NO as an effector molecule of parasite killing: modulation of its synthesis by cytokines

It has recently been appreciated that NO, a molecule previously known to play a physiologic role in blood pressure regulation, is a major effector molecule of macrophage cytotoxicity against a variety of microbial targets, including protozoan and helminth parasites. NO production by macrophages is arginine dependent and catalyzed by a cytokine-inducible form of the NO synthase. This activity is positively controlled by several up-regulatory stimuli (including IFN-gamma, TNF-alpha, IL-2) and negatively controlled by others (principally IL-10, IL-4, TGF-beta). Other cell types, such as endothelial cells and hepatocytes, display a similar capacity for NO production in response to cytokine stimulation. In murine models of leishmaniasis and schistosomiasis, in vivo NO synthesis correlates with protective immunity against infection. The effector molecule that plays a similar role in cell-mediated immunity in man has not yet been identified.

The role of cytokines in the pathogenesis of hepatic granulomatous disease in Schistosoma mansoni infected mice

Cytokines are important in the cell-mediated response to Schistosoma mansoni eggs. We have found that Th2 cytokine responses (e.g. IL-4 and IL-5) are augmented after egg laying begins while Th1 responses (IL-2 and IFN-gamma) are down regulated in S. mansoni infected mice. Treatment of mice with anti-IL-5 monoclonal antibodies (Mab) suppressed the eosinophil response almost completely but did not affect granuloma size and slightly increased hepatic fibrosis. Anti-IL-4 treatment abolished IgE responses in infected mice and decreased hepatic fibrosis slightly. Anti-IFN-gamma treatment had no effect on hepatic pathology. Anti-IL-2 treatment decreased granuloma size significantly and decreased hepatic fibrosis markedly. Anti-IL-2 treatment dramatically decreased IL-5 secretion by splenic cells in vitro and decreased peripheral blood and tissue eosinophilia. In contrast IL-4 secretion was unaffected and serum IgE was normal or increased. IL-2 and IFN-gamma secretion by splenic cells of treated mice were slightly but not significantly increased suggesting that anti-IL-2 treatment is affecting Th2 rather than Th1 responses.

Endothelial cells are activated by cytokine treatment to kill an intravascular parasite, Schistosoma mansoni, through the production of nitric oxide

Like many pathogens that undergo an intravascular stage of development, larvae of the helminth parasite Schistosoma mansoni migrate through the blood vessels, where they are in close contact with endothelial cells. In vitro exposure of murine endothelial cells to various cytokines (interferon gamma, tumor necrosis factor alpha, and interleukin 1 alpha or 1 beta) resulted in their activation to kill schistosomula through an arginine-dependent mechanism involving production of nitric oxide (NO). Cytokine-treated endothelial cells showed increased expression of mRNA for the inducible form of the NO synthase, and both NO production and larval killing were suppressed by treatment with competitive inhibitors. The effector function of cytokine-treated endothelial cells was similar to that of activated inflammatory tissue macrophages, although activation appeared to be differentially regulated in these two cell types. Activated endothelial cells killed older (18-day) forms of the parasite, such as those currently thought to be a primary target of immune elimination in the lungs of mice previously vaccinated with radiation-attenuated cercariae, as well as newly transformed larvae. In C57BL/6 mice, which become resistant to S. mansoni infection as a result of vaccination with irradiated cercariae, endothelial cell morphology characteristic of activation was observed in the lung by 1-2 weeks after challenge infection. Similar endothelial cell changes were absent in P-strain mice, which do not become resistant as a result of vaccination. Together, these observations indicate that endothelial cells, not traditionally considered to be part of the immune system, may play an important role in immunity to S. mansoni and, by means of NO-dependent killing, could serve as effectors of resistance to other intravascular pathogens.

Role of interleukin-10 in T helper cell dysfunction in asymptomatic individuals infected with the human immunodeficiency virus

The loss of T helper cell (TH) function in asymptomatic HIV type 1-infected individuals occurs before the decline in CD4+ T cells. At least part of the loss in TH function results from changes in immunoregulatory cytokine profiles. To investigate the role of IL-10 in such dysregulation, we tested whether: (a) expression of IL-10-specific mRNA would be upregulated in PBMC from asymptomatic, HIV-infected (HIV+) individuals; (b) PBMC from these same individuals would produce increased levels of IL-10 when stimulated in vitro with phytohemagglutinin; and (c) defective antigen-specific TH function could be restored by anti-IL-10 antibody. We observed that IL-10-specific mRNA was marginally upregulated, and increased levels of IL-10 were produced by PBMC from HIV+ individuals compared with PBMC from uninfected individuals. Those individuals whose TH function was more severely compromised produced higher levels of IL-10. Additionally, defective antigen-specific TH function in vitro could be reversed by anti-IL-10 antibody, including the response to HIV envelope synthetic peptides. Furthermore, the antigen-specific TH responses of HIV-uninfected PBMC could be reduced with IL-10, a process reversed by anti-IL-10. These results confirm that the early loss of TH function in HIV+ individuals is due at least in part to cytokine-induced immune dysregulation, and support the hypothesis of a switch from a predominant type 1 state to a predominant type 2 condition in HIV infection.

Emergence of NK1.1+ cells as effectors of IFN-gamma dependent immunity to Toxoplasma gondii in MHC class I-deficient mice

CD8+ T lymphocytes have been reported to play a major role in the protective immune response against acute infection with Toxoplasma gondii. In order to further assess the role of CD8+ cells in resistance against this protozoan we examined the ability of beta 2m-deficient mice, which fail to express MHC class I molecules and peripheral CD8+ lymphocytes, to survive tachyzoite challenge following vaccination with an attenuated parasite mutant. Surprisingly, vaccination of beta 2m-deficient mice induced strong resistance to lethal challenge, with > 50% surviving beyond 3 months. Vaccinated beta 2m-deficient mice, but not control heterozygotes, showed a five- to six-fold expansion in spleen cell number and approximately 40% of the splenocytes were found to express the NK markers NK1.1 and asialo GM1. Spleen cells from the vaccinated beta 2m-deficient animals failed to kill either infected host cells or the NK target YAC-1. However, high levels of IFN-gamma were secreted when the cells were cultured in vitro with soluble T. gondii lysate, and this response was abolished by NK1.1+ but not CD4+ and CD8+ lymphocyte depletion, implicating the NK1.1+ population as the major source of IFN-gamma. More importantly, vaccine-induced immunity in beta 2m-deficient mice was completely abrogated by in vivo administration of antibody to NK1.1, asialo GM1, or IFN-gamma. Together, the data suggest that in class I-deficient mice vaccinated against T. gondii, the absence of CD8+ effector cells is compensated for by the emergence of a population of NK1.1+ and asialo GM1+ cells which lack cytolytic activity, and that the protective action of these cells against the parasite is attributable to IFN-gamma production. The induction of this novel NK population may provide an approach for controlling opportunistic infections in immunocompromised hosts.

Interleukin 12 acts directly on CD4+ T cells to enhance priming for interferon gamma production and diminishes interleukin 4 inhibition of such priming

Naive CD4+ T cells produce interleukin 2 (IL-2) but little IL-4 or interferon gamma (IFN-gamma). In vitro, they develop into IL-4 or IFN-gamma producers depending on the conditions of the priming culture. Using T-cell receptor transgenic CD4+ T cells, the role of IL-12 and IL-4 in antigen-specific priming was examined. IL-12 substantially enhanced the ability of naive CD4+ T cells to develop into cells that produced IFN-gamma upon restimulation. However, it was not essential since anti-IL-12 antibodies failed to block the priming for IFN-gamma observed in the absence of exogenous IL-12. When both IL-12 and IL-4 were present in the priming culture, IL-12 did not inhibit priming for IL-4 production. In contrast, IL-4 diminished but did not abolish priming for IFN-gamma production. In an accessory cell-independent priming system, IL-12 strikingly augmented priming for IFN-gamma production, indicating that it acts directly on T cells. IFN-gamma itself did not enhance priming for IFN-gamma production in either accessory cell-dependent or independent systems. In an accessory cell-dependent system, the IL-12-mediated enhancement was not blocked by adding neutralizing anti-IFN-gamma monoclonal antibody. However, in an accessory cell-independent system, anti-IFN-gamma antibody did inhibit priming for IFN-gamma production leaving open a role for IFN-gamma in the priming process. These data indicate that IL-12 has a major effect on the inductive phase of T-cell priming by enhancing commitment to IFN-gamma production and thus can profoundly influence the state of immunity that develops.

