Abstract PL07-03: Mechanisms of protective tumor immunity.

Abstract

Abstract Efficacious cancer immunotherapies will likely require combinations of strategies that enhance tumor antigen presentation and antagonize negative immune regulatory circuits. We demonstrated that vaccination with irradiated, autologous melanoma cells engineered to secrete GM-CSF followed by antibody blockade of CTLA-4 accomplishes clinically significant tumor destruction with minimal toxicity in a majority of stage IV metastatic melanoma and some advanced ovarian carcinoma patients. The extent of tumor necrosis in post-treatment biopsies was linearly related to the natural logarithm of the ratio of infiltrating CD8+ effector T cells to FoxP3+ Tregs, suggesting that further Treg inhibition might increase the frequency of clinical responses. Through an analysis of cytokine deficient mice, we delineated a critical role for GM-CSF in Treg homeostasis. GM-CSF is required for the expression of the phosphatidylserine binding protein MFG-E8 in antigen presenting cells, whereas the uptake of apoptotic cells by phagocyte-derived MFG-E8 maintains peripheral Treg activity. The pharmacologic inhibition of MFG-E8 function through genetic or engineering-based approaches blocks Treg induction, which intensifies vaccine-induced responses, leading to the regression of established tumors in mice. The clinical translation of these therapeutic strategies to Phase I testing in humans is underway. The detailed analysis of patients achieving sustained clinical benefits from irradiated, autologous GM-CSF secreting tumor cell vaccines and CTLA-4 antibody blockade also affords a rich opportunity to identify antigens associated with immune-mediated tumor destruction and to delineate mechanisms of therapeutic immunity. We elucidated several of the molecular pathways that underlie these antitumor effects, including the NKG2D system and multiple secreted/or cell surface proteins that contribute to tumor promoting inflammation. The therapy-induced antibodies manifest functional activity in vitro, antagonizing tumor cell survival, invasive potential, and angiogenesis. These findings support a key role for humoral immunity in tumor destruction, and highlight interest in more detailed characterization of the antitumor B cell repertoire in vaccinated patients. Citation Information: Mol Cancer Ther 2013;12(11 Suppl):PL07-03. Citation Format: Glenn Dranoff. Mechanisms of protective tumor immunity. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr PL07-03.