# Worms and the Treatment of Inflammatory Bowel Disease:  Are

Authors: Nathalie E. Ruyssers; Benedicte Y. De Winter; Joris G. De Man; Alex Loukas; Arnold G. Herman; Paul A. Pelckmans; Tom G. Moreels
Journal: Clinical and Developmental Immunology
 (2008)

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## Abstract

The lack of exposure to helminth infections, as a result of improved living
standards and medical conditions, may have contributed to the increased incidence of
IBD in the developed world. Epidemiological, experimental, and clinical data sustain the
idea that helminths could provide protection against IBD. Studies investigating the underlying
mechanisms by which helminths might induce such protection have revealed the importance
of regulatory pathways, for example, regulatory T-cells. Further investigation on how helminths
influence both innate and adaptive immune reactions will shed more light on the complex
pathways used by helminths to regulate the hosts immune system. Although therapy with
living helminths appears to be effective in several immunological diseases, the disadvantages
of a treatment based on living parasites are explicit. Therefore, the identification and
characterization of helminth-derived immunomodulatory molecules that contribute to the
protective effect could lead to new therapeutic approaches in IBD and other immune diseases.


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## Body

## 1. Inflammatory Bowel Diseases AND The Hygiene Hypothesis

Inflammatory bowel diseases (IBDs), such as Crohn’s disease and
ulcerative colitis, are chronic  immune diseases of the gastrointestinal tract.
Although the aetiologies of these diseases still remain unknown, the current
hypothesis indicates that IBD results from an uncontrolled immune response to
the normal gut flora [1, 2].
Genetic factors and environmental factors both contribute to the damaging mucosal
immune response [3, 4].The incidence of IBD has steadily increased in the developed world since
1950 [5, 6]. According
to the hygiene hypothesis, this is directly related to the higher
hygienic standards in these countries [7, 8]. It is suggested that the lack of exposure to
infectious agents like helminths, as a result of improved living standards and medical
conditions, modulates the development of the immune system and thereby increases the
risk of immune diseases [9, 10].The hygiene hypothesis was initially proposed by Strachan in 1989 for
hay fever [11] and additional epidemiological studies
were performed to further investigate the link between this hygiene concept and the
incidence of other immunological diseases. As a consequence, the hygiene hypothesis
is now proposed for several immunological disorders such as asthma and allergic
diseases [12], cardiovascular diseases
[13], Type 1 diabetes mellitus
[14], multiple sclerosis
[15], and IBD [16].The hygiene hypothesis for IBD is clearly supported by the geographical
distribution of the disease. There is a well described north-south gradient for
the incidence of IBD. Northern Europe and North America have the highest IBD
incidence rates whereas Crohn’s disease and ulcerative colitis remain scarce in
South America, Africa, and Asia [6, 17]. However, the gap between high- and low-incidence
areas in northern versus southern regions is narrowing. In Asia, for example,
incidence rates still remain low as compared to Europe, but they are rapidly
increasing [18]. Changing lifestyle is thought to be the
major cause of the disease increase in low-incidence areas
[18]. The most important factor to explain these geographical differences is the socioeconomic
level [16]. IBD is more
frequently seen among patients with a higher socioeconomic status [19, 20]. Higher socioeconomic levels can be associated with
better sanitation conditions, high-quality water, and better medical standards
[2].Another factor supporting the hygiene hypothesis is the inverse relationship
between infant mortality rates and the incidence of IBD. Infant mortality might
be linked to worse hygiene and medical conditions. Countries with high infant
mortality rates consequently have lower reported incidence of IBD
[21].As mentioned previously, better hygienic circumstances translate into
diminished exposure to infectious agents like helminths. The absence of such
parasitic infections during childhood renders the immune system more prone to
allergic and immune diseases. Thus infections seem to activate an important
protective factor against these disorders [7].
Identifying the nature of this protective effect and implementing this notion in
therapeutic strategies against IBD and other immune diseases is now the
challenge for basic research.


