Douglas C. Palmer

Depletion of immune elements before adoptive cell transfer (ACT) can dramatically improve the antitumor efficacy of transferred CD8+ T cells, but the specific mechanisms that contribute to this enhanced immunity remain poorly defined. Elimination of CD4+CD25+ regulatory T (T reg) cells has been proposed as a key mechanism by which lymphodepletion augments ACT-based immunotherapy. We found that even in the genetic absence of T reg cells, a nonmyeloablative regimen substantially augmented CD8+ T cell reactivity to self-tissue and tumor. Surprisingly, enhanced antitumor efficacy and autoimmunity was caused by increased function rather than increased numbers of tumor-reactive T cells, as would be expected by homeostatic mechanisms. The gammaC cytokines IL-7 and IL-15 were required for augmenting T cell functionality and antitumor activity. Removal of gammaC cytokine-responsive endogenous cells using antibody or genetic means resulted in the enhanced antitumor responses similar to those seen after nonmyeloablative conditioning. These data indicate that lymphodepletion removes endogenous cellular elements that act as sinks for cytokines that are capable of augmenting the activity of self/tumor-reactive CD8+ T cells. Thus, the restricted availability of homeostatic cytokines can be a contributing factor to peripheral tolerance, as well as a limiting resource for the effectiveness of tumor-specific T cells.

Central memory CD8+ T cells (T(CM)) and effector memory CD8+ T cells (T(EM)) are found in humans and mice; however, their relative contributions to host immunity have only recently been examined in vivo. Further, the ability of T(CM) to treat an established tumor or infection has yet to be evaluated. To address the therapeutic potential of different tumor-reactive CD8+ T cell memory subsets, we used an established model for the in vitro generation of T(CM) and T(EM) by using IL-15 and IL-2, respectively. Adoptively transferred T(CM) exhibited a potent in vivo recall response when combined with tumor-antigen vaccination and exogenous IL-2, leading to the eradication of large established tumors. By contrast, T(EM) were far less effective on a per-cell basis. Microarray analysis revealed that the signature of highly in vivo effective antitumor T cells included the overexpression of genes responsible for trafficking to secondary lymphoid tissues. This gene expression profile correctly predicted the in vitro and in vivo lymphoid-homing attributes of tumor-reactive T cells. Furthermore, we found that homing to secondary lymphoid tissue is required for optimal tumor treatment. Our findings indicated that highly in vivo effective antitumor T cells were those that initially targeted secondary lymphoid tissue, rather than tumor sites, as had previously been postulated. Thus, tumor-reactive CD8+ T cell populations with the phenotypic and functional attributes of T(CM) may be superior to T(EM)/effector T cells for adoptive immunotherapies using concomitant tumor-antigen vaccination.

Naive CD8+ T cells rely upon oxidation of fatty acids as a primary source of energy. After antigen encounter, T cells shift to a glycolytic metabolism to sustain effector function. It is unclear, however, whether changes in glucose metabolism ultimately influence the ability of activated T cells to become long-lived memory cells. We used a fluorescent glucose analog, 2-NBDG, to quantify glucose uptake in activated CD8+ T cells. We found that cells exhibiting limited glucose incorporation had a molecular profile characteristic of memory precursor cells and an increased capacity to enter the memory pool compared with cells taking up high amounts of glucose. Accordingly, enforcing glycolytic metabolism by overexpressing the glycolytic enzyme phosphoglycerate mutase-1 severely impaired the ability of CD8+ T cells to form long-term memory. Conversely, activation of CD8+ T cells in the presence of an inhibitor of glycolysis, 2-deoxyglucose, enhanced the generation of memory cells and antitumor functionality. Our data indicate that augmenting glycolytic flux drives CD8+ T cells toward a terminally differentiated state, while its inhibition preserves the formation of long-lived memory CD8+ T cells. These results have important implications for improving the efficacy of T cell-based therapies against chronic infectious diseases and cancer.

Many tumor-associated antigens are derived from nonmutated "self" proteins. T cells infiltrating tumor deposits recognize self-antigens presented by tumor cells and can be expanded in vivo with vaccination. These T cells exist in a functionally tolerant state, as they rarely result in tumor eradication. We found that tumor growth and lethality were unchanged in mice even after adoptive transfer of large numbers of T cells specific for an MHC class I-restricted epitope of the self/tumor antigen gp100. We sought to develop new strategies that would reverse the functionally tolerant state of self/tumor antigen-reactive T cells and enable the destruction of large (with products of perpendicular diameters of >50 mm2), subcutaneous, unmanipulated, poorly immunogenic B16 tumors that were established for up to 14 d before the start of treatment. We have defined three elements that are all strictly necessary to induce tumor regression in this model: (a) adoptive transfer of tumor-specific T cells; (b) T cell stimulation through antigen-specific vaccination with an altered peptide ligand, rather than the native self-peptide; and (c) coadministration of a T cell growth and activation factor. Cells, vaccination, or cyto-kine given alone or any two in combination were insufficient to induce tumor destruction. Autoimmune vitiligo was observed in mice cured of their disease. These findings illustrate that adoptive transfer of T cells and IL-2 can augment the function of a cancer vaccine. Furthermore, these data represent the first demonstration of complete cures of large, established, poorly immunogenic, unmanipulated solid tumors using T cells specific for a true self/tumor antigen and form the basis for a new approach to the treatment of patients with cancer.