Helminthic infection has become rare in highly industrialized nations. Concurrent with the decline in helminthic infection has been an increase in the prevalence of inflammatory disease. Removal of helminths from our environment and their powerful effects on host immunity may have contributed to this increase. Several helminth species can abrogate disease in murine models of inflammatory bowel disease, type 1 diabetes, multiple sclerosis, and other conditions. Helminths evoke immune regulatory pathways often involving dendritic cells, regulatory T cells, and macrophages that help to control disease. Cytokines, such as IL-4, IL-10, and TGF-β, have a role. Notable is the helminthic modulatory effect on innate immunity, which impedes development of aberrant adaptive immunity. Investigators are identifying key helminth-derived immune modulatory molecules that may have therapeutic usefulness in the control of inflammatory disease.

Helminths are worm-like animal parasites that have adapted over many millions of years to live in the gastrointestinal tract, blood, lungs, or other tissues of various species. Their long-term survival requires intricate regulatory interactions between parasite and host immunity. In developed countries, the 20th century brought unprecedented advancements in living standards associated with substantial improvements in agricultural practices and water and food quality. This disrupted the life cycle of various helminths, leading to deworming of the population. The long-standing close association between these parasites and their specific hosts may have led to immune interdependency through the process of coevolution. Epidemiologic data and animal experimentation suggest that elimination of helminths contributes to the increasing prevalence of some immune-mediated diseases in regions with ever-improving sanitation. Diseases with increasing frequency include ulcerative colitis, Crohn’s disease, type 1 diabetes (T1D), multiple sclerosis (MS), rheumatoid arthritis (RA), asthma, and food allergy. Studies, mostly in animal models of human disease, are providing insight into how helminths mediate protection from these conditions.

Inflammatory bowel disease (IBD) is the collective term for ulcerative colitis and Crohn’s disease. These diseases probably are the consequence of an inappropriately aggressive mucosal adaptive immune response to substances normally in the intestinal lumen. IBD became a significant health problem in highly developed countries in the 20th century, and it is spreading in underdeveloped countries (1, 2). Epidemiologic studies and clinic trials suggest that natural helminthic infection protects people from IBD (3).

Helminths modulate intestinal inflammation through activation of interactive immune regulatory circuits involving regulatory T cells (Tregs), dendritic cells (DCs), macrophages, and several cytokines (Fig. 1). There are many helminth species inhabiting different regions of their host. Some have complex life cycles, traveling through the bloodstream and/or various tissues of the body, whereas others enter through the mouth and stay in the lumen of the gastrointestinal tract. Many only inhabit a very limited range of hosts. One may expect, with wide diversity among species, that these organisms developed distinctively creative ways to modulate host immunity. Thus, it is surprising that evolution has endowed a number of them with similar approaches to quell host immunity.

FIGURE 1.

Helminths activate regulatory circuits that limit inflammation in IBD. IBD results from overresponsiveness of adaptive immune pathways to normal constituents of intestinal contents. H. polygyrus bakeri infection induces regulatory DCs and macrophages, and it activates Tregs (CD4+Foxp3+) in the gut to inhibit effector T cell responses. IL-10 coming from intestinal Tregs is particularly important. TGF-β and IL-4 also participate in the regulation. Intestinal helminthic infections alter the composition of gut flora. Although yet unproven, changes in intestinal flora could impact mucosal immune function, leading to protection from IBD.

FIGURE 1.

Helminths activate regulatory circuits that limit inflammation in IBD. IBD results from overresponsiveness of adaptive immune pathways to normal constituents of intestinal contents. H. polygyrus bakeri infection induces regulatory DCs and macrophages, and it activates Tregs (CD4+Foxp3+) in the gut to inhibit effector T cell responses. IL-10 coming from intestinal Tregs is particularly important. TGF-β and IL-4 also participate in the regulation. Intestinal helminthic infections alter the composition of gut flora. Although yet unproven, changes in intestinal flora could impact mucosal immune function, leading to protection from IBD.

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Animal models of IBD suggest that Tregs help to prevent excessive intestinal inflammation (4). The murine gut harbors large numbers of Foxp3+CD4+ Tregs. The colon and terminal ileum contain most of the intestinal flora. In the distal bowel, ∼25% of the lamina propria CD4+ T cells express Foxp3, and the Foxp3+ T cells are the major source of IL-10 (5). These cells likely function to restrain the host immune response to the normal intestinal flora.

