Increased susceptibility of Stat4-deficient and enhanced resistance in Stat6-deficient mice to infection with Trypanosoma cruzi

Signaling by IL-12 and IL-23 and the immunoregulatory roles of STAT4

Produced in response to a variety of pathogenic organisms, interleukin (IL)-12 and IL-23 are key immunoregulatory cytokines that coordinate innate and adaptive immune responses. These dimeric cytokines share a subunit, designated p40, and bind to a common receptor chain, IL-12R beta 1. The receptor for IL-12 is composed of IL-12R beta 1 and IL-12R beta 2, whereas IL-23 binds to a receptor composed of IL-12R beta 1 and IL-23R. Both cytokines activate the Janus kinases Tyk2 and Jak2, the transcription factor signal transducer and activator of transcription 4 (STAT4), as well as other STATs. A major action of IL-12 is to promote the differentiation of naive CD4+ T cells into T-helper (Th) 1 cells, which produce interferon (IFN)-gamma, and deficiency of IL-12, IL-12R subunits or STAT4 is similar in many respects. In contrast, IL-23 promotes end-stage inflammation. Targeting IL-12, IL-23, and their downstream signaling elements would therefore be logical strategies for the treatment of immune-mediated diseases.

STATs as critical mediators of signal transduction and transcription: lessons learned from STAT5

Signal transducers and activators of transcription (Stats) comprise a family of seven transcription factors that are activated by a variety of cytokines, hormones and growth factors. Stats are activated through tyrosine phosphorylation, mainly by Jak kinases, that lead to their dimerization, nuclear translocation and regulation of target gene expression. Stat5 was originally identified as a transcription factor that regulates the beta-casein gene in response to prolactin (PRL), but Stat5 is activated also by several other cytokines and growth factors. The molecular mechanisms that underlie Stat5-mediated transcription involve interactions and cooperation with sequence specific transcription factors and transcriptional coregulators. Our studies identified p100 protein as a coactivator for Stat5, and suggest the existence of a positive regulatory loop in PRL-induced transcription, where PRL stabilizes p100 protein, which in turn can cooperate with Stat5 in transcriptional activation. Suppressors of cytokine signaling (SOCS) proteins are important negative regulators of Stats. A target gene for Stat5, the serine/threonine kinase Pim-1, was found to cooperate with SOCS-1 and SOCS-3 to inhibit Stat5 activity suggesting that Pim-1 together with SOCS-1 and SOCS-3 are components of a negative feedback mechanism that allows Stat5 to regulate its own activation.

Parasite-induced Th2 polarization is associated with down-regulated dendritic cell responsiveness to Th1 stimuli and a transient delay in T lymphocyte cycling

The nature of the signals that bias Th effector choice is still not completely understood. Using parasite extracts from pathogens known to induce polarized Th1 or Th2 responses and an in vitro experimental model for priming murine CD4(+) cells, we demonstrated that splenic dendritic cells (DC), but not B cells, promote Th1/Th2 differentiation of naive CD4(+) lymphocytes. Th polarization in this system was found not to depend on DC secretion of the polarizing cytokines IL-12/IL-4, but instead correlated with distinct states of DC activation induced by the different parasite preparations. As expected, conditioning of DC for Th1 development was associated with up-regulation of costimulatory molecules and enhanced chemokine production and required intact MyD88 signaling. In contrast, conditioning of DC for Th2 differentiation correlated with down-regulation of many of the same functions and was MyD88 independent. This dampened DC activation was accompanied in the cocultures by a reduction in the frequency of CD4(+) lymphocytes exiting the first division of the cell cycle. When the latter was mimicked by drug-induced arrest of peptide-primed CD4(+) cells after the S phase of the first cycle, a marked Th2 polarization was also observed. Together, these findings suggest that the emergence of IL-4-producing CD4(+) lymphocytes results from a suppression in DC function leading to a temporary delay in initial T cell cycling.

A STAT4-dependent Th1 response is required for resistance to the helminth parasite Taenia crassiceps

To determine the role of STAT4-dependent Th1 responses in the regulation of immunity to the helminth parasite Taenia crassiceps, we monitored infections with this parasite in resistant mice lacking the STAT4 gene. While T. crassiceps-infected STAT4(+/+) mice rapidly resolved the infection, STAT4(-/-) mice were highly susceptible to infection and displayed large parasite loads. Moreover, the inability of STAT4(-/-) mice to control the infection was associated with the induction of an antigen-specific Th2-type response characterized by significantly higher levels of Th2-associated immunoglobulin G1 (IgG1) and total IgE as well as interleukin-4 (IL-4), IL-10, and IL-13 than those in STAT4(+/+) mice, who produced significantly more gamma interferon. Furthermore, early after infection, macrophages from STAT4(-/-) mice produced lower levels of the pro-inflammatory cytokines IL-12, tumor necrosis factor alpha, IL-1 beta, and nitric oxide (NO) than those from STAT4(+/+) mice, suggesting a pivotal role for macrophages in mediating protection against cysticercosis. These findings demonstrate a critical role for the STAT4 signaling pathway in the development of a Th1-type immune response that is essential for mediating protection against the larval stage of T. crassiceps infection.

