PO-452 Selective CD47 immune checkpoint blockade on tumour cells with bispecific antibodies to effectively control tumour growth: primary role of the phagocytes

Introduction CD47 is a membrane protein overexpressed on tumour cells and considered as an innate immune checkpoint. CD47 interacts with SIRPα on myeloid cells and induces a ‘don’t eat me’ signal limiting cell clearance by phagocytes. Blocking the CD47-SIRPα axis is thus an attractive approach to foster tumour cell killing by tumor-associated macrophages (TAMs) and boost cross-priming of anti-tumour T cells by dendritic cells (DC). However, CD47 targeting is hindered by haematological toxicity due to its ubiquitous expression. To focus CD47 blockade on tumour cells, we generated two bispecific antibodies (biAbs) pairing a high affinity arm targeting a tumour associated antigen (TAA), i.e., CD19 or mesothelin (MSLN), to an optimised lower affinity arm targeting CD47 and investigated their efficacy and mechanism of action (MoA) in xenograft tumour models. Material and methods BiAbs targeting CD47xCD19 or CD47xMSLN were tested in immunodeficient mice implanted with the following human tumour cell lines: Raji (CD19+, lymphoma), OVCAR-3 (MSLN+, ovarian) or MSLN-transfected HepG2 (hepatic). Analysis by flow cytometry was used to evaluate the impact of the biAbs on the tumour microenvironment (TME). The contribution of macrophages to tumour growth inhibition was addressed by clodronate-mediated depletion. Finally, biAbs-stimulated cross-priming of anti-tumour T-cells by DC was assessed in vitro . Results and discussions BiAbs controlled the growth of CD19 +lymphomas and MSLN +tumours. In the Raji xenograft model, treatment with the CD47xCD19 biAb did not increase the number of TAMs, but enhanced their tumoricidal activity (i.e., more macrophages engulfing tumour cells). The treatment also reduced the proportion of CD11b + Ly6g + granulocytic myeloid-derived suppressor cells. In contrast, in the MSLN-transfected HepG2 xenograft model, treatment with the CD47xMSLN biAb induced a significant increase of macrophages in the tumour. Depletion of phagocytic cells with clodronate in both models impaired the anti-tumour activity of the biAbs, highlighting the role of macrophages in the MoA. In the antigen-cross presentation assay, the biAbs promoted cross-priming of tumour antigen-specific T-cells, suggesting the potential of CD47xTAA biAbs to trigger anti-tumour T cell responses. Conclusion CD47 targeting biAbs, tethered to cancer cells by their TAA binding arm, remodel the TME and control the lymphoma and MSLN +tumours. Furthermore, the observed induction of the adaptive immune response may further enhance the antitumor efficacy of such biAbs in patients.