## 2. Immunology of the Gastrointestinal Tract

### 2.1. Initiating Innate and Adaptive Immune Responses to Enteric Antigens in the Gut

The gastrointestinal tract is continuously exposed to a wide range of dietary and
environmental antigens, both harmless and pathogenic. Mounting protective immune
responses against harmful pathogens whilst also preventing excessive responses to
harmless antigens from food and bacterial flora is one of the major dichotomous functions
of the mucosal immune system [22].There are different levels of host defence that pathogens have to
trespass to induce inflammation. Numerous mechanisms are acting to form a
physical barrier to prevent micro-organisms from gaining access to the
underlying tissues. Production of saliva and mucus, gastric and pancreatic
juices, intestinal peristalsis, and epithelial cells all contribute to the
elimination of pathogens from the gut lumen [23, 24]. Tight junctions between epithelial cells form a
barrier to prevent bacterial pathogens from invading the gut tissue
[25–27]. Once a pathogen breaks through this physical
barrier, innate and adaptive immune responses work closely together to
eliminate the intruder [28].Antigens in the gut lumen can be taken up via different transport routes
[29]. The innate immune system will respond to
pathogen associated molecular patterns (PAMPs). As a part of the innate immune system,
phagocytes like monocytes, macrophages and dendritic cells, and cytotoxic cells like
natural killer cells rapidly control the invasion [30].
The adaptive immune system responds to antigens which have been presented by cells
of the innate immune system [30]. Once
antigens are taken up by antigen presenting cells, such as dendritic cells,
fragments of the antigen are presented to T-cells locally or in mesenteric
lymph nodes (MLNs) after migration of the antigen presenting cells
[31, 32]. Adaptive
immune responses are initiated by stimulation of lymphocytes. T-cells will help
B lymphocytes to secrete immunoglobulins, the antigen-specific antibodies that
are responsible for eliminating extracellular pathogens. On the other hand, T
lymphocytes eradicate intracellular pathogens and mediate, for example, antihelminth
and allergic responses [23]. Adaptive immune responses improve on
repeated exposure to a given antigen by the formation of B and T memory cells
[28].


### 2.2. T Cell Subsets and the Immunological Basis of Helminth Therapy in IBD

T lymphocytes are characterized by their cell-surface antigens called CD (cluster
of differentiation) antigens. A common CD antigen found on all T-cells is the CD3
molecule which forms an essential part of the T-cell receptor and is important in the
recognition of antigens presented by antigen presenting cells
[33]. Within this pool of T lymphocytes, a difference
is made between cytotoxic T-cells (CD8+) and helper T-cells (CD4+).
CD4+ T-cells can orchestrate the functional activity of both innate and
adaptive immune systems by “helping” macrophages, NK cells, CD8+ T cells, and
B cells. These T helper (Th) cells can be divided into several subsets of CD4+
cells and each subset is suited for coordinating the effector activities that best
combat the invading pathogen [34]. CD4+ T lymphocytes
can be classified into distinct populations based on the cytokines they produce
(Figure 1) [35–37].Figure 1
T-cell subsets. Naïve CD4+ T cells are stimulated by antigen
presenting cells and the cytokine environment to proliferate into a certain subset.
There are three distinct effector T-cell subsets (red): Th1, Th2, and Th17. CD4+
regulatory T-cells (green) can be subdivided in CD4+CD25+ Treg,
Tr1, and Th3 cells. Crohn’s disease is characterized by Th1, Th17 inflammation,
whereas helminths induce Th2 and regulatory T-cells
(modified from [36, 37]).Effector CD4+ T cells are divided into three distinct lineages. T helper
1 (Th1) cells are engaged in the eradication of intracellular pathogens (e.g., intracellular
bacteria and viruses) and are characterized by the production of IL-2, IL-12, and
IFN-γ [38]. Gastrointestinal
inflammation during Crohn’s disease is Th1 mediated
[23]. T helper 2 (Th2) cells stimulate B-cell antibody
production, eosinophil recruitment and mucosal expulsion mechanisms and are
characterized by the secretion of IL-4, IL-5, and IL-13
[38]. Th2 cells enhance elimination of parasitic
helminth infections and support allergic responses. During helminth infection,
the host evokes a strong Th2 immune response to provide protection against worm
colonization [39]. The cytokines produced by Th1 and
Th2 cells crossregulate each other’s development and activity, for example, IFN-γ produced by Th1 cells amplifies Th1 development
and inhibits proliferation of Th2 cells
[35]. In this way, helminths can evoke an immune
response that might be able to attenuate the Th1 response found during Crohn’s disease.A third lineage of effector CD4+ cells has been recently discovered and
is characterized by the production of IL-17, the Th17 cell. IL-17 induces
expression of many innate inflammatory mediators such as IL-6, acute phase
proteins, granulocyte-colony stimulating factor, and prostaglandin E2. Th1 and
Th2 cytokines can inhibit Th17 development, while Th1 and Th2 effector cells seem
resistant to IL17 expression [40]. It is now
clear that the Th17 pathway is critical for the development of inflammation.
IL17 is elevated in a variety of inflammatory conditions as shown for
rheumatoid arthritis, asthma, and recently IBD [41].
Furthermore, it has been shown that IL-23 supports the proliferation of Th17 cells.
IL-23 is mainly produced by activated myeloid cells such as macrophages and dendritic
cells. The discovery of this new IL-23/IL-17 pathway was a major breakthrough
in the immunopathogenesis of IBD and the exact role of this axis needs to be
further defined [41, 42].
Investigation of the effect of helminth infections on the IL-23/IL-17 pathway may
uncover additional immunological pathways by which helminths can provide protection
against immune disorders.Aside from these effector T-cells, another population of CD3+ cells called
regulatory T (Treg) cells have been described. Treg cells have immunosuppressive
function and cytokine profiles distinct from either Th1, Th2, or Th17 T-cells
[43]. By suppressing excessive Th1, Th2, or Th17
immune responses, Treg cells play an important role in the maintenance of
self-tolerance, thus preventing autoimmune diseases, as well as inhibiting harmful
inflammatory diseases such as asthma and inflammatory bowel disease
[44]. There is emerging evidence that distinct
subgroups of CD4+, CD8+, and natural killer T cells mediate immune
regulatory mechanisms [45]. The most attention is being
paid to the CD4+ Treg cells which can be subdivided into different subsets.
These include the natural CD4+CD25+ Treg cells, which inhibit immune
responses through cell-cell contact and through the production of immunosuppressive
cytokines, type 1 Tr (Tr1) cells which secrete high levels of IL-10 and
type 3 T (Th3) cells which primarily secrete TGF-β [43].
Treg lymphocytes suppress the differentiation of both Th1 and Th2 lymphocytes and are
considered real gatekeepers of the mucosal immune response
[2].The balance between Th1, Th2, and Treg cells is of special interest in the
gastrointestinal immune system. The gut provides a unique microenvironment prone to
Treg cell differentiation. This microenvironment is characterized by the constant
exposure to commensal flora and food antigens and by the presence of immunomodulatory
factors and cytokines that participate in the differentiation of the mucosal immune
system [22]. Defects of the regulatory mechanisms may
lead to development of specific Th1- or Th2- mediated diseases.Given that helminths induce a distinct immunological mechanism compared
to IBD, worms can be used as immunomodulators to downregulate the immune
response in IBD. Helminths induce Th2 and Treg cells which are capable of
suppressing Th1 effector cells, the cells responsible for maintenance of
inflammation in IBD patients.