Molecular and cellular control of T1/T2 immunity at the interface between antimicrobial defense and immune pathology

The immune system evolved to rapidly recognize infectious threats and promptly mobilize cellular effectors to the infection site. Establishment of a robust T1-type immunity is a prerequisite for effective defense against most viruses and intracellular bacteria. However, accumulating evidence shows that T1 and T2 responses during such infections are not mutually exclusive. A possibility may be that the dual T1-T2 nature of antiviral immune responses is merely a byproduct of less than perfect crossregulatory mechanisms. Herein, we discuss molecular and cellular mechanisms of T-cell differentiation along with recent evidence supporting the hypothesis that rather than representing an epiphenomenon, coinduction of virus-specific T2 cells plays a significant homeostatic role. Thus, molecular pathways that regulate IL-4 production during influenza virus infection monitor T1-mediated immune responses in vital organs such as lungs and prevent immune pathology that may otherwise interfere with recovery from disease. Such evidence suggests that coinduction of T2 immunity maintains immune homeostasis during T1-mediated defense reactions. Finally, we outline implications on the earlier concept of T1/T2 dichotomy, supporting a model in which these two subsets, rather than being mutually antagonistic, together facilitate the recovery from infection.

Importance of CD8 T cell-mediated immune response during intracellular parasitic infections and its implications for the development of effective vaccines

Obligatory intracellular parasites such as Plasmodium sp, Trypanosoma cruzi, Toxoplasma gondii and Leishmania sp are responsible for the infection of hundreds of millions of individuals every year. These parasites can deliver antigens to the host cell cytoplasm that are presented through MHC class I molecules to protective CD8 T cells. The in vivo priming conditions of specific CD8 T cells during natural infection are largely unknown and remain as an area that has been poorly explored. The antiparasitic mechanisms mediated by CD8 T cells include both interferon-gamma-dependent and -independent pathways. The fact that CD8 T cells are potent inhibitors of parasitic development prompted many investigators to explore whether induction of these T cells can be a feasible strategy for the development of effective subunit vaccines against these parasitic diseases. Studies performed on experimental models supported the hypothesis that CD8 T cells induced by recombinant viral vectors or DNA vaccines could serve as the basis for human vaccination. Regimens of immunization consisting of two different vectors (heterologous prime-boost) are much more efficient in terms of expansion of protective CD8 T lymphocytes than immunization with a single vector. The results obtained using experimental models have led to clinical vaccination trials that are currently underway.

Myosin Autoimmunity Is Not Essential for Cardiac Inflammation in Acute Chagas’ Disease

Infection with the protozoan parasite Trypanosoma cruzi leads to acute myocarditis that is accompanied by autoimmunity to cardiac myosin in susceptible strains of mice. It has been difficult to determine the contribution of autoimmunity to tissue inflammation, because other inflammatory mechanisms, such as parasite-mediated myocytolysis and parasite-specific immunity, are coincident during active infection. To begin to investigate the contribution of myosin autoimmunity to myocarditis, we selectively inhibited myosin autoimmunity by restoring myosin tolerance via injection of myosin-coupled splenocytes. This tolerization regimen suppressed the strong myosin-specific delayed-type hypersensitivity (DTH) that normally develops in infected mice, although it did not affect myosin-specific Ab production. Suppression of myosin autoimmunity had no effect on myocarditis or cardiac parasitosis. In contrast, myosin tolerization completely abrogated myocarditis in mice immunized with purified myosin, which normally causes severe autoimmune myocarditis. In this case, myosin-specific DTH and Ab production were significantly reduced. We also examined the contribution of T. cruzi-specific immunity to inflammation by injection of T. cruzi-coupled splenocytes before infection. This treatment reduced T. cruzi DTH, although there was no effect on parasite-specific Ab production. Interestingly, cardiac inflammation was decreased, cardiac parasitosis was significantly increased, and mortality occurred earlier in the parasite-tolerized animals. These results indicate that myosin-specific autoimmunity, while a potentially important inflammatory mechanism in acute and chronic T. cruzi infection, is not essential for inflammation in acute disease. They also confirm previous studies showing that parasite-specific cell-mediated immunity is important for myocarditis and survival of T. cruzi infection.

