In order to analyse the humoral immune response to the commensal yeast Pityrosporum ovale, we developed a western immunoblot technique with a salt soluble extract of P. ovale cytoplasm. In the present study, we tested sera from patients with psoriasis (n= 15), seborrhoeic dermatitis (n= 10), pityriasis versicolor (n= 8), and normal controls (n = 10). Seventy-three per cent (11/15) of the patients with psoriasis showed specific reactivity with a protein derived from P. ovale of estimated molecular mass 120 kDa, and 46% (7/15) of the cases recognized a 100-kDa protein. Sera from pityriasis versicolor and normal donors showed nonspecific reactivity with several bands of lower molecular weight.

To characterize the location of the 100 and 120-kDa proteins, we performed a lyticase digestion of the cell wall, and analysed the soluble digested products by western blotting. The sera from psoriasis patients detected several bands in the range 100–120 kDa. The finding of the immunoreactive 120-kDa protein in this fraction suggests its location at the space between cell wail and membrane (periplasmic space).

As a control, we performed an extraction of the cytoplasmic proteins of the dimorphic yeast Candida albicans. C. albicans showed a different pattern of banding in SDS–PAGE. Immunoblots with C. albicans did not allow the detection of any related band. A smear was observed in the high molecular weight range consistent with the presence of lipopolysaccharides.

The role of the immune response in infection by P. ovale has not yet been fully explored. The function of the antibodies recognizing 100-120-kDa bands in the majority of the patients with psoriasis seems to represent a specific immune response to the yeast phase of P. ovale.