## 2.1. Initiating Innate and Adaptive Immune Responses to Enteric Antigens in the Gut

The gastrointestinal tract is continuously exposed to a wide range of dietary and
environmental antigens, both harmless and pathogenic. Mounting protective immune
responses against harmful pathogens whilst also preventing excessive responses to
harmless antigens from food and bacterial flora is one of the major dichotomous functions
of the mucosal immune system [22].There are different levels of host defence that pathogens have to
trespass to induce inflammation. Numerous mechanisms are acting to form a
physical barrier to prevent micro-organisms from gaining access to the
underlying tissues. Production of saliva and mucus, gastric and pancreatic
juices, intestinal peristalsis, and epithelial cells all contribute to the
elimination of pathogens from the gut lumen [23, 24]. Tight junctions between epithelial cells form a
barrier to prevent bacterial pathogens from invading the gut tissue
[25–27]. Once a pathogen breaks through this physical
barrier, innate and adaptive immune responses work closely together to
eliminate the intruder [28].Antigens in the gut lumen can be taken up via different transport routes
[29]. The innate immune system will respond to
pathogen associated molecular patterns (PAMPs). As a part of the innate immune system,
phagocytes like monocytes, macrophages and dendritic cells, and cytotoxic cells like
natural killer cells rapidly control the invasion [30].
The adaptive immune system responds to antigens which have been presented by cells
of the innate immune system [30]. Once
antigens are taken up by antigen presenting cells, such as dendritic cells,
fragments of the antigen are presented to T-cells locally or in mesenteric
lymph nodes (MLNs) after migration of the antigen presenting cells
[31, 32]. Adaptive
immune responses are initiated by stimulation of lymphocytes. T-cells will help
B lymphocytes to secrete immunoglobulins, the antigen-specific antibodies that
are responsible for eliminating extracellular pathogens. On the other hand, T
lymphocytes eradicate intracellular pathogens and mediate, for example, antihelminth
and allergic responses [23]. Adaptive immune responses improve on
repeated exposure to a given antigen by the formation of B and T memory cells
[28].