Distinct requirements for the naturally occurring splice forms Stat4alpha and Stat4beta in IL-12 responses

Signal transducer and activator of transcription (Stat)4 is a signaling molecule required for normal responses to interleukin-12 (IL-12) and is critically involved in inflammatory responses. We have isolated an alternatively spliced isoform of Stat4, termed Stat4beta, which lacks 44 amino acids at the C-terminus, encompassing the putative transcriptional activation domain. To assess the in vivo roles of these Stat4 isoforms, we generated transgenic Stat4-deficient mice expressing Stat4alpha or Stat4beta. Our results indicate that T-cell-specific expression of Stat4alpha or Stat4beta can mediate many aspects of IL-12 signaling including the differentiation of Th1 cells. However, Stat4alpha is required for normal levels of IL-12-induced interferon-gamma production from Th1 cells. Microarray analysis identified 98 genes induced by both Stat4 isoforms, 32 genes induced only by Stat4alpha and 29 genes induced only by Stat4beta. Some induced genes correlate with specific functions including the ability of Stat4beta, but not Stat4alpha, to mediate IL-12-stimulated proliferation. Thus, Stat4alpha and Stat4beta have distinct roles in mediating responses to IL-12.

INHIBITION OF TRYPANOSOMA CRUZI EGRESS FROM INFECTED FIBROBLASTS IS MEDIATED BY CD4+ AND μ+ IMMUNE CELLS

Trypanosoma cruzi, the causative agent of Chagas disease, is able to reproduce intracellularly in many host cell types while in the mammalian host. Although cellular immunity is known to be important in resistance to infection, the ability of immune cells to interfere with the completion of the intracellular growth cycle of T. cruzi has not been described. Using a tissue culture system to study the parasite growth cycle, we have found that spleen cells from infected mice are able to decrease the number of parasites released from infected fibroblasts. Spleen cells from mice infected for as few as 14 days and as long as 300 days display this inhibitory ability. Parasite egress from infected cells is inhibited by factor(s) released by immune cells during coculture with infected fibroblasts. Immune cell depletion studies indicate that the inhibitory activity requires the presence of both CD4+ T cells and mu+ B cells. These results suggest a direct ability of immune cells to somehow interfere with the completion of the intracellular cycle, and this ability may play a role in control of this parasite.

Immunization with recombinant paraflagellar rod protein induces protective immunity against Trypanosoma cruzi infection

In the present study, we have produced recombinant paraflagellar rod proteins (PFR) and report their use for successful vaccination of mice against Trypanosoma cruzi. This protection is associated with a highly polarized type 1 cytokine production profile. Additionally, we have analyzed the gene sequence encoding PFR-2 to determine the degree of conservation among seven highly diverse strains of T. cruzi, and found it to be highly conserved. The results presented here indicate that the PFR antigens are highly conserved, and immunization with rPFR-1, PFR-2, or an equimolar mix of the PFR-1, -2, and -3 proteins provides protective immunity against Trypanosoma cruzi.

Immunization with cDNA expressed by amastigotes of Trypanosoma cruzi elicits protective immune response against experimental infection

Immunization of mice with plasmids containing Trypanosoma cruzi genes induced specific antibodies, CD4(+) Th1 and CD8(+) Tc1 cells, and protective immunity against infection. In most cases, plasmids used for DNA vaccination contained genes encoding antigens expressed by trypomastigotes, the nonreplicative forms of the parasite. In this study, we explored the possibility of using genes expressed by amastigotes, the form of the parasite which replicates inside host cells, for experimental DNA vaccination. For that purpose, we selected a gene related to the amastigote surface protein 2 (ASP-2), an antigen recognized by antibodies and T cells from infected mice and humans, for our study. Using primers specific for the asp-2 gene, four distinct groups of genes were amplified from cDNA from amastigotes of the Y strain of T. cruzi. At the nucleotide level, they shared 82.3 to 89.9% identity with the previously described asp-2 gene. A gene named clone 9 presented the highest degree of identity with the asp-2 gene and was selected for immunological studies. Polyclonal antisera raised against the C terminus of the recombinant protein expressed by the clone 9 gene reacted with an antigen of approximately 83 kDa expressed in amastigotes of T. cruzi. Immunization of BALB/c mice with eukaryotic expression plasmids containing the clone 9 gene elicited specific antibodies and CD4(+) T-cell-dependent gamma interferon secretion. Upon challenge with trypomastigotes, mice immunized with plasmids harboring the clone 9 gene displayed reduced parasitemia and survived lethal infection. We concluded that amastigote cDNA is an interesting source of antigens that can be used for immunological studies, as well as for vaccine development.

Resistant mice lacking interleukin-12 become susceptible to Trypanosoma cruzi infection but fail to mount a T helper type 2 response

