A cell-spreading inhibitor exists in serum and in epidermal basal cells

A unique monoclonal antibody was obtained by immunizing mice with complement-inactivated fetal bovine serum (FBS). This antibody, named SI-1, stained epidermal basal cells of humans, pig, guinea pig, and rat by an indirect immunofluorescence technique after pretreatment of cryostat sections with alkali buffer (pH 9.6). After dissociating pig epidermal cells by trypsin, the SI-1 antibody stained exclusively and strongly one type of uniquely shaped cells. They were small and hanging-bell or columnar in shape with one convoluted side on the base, consisting of less than 2.8% of the dissociated epidermal cell population. The antigen contained in FBS was partially purified by affinity chromatography using the SI-1 antibody. The affinity-purified antigen inhibited the spreading of PAM cells, a spontaneously transformed murine keratinocyte line, in serum-free medium in a dose-dependent manner at concentrations of 10(-5) to 10 ng/ml. The antigen also inhibited the spreading of trypsinized pig epidermal cells in the range of 10(-2) to 10(3) ng/ml in the presence of 0.05% FBS. Although there have been a few reports indicating that serum inhibited both spreading and attachment, a specific factor in serum has not been purified before. This is, to our knowledge, the first presentation of a cell-spreading inhibitor contained in serum.