Highly CD81-expressing stromal cells characterize a tumor-suppressive immune microenvironment in invasive breast cancer

BACKGROUND: CD81, a member of the tetraspanin family, is implicated in tumor progression, and its elevated expression in tumor cells has been associated with poor prognosis across multiple cancers. However, the functional significance of CD81 within the stromal compartments of the tumor microenvironment (TME) remains unclear. This study investigated the clinical relevance and mechanistic role of stromal CD81 (sCD81) in invasive breast cancer. METHODS: A total of 462 human primary invasive breast cancer samples, along with 13 normal breast tissues and 10 ductal carcinoma in situ samples collected between 2000 and 2013, were analyzed. Immunohistochemistry and spatial analyses were used to evaluate sCD81 expression, and its association with cancer-associated fibroblast (CAF) subsets and immune components in the TME, as well as correlation with clinicopathological correlations and survival outcome. RESULTS: sCD81 expression was significantly higher in invasive breast cancer than in normal breast stromal tissue but did not differ across clinical subtypes. High sCD81 expression was associated with significantly longer disease-free survival, and multivariate analysis identified sCD81 as an independent prognostic factor alongside nuclear grade, estrogen receptor status, and lymph node metastasis. Co-expression analyses demonstrated that sCD81-high stromal cells exhibited a CD34-positive, FAP-low, and PDGFRα-low phenotype, indicating that they are distinct from established CAF subtypes. Both sCD81 and podoplanin (PDPN) expression correlated with increased immune cell infiltration. However, PDPN-positive CAFs promoted an immunosuppressive TME characterized by enrichment of regulatory T cells and M2-like macrophages, whereas CD81-high stromal cells did not exert such effects. Spatial analyses revealed that CD81-high stromal cells were preferentially localized near tumor vasculature and CCL19-expressing cells, suggesting a potential association with T-cell recruitment into the TME through CCL19-mediated mechanisms. CONCLUSIONS: These findings identify stromal CD81 expression as a novel prognostic marker in invasive breast cancer and highlight its association with a tumor-suppressive immune microenvironment, distinct from the immunosuppressive phenotype typically associated with PDPN-positive CAFs.