Abstract 2884: Optimized CD28 bispecific antibodies for targeted activation of T cells within the tumor microenvironment
Authors: Majocchi Sara , Lloveras Pauline , Burnet-Merlin Coline , Nouveau Lise , Pleche Nicolas , Malinge Pauline , Batista Claudia , Daviet Christelle , Charreton Maud , Pontini Guillemette , Gueneau Franck , Moine Valery , Magistrelli Giovanni , Shang Limin , Ferlin Walter , Masternak Krzysztof 
Published: June 2022

Abstract
Tumor-targeted CD28 bispecific antibodies (bsAbs) are designed to co-stimulate T cells specifically within the tumor microenvironment. By bridging T cells to malignant cells expressing a selected tumor-associated antigen (TAA), CD28 bsAbs deliver the so-called signal 2 to T cells unleashing their full cytotoxic potential. Contrary to bivalent CD28 monoclonal antibody (mAb) superagonists, the activation of CD28 with monovalent CD28 bispecifics requires the binding of a TAA expressed by the cancer cells to induce CD28 clustering. Preclinical studies have shown the benefit of adding costimulatory CD28 bsAbs for the treatment of solid tumors, boosting the efficacy of bispecific T cell engagers (CD3 bsAbs) or PD-(L)1 checkpoint inhibitors.
Using our proprietary fully human κλ body antibody platform, which relies on a common heavy chain and on two distinct light chains driving specificity and affinity (i.e., one kappa and one lambda), we have identified a panel of agonist anti-CD28 kappa arms. Using our existing lambda anti-TAA arms (e.g., targeting CD19, MSLN, HER2, EGFR), an array of TAA-CD28 κλ bodies targeting different tumors were generated. The capacity of these CD28 κλ bodies to provide signal 2 to T cells was assessed both in vitro and in vivo in combination with CD3 bsAbs. In the presence of cancer cells and corresponding TAA-CD3 bsAbs, the CD28 κλ bodies enabled the activation of primary human CD4 and CD8 T cells, leading to T cell proliferation and secretion of cytokines such as IL-2 and IFNγ, and granzyme B. Importantly, when used alone or in combination with an untargeted CD3 bsAb (that is, in the absence of signal 1), CD28 κλ bodies showed no sign of T cell activation, potentially limiting peripheral toxicity. The resulting change in the T cell activation status led to a potent in vitro T cell-dependent cellular cytotoxicity (TDCC) against TAA-expressing cancer cell lines. CD28 κλ bodies targeting gastrointestinal tumors were tested in combination with CD3 bsAbs in xenogeneic tumor models and synergistically improved the antitumoral activity of T cell engagers, leading to an enhanced tumor control. Thus, via the identification of optimal agonist anti CD28 arms, CD28 bsAbs targeting multiple TAAs were designed for tumor-specific activation of the immune system. The resulting TAA-CD28 κλ bodies can enhance the antitumor response induced by CD3-retargeting bsAbs via the induction of T cell proliferation and activation, increased cytokine secretion and boosted anti-tumoral cytotoxicity. Other anti-TAA arms are being explored, to expand the panel of TAAs that could be easily paired with our anti-CD28 platform arms.
Citation Format: Sara Majocchi, Pauline Lloveras, Coline Burnet-Merlin, Lise Nouveau, Nicolas Pleche, Pauline Malinge, Claudia Batista, Christelle Daviet, Maud Charreton, Guillemette Pontini, Franck Gueneau, Valery Moine, Giovanni Magistrelli, Limin Shang, Walter Ferlin, Krzysztof Masternak. Optimized CD28 bispecific antibodies for targeted activation of T cells within the tumor microenvironment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2884.