Myeloid-derived MIF is a central regulator of MDSC-driven T-cell dysfunction in head and neck squamous cell carcinoma.

Head and neck squamous cell carcinoma (HNSCC) exhibits a profoundly immunosuppressive tumor microenvironment (TME) enriched for myeloid-derived suppressor cells (MDSCs), regulatory T cells (Tregs), and dysfunctional CD8⁺ T cells, limiting therapeutic benefit from immune checkpoint blockade. Macrophage migration inhibitory factor (MIF) is elevated in HNSCC and linked to poor outcomes, yet the cellular source and functional role of tumor-promoting MIF in shaping antitumor immunity remain unclear. We used a myeloid-specific MIF knockout mouse (mMIF KO) in an orthotopic MOC2 HNSCC model and evaluated immune function using ex vivo co-cultures of tumor-derived MDSCs with naïve T cells under Treg-skewing, basal, or Th1-polarizing conditions. Myeloid-restricted MIF deletion significantly reduced tumor growth and increased CD8⁺ T cell infiltration, accompanied by reduced CTLA4, TIGIT, and TIM3 expression, with elevated PD1 consistent with antigen-engaged effector activation. High-dimensional immune profiling revealed expansion of cytotoxic CD8⁺ T cell states (GranzymeBhi/Perforinhi) and contraction of IL10-producing CD4⁺ regulatory (Tr1-like) populations, findings corroborated by decreased intratumoral CD4⁺FoxP3⁺Tregs. Within the myeloid compartment, MIF deletion selectively depleted polymorphonuclear MDSCs (PMN-MDSCs), including CSF1Rhi and PD-L1hi subsets. Functionally, tumor-derived MDSCs lacking MIF showed impaired capacity to induce Tregs and to promote CD8⁺ T cell exhaustion in ex vivo co-cultures, effects partially overcome by TGFβ/Th1-polarizing signals. These findings identify myeloid-derived MIF as a central regulator of the MDSC-Treg-CD8⁺ axis in HNSCC and support myeloid-targeted MIF inhibition as a strategy to reprogram the TME and enhance responses to immune checkpoint blockade.