Heligmosomoides polygyrus bakeri is a luminal murine helminth that lives in the proximal small bowel, with only the larval stages superficially invading the epithelial lining. This parasite expands the number of Foxp3+ T cells in the mesenteric lymph nodes (MLNs) (6, 7) and intestinal lamina propria of its murine host (5). The costimulatory receptor ICOS facilitates this Treg expansion (8). This helminth also “activates” Tregs, making them highly regulatory (5). Rag mice reconstituted with CD25CD4+ T cells develop intestinal inflammation as a result of a lack of Tregs. Foxp3+ T cells in intestines of healthy wild-type mice are not very regulatory and afford no protection from colitis when transferred into this model of IBD. However, Foxp3+ T cells isolated from the colon, terminal ileum, or MLNs of H. polygyrus bakeri–infected wild-type mice populate the gut and MLNs of the Rag recipients more readily and prevent disease (5).

H. polygyrus bakeri infection induces intestinal Tregs to express several genes, as revealed using microarray and real-time PCR analysis and/or ELISA. Among these are IL-10 (5) and GATA3 (J.V. Weinstock, unpublished observations). The latter is noteworthy, because GATA3 is required for Tregs to accumulate at sites of inflammation. Moreover, it helps to sustain high-level Foxp3 and 10 expression, which are needed for Tregs to protect mice from colitis (911).

In the CD25CD4+ T cell transfer model of IBD, the Foxp3+IL10+CD4+ T cell subset is essential for controlling disease (5, 1214). IFN-γ is a driver of colitis in most IBD models. In the gut, H. polygyrus bakeri–activated Tregs control colitis, in part through secretion of IL-10, which inhibits the production of IFN-γ from mucosal effector T cells. Other mechanisms of action are likely as well. As with H. polygyrus bakeri infection (15), other helminthic species, such as Hymenolepis diminuta (16) and Schistosoma mansoni (17), also induce IL-10 secretion. Litomosoides sigmodontis suppressed B cell responses in the host through induction of IL-10 and Tregs (18, 19).

Helminthic induction of IL-10 synthesis is not an essential factor for controlling IBD. Helminths still prevent colitis and suppress ongoing disease in IL10−/− mice (6, 20). This suggests that helminths activate other important immune regulatory pathways that are IL-10/Treg independent.

Helminths also stimulate various immune cell types to produce other cytokines that hinder the development of T cell subtypes that are implicated in IBD pathogenesis. H. polygyrus bakeri infection stimulates the mucosa to make TGF-β. Transgenic mice producing T cells with disrupted TGF-βR signaling develop colitis. In this transgenic mouse, H. polygyrus bakeri infection cannot dampen the mucosal Th1 response or prevent colitis (21). This shows that H. polygyrus bakeri regulation of mucosal inflammation requires T cells that respond to TGF-β.

Helminths trigger Th2-type responses that have a role in colitis prevention. Trinitrobenzene sulfate (TNBS) mixed with alcohol and given rectally induces murine colitis. Helminths, like S. mansoni and H. polygyrus bakeri, protect mice from TNBS colitis by limiting the colonic IFN-γ and IL-12 response. Helminths stimulate the expansion of Th2 cells that make IL-4. Disruption of the Th2 pathway enhances Th1 cell differentiation and colitis, showing the importance of Th2 cytokines for disease control in this model (17). Isolated helminth products can stimulate these pathways. For example, exposure to schistosome-derived recombinant glutathione S transferase decreases TNBS-induced colitis, inhibits T cell IFN-γ, and promotes IL-4 and IL-10 production (M. Capron, personal communication). There is a colitis model driven by Th2-type cytokines (oxazolone-induced colitis) in which infection with H. diminuta makes the inflammation worse (22). However, helminthic infections can curtail allergic reactions driven by the Th2 pathway (see below). Thus, there are mechanisms of regulation independent of Th2 cytokines.

IL-17 has a role in driving colitis. H. polygyrus bakeri blocks IL-17 secretion, in part through stimulating IL-4 production and, to some extent, IL-10, which affects Th17 cell function (23). Disruption of Stat6 signaling specifically in T cells negates the ability of H. polygyrus bakeri infection to reverse established CD25lo T cell transfer colitis and inhibit IL-17 production (D.E. Elliott, manuscript in preparation). Exposure to helminths also dampens MLN T cell responsiveness to IL-6 through suppression of T cell IL-6Ra expression and induction of SOCS3 (D.E. Elliott, manuscript in preparation). Induction of Th2 circuits and suppression of IL-6/Stat3 signaling constrain Th17 activity.

CD8+ T cells also may have some role in helminthic control of IBD. After H. polygyrus bakeri infection, CD8+ Tregs can reduce the severity of colitis. They inhibit T lymphocyte proliferation through direct cell contact and class I MHC interactions without the need for IL-10 or TGF-β (24). CD8+ Tregs are implicated in the control of several diseases featuring immune dysregulation (25, 26).