Acute cerebral toxoplasmosis is induced by in vivo neutralization of TNF-alpha and correlates with the down-regulated expression of inducible nitric oxide synthase and other markers of macrophage activation

C57BL/6 mice infected with the ME-49 strain of Toxoplasma gondii develop a progressive encephalitis culminating in 100% mortality between 12 and 15 wk after intraperitoneal inoculation of the parasite. Moreover, when injected at 4 wk after infection with anti-IFN-gamma mAb, progression of toxoplasmic encephalitis is markedly accelerated, resulting in death of the animals by 9 to 12 days posttreatment. In this study, we investigated the expression of mRNAs encoding cytokines as well as lymphocyte and macrophage markers during the development of toxoplasmic encephalitis. High levels of lymphocyte CD4 and CD8 surface Ag transcript were detected in the brains of mice throughout the infection. In addition from 2 to 4 wk we found elevations of Th1 (IFN-gamma and IL-2) but not of Th2 (IL-4 and IL-5) cytokine mRNAs. The elevation in Th1 cytokines was accompanied by increases in the expression of monokine (IL-1, IL-6, IL-10, granulocyte macrophage-colony stimulating factor [GM-CSF], and TNF-alpha) mRNAs, as well as markers expressed by activated macrophages (major histocompatibility class II [Ia], inducible nitric oxide synthase [iNOS] and macrophage activation gene 1 [Mag-1]). Interestingly, after 8 wk of infection with T. gondii we observed a dramatic decrease of Th1 cytokine and most monokine (IL-1, IL-6, GM-CSF, and TNF-alpha) as well as Mag-1 and iNOS mRNA levels. This down-regulation was associated with enhanced necrosis and neutrophilic infiltrates in the brain accompanied by increased expression of genes expressed specifically by the tachyzoite stage of T. gondii (T. gondii surface antigen 1 [SAG-1] and T. gondii surface antigen 2 [SAG-2]). Similarly, in mice chronically infected with T. gondii and treated with anti-IFN-gamma mAb the resulting pathology was associated with decreased expression of TNF-alpha and iNOS and increased expression of SAG-1 and SAG-2. Moreover, treatment with anti-TNF-alpha mAb also resulted in enhanced pathology, which correlated with low levels of iNOS mRNA and high levels of tachyzoite-specific mRNAs. Together these results suggest that reactivation of T. gondii results from a down-regulation of IFN-gamma and TNF-alpha expression leading to decreased macrophage or microglial cell activation, release of parasite growth, and subsequent tissue damage.

Acute cerebral toxoplasmosis is induced by in vivo neutralization of TNF-alpha and correlates with the down-regulated expression of inducible nitric oxide synthase and other markers of macrophage activation

C57BL/6 mice infected with the ME-49 strain of Toxoplasma gondii develop a progressive encephalitis culminating in 100% mortality between 12 and 15 wk after intraperitoneal inoculation of the parasite. Moreover, when injected at 4 wk after infection with anti-IFN-gamma mAb, progression of toxoplasmic encephalitis is markedly accelerated, resulting in death of the animals by 9 to 12 days posttreatment. In this study, we investigated the expression of mRNAs encoding cytokines as well as lymphocyte and macrophage markers during the development of toxoplasmic encephalitis. High levels of lymphocyte CD4 and CD8 surface Ag transcript were detected in the brains of mice throughout the infection. In addition from 2 to 4 wk we found elevations of Th1 (IFN-gamma and IL-2) but not of Th2 (IL-4 and IL-5) cytokine mRNAs. The elevation in Th1 cytokines was accompanied by increases in the expression of monokine (IL-1, IL-6, IL-10, granulocyte macrophage-colony stimulating factor [GM-CSF], and TNF-alpha) mRNAs, as well as markers expressed by activated macrophages (major histocompatibility class II [Ia], inducible nitric oxide synthase [iNOS] and macrophage activation gene 1 [Mag-1]). Interestingly, after 8 wk of infection with T. gondii we observed a dramatic decrease of Th1 cytokine and most monokine (IL-1, IL-6, GM-CSF, and TNF-alpha) as well as Mag-1 and iNOS mRNA levels. This down-regulation was associated with enhanced necrosis and neutrophilic infiltrates in the brain accompanied by increased expression of genes expressed specifically by the tachyzoite stage of T. gondii (T. gondii surface antigen 1 [SAG-1] and T. gondii surface antigen 2 [SAG-2]). Similarly, in mice chronically infected with T. gondii and treated with anti-IFN-gamma mAb the resulting pathology was associated with decreased expression of TNF-alpha and iNOS and increased expression of SAG-1 and SAG-2. Moreover, treatment with anti-TNF-alpha mAb also resulted in enhanced pathology, which correlated with low levels of iNOS mRNA and high levels of tachyzoite-specific mRNAs. Together these results suggest that reactivation of T. gondii results from a down-regulation of IFN-gamma and TNF-alpha expression leading to decreased macrophage or microglial cell activation, release of parasite growth, and subsequent tissue damage.

A partial cDNA clone of trypomastigote decay-accelerating factor (T-DAF), a developmentally regulated complement inhibitor of Trypanosoma cruzi, has genetic and functional similarities to the human complement inhibitor DAF

Resistance to complement-mediated lysis in Trypanosoma cruzi is due to the expression of complement-regulatory factors by the virulent developmental forms of this protozoan parasite. An 87- to 93-kDa molecule, which we have termed T-DAF (trypomastigote decay-accelerating factor), is present on the surface of the parasite and inhibits complement activation in a manner functionally similar to the mammalian complement regulatory component, decay-accelerating factor. In this report, we characterized monospecific polyclonal and monoclonal antibodies which were obtained from mice and rabbits immunized with fast protein liquid chromatography-purified T-DAF. These polyclonal antibodies were shown to inhibit T-DAF activity and were capable of inducing lysis of the parasites. Both the polyclonal and monoclonal antibodies were used to screen a cDNA expression library prepared from T. cruzi trypomastigote mRNA. From this library, we obtained a partial lambda gt11 cDNA clone which showed genetic and functional similarity to the human C3 convertase inhibitor DAF (A. Nicholson-Weller, J. Burge, D. T. Fearon, P. F. Weller, and K. F. Austen, J. Immunol. 129:184-189, 1982).

Analysis of cytokine mRNA expression during primary granuloma formation induced by eggs of Schistosoma mansoni

Granulomas induced by parasite eggs are the primary lesions in mice infected with Schistosoma mansoni. Temporal analysis of cytokine mRNA expression in lung tissue containing synchronized granulomas demonstrated a Th0-like pattern of lymphokine expression. IFN-gamma, IL-1 beta, and IL-6 were the primary cytokines induced by day 1 in developing lung granulomas initiated by i.v. egg injection. These changes were followed by increases in expression of IL-2, IL-4, and IL-10 mRNA on day 3 and TNF-alpha and IL-5 mRNA on day 6. Nearly all cytokine mRNA reached maximal levels by day 6, which preceded the peak in granuloma size seen on day 14. In vivo treatment of egg-injected mice with either anti-IL-2 or anti-IL-4 antibodies significantly diminished the size of circumoval granulomas in the lungs. Both groups of antibody-treated animals displayed a marked reduction in IL-4 as well as IL-5 mRNA expression, although IFN-gamma and IL-2 mRNA levels were unchanged or slightly increased. These findings confirm previous observations suggesting a role for IL-2 in egg-induced pathology via the generation of Th2-associated responses, and also indicate a primary function for IL-4 in granuloma formation. Analysis of responses after injection of eggs into nude mice demonstrated that only the Th2 cytokines IL-4 and IL-5 are exclusively dependent on T cells for their induction. Taken together, these data suggest that Th2 cells producing IL-4 play a major role in egg granuloma formation, and that the induction and ultimate down-modulation of Th2-like responses may be influenced by non-T-cell-derived cytokines.

