TARGETING AN IMMUNORECESSIVE TUMOR EPITOPE FOR ADOPTIVE IMMUNOTHERAPY OF ADVANCED-STAGE BRAIN TUMORS INDUCED BY SV40 T ANTIGEN
Open Access
- Author:
- Ryan, Christina Marie
- Graduate Program:
- Microbiology and Immunology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- April 25, 2007
- Committee Members:
- Todd Schell, Committee Chair/Co-Chair
Sarah Bronson, Committee Member
Mary Judith Tevethia, Committee Member
Christopher Charles Norbury, Committee Member
Shao Cong Sun, Committee Member
Witold B. Rybka, Committee Member - Keywords:
- tumor immunology
vaccination
cytotoxic T cells - Abstract:
- The potential to recruit tumor-specific CD8+ T lymphocytes (T-CD8 cells) for the control of cancer has been investigated intensively for many years but has resulted in only limited success in human clinical trials. Such lack of translational success might be due in part to the use of particular animal models involving transplantable tumors, which fail to recapitulate natural cancer. This thesis project makes use of line SV11 mice, which express the oncoprotein SV40 large T antigen (Tag) as a transgene, in order to study the tumor-specific T-CD8 cell response in mice that develop spontaneously-arising choroid plexus tumors. Similar to some cancer patients, the T-CD8 cell response to the major tumor antigen in these mice - the Tag - is hindered by the development of T cell tolerance. Although T-CD8 cells specific for the three immunodominant Tag epitopes are deleted from the repertoire of SV11 mice during T cell development, a subset of Tag-specific T-CD8 cells, specific for the immunorecessive epitope V, survive in SV11 mice. This residual population of T-CD8 cells is composed of a mixture of both low and high avidity epitope V specific cells that can be recruited following a prime and boost immunization approach. To determine the effect of the tumor-bearing environment on higher avidity epitope V specific T-CD8 cells, the fate and anti-tumor potential of naïve T cell receptor transgenic T-CD8 cells (TCR-V cells) following adoptive transfer into SV11 mice with advanced tumors was investigated. TCR-V cells recognized the endogenous Tag post transfer into tumor-bearing SV11 mice, resulting in proliferation and acquisition of effector functions, but failed to accumulate in either the lymphoid tissues or at the tumor site. Specific immunization of SV11 mice following adoptive transfer of naïve TCR-V cells resulted in a modest expansion in the lymphoid organs, but failed to induce significant T cell accumulation at the tumor site. Approaches aimed at augmenting epitope V targeted adoptive immunotherapy of SV11 mice bearing advanced stage tumors were thus investigated, and it was found that the cellular source of antigen, mode of antigen presentation, and timing of immunization played critical roles in eliciting the TCR-V cell response in vivo. The results demonstrate that the anti-tumor response of TCR-V T cells in tumor-bearing mice could be significantly enhanced by activation against both endogenous and exogenous sources of Tag. Specifically, strategically-timed cellular immunization targeted against the immunorecessive epitope promoted dramatic accumulation of functional TCR-V cells at the tumor site, which was associated with prolonged survival of SV11 mice. Tumor infiltration correlative with delayed tumor progression was also induced following the administration of an agonistic anti-CD40 mAb, suggesting that TCR-V cells were effectively triggered and primed against the endogenous Tag following activation of resident antigen presenting cells, even in the presence of advanced tumors. Although both therapeutic approaches yielded comparable effects on the enhancement of SV11 survival, high levels of TCR-V cells did not persist long-term following either treatment regimen, and mice eventually succumbed to tumor burden. However, in contrast to either regimen administered individually, a combinatorial immunotherapeutic approach, consisting of both anti-CD40 pre-conditioning and immunization synergistically promoted long-term maintenance of TCR-V cells in the brain and extended SV11 survival. These data implied that adoptive T-CD8-mediated immunotherapy against spontaneously-arising tumors was optimized by enhanced sensitization against the endogenous tumor antigen and well-timed exogenous immunization. It was also found that T-CD8 cell persistence at the tumor site did not always predict therapeutic efficacy, and that tumor burden at the onset of combinatorial immunotherapy may pose limitations on the capacity for TCR-V cells to control disease progression. Exploration of mechanisms contributing to the ultimate loss of tumor control despite long-term maintenance of TCR-V cells in SV11 mice post combinatorial immunotherapy, revealed that eventually the tumor environment in SV11 mice was characterized by the accumulation of anergic TCR-V cells and tumor infiltration by bone marrow-derived DCs expressing inhibitory receptors. These findings implicated TCR-V cell anergy - associated with persistent up-regulation of the inhibitory receptor PD-1 on tumor-infiltrating TCR-V cells compared with peripheral TCR-V cells - in the promotion of tumor escape from immune surveillance. However, the anergic phenotype was reversible, evidenced by the ability to restore function of non-responsive TCR-V cells ex vivo through blockade of a PD-1 receptor ligand. Furthermore, in spite of the failure of TCR-V cells from the initial transfer to exert continued anti-tumor function, SV11 tumors remained receptive to a second round of combinatorial therapy that included a fresh adoptive transfer of naïve TCR-V cells. These findings implicate critical checkpoints during tumor progression at which therapeutic intervention is necessary, namely (i.) optimizing initial enhancement of the priming event against endogenous tumor antigen, (ii.) providing appropriately-timed immunization to prolong the anti-tumor response, and (iii.) monitoring disease at late time points to overcome negative regulation of persisting cells. These studies demonstrate the potential for strategically-executed immunotherapeutic manipulations to dramatically augment a relatively weak anti-tumor response of immunorecessive epitope-specific T-CD8 cells in the face of advanced tumors, and imply that similar combinatorial therapeutic approaches toward cancer patients could yield promising results in clinical practice.