## 2.2. T Cell Subsets and the Immunological Basis of Helminth Therapy in IBD

T lymphocytes are characterized by their cell-surface antigens called CD (cluster
of differentiation) antigens. A common CD antigen found on all T-cells is the CD3
molecule which forms an essential part of the T-cell receptor and is important in the
recognition of antigens presented by antigen presenting cells
[33]. Within this pool of T lymphocytes, a difference
is made between cytotoxic T-cells (CD8+) and helper T-cells (CD4+).
CD4+ T-cells can orchestrate the functional activity of both innate and
adaptive immune systems by “helping” macrophages, NK cells, CD8+ T cells, and
B cells. These T helper (Th) cells can be divided into several subsets of CD4+
cells and each subset is suited for coordinating the effector activities that best
combat the invading pathogen [34]. CD4+ T lymphocytes
can be classified into distinct populations based on the cytokines they produce
(Figure 1) [35–37].Figure 1
T-cell subsets. Naïve CD4+ T cells are stimulated by antigen
presenting cells and the cytokine environment to proliferate into a certain subset.
There are three distinct effector T-cell subsets (red): Th1, Th2, and Th17. CD4+
regulatory T-cells (green) can be subdivided in CD4+CD25+ Treg,
Tr1, and Th3 cells. Crohn’s disease is characterized by Th1, Th17 inflammation,
whereas helminths induce Th2 and regulatory T-cells
(modified from [36, 37]).Effector CD4+ T cells are divided into three distinct lineages. T helper
1 (Th1) cells are engaged in the eradication of intracellular pathogens (e.g., intracellular
bacteria and viruses) and are characterized by the production of IL-2, IL-12, and
IFN-γ [38]. Gastrointestinal
inflammation during Crohn’s disease is Th1 mediated
[23]. T helper 2 (Th2) cells stimulate B-cell antibody
production, eosinophil recruitment and mucosal expulsion mechanisms and are
characterized by the secretion of IL-4, IL-5, and IL-13
[38]. Th2 cells enhance elimination of parasitic
helminth infections and support allergic responses. During helminth infection,
the host evokes a strong Th2 immune response to provide protection against worm
colonization [39]. The cytokines produced by Th1 and
Th2 cells crossregulate each other’s development and activity, for example, IFN-γ produced by Th1 cells amplifies Th1 development
and inhibits proliferation of Th2 cells
[35]. In this way, helminths can evoke an immune
response that might be able to attenuate the Th1 response found during Crohn’s disease.A third lineage of effector CD4+ cells has been recently discovered and
is characterized by the production of IL-17, the Th17 cell. IL-17 induces
expression of many innate inflammatory mediators such as IL-6, acute phase
proteins, granulocyte-colony stimulating factor, and prostaglandin E2. Th1 and
Th2 cytokines can inhibit Th17 development, while Th1 and Th2 effector cells seem
resistant to IL17 expression [40]. It is now
clear that the Th17 pathway is critical for the development of inflammation.
IL17 is elevated in a variety of inflammatory conditions as shown for
rheumatoid arthritis, asthma, and recently IBD [41].
Furthermore, it has been shown that IL-23 supports the proliferation of Th17 cells.
IL-23 is mainly produced by activated myeloid cells such as macrophages and dendritic
cells. The discovery of this new IL-23/IL-17 pathway was a major breakthrough
in the immunopathogenesis of IBD and the exact role of this axis needs to be
further defined [41, 42].
Investigation of the effect of helminth infections on the IL-23/IL-17 pathway may
uncover additional immunological pathways by which helminths can provide protection
against immune disorders.Aside from these effector T-cells, another population of CD3+ cells called
regulatory T (Treg) cells have been described. Treg cells have immunosuppressive
function and cytokine profiles distinct from either Th1, Th2, or Th17 T-cells
[43]. By suppressing excessive Th1, Th2, or Th17
immune responses, Treg cells play an important role in the maintenance of
self-tolerance, thus preventing autoimmune diseases, as well as inhibiting harmful
inflammatory diseases such as asthma and inflammatory bowel disease
[44]. There is emerging evidence that distinct
subgroups of CD4+, CD8+, and natural killer T cells mediate immune
regulatory mechanisms [45]. The most attention is being
paid to the CD4+ Treg cells which can be subdivided into different subsets.
These include the natural CD4+CD25+ Treg cells, which inhibit immune
responses through cell-cell contact and through the production of immunosuppressive
cytokines, type 1 Tr (Tr1) cells which secrete high levels of IL-10 and
type 3 T (Th3) cells which primarily secrete TGF-β [43].
Treg lymphocytes suppress the differentiation of both Th1 and Th2 lymphocytes and are
considered real gatekeepers of the mucosal immune response
[2].The balance between Th1, Th2, and Treg cells is of special interest in the
gastrointestinal immune system. The gut provides a unique microenvironment prone to
Treg cell differentiation. This microenvironment is characterized by the constant
exposure to commensal flora and food antigens and by the presence of immunomodulatory
factors and cytokines that participate in the differentiation of the mucosal immune
system [22]. Defects of the regulatory mechanisms may
lead to development of specific Th1- or Th2- mediated diseases.Given that helminths induce a distinct immunological mechanism compared
to IBD, worms can be used as immunomodulators to downregulate the immune
response in IBD. Helminths induce Th2 and Treg cells which are capable of
suppressing Th1 effector cells, the cells responsible for maintenance of
inflammation in IBD patients.


