Abstract Immunosuppressive activity of myeloid cells within the tumor microenvironment (TME) contributes significantly to tumor immune evasion. Leukocyte immunoglobulin-like receptor subfamily B (LILRB) members are inhibitory receptors primarily expressed on myeloid cells. LILRB1, LILRB2 and LILRB4 are upregulated on tumor-infiltrating myeloid cells across many cancers. LILRB1 and LILRB2 suppress cytotoxic T cell and NK cell activity through interactions with their ligands such as HLA-G and HLA-G/β2m. LILRB4 interacts with extracellular matrix proteins and subsequently promotes myeloid-mediated immunosuppression, leading to impairment of anti-tumor immunity. Although immune checkpoint inhibitors (ICIs) show broad clinical benefit, many patients are refractory or develop acquired resistance. ICI-resistant tumors often exhibit increased polarization of immunostimulatory M1 to immunosuppressive M2 tumor-associated macrophages. As macrophages are abundant in the TME across many cancer types, reprogramming of tumoricidal macrophages and their engagement with tumor cells could be a promising approach to elicit effective anti-tumor responses. Since high LILRBs expression correlate with poor prognosis and ICI non-responsiveness, and PD-L1 is highly expressed in multiple cancer types or positively correlates with LILRBs expression, we designed GI-128, a macrophage-engaging bispecific antibody composed of an anti-PD-L1 sdAb (single domain antibody) and an anti-LILRBs sdAb. Here, we showed that GI-128 candidates exhibited high affinity for both PD-L1 and LILRB1/2/4 and effectively suppressed their receptor-ligand interactions against each target in cell-based blockade assay or cell-based reporter assay. In functional assays using immune cells, GI-128 candidates enhanced macrophage-mediated phagocytosis of tumor cells. Furthermore, they promoted pro-inflammatory cytokine production in LPS-stimulated PBMC-derived monocytes and induced M1 polarization during macrophage differentiation. In addition, a trans-binding assay showed that GI-128 candidates bound simultaneously to PD-L1-expressing cells and to LILRB1-, LILRB2- or LILRB4-expressing cells, implying their functionality as macrophage-engaging molecules. These findings suggest that GI-128, a bispecific macrophage engager, has a promising potential to reinvigorate tumoricidal macrophage function in immunosuppressed PD-L1-positive tumors. In vivo anti-tumor efficacy of GI-128 is being investigated. Citation Format: Seong Hun Kim, Eunji Ahn, Min Sang Yoon, Ji Cheol Kim, Hye Jun Park, Wonjae Lee, Hyuckjun Mok, Eun Jin Lee, Jisoo Kim, Young Min Oh, Kook Hwan Kim, Myoung Ho Jang. Discovery and characterization of GI-128: A PD-L1 x LILRB1/2/4 bispecific antibody as a macrophage engager abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 4412.
Kim et al. (Fri,) studied this question.