Analysis of cytokine mRNA expression during primary granuloma formation induced by eggs of Schistosoma mansoni

Granulomas induced by parasite eggs are the primary lesions in mice infected with Schistosoma mansoni. Temporal analysis of cytokine mRNA expression in lung tissue containing synchronized granulomas demonstrated a Th0-like pattern of lymphokine expression. IFN-gamma, IL-1 beta, and IL-6 were the primary cytokines induced by day 1 in developing lung granulomas initiated by i.v. egg injection. These changes were followed by increases in expression of IL-2, IL-4, and IL-10 mRNA on day 3 and TNF-alpha and IL-5 mRNA on day 6. Nearly all cytokine mRNA reached maximal levels by day 6, which preceded the peak in granuloma size seen on day 14. In vivo treatment of egg-injected mice with either anti-IL-2 or anti-IL-4 antibodies significantly diminished the size of circumoval granulomas in the lungs. Both groups of antibody-treated animals displayed a marked reduction in IL-4 as well as IL-5 mRNA expression, although IFN-gamma and IL-2 mRNA levels were unchanged or slightly increased. These findings confirm previous observations suggesting a role for IL-2 in egg-induced pathology via the generation of Th2-associated responses, and also indicate a primary function for IL-4 in granuloma formation. Analysis of responses after injection of eggs into nude mice demonstrated that only the Th2 cytokines IL-4 and IL-5 are exclusively dependent on T cells for their induction. Taken together, these data suggest that Th2 cells producing IL-4 play a major role in egg granuloma formation, and that the induction and ultimate down-modulation of Th2-like responses may be influenced by non-T-cell-derived cytokines.

Fc epsilon receptor-positive cells are a major source of antigen-induced interleukin-4 in spleens of mice infected with Schistosoma mansoni

When cultured in vitro with either mitogen or parasite antigens, spleen cells from mice infected with Schistosoma mansoni produce significantly higher levels of IL-4 than splenocytes from control animals. Previous studies suggested that this increase in IL-4 production occurs because of a selective expansion of T helper type 2 (Th2) cells in infected mice. However, these experiments employed unfractionated spleen populations rather than purified T lymphocytes. Here we demonstrate that T-depleted spleen cells from infected animals synthesize high levels of interleukin-4 (IL-4), but no IL-5 when stimulated with parasite antigen in vitro. Nevertheless, when purified by sorting, T cells and non-B, non-T (NBNT) populations produced similar amounts of IL-4 in response to parasite antigen. The IL-4 producing NBNT cells were found to belong to an Fc epsilon receptor (Fc epsilon R)-positive population which after sort purification produced high levels of IL-4 (between 1000 and 2000 U of per 5 x 10(3) cells). FACS analysis revealed that these Fc epsilon R+ cells make up 0.53% of splenic NBNT cells in control animals while in 8-9-week-infected animals they increase to 3.8% of that population. In contrast, in mice with 8-week unisexual worm infections these cells comprise only 1.71% of NBNT cells, indicating that eggs are a major stimulus of the response. The expansion of Fc epsilon R+ cells and their production of IL-4 could be an important factor regulating the selection and induction of different CD4+ subsets in schistosome-infected hosts.

Interleukin 12 is required for the T-lymphocyte-independent induction of interferon gamma by an intracellular parasite and induces resistance in T-cell-deficient hosts

Immunity against the intracellular protozoan Toxoplasma gondii is highly dependent on interferon gamma (IFN-gamma). We have previously shown that, in addition to T lymphocytes, natural killer (NK) cells can be stimulated by the parasite to produce this cytokine by a reaction requiring adherent accessory cells and tumor necrosis factor alpha. We now demonstrate that a recently characterized cytokine, interleukin 12 (IL-12), is also necessary for parasite-induced IFN-gamma synthesis by NK cells. Anti-IL-12 antibodies completely inhibited T. gondii or bacterial endotoxin-stimulated IFN-gamma production by NK-enriched spleen cells from severe combined immunodeficient mice. Moreover, potent NK cytokine responses were induced by the combination of IL-12 and tumor necrosis factor alpha. In addition, adherent spleen cells from scid/scid mice or thyoglycollate-elicited macrophages from BALB/c animals produced high levels of both IL-12 (p40) and tumor necrosis factor alpha mRNAs when exposed to either live tachyzoites, parasite extracts, or endotoxin, confirming that these cytokines are produced by accessory cells. Finally, in vivo studies showed that treatment with recombinant IL-12 results in prolonged survival of scid mice after infection with T. gondii by means of a response dependent on both IFN-gamma and NK cells. Together the data argue that IL-12 is required for the T-cell-independent triggering of NK cells by intracellular parasites and that the cytokine may be useful for inducing this protective pathway in immunodeficient hosts.

Detection of in vivo expression of interleukin-10 using a semi-quantitative polymerase chain reaction method in Schistosoma mansoni infected mice

A modified polymerase chain reaction (PCR) assay for analysis of cytokine gene expression from reverse-transcribed (R/T) RNA obtained from small numbers of cells is described in detail. This method employs a previously described dot-blot format and utilizes a target specific radioactive oligonucleotide probe which hybridizes to the PCR amplified product, thus increasing both specificity and sensitivity. This obviates the need for repeated electrophoresis gels and easily accommodates large experiments (e.g., numerous samples or kinetic studies), using small amounts of RNA from low cell numbers. Manipulation of many samples is further enhanced with the use of a PCR thermocycler, which like the dot-blot apparatus is designed in a 96-well format. We describe the use of the house-keeping enzyme hypoxanthine phosphoribosyltransferase (HPRT) as an internal standard, which is especially suitable since its range of detectability of expression is similar to that of the cytokines under test. This enables one to obtain an accurate measure of losses or degradation of RNA, as well as controlling for efficiency of the R/T and PCR reactions. These reactions are further controlled by inclusion of a standard curve consisting of a titration of a known amount of RNA from a cell line expressing the cytokine under test. As well as controlling for the R/T-PCR, this standard curve also enables one to obtain a semi-quantitative measure of cytokine expression by different cell populations during an immune response. We show that this method can be used successfully for studying differential expression of IL-10 in different microenvironments during infection of mice with Schistosoma mansoni.

Host resistance to Toxoplasma gondii: model for studying the selective induction of cell-mediated immunity by intracellular parasites

Cell-mediated immunity (CMI) provides a major host defense against infectious disease and malignancy. Nevertheless, the mechanisms underlying the induction of CMI remain poorly understood and this lack of knowledge remains an important impasse in the design of rationally based vaccines and immunotherapies. Toxoplasma gondii is an intracellular pathogen that, as part of its normal life cycle, induces a potent CMI response leading to host resistance. Studies on the interaction of this parasite with the immune system therefore provide a unique approach for identifying events that selectively lead to the triggering of host-protective CMI. In addition, this knowledge may provide new strategies for immunotherapy of opportunistic infections in immunodeficient hosts.

Toxoplasma gondii induces a T-independent IFN-gamma response in natural killer cells that requires both adherent accessory cells and tumor necrosis factor-alpha

Spleen cells from scid mice produce high levels of IFN-gamma when exposed to either live tachyzoites of Toxoplasma gondii or a soluble parasite extract. Small numbers of parasites are sufficient to stimulate this response, which is also induced by cell-free supernatants of cultured tachyzoites. The parasite molecules responsible for triggering IFN-gamma production are heat-labile but resistant to freezing and thawing. Depletion of NK cells or adherent cells from the splenocyte population abolishes the response. Moreover, cultured bone marrow-derived NK cells are stimulated by Toxoplasma to produce IFN-gamma, but only when supplemented with adherent peritoneal washout or thioglycollate-induced exudate cells. Supernatants of macrophages preincubated with T. gondii extract also induce IFN-gamma synthesis by cultured NK cells. Addition of neutralizing mAb against TNF-alpha abolishes the IFN-gamma response of scid spleen cells exposed to the parasite or of NK cells incubated with supernatants of adherent cells stimulated with T. gondii extract. Moreover, splenic adherent cells produce low levels of TNF-alpha in response to the parasite. Nevertheless, TNF-alpha alone is not sufficient to trigger IFN-gamma production from purified NK cell populations. These findings provide the first example of the stimulation of T-independent IFN-gamma production by a protozoan. The ability of T. gondii to trigger this pathway may underlie its induction of strong IFN-gamma-dependent nonspecific and specific cell-mediated immunity.

