Cell fusion upregulates PD-L1 expression for evasion from immunosurveillance

Area of Science:

• Cancer Biology
• Stem Cell Biology
• Immunology

Background:

• Mesenchymal stem cells (MSCs) are crucial for tissue repair but rarely fuse with somatic cells.
• Bladder cancer cells (UMUC-3) co-cultured with bone marrow-derived MSCs (BM-MSCs) showed a ~5% fusion rate.
• These fusion cells maintained high tumorigenicity.

Purpose of the Study:

• To investigate the mechanisms by which cell fusion promotes tumorigenesis.
• To identify genetic and molecular alterations in MSC-cancer cell fusion products.
• To explore the therapeutic potential of targeting pathways induced by cell fusion.

Main Methods:

• Co-culture of UMUC-3 bladder cancer cells with BM-MSCs.
• Establishment and characterization of eleven fusion cell clones.
• Genomic analysis (Mb-scale deletions, copy number gains).
• In vitro growth and anchorage-independent growth assays.
• Xenograft experiments in a mouse model.
• Gene expression profiling and identification of altered genes.
• PD-L1 knockout experiments and analysis of cytokine expression (CCL2).

Main Results:

• Fusion cells exhibited loss of cancer cell deletions but retained copy number gains, indicating genomic instability with 3-polar spindles.
• Fusion cells demonstrated enhanced in vitro proliferation, anchorage-independent growth, and efficient tumor formation in vivo.
• 264 genes were specifically altered in fusion cells, including interferon-stimulated genes (ISGs).
• PD-L1 was induced independently of STAT1, regulated by histone modification, potentially inhibiting macrophage phagocytosis.
• Overexpression of CCL2 promoted tumor progression by creating tumor-associated macrophages (TAMs).

Conclusions:

• Cell fusion between MSCs and bladder cancer cells promotes tumorigenesis through genomic instability and altered gene expression.
• Induced PD-L1 expression in fusion cells contributes to immune evasion.
• CCL2-mediated TAM recruitment further drives tumor progression.
• These findings reveal novel mechanisms linking cell fusion to tumorigenesis and highlight PD-L1 as a potential therapeutic target in cancer immunotherapy.