Rejection of tumors expressing higher levels of the gp33 antigen required TILs targeting the tumor stroma. (A) As assessed by EGFP fluorescence, antigenic cancer cells in gp33-Hi tumors were eliminated between 4 and 6 d after T cell transfer. Shaded histograms are Neo tumor cells; unshaded histograms are gp33-Hi tumor cells. (B) gp33-Hi tumors contained more nonviable cells than gp33-Lo tumors after adoptive transfer of P14 T cells, as analyzed by PI uptake. (C) gp33-Hi tumors in which ALVs were eliminated contained more nonviable stromal cells than gp33-Lo tumors, as determined by CD11b staining. The numbers in the upper-right and lower-right quadrants indicate the percentages of CD11b+ PI+ and CD11b+ PI- cells, respectively. (D) gp33-Hi tumors escaped a CTL response when the stroma did not express the correct MHC-presenting molecule. (E) ALVs escaped a CTL response when the stroma did not express the correct MHC-presenting molecule. For these measurements, tumors from mice in (D) were reisolated and tested for gp33 antigen expression. Unshaded histograms represent the Neo cells. Shaded histograms represent the gp33-Hi cancer cells indicated.