Toxoplasma gondii induces a T-independent IFN-gamma response in natural killer cells that requires both adherent accessory cells and tumor necrosis factor-alpha

Spleen cells from scid mice produce high levels of IFN-gamma when exposed to either live tachyzoites of Toxoplasma gondii or a soluble parasite extract. Small numbers of parasites are sufficient to stimulate this response, which is also induced by cell-free supernatants of cultured tachyzoites. The parasite molecules responsible for triggering IFN-gamma production are heat-labile but resistant to freezing and thawing. Depletion of NK cells or adherent cells from the splenocyte population abolishes the response. Moreover, cultured bone marrow-derived NK cells are stimulated by Toxoplasma to produce IFN-gamma, but only when supplemented with adherent peritoneal washout or thioglycollate-induced exudate cells. Supernatants of macrophages preincubated with T. gondii extract also induce IFN-gamma synthesis by cultured NK cells. Addition of neutralizing mAb against TNF-alpha abolishes the IFN-gamma response of scid spleen cells exposed to the parasite or of NK cells incubated with supernatants of adherent cells stimulated with T. gondii extract. Moreover, splenic adherent cells produce low levels of TNF-alpha in response to the parasite. Nevertheless, TNF-alpha alone is not sufficient to trigger IFN-gamma production from purified NK cell populations. These findings provide the first example of the stimulation of T-independent IFN-gamma production by a protozoan. The ability of T. gondii to trigger this pathway may underlie its induction of strong IFN-gamma-dependent nonspecific and specific cell-mediated immunity.

Schistosoma japonicum-infected mice show reduced hepatic fibrosis and eosinophilia and selective inhibition of interleukin-5 secretion by CD4+ cells after treatment with anti-interleukin-2 antibodies

Schistosoma japonicum-infected mice were injected with antibodies to interleukin-2 (IL-2) and/or IL-2 receptor to clarify the role of IL-2 on the granulomatous reaction around schistosome eggs in the liver. Granulomas were of normal or slightly increased size in animals subjected to IL-2 blockade, but hepatic fibrosis was markedly decreased in treated animals 10 weeks after infection. Anti-IL-2 treatment significantly decreased the in vitro secretion of IL-5 by antigen-stimulated spleen cells, and peripheral eosinophilia and tissue eosinophilia were diminished. Secretion of IL-2, IL-4, and gamma interferon was unaffected. Our results indicate that IL-2 is not an essential determinant of granuloma size in S. japonicum-infected mice but that, as in Schistosoma mansoni infection, the development of hepatic fibrosis is critically dependent on IL-2 levels and granuloma size and hepatic fibrosis are differentially regulated.

Helminth infection results in decreased virus-specific CD8+ cytotoxic T-cell and Th1 cytokine responses as well as delayed virus clearance

During the time of egg deposition, schistosome-infected mice exhibit a downregulation in interleukin 2 and interferon gamma production toward parasite antigens, mitogens, and foreign nonparasite protein antigens. To determine whether this imbalance in cytokine response would impact on CD8+ cytotoxic T-lymphocyte (CTL) responses, as well as on immune clearance of viral infections, we challenged Schistosoma mansoni-infected BALB/c mice, when cytokine imbalance was prominent, with a recombinant vaccinia virus expressing human immunodeficiency virus type 1 gp160. In contrast to control vaccinia-infected animals, S. mansoni plus vaccinia-infected mice did not produce significant Th1 cytokine responses upon in vitro stimulation with recombinant gp120, consistent with previous results for nonparasite antigens. However, more striking was the downregulation of the virus-specific CTL response not previously studied. Spleen cells from vaccinia-infected control mice displayed strong CD8+ cytolytic activity against gp160-transfected fibroblasts and fibroblasts pulsed with a peptide (P18) representing a CTL epitope of gp160. In contrast, mice coinfected with S. mansoni and vaccinia manifested absent or markedly reduced in vitro CTL activity even in the presence of exogenous interleukin 2. To determine whether this immune dysregulation might impact on viral clearance, we measured virus titers in tissues as a function of time. Mice infected with vaccinia virus alone rapidly cleared the virus, whereas in animals coinfected with S. mansoni, viral clearance was delayed by as much as 3 weeks in the liver and by several days in the spleen and lungs. These observations suggest that helminth infection may influence immune responses to concurrent viral infections.

Bone marrow macrophages process exogenous Toxoplasma gondii polypeptides for recognition by parasite-specific cytolytic T lymphocytes

CD8+ T cells from mice vaccinated with an attenuated strain of Toxoplasma gondii have previously been shown to have cytolytic activity against bone marrow macrophages (BMM phi) preincubated with a soluble tachyzoite extract. In the present study, we show that class I-transfected L cells differ from BMM phi in that although both cell types are recognized CTL after infection, only BMM phi are killed after sensitization with soluble tachyzoite extract. Gel filtration studies indicated that the T. gondii Ag responsible for sensitization of BMM phi are macromolecules of M(r) > or = 12,000. In contrast, peptides derived by tryptic digestion of this material were found to sensitize both transfected L cells and BMM phi. Although exogenous beta 2-microglobulin markedly enhanced peptide sensitization of BMM phi, no such effect was observed using the macromolecular preparation. This result suggests a requirement for cellular internalization in the processing by BMM phi of soluble Ag for class I-restricted recognition. In related experiments, infected and Ag-sensitized BMM phi were found to express cross-reactive T. gondii epitopes, as determined by cold target inhibition studies. Supernatant derived by 100,000 x g centrifugation of tachyzoite extract had potent sensitizing activity, and after anion exchange chromatography most of the activity was associated with a single fraction. The p30 Ag was not detected by immunoblot analysis in the biologically active supernatant and chromatographic fractions. These findings establish the feasibility of identifying the parasite Ag recognized by CD8+ effectors by direct fractionation of T. gondii proteins coupled with sensitization of BMM phi targets.

Bone marrow macrophages process exogenous Toxoplasma gondii polypeptides for recognition by parasite-specific cytolytic T lymphocytes

CD8+ T cells from mice vaccinated with an attenuated strain of Toxoplasma gondii have previously been shown to have cytolytic activity against bone marrow macrophages (BMM phi) preincubated with a soluble tachyzoite extract. In the present study, we show that class I-transfected L cells differ from BMM phi in that although both cell types are recognized CTL after infection, only BMM phi are killed after sensitization with soluble tachyzoite extract. Gel filtration studies indicated that the T. gondii Ag responsible for sensitization of BMM phi are macromolecules of M(r) > or = 12,000. In contrast, peptides derived by tryptic digestion of this material were found to sensitize both transfected L cells and BMM phi. Although exogenous beta 2-microglobulin markedly enhanced peptide sensitization of BMM phi, no such effect was observed using the macromolecular preparation. This result suggests a requirement for cellular internalization in the processing by BMM phi of soluble Ag for class I-restricted recognition. In related experiments, infected and Ag-sensitized BMM phi were found to express cross-reactive T. gondii epitopes, as determined by cold target inhibition studies. Supernatant derived by 100,000 x g centrifugation of tachyzoite extract had potent sensitizing activity, and after anion exchange chromatography most of the activity was associated with a single fraction. The p30 Ag was not detected by immunoblot analysis in the biologically active supernatant and chromatographic fractions. These findings establish the feasibility of identifying the parasite Ag recognized by CD8+ effectors by direct fractionation of T. gondii proteins coupled with sensitization of BMM phi targets.

Effect of rifampicin on the enterohepatic recycling of estrogen in female tuberculous patients

Enterohepatic recycling of estrogen after oral administration of 1 mg non-radioactive estriol was studied in ten women taking rifampicin as one of the antitubercular drugs and in fourteen healthy women selected as control subjects. The extent of enterohepatic recycling of estriol (E(3)) during early follicular phase of menstrual cycle was assessed by monitoring for 48 hours the urinary excretion of its two major metabolites i.e., estriol-16alpha-glucuronide (E(3)-16alpha-G) and estriol-3-glucuronide (E(3)-3-G). Of these metabolites the former is synthesized in liver as well as in the gut where as later is synthesized exclusively in the gut, hence the change in urinary level of H(3)-3-G with respect to E(3)-16alpha-G was considered to reflect the extent of enterohepatic recycling of E(3). The levels of both metabolites in urine were determined by radioimmunoassay. Lower values of variables including E(3)-3-G output, E(3)-3-G output as % of the total (E(3)-16alpha-G + E(3)-3-G) excretion and total E(3) (E(3)-16alpha-G + E(3)-3-G) output as % dose and the diurnal variation in urinary excretion of these metabolites show that the final elimination of E(3) from the body was quicker in these patients as compared with the control subjects, probably due to the reduced extent of enterohepatic recycling of E(3) as a result of long term chemotherapy with rifampicin.