Using another model of colitis, it was shown that helminths also control IBD through activation of intestinal regulatory DCs. T cell– and B cell–deficient Rag mice reconstituted with IL10−/− T cells develop colitis because their Tregs cannot make IL-10. Rag mice infected with H. polygyrus bakeri and then dewormed with a pharmaceutical agent before IL10−/− T cell reconstitution are protected from colitis (20). Moreover, the intestinal mucosa makes less colitogenic cytokines, such as IFN-γ and IL-17, after this brief H. polygyrus bakeri exposure, even if the animals are configured to remain free from colitis. This infers that H. polygyrus bakeri can act through cells of innate immunity to render animals resistant to disease.

The mechanism underlying this protection involves induction of regulatory DCs in the intestinal mucosa (20). Activation of these cells does not require the aid of T or B cells. Compared with DCs from uninfected animals, intestinal DCs after H. polygyrus bakeri infection only weakly support Ag-driven IFN-γ secretion. Furthermore, DCs isolated from the intestines or MLNs of H. polygyrus bakeri–infected Rag mice transferred into colitis-susceptible mice block colitis and mucosal Ag–induced IFN-γ and IL-17 responses (27).

The mechanism through which these regulatory DCs quell colitis has been characterized in part. The regulatory DCs do not prevent effector T cells from populating the gut or MLNs, but they do inhibit their function. The regulatory DCs, through a cell contact–dependent mechanism, interfere with the interaction of proinflammatory DCs with their effector T cell counterparts. This prevents Ag-induced IFN-γ and IL-17 secretion. IL-4, IL-10, and TGF-β, as well as Tregs, are not essential for this regulatory process (27).

H. polygyrus bakeri residing in the proximal bowel induces substantial changes in the activation state of DCs residing in the distal intestine (Fig. 2). Microarray analysis shows substantial downmodulation of Jak1/2 and other intracellular signaling pathways (several MAPKs) important for the induction of proinflammatory cytokines that drive effector T cell activation. Many components of the MHC Ag-presenting complex (e.g., CD40, CD80, CD86, H2) are also reduced. In addition, the DCs display decreased expression for molecules associated with the receptor-signaling pathways for IL-1, CSF, IL-6, and TGF-β (20) (J.V. Weinstock, unpublished observations).

FIGURE 2.

Effects of H. polygyrus bakeri infection on the function of DCs. H. polygyrus bakeri infection blocks CLEC and TLR expression and promotes REG secretion in gut DCs. The infection also inhibits DC intracellular signaling pathways (Jak1 and Jak2, and several MAPKs) important for proinflammatory cytokine production. There also is disruption in the signaling pathways for IL-1, IL-6, TGF-β, and CSF. The MHC complex is downmodulated as well (CD40, CD80, CD86, MHCII). As a result of these changes, intestinal DCs are less able to activate effector T cells.

FIGURE 2.

Effects of H. polygyrus bakeri infection on the function of DCs. H. polygyrus bakeri infection blocks CLEC and TLR expression and promotes REG secretion in gut DCs. The infection also inhibits DC intracellular signaling pathways (Jak1 and Jak2, and several MAPKs) important for proinflammatory cytokine production. There also is disruption in the signaling pathways for IL-1, IL-6, TGF-β, and CSF. The MHC complex is downmodulated as well (CD40, CD80, CD86, MHCII). As a result of these changes, intestinal DCs are less able to activate effector T cells.

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An intriguing discovery is the effect of H. polygyrus bakeri infection on intestinal DC innate immune receptor expression. DCs are the critical link between innate and adaptive immunity (28). They sample Ags in the intestinal lumen and present these Ags to T cells, inducing their differentiation and proliferation or perhaps rendering them inert. DCs sense threats in the environment through germline-encoded pattern recognition receptors that bind motifs on bacteria, fungi, viruses, or stressed hosts cells. Engagement of these receptors on or in DCs alters their function. There are four families of such receptors that include TLRs and C-type lectin receptors (CTLRs). Microarray analysis revealed that the intestinal DCs expressed several TLRs, and H. polygyrus bakeri infection decreased the expression of several TLR subtypes (9, 13). Also, there is downmodulation of LPS-binding protein and CD14, important components of the TLR4-signaling complex (Fig. 2).

