Regulation and biological function of helminth-induced cytokine responses

The immunological hallmarks of infection with parasitic belmintbs, namely eosinophilia, mastocytosis and increased IgE synthesis, all appear to be induced by cytokines from the T_H2 subset of CD4⁺ T cells: IgE production is stimulated by interleukin 4 (IL-4), eosinophilia by IL-5 and mastocytosis by IL-3 and IL-4. Here, Fred Finkelman and colleagues argue that the functional significance of the eosinophilia-mastocytosis-IgE axis in helminth infection is unclear and suggest that in some worm infections T_H2-cell cytokines may contribute to host protection, while in others they may promote parasite survival.     

New weapons in the war on worms: identification of putative mechanisms of immune-mediated expulsion of gastrointestinal nematodes

Parasitic nematode infections of humans and livestock continue to impose a significant public health and economic burden worldwide. Murine models of intestinal nematode infection have proved to be relevant and tractable systems to define the cellular and molecular basis of how the host immune system regulates resistance and susceptibility to infection. While susceptibility to chronic infection is propagated by T helper cell type 1 cytokine responses (characterised by production of IL-12, IL-18 and interferon-gamma), immunity to intestinal-dwelling adult nematode worms is critically dependent on a type 2 cytokine response (controlled by CD4+T helper type 2 cells that secrete the cytokines IL-4, IL-5, IL-9 and IL-13). However, the immune effector mechanisms elicited by type 2 cytokines in the gut microenvironment that precipitate worm expulsion have remained elusive. This review focuses on new studies that implicate host intestinal epithelial cells as one of the dominant immune effector cells against this group of pathogens. Specifically, three recently identified type 2 cytokine-dependent pathways that could offer insights into the mechanisms of expulsion of parasitic nematodes will be discussed: (i) the intelectins, a new family of galactose-binding lectins implicated in innate immunity, (ii) the resistin-like molecules, a family of small cysteine-rich proteins expressed by multiple cell types, and (iii) cytokine regulation of intestinal epithelial cell turnover. Identifying how the mammalian immune response fights gastrointestinal nematode infections is providing new insights into host protective immunity. Harnessing these discoveries, coupled with identifying what the targets of these responses are within parasitic nematodes, offers promise in the design of a new generation of anti-parasitic drugs and vaccines.     

Critical role of IL-25 in nematode infection-induced alterations in intestinal function

IL-25 (IL-17E) is a member of the IL-17 cytokine family. IL-25-deficient mice exhibit impaired Th2 immunity against nematode infection, implicating IL-25 as a key component in mucosal immunity. The sources of IL-25 and mechanisms responsible for the induction of Th2 immunity by IL-25 in the gastrointestinal tract remain poorly understood. There is also little information on the regulation of IL-25 during inflammation or its role in gut function. In the current study, we investigated the regulation of IL-25 during Nippostrongylus brasiliensis infection and the contribution of IL-25 to the infection-induced alterations in intestinal function. We found that epithelial cells, but not immune cells, are the major source of IL-25 in the small intestine. N. brasiliensis infection-induced upregulation of IL-25 depends upon IL-13 activation of STAT6. IL-25(-/-) mice had diminished intestinal smooth muscle and epithelial responses to N. brasiliensis infection that were associated with an impaired Th2 protective immunity. Exogenous IL-25 induced characteristic changes similar to those after nematode infection but was unable to restore the impaired host immunity against N. brasiliensis infection in IL-13(-/-) mice. These data show that IL-25 plays a critical role in nematode infection-induced alterations in intestinal function that are important for host protective immunity, and IL-13 is the major downstream Th2 cytokine responsible for the IL-25 effects.     