## 3. Helminths as Therapeutic Agents in IBD

### 3.1. Experimental and Clinical Studies Supporting Helminth-Based Therapy

Helminths colonize more than one third of the world population
[46]. In developed countries, these parasites have
been largely eradicated as a public health concern due to the availability of
efficacious drugs and better sanitation conditions
[47]. In developing countries, however, helminth
colonization is still common [48]. As shown by
epidemiological studies, there is an inverse relation between the frequency of
worm colonization and the prevalence of IBD [46].
It was Elliott et al. who first proposed the hypothesis that the loss of exposure to
parasitic worms increased the risk of IBD [49, 50].Preliminary data of Elliott et al. illustrating a protective response ofSchistosoma mansoni infection on trinitrobenzene sulphate
(TNBS)-induced colitis in mice [49] have led to
several experimental animal studies investigating the effect of helminth infections
on IBD [2]. The first full study on helminth
modulation of experimentally induced colitis was published by Reardon et al. in 2001. They showed that infection of mice with the tapeworm
Hymenolepis diminuta ameliorated dextran sodium sulphate
(DSS)-induced colitis [51]. Khan et al. subsequently
showed that infection with the nematode, Trichinella spiralis,
protected mice from colitis induced by intrarectal challenge with dinitrobenzene
sulphate (DNBS) [52]. Elliott et al. demonstrated that
schistosome eggs had a protective effect on TNBS-induced colitis in mice
[53] and that Heligmosomoides polygyrus
could reduce established colitis in IL-10 deficient mice
[54]. We previously demonstrated a protective effect
of infection with the blood fluke, Schistosoma mansoni, on
trinitrobenzene sulfonic acid (TNBS)-induced colitis in rats
[55]. Taken together, different helminth parasites
(nematode, cestode, and trematode) can ameliorate colitis in different experimental
animal models [56]. Furthermore, helminths also
protect against other immunological diseases as shown in rodent models for asthma
[57], type 1 diabetes mellitus
[14], and experimental autoimmune encephalomyelitis
[17, 58].Based on the promising findings of helminth infections on experimental
colitis, clinical studies were initiated. Treatment of patients with the porcine
whipworm,Trichuris suis, resulted in clinical amelioration of
both Crohn’s disease and ulcerative colitis [59, 60]. In the same line, a proof of concept study showed
clinical efficacy of experimental infection with the human hookworm
Necator americanus on Crohn’s disease
[61]. Clinical trials of Necator americanus
in asthma are being organized [62]. An international
multicentre clinical trial is in preparation (awaiting FDA approval) to further
investigate the clinical efficacy of helminth-based therapy in IBD
[2].