Two classes of CTLRs are regenerating islet-derived receptors (REGs) and C-type lectin-like domain-containing receptors (CLECs). REGs are secretory proteins that act on the structure bearing the ligand, without modulating the function of their cell of origin (29). Secretory REGs, like 3b and 3g, bind intestinal bacteria and other organisms, leading to their demise (30, 31). REG−/− mice show the importance of some of these REGs (30). H. polygyrus bakeri infection greatly increases the expression of all REGS (REG1, REG3a, REG3b, REG3g, REG4) displayed by intestinal DCs. High REG protein secretion by intestinal DCs would help to keep organisms away from the DC membrane. REG4 has antiapoptosis properties that protect host cells from death (32, 33).

Host cells express CLECs as transmembrane proteins. When CLECs engage their ligands, the receptors trigger intracellular-signaling pathways to alter cell function (34). CLEC7A engages components of fungi and some bacteria, whereas CLEC9A binds dead or dying cells (35). When engaged, some CLEC receptors, like CLEC7A and CLEC9A, activate DCs, promoting T cell activation (35). H. polygyrus bakeri infection profoundly inhibits the expression of nearly all of the CLECs expressed by intestinal DCs (4N, 7A, 9A, and 12A). Therefore, decreased CLEC (e.g., 7A, 9A) expression associated with heightened REG secretion makes it less likely that organisms and necrotic cells will approach DCs and encounter membrane-bound, proinflammatory CLEC receptors. Thus, helminthic regulation of TLRs and CTLRs (CLECs and REGs) may render intestinal DCs less likely to activate adaptive immunity and, subsequently, IBD.

Helminths also protect from IBD through induction of alternatively activated macrophages. Helminths induce the production of IL-10 and Th2 cytokines, such as IL-4 and IL-5, which activate macrophages in distinct ways (36). These alternatively activated macrophages make IL-10, TGF-β, and other immunomodulatory factors that can modulate Th1-type inflammation (37).

Another model of IBD is dextran sodium sulfate (DSS)-induced enteritis. DSS administered orally to rodents damages the intestinal epithelial lining, inducing gut inflammation.

Infection of BALB/c mice with S. mansoni protects from DSS-induced injury through induction of regulatory macrophages. The adult schistosome flukes, living in the portal vein, induce this protection. The protective process does not require Tregs or regulatory cytokines, such as TGF-β and IL-10 (38).

A cysteine protease inhibitor (cystatin) of filarial nematodes protects mice from DSS-induced colitis (39). Macrophages and IL-10 are necessary for this protection, as suggested by a lung inflammatory model. Cystatin activates intracellular-signaling pathways, such as ERK and p38, which induce macrophages to make IL-10 and IL-12p40 (40).

In the IL10−/− Rag model of IBD, H. polygyrus bakeri infection induces regulatory macrophages in the gut of Rag mice. These cells are induced even if mice are not reconstituted with T or B cells. Thus, this induction does not require participation of adaptive immunity. These intestinal macrophages inhibit Ag-induced IL-17 and IFN-γ secretion by a contact-dependent mechanism. Also, when transferred into Rag mice, they protect animals from colitis (J.V. Weinstock, A.M. Blum, and L. Hang, unpublished observations).

In another study using dinitrobenzene sulfonic acid (DNBS) instead of TNBS to induce IBD, infection with H. diminuta protects mice from colitis through induction of alternatively activated macrophages in the colon. Alternatively activated macrophages transferred into mice protect from DNBS-induced injury, attesting to their role in the regulatory process. Extracts from H. diminuta worms injected i.p. also provide protection and suppress macrophage function in vitro (41, 42).

Thus, macrophages activated by helminth infection can suffice to protect from IBD (42). This also suggests that some helminths make soluble factors that can mediate this process in lieu of live organisms. In the DNBS model, alternatively activated macrophages work through an IL-10–dependent mechanism to control colitis (43).

To modulate colitis, helminths must release soluble factors or communicate with the host in some other fashion. The presence of worm-derived soluble factors is supported by experiments that use extracts from H. diminuta worms (41) or dead schistosome OVA to protect mice from colitis (17). Helminths produce a number of products with immune-modulatory properties (7, 4449). For instance, helminths produce molecules that induce Tregs (7). To induce regulatory cells, intestinal helminths must breach the mucosal barrier to engage the immune system. This communication may occur through several possible mechanisms.

DCs advance dendrites across the epithelial barrier, which could permit intestinal helminths in the intestines to communicate directly with these cells. Supporting this hypothesis are data showing that H. polygyrus bakeri and other helminths release factors that affect the state of DC activation (49, 50). This, in turn, can result in decreased Ab responses (50) and stimulation of Treg development.

