infection in an immunocompromised murine model was evaluated. Chronically infected mice were challenged with LP-BM5 virus, and the kinetics of CD8(+) T-cell function was studied. At 10 weeks after viral infection, mice showed obvious signs of systemic illness and began to die. At this stage, CD8(+) T cells were unresponsive to antigenic stimulation and unable to kill Toxoplasma-infected targets. IFN-gamma production by the CD8(+) T cells from dual-infected animals reached background levels, and a dramatic fall in the frequency of precursor cytotoxic T lymphocytes was observed. Histopathological analysis of the tissues demonstrated signs of disseminated toxoplasmosis as a result of reactivation of infection. However, treatment of the dual-infected animals with immune CD8(+) T cells at 5 weeks post-LP-BM5 challenge prevented the reactivation of toxoplasmosis, and mice continued to live. Our study for the first time demonstrates a therapeutic role for CD8(+) T cells against an opportunistic infection in an immunocompromised state.

The CD28/B7 Interaction Is Not Required for Resistance to Toxoplasma gondii in the Brain but Contributes to the Development of Immunopathology

Infection of C57BL/6 mice with Toxoplasma gondii leads to chronic encephalitis characterized by infiltration into the brain of T cells that produce IFN-γ and mediate resistance to the parasite. Our studies revealed that expression of B7.1 and B7.2 was up-regulated in brains of mice with toxoplasmic encephalitis (TE). Because CD28/B7-mediated costimulation is important for T cell activation, we assessed the contribution of this interaction to the production of IFN-γ by T cells from brains and spleens of mice with TE. Stimulation of splenocytes with Toxoplasma Ag or anti-CD3 mAb resulted in production of IFN-γ, which was inhibited by 90% in the presence of CTLA4-Ig, an antagonist of B7 stimulation. However, production of IFN-γ by T cells from the brains of these mice was only slightly reduced (20%) by the addition of CTLA4-Ig. To address the role of the CD28/B7 interaction during TE, we compared the development of disease in C57BL/6 wild-type (wt) and CD28−/− mice. Although the parasite burden was similar in wt and CD28−/− mice, CD28−/− mice developed less severe encephalitis and survived longer than wt mice. Ex vivo recall responses revealed that mononuclear cells isolated from the brains of chronically infected CD28−/− mice produced less IFN-γ than wt cells, and this correlated with reduced numbers of intracerebral CD4+ T cells in CD28−/− mice compared with wt mice. Taken together, our data show that resistance to T. gondii in the brain is independent of CD28 and suggest a role for CD28 in development of immune-mediated pathology during TE.

Coinfection with Toxoplasma gondii inhibits antigen-specific Th2 immune responses, tissue inflammation, and parasitism in BALB/c mice infected with Leishmania major

Lesion size, cellular infiltration, and tissue parasitism in the footpads of BALB/c mice infected with Leishmania major were all dramatically inhibited during acute but not chronic infection with Toxoplasma gondii. Similarly, acute but not chronic toxoplasmosis at the time of infection with L. major had a strong inhibitory effect on development of acquired immune responses mediated by Th2 lymphocytes. In contrast, no major changes in Leishmania-specific Th1-mediated responses were observed in mice coinfected with T. gondii.

Changes in cytokine levels during reactivation of Toxoplasma gondii infection in lungs

We studied cytokine proteins and mRNAs in mice with two forms of Toxoplasma gondii pneumonia resulting from reactivation of infection. In the first form, mice were infected with T. gondii, developed and recovered from systemic disease, and then developed pneumonia 3 weeks later. As pulmonary inflammation developed, levels of cytokine mRNAs for gamma interferon (IFN-gamma), interleukin-2 (IL-2), IL-4, and IL-10 increased in bronchoalveolar lavage (BAL) cells or lung tissue, and the level of IFN-gamma protein increased in BAL fluid. The second form of pneumonia occurred as a complication of primary cytomegalovirus (CMV) disease in mice with dormant T. gondii infection. During CMV disease, IL-2 mRNA levels decreased in lung tissue, IL-10 protein levels increased in lung tissue, and IL-10 protein levels increased in BAL fluid. As the mice recovered from CMV disease, T. gondii infection was reactivated in the lungs and was manifested as T. gondii pneumonia. During CMV-induced T. gondii pneumonia, IFN-gamma, IL-2, IL-4, and IL-10 mRNA levels increased in BAL cells or lung tissue, and both IFN-gamma and IL-2 protein levels increased in BAL fluid. We concluded that both forms of T. gondii pneumonia are accompanied by increases in both type 1 T-helper and type 2 T-helper cytokine levels in lungs. The mechanism of CMV-induced reactivation of T. gondii infection in lungs may involve local decreases in IL-2 levels and/or increases in IL-10 levels.

