Abstract B046: Therapeutic mechanisms of anti-4-1BB antibodies in cancer: agonism versus regulatory T cell depletion
Authors: Remer Marcus , Rogel Anne , Buchan Sarah , Johnson Peter W.M. , Al-Shamkhani Aymen 
Published: November 2016

Abstract
4-1BB (CD137) is a T cell activating receptor belonging to the TNF receptor superfamily that can be targeted by mAb to eradicate established tumors. 4-1BB is expressed on a multitude of hematopoietic cells, including activated conventional CD4 and CD8 T cells, regulatory T (Treg) cells, dendritic cells and NK cells. 4-1BB signaling promotes the proliferation, survival and effector function of conventional activated T cells and therapeutic 4-1BB antibodies are thought to mediate their effects through receptor agonism. However, whilst investigating the expression pattern of a panel of activated T cell markers on tumor infiltrating T cells, we unexpectedly found that intra-tumoral Treg cells express high levels of 4-1BB when compared with conventional T cells. This finding raised the possibility that anti-4-1BB antibodies mediate their antitumor effects through depletion of intra-tumoral Treg cells. Moreover, precedence for a Treg cell dependent mechanism exists for mAb targeting the related receptors OX40 and GITR. The choice of antibody isotype is critical as the therapeutic mechanism of action is dependent on the engagement of activatory and inhibitory FcγRs. Murine IgG1 mAb confer good agonistic potential through co-engagement of inhibitory FcγRs leading to receptor cross-linking and activation to promote CD8+ T cell mediated antitumor immunity. In contrast, murine IgG2a mAb have the potential to deplete target cells through activation of FcγR-expressing macrophages within the tumor microenvironment. To address the question of whether tumor protection with anti-4-1BB antibodies is mediated by agonism or selective Treg cell depletion, we generated a novel mouse strain with conditional deletion of 4-1BB on Foxp3+ Treg cells (4-1BBFoxp3-/−). The phenotype was validated by showing complete loss of 4-1BB expression on intra-tumoral CD4+ Foxp3+ cells whilst being conserved on conventional T cell subsets. To examine whether 4-1BB expression on intra-tumoral Treg cells was required for the therapeutic activity of anti-4-1BB mAb, 4-1BBFoxp3-/- or control mice were inoculated with EG7 thymoma cells and subsequently treated with either mIgG1 or mIgG2a anti-4-1BB mAb. The EG7 tumors grew equally well in 4-1BBFoxp3-/- and control mice given IgG isotype control Ab, indicating that the loss of 4-1BB expression on Treg cells does not alter the tumor immune environment. Treatment with mIgG1 or mIgG2a anti-4-1BB mAb resulted in tumor regression then complete clearance in the majority of control mice. Equivalent response to therapy was observed in the 4-1BBFoxp3-/- mice, suggesting that 4-1BB expression on Treg cells is not essential for the therapeutic activity of mIgG1 or mIgG2a anti-4-1BB mAb. Specifically, the mIgG2a anti-4-1BB mAb maintained its antitumor activity in the absence of 4-1BB-expressing Treg cells, suggesting that the therapeutic activity is driven by effector T cell costimulation and not differential intra-tumoral Treg cell deletion. These results establish T cell costimulation as a key mechanism for the therapeutic activity of anti-4-1BB mAb in cancer and have clinical relevance in optimizing human anti-4-1BB mAb for delivering costimulation.
Citation Format: Marcus Remer, Anne Rogel, Sarah Buchan, Peter W.M. Johnson, Aymen Al-Shamkhani. Therapeutic mechanisms of anti-4-1BB antibodies in cancer: agonism versus regulatory T cell depletion [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr B046.