B7-1 EXPRESSION BY A NONANTIGEN PRESENTING CELL-DERIVED TUMOR

The existence of a naturally occurring immunosurveillance against neop lastic cells is controversial, A difficulty with this concept is that tumor-specific antigen-reactive T cells would not be expected to becom e activated after encountering tumor cells, since T cells that bind to antigen in the absence of the costimulation provided by antigen-prese nting cells may be inactivated, We studied a transgenic model of tumor igenesis where T cells reactive to a particular tumor-specific antigen are lost prior to the development of non-antigen-presenting cell-deri ved tumors; therefore, the tumors that develop are not subjected to im munosurveillance. We found that a tumor cell line derived from one suc h tumor expresses the T-cell costimulatory molecule B7-1, the expressi on of which is normally restricted to antigen-presenting cells, In add ition, we found that several immortalized cell lines, which are nontum origenic and thus have suffered only early genetic events in the tumor igenesis process, express B7. This suggests that a host cell can be in duced to express surface B7-1 molecules after suffering an oncogenic i nsult, which might possibly be a primary mechanism of immunosurveillan ce against tumors.