Monoclonal antibodies against SSEA-1 antigen: binding properties and inhibition of human natural killer cell activity against target cells bearing SSEA-1 antigen.

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Abstract
We describe the properties of three monoclonal antibodies (Mab) against stage-specific embryonic antigen-1 (SSEA-1) in terms of their binding activity to HL60, K562, OTF9, and SOTF9 tumor target cells and their functional activity in modulating human natural killer (NK) cytotoxicity assays in vitro against these target cells. Indirect binding, competition, and Western blot analyses indicate that the Mab AEC3A1-9 (3A1), ASSEA-1, and AECAB1-32 (AB1) recognize cell-defined SSEA-1 antigen with activity characteristic of the cell source (HL60 greater than OTF9 greater than K562 much greater than SOTF9). The addition of anti-SSEA-1 Mab to the NK cytotoxicity assay resulted in an inhibition of LU per 1 X 10(6) PBL that correlated closely with the expression of SSEA-1 antigen on the target cell. No significant inhibition was seen for seven other Mab. Inhibition of NK activity (greater than 30%) was observed in the presence of anti-SSEA-1 Mab for 18 of 21 and 6 of 7 human donors examined for HL60 and OTF9 target cells, respectively. The pretreatment of fixed competing cells with anti-SSEA-1 Mab reduced the efficacy of those cells to act as cold competitors in a standard NK cytotoxic assay. Taken together these data suggest that SSEA-1 determinants are important at some stage in the cytolysis produced by NK cells.