<正> It's well known that interleukin-2 (IL-2) plays animportant role in eliciting antitumor immunityparticularly mediated by T cells. In addition, theexpression of MHC can be enhanced by IL-2 genetransfection in tumor cells. Recent studies have indicatedthat at least two signals are required for the activation ofnaive T cells by antigen-bearing target cells: an antigen-sopific signal and a crystimulatory signal. B7-l is an