Direct interactions between intestinal helminths and the gut epithelium is another possible mechanism of action. The intestinal epithelium releases regulatory molecules and sits close to immunocytes. Infection with Trichuris muris stimulates intestinal epithelial cells to make thymic stromal lymphopoietin, which can interact with the lamina propria DCs, promoting a Th2 response and worm expulsion. It also limits IL-12 and IFN-γ production in DSS-induced colitis, reducing pathology (51).

Although some intestinal helminths have no fixed association with the epithelial lining, others interact closely with the mucosal barrier (e.g., hookworm) or place holdfasts beneath the epithelial lining (e.g., whipworm). This affords further access for direct communication with T cells to induce Tregs (7) or with other cells to promote regulation (49).

Gut bacteria are important for the health of the mucosal immune system and readily interact with intestinal DCs and other cells (52). H. polygyrus bakeri infection modifies the distribution and abundance of some intestinal bacteria. There is an increase in Lactobacillaceae. Various bacterial species within this group inhibit intestinal inflammation in models of colitis (53). Rhesus monkeys develop colitis. Trichuris trichiura infection results in a milder colitis associated with reduced bacterial attachment to the epithelial surface and changes to the composition of microbial communities attached to the intestinal mucosa (54).

Helminths also may protect via enhancement of mucosal barrier function (55). Trichuris infection stimulates IL-22 production in the mucosa, which is a molecule associated with epithelial repair and enhancement of the overlying mucous layer (54, 56).

There are helminth species that suppress colitis while living in regions of the host distant from the intestines. Their mode of communication with host immunity could be different. For example, the filaria Brugia malayi resides in lymphatics and releases copious amounts of asparaginyl-tRNA synthetase, which can block IL-8 signaling in human and murine leukocytes and suppress murine T cell–transfer colitis (57).

In summary, animal models suggest that helminths control colitis via induction of several distinct immune-regulatory pathways. This includes promotion of Treg function through induction of Gata3, IL-33R, and IL-10 expression, generation of regulatory DCs with a unique phenotype (Fig. 2), and induction of regulatory macrophages. Also, it appears that IL-4 and TGF-β, as well as their signaling pathways, have a role. However, these regulatory pathways are not necessarily called into play simultaneously with similar importance in each distinct IBD animal model.

Genome-wide association studies demonstrated susceptibility gene overlap between IBD and other autoimmune and immune-mediated inflammatory diseases (58). MS, T1D, RA, and asthma, like IBD, have emerged in populations benefiting from advanced socioeconomic development. This suggests that the environmental factors that impacted immune pathways and increased the risk for IBD also increased the risk for other immune-mediated illnesses. Animal models of these organ-specific inflammatory diseases show that many of the helminth-induced regulatory circuits that regulate murine colitis suppress inflammation in these diseases as well.

Mice or rats immunized with myelin-associated Ags develop experimental autoimmune encephalitis (EAE), a model of MS (59). Mice exposed to viable S. mansoni or dead OVA are protected from developing EAE (60, 61). Schistosome exposure suppresses splenocyte and CNS cell production of IL-12p40, IFN-γ, and TNF-α while increasing TGF-β, IL-10, and IL-4. Infection with H. polygyrus bakeri (62) or Fasciola hepatica (63) or treatment with soluble Trichuris suis adult or larval Trichinella spiralis homogenate (64) also suppresses EAE disease scores, with similar changes in the cytokine profile. T. spiralis infection also affords protection in a rat EAE model (65). Draining popliteal lymph node cells from parasite-exposed rats produce less IFN-γ and IL-17 and more IL-10 and IL-4 in response to concanavalin A stimulation compared with cells from helminth-naive animals. Infection also increases the number of CD4+CD25+Foxp3+ T cells in the spleen. Adoptive transfer of splenic T cells from infected rats into helminth-naive rats protects recipients from developing EAE (65).

As discussed above, helminths produce factors that mediate this protection. Adoptive transfer of bone marrow–derived DCs exposed to excretory/secretory products from cultured T. spiralis muscle cyst larvae also protects against EAE (66). Protection is associated with decreased DC IL-12p70 production and increased DC IL-10 production. Splenocytes from rats that receive helminth product–exposed DCs prior to EAE challenge have more Foxp3+ T cells, make less IL-17A and IFN-γ, and produce more IL-4, IL-10, and TGF-β than do splenocytes from rats that receive medium-alone–exposed DCs (66).

Infection with Taenia crassiceps also inhibits development of EAE (67). Inhibition is associated with suppression of TNF-α and induction of alternatively activated macrophages. Factors released by T. crassiceps cysticerci impair LPS-stimulated bone marrow–derived IL-12 and TNF-α production in a cRAF-dependent manner (68).