Antibodies to IL-3 and IL-4 suppress helminth-induced intestinal mastocytosis

Rodents infected with the nematode parasite Nippostrongylus brasiliensis (Nb) develop intestinal mastocytosis, eosinophilia, and elevated serum IgE levels. Although IL-4 and IL-5 are necessary for stimulation of IgE synthesis and eosinophilia, respectively, the cytokines that regulate gut mast cell hyperplasia have not been identified. To address this question, 6- to 8-wk-old BALB/c mice were injected on day 0 and day 7 of Nb infection with a rat anti-mouse IL-4 mAb, and with polyclonal sheep (day 0) and rabbit (day 7) anti-mouse IL-3 IgG antibodies. Additional Nb-infected mice received equal doses of isotype- and species-matched control antibodies. Mice were sacrificed on days 12 or 13 post-infection, and mucosal mast cells (MMC) in sections of the small intestine were enumerated. Nb infection induced a 25- to 40-fold increase in MMC over that observed in uninfected controls. Anti-IL-3 or anti-IL-4 alone suppressed the Nb-induced MMC response by 40 to 50%, whereas both antibodies combined suppressed the MMC response by 85 to 90%. Anti-IL-3 alone had no effect on the serum IgE levels, which were essentially abrogated in the Nb-infected mice treated with anti-IL-4. Blood eosinophilia was not affected by treatment with anti-IL-3 and/or anti-IL-4. These studies demonstrate that IL-3 and IL-4 are physiologically important stimuli of mastocytosis in vivo, and suggest therapeutic interventions that may counteract adverse host responses to allergens as well as to parasites.     

Induction of enhanced immunity to intestinal nematodes using IL-9-producing dendritic cells

Dendritic cells can be considered natural adjuvants and are able to act as cellular vaccines to protect against disease. Adoptive transfer of Ag-pulsed bone marrow-derived dendritic cells (BMDCs) enhanced expulsion of the intestinal nematode, Trichinella spiralis, from the small intestine. IL 9 is a critical cytokine in protective immunity to intestinal nematode infection and is believed to enhance Th2 immune responses. Deriving dendritic cells from an IL-9 transgenic (IL-9t) mouse has enabled a detailed investigation of the importance of IL-9 during Ag presentation. Indeed, IL-9t dendritic cells significantly enhanced T cell proliferation and Th2 responses and, after adoptive transfer, enhanced parasite-specific IgG1 and intestinal mastocytosis in vivo, leading to accelerated expulsion of adult worms from the intestine. Overall, this paper demonstrates that dendritic cell vaccination can be used to successfully protect the host against intestinal nematode infection and suggests that IL-9 can act as a potent type 2 adjuvant during Ag presentation and the early stages of Th2 activation.     

IL-18 regulates intestinal mastocytosis and Th2 cytokine production independently of IFN-gamma during Trichinella spiralis infection

Expulsion of the gastrointestinal nematode Trichinella spiralis is associated with pronounced mastocytosis mediated by a Th2-type response involving IL-4, IL-10, and IL-13. Here we demonstrate that IL-18 is a key negative regulator of protective immune responses against T. spiralis in vivo. IL-18 knockout mice are highly resistant to T. spiralis infection, expel the worms rapidly and subsequently develop low levels of encysted muscle larvae. The increased speed of expulsion is correlated with high numbers of mucosal mast cells and an increase in IL-13 and IL-10 secretion. When normal mice were treated with rIL-18 in vivo, worm expulsion was notably delayed, and the development of mastocytosis and Th2 cytokine production was significantly reduced. The treatment had no effect on intestinal eosinophilia or goblet cell hyperplasia but specifically inhibited the development of mastocytosis. Addition of rIL-18 to in vitro cultures of bone marrow-derived mast cells resulted in a significant reduction in cell yields as well as in the number of IL-4-secreting mast cells. In vivo treatment of T. spiralis-infected IFN-gamma knockout mice with rIL-18 demonstrated that the inhibitory effect of IL-18 on mastocytosis and Th2 cytokine secretion is independent of IFN-gamma. Hence, IL-18 plays a significant biological role as a negative regulator of intestinal mast cell responses and may promote the survival of intestinal parasites in vivo.     

Breach of IL-12 monopoly in the initiation of type 1 immunity to intracellular infections: IL-12 is not required.