CD8+ T-cell interactions with Toxoplasma gondii: implications for processing of antigen for class-I-restricted recognition

CD8+ T lymphocytes contribute to the control of acute and chronic T. gondii infection, and the activity of these cells is closely related to IFN gamma production. However, it is not clear whether CD8+ lymphocytes mediate protection solely by production of IFN gamma, or whether the lymphokine synergizes with CD8+ CTL activity. Interestingly, sensitization of bone marrow macrophages for CTL lysis can occur either as a result of infection or incubation with soluble T. gondii antigen. Thus, this system offers a powerful approach for analysing antigen processing pathways employed by intracellular protozoa, as well as for identifying the peptide epitopes which serve as targets for CD8(+)-dependent immunity.

The microbicidal activity of interferon-gamma-treated macrophages against Trypanosoma cruzi involves an L-arginine-dependent, nitrogen oxide-mediated mechanism inhibitable by interleukin-10 and transforming growth factor-beta

The present study was carried out to determine the effector mechanism of anti-Trypanosoma cruzi activity by interferon (IFN)-gamma plus lipopolysaccharide (LPS)-treated macrophages. A macrophage cell line (IC-21) that failed to mount an appreciable oxidative burst was nevertheless found able to control T. cruzi growth after exposure to IFN-gamma alone or IFN-gamma plus LPS. Moreover, microbicidal functions of both inflammatory macrophages and IC-21 against T. cruzi was found to be inhibited in the presence of NG-monomethyl-L-arginine (NGMMA), a competitive inhibitor of L-arginine. Addition of supplemental L-arginine to the culture overcame the capacity of NGMMA to block activated macrophage anti-T. cruzi functions. The ability of NGMMA to reverse both parasite growth inhibition and killing by IFN-gamma plus LPS-activated macrophages was found to correlate with the suppression of nitrite accumulation in the culture supernatants. Together, these results implicate the L-arginine-dependent production of nitric oxide in T. cruzi killing by activated macrophages. We also tested the ability of interleukin(IL)-10 and transforming growth factor (TGF)-beta, to block regulation of T. cruzi growth in this system. Both IL-10 and TGF-beta inhibited anti-parasite function by IFN-gamma-activated macrophages, with an optimal dose of 100 units/ml and 0.5 ng/ml, respectively. Moreover, when used in combination, suboptimal doses of IL-10 and TGF-beta were found to produce a synergistic inhibitory effect in the regulation of T. cruzi growth. The ability of IL-10 and TGF-beta to suppress microbicidal function was also positively correlated with inhibition of nitrite generation in macrophage culture supernatants. These results predict an in vivo role for IL-10 and TGF-beta in promoting parasite survival in the face of the host cell-mediated immune response.

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

Mice infected with LP-BM5 murine leukemia viruses develop a syndrome, termed mouse AIDS (MAIDS), characterized by increasingly severe immunodeficiency and progressive lymphoproliferation. Virus-infected mice were examined for the ability to resist acute infection and to control chronic infection with the protozoan Toxoplasma gondii, a major opportunistic pathogen of individuals infected with human immunodeficiency virus. Mice infected with the retroviruses for 2 or 4 weeks responded normally to challenge with the parasite, but mice inoculated with the protozoan 8 or 12 weeks after viral infection died with acute disease due to T. gondii. Increased sensitivity to acute infection was associated with a reduced ability to produce gamma interferon (IFN-gamma) and with established changes in CD4+ T-cell function. Mice latently infected with T. gondii and then inoculated with the retrovirus mixture were found to reactivate the parasite infection, with 30 to 40% of dually infected animals dying between 5 and 16 weeks after viral infection. Reactivation was associated with reduced proliferation and impaired production of IFN-gamma in response to stimulation with soluble T. gondii antigens or to concanavalin A. Continuing resistance to lethal reactivation in the remaining mice was shown to require CD8+ T cells and expression of IFN-gamma. In addition, it was found that chronic infection with T. gondii altered the course of MAIDS by inhibiting the progression of splenomegaly and immunodeficiency and reducing the expression of both the helper and etiologic defective viruses. These results support previous studies which indicate that infection with T. gondii is controlled by synergistic interactions between CD4+ and CD8+ T cells, the functions of which are progressively impaired during the course of MAIDS.

Role of T-cell derived cytokines in the downregulation of immune responses in parasitic and retroviral infection

Parasitic infection is frequently accompanied by a downregulation in host cell-mediated immunity. Recent studies suggest that this modulation of helper T cells and effector cell function can at least in part be attributed to the action of a set of inhibitory cytokines produced by T lymphocytes as well as by a number of other cell types. The best characterized of these inhibitory lymphokines are IL-4, IL-10 and TGF-beta. Interestingly, both IL-4 and IL-10 are produced by the Th2 but not the Th1 subset of CD4+ helper cells. The former subset dominates in many situations of chronic or exacerbated parasitic infection and is thought to suppress Th1 function as a consequence of the cross-regulatory activity of these two cytokines. The latter hypothesis is supported by recent experiments demonstrating that mAb-mediated neutralization of IL-10 reverses suppressed IFN-gamma responses and/or disease susceptibility in mice with parasitic infections. In vivo neutralization of TGF-beta has also been reported to increase host resistance to parasite challenge. In addition to suppressing T-cell differentiation, function or proliferation, IL-4, IL-10 and TGF-beta each inhibit the ability of IFN-gamma to activate macrophages for killing of both intracellular and extracellular parasites. Moreover, the three cytokines are able to synergize with each other in downregulating these parasiticidal effects. Interestingly, each of the cytokines inhibits the production of reactive nitrogen oxides, an effector mechanism previously demonstrated to play a major role in parasite killing by activated macrophages. In the case of IL-10, this suppression of nitrogen oxide production appears to result from an inhibition of TNF-alpha synthesis leading to defective macrophage stimulation. While distant from parasites in their biology and phylogeny, some retroviruses also appear to induce an over-production in downregulatory cytokines which is closely associated with the onset of immunodeficiency. Thus, in an animal model involving infection of mice with LP-BM5 MuLV and in human HIV infection, Th2 (IL-10 and/or IL-4) cytokine synthesis is increased while Th1 (IFN-gamma and/or IL-2) cytokine production is suppressed. These observations suggest that cytokine-mediated cross-regulation may play a role in the pathogenesis of acquired immune deficiency disease, contributing both to the progression of retroviral infection and the increase in susceptibility to opportunistic infections and malignancy. Observations of similar cytokine cross-regulatory activities in organisms as diverse as helminths, protozoa and retroviruses predict that comparable mechanisms may operate in a wide variety of infectious diseases.

Interleukin 10 inhibits macrophage microbicidal activity by blocking the endogenous production of tumor necrosis factor alpha required as a costimulatory factor for interferon gamma-induced activation

Interleukin 10 (IL-10) inhibits interferon gamma-induced macrophage activation for cytotoxicity against larvae of the human parasite Schistosoma mansoni by suppressing production of the toxic effector molecule nitric oxide (NO). In this study, the mechanism of IL-10 action was identified as inhibition of endogenous tumor necrosis factor alpha (TNF-alpha) production by interferon gamma-activated macrophages. TNF-alpha appears to serve as a cofactor for interferon gamma-mediated activation, since both schistosomulum killing and NO production were inhibited by anti-TNF-alpha antibody, whereas TNF-alpha alone was unable to stimulate these macrophage functions. IL-10 blocked TNF-alpha production by interferon gamma-treated macrophages at the levels of both protein and mRNA synthesis. Addition of exogenous TNF-alpha reversed IL-10-mediated suppression of macrophage cytotoxic activity as well as NO production. Likewise, addition of a macrophage-triggering agent (bacterial lipopolysaccharide or muramyl dipeptide), which induced the production of TNF-alpha, also reversed the suppressive effect of IL-10 on cytotoxic function. In contrast to IL-10, two other cytokines, IL-4 and transforming growth factor beta, which also inhibit macrophage activation for schistosomulum killing and NO production, did not substantially suppress endogenous TNF-alpha production. These results, therefore, describe a separate pathway by which macrophage microbicidal function is inhibited by the down-regulatory cytokine IL-10.