### 3.2. The Use of  Helminth-Derived Molecules as Therapeutic Agents

Although helminth infections appear to be effective against IBD, treatment
of patients with living helminths may envision drawbacks. Persistent infection
and/or invasion of the parasite (particularly zoonotic ones) to other tissues
in the human host, where they might cause pathology, should be considered
[63, 64]. In 2006,
Kradin et al. reported that treatment of a pediatric Crohn’s disease patient with
five oral doses of Trichuris suis ova caused infection with living sexually immature worms
in the ileocecal region and a sexually mature male worm within the cecum
[64]. Although helminths may be beneficial in the treatment of IBD, using living helminth ova
can lead to infection, therefore, therapeutic human helminth colonization needs
to be closely examined for potential adverse side effects. Furthermore, intestinal
helminths influence gastrointestinal physiology. Infection with certain nematodes
may induce enhanced intestinal propulsive activity, goblet cell hyperplasia,
and increased mucus secretion [65]. As a consequence,
intestinal helminths may alter gastrointestinal motility, possibly resulting in
intestinal symptoms like diarrhoea and abdominal cramps [65]. Moreover,
the idea of being infected with a living parasite could be psychologically hard
to accept for some patients. Therefore, treatment with immunologically active
helminth molecules might overcome the possible disadvantages of a therapy with
living parasites.Identification and characterization of helminth-derived immunomodulatory
molecules that contribute to the anticolitis effect could lead to new
therapeutic approaches in IBD without the need for helminth infection
[56, 66].
Using parasite extracts or synthetic drugs designed to mimic the disease-modulating
effect of helminth molecules also allows greater flexibility in dosing routes
and therapeutic applications [67].Helminths possess evolved mechanisms to turn off proinflammatory cascades
by secreting and expressing certain molecules [37].
Multiple studies have characterized a broad spectrum of helminth-derived
immunomodulatory products. A detailed review of these products is beyond the scope
of this paper so we will bring only some molecules of interest into focus.
Maizels et al. showed that the filarial nematode Brugia malayi
produces homologues of the mammalian cytokine TGF-β. Bm-tgh-2 is secreted by adult worms
and binds to mammalian TGF-β
receptors thus performing an immunomodulatory function in the host
[39]. Helminths secrete cysteine protease inhibitors
which interfere with antigen presentation and increase IL-10 secretion from macrophages
[68]. Helminth-derived carbohydrates contribute to the
induction of Th2 immune responses [69].
Lacto-N-fucopentaose III is the predominant carbohydrate component of
Schistosma mansoni egg antigens and this glycan stimulates the
secretion of Th2 cytokines [70]. Harnett et al.
recently showed that the phosphorylcholine part of the glycoconjugate ES-62,
secreted by filarial nematodes, is responsible for its anti-inflammatory action in
arthritis [71]. Research focusing on the development
of vaccines against helminth infections also showed the effectiveness of helminth
antigens as immunomodulators. Vaccination studies against Schistosomiasis are focusing on the protective effect of several Schistosoma antigens [72]. Vaccination
studies against human hookworm infections tested recombinant excretory/secretory (ES)
products from L3 larval stages of Ancylostoma caninum and promising
results were observed [73–75]. Furthermore, vaccination studies against
hookworm infection revealed that administration of a cocktail of recombinant antigens
has an improved protective effect compared to the protection achieved with
separate antigens [76].In respect to IBD, there is need for in-depth experimental studies on the effect
of helminth antigens on colitis. We are currently investigating the therapeutic
potential of protein mixtures ofSchistosoma mansoni and
Ancylostoma caninum on TNBS-induced colitis in mice. As shown in
Figure 2, preliminary experiments showed that both
S. mansoni soluble worm proteins and A. caninum
ES products attenuated TNBS-induced inflammation of the murine colon
[77]. These results indicate that the beneficial effect
of treatment with living worms on experimental colitis may be reproduced with soluble
extracts of helminths. Yang et al. showed that Schistosoma japonicum
egg antigens inhibited the development of asthma in a murine model
[57]. S. mansoni antigens are also able to modulate
innate immune responses and prevent onset of type 1 diabetes [78]. These studies indicate
that treatment with helminth extracts may be as effective as treatment with living
helminths and that the achieved  protection is not specific for just one
helminth species. Isolated helminth proteins may provide a more readily
acceptable form of therapy for patients than living worms.Figure 2
Effect of Schistosoma mansoni soluble worm
proteins (SmSWPs) and Ancylostoma caninum excretory/secretory
products (AcESPs) on myeloperoxidase (MPO) activity. MPO activity was measured to
monitor the degree of myeloid cell infiltration in the colon. Data are presented as
units MPO per gram of colon tissue and 1 unit equals the amount of MPO necessary
to degrade 1 μ mol of H2O2 to H2O per minute at 25°C. TNBS-induced colitis caused a significant increase in MPO activity compared to control mice treated with phosphate-buffered saline (PBS). Intraperitoneal injection of
helminth-derived products significantly ameliorated inflammation as shown by the significant decrease in MPO activity. Treatment of control mice with SmSWP or AcESP had no effect (data not shown). *p≤.05, significantly different from contol PBS; #p≤.05,
significantly different from TNBS-PBS; two way ANOVA, n=7−10 [77].


