In vivo optical imaging of folate receptor-β in head and neck squamous cell carcinoma.

OBJECTIVES/HYPOTHESIS Folate receptor (FR) expression, although known to be elevated in many types of cancer and inflammatory cells, has not been well characterized in head and neck squamous cell carcinoma (HNSCC). We hypothesized that tumor infiltrating inflammatory cells expressing FR-β could allow fluorescent visualization of HNSCC tumors using folate conjugated dyes even when FR expression in cancer cells is low. STUDY DESIGN Retrospective review of clinical pathologic specimens and in vivo animal study. METHODS A tissue microarray with tumor and tumor-free tissue from 22 patients with HNSCC was stained with antibodies to FR-α and FR-β. We characterized FR-β(+) cells by examining CD45, CD68, CD206, and transforming growth factor (TGF)-β expression. To investigate fluorescent imaging, mice with orthotopic tumor xenografts were imaged in vivo after intravenous injections of folate conjugated fluorescein isothiocyanate (folate-FITC) and were histologically evaluated ex vivo. RESULTS All tumor samples demonstrated significant FR-β staining and negligible FR-α staining. FR-β(+) cells found in tumors coexpressed CD68 and had increased expression of CD206 and TGF-β characteristic of tumor-associated macrophages. In the xenograft models, tumors showed strong in vivo fluorescence after folate-FITC injection in contrast to surrounding normal tissues. Histologic examination of the xenograft tissue similarly showed folate-FITC uptake in areas of inflammatory cellