A human monoclonal, mu, kappa, cold agglutinin antibody of the rare specificity Pr h (serum and proper eluates) was used in immunofluorescence and immunoperoxidase techniques and in immunoelectron microscopy on rabbit lip specimens. Pr h antibody strongly reacted with scattered cells in epidermis, which were demonstrated to be Merkel cells by electron microscopy; no nerve fibers were stained. In immunoelectron microscopy (IEM), a strong reaction was seen within the cytoplasm and around the

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... les. This is the first IEM staining of Merkel cells (MC) so far reported; it demonstrates the expression of a carbohydrate differentiation antigen in MC. The availability of a potent monoclonal antibody reacting with MC but not with neighboring epidermal cells in rabbit lip offers a new tool for the study of several aspects of MC biology, including antigenic properties and kinetics. We have previously described the localization of Pr antigens (Pr ag) within the epidermis; Pr ag are glycoconjugates of the red blood cell (RBC) membranes-which are traced by human monoclonal cold agglutinin antibodies [1]. One of these, Pr h, was found to be expressed in the basal cells of the human epidermis [2]. Pr h ag was also detected in animal epidermis, with distinct distributions according to both species and type of differentiation of the epithelium (2] . The expression of the Pr h ag within rabbit lip epidermis (RLE) specimens was striking! On the mucous side, nearly all basal cells were found to express the antigen; on the parakeratotic (PK) and on t he orthokeratotic (OK) sides, however, the Pr h ag was not uniformly distributed within the basal cells. Only a few scattered cells were strongly positive on the OK side and clusters of strongly positive cells were observed on the epidermal crests of PK sides [2] . As the nature of these isolated cells had not been established and since we were working with a monoclonal antibody which had clearly been demonstrated to react with keratinocytes, one possibility was t hat these isolated cells could be keratinocytes antigenically distinct from t he surrounding ones; this would be in agreement with the concept of a func-