## 3.1. Experimental and Clinical Studies Supporting Helminth-Based Therapy

Helminths colonize more than one third of the world population
[46]. In developed countries, these parasites have
been largely eradicated as a public health concern due to the availability of
efficacious drugs and better sanitation conditions
[47]. In developing countries, however, helminth
colonization is still common [48]. As shown by
epidemiological studies, there is an inverse relation between the frequency of
worm colonization and the prevalence of IBD [46].
It was Elliott et al. who first proposed the hypothesis that the loss of exposure to
parasitic worms increased the risk of IBD [49, 50].Preliminary data of Elliott et al. illustrating a protective response ofSchistosoma mansoni infection on trinitrobenzene sulphate
(TNBS)-induced colitis in mice [49] have led to
several experimental animal studies investigating the effect of helminth infections
on IBD [2]. The first full study on helminth
modulation of experimentally induced colitis was published by Reardon et al. in 2001. They showed that infection of mice with the tapeworm
Hymenolepis diminuta ameliorated dextran sodium sulphate
(DSS)-induced colitis [51]. Khan et al. subsequently
showed that infection with the nematode, Trichinella spiralis,
protected mice from colitis induced by intrarectal challenge with dinitrobenzene
sulphate (DNBS) [52]. Elliott et al. demonstrated that
schistosome eggs had a protective effect on TNBS-induced colitis in mice
[53] and that Heligmosomoides polygyrus
could reduce established colitis in IL-10 deficient mice
[54]. We previously demonstrated a protective effect
of infection with the blood fluke, Schistosoma mansoni, on
trinitrobenzene sulfonic acid (TNBS)-induced colitis in rats
[55]. Taken together, different helminth parasites
(nematode, cestode, and trematode) can ameliorate colitis in different experimental
animal models [56]. Furthermore, helminths also
protect against other immunological diseases as shown in rodent models for asthma
[57], type 1 diabetes mellitus
[14], and experimental autoimmune encephalomyelitis
[17, 58].Based on the promising findings of helminth infections on experimental
colitis, clinical studies were initiated. Treatment of patients with the porcine
whipworm,Trichuris suis, resulted in clinical amelioration of
both Crohn’s disease and ulcerative colitis [59, 60]. In the same line, a proof of concept study showed
clinical efficacy of experimental infection with the human hookworm
Necator americanus on Crohn’s disease
[61]. Clinical trials of Necator americanus
in asthma are being organized [62]. An international
multicentre clinical trial is in preparation (awaiting FDA approval) to further
investigate the clinical efficacy of helminth-based therapy in IBD
[2].