The Th1 to Th2 shift induced by Schistosoma mansoni does not exacerbate murine aids (MAIDS)

Schistosoma mansoni infection in mice is associated with a switch from a Th1 to a Th2-type cytokine response. The role of Th1 and Th2 responses in immune dysregulations associated with AIDS and murine AIDS (MAIDS) is controversial, but a Th2 bias could be associated with disease progression, raising the hypothesis that helminth infections might accelerate the retroviral disease progression. Here, we used the murine model of AIDS to evaluate the course of the viral disease during co-infection with S. mansoni. C57BL/6 mice were infected with S. mansoni cercariae 8 weeks before intravenous challenge with the LP-BM5 retroviral complex. MAIDS did not progress faster in co-infected mice, in terms of spleen and inguinal lymphadenopathy size, ecotropic virus titres in the spleen, or in vitro proliferative responses to mitogen. Th2 cytokine production was not enhanced in co-infected animals, except for an isolated increase in IL-4 production 21 weeks after LP-BM5 infection. Co-infected animals had significantly lower lymph node and spleen weights than mice infected with LP-BM5 only. MAIDS did not influence the granulomatous response to S. mansoni in the liver of co-infected mice. Finally, infection with S. mansoni neither enhanced Th2 cytokine production nor accelerated MAIDS progression in animals subsequently challenged with LP-BM5.

Regulation and function of T-cell-mediated immunity during Toxoplasma gondii infection

The intracellular protozoan Toxoplasma gondii is a widespread opportunistic parasite of humans and animals. Normally, T. gondii establishes itself within brain and skeletal muscle tissues, persisting for the life of the host. Initiating and sustaining strong T-cell-mediated immunity is crucial in preventing the emergence of T. gondii as a serious pathogen. The parasite induces high levels of gamma interferon (IFN-gamma) during initial infection as a result of early T-cell as well as natural killer (NK) cell activation. Induction of interleukin-12 by macrophages is a major mechanism driving early IFN-gamma synthesis. The latter cytokine, in addition to promoting the differentiation of Th1 effectors, is important in macrophage activation and acquisition of microbicidal functions, such as nitric oxide release. During chronic infection, parasite-specific T lymphocytes release high levels of IFN-gamma, which is required to prevent cyst reactivation. T-cell-mediated cytolytic activity against infected cells, while easily demonstrable, plays a secondary role to inflammatory cytokine production. While part of the clinical manifestations of toxoplasmosis results from direct tissue destruction by the parasite, inflammatory cytokine-mediated immunopathologic changes may also contribute to disease progression.

Cryptococcus neoformans infection in mice previously infected with LP-BM5 MuLV, the agent of murine AIDS (MAIDS)

We studied susceptibility to experimental systemic cryptococcosis in mice previously infected with the retroviral complex LP-BM5 (responsible for murine AIDS). LP-BM5 was inoculated to C57B1/6 mice by intravenous (i.v.) injection 8 weeks before an i.v. challenge with 4 x 10(3) CFU of Cryptococcus neoformans. Uninfected and singly infected mice were used as controls. LP-BM5 infection did not result in a significant increase in fungal burdens in the lungs or brains of co-infected animals compared to mice infected with C. neoformans alone. However, mortality was enhanced in the co-infected animals. The kinetics of splenocyte subsets differed in co-infected mice and LP-BM5-infected mice; the increase in CD4+, CD8+ and Ly5+ cells was only moderate in the former. Cytokine production by concanavalin A (Con A)-stimulated splenocytes from co-infected mice showed a marked decrease in the Th1 response (IFN-gamma, IL-2) and an increase in the Th2 response (IL-4, IL-10). Furthermore, cryptococcosis altered the course of MAIDS, inhibiting splenomegaly. This effect was not related to a decrease in ecotropic virus titres in the spleen or to improved in vitro responsiveness of spleen cells to Con A. The marked decrease in IFN-gamma production in co-infected animals could partly explain the inhibition of LP-BM5-induced splenomegaly. This model of murine retroviral infection does not seem to be suitable for studying cryptococcosis in immunosuppressed animals, but remains valuable for investigating in vivo interactions between two pathogens.