IL-12 is believed to play an important role in type 1 T-cell differentiation and type 1 cytokine IFN-gamma release by T- and NK-cells and macrophages in host defense against intracellular infections by bacteria, parasites, fungi and viruses. However, recent studies by us and others have provided unequivocal evidence that while IL- 12 is critically required for the development of type 1 immunity to the majority of intracellular bacterial, parasitic and fungal infections, it is not required for anti-viral type 1 immune responses. These findings have provoked our re-thinking about the role of IL-12 in type 1 immunity and the search for additional cytokines capable of initiating anti-viral type 1 immunity. We hypothesize that there exist multiple cytokines including IL-12 which play a redundant role in the initiation of type 1 immunity against viral infection. These cytokines are likely released from not only antigen-presenting macrophages/dendritic cells but many other cell types, which suits the mode of viral infection. The existence of multiple factors capable of driving type 1 immunity endows the host with additional safeguards to cope with prevalent viral foes.     

IL-27 suppresses Th2 cell development and Th2 cytokines production from polarized Th2 cells: a novel therapeutic way for Th2-mediated allergic inflammation

IL-27 up-regulates Th1 but down-regulates Th2 responses. However, its molecular mechanism and regulatory effects on polarized Th2 cells remain unclear. In this study, we have revealed that IL-27 inhibits Th2 cell development as well as Th2 cytokines production from already polarized Th2 cells by down-regulation of GATA-3 and up-regulation of T-bet expression simultaneously. In vivo daily IL-27 treatment for 1 wk after Leishmania major infection protects BALB/c mice from footpad swelling by diminishing parasite burden via reciprocal regulation of Th1 and Th2 responses. Furthermore, IL-27 stimulation causes marked reduction in the capacity of host mouse to mount a Th2 response against Strongyloides venezuelensis infection. Thus, IL-27-treated mice failed to develop intestinal mastocytosis after S. venezuelensis infection and exhibited a marked delay in parasite expulsion. Finally, intranasal administration of IL-27 inhibits OVA-induced airway hyperresponsiveness and inflammation in OVA-sensitized animals. Thus, IL-27 could provide us with a novel therapeutic way for treating Th2-associated diseases such as bronchial asthma.     

IL-7 and IL-15 do not synergize during CD8 T cell recall response against an obligate intracellular parasite

Long-term protection against Toxoplasma gondii is dependent on robust CD8⁺ T cell immunity. In the absence of this response, the host is unable to maintain chronicity, which results in recrudescence of infection and possible death. Factors needed for the persistence of protective CD8⁺ T cells against the parasite need to be evaluated. Previous studies from our laboratory have reported that synergism between γ chain cytokines like IL-7 and IL-15 is critical for the generation of CD8⁺ T cell response needed for protection during acute infection. In this study we report that the situation is different during the recall response where CD8⁺ T cell response is almost entirely dependent on IL-15, with IL-7 at best playing a minor role. In the absence of IL-15, CD8⁺ T cells fail to respond optimally to parasitic re-challenge and hosts are unable to control their replication, which leads to their death. Thus T. gondii infection may represent a unique situation where CD8⁺ T cell response during secondary challenge is primarily dependent on IL-15 with other γ chain cytokines having nominal effect. These findings provide important information regarding factors involved in the generation of protective immunity against T. gondii with strong implications in developing immunotherapeutic agents against the pathogen.     

IL-13 mediates in vivo IL-9 activities on lung epithelial cells but not on hematopoietic cells