Interleukin-12 (IL-12) is essential to resistance to Trypanosoma cruzi infection because it stimulates the synthesis of interferon-gamma (IFN-gamma) that activates macrophages to a parasiticidal effect. Investigation of mice deprived of IL-12 genes (IL-12 knockout mice) has confirmed the important role of IL-12 and IFN-gamma in controlling parasitism in T. cruzi infection. However, it has not yet been addressed whether a shift towards a T helper type 2 (Th2) pattern of cytokine response occurred in these mice that might have contributed to the aggravation of the infection caused by IL-12 deprivation. We examined the course of T. cruzi (Y strain) infection and the regulation of cytokine responses and nitric oxide production in C57BL/6 IL-12 p40-knockout mice. The mutant mice were extremely susceptible to the infection as evidenced by increased parasitaemia, tissue parasitism and mortality in comparison with the control C57BL/6 mouse strain (wild-type) that is resistant to T. cruzi. A severe depletion of parasite-antigen-specific IFN-gamma response, without an increase in IL-4 or IL-10 production, accompanied by reduced levels of nitric oxide production was observed in IL-12 knockout mice. We found no evidence of a shift towards a Th2-type cytokine response. In IL-12 knockout mice, the residual IFN-gamma production is down-regulated by IL-10 but not by IL-4 and nitric oxide production is stimulated by tumour necrosis factor-alpha. Parasite-specific immunoglobulin G1 antibody levels were similar in IL-12 knockout and wild-type mice, whereas IL-12 knockout mice had much higher levels of immunoglobulin G2b.

Evidence inconsistent with a negative influence of T helper 2 cells on protection afforded by a dominant T helper 1 response against Mycobacterium tuberculosis lung infection in mice

Mice incapable of generating an efficient Th2 response because of functional deletion of the genes for signal transducer and activation of transcription 6 (Stat6), interleukin-4 receptor alpha chain (IL-4Ralpha), or IL-4 plus IL-13 (IL-4/IL-13) were no more resistant than wild-type (WT) mice to airborne infection with virulent Mycobacterium tuberculosis. WT mice were able to control infection and hold it at a stationary level following 20 days of log linear M. tuberculosis growth. Likewise, infection was kept under control and was held at the same stationary level in IL-4/IL-13(-/-) mice but progressed to a slightly higher level in Stat6(-/-) and IL-4Ralpha(-/-) mice. The onset of stationary-level infection in WT mice was associated with the expression of Th1-mediated immunity, as evidenced by an approximately 100- to 1,000-fold increase in the lungs in the synthesis of mRNA for IL-12, gamma interferon (IFN-gamma), and inducible nitric oxide synthase (NOS2) that was sustained for at least 100 days. IL-12 is essential for the induction of Th1 immunity, IFN-gamma is a key Th1 cytokine involved in mediation of immunity, and NOS2 is an inducible enzyme of macrophages and is needed by these cells to express immunity. In response to infection, the lungs of Stat6(-/-) mice showed increases in synthesis of mRNA for IL-12, IFN-gamma, and NOS2 similar to that seen in WT mice. In IL-4/IL-13(-/-) mice, however, synthesis of mRNA for IFN-gamma and NOS2 reached higher levels than in WT mice. These results argue against the notion that a Th2 response is partly or wholly responsible for the inability of Th1-mediated immunity to resolve infection with a virulent strain of M. tuberculosis.

Neonatal tolerance in the absence of Stat4- and Stat6- dependent Th cell differentiation

Neonatal tolerance to specific Ag is achieved by nonimmunogenic exposure within the first day of life. The mechanism that regulates this tolerance may provide the basis for successful organ transplantation and has recently been thought to be immune deviation from the inflammatory Th1 response to a Th2 response. To test the importance of Th2 cells in the establishment of neonatal tolerance, we examined neonatal tolerance in Stat4- and Stat6-deficient mice, which have reduced Th1 and Th2 cell development, respectively. Neonatal tolerance of both the T and B cell compartments in Stat4- and Stat6-deficient mice was similar to that observed in wild-type mice. Cytokine production shifted from a Th1 to a Th2 response in wild-type mice tolerized as neonates. In contrast, tolerance was observed in Stat6-deficient mice despite maintenance of a Th1 cytokine profile. These results suggest that cells distinct from Stat6-dependent Th2 cells are required for the establishment of neonatal tolerance.

IL-12, while beneficial, is not essential for the host response to VSV encephalitis

In this report, the role of STAT4 and local production of interleukin (IL)-12 in the central nervous system (CNS) were examined during experimental vesicular stomatitis virus (VSV) encephalitis. We have previously shown that IL-12 treatment is beneficial both in vitro and in vivo during experimental VSV infection. This inhibition of VSV replication was dependent on the production of nitric oxide (NO) by the neuronal isoform of nitric oxide synthase (NOS-1). In vitro, IL-12 induces the phosphorylation and nuclear localization of STAT4 in neuroblastoma cell lines. STAT4 expression was not required for host survival or clearance of virus during experimental VSV encephalitis. Taken together, these data suggest that while neurons can respond directly to IL-12 in vitro by signaling through STAT4, STAT4 is not required for survival. It is likely that redundant innate host inflammatory cytokine responses compensate for the absence of IL-12 signaling.