Modulation of murine AIDS-related pathology by concurrent antibody treatment and coinfection with Leishmania major

Infection of C57BL/6 mice with a mixture of murine leukemia viruses (MuLVs) designated LP-BM5 MuLV leads to a disease characterized by progressive immunodeficiency and lymphoproliferation, known as murine AIDS (MAIDS). The development of MAIDS is associated with increased B-cell lymphoblast proliferation, but there is reason to believe that T-cell function and, particularly, T-cell-derived cytokines may also play a role. We have previously shown that concurrent infection with Leishmania major (which induces a strongly polarized Th1 response in C57BL/6 mice) and LP-BM5 MuLV modulates the disease induced by both infections. Here we show by treatment of mice with anticytokine antibodies that this modulation is largely exerted through the balance of Th1 and Th2 cytokines. Infected mice treated with antibodies to interleukin-4 and interleukin-10 exhibited a delayed development of MAIDS-related pathology and maintained T-cell responsiveness longer than mice treated with control antibody. Gamma interferon induced by coinfection with L. major synergized with anti-IL-4 treatment to inhibit the development of MAIDS pathology. Conversely, treatment with anti-gamma interferon led to a significant increase in splenomegaly and lymphadenopathy and slightly exacerbated loss of T-cell function. These data suggest that the production of Th2-associated cytokines may promote MAIDS pathology, while Th1-associated cytokines may help control the disease.

Oral carriage of Candida albicans in murine AIDS

Oral candidiasis is a common fungal infection in patients infected with the human immunodeficiency virus (HIV). Although rare at the time of primary HIV infection, it is frequently found throughout the asymptomatic phase and is predictive of progressive immunodeficiency. However, the precise immune defect which results in outgrowth of commensal Candida albicans in HIV infection has not been identified. Mice infected with the Du5H(G6T2) mixture of mouse leukemia viruses develop a syndrome, designated murine AIDS (MAIDS), that has many of the immune abnormalities found in HIV infection. Retrovirus-infected C57BL/6 mice were examined for their ability to resist the development of oral candidiasis from the carrier state established after a self-limiting acute infection and to clear a subsequent secondary inoculum of oral C. albicans. Most of the mice orally colonized with C. albicans and then inoculated with the retrovirus mixture maintained a low-level oral carriage of C. albicans, while 30% of coinfected mice developed recurring 2- to 3-week episodes of acute Candida proliferation, separated by transient recoveries to the carrier state. The frequencies of CD4+ and CD8+ lymphocytes were, respectively, unchanged and significantly decreased (P < 0.05) in both cervical lymph nodes and spleens of coinfected mice compared to the corresponding frequencies in C. albicans-carrying, virus-free, age-matched control animals. Secretion of gamma interferon by concanavalin A (ConA)-stimulated spleen cells from Candida-carrying, retrovirus-infected mice was significantly decreased (P < 0.05) compared to that of C. albicans-carrying, retrovirus-free mice, in accordance with known abnormalities associated with MAIDS. However, production of this cytokine by ConA-stimulated or unstimulated cervical lymph node cells from coinfected mice was enhanced compared to that of virus-free animals colonized with C. albicans. Acquired resistance to reinfection with C. albicans was maintained in retrovirus-infected mice and was associated with a mucosal recruitment of CD8+ cells not observed in control mice. These results suggest that alterations in mucosal immunity which occur in MAIDS differ substantially from defects observed at other sites and that surrogate epithelial defense mechanisms may function locally to limit Candida proliferation.

Th0-like CD4+ T cells protect mice with murine retrovirus-induced immunodeficiency syndrome (MAIDS) against co-infection with Listeria monocytogenes

We examined the host defence mechanism against infection with Listeria monocytogenes, a facultative intracellular bacterium, in mice with murine acquired immunodeficiency syndrome (MAIDS) caused by LP-BM5 murine leukaemia virus (MuLv) infection. Although LP-BM5 MuLV infection in C57BL/6 mice leads to a stage of immunodeficiency characterized by severe compromise of cell-mediated immunity, the mice with established MAIDS infected with LP-BM5 8 weeks previously, showed resistance to an intraperitoneal infection with Listeria monocytogenes. These MAIDS mice also showed resistance to a lethal dose of secondary listerial challenge, while the delayed-type hypersensitivity response to heat-killed Listeria (HKL.) was severely impaired in MAIDS mice. The resistance of MAIDS mice to listerial infection was mediated by CD4+ alpha beta T cells but neither by gamma delta T cells nor natural killer (NK) cells. Interferon-gamma (IFN-gamma) and interleukin-10 (IL-10) were produced by CD4+ T cells from Listeria-infected MAIDS mice in response to the in vitro stimulation with HKL, whereas IFN-gamma but not IL-10 were produced by those from Listeria-infected control mice. These results suggest that T-helper 0 (Th0)-like immune responses of CD4+ T cells occur and participate in host defence mechanisms against listerial infection in MAIDS mice.

