YKL-40 (also named chitinase 3 like-1 protein [CHI3L1]) is a secreted chitinase-like protein which is upregulated in cancers and suggested to have pro-tumorigenic activity. YKL-40 lacks enzymatic function, but it can bind carbohydrates such as chitin. Chitooligosaccharides (COS) derived from deacetylation and hydrolysis of chitin might be used for the blockade of YKL-40 function. Here, public single-cell RNA sequencing datasets were used to elucidate the cellular source of YKL-40 gene expression in human tumors. Fibroblasts and myeloid cells were the primary sources of YKL-40. Screening of YKL-40 gene expression in syngeneic mouse cancer models showed the highest expression in the Lewis lung carcinoma (LL2) model. LL2 was used to investigate COS monotherapy and combinations with immune checkpoint inhibitors (anti-PD-L1 and anti-CTLA-4) (ICIs) and radiotherapy (8 Gy × 3) (RT). COS tended to reduce plasma YKL-40 levels, but it did not affect tumor growth. LL2 showed minimal responses to ICIs, or to RT alone. Interestingly, ICIs combined with COS led to delayed tumor growth. RT also enhanced the efficacy of ICIs; however, the addition of COS did not further delay the tumor growth. COS may exert their anti-tumorigenic effects through the inhibition of YKL-40, but additional functions of COS should be investigated.

Background/Aim: Lung adenocarcinoma (LUAD), a predominant subtype of non-small cell lung cancer (NSCLC), is characterized by a complex tumor microenvironment (TME) that drives immune evasion and contributes to variable clinical outcomes. This study investigates the role of tumor-derived macrophage migration inhibitory factor (MIF) on immune modulation and prognosis in LUAD. Materials and Methods: Single-cell RNA sequencing (scRNA-seq) data from three LUAD tumors (E-MTAB-6149; 29,936 cells) were analyzed with Seurat to identify cell types and marker genes. Cell–cell communication was assessed using CellChat, and prognostic significance was evaluated in the TCGA-LUAD cohort with GEPIA2. Results: Malignant cells showed the highest expression of MIF, which may interact with CD74⁺CXCR4⁺ and CD74⁺CD44⁺ receptor complexes on B cells, T cells, and myeloid cells, consistent with known MIF-mediated signaling pathways. A predominant B cell subset (59%) expressing CD74 or CXCR4 (termed as MIFR⁺ B cells) showed elevated expression of MHC class II genes (such as HLA-DRA and HLA-DPB1) and co-stimulatory genes (such as CD40 and CD83), indicating antigen-presenting cell (APC)-like functions. High MIF expression was associated with a poor prognosis in TRU-subtype LUAD [hazard ratio (HR)=2.5, p-value=0.029], while MIFR⁺ B cell signatures was correlated with improved survival. Conclusion: Tumor-derived MIF is associated with poor prognosis, likely by suppressing the immune system, but it also promotes the induction of APC-like B cells, which are associated with improved outcomes, highlighting its dual role in LUAD. These findings position MIF as a potential therapeutic target and suggest that MIFR⁺ B cells could serve as important prognostic markers.