## 3.2. The Use of  Helminth-Derived Molecules as Therapeutic Agents

Although helminth infections appear to be effective against IBD, treatment
of patients with living helminths may envision drawbacks. Persistent infection
and/or invasion of the parasite (particularly zoonotic ones) to other tissues
in the human host, where they might cause pathology, should be considered
[63, 64]. In 2006,
Kradin et al. reported that treatment of a pediatric Crohn’s disease patient with
five oral doses of Trichuris suis ova caused infection with living sexually immature worms
in the ileocecal region and a sexually mature male worm within the cecum
[64]. Although helminths may be beneficial in the treatment of IBD, using living helminth ova
can lead to infection, therefore, therapeutic human helminth colonization needs
to be closely examined for potential adverse side effects. Furthermore, intestinal
helminths influence gastrointestinal physiology. Infection with certain nematodes
may induce enhanced intestinal propulsive activity, goblet cell hyperplasia,
and increased mucus secretion [65]. As a consequence,
intestinal helminths may alter gastrointestinal motility, possibly resulting in
intestinal symptoms like diarrhoea and abdominal cramps [65]. Moreover,
the idea of being infected with a living parasite could be psychologically hard
to accept for some patients. Therefore, treatment with immunologically active
helminth molecules might overcome the possible disadvantages of a therapy with
living parasites.Identification and characterization of helminth-derived immunomodulatory
molecules that contribute to the anticolitis effect could lead to new
therapeutic approaches in IBD without the need for helminth infection
[56, 66].
Using parasite extracts or synthetic drugs designed to mimic the disease-modulating
effect of helminth molecules also allows greater flexibility in dosing routes
and therapeutic applications [67].Helminths possess evolved mechanisms to turn off proinflammatory cascades
by secreting and expressing certain molecules [37].
Multiple studies have characterized a broad spectrum of helminth-derived
immunomodulatory products. A detailed review of these products is beyond the scope
of this paper so we will bring only some molecules of interest into focus.
Maizels et al. showed that the filarial nematode Brugia malayi
produces homologues of the mammalian cytokine TGF-β. Bm-tgh-2 is secreted by adult worms
and binds to mammalian TGF-β
receptors thus performing an immunomodulatory function in the host
[39]. Helminths secrete cysteine protease inhibitors
which interfere with antigen presentation and increase IL-10 secretion from macrophages
[68]. Helminth-derived carbohydrates contribute to the
induction of Th2 immune responses [69].
Lacto-N-fucopentaose III is the predominant carbohydrate component of
Schistosma mansoni egg antigens and this glycan stimulates the
secretion of Th2 cytokines [70]. Harnett et al.
recently showed that the phosphorylcholine part of the glycoconjugate ES-62,
secreted by filarial nematodes, is responsible for its anti-inflammatory action in
arthritis [71]. Research focusing on the development
of vaccines against helminth infections also showed the effectiveness of helminth
antigens as immunomodulators. Vaccination studies against Schistosomiasis are focusing on the protective effect of several Schistosoma antigens [72]. Vaccination
studies against human hookworm infections tested recombinant excretory/secretory (ES)
products from L3 larval stages of Ancylostoma caninum and promising
results were observed [73–75]. Furthermore, vaccination studies against
hookworm infection revealed that administration of a cocktail of recombinant antigens
has an improved protective effect compared to the protection achieved with
separate antigens [76].In respect to IBD, there is need for in-depth experimental studies on the effect
of helminth antigens on colitis. We are currently investigating the therapeutic
potential of protein mixtures ofSchistosoma mansoni and
Ancylostoma caninum on TNBS-induced colitis in mice. As shown in
Figure 2, preliminary experiments showed that both
S. mansoni soluble worm proteins and A. caninum
ES products attenuated TNBS-induced inflammation of the murine colon
[77]. These results indicate that the beneficial effect
of treatment with living worms on experimental colitis may be reproduced with soluble
extracts of helminths. Yang et al. showed that Schistosoma japonicum
egg antigens inhibited the development of asthma in a murine model
[57]. S. mansoni antigens are also able to modulate
innate immune responses and prevent onset of type 1 diabetes [78]. These studies indicate
that treatment with helminth extracts may be as effective as treatment with living
helminths and that the achieved  protection is not specific for just one
helminth species. Isolated helminth proteins may provide a more readily
acceptable form of therapy for patients than living worms.Figure 2
Effect of Schistosoma mansoni soluble worm
proteins (SmSWPs) and Ancylostoma caninum excretory/secretory
products (AcESPs) on myeloperoxidase (MPO) activity. MPO activity was measured to
monitor the degree of myeloid cell infiltration in the colon. Data are presented as
units MPO per gram of colon tissue and 1 unit equals the amount of MPO necessary
to degrade 1 μ mol of H2O2 to H2O per minute at 25°C. TNBS-induced colitis caused a significant increase in MPO activity compared to control mice treated with phosphate-buffered saline (PBS). Intraperitoneal injection of
helminth-derived products significantly ameliorated inflammation as shown by the significant decrease in MPO activity. Treatment of control mice with SmSWP or AcESP had no effect (data not shown). *p≤.05, significantly different from contol PBS; #p≤.05,
significantly different from TNBS-PBS; two way ANOVA, n=7−10 [77].


## 4. Concluding Remarks

The hygiene hypothesis suggests an inverse relationship between
parasitic infections and the incidence of IBD. Epidemiological, experimental,
and clinical data sustain the idea that helminths could provide protection
against IBD. The importance of regulatory pathways such as regulatory T-cells,
by which helminths induce such protection have been described. However, the complex
pathways helminths activate to regulate the host’s immune system need further
investigation. Helminths influence innate as well as adaptive immune responses
and this knowledge can contribute to new therapeutic approaches of helminth-induced
protection. Therapy with living helminths appears to be effective in several
immunological diseases. A logical  next
step, to avoid the possible disadvantages of a treatment with living parasites,
is the identification and characterization of helminth-derived immunosuppressive
molecules that contribute to the protective effect.



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*Source: 567314-2008-05-21.xml*