**5. Influence of helminth infections on human atopic disorders**

Compared to studies on human helminth infections and autoimmunity, reports on the interrelationships between helminth infections and atopic disorders are much more common. However, as there is considerable variation in results among the reports, systematic reviews and meta-analyses are important. According to reviews of crosssectional studies on the relationships between current parasitic infections and athma and atopy (Leonardi-Bee et al., 2006; Flohr et al., 2009; Feary et al., 2011), both intestinal helminths (Hookworms, *Ascaris*, *Trichuris*, *Strongyloides*, *Enterobius*) and schistosomes significantly lowered reactivity in the skin prick test. In contrast, only hookworm infections lowered the risk of asthma significantly; odds ratio (OR) = 0.50 (Leonardi-Bee et al., 2006). It is worth noting that *Ascaris* infections heightened the risk of asthma (OR = 1.34). Other geohelminths had no significant effects on the risk of asthma.

The evidence obtained in cross-sectional studies is indirect. Direct evidence of ameliorating effects of helminths can be obtained by intervention studies. According to the review literature above (Flohr et al., 2009), in some intervention studies, allergic skin sensitization increased after de-worming treatment. This finding was reproducibly observed in independent studies in Venezuela (Lynch et al., 1993), Gabon (van den Biggelaar et al., 2004) and Vietnam (Flohr et al., 2010). However, in a study in Ecuador (Cooper et al., 2006), there was no increase in the prevalence of atopy or clinical allergies after de-worming treatment. Furthermore, in the study in Vietnam (Flohr et al., 2010), the clinical symptoms of allergy did not worsen in the treated group despite the increased sensitization. Remarkably, a study in Venezuela (Lynch et al., 1997) showed a clinical *improvement* in asthma after regular antihelminthic treatment. Taken together with cross-sectional studies of allergies in helminthinfected individuals, the overall results could be summarized as follows: 1) In general, parasitic helminths suppress skin sensitization. 2) However, parasitic helminths do not always suppress clinical allergies and can sometimes worsen allergic symptoms. Regarding the timing of helminth infections, a study in Brazil is especially noteworthy (Rodrigues et al., 2008). In the study, heavy infections with *T. trichiura* in early childhood were shown to reduce reactivity to allergens in later childhood. Even if the child was not infected at the time of the skin test, this protective effect was observed. Cooper et al. (2009) summarized studies on helminth infections and clinical allergies in a review, in which he stated that different helminths have different effects on allergies depending on the timing of exposure. According to the review, *Trichuris* and hookworms are protective, whereas *Ascaris* and *Toxocara* may be risk factors in certain situations. Further studies, especially de-worming intervention studies or therapeutic clinical trials using helminths, may be necessary to establish a general view of the anti-allergic effects of helminths.

#### **6. T cell subsets and autoimmunity**

Until several years ago, major autoimmune diseases such as MS, T1D, RA and CD and animal models thereof had been classified as Th1-dependent diseases. Recently, however,

Parasitic Helminths as Potential Therapeutic Agents Against Autoimmune Disorders 597

protective effect. In general, a down-regulation of both Th17 and Th1 cytokine expression has been demonstrated (Walsh et al., 2009; Wu et al., 2010; Reyes et al., 2011) except in papers published before the emergence of the Th17 concept (Sewell et al., 2003; La Flamme et al., 2003). Regarding cellular involvement in the suppression, B cells highly expressing CD23 were shown to be responsible for EAE suppression (Wilson et al., 2010) in adoptive transfer experiments. In that study, B cells from IL-10-deficient mice as well as from wildtype mice conferred protection against EAE. The involvement of AAMΦ is also plausible, because AAMΦ markers are increased in the brain in *T. crassiceps*-infected EAE mice (Reyes et al., 2011). In addition, abrogation of schistosome-induced anti- encephalitogenic effects in

STAT6-deficient mice (Sewell et al., 2003) might support the importance of AAM Φ.

