FUNCTIONAL-ANALYSIS OF RAN TC4 AS A PROTEIN REGULATING T-CELL COSTIMULATION/

Antigen (Ag)-triggered activation of T cells requires engagement of bo th the T-cell Ag receptor and a costimulatory receptor, for which CD28 can function as a prototypical example. CD80 and CD86 represent ligan ds for this receptor, and although they are present on professional Ag -presenting cells, these molecules are absent from most tumors. Yet so me tumors are still able to costimulate a T-cell response, while other s cannot. Therefore, a key question concerns the molecular basis for t he costimulation of T cells by those tumor cells not expressing the CD 28 ligands CD80 and CD86. Upon screening a cDNA library of such a tumo r cell line in a transient COS cell transfection assay for costimulato ry activity, we identified Ran/TC4 as a protein whose overexpression r esults in costimulatory activity. Ran/TC4 is a ubiquitously expressed member of the Ras gene superfamily of small guanosine triphosphate-bin ding proteins and is involved in nuclear transport; Ran/TC4 cDNA-trans fected COS cells specifically costimulate CD8 T cells and not CD4 T ce lls. Transfection of Ran/TC4 into the costimulation-deficient murine R MA lymphoma cell line introduced costimulatory capacity for CD8 T cell s and resulted in markedly elevated levels of nuclear Ran/TC4 protein expression. in addition, in vivo priming of mice with Ran/TC4-transfec ted RMA cells induced protection against wild-type (wt) RMA tumor cell s. Ran/TC4-transfected RMA cells and wt RMA tumor cells exhibit compar able in viva growth rates in mice lacking T and B cells, and Ran/TC4-m ediated tumor rejection thus involves B and/or T cells. This possibili ty is substantiated by the observation that T cells from normal mice c hallenged with Ran/TC4-transfected RMA cells can mount a cytotoxic T-c ell response not only against the Ran/TC4-transfected tumor cells but also against wt RMA tumor cells. Based an these results, we conclude t hat gene transfer-mediated elevations in Ran/TC4 can confer costimulat ory function for CD8 T cells to tumor cells. This finding suggests a n ovel application of Ran/TC4 as a protein capable of regulating costimu lation in tumor cells.