Clinical studies involving patients with MS from helminth-endemic areas show that those with active helminthic infections have attenuated disease compared with uninfected patients. Treatment of helminthic infections results in worsening MS activity that is associated with an increase in the fraction of PBMCs making IFN-γ and IL-12 and a decrease in the fraction producing IL-10 and TGF-β (69). Helminth removal also decreases the frequency of circulating CD4+CD25+Foxp3+ T cells. Patients with MS and active helminth infections had an increased frequency of spinal fluid Foxp3+ Treg cells and higher serum retinoic acid levels compared with healthy controls or uninfected MS patients (70). Exposure of PBMC-derived DCs to schistosome soluble egg Ags (SEAs) induced enzymes involved in retinoic acid synthesis, likely by a TLR2 activation–dependent pathway. LPS-stimulated DCs derived from PBMCs of helminth-infected patients made less IL-6, IL-12p70, IL-23, and TNF-α than did DCs from uninfected patients, and levels were further reduced, in a SOCS3-dependent fashion, by exposure to SEA. SEA-exposed DCs, cocultured with autologous CD25 T cells, reduce T cell STAT3 activation while increasing SMAD3 activation and Foxp3 expression (70).

These studies show that helminths can suppress other organ-specific inflammatory diseases beyond colitis. In addition, they show that infection with diverse helminths (nematodes, trematodes, and cestodes) can suppress a specific disease. These different classes of helminths evolved independently and may use different products to influence immune-regulatory pathways. It will be interesting to determine how divergent or convergent are the mechanisms used by different helminths.

T1D develops spontaneously in autoimmune-prone NOD mice, or it can be elicited after serial injection of low-dose streptozotocin (STZ; an islet β cell toxin) in other strains (71). Infection with S. mansoni, T. spiralis, H. polygyrus bakeri, or L. sigmodontis protects NOD mice from insulitis (7275). However, the mechanisms of protection may differ among species.

Young NOD mice exposed to schistosome OVA alone or to SEA are protected from developing diabetes. SEA-induced protection is associated with increased pancreatic mononuclear cell expression of TGF-β, IL-4, and IL-10 mRNA (76). SEA treatment increases the number of CD4+CD25+Foxp3+ T cells in the pancreas and spleen. Splenocytes from SEA-treated NOD mice do not produce disease when transferred into NOD.scid recipients (76). Depletion of Tregs from these splenocytes restores their pathogenicity, showing that this is a critical pathway for protection.

Intraperitoneal injection of excretory/secretory Ags from F. hepatica also provides protection from insulitis in NOD mice (77). Disease prevention is associated with the induction of IL-10–secreting B cells and transcripts indicative of M2 macrophage induction in pancreatic lymph node cell populations.

Infection with L. sigmodontis delays diabetes in IL-4–deficient NOD mice and is associated with increased numbers of splenic CD4+CD25+Foxp3+ Tregs. Like splenocytes from SEA-treated NOD mice, splenocytes from L. sigmodontis–infected mice do not produce diabetes when transferred into NOD.scid recipients. However, unlike the SEA model, depletion of Tregs does not restore pathogenicity, suggesting that this is not a critical pathway for L. sigmodontis–mediated protection. Instead, blockade of TGF-β function abrogates protection, indicating that this pathway is critical for this helminth and disease model (75).

Infection with H. polygyrus bakeri also protects IL-4–deficient NOD mice from developing diabetes (78). Protection is associated with induction of IL-10 production by CD127hiFoxp3 T cells present in pancreatic lymph nodes. Blockade of IL-10 function in vivo in IL-4–deficient (but not IL-4–sufficient) NOD mice abrogates protection (78). This suggests that helminth-amplified IL-4 and IL-10 circuits can independently provide protection.

Infection with T. crassiceps decreases insulitis and protects BALB/C and C57BL/6 mice from STZ-induced diabetes. T. crassiceps–induced protection is associated with an increase in IL-4 and alternatively activated macrophages but not with induction of Tregs (79).

Infection with H. polygyrus bakeri also affords protection from STZ-induced diabetes in C57BL/6 mice (80). H. polygyrus bakeri–induced protection remains intact in STAT6- or IL-10–deficient mice, suggesting that IL-4 and IL-10, individually, are not required for protection in this model.

These studies show that helminths residing in different host tissues can suppress the same organ-specific inflammation. Critical mechanisms for protection from T1D appear to vary among various helminthic species. Also, they use mechanisms that differ from those that protect from colitis or EAE. This may reflect the varying importance of specific regulatory pathways for each model system.

