Background: Cell surface glycoproteins of the CD44 family play roles in cell-cell and cell-matrix interactions. Their aberrant expression has been implicated in tumor invasion and metastasis of a variety of neoplasms, but not, to date, of thymic epithelial tumors.
Methods: To investigate the expression of CD44 molecules, immunohistochemical staining using monoclonal antibodies against human CD44 standard form (CD44 s) and two common splicing variant (CD44v) isoforms, CD44v5 and CD44v6, was performed on 64 resected thymomas and 20 normal thymuses. These tumors were categorized histologically according to the World Health Organization (WHO) histologic classification, and the pathologic staging was classified according to the definitions of Masaoka.
Results: The positive expression rates in these patients were as follows: CD44 s (normal thymuses, 10%; thymomas, 22%), CD44v5 (normal thymuses, 0%; thymomas, 67%), and CD44v6 (normal thymuses, 0%; thymomas, 26%). CD44 s and CD44v5 immunoreactivity showed a positive correlation with tumor stages (p = 0.034 and 0.027, respectively). The CD44v5 expression of neoplastic cells in tumor capsules has significant correlation with tumor stages (II, 5%; III, 70%; IVA, 100%; p < 0.001). On the basis of univariate survival analysis, the Masaoka staging system, WHO histologic classification, and CD44v5 expression showed a statistically significant positive relation to survival (p < 0.001, 0.002, 0.011, respectively). Using Cox's regression model, increasing CD44v5 expression, the Masaoka staging, and the WHO classification system were found to be significant independent prognostic factors.
Conclusions: CD44v5 expression is independently positively correlated with the aggressiveness of thymic epithelial tumors. The expression of CD44v5 may be a potential trigger of tumor invasion in thymomas.