Simultaneous depletion of CD4+ and CD8+ T lymphocytes is required to reactivate chronic infection with Toxoplasma gondii

C57BL/6 mice chronically infected with an avirulent strain (ME-49) of Toxoplasma gondii were used to study the mechanisms by which T lymphocytes and IFN-gamma prevent reactivation of latent infection. Infected animals were treated with mAb, either anti-CD8, anti-CD4, anti-CD4 plus anti-CD8, anti-IFN-gamma, or anti-CD4 plus anti-IFN-gamma and the mice followed for survival, histopathology, cyst numbers, and spleen cell cytokine responses. In agreement with previously published findings, treatment with anti-IFN-gamma antibodies fully reactivated the asymptomatic infection, inducing massive necrotic areas in the brain with the appearance of free tachyzoites and death of all animals within 2 wk. Mice treated with the combination of anti-CD4 plus anti-CD8 antibodies showed augmented pathology and mortality nearly identical to the anti-IFN-gamma- treated animals. In contrast, treatment with anti-CD4 or anti-CD8 mAb alone failed to result in significantly enhanced brain pathology or mortality. In additional experiments, full reactivation of infection was observed in mice treated with anti-CD4 plus anti-IFN-gamma indicating that CD4+ lymphocytes are not required for the pathology resulting from IFN-gamma neutralization. Cytokine measurements on parasite Ag-stimulated spleen cells from mAb-treated mice indicated that both CD4+ and CD8+ cells produce IFN-gamma whereas only CD4+ cells contribute to parasite Ag-induced IL-2 synthesis. Together, these results suggest that CD4+ and CD8+ lymphocytes act additively or synergistically to prevent reactivation of chronic T. gondii infection probably through the production of IFN-gamma.

Simultaneous depletion of CD4+ and CD8+ T lymphocytes is required to reactivate chronic infection with Toxoplasma gondii

C57BL/6 mice chronically infected with an avirulent strain (ME-49) of Toxoplasma gondii were used to study the mechanisms by which T lymphocytes and IFN-gamma prevent reactivation of latent infection. Infected animals were treated with mAb, either anti-CD8, anti-CD4, anti-CD4 plus anti-CD8, anti-IFN-gamma, or anti-CD4 plus anti-IFN-gamma and the mice followed for survival, histopathology, cyst numbers, and spleen cell cytokine responses. In agreement with previously published findings, treatment with anti-IFN-gamma antibodies fully reactivated the asymptomatic infection, inducing massive necrotic areas in the brain with the appearance of free tachyzoites and death of all animals within 2 wk. Mice treated with the combination of anti-CD4 plus anti-CD8 antibodies showed augmented pathology and mortality nearly identical to the anti-IFN-gamma- treated animals. In contrast, treatment with anti-CD4 or anti-CD8 mAb alone failed to result in significantly enhanced brain pathology or mortality. In additional experiments, full reactivation of infection was observed in mice treated with anti-CD4 plus anti-IFN-gamma indicating that CD4+ lymphocytes are not required for the pathology resulting from IFN-gamma neutralization. Cytokine measurements on parasite Ag-stimulated spleen cells from mAb-treated mice indicated that both CD4+ and CD8+ cells produce IFN-gamma whereas only CD4+ cells contribute to parasite Ag-induced IL-2 synthesis. Together, these results suggest that CD4+ and CD8+ lymphocytes act additively or synergistically to prevent reactivation of chronic T. gondii infection probably through the production of IFN-gamma.

The effect of polymerization of methylmethacrylate on metal-backed and non-metal-backed acetabular components

Aseptic loosening of the acetabular component has been identified as the main cause of failure of cemented total hip arthroplasty in the long term. Wear of the high-density polyethylene has been implicated as a major contributory factor to loosening. The etiology of wear is multifactorial. The exothermic phase of polymerization of polymethylmethacrylate is associated with a 5.9% volume expansion. Using double-exposure holographic interferometry, the authors examined the influence of polymerization of cement on the internal geometry of a metal-backed cup and two non-metal-backed cups (Charnley 22 mm, Muller 32 mm). The non-metal-backed cups exhibited marked heat concentration points and disrupted fringe patterns, indicating distortion of the internal geometry of the cup. In contradistinction, the metal-backed cup showed good heat dissipation and regular fringe patterns, indicating minimal internal distortion. The differences were not quantified but were visually dramatic. The authors believe this may be an important contributory factor to cup wear in cemented total hip arthroplasty.

IL-10 synergizes with IL-4 and transforming growth factor-beta to inhibit macrophage cytotoxic activity

After activation with IFN-gamma, thioglycollate-elicited murine peritoneal macrophages kill schistosomula of Schistosoma mansoni in vitro by an L-arginine-dependent mechanism which involves the production of reactive nitrogen oxides (NO). In the present study we demonstrate that the regulatory cytokines IL-10, IL-4, and transforming growth factor-beta (TGF-beta) are potent inhibitors of this extracellular killing function of activated macrophages. Each cytokine was found to suppress killing of schistosomula in a dose-dependent fashion. The activity of IL-10 was not permanent, because subsequent treatment with additional IFN-gamma 2 to 6 h later reversed the inhibition of macrophage larval killing. More importantly, the combination of suboptimal levels of any two of these three cytokines was found to give a potent synergistic suppression of schistosomulum killing by IFN-gamma-treated macrophages. Similarly, IL-10, IL-4, or TGF-beta alone blocked the production of NO, and when used in combination these cytokines exhibited an enhanced inhibitory effect on nitrite production. Macrophage-mediated killing of schistosomula through the generation of NO has been shown previously to be a major effector mechanism of schistosome immunity. The results presented here suggest that the suppression of this mechanism by induction of the regulatory cytokines IL-10, IL-4, and TGF-beta, which are known to be produced during schistosome infection, may be an important strategy used by the parasite to evade macrophage-mediated immune destruction.

IL-10 synergizes with IL-4 and transforming growth factor-beta to inhibit macrophage cytotoxic activity

After activation with IFN-gamma, thioglycollate-elicited murine peritoneal macrophages kill schistosomula of Schistosoma mansoni in vitro by an L-arginine-dependent mechanism which involves the production of reactive nitrogen oxides (NO). In the present study we demonstrate that the regulatory cytokines IL-10, IL-4, and transforming growth factor-beta (TGF-beta) are potent inhibitors of this extracellular killing function of activated macrophages. Each cytokine was found to suppress killing of schistosomula in a dose-dependent fashion. The activity of IL-10 was not permanent, because subsequent treatment with additional IFN-gamma 2 to 6 h later reversed the inhibition of macrophage larval killing. More importantly, the combination of suboptimal levels of any two of these three cytokines was found to give a potent synergistic suppression of schistosomulum killing by IFN-gamma-treated macrophages. Similarly, IL-10, IL-4, or TGF-beta alone blocked the production of NO, and when used in combination these cytokines exhibited an enhanced inhibitory effect on nitrite production. Macrophage-mediated killing of schistosomula through the generation of NO has been shown previously to be a major effector mechanism of schistosome immunity. The results presented here suggest that the suppression of this mechanism by induction of the regulatory cytokines IL-10, IL-4, and TGF-beta, which are known to be produced during schistosome infection, may be an important strategy used by the parasite to evade macrophage-mediated immune destruction.

Treatment with anti-IL-2 antibodies reduces hepatic pathology and eosinophilia in Schistosoma mansoni-infected mice while selectively inhibiting T cell IL-5 production

Granulomas around Schistosoma mansoni eggs are a principal cause of morbidity in mice infected with this helminth. In vivo treatment of infected mice with anti-IL-2 antibodies, with or without anti-IL-2 receptor antibodies, significantly diminished the size of circumoval granulomas in the liver and decreased hepatic fibrosis to half that in untreated mice. Antibody-treated animals also displayed a marked reduction in both peripheral blood and tissue eosinophilia while IgE levels were unchanged or increased. Spleen cell cytokine production in response to Ag or mitogen stimulation was selectively altered by in vivo anti-IL-2 administration. IL-5 responses were dramatically reduced, whereas IL-4, IL-2, and IFN-gamma responses were not consistently changed. These findings confirm previous observations, suggesting a role for IL-2 in egg-induced pathology but indicate that the primary function of this cytokine in schistosome-infected mice may be in the generation of Th2- rather than Th1-associated responses.