Collagen-induced arthritis (CIA) is a murine model of RA that develops in mice immunized with type II collagen in CFA (81). Infection with S. mansoni before collagen sensitization protects mice from developing polyarticular arthritis (82). This protection is associated with reduced IFN-γ, TNF-α, and IL-17 production, but increased IL-4 and IL-10 production, by splenocytes compared with collagen-sensitized helminth-naive mice (82). Schistosoma japonicum also protects mice from CIA. The infection results in suppressed IFN-γ secretion, but augmented IL-4 and IL-10 secretion, by mitogen-stimulated splenocytes (83, 84).

Another rodent arthritis model is monoarticular inflammation provoked by injection of CFA (without collagen) into a knee joint. Treatment with a 16-kDa recombinant protein derived from S. japonicum (rSj16) protects rats from CFA-induced joint inflammation. Protection is associated with a reduction in serum TNF-α, NO, and IL-1β and restoration of IL-10 levels compared with untreated arthritic and control rats (85).

A 62-kDa phosphocholine-containing glycoprotein (ES-62) isolated from Acanthocheilonema viteae prevents and treats established CIA (86). Collagen-stimulated draining lymph node cells from ES-62–treated mice make less IFN-γ and TNF-α and more IL-10 than do cells from untreated mice. Treatment of mice with ES-62 before CIA induction results in reduced serum IL-17 levels and fewer Th17 cells in the draining lymph node and affected joints (87). Bone marrow–derived DCs stimulated with LPS and ES-62 make less TNF-α, IL-6, and IL-23 and have reduced ability to induce IL-17 production by OT-II T cells in vitro. ES-62 also works directly on polarized Th17 cells to reduce IL-17 production and MyD88 expression (87). A synthetic small molecule modeled after ES-62 (N-(2-[(4-bromobenzyl)sulfonyl]ethyl)-N,N-dimethylamine, 11a) inhibits development of CIA and suppresses the release of IL-12p40 and IL-6 from LPS-stimulated macrophages (88).

Culturing CpG-stimulated bone marrow–derived DCs with F. hepatica total extract increases IL-10 and TGF-β production and suppresses IL-12p70, IL-23, IL-6, and TNF-α production compared with DCs stimulated without extract (89). F. hepatica extract–treated DCs pulsed with collagen and then given to mice suppress CIA and inhibit IL-17 and IFN-γ but augment IL-4, IL-10, and TGF-β production by collagen-stimulated draining lymph node cells. Extract-treated DCs increase the frequency of CD25+Foxp3+ Tregs in draining lymph nodes, and transfer of these T cells suppressed CIA in recipients (89).

H. diminuta infection before intra-articular CFA challenge reduces joint swelling and speeds the resolution of inflammation in mice (90). H. diminuta exposure suppresses CFA-induced TNF-α mRNA expression in challenged joints. Infection increases splenocyte IL-4 and IL-10 production, and H. diminuta infection does not protect IL-10–deficient mice from CFA arthritis (90), suggesting that IL-10 is important for this protection.

The same group found that H. diminuta worsens joint inflammation in another arthritis model in which disease results from injecting BALB/c mice with serum from arthritogenic K/BxN mice that contains Abs against autologous glucose-6-phosphate isomerase (91). Mast cells are required for joint inflammation in the K/BxN model, and worsening of arthritis is likely due to helminth-induced mast cell activation (91).

These studies again confirm the broad nature of helminth-associated immune regulation. In addition, they demonstrate the central protective roles for IL-10 and TGF-β production and suppression of IFN-γ and IL-17 circuitry by helminths in control of arthritis.

Animal models and clinical studies indicate that dysregulated Th1/Th17 responses underlie IBD, MS, T1D, and RA. Because helminth infection directly suppresses those cytokine pathways, helminth-associated suppression of these diseases appears to be somewhat straightforward.

In contrast, allergy and asthma appear to result from excessive Th2-type inflammation. Because helminth infections usually stimulate strong Th2 responses, it is counterintuitive that helminthic exposure would lessen allergic inflammation. However, studies comparing groups treated to remove helminths with untreated controls suggest that helminth infection decreases the prevalence of atopy, at least as measured by skin prick test positivity (92).

Animal models indicate that helminths induce regulatory pathways that can suppress atopic disease. A major murine model of allergic inflammation is airway hyperresponsiveness (AHR) induced by respiratory exposure to an Ag previously used to sensitize the animals (93). This sensitization uses Ag mixed with alum adjuvant.

