Multifactorial effects of LGALS1 blockade sensitize tumors to immune checkpoint inhibitor

Head and neck squamous cell carcinoma (HNSCC) accounts for 90% of all head and neck cancers. While immune checkpoint inhibitors (ICIs) have improved HNSCC outcomes, response rates remain low (10–20%). Identifying novel targets to sensitize HNSCC to ICI therapy is therefore a critical unmet clinical need. To identify potential new targets, we analyzed human HNSCC clinical samples and mouse HNSCC models. We evaluated blood-derived samples from ICI-treated HNSCC patients and tumor-bearing mice (responders vs. non-responders to anti-PD-L1 treatment). We tested the efficacy of combined LGALS1 inhibition and anti-PD-L1 therapy, characterizing immune profiles via flow cytometry. Additionally, we employed mechanistic assays to determine the effects of LGALS1 blockade on myeloid cell differentiation and CD8 T cell activation. We showed that LGALS1 is upregulated in HNSCC in a subtype- and stage-dependent manner. Contrary to prior findings focusing on extracellular LGALS1, our study reveals that LGALS1 expression within SCC tumor cells, tumor-infiltrating myeloid cells, and activated CD8 T cells is predominantly intracellular. In mouse models, lower intracellular LGALS1 in myeloid cells and higher plasma LGALS1 levels correlated with anti-PD-L1 unresponsiveness; consistently, elevated serum LGALS1 predicted poor survival in ICI-treated HNSCC patients. Pharmacological LGALS1 inhibition sensitized tumors to anti-PD-L1 and markedly prolonged recipient survival. Notably, LGALS1 inhibition did not alter expression levels; instead, its efficacy depended on tumor-intrinsic and tumor-extrinsic mechanisms. The combination of LGALS1 blockade and anti-PD-L1 effectively remodeled the myeloid compartment, significantly decreasing the frequency of total CD11b + cells and tumor-associated macrophages (TAMs) while shifting the remaining TAMs toward an M1-like phenotype. These changes, coupled with enhanced CD8 and CD4 T cell infiltration and effector function, promoted a less immunosuppressive tumor microenvironment. In vitro co-culture assays demonstrated that LGALS1 inhibition shifted TAM polarization toward an M1-like phenotype. Furthermore, LGALS1 blockade directly activated CD8 T cells via a STAT3-dependent mechanism. We conclude that LGALS1 possesses significant prognostic value for predicting ICI response in HNSCC. LGALS1 may represent a multimodal therapeutic target to sensitize tumors to immunotherapy, as its blockade simultaneously modulates tumor cells, myeloid cells, and CD8 T cells to overcome multi-layered resistance and promote robust anti-tumor immunity.