Fehlendes granulozytares Infiltrat bei der Pityriasis versicolor - ein Hinweis fur eine spezifische antiinflammatorische Aktivitat des Erregers? Missing granulocytic infiltrate in pityriasis versicolor - indication of specific anti-inflammatory activity of the pathogen?

Skin diseases associated with Malassezia yeasts: facts and controversies

The implication of the yeast genus Malassezia in skin diseases has been characterized by controversy, since the first description of the fungal nature of pityriasis versicolor in 1846 by Eichstedt. This is underscored by the existence of Malassezia yeasts as commensal but also by their implication in diseases with distinct absence of inflammation despite the heavy fungal load (pityriasis versicolor) or with characteristic inflammation (eg, seborrheic dermatitis, atopic dermatitis, folliculitis, or psoriasis). The description of 14 Malassezia species and subsequent worldwide epidemiologic studies did not reveal pathogenic species but rather disease-associated subtypes within species. Emerging evidence demonstrates that the interaction of Malassezia yeasts with the skin is multifaceted and entails constituents of the fungal wall (melanin, lipid cover), enzymes (lipases, phospholipases), and metabolic products (indoles), as well as the cellular components of the epidermis (keratinocytes, dendritic cells, and melanocytes). Understanding the complexity of their interactions will highlight the controversies on the clinical presentation of Malassezia-associated diseases and unravel the complexity of skin homeostatic mechanisms.

[Pityriasis versicolor : new aspects of an old disease]

Pityriasis versicolor (PV) is one of the most common infectious skin diseases, as well as the most common dermatosis associated with pigmentation alterations of the skin. PV is prevalent in 1% of the population living in temperate climate zones and more common during the summer. In tropical areas, PV is found in up to 50% of all patients consulting a dermatologist. Of the known Malassezia species, M. globosa is currently felt to play a key role in the pathogenesis of PV, as it is most commonly found in PV lesions. In addition, its round-shaped cells may contribute to the characteristic histology of the disease ("spaghetti and meatballs"). However, the clinical appearance of PV including hyper- and hypopigmentation, fluorescence of the lesions, as well as a lack of inflammation despite high fungal load cannot fully be explained by the presence of M. globosa, which is also found on healthy skin. In M. furfur a tryptophan-dependent metabolic pathway generates a number of indole pigments, which may be associated with the clinical appearance of PV. In the model organism Ustilago maydis it was shown that the formation of the indole compounds occurs spontaneously after initial conversion of tryptophan into indole pyruvate controlled by the key enzyme aminotransferase Tam 1. We review the present knowledge of PV and highlight the potential role of Tam1 in explaining the poorly understood aspects of the disease. Promising therapeutic results using the application of Tam1 inhibitors to treat PV support the enzyme's important role in the disease pathogenesis.

Malassezia virulence determinants

PURPOSE OF REVIEW

Malassezia yeasts are associated with a number of dermatologic and systemic diseases in humans and animals. Pityriasis versicolor is amongst these diseases and represents one of the most common human skin diseases. Beyond that, the role of Malassezia yeasts in the pathogenesis of other skin diseases such as psoriasis, seborrheic dermatitis and confluent and reticulate papillomatosis is discussed but remains less clear. Clear pathogenetic mechanisms of the above-mentioned diseases are not known so far. The review presents new findings on virulence factors of Malassezia yeasts, shedding light on the pathogenesis of Malassezia-associated diseases.

RECENT FINDINGS

Several virulence factors in Malassezia yeasts are known, based on their enzymatic lipolytic activity resulting in the production of distinct metabolites and special cell wall features. Recently, a secondary metabolic pathway possibly implicated in the pathogenesis of pityriasis versicolor was described.

SUMMARY

The article presents virulence factors of Malassezia yeasts ranging from irritant metabolic byproducts to highly bioactive indole derivatives and attempts to clarify their pathogenic implications in the different diseases. Special emphasis is given to the pathogenesis of pityriasis versicolor, as it represents the disease wherein the causative relationship with Malassezia yeasts appears the most obvious.

