Simple SummaryIn cancer patients, immunotherapy using targeted antibodies is often not effective in the long term due to resistance mechanisms developed by cancer cells. Tumors often overexpress certain molecules on the cell surface, that bind to specific receptors on immune cells, such as myeloid cells that can inhibit the immune response against cancer cells. One of such interactions involves sialic acid molecules on cancer cells and specific receptors called Siglecs on myeloid cells. This interaction prevents neutrophils (a type of myeloid cell) from effectively killing tumor cells. To improve the effectiveness of immunotherapy using a specific type of antibody called IgA, we investigated ways to disrupt the interaction between sialic acid and Siglecs. Removal of sialic acids from the tumor cells, a process called desialylation, enhanced the ability of neutrophils to kill tumor cells in the presence of IgA antibodies. Additionally, combining desialylation with blocking the CD47/SIRPα interaction, that was also reported as an inhibitory interaction on neutrophils, further improved the immune response. The study suggests that a combination of blocking different inhibitory interactions, such as CD47/SIRPα and sialic acids/Siglec, may be necessary to optimize cancer immunotherapy, considering the ways in which tumor cells evade the immune system.AbstractImmunotherapy with targeted therapeutic antibodies is often ineffective in long-term responses in cancer patients due to resistance mechanisms such as overexpression of checkpoint molecules. Similar to T lymphocytes, myeloid immune cells express inhibitory checkpoint receptors that interact with ligands overexpressed on cancer cells, contributing to treatment resistance. While CD47/SIRPα-axis inhibitors in combination with IgA therapy have shown promise, complete tumor eradication remains a challenge, indicating the presence of other checkpoints. We investigated hypersialylation on the tumor cell surface as a potential myeloid checkpoint and found that hypersialylated cancer cells inhibit neutrophil-mediated tumor killing through interactions with sialic acid-binding immunoglobulin-like lectins (Siglecs). To enhance antibody-dependent cellular cytotoxicity (ADCC) using IgA as therapeutic, we explored strategies to disrupt the interaction between tumor cell sialoglycans and Siglecs expressed on neutrophils. We identified Siglec-9 as the primary inhibitory receptor, with Siglec-7 also playing a role to a lesser extent. Blocking Siglec-9 enhanced IgA-mediated ADCC by neutrophils. Concurrent expression of multiple checkpoint ligands necessitated a multi-checkpoint-blocking approach. In certain cancer cell lines, combining CD47 blockade with desialylation improved IgA-mediated ADCC, effectively overcoming resistance that remained when blocking only one checkpoint interaction. Our findings suggest that a combination of CD47 blockade and desialylation may be necessary to optimize cancer immunotherapy, considering the upregulation of checkpoint molecules by tumor cells to evade immune surveillance.