Retrovirus-elicited interleukin-12 and tumour necrosis factor-alpha as inducers of interferon-gamma-mediated pathology in mouse AIDS

Spleen cells from mice resistant or sensitive to mouse acquired immune deficiency syndrome (MAIDS) were examined for cytokine mRNA. In MAIDS-resistant BALB/c mice, cytokine transcripts peaked at 1 week after infection with Type 1 cytokines [interleukin-2 (IL-2), tumour necrosis factor-alpha (TNF-alpha), interferon-gamma (IFN-gamma), IL-12], dominating over Type 2 cytokines (IL-4, IL-10). Expression of cytokines other than IL-2 later declined to levels seen in uninfected mice. In MAIDS-sensitive B6 mice, transcripts for all cytokines were increased at 1 week and, except for IL-2, increased progressively. Spontaneous production of IFN-gamma protein was associated with enhanced mRNA expression at 1 week after infection of either strain, but none was detectable in association with even higher levels of transcripts at later times after infection of B6 mice. Spleen cells from longer-term-infected B6 mice, however, produced substantial amounts of IFN-gamma following treatment with lipopolysaccharide (LPS) or IL-12. Inclusion of anti-IL-12 or anti-TNF-alpha antibodies blocked induction of IFN-gamma by LPS. Induction of IFN-gamma by IL-12 was potentiated by TNF-alpha following stimulation of intact spleen cells and purified CD4+ or CD8+ T cells, as well as negatively selected CD4-8- cells from infected B6 mice. Further studies showed that IFN-gamma knockout mice on a B6 background developed MAIDS with a prolonged time-course, whereas BALB/c knockout mice were unchanged in their resistance to MAIDS. These studies suggest that continuing low-level expression of IFN-gamma, stimulated by IL-12 and TNF-alpha, contributes to the susceptibility of B6 mice to MAIDS but is not required for the resistance of BALB/c mice to disease.

Effects of exogenous, nonleukemogenic, ecotropic murine leukemia virus infections on the immune systems of adult C57BL/6 mice

Mouse AIDS (MAIDS) develops in mice infected with a mixture of replication-competent ecotropic and mink lung cell focus-inducing murine leukemia viruses and an etiologic replication-defective virus. Helper viruses are not required for induction of MAIDS, but the time course of disease is accelerated in their presence. To understand the possible contributions of ectropic murine leukemia viruses to MAIDS pathogenesis, we biologically cloned a series of viruses from the MAIDS-inducing LP-BM5 virus mixture. These viruses were examined for replication in tissues of infected mice and for effects on the immune system. All virus stocks replicated efficiently in mice. Infected animals showed slight lymphadenopathy and splenomegaly due primarily to B-cell proliferation associated with differentiation to immunoglobulin secretion resulting in twofold increases in serum immunoglobulin M levels; however, B-cell responses to helper T-cell-independent antigens were increased rather than decreased as in MAIDS. Analyses of CD8+ T-cell function showed that cytotoxic T-lymphocyte responses to alloantigens were comparable in control and infected mice. Finally, we showed that infection resulted in enhanced expression of transcripts for interleukin-10, interleukin-4, and gamma interferon. These cytokines can all contribute to B-cell activation and may promote the expansion of a target cell population for the MAIDS defective virus.

Hot water extracts of Chlorella vulgaris reduce opportunistic infection with Listeria monocytogenes in C57BL/6 mice infected with LP-BM5 murine leukemia viruses

The bacterial elimination after infection with Listeria monocytogenes was impaired in mice with murine acquired immunodeficiency syndrome (MAIDS) by infection with LP-BM5 murine leukemia virus. Oral administration of hot water extracts of Chlorella vulgaris (CVE) restored the capacity of MAIDS mice to eliminate L. monocytogenes in association with improvement of the deteriorated immune response to L. monocytogenes. DTH response to Listeria in CVE-treated MAIDS mice was significantly higher than that of MAIDS mice after Listeria infection in association with increases in number of CD4+CD8- and CD4-CD8+ alpha beta T-cells in the infected sites. CVE might be effective in the treatment of opportunistic infection in retrovirus-induced immunodeficient patients.