Increased IL-9 expression, either systemically or under the control of lung-specific promoter, induces an asthma-like phenotype, including mucus overproduction, mastocytosis, lung eosinophilia, and airway hyperresponsiveness. These activities correlate with increased production of other Th2 cytokines such as IL-4, IL-5, and IL-13 in IL-9 Tg mice. To determine the exact role of IL-13 in this phenotype, mice overexpressing IL-9 were crossed with IL-13-deficient mice. In these animals, IL-9 could still induce mastocytosis and B lymphocyte infiltration of the lungs. Although IL-9-induced eosinophilia in the peritoneal cavity was not diminished in the absence of IL-13, IL-13 was required for IL-9 to increase eotaxin expression and lung eosinophilia. Mucus production and up-regulation of lung epithelial genes upon IL-9 overexpression were completely abolished in the absence of IL-13. Using hemopoietic cell transfer experiments with recipients that overexpressed IL-9 but were deficient in the IL-9 receptor (IL-9R), we could demonstrate that the effect of IL-9 on lung epithelial cells is indirect and could be fully restored by transfer of hemopoietic cells expressing IL-9R. Mucus production by lung epithelial cells was only up-regulated when hemopoietic cells simultaneously expressed functional IL-9R and IL-13 genes, indicating that IL-13 is not a cofactor but a direct mediator of the effect of IL-9 on lung epithelial cells. Taken together, these data indicate that IL-9 can promote asthma through IL-13-independent pathways via expansion of mast cells, eosinophils, and B cells, and through induction of IL-13 production by hemopoietic cells for mucus production and recruitment of eosinophils by lung epithelial cells.     

The role of IL-5 in the immune responses to nematodes in rodents

In this review, Masataka Korenoga and Isoo Toda discuss the role of interleukin 5 (IL-5) in immune responses caused by parasitic infections. Recent studies have shown that eosinophilia in parasitic infections is dependent on IL-5. Although eosinophils have been thought to be potent effector cells (from results of experiments, in vitro), their role in protective immunity to parasites is controversial.     

Stat6 signaling promotes protective immunity against Trichinella spiralis through a mast cell- and T cell-dependent mechanism

Studies in mice infected with the gastrointestinal nematode parasite Nippostrongylus brasiliensis demonstrated that IL-4/IL-13 activation of Stat6 suppresses development of intestinal mastocytosis and does not contribute to IL-4/IL-13 production, but is still essential for parasite expulsion. Because expulsion of another gastrointestinal nematode, Trichinella spiralis, unlike N. brasiliensis expulsion, is mast cell dependent, these observations suggested that T. spiralis expulsion would be Stat6 independent. Instead, we find that Stat6 activation by IL-4/IL-13 is required in T. spiralis-infected mice for the mast cell responses that induce worm expulsion and for the cytokine responses that induce intestinal mastocytosis. Furthermore, although IL-4 induces N. brasiliensis expulsion in the absence of B cells, T cells, and mast cells, mast cells and T cells are required for IL-4 induction of T. spiralis expulsion. Thus, Stat6 signaling is required for host protection against N. brasiliensis and T. spiralis but contributes to expulsion of these two worms by different mechanisms. The induction of multiple effector mechanisms by Stat6 signaling provides a way for a cytokine response induced by most gastrointestinal nematode parasites to protect against most of these parasites, even though different effector mechanisms are required for protection against different nematodes.     

Th2-mediated host protective immunity to intestinal nematode infections.

Despite many years of study, relatively little is known about the effector mechanisms that operate against intestine-dwelling nematodes. Most of the current understanding comes from studies of laboratory model systems in rodents. It is clear that when an intestinal helminth infection takes place the immune system generates a strong Th2-mediated response, which regulates a variety of responses characteristic of helminth infections such as eosinophilia, intestinal mastocytosis and elevated IgE production. The ability to modulate the host''s immune response in vivo with cytokine-specific monoclonal antibodies and recombinant cytokines, together with the use of animals with disruption of key genes involved in the immune response, have provided powerful tools with which to dissect the potential effector mechanisms operating. In the absence of a T-cell compartment the host is unable to expel the parasite. If a Th1-dominated response is generated, protective immunity is almost universally compromised. Thus, it it would appear that some aspect of a Th2-mediated response controls effector mechanisms. Although it is clear that for some infections the mast cell appears to be involved in protection, probably through the generation of a non-specific inflammatory response, how these cells become activated remains unclear. Data from infections in transgenic animals suggest that activation is not through the high-affinity receptor for IgE. Such studies also call into doubt the importance of conventional interactions between effector leucocytes and antibody. There is little evidence to support a protective role for eosinophilia in any system. New data also imply that, although interleukin 4 (IL-4) is generally important (and can exert effects independent of an adaptive immune response), it is not always sufficient to mediate protection; other Th2 cytokines (e.g. IL-13) may warrant closer investigation. It is apparent that a number of potential Th2-controlled effector mechanisms (some of which may be particularly important at mucosal surfaces) remain to be explored. Overall, it is likely that worm expulsion is the result of a combination of multiple mechanisms, some of which are more critical to some species of parasite than to others.     

