Immune Cell-Specific and Isoform-Selective Regulation of CD44 in Pancreatic Ductal Adenocarcinoma Links Lymph Node Variant Loss and Exosomal CD44 to Clinical Outcome in Pancreatic Ductal Adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is characterized by immune cell dysfunction and poor prognosis. CD44, a cell surface glycoprotein with multiple splice variants, has been implicated in tumor progression, but its compartment-specific roles in PDAC remain unclear. CD44 standard and variant isoform expression was analyzed in patient-derived lymph nodes (LNs) by quantitative PCR. Immune cell-specific CD44 expression was assessed by flow cytometry in LNs and peripheral blood. Soluble and exosome-associated CD44 (exo-CD44) were measured in plasma. Clinical associations and survival analyses were performed. Transcriptomic, immune infiltration, immune checkpoint, and drug sensitivity analyses were conducted using TCGA-PAAD and pharmacogenomic datasets. CD44 standard isoform expression was unchanged in PDAC LNs, whereas multiple CD44 variant isoforms (v4-v10) were significantly reduced and associated with metastatic disease and poor survival, particularly CD44v5, v6, v7, and v10. CD44 expression was enriched in CD45⁺ immune cells, with highest levels in CD4⁺ T cells in both LNs and blood. Soluble CD44 levels showed no clinical associations. In contrast, exo-CD44 levels were reduced overall in PDAC but increased in patients with distant metastasis, positive resection margins, systemic inflammation, and reduced survival. High CD44 expression was associated with advanced disease, immune cell infiltration, immune checkpoint gene expression, reduced sensitivity to gemcitabine, paclitaxel, rapamycin, and FMK, and distinct CTLA4/PD-L1 checkpoint profiles. CD44 exhibits compartment-specific regulation in PDAC, linking immune remodeling, exosome signaling, and therapeutic resistance to adverse clinical outcome.