Up-regulation of T helper 2 and down-regulation of T helper 1 cytokines during murine retrovirus-induced immunodeficiency syndrome enhances susceptibility of a resistant mouse strain to Leishmania amazonensis

Resistance to and recovery from leishmania infection is dependent on cell-mediated immunity. C57BL/6 mice are resistant to Leishmania amazonensis (La) infection but susceptible to LP-BM5 murine leukemia virus (MuLV) infection. MuLV infection leads to a state of immunodeficiency characterized by severe compromise of cell-mediated immunity. When infected with La alone, C57BL/6 mice developed a small transient lesion that evolved to spontaneous healing or a lesion with extremely slow growth. Lesions were predominantly comprised of a lympho-macrophagic infiltrate with few parasitized macrophages. When infected with La and, 4 weeks later, with MuLV (La-MuLV), the mice developed a large uncontrolled nonhealing lesion containing vacuolated and heavily parasitized macrophages. In contrast, mice infected with MuLV first and La 4 weeks later (MuLV-La) developed a small but persistent lesion, characterized histologically by a small number of heavily parasitized macrophages and few lymphocytes. Eight weeks after MuLV infection, both had similar immunological profiles with decreased lymphocyte proliferation, diminished production of interferon-gamma, and high production of interleukins 4 and 10. At the time of L. amazonensis infection, La-MuLV animals have a normal T cell function whereas in MuLV-La mice this function is already impaired; this may influence the recruitment of macrophages to the site of leishmania injection.

Interactions between murine AIDS (MAIDS) and toxoplasmosis in co-infected mice

We coinfected C57B1/6 mice with LP-BM5 murine leukaemia viruses, responsible for murine AIDS (MAIDS), and an avirulent strain of Toxoplasma gondii. Virus-infected mice were infected perorally on day 30 with 10 cysts of T. gondii, and T. gondii-infected mice were challenged with LP-BM5 on day 20, 30 or 60 after parasite inoculation. Uninfected and singly infected mice were used as controls. The kinetics of parasite burden in blood, lungs and brain, together with blood lymphocyte subsets, and spleen and lymph node weights, were serially determined in each group of mice. The kinetics of parasite counts in mice infected by LP-BM5 then by T. gondii were similar to those in mice infected by T. gondii only, except for lung counts, which reached higher values than in animals infected with T. gondii alone, then fell and re-increased until the end of the experiment. The only significant change in parasite burdens when mice were first infected by T. gondii and then by LP-BM5, compared with T. gondii controls, was an increase in lung counts in mice challenged with LP-BM5 20 days after T. gondii inoculation. Whatever the schedule of co-infection, the kinetics of lymphocyte subsets in co-infected mice differed from those in T. gondii- or LP-BM5-infected mice; in dually infected mice CD4+ and CD8+ cell counts were intermediate between values in mice singly infected by the parasite or the virus. Enlargement of spleen and lymph nodes, which is a major criterion of MAIDS progression, was significantly less marked in co-infected mice than in mice infected with LP-BM5 alone. These data point to cross-regulation of T. gondii and LP-BM5 infections, which results in increased susceptibility to T. gondii, and may alter the progression of MAIDS.

Murine AIDS protects mice against experimental cerebral malaria: down-regulation by interleukin 10 of a T-helper type 1 CD4+ cell-mediated pathology

The retrovirus LP-BM5 murine leukemia virus induces murine AIDS in C57BL/6 mice that has many similarities with human AIDS; Plasmodium berghei ANKA causes experimental cerebral malaria in the same strain of mice. The outcome of malaria infection was studied in mice concurrently infected with the two pathogens. The retrovirus significantly reduced the gravity of the neurological manifestations associated with Plasmodium berghei ANKA infection. The protection against experimental cerebral malaria induced by murine AIDS increased with duration of viral infection and, hence, with the severity of the immunodeficiency. Interleukin 10, principally from splenic T cells, was shown to play a crucial role in this protection.

Consequences of secondary or co-infections for immunity

While remarkable progress has been made using genetically altered mice to understand the importance of different cytokines in protecting against experimental infections or co-infections, an examination of the opportunistic infections that occur during HIV infection of humans does not yet show a clear picture of cytokine imbalance. Opportunistic infections appear to result from impairments in cells mediating innate resistance, such as natural killer cells, macrophages, and neutrophils. Some of these defects may not be corrected even if CD4+ T cells were suddenly restored to normal. The lessons from immunodeficient and gene knockout mice now need to be put to the test in the clinic.

