Abnormal sequence of expression of differentiation markers in psoriatic epidermis: inversion of two steps in the differentiation program?

This immunohistologic study was undertaken to compare epidermal differentiation in normal and psoriatic skin. Although basal cells retain a normal phenotype in this disease, suprabasal layers exhibit abnormal sets of differentiation markers. The 67-kD keratin and Bd5 antigen, which are found in normal epidermis immediately above the basal layer, appear several layers higher in involved psoriatic epidermis. On the contrary, KF2 antigen, which is found in the upper spinous layers of normal epidermis, appears more precociously in psoriatic epidermis. Paradoxically, in this disease characterized by the absence of a granular layer, some markers specific for this layer in normal skin, such as involucrin and transglutaminase, appear in lower skin cell layers, while other granular markers, such as filaggrin, are either absent or found in the parakeratotic scales. These results point out the existence in psoriasis of a suprabasal cell population characterized by a set of markers that are never coexpressed in normal epidermis. The existence of this abnormal population of cells can be explained as the result of the inversion of two steps in the differentiation program. Thus, instead of an inability to express a given differentiation marker, psoriasis seems to be characterized by an abnormal sequence of expression of these markers.