The role of Malassezia in the pathogenesis of atopic eczema/dermatitis syndrome

Sammanfattning: Atopic eczema/dermatitis syndrome (AEDS) is a chronically relapsing skin disease, which is often associated with elevated levels of serum lgE. The immunopathology of AEDS is at present unclear, but it is known that lymphocytes infiltrating the AEDS lesions are predominantly of the CD4+ T helper phenotype. Individuals with AEDS often have lgE antibodies to the yeast Malassezia, earlier denoted Pityrosporum, a member of the normal cutaneous flora. The aim of this study was to investigate the role of Malassezia in the pathogenesis of AEDS, particularly with a focus on T-cell stimulating activity. Peripheral blood mononuclear cells (PBMC) from AEDS patients with Malassezia specific serum lgE showed a dose dependent, proliferative response against Malassezia, which was significantly higher than the response in PBMC from healthy individuals. Malassezia reactive T-cell clones were established from the skin and blood of two AEDS patients with high serum IgE-levels against Malassezia. The majority of the clones were CD4+, and the skin-derived T-cell clones produced more Th2 cytokines than the clones derived from blood. Furthermore, Malassezia reactive T-cell lines could be established both from AEDS patients with Malassezia specific serum lgE and non-atopic healthy individuals. The T-cell lines derived from AEDS patients showed a significantly higher production of M cytokines than those derived from healthy individuals. To investigate if the T-cell stimulating activity of Malassezia could be due to components of the yeast having superantigen activity the usage of T-cell receptor P-chain V-gene segments (TCRBV) was compared in freshly prepared PBMC and Malassezia reactive T-cell lines from AEDS patients and healthy controls. It could not be excluded that parts of the T-cell response against Malassezia are driven by superantigens, but with the methods available, 1 did not find any evidence for superantigen activity in extracts of Malassezia. Instead these data support the importance of classical MHC restricted allergens. To study the occurrence of reactivity to Malassezia extract and three recombinant Malassezia allergens (rMal s 1, rMal s 5 and rMal s 6), measured as specific serum IgE, positive skin prick test (SPT) and atopy patch test (APT) reactions, 132 adult AEDS patients, selected without knowledge of their serum lgE levels, were investigated at three Swedish University Hospitals. Sixty-seven percent of the AEDS patients, but none of 33 healthy controls, reacted with specific serum lgE, positive SPT andlor positive APT reaction to any of the tested Malassezia allergens. Addition of APT to the test battery revealed a previously overlooked impact of Malassezia-hypersensitivity also in subgroups of AEDS patients without head and neck dermatitis or without high total serum lgE. Forty AEDS patients and 16 healthy controls were additionally investigated for their in vitro PBMC response to Malassezia. The AEDS patients were subdivided according to their in vivo reaction to Malassezia extract into three groups: SPT+/APT+ (n=12), SPT+/APT- (n=12), and SPT- /APT(n=16). The SPT+/APT+ and the SPT+/APT-, but not the SPT-/APT- patients, had a significantly higher PBMC-proliferation to Malassezia than the healthy controls. Interestingly, the PBMCproliferation to Malassezia in the SPT+/APT+ group were significantly higher than in the SPT+/APT- group. Furthermore, in response to in vitro Malassezia-stimulation both PBMC proliferation and M cytokine-production correlated with the APT reactions to Malassezia, strongly suggesting a relationship between circulating T-cells, with a Th2-like cytokine profile, and positive APT reactions. In conclusion, AEDS patients with Malassezia-specific serum IgE, unlike healthy individuals, have acquired a Th2-like T-cell response to classical antigens in Malassezia, that may be of importance for maintaining their skin inflammation. This hypothesis gets further support in the correlation between in vivo APT reaction and in vitro PBMC reactivity to Malassezia. The addition of APT and rMaI allergens to the test battery will improve the possibility to identify AEDS patients with Malasseziahypersensitivity.