*Trichinella pseudospiralis* Infection IL-17↓, IL-6↓, IL-1↓, IFN-↓, TNF-↓ Wu et al., 2010 *Schistosoma japonicum* Egg Ag i.p. IFN-↓, IL-4↑ Zheng et al., 2008

*Trichinella spiralis* Infection Gruden-Movsesijan et al., 2008

Dependent on STAT6

Independent of IL-10

Non-obese diabetic (NOD) mice have been used widely as an animal model of T1D. Spontaneous destruction of pancreatic β-cells and subsequent hyperglycemia are observed in NOD mice. The pathogenesis of T1D in this mouse has been studied extensively, however, there is still considerable controversy over the relative contribution of Th1 (or IFNγ) and Th17 (or IL-17). Anti-IFN-γ treatment rendered NOD mice resistant to cyclophosphamide (CY)-accelerated diabetes (Debray-Sachs et al., 1991). In contrast, in IFNγ deficient NOD mice, neither insulitis nor diabetes was prevented although the onset was delayed (Hultgren et al., 1996; Gysemans et al., 2008). These findings suggest that IFN-γ is involved in, but not essential to, the pathogenesis. IFN-γ is not itself detrimental to β-cells, but induces apoptosis when acting with IL-1β or TNF-α (Gysemans et al., 2008). Regarding this pro-apoptotic effect, dual roles of IFN-γ in NOD mice have been indicated; i.e. IFN-γ induces β-cell destruction via STAT-1 but protects β-cells via IRF-1 (Gysemans et al., 2008). On the other hand, pathological roles of IL-17 have been also suggested in mice (Miljkovic et al., 2005: Emamaullee et al., 2009) and humans (Honkanen et al., 2010). Anti-IL-17 antibody prevented diabetes in NOD mice when administered around the time of onset (Emamaullee et al., 2009). At present, it is reasonable to conclude that both Th1 and Th17 play some role

*Taenia crassiceps* Infection IL-17↓, TNF-↓, IL-4↑, IL-10↑ Reyes et al., 2011 AAM markers↑

Infection IL-12/23 p40↓, IFN-↓, TNF-↓, IL-4↑ La Flamme et al., 2003

B cells, Independent of IL-10 Wilson et al., 2010

Sewell et al., 2003

Walsh et al., 2009

Helminth Treatment Proposed mechanism Refs

infected mouse cells

*Schistosoma mansoni* Egg i.p. IFN-↓, IL-4↑, TGF-↑, IL-10↑,

*Fasciola hepatica* Infection IFN-↓, IL-17↓, Dependent on TGF-,

Table 1. Suppressive effect of parasitic helminths on EAE.

*Heligmosomoides polygyrus* Adoptive transfer of

↓:down-regulation, ↑:up-regulation

in diabetogenesis in NOD mice.

**7.2 Experimental T1D** 

the finding of a critical role for IL-23 in the pathogenesis of some experimental forms of autoimmune diseases (Cua et al., 2003; Murphy et al., 2003) and subsequent discovery of a pathogenic T cell subset producing IL-17 (Th17) (Langrish et al., 2005; Park et al., 2005) has led us to revisit the "Th1/Th2 paradigm". The simplified "Th1/Th2 paradigm" still explains many immunological phenomena, but there is accumulating evidence of the importance of Th17 in the pathogenesis of autoimmunity. By using IL-17-deficient mice, essential roles of IL-17 in the pathogenesis of autoimmune disease have been demonstrated directly (Nakae et al., 2003a, 2003b; Komiyama et al., 2003). Development of the Th17 lineage is antagonized by both Th1-related cytokines (IL-12 and IFN-γ) and a Th2 cytokine (IL-4) (Park et al., 2005; Nakae et al., 2007). Conversely, IL-23 and IL-17 negatively regulate Th1 differentiation (Nakae et al., 2007). Therefore, the balance of these Th subsets is much more complex than previously believed. Complicating the situation further is the recent finding that Th17 is not a stable subset and can be changed to the Th1 phenotype; i.e. plasticity of Th17 (Kurschus et al., 2010; Dong, 2011). The fate of Th17 cells depends on their surrounding environment (Dong, 2011; Lee et al., 2009) and Th17's pathogenic nature depends on the conversion to Th1 cells, in the case of experimental T1D (Martin-Orozco et al., 2009; Bending et al., 2009). The relative importance of each Th subset to the pathogenesis may differ with the disease model, however in most cases, the pathogenicity of Th1 and Th17 is still under debate. There is a report that a transcription factor, T-bet, is essential for the encephalitogenicity of T cells rather than cytokine production (Yang et al., 2009). In contrast, T-bet seems to be a negative regulator in experimental autoimmune myocariditis (Rangachari et al., 2006). Given the unstable nature of the Th17 subset and disease heterogeneity of individual patients, antagonism of both the Th1 and Th17 subsets would be a better choice for the successful suppression of autoimmune diseases. From this viewpoint, parasitic helminths may have ideal immunomodulatory activities for treatment of autoimmunity.
