Abstract
Malassezia spp. are believed to be the causative agents of pityriasis versicolor and are strongly implicated in seborrhoeic dermatitis. The yeast also forms part of the normal human cutaneous microflora. We have previously shown that when Malassezia yeast cells are incubated with human peripheral blood mononuclear cells (PBMCs), they are capable of reducing the levels of pro-inflammatory cytokines produced. In order to test the hypothesis that this immunoevasive phenomenon may be related to the unusually high level of lipid in the Malassezia yeast cell wall, we have compared the immunomodulatory capacity of normal and lipid-depleted yeast cells. Stationary phase yeast cells of Malassezia sympodialis, M. globosa and M. restricta were treated with chloroform:methanol to extract the surface lipids. The lipid-depleted and non-depleted yeast cells were then co-cultured with human PBMCs from three different human donors at a ratio of 20 yeasts per leukocyte for 24 h. The levels of interleukin (IL)-1β, IL-6 and tumour necrosis factor (TNF)-α were then determined by enzyme-linked immunosorbent assay (ELISA). The results demonstrated that extraction of lipid reversed the yeast cell capacity to reduce the levels of pro-inflammatory cytokines. The levels of IL-1β, IL-6 and TNF-α produced in response to lipid-extracted Malassezia of all three species were either no different from or significantly greater (P<0·05; ANOVA) than the constitutive control levels. These results suggest that the lipid microfibrillar layer of Malassezia may prevent the yeast cells from inducing inflammation and provide an explanation for the normal commensal status of the organism on human skin. The hypothesis that the lipid layer is absent or altered in seborrhoeic dermatitis may provide an explanation for the inflammatory nature of this dermatosis.
