Tumors expressing lower levels of gp33 antigen escape rejection as ALVs. 2C mice were injected with 10⁶ gp33-Lo cells on the left flank and with 10⁶ gp33-Hi cells on the right flank. Fourteen days after tumor challenge, 2C mice (mice 1, 2, and 3) received approximately 10⁷ naive P14 T cells. Mouse 4 did not receive T cells. (A) gp33-Hi tumors were rejected, but gp33-Lo tumors escaped. The left and center pictures are of the same mouse, taken 2 wk apart. (B) P14 T cells delayed the growth of gp33-Lo tumors but rejected similarly sized gp33-Hi tumors. (C) gp33-Lo tumors escaped a CTL response despite a substantial expansion of P14 T cells (V-alpha2+ CD8+ cells) in the PBLs. (D) ALVs escaped a CTL response when the parental cancer cells expressed low levels of antigen.