Impairment of the cellular immune response in acute murine toxoplasmosis: regulation of interleukin 2 production and macrophage-mediated inhibitory effects

Depression of the cellular immune response to Toxoplasma gondii has been reported in both mice and humans. The present study was undertaken to determine the kinetics and mechanism of the observed downregulation of interleukin 2 (IL-2) production during experimental murine toxoplasmosis. For these investigations, the cell-mediated immune response to the wild type (PTg) was compared with that to the less-virulent mutant parasite (PTgB), which is deficient in the major surface antigen, p30 (SAG-1). Spleen cells from infected A/J mice failed to proliferate in response to Toxoplasma antigens during the first week of infection. Both PTg- and PTgB-infected A/J mice exhibited a significant reduction in the concanavalin A (Con A)-induced lymphoproliferative response. Further, the response of splenocytes from mice infected with the wild-type parasite was significantly diminished compared with that of mice infected with PTgB. The lymphoproliferative response to Con A reached its nadir at day 7 and remained below control levels for at least 14 days postinfection. By day 21 postinfection, the response to Con A and to Toxoplasma antigens was restored to the level observed prior to day 7. Con A-stimulated culture supernatants of spleen cells from mice on day 7 postinfection contained significantly less IL-2 than normal mice. There was no significant difference in the numbers of binding sites or capacity of high-affinity IL-2 receptors between infected and normal mouse splenocytes as determined by Scatchard analysis. Exogenous IL-2 at different concentrations failed to restore the proliferative response of lymphocytes from infected mice to Con A. Adherent macrophages from 7-day-infected mice were able to suppress IL-2 production by normal splenocytes following stimulation with Con A. The inhibitory activity mediated by infected cells was reversed by the antibody to IL-10 but not transforming growth factor beta. There were insignificant levels of nitric oxide production in both infected and normal splenocytes. These results indicate that during acute murine toxoplasmosis, there is a well-defined period (day 7) during which both the T-cell mitogen and parasite antigen-associated lymphoproliferative response are reduced. Further, there is a reduction in the production of IL-2 and an increase in IL-10, which appear to mediate, in part, the observed downregulation of immunity to T. gondii.

Feline immunodeficiency virus predisposes cats to acute generalized toxoplasmosis

This study was designed to examine the effects of a pre-existing, clinically asymptomatic feline immunodeficiency virus (FIV) infection on a primary challenge with Toxoplasma gondii. Parenteral challenge of FIV-infected cats with tachyzoites of the ME49 strain of T. gondii caused a precipitous drop in all lymphocytes (CD4+, CD8+, and B cells) and generalized severe toxoplasmosis. The predominant postmortem lesions included acute and often fatal interstitial pneumonia, dominated histologically by macrophages, and multifocal to coalescing hepatic necrosis. Immunohistochemistry revealed numerous T. gondii antigen and tachyzoites in macrophages and other cell types in the lung lesions. The proliferative response of peripheral blood mononuclear cells to specific (T. gondii antigen) and nonspecific (Concanavalin A) mitogens was defective in the dually infected cats, suggesting marked immunosuppression. In contrast to the dually infected cats, cats infected only with T. gondii developed a transient, mild clinical disease characterized by anorexia, lethargy, and multifocal chorioretinitis. Lymphocyte changes in T. gondii-infected cats included an early pan-lymphopenia followed by reestablishment of all lymphocyte subset profiles. These cats also showed a reduced proliferative response to Concanavalin A at 1 week after challenge, but a measurable in vivo response to T. gondii antigens, as evidenced by in vitro lymphocyte proliferation in the absence of a mitogenic stimulus. These results show that infection of cats with FIV-NCSU, markedly enhances their susceptibility to a primary T. gondii infection and provides a model to study the mechanisms of the underlying immunological defect(s) occurring early after HIV infection that may predispose individuals to development of acquired immunodeficiency syndrome and associated diseases.

Immunity to Toxoplasma gondii

Recent advances in our understanding of the immunological mechanisms involved during infection with Toxoplasma gondii include evidence for the role of different subsets of lymphocytes and cytokines in acute infection as well as in reactivation of chronic infection. The mechanisms of presentation of T. gondii antigen have been clarified recently, and animal models of toxoplasmosis that mimic disease observed in AIDS patients developed.