Role of cytokines in autoimmune myocarditis and cardiomyopathy

Cellular as well as humoral autoimmune responses are critically associated with the pathogenesis and progression of myocarditis and cardiomyopathy. Cytokines appear to play critical roles in accentuating or regulating autoimmune mechanisms in these disorders. However, depending on the triggers of autoimmune responses against the heart, such as viral or parasitic infections and experimental immunization with cardiac myosin, the effect of each cytokine on autoimmune myocardial disease may vary. Cytokines may represent new therapeutic targets in the treatment and prevention of autoimmunity-mediated myocarditis and cardiomyopathy, though the etiology and variability in the type of autoimmune responses should be taken into account in the development of cytokine/anti-cytokine treatment of these disorders.

Th1 cells regulate hematopoietic progenitor cell homeostasis by production of oncostatin M

Regulation of hematopoietic progenitor cell homeostasis is crucial for maintenance of innate immunity and the ability of the body to respond to injury and infection. In this report, we demonstrate that progenitor cell numbers and cycling status in vivo are dramatically increased in mice deficient in Stat6 and decreased in mice deficient in Stat4, targeted mutations which also alter T helper cell polarization. Experiments using mice that have T cell restricted transgenic expression of Stat4 or Stat6 or have been in vivo depleted of T cell subsets demonstrate that CD4(+) T cells regulate progenitor cell activity. Injection of the Th1 cytokine Oncostatin M but not other cytokines into Stat4-deficient mice recovers progenitor cell activity to wild-type levels. Thus, T helper cells actively regulate hematopoietic progenitor cell homeostasis.

Cutting edge: susceptibility to the larval stage of the helminth parasite Taenia crassiceps is mediated by Th2 response induced via STAT6 signaling

Using STAT6(-/-) BALB/c mice, we analyzed the role of STAT6-induced Th2 response in determining the outcome of murine cysticercosis caused by the helminth parasite Taenia crassiceps. After T. crassiceps infection, wild-type BALB/c mice developed a strong Th2-like response; produced high levels of IgG1, IgE, IL-4, as well as IL-13; and remained susceptible to T. crassiceps. In contrast, similarly infected STAT6(-/-) mice mounted a strong Th1-like response; produced high levels of IgG2a, IL-12, IFN-gamma, as well as nitric oxide; and efficiently controlled T. crassiceps infection. These findings demonstrate that Th2-like response induced via STAT6-mediated signaling pathway mediates susceptibility to T. crassiceps and, furthermore, that unlike the case in most helminths, immunity against T. crassiceps is mediated by a Th1-like rather than Th2-like response.

In the absence of IL-12, CD4(+) T cell responses to intracellular pathogens fail to default to a Th2 pattern and are host protective in an IL-10(-/-) setting

IL-12-deficient mice exposed to nonlethal infections with intracellular pathogens or repeatedly immunized with a pathogen extract developed lowered but nevertheless substantial numbers of IFN-gamma(+) CD4(+) T cells compared to those observed in wild-type animals. Moreover, the CD4(+) responses in these knockout animals failed to default to a Th2 pattern. The protective efficacy of the Th1 cells developing in an IL-12-deficient setting was found to be limited by IL-10 since mice doubly deficient in IL-10 and IL-12 survived, while animals deficient in IL-12 alone succumbed to pathogen challenge. In contrast to IL-12 knockout mice, MyD88-deficient animals exposed to a Th1 microbial stimulus developed a pure Th2 response, arguing that this signaling element plays a more critical function than IL-12 in determining pathogen-induced CD4 polarization.

The Trypanosoma cruzi trans-sialidase is a T cell-independent B cell mitogen and an inducer of non-specific Ig secretion

Polyclonal lymphocyte activation and hypergammaglobulinemia characterize the acute phase of many parasitic diseases, including Chagas' disease, a debilitating condition caused by Trypanosoma cruzi. Polyclonal lymphocyte activation correlates with disease susceptibility inT. cruzi infection. Thus, identifying factors that drive such reactivities should provide insight into mechanisms of parasite evasion from host immunity and of disease pathogenesis. Sensitization of mice with small doses of T. cruzi trans-sialidase (TS) turns the mice into highly susceptible hosts to T. cruzi. In addition, TS heterologously expressed in Leishmania major greatly enhances virulence of the parasite to mice. In attempt to study the mechanism of TS-induced virulence, we found that TS and its C-terminal long tandem repeat (LTR) are T-independent polyclonal activators for mouse B cells. While B cells deficient/defective in L-6, CD40 or Toll-like receptor-4 are similarly activated by TS as compared to wild-type cells, B cells from Bruton's tyrosine kinase-defective X-linked immunodeficient mice are remarkably insensitive to TS activation. TS-induced B cell activation in vitro is accompanied by Ig secretion independent of T cells. Furthermore, administration of TS into normal mice leads to non-specific Ig secretion that peaks 4-6 days after injection. Thus TS, through its LTR, induces abnormal polyclonal B cell activation and Ig secretion, which could explain in part its virulence-enhancing activity.