Mammalian antimicrobial peptide influences control of cutaneous Leishmania infection

Cathelicidin-type antimicrobial peptides (CAMP) are important mediators of innate immunity against microbial pathogens acting through direct interaction with and disruption of microbial membranes and indirectly through modulation of host cell migration and activation. Using a mouse knock-out model in CAMP we studied the role of this host peptide in control of dissemination of cutaneous infection by the parasitic protozoan Leishmania. The presence of pronounced host inflammatory infiltration in lesions and lymph nodes of infected animals was CAMP-dependent. Lack of CAMP expression was associated with higher levels of IL-10 receptor expression in bone marrow, splenic and lymph node macrophages as well as higher anti-inflammatory IL-10 production by bone marrow macrophages and spleen cells but reduced production of the pro-inflammatory cytokines IL-12 and IFN-γ by lymph nodes. Unlike wild-type mice, local lesions were exacerbated and parasites were found largely disseminated in CAMP knockouts. Infection of CAMP knockouts with parasite mutants lacking the surface metalloprotease virulence determinant resulted in more robust disseminated infection than in control animals suggesting that CAMP activity is negatively regulated by parasite surface proteolytic activity. This correlated with the ability of the protease to degrade CAMP in vitro and co-localization of CAMP with parasites within macrophages. Our results highlight the interplay of antimicrobial peptides and Leishmania that influence the host immune response and the outcome of infection.     

Examination of epithelial tissue cytokine response to natural peste des petits ruminants virus (PPRV) infection in sheep and goats by immunohistochemistry

In this study, we aimed to evaluate expression of IL-4, IL-10, TNF-α, IFN-γ and iNOS in lingual, buccal mucosa and lung epithelial tissue using immunoperoxidase technique and to compare with the tissues of control animals. The tissues used in the study were collected from 17 PPRV-affected and 5 healthy sheep and goats. In PPRV positive animals, the lungs, lingual and buccal mucosa had significantly higher iNOS, IFN-γ and TNF-α expressions compared to control group animals. There was no significant difference between PPRV positive and control groups for IL-4 and IL-10 expressions of epithelial tissues. In conclusion, the epithelial tissues infected by PPRV showed significant iNOS, IFN-γ and TNF-α expressions and they might play an important role in the initiation and regulation of cytokine response, as they take place in the first host barrier to be in contact with PPRV. It is suggested that the more epithelial damage produced by PPRV the more cytokine response may result in the infected epithelial cells. The first demonstration of iNOS expression and epithelial cytokine response to PPRV in natural cases is important because it may contribute to an early initiation of systemic immunity against PPRV infection, in addition to direct elimination of the virus during the initial epithelial phase of the infection.     

IL-17A-producing gammadeltaT cells promote CTL responses against Listeria monocytogenes infection by enhancing dendritic cell cross-presentation

Interleukin-17A-producing T cells, especially Th17, have been shown to be involved in inflammatory autoimmune diseases and host defense against extracellular infections. However, whether and how IL-17A or IL-17A-producing cells can help protection against intracellular bacteria remains controversial, especially how it regulates the adaptive immunity besides recruitment of neutrophils in the innate immune system. By infecting IL-17A-deficient mice with Listeria monocytogenes, we show in this study that IL-17A is required for the generation of Ag-specific CD8(+) CTL response against primary infection, but not for the generation of memory CD8(+) T cells against secondary challenge. Interestingly, we identify γδT cells, but not conventional CD4(+) Th17 cells, as the main cells for innate IL-17A production during L. monocytogenes infection. Furthermore, γδT cells are found to promote Ag-specific CD8(+) T cell proliferation by enhancing cross-presentation of dendritic cells through IL-17A. Adoptive transfer of Il17a(+/+) γδT cells, but not Il17a(-/-) γδT cells or Il17a(+/+) CD4(+) T cells, were sufficient to recover dendritic cells cross-presentation and defective CD8(+) T cell response in Il17a(-/-) mice. Our findings indicate an important role of infection-inducible IL-17A-producing γδT cells and their derived IL-17A against intracellular bacterial infection, providing a mechanism of IL-17A for regulation of innate and adaptive immunity.     