Infection with H. polygyrus bakeri during or prior to OVA sensitization inhibits subsequent airway reactivity (94) and inflammation (94, 95) upon aerosol challenge. Transfer of MLN cells or splenocytes from infected mice into helminth-naive animals inhibits airway inflammation, showing activation of regulatory cells. H. polygyrus bakeri exposure increases the percentage of CD4+CD25+Foxp3+ T cells in the mesenteric and thoracic lymph nodes (94, 95). In addition, H. polygyrus bakeri colonization induces a CD19+CD23+ regulatory B cell population that can adoptively transfer suppression of airway inflammation, independent of IL-10 production (62). Excretory/secretory products from cultured adult H. polygyrus bakeri worms also inhibit OVA-stimulated airway inflammation and hyperreactivity when given at the time of OVA alum sensitization (96). This protection is associated with reduced IL-4, IL-5, IL-13, and IFN-γ levels in bronchoalveolar lavage fluid, suppression of the OVA-induced increase in alternatively activated macrophage markers, and reduced Teffector/Treg ratios in lung tissue (96).

Exposure to other helminths, such as S. mansoni (9799), S. japonicum (100), or T. spiralis (101), also affords protection from allergic airway reactivity and inflammation. Helminth exposure is associated with decreased OVA-stimulated IL-4 and IL-5, but increased IL-10 and TGF-β production, as measured in either bronchoalveolar lavage fluid or supernatants from cultured pulmonary draining lymph node cells or splenocytes. Splenic CD11c+ DCs isolated from S. japonicum–infected mice transfer protection to helminth-naive mice (100). Transfer of splenic T cells from T. spiralis–infected mice, which contain >2-fold higher percentages of CD4+CD25+Foxp3+ cells, provides partial protection against OVA-induced airway inflammation (102). S. mansoni infection also induces CD4+CD25+Foxp3+ Tregs, and targeted in vivo depletion of Foxp3-expressing cells negates the protective influence of infection, supporting the importance of Tregs for this protection (99). In addition, like H. polygyrus bakeri, exposure to S. mansoni induces CD19+CD23+ regulatory B cells that can transfer protection from AHR (103). These regulatory B cells express CD1d, require intact IL-10 production, and act, in part, by increasing the number of pulmonary CD4+CD25+Foxp3+ Tregs in the lungs (103).

Treatment of mice with ES-62 from A. viteae also protects from OVA-AHR and pulmonary inflammation (46, 104) in association with reduced mast cell degranulation, lower OVA-stimulated IL-4, IL-5, and IL-13 production by draining lymph node cells, and decreased Th17 cells compared with ES-62–naive mice. Treatment with anti–IFN-γ abrogates protection from airway inflammation/reactivity and reverses changes in cytokine profile and Th17 frequency (46), indicating that ES-62 induces counter-regulatory Th1 circuitry in this model.

Helminth infections exerted a strong selective pressure on our genome (105). Many host factors that confer risk for immune-mediated disease evolved under the selection pressure of helminths (106). Thus, it is plausible that eradication of helminthic infections and the loss of their immune-modulatory effects promoted the development of some of the immunological diseases.

There are numerous animal models representing a diverse range of diseases for which helminths prevent and/or abrogate inflammation in various organs. Many helminth species mediate protection, evoking similar immune-regulatory mechanisms. Common themes include modulation of DC function, activation of Tregs, alteration of macrophage activity, and enhancement of regulatory cytokine synthesis. Several of these mechanisms appear to function concurrently and independently of each other. Thus, the loss of any one regulatory pathway will not necessarily abrogate protection from disease. A complex array of different gene interactions, environmental factors, and aberrant host immune responses drive immunological diseases. Thus, the mechanisms of protection should not be expected to be the same for all diseases, mouse strains, and humans. The vast array of independent regulatory circuits that helminths engage may explain why they affect many disease states.

A number of investigations are underway to identify the helminth-derived molecular signals that mediate host immune modulation. This could lead to new pharmaceutical agents that target unique immune regulatory pathways, which will allow safe control or the prevention of some immune-mediated illnesses.

This work was supported by National Institutes of Health Grants DK38327 and DK058755, the Veterans Administration Medical Center, the Schneider family, and the Gilman family.

Abbreviations used in this article:

AHR

airway hyperresponsiveness

CIA

collagen-induced arthritis

CLEC

C-type lectin-like domain-containing receptor

CTLR

C-type lectin receptor

DC

dendritic cell

DNBS

dinitrobenzene sulfonic acid

DSS

dextran sodium sulfate

EAE

experimental autoimmune encephalitis

IBD

inflammatory bowel disease

MLN

mesenteric lymph node

MS

multiple sclerosis

RA

rheumatoid arthritis

REG

regenerating islet-derived receptor

SEA

schistosome soluble egg Ag

STZ

streptozotocin

T1D

type 1 diabetes

TNBS

trinitrobenzene sulfate

Treg

regulatory T cell.

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The authors have no financial conflicts of interest.