Infection with Schistosoma mansoni alters Th1/Th2 cytokine responses to a non-parasite antigen

Schistosoma mansoni infection in the mouse has been shown to be accompanied by a down-regulation in parasite-Ag- and mitogen-induced Th1 cytokine secretion (IL-2 and IFN-gamma) with a simultaneous increase in the production of Th2 cytokines (IL-4, IL-5, and IL-10), suggesting a generalized imbalance in lymphocyte function. In the present study, we examined whether infection with S. mansoni would also influence the character of immune responses to a non-parasite Ag, sperm whale myoglobin (SwMb). When spleen cells (SC) from schistosome-infected SwMb-immunized animals were stimulated with SwMb, their production of IL-2 and IFN-gamma per CD4+ cell was found to be significantly reduced (by 45% and 59%, respectively) compared with the responses observed in immunized uninfected animals. Moreover, SwMb-induced secretion of IL-4 per CD4+ cell was increased threefold in SC cultures from infected mice. No myoglobin-induced IL-5 was detected in the same cultures. Addition to SC cultures of a neutralizing mAb specific for IL-10 partly restored the suppressed IFN-gamma response to SwMb seen in infected mice, suggesting a role for IL-10 in the observed down-regulation. S. mansoni-infected mice also showed an impaired antibody response to SwMb, with levels ranging from 10% to 27% of those observed in uninfected mice, although no differences in IgG isotype were evident. Taken together, these results suggest that infection with S. mansoni induces a down-regulation of Th1 responses and elevation of Th2 responses to unrelated foreign immunogens as well as to parasite Ag themselves. One implication of these findings is that helminth-infected individuals may have altered cell-mediated immune function to other microbial agents.

Treatment with anti-IL-2 antibodies reduces hepatic pathology and eosinophilia in Schistosoma mansoni-infected mice while selectively inhibiting T cell IL-5 production

Granulomas around Schistosoma mansoni eggs are a principal cause of morbidity in mice infected with this helminth. In vivo treatment of infected mice with anti-IL-2 antibodies, with or without anti-IL-2 receptor antibodies, significantly diminished the size of circumoval granulomas in the liver and decreased hepatic fibrosis to half that in untreated mice. Antibody-treated animals also displayed a marked reduction in both peripheral blood and tissue eosinophilia while IgE levels were unchanged or increased. Spleen cell cytokine production in response to Ag or mitogen stimulation was selectively altered by in vivo anti-IL-2 administration. IL-5 responses were dramatically reduced, whereas IL-4, IL-2, and IFN-gamma responses were not consistently changed. These findings confirm previous observations, suggesting a role for IL-2 in egg-induced pathology but indicate that the primary function of this cytokine in schistosome-infected mice may be in the generation of Th2- rather than Th1-associated responses.

Infection with Schistosoma mansoni alters Th1/Th2 cytokine responses to a non-parasite antigen

Schistosoma mansoni infection in the mouse has been shown to be accompanied by a down-regulation in parasite-Ag- and mitogen-induced Th1 cytokine secretion (IL-2 and IFN-gamma) with a simultaneous increase in the production of Th2 cytokines (IL-4, IL-5, and IL-10), suggesting a generalized imbalance in lymphocyte function. In the present study, we examined whether infection with S. mansoni would also influence the character of immune responses to a non-parasite Ag, sperm whale myoglobin (SwMb). When spleen cells (SC) from schistosome-infected SwMb-immunized animals were stimulated with SwMb, their production of IL-2 and IFN-gamma per CD4+ cell was found to be significantly reduced (by 45% and 59%, respectively) compared with the responses observed in immunized uninfected animals. Moreover, SwMb-induced secretion of IL-4 per CD4+ cell was increased threefold in SC cultures from infected mice. No myoglobin-induced IL-5 was detected in the same cultures. Addition to SC cultures of a neutralizing mAb specific for IL-10 partly restored the suppressed IFN-gamma response to SwMb seen in infected mice, suggesting a role for IL-10 in the observed down-regulation. S. mansoni-infected mice also showed an impaired antibody response to SwMb, with levels ranging from 10% to 27% of those observed in uninfected mice, although no differences in IgG isotype were evident. Taken together, these results suggest that infection with S. mansoni induces a down-regulation of Th1 responses and elevation of Th2 responses to unrelated foreign immunogens as well as to parasite Ag themselves. One implication of these findings is that helminth-infected individuals may have altered cell-mediated immune function to other microbial agents.

Regulation of immunity to parasites by T cells and T cell-derived cytokines

Parasitic protozoa and helminths are a diverse group of organisms which together form a major cause of infectious disease in humans and livestock. Studies in animal models have revealed that T lymphocytes and the cytokines they produce play a crucial role in determining the outcome of parasitic infection in terms of both protective immunity and immunopathology. Of particular interest is recent evidence that different parasitic infections in the context of different host genetic background can trigger polarized CD4+ T cell subset responses. The set of cytokines produced by these different T helper responses, in turn, can have opposing effects on the parasite, resulting in either control of infection or promotion of disease. Moreover, cytokines produced by one CD4+ subset can block either the production and/or activity of the cytokines produced by the other subset. The establishment of this state of cross-regulation may be important for parasite survival. CD8+ T cells also appear to play a dual effector/regulatory role in parasite immunity and immunopathology, although the mechanisms underlying their induction and function are less well understood. CD(8+)-mediated cytolytic killing functions have now been demonstrated against a number of different intracellular protozoa, although IFN-gamma produced by the same effector cells may also be critical in host community. In addition to providing highly relevant models for studying the selection and immunobiologic function of T-cell subsets, research on T lymphocyte-parasite interactions is crucial for the design of effective vaccines and immunotherapies and thus has broad practical as well as theoretical ramifications.

IL-10 inhibits parasite killing and nitrogen oxide production by IFN-gamma-activated macrophages

IL-10, a cytokine produced by CD4+ T lymphocytes belonging to the Th-2 subset, has previously been shown to inhibit the synthesis of IFN-gamma by both T cells and NK cells. We now demonstrate that IL-10 can also down-regulate IFN-gamma-dependent immunity by blocking the ability of that lymphokine to activate macrophages. Thus, IL-10, in a dose-dependent manner, inhibits the microbicidal activity of IFN-gamma-treated inflammatory macrophages against intracellular Toxoplasma gondii as well as the extracellular killing of schistosomula of Schistosoma mansoni. This suppression correlates with the inhibition by IL-10 of IFN-gamma-induced production of toxic nitrogen oxide metabolites, an effector mechanism previously implicated in the killing by macrophages of both parasite targets. IL-10 inhibition of nitric oxide production was shown to occur when the cytokine is given before or together with the IFN-gamma-activating stimulus, but not after its removal from the cultures and to require 12 h of contact for maximal suppressive effect on macrophage function. These results, taken together with previous findings on the down-regulation of Th1 lymphokine production by IL-10, indicate that the induction of IL-10 may be an important strategy by which parasites evade IFN-gamma-dependent, cell-mediated immune destruction.

IL-10 inhibits parasite killing and nitrogen oxide production by IFN-gamma-activated macrophages

IL-10, a cytokine produced by CD4+ T lymphocytes belonging to the Th-2 subset, has previously been shown to inhibit the synthesis of IFN-gamma by both T cells and NK cells. We now demonstrate that IL-10 can also down-regulate IFN-gamma-dependent immunity by blocking the ability of that lymphokine to activate macrophages. Thus, IL-10, in a dose-dependent manner, inhibits the microbicidal activity of IFN-gamma-treated inflammatory macrophages against intracellular Toxoplasma gondii as well as the extracellular killing of schistosomula of Schistosoma mansoni. This suppression correlates with the inhibition by IL-10 of IFN-gamma-induced production of toxic nitrogen oxide metabolites, an effector mechanism previously implicated in the killing by macrophages of both parasite targets. IL-10 inhibition of nitric oxide production was shown to occur when the cytokine is given before or together with the IFN-gamma-activating stimulus, but not after its removal from the cultures and to require 12 h of contact for maximal suppressive effect on macrophage function. These results, taken together with previous findings on the down-regulation of Th1 lymphokine production by IL-10, indicate that the induction of IL-10 may be an important strategy by which parasites evade IFN-gamma-dependent, cell-mediated immune destruction.

