Abstract 1176: Myeloid-specific disruption of a2,6-linked sialic acid enhances their regulatory and tumorigenic functions by unmasking Gal-1 specific ligands

Abstract Myeloid cells (MCs) promote tumor progression by suppressing antitumor immunity and inducing angiogenesis. GAL1 is a carbohydrate-binding protein associated with poor prognosis, and immune and vascular reshaping across various cancers. Its binding can be disrupted by ST6GAL1, a sialyltransfase that adds α(2, 6)-linked sialic acid to poly-LacNAc structures. We studied the impact of ST6GAL1 in modulating the immunoregulatory and tumorigenic activity of tumor-associated MCs by modulating the exposure of GAL1 ligands. To study the impact of GAL1-dependent circuits on MCs function, PMN- and M-MCs were isolated from tumors, tumor-draining lymph nodes (TDLN), and spleens from a CT26 colorectal cancer (CRC) model mice by Percoll gradient. Their glycophenotype was analyzed using a lectin array focused on glycan structures known to modulate GAL1 binding. We observed higher GAL1 binding to tumor MCs compared to TDLN or spleen MCs, supported by increased poly-LacNAc structures, core-2 O-glycan branching, and complex N-glycans, together with a decrease in α(2, 6)-linked sialic acid in PMN-MCs over M-MCs in all tissues (p<0.05). These niche-dependent variations were absent in tumor-free mice. To explore this in patients we used single-cell transcriptomic data from two cohorts of melanoma (Sade-Feldman 2018) and CRC (Li 2023) patients treated with immune checkpoint inhibitors (ICI) (accession numbers GSE120575 and GSE205506 respectively) and used a published “Dysfunctional lymphocyte” signature (Sade-Feldman 2018) and an in-house “Regulatory MCs” (rMC) signature (data not shown) to assess the interplay between the myeloid and T cell compartments. MCs from non-responders showed lower ST6GAL1 expression levels before treatment when compared to responders (p<0.001). This correlated with a higher rMC score and dysfunctional lymphocyte signature. Furthermore, we found higher LGALS1 expression in fibroblasts and MCs of these patients. Finally, we studied the impact of ST6GAL1 expression in modulating MC functions in vitro using bone-marrow (BM) derived MCs from St6gal1-/- or WT mice exposed to GAL1, and in vivo using LysMCre-St6gal1fl/fl and control St6gal1fl/fl mice. ST6GAL1 deficiency in BM-derived MCs enhanced the regulatory function of both MC subtypes. LysMCre-St6gal1fl/fl mice showed accelerated tumor growth, with higher VEGF and lower IFN-γ secretion than controls (p<0.01). These effects were suppressed by an anti-GAL1 neutralizing mAb. In conclusion, α(2, 6) sialylation controls the regulatory and tumorigenic capacities of MCs by masking GAL1-specific ligands. Thus, targeting glycosylation-dependent GAL1-driven circuits may contribute to reprogramming MC functions and improve the efficacy of both ICI and anti-angiogenic treatments. Citation Format: Joaquin P. Merlo, Camila A. Bach, Sabrina G. Gatto, Rosa M. Morales, Karina V. Mariño, Gabriel A. Rabinovich, Ada G. Blidner. Myeloid-specific disruption of a2, 6-linked sialic acid enhances their regulatory and tumorigenic functions by unmasking Gal-1 specific ligands [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 1176.