**4. Mechanisms by which fungi act as an exacerbating factor for atopic dermatitis**

#### **4.1 Antigen-specific inflammation caused via activation of antigen-specific T cells**

Allergy to fungi such as *Candida* spp. and *Malassezia* spp. has been implicated as an exacerbating or intractable factor in the symptoms of AD (Savolainen et al., 1993; Tanaka et al., 1994; Kitamura et al., 1997; Morita et al., 1999; Linder et al., 2000; Faergemann 2002; Kanda et al., 2002; Svejgaard et al., 2004). *Candida* spp. are indigenous fungi inhabiting the oral cavity, digestive tract and vagina. Healthy people are thought to acquire the Th1 type immunity against *Candida* spp. (Tanaka et al., 1994; Romani et al., 1995). For instance, the activation of Th1-type CD4+ cell induces phagocyte-dependent immunity, which apparently represents an important mechanism of anti-*Candida* resistance, and it was demonstrated that healthy subjects with a normal immune response show high peripheral blood lymphocyte proliferative responses as well as positive scarification patch tests to *C. albicans* antigen, suggesting the dominant presence of Thl type T cells specific to *C. albicans* antigen. It is well known that Thl clones secrete IL-2 and IFN-γ and preferentially induce delayed type hypersensitivity (Stout & Bottomly, 1989), while Th2 clones produce IL-4, IL-5 (Mosmann et al., 1986) and IL-10 (Fiorentino et al., 1989) and help to promote IgE production (Boom et al., 1988; Killar et al., 1987). In AD patients, Th1-type immunity has been shown to shift to Th2 type (Fig. 1) since the patients immediately react to skin testing using *Candida*-antigen (Tanaka et al., 1994; Kitamura et al., 1997), and *Candida*-specific IgE increases with the severity of the symptoms of AD (Tanaka et al., 1994). Specifically, AD patients displayed a significantly lower incidence of positive patch test reactions to *C. albicans* allergen than the healthy control subjects, and the patients with negative *C. albicans* patch tests tended to have

control dogs (without *Malassezia* dermatitis or otitis) did not differ significantly from those in atopic dogs with *Malassezia* dermatitis. No significant correlation was found between the lymphocyte blastogenic response and the type-1 hypersensitivity response to *M. pachydermatis* within any of the groups, suggesting that modification of the dysregulated immune response toward *M. pachydermatis* may assist in the reduction of pathologic changes associated with an AD phenotype in dogs. In another study, Chen et al. (2002) compared IgE responses to separated proteins of *M. pachydermatis* in atopic dogs with *Malassezia* dermatitis and clinically normal dogs. The results of their study showed that the majority of atopic dogs with *Malassezia* dermatitis have a greater IgE response than normal dogs, suggesting an IgE-mediated immune response may be clinically important in the pathogenesis of the disease. In felines, *Malassezia* spp. have been more frequently isolated from healthy ear canals and skin in feline leukaemia (FeLV)- or feline immunodeficiency virus (FIV)-infected cats than in those noninfected (Sierra et al., 2000). In addition, *Malassezia* spp. overgrowth has been described in feline localized benign exfoliative skin diseases, such as chin acne and the idiopathic facial dermatitis of Persian cats (Jazic et al., 2006; Bond et al*.*, 2000). Based on these findings, Ordeix et al. (2007) conducted a multicentre, retrospective and descriptive study to document *Malassezia* spp. overgrowth in allergic cats. Their results suggested that *Malassezia* spp. overgrowth may represent a secondary cutaneous problem in allergic cats particularly in those with greasy adherent brownish scales on their skin. The favorable response to treatment with antifungal agent alone suggests that, as in dogs, *Malassezia* spp. may be partly responsible for both

**4. Mechanisms by which fungi act as an exacerbating factor for atopic** 

**4.1 Antigen-specific inflammation caused via activation of antigen-specific T cells**  Allergy to fungi such as *Candida* spp. and *Malassezia* spp. has been implicated as an exacerbating or intractable factor in the symptoms of AD (Savolainen et al., 1993; Tanaka et al., 1994; Kitamura et al., 1997; Morita et al., 1999; Linder et al., 2000; Faergemann 2002; Kanda et al., 2002; Svejgaard et al., 2004). *Candida* spp. are indigenous fungi inhabiting the oral cavity, digestive tract and vagina. Healthy people are thought to acquire the Th1 type immunity against *Candida* spp. (Tanaka et al., 1994; Romani et al., 1995). For instance, the activation of Th1-type CD4+ cell induces phagocyte-dependent immunity, which apparently represents an important mechanism of anti-*Candida* resistance, and it was demonstrated that healthy subjects with a normal immune response show high peripheral blood lymphocyte proliferative responses as well as positive scarification patch tests to *C. albicans* antigen, suggesting the dominant presence of Thl type T cells specific to *C. albicans* antigen. It is well known that Thl clones secrete IL-2 and IFN-γ and preferentially induce delayed type hypersensitivity (Stout & Bottomly, 1989), while Th2 clones produce IL-4, IL-5 (Mosmann et al., 1986) and IL-10 (Fiorentino et al., 1989) and help to promote IgE production (Boom et al., 1988; Killar et al., 1987). In AD patients, Th1-type immunity has been shown to shift to Th2 type (Fig. 1) since the patients immediately react to skin testing using *Candida*-antigen (Tanaka et al., 1994; Kitamura et al., 1997), and *Candida*-specific IgE increases with the severity of the symptoms of AD (Tanaka et al., 1994). Specifically, AD patients displayed a significantly lower incidence of positive patch test reactions to *C. albicans* allergen than the healthy control subjects, and the patients with negative *C. albicans* patch tests tended to have

pruritus and cutaneous lesions in allergic cats.

**dermatitis** 

higher levels of total serum IgE including anti-*C. albicans* IgE antibody. In other words, the delayed-type hypersensitivity to *C. albicans* antigen, which is highly prevalent in atopics without dermatitis as well as non-atopics, was reduced in most of the AD patients.

Fig. 1. Shift of Th1-type immunity to Th2-type immunity in allergic diseases including atopic dermatitis (AD). In healthy individuals, dendritic cells present fungal antigen to naive T cells which in turn differentiate to Th1 type cells, resulting in the cellular immune response. In AD patients, Th1-type immunity shifts to Th2-type immunity in which Th2 clones produce IL-4, IL-5 and IL-10 and induce IgE production.

The lipophilic fungus *M. furfur* indigenously inhabits the seborrheic region of the body, such as head, neck and upper part of the back. It was also reported that the fungus may be implicated in rosacea-like dermatitis and edematous erythema, which are chromic and intractable symptoms characteristic to the face with adult-type AD (Mukai et al., 1997), and that *Malassezia*-specific IgE level is high in the head and neck of AD patients (Bayrou et al., 2005; Darabi et al., 2009). Regarding the 11 currently recognized *Malassezia* species as an exacerbating factor in AD, *M. globosa* and *M. restricta* are found to frequently colonize the skin of AD patients. For instance, specific IgE antibodies against eight *Malassezia* species (*M. dermatitis*, *M. furfur*, *M. globosa*, *M. obtusa*, *M. pachydermatis*, *M. slooffiae*, *M. sympodialis*, and *M. restricta*) in sera from AD patients were examined using an enzyme-linked immunosorbent assay, and it was found that the specific IgE value against *M. restricta* was greater than those against the other *Malassezia* species (Kato et al., 2006).
