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A Novel surface molecule expressed by long‐term cultured T and natural killer cells is involved in cell activation

Two monoclonal antibodies (mAb), termed ED6 and LD6, were obtained by immunizing mice with cytotoxic T cell lines expressing the T cell receptor (TcR)γ/δ. These mAb were selected according to their ability to trigger the cytolytic program of the immunizing cell lines in a redirected killing assay. Both mAb recognized molecule(s) expressed on the surface of most long‐term cultured TcR γ/δ+, TcR α/β+ and CD3−CD16+ lymphocytes, while it was absent on resting peripheral blood lymphocytes. In addition both mAb reacted with neoplastic B cell lines, Epstein‐Barr virus‐transformed B cell lines, small cell lung cancer and glioma cell lines, while no surface reactivity was detected on ovarian, breast, colon and non‐small cell lung cancer lines. The functional activity of these mAb was studied by two cytolytic assays. Both mAb were able to trigger the cytolytic program of CD3+TcR γ/δ+ polyclonal cell lines and of a CD3−CD16+ NK cell clone against the murine mastocytoma target cell line P815 (Fc receptor+) in a 4‐h 51Cr‐release assay. In addition, ED6 and LD6 hybridomas were lysed by TcRγ/δ+ effector cells while other hybridomas (obtained from the same fusion) were not lysed. ED6 and LD6 mAb (in the presence of submitogenic doses of the phorbol 12‐myristate 13‐acetate) also induced the secretion of interleukin 2 by ED6/LD6+ T cell clones expressing TcR γ/δ or α/β. mAb‐induced surface antigen modulation experiments showed that the antigenic determinant recognized by ED6 and LD6 co‐modulated, thus indicating that the two mAb probably recognize the same or closely associated molecules. The molecular characteristics of the antigen recognized by the mAb were investigated by Western blot analysis. The LD6 mAb recognized a major band of approximately 65 kDa, both under nonreducing and reducing conditions. These data indicate that ED6 and LD6 mAb recognize a novel non‐lineage‐specific activation antigen which is involved in the induction of the functional program of long‐term cultured T or natural killer cells.