Interleukin-6 is required for parasite specific response and host resistance to Trypanosoma cruzi

Infection with Trypanosoma cruzi, the agent of Chagas' disease, results in elevated levels of interleukin-6 (IL-6) in serum and infected tissues. However, it remains unknown whether IL-6 plays a role in host defence against T. cruzi. To determine whether IL-6 underlies disease progression, we followed the time course of T. cruzi-infected mice bearing IL-6 +/+ and minus sign/minus sign genotypes, respectively. We found that IL-6 minus sign/minus sign mice were more susceptible to T. cruzi infection as they exhibited about 3-fold higher parasitaemia and died earlier than wild-type animals. Unlike what might be expected, T. cruzi-infected IL-6 minus sign/minus sign mice did not show at peak infection a decrease in the secretion of IFN-gamma, a Th1 cytokine crucial for controlling the parasite. Instead, they exhibited a much reduced splenocyte recall response to T. cruzi antigens. Our results suggest that IL-6 mediates anti-parasite protective responses against T. cruzi.

STAT4 is required for antibacterial defense but enhances mortality during polymicrobial sepsis

The signal transducer and activator of transcription factor 4 (STAT4) pathway mediates the intracellular effects of interleukin-12 (IL-12), leading to the production of gamma interferon, induction of a T helper type 1 response, and increased natural killer cell cytotoxicity. The purpose of this study was to determine the role of the STAT4 pathway during polymicrobial peritonitis in the cecal ligation and puncture (CLP) model. CLP was performed on STAT4-deficient (STAT4(-/-)) and wild-type control (BALB/c) mice. At 4 h after CLP, STAT4(-/-) mice had significantly higher bacterial counts in the peritoneal lavage fluid, liver, and blood. This difference persisted for 18 h in the peritoneal lavage fluid and blood. Neutrophil migration to the site of infection and into remote tissues was unaffected. Despite higher bacterial counts locally and systemically, STAT4(-/-) mice had a lower mortality rate than BALB/c controls. In contrast, blockade of IL-12 in BALB/c mice was detrimental to host survival. A blunted serum IL-12 response at 18 h after CLP was exhibited in STAT4(-/-) mice. These results suggest several critical roles for the STAT4 pathway in the resolution of polymicrobial infections. Additionally, the disparate effects observed with IL-12 blockade and STAT4 deficiency on host survival suggest that IL-12 may activate alternate pathways promoting survival.

Pivotal role of interleukin-12 and interferon-gamma axis in controlling tissue parasitism and inflammation in the heart and central nervous system during Trypanosoma cruzi infection

The role of cytokines in the control of tissue parasitism and pathogenesis of experimental Chagas' disease was investigated. Wild-type and different cytokine as well as inducible nitric oxide synthase (iNOS) knockout mice were infected with the Colombian strain of Trypanosoma cruzi, and the kinetics of tissue parasitism, inflammatory reaction, parasitemia, and mortality were determined. We demonstrate the pivotal role of the interleukin (IL)-12/interferon (IFN)-gamma/iNOS axis and the antagonistic effect of IL-4 in controlling heart tissue parasitism, inflammation, and host resistance to acute infection with T. cruzi. Further, the heart and central nervous system were shown the main sites of reactivation of T. cruzi infection in mice lacking functional genes for IFN-gamma and IL-12, respectively. Our results also show that in contrast to IFN-gamma knockout (KO) mice, splenocytes from IL-12 KO mice infected with T. cruzi produced low levels of IFN-gamma upon stimulation with antigen. Consistently, high levels of anti-T. cruzi IgG2a antibodies were detected in the sera from IL-12 KO, but not from IFN-gamma KO mice, infected with the Colombian strain of T. cruzi. Thus, our results suggest that the level of IFN-gamma deficiency is a major determinant of the site of reactivation of T. cruzi infection in immunocompromised host.

DNA sequences encoding CD4+ and CD8+ T-cell epitopes are important for efficient protective immunity induced by DNA vaccination with a Trypanosoma cruzi gene

Immunization of BALB/c mice with a plasmid containing the gene for Trypanosoma cruzi trans-sialidase (TS) induced antibodies that inhibited TS enzymatic activity, CD4+ Th1 and CD8+ Tc1 cells, and protective immunity against infection. We used this model to obtain basic information on the requirement of CD4 or CD8 or B-cell epitopes for an effective DNA-induced immunity against T. cruzi infection. For that purpose, mice were immunized with plasmids containing DNA sequences encoding (i) the entire TS protein, (ii) the TS enzymatic domain, (iii) the TS CD4+ T-cell epitopes, (iv) the TS CD8+ T-cell epitope, or (v) TS CD4+ and CD8+ T-cell epitopes. Plasmids expressing the entire TS or its enzymatic domain elicited similar levels of TS-inhibitory antibodies, gamma interferon (IFN-gamma)-producing T cells, and protective immunity against infection. Although the plasmid expressing TS CD4 epitopes was immunogenic, its protective efficacy against experimental infection was limited. The plasmid expressing the CD8 epitope was poorly immunogenic and provided little protective immunity. The reason for the limited priming of CD8+ T cells was due to a requirement for CD4+ T cells. To circumvent this problem, a plasmid expressing both CD4+ and CD8+ T-cell epitopes was produced. This plasmid generated levels of IFN-gamma-producing T cells and protective immunity comparable to that of the plasmid expressing the entire catalytic domain of TS. Our observations suggest that plasmids expressing epitopes recognized by CD4+ and CD8+ T cells may have a better protective potential against infection with T. cruzi.

