Central Role of Tumor-Associated CD8+ T Effector/Memory Cells in Restoring Systemic Antitumor Immunity

Sustained delivery of IL-12 and GM-CSF to tumors induces the activation of tumor-resident CD8(+) T effector/memory cells (Tem) followed by cytotoxic CD8(+) T effector cell expansion. To determine whether the secondary effectors expanded from tumor-associated Tem or were primed de novo, activation kinetics of tumor-draining lymph node (TDLN) CD8(+) T cells were analyzed. Treatment promoted a 4-fold increase in the numbers of TDLN CD8(+) T cells displaying a CD69(+)CCR5(+)CD62L(-) periphery-homing effector phenotype by day 4 posttherapy. Pulse labeling of tumor and TDLN T cells with BrdU confirmed that proliferation occurred exclusively within the draining lymph nodes between days 1 and 4 with subsequent migration of primed CD8(+) T effectors to tumors on day 7. Day 4 CD8(+) T effector cells preferentially homed to and lysed experimental, but not control, tumors, establishing tumor specificity. To determine whether the secondary CD8(+) T effector cell response was dependent on activation of tumor-resident CD8(+) Tem, mice that were selectively depleted of tumor-infiltrating CD8(+) T cells were treated and monitored for T effector priming. In the absence of tumor-resident CD8(+) Tem, T effector cell expansion was completely abrogated in the TDLN, revealing that restoration of CD8(+) Tem function was critical to the induction of secondary T effectors. T cell priming failed to occur in IFN-gamma or perforin knockout mice, demonstrating that the requirement for Tem activation was associated with induction of Tem cytotoxicity. These data confirm that intratumoral IL-12 plus GM-CSF induces de novo priming of tumor-specific CD8(+) T effector cells in the TDLN and establish the critical role of preexisting intratumoral CD8(+) Tem in driving this process.