Lack of PPARγ in myeloid cells confers resistance to Listeria monocytogenes infection

The peroxisomal proliferator-activated receptor γ (PPARγ) is a nuclear receptor that controls inflammation and immunity. Innate immune defense against bacterial infection appears to be compromised by PPARγ. The relevance of PPARγ in myeloid cells, that organize anti-bacterial immunity, for the outcome of immune responses against intracellular bacteria such as Listeria monocytogenes in vivo is unknown. We found that Listeria monocytogenes infection of macrophages rapidly led to increased expression of PPARγ. This prompted us to investigate whether PPARγ in myeloid cells influences innate immunity against Listeria monocytogenes infection by using transgenic mice with myeloid-cell specific ablation of PPARγ (LysMCre×PPARγ(flox/flox)). Loss of PPARγ in myeloid cells results in enhanced innate immune defense against Listeria monocytogenes infection both, in vitro and in vivo. This increased resistance against infection was characterized by augmented levels of bactericidal factors and inflammatory cytokines: ROS, NO, IFNγ TNF IL-6 and IL-12. Moreover, myeloid cell-specific loss of PPARγ enhanced chemokine and adhesion molecule expression leading to improved recruitment of inflammatory Ly6C(hi) monocytes to sites of infection. Importantly, increased resistance against Listeria infection in the absence of PPARγ was not accompanied by enhanced immunopathology. Our results elucidate a yet unknown regulatory network in myeloid cells that is governed by PPARγ and restrains both listeriocidal activity and recruitment of inflammatory monocytes during Listeria infection, which may contribute to bacterial immune escape. Pharmacological interference with PPARγ activity in myeloid cells might represent a novel strategy to overcome intracellular bacterial infection.     

Do basophils play a role in immunity against parasites?

The link between parasites and eosinophilia has been known for more than a century, although the role of eosinophils in host protection is still an open issue. Much less appreciated, however, is the concurrent systemic induction of a related cell type, the basophil, in parasitized hosts. To date, little is known about the role of basophils in immunity against parasites, but recent evidence points to a possible crucial role in the initiation of T-helper type 2 responses in the host. In this article, we review the current understanding of parasitic infections and basophils and discuss their putative role in immunity.     

Heat-killed Lactobacillus gasseri TMC0356 protects mice against influenza virus infection by stimulating gut and respiratory immune responses

This study investigated whether heat-killed Lactobacillus protects host animal against influenza virus infection and stimulates their immunity. Heat-killed Lactobacillus gasseri TMC0356 was orally administered to BALB/c mice for 19 days; the mice were intranasally infected with Flu A/PR/8/34 (H1N1) on day 14, and clinical symptoms were monitored. After 6 days, the mice were sacrificed, and pulmonary virus titres were determined. Splenic activation of natural killer (NK) cells and the mRNA expression of cytokines and other immune molecules in the lung and Peyer's patch (PP) were analysed. Clinical symptom scores of mice orally fed TMC0356 ameliorated significantly (P < 0.01); their pulmonary virus titres decreased significantly compared with those of control mice (P < 0.05); their mRNA expression of interleukin (IL)-12, IL-15 and IL-21 in PP and the pulmonary mRNA expression of IFN-γ, TNF, IL-12a, IL-12rbl, IL-2rb and perforin 1 increased significantly (P < 0.05). Oral administration of heat-killed lactobacilli may protect against influenza virus infection by stimulating local and systemic immune responses. Cellular components of lactobacilli may be pivotal in protecting against viral infection by enhancing gut and respiratory immune responses.