The immunization of A2/K(b) transgenic mice with the KMP11-HSP70 fusion protein induces CTL response against human cells expressing the T. cruzi KMP11 antigen: identification of A2-restricted epitopes

Cytotoxic T lymphocyte response against Jurkat-A2/K(b) cells expressing the T. cruzi KMP11 protein has been evaluated after immunization of C57BL/6-A2/K(b) transgenic mice with the KMP11 and KMP11-HSP70 recombinant proteins. The results show that mice immunized with KMP11 covalently fused to the T. cruzi HSP70 protein, but not mice immunized with KMP11 alone, elicit a CTL response against the Jurkat-A2/K(b) cells expressing the KMP11 protein. The data also show that spleen cells from mice immunized with the fusion protein and stimulated with the K1 peptide induce lysis of both the Jurkat-A2/K(b) cells transfected with the KMP11 coding gene and the Jurkat-A2-K(b) cells pulsed with the K1 peptide. Splenocytes stimulated with the K3 peptide induce lysis of the Jurkat-A2/K(b) cells loaded with the K3 peptide but they do not recognize the target cells expressing the KMP11 protein. Similar results were obtained using lymph node from mice immunized with the peptides. Thus, we believe there are two cytotoxic T cell epitopes restricted to the A2 molecule (K1(KMP11) (4-12) and K3(KMP11) (41-50)) in the KMP11 protein, and suggest that the K1 peptide could be considered an immunodominant antigen whilst the K3 peptide may be regarded as a cryptic epitope. The fact that the CTL lines induced in B6-A2/K(b) mice recognize human cells expressing KMP11 protein, indicates that the KMP11 antigen fused to HSP70 could be a good candidate for the induction of immunoprotective cytotoxic responses against T. cruzi natural infection.

Th1/Th2 effector choice in parasitic infection: decision making by committee

Parasitic infections frequently result in highly polarized CD4+ T cell responses characterized by dominant Th1 or Th2 cytokine production profiles. Although previously thought to be strictly dependent on signaling by the differentiative cytokines, IL-12 and IL-4, recent data indicate that this polarization may be primarily decided instead by a series of different factors intrinsic to the pathogen-antigen-presenting-cell interaction that influence T cell priming.

Enhanced resistance in STAT6-deficient mice to infection with ectromelia virus

We inoculated BALB/c mice deficient in STAT6 (STAT6(-/-)) and their wild-type (wt) littermates (STAT6(+/+)) with the natural mouse pathogen, ectromelia virus (EV). STAT6(-/-) mice exhibited increased resistance to generalized infection with EV when compared with STAT6(+/+) mice. In the spleens and lymph nodes of STAT6(-/-) mice, T helper 1 (Th1) cytokines were induced at earlier time points and at higher levels postinfection when compared with those in STAT6(+/+) mice. Elevated levels of NO were evident in plasma and splenocyte cultures of EV-infected STAT6(-/-) mice in comparison with STAT6(+/+) mice. The induction of high levels of Th1 cytokines in the mutant mice correlated with a strong natural killer cell response. We demonstrate in genetically susceptible BALB/c mice that the STAT6 locus is critical for progression of EV infection. Furthermore, in the absence of this transcription factor, the immune system defaults toward a protective Th1-like response, conferring pronounced resistance to EV infection and disease progression.

The role of endothelin in the pathogenesis of Chagas' disease

Infection with Trypanosoma cruzi causes a generalised vasculitis of several vascular beds. This vasculopathy is manifested by vasospasm, reduced blood flow, focal ischaemia, platelet thrombi, increased platelet aggregation and elevated plasma levels of thromboxane A(2) and endothelin-1. In the myocardium of infected mice, myonecrosis and a vasculitis of the aorta, coronary artery, smaller myocardial vessels and the endocardial endothelium are observed. Immunohistochemistry studies employing anti-endothelin-1 antibody revealed increased expression of endothelin-1, most intense in the endocardial and vascular endothelium. Elevated levels of mRNA for prepro endothelin-1, endothelin converting enzyme and endothelin-1 were observed in the infected myocardium. When T. cruzi-infected mice were treated with phosphoramidon, an inhibitor of endothelin converting enzyme, there was a decrease in heart size and severity of pathology. Mitogen-activated protein kinases and the transcription factor activator-protein-1 regulate the expression of endothelin-1. Therefore, we examined the activation of mitogen-activated protein kinases in the myocardium by T. cruzi. Western blot demonstrated an extracellular signal regulated kinase. In addition, the activator-protein-1 DNA binding activity, as determined by electrophoretic mobility shift assay, was increased. Increased expression of cyclins A and cyclin D1 was observed in the myocardium, and immunohistochemistry studies revealed that interstitial cells and vascular and endocardial endothelial cells stained intensely with antibodies to these cyclins. These data demonstrate that T. cruzi infection of the myocardium activates extracellular signal regulated kinase, activator-protein-1, endothelin-1, and cyclins. The activation of these pathways is likely to contribute to the pathogenesis of chagasic heart disease. These experimental observations suggest that the vasculature plays a role in the pathogenesis of chagasic cardiomyopathy. Additionally, the identification of these pathways provides possible targets for therapeutic interventions to ameliorate or prevent the development of cardiomyopathy during T. cruzi infection.

