Helper-independent, L-selectin(low) CD8⁺ T cells with broad anti-tumor efficacy are naturally sensitized during tumor progression

We recently reported that the CD4⁺ T cell subset with low L-selectin expression (CD62L(low))in tumor-draining lymph nodes (TDLN) can be culture activated and adoptively transferred to eradicate established pulmonary and intracranial tumors in syngeneic mice, even without coadministration of IL-2. We have extended these studies to characterize the small subset of L-selectin(low) CD8⁺ T cells naturally present in TDLN of mice bearing weakly immunogenic tumors. Isolated L-selectin(low) CD8⁺ T cells displayed the functional phenotype of helper-independent T cells, and when adoptively transferred could consistently eradicate, like L-selectin(low) CD4⁺ T cells, both established pulmonary and intracranial tumors without coadministration of exogenous IL-2. Whereas adoptively transferred L-selectin(low) CD4⁺ T cells were more potent on a cell number basis for eradicating 3-day intracranial and s.c. tumors, L-selectin(low) CD8⁺ T cells were more potent against advanced (10-day) pulmonary metastases. Although the presence of CD4⁺ T cells enhanced generation of L-selectin(low) CD8⁺ effector T cells, the latter could also be obtained from CD4 knockout mice or normal mice in vivo depleted of CD4⁺ T cells before tumor sensitization. Culture-activated L-selectin(low) CD8⁺ T cells did not lyse relevant tumor targets in vitro, but secreted IFN-γ and GM-CSF when specifically stimulated with relevant tumor preparations. These data indicate that even without specific vaccine maneuvers, progressive tumor growth leads to independent sensitization of both CD4⁺ and CD8⁺ anti-tumor T cells in TDLN, phenotypically L-selectin(low) at the time of harvest, each of which requires only culture activation to unmask highly potent stand-alone effector function.