Pathogenesis of dermatophytosis and tinea versicolor

Dermatophytoses are infections caused by keratinophilic fungi known as dermatophytes. Several steps are required for infection to take place: contact, adherence, and invasion of keratin layers. The severity of the infection depends on the type of agent, environmental factors, and the host immunologic status. Tinea versicolor is caused by the Malassezia spp yeasts, which are microorganisms that belong to normal biota in seborrheic areas, but some contributing factors, such as the application of oily preparations, creams, an increase in ambient humidity, corticosteroid abuse, or genetic predisposition can induce its overgrowth in both filamentous and yeast structures. Exposure to sunlight stimulates the production of azelaic acid, which causes the appearance of hypopigmented spots. Currently, there is no scientific explanation for hyperpigmented lesions.

[Generation of reactive oxygen species in vitro by Malassezia yeasts]

BACKGROUND AND METHODS

Free oxygen species are able to destroy cells due to their cytotoxic effect, which is based on lipid peroxidation, enzyme oxidation and protein oxidation. We analyzed the ability of six different Malassezia (M.) species to generate reactive oxygen species (ROS), using the lucigenin-amplified chemiluminescence. Further investigations showed the effect of different concentrations of terbinafine on ROS generation by M. furfur and M. pachydermatis after 10, 30 and 60 minutes.

RESULTS

All of the investigated Malassezia species (M. furfur, M. globosa, M. sympodialis, M. slooffiae, M. obtusa, M. pachydermatis) was able to generate reactive oxygen species. Terbinafine inhibited free radical production by M. furfur and M. pachydermatis.

CONCLUSION

Dependent on their cell concentrations, M. species have the ability to generate ROS. The ability of the different Malassezia species to produce ROS should be considered as one further virulence factor of this yeast. The antifungal agent terbinafine (concentrations from 100 microg to 1 microg ml -1) reduced ROS production by M. furfur and M. pachydermatis. Terbinafine acts as oxygen species scavenger. This could be an additional mechanism supporting the classic antifungal effect of this agent, the inhibition of ergosterol synthesis.

Quantification of ultraviolet protective effects of pityriacitrin in humans

Pityriacitrin (PIT), produced by Malassezia yeasts, is an UV absorbing substance that might cause hypopigmentation in pityriasis versicolor alba. We aimed to investigate the UV protective effect of PIT in humans using in vitro and in vivo test methods. Spectrophotometry of PIT cream and the vehicle was performed in the wavelength range from 290 to 400 nm. UV transmission and the sun protection factor (SPF) were assessed for different cream formulations. Using colorimetry we evaluated erythema and pigmentation following irradiation of cream-protected and non-protected skin of healthy subjects. UVB as well as UVA transmission decreased with increasing PIT concentrations. An increase of PIT concentration of 1.25, 2.5, and 5% was associated with slightly increasing SPFs of 1.4, 1.5, and 1.7, respectively. Our in vivo tests confirmed the validity of the SPF of PIT 5% cream determined in vitro. In conclusion, the UV protective effect of PIT is all in all very weak suggesting that PIT is likely only an inferior cofactor in the development of hypopigmentation in pityriasis versicolor alba lesions following sun exposure.

Recent developments in the immunology and biology ofMalasseziaspecies

Malassezia spp. are members of the normal cutaneous flora, but are also associated with several cutaneous diseases. Recent studies of the interaction of Malassezia spp. with melanocytes, fibroblasts, keratinocytes and dendritic cells have highlighted their potential to modulate the immune response directed against them. In normal skin they may downregulate the inflammatory response, allowing them to live as commensals. In contrast, in atopic/eczema dermatitis syndrome and psoriasis, they may elicit an inflammatory response that contributes to the maintenance of lesions. Future research may define ways to influence this inflammatory cycle and hence to control or prevent exacerbations of these diseases.