Parasite persistence in the aetiology of Chagas disease

Two primary hypotheses are proposed to account for pathogenesis in chronic Trypanosoma cruzi infections: that the persistence of T. cruzi at specific sites in the infected host results in chronic inflammatory reactivity and that T. cruzi infection induces immune responses which are targetted at self tissues. The data supporting parasite persistence as the primary cause of disease in T. cruzi infection have been recently reviewed and the reader is referred to this review for extensive documentation of most of the arguments outlined herein. This manuscript will briefly reiterate the main points of this previous review, adding additional data that have been presented since its publication. Then, philosophical and practical arguments on why Chagas disease should be investigated and treated as a parasitic infection and not as an autoimmune disease are presented. This is admittedly an 'opinion piece' and not a balanced review of the literature on Chagas disease. There are substantial data other than those reviewed here, which have been presented in support of the autoimmunity hypothesis. It is left to others to review that body of literature.

Antigen-specific Th1 but not Th2 cells provide protection from lethal Trypanosoma cruzi infection in mice

Infection with Trypanosoma cruzi results in the development of both type 1 and type 2 patterns of cytokine responses during acute and chronic stages of infection. To investigate the role of Th1 and Th2 subsets of CD4(+) T cells in determining the outcome of T. cruzi infection in mice, we have developed T. cruzi clones that express OVA and have used OVA-specific TCR-transgenic T cells to generate OVA-specific Th1 and Th2 cells. BALB/c mice receiving 10(7) OVA-specific Th1 cells and then challenged with OVA-expressing T. cruzi G-OVA.GPI showed significantly lower parasitemia and increased survival in comparison to mice that received no cells. In contrast, recipients of OVA-specific Th2 cells developed higher parasitemias, exhibited higher tissue parasitism and inflammation, and had higher mortality than recipients of Th1 cells after infection with T. cruzi G-OVA.GPI. Mice receiving a mixture of both Th1 and Th2 OVA-specific cells also were not protected from lethal challenge. The protective effect of the OVA-specific Th1 cells was OVA dependent as shown by the fact that transfer of OVA-specific Th1 or Th2 cells failed to alter the course of infection or disease in mice challenged with wild-type T. cruzi. Immunohistochemical analysis of OVA-specific Th1 and Th2 cells at 4, 15, and 30 days postinfection revealed the persistence and expansion of these cells in mice challenged with T. cruzi G-OVA.GPI but not in mice infected with wild-type T. cruzi. We conclude that transfer of Ag-specific Th1 cells but not Th2 cells protect mice from a lethal infection with T. cruzi.

Pivotal role of signal transducer and activator of transcription (Stat)4 and Stat6 in the innate immune response during sepsis

Signal transducer and activator of transcription (Stat)4 and Stat6 are transcription factors that provide type 1 and type 2 response, respectively. Here, we explored the role of Stat4 and Stat6 in innate immunity during septic peritonitis. Stat4-/- and Stat6-/- mice were resistant to the lethality compared with wild-type (WT) mice. At the mechanistic level, bacterial levels in Stat6-/- mice were much lower than in WT mice, which was associated with increased peritoneal levels of interleukin (IL)-12, tumor necrosis factor (TNF)-alpha, macrophage-derived chemokine (MDC), and C10, known to enhance bacterial clearance. In Stat4-/- mice, hepatic inflammation and injury during sepsis were significantly ameliorated without affecting local responses. This event was associated with increased hepatic levels of IL-10 and IL-13, while decreasing those of macrophage inflammatory protein (MIP)-2 and KC. Sepsis-induced renal injury was also abrogated in Stat4-/- mice, which was accompanied by decreased renal levels of MIP-2 and KC without altering IL-10 and IL-13 levels. Thus, Stat6-/- and Stat4-/- mice appeared to be resistant to septic peritonitis by enhancing local bacterial clearance and modulating systemic organ damage, respectively, via balancing cytokine responses. These results clearly highlight an important role of local type 1 and systemic type 2 cytokine response in protective immunity during sepsis, which can be regulated by Stat proteins.