CD4+ subset regulation in viral infection. Preferential activation of Th2 cells during progression of retrovirus-induced immunodeficiency in mice

Progressive lymphoproliferation and increasingly severe immunodeficiency are prominent features of a syndrome, designated mouse AIDS, which develops in susceptible strains of mice infected with the mixture of murine leukemia viruses, termed LP-BM5. Development of splenomegaly and lymphadenopathy, caused primarily by increases in B cell immunoblasts, requires the presence of CD4+ T cells and is assumed to be mediated by lymphokines produced by these cells inasmuch as progression of disease is markedly inhibited by treatment of infected mice with cyclosporin A. Studies of spleen cells from infected mice revealed spontaneous production of cytokines (IFN-gamma, IL-2, IL-4, IL-5, and IL-10) characteristic of Th0 (or a mixture of Th1 and Th2) T helper cells at 1 wk after infection. At later times, IFN-gamma and IL-2, characteristic products of Th1 helper clones, were expressed poorly, either spontaneously or after stimulation of cells with Con A. In contrast, IL-4, IL-5, IL-6, and IL-10, cytokines typically synthesized by Th2 cells, were produced in response to Con A or spontaneously through 18 wk post-infection. Increased serum IgE levels and enhanced IL-10 mRNA expression were consistent with expression of Th2 cytokines at biologically significant levels in vivo. Selective depletion of T cell subsets before stimulation with Con A showed that CD4+ T cells were the primary source of IL-2, IL-4, IL-10, and, to a lesser extent, IFN-gamma in spleens and lymph nodes of normal or infected mice. These results suggest that persistent activation of CD4+ T cells with the lymphokine profile of Th2 helper clones is responsible for chronic B cell stimulation, down-regulation of Th1 cytokines, and impaired CD8+ T cell function in mouse AIDS. This provides the first demonstration that, like many parasitic infections, viruses encoding potent antigenic stimuli can markedly affect the balance of Th subset expression.

CD4+ subset regulation in viral infection. Preferential activation of Th2 cells during progression of retrovirus-induced immunodeficiency in mice

Progressive lymphoproliferation and increasingly severe immunodeficiency are prominent features of a syndrome, designated mouse AIDS, which develops in susceptible strains of mice infected with the mixture of murine leukemia viruses, termed LP-BM5. Development of splenomegaly and lymphadenopathy, caused primarily by increases in B cell immunoblasts, requires the presence of CD4+ T cells and is assumed to be mediated by lymphokines produced by these cells inasmuch as progression of disease is markedly inhibited by treatment of infected mice with cyclosporin A. Studies of spleen cells from infected mice revealed spontaneous production of cytokines (IFN-gamma, IL-2, IL-4, IL-5, and IL-10) characteristic of Th0 (or a mixture of Th1 and Th2) T helper cells at 1 wk after infection. At later times, IFN-gamma and IL-2, characteristic products of Th1 helper clones, were expressed poorly, either spontaneously or after stimulation of cells with Con A. In contrast, IL-4, IL-5, IL-6, and IL-10, cytokines typically synthesized by Th2 cells, were produced in response to Con A or spontaneously through 18 wk post-infection. Increased serum IgE levels and enhanced IL-10 mRNA expression were consistent with expression of Th2 cytokines at biologically significant levels in vivo. Selective depletion of T cell subsets before stimulation with Con A showed that CD4+ T cells were the primary source of IL-2, IL-4, IL-10, and, to a lesser extent, IFN-gamma in spleens and lymph nodes of normal or infected mice. These results suggest that persistent activation of CD4+ T cells with the lymphokine profile of Th2 helper clones is responsible for chronic B cell stimulation, down-regulation of Th1 cytokines, and impaired CD8+ T cell function in mouse AIDS. This provides the first demonstration that, like many parasitic infections, viruses encoding potent antigenic stimuli can markedly affect the balance of Th subset expression.

Analysis of egg granuloma formation in Schistosoma japonicum-infected mice treated with antibodies to interleukin-5 and gamma interferon

Schistosoma japonicum-infected mice were treated with antibodies to interleukin-5 (IL-5) or gamma interferon (IFN-gamma) from week 3 or 4 to week 10 of infection. Neither antibody affected egg production by the parasite, and neither had a consistent effect on the secretion of IFN-gamma or IL-5 cell-related cytokines by spleen cells from infected mice. Mice treated with antibody to murine IL-5 had only rare eosinophils in hepatic circumoval granulomas. Granulomas around single eggs were reduced in volume by a third, but hepatic fibrosis was unaffected. Treatment with antibody to murine IFN-gamma also reduced the size of granulomas and also did not affect hepatic fibrosis, which was measured as hydroxyproline. Our results, taken together with the studies of others, indicate that a complex interaction of cytokines affects granuloma size and that the size and fibrosis of granulomas are to some extent regulated independently.

Production of IL-10 by CD4+ T lymphocytes correlates with down-regulation of Th1 cytokine synthesis in helminth infection

After the onset of parasite egg deposition, mice infected with the helminth Schistosoma mansoni mount strong Th2 cytokine responses in the absence of significant Th1 cytokine synthesis. To examine the basis of this immunoregulatory state, spleen or lymph node cells from schistosome-infected mice were stimulated with parasite-specific Ag and the supernatants tested for their capacity to suppress IFN-gamma synthesis by a Th1 cell line. Strong inhibition was observed that was neutralized by a mAb against IL-10, a cytokine previously shown to down-regulate Th1 cytokine synthesis. By means of ELISA measurements the production of IL-10 in schistosome infection was confirmed and shown to depend on CD4+ T cells. IL-10 synthesis stimulated by either mitogen or Ag was observed only at those stages of infection when Th2 response induction and Th1 cytokine down-regulation also occurred and was not detected in mice vaccinated with attenuated parasites. Moreover, the addition of the neutralizing anti-IL-10 mAb to Ag-stimulated spleen cell cultures from infected mice caused a dramatic augmentation in IFN-gamma synthesis. These findings suggest that IL-10 is responsible for the down-regulation of Th1 responses observed in schistosome infections, a phenomenon that may enable the parasite to escape potentially harmful cell-mediated responses.

Production of IL-10 by CD4+ T lymphocytes correlates with down-regulation of Th1 cytokine synthesis in helminth infection

After the onset of parasite egg deposition, mice infected with the helminth Schistosoma mansoni mount strong Th2 cytokine responses in the absence of significant Th1 cytokine synthesis. To examine the basis of this immunoregulatory state, spleen or lymph node cells from schistosome-infected mice were stimulated with parasite-specific Ag and the supernatants tested for their capacity to suppress IFN-gamma synthesis by a Th1 cell line. Strong inhibition was observed that was neutralized by a mAb against IL-10, a cytokine previously shown to down-regulate Th1 cytokine synthesis. By means of ELISA measurements the production of IL-10 in schistosome infection was confirmed and shown to depend on CD4+ T cells. IL-10 synthesis stimulated by either mitogen or Ag was observed only at those stages of infection when Th2 response induction and Th1 cytokine down-regulation also occurred and was not detected in mice vaccinated with attenuated parasites. Moreover, the addition of the neutralizing anti-IL-10 mAb to Ag-stimulated spleen cell cultures from infected mice caused a dramatic augmentation in IFN-gamma synthesis. These findings suggest that IL-10 is responsible for the down-regulation of Th1 responses observed in schistosome infections, a phenomenon that may enable the parasite to escape potentially harmful cell-mediated responses.

CD8+ T cells from mice vaccinated against Toxoplasma gondii are cytotoxic for parasite-infected or antigen-pulsed host cells

Mice vaccinated with a live temperature sensitive mutant (TS-4) of Toxoplasma gondii develop complete resistance to subsequent challenge with a highly virulent Toxoplasma strain (RH). Because CD8+ T cells have been demonstrated to be critical to this protective immunity in vivo, the involvement of cytotoxic T lymphocytes in the killing of infected cells in vaccinated mice was investigated. After restimulation in vitro, splenic T cells from vaccinated mice of either the BALB/c or C57BL/6 strains were found to kill syngeneic bone marrow-derived macrophages infected with TS-4 tachyzoites or preincubated with soluble T. gondii Ag. Unimmunized control mice or mice vaccinated with heat-killed TS-4 tachyzoites failed to generate significant CTL activity in vitro. Moreover, the observed lytic reaction was found to be target specific, not killing uninfected or unpulsed macrophages even when included as bystanders in the assay. Target lysis did not depend on the production by the effector cells of either a cytotoxic supernatant factor or IFN-gamma. Depletion of CD8+ cells from the splenic effector cell population, however, abrogated the cytotoxic activity, whereas depletion of CD4+ cells had little effect. The MHC restriction of the Toxoplasma-specific cytolytic reaction was confirmed in studies using effector cells from BALB/c mice and targets from congenic or mutant haplotype strains. These experiments indicated that target killing is primarily restricted by genes mapping within the H-2D/Ld loci. Together, these results establish MHC-restricted cytolysis as a major parameter of CD8+ effector function against T. gondii and indicate that, in the case of this protozoan, Ag presentation to CD8+ lymphocytes can occur as a result of either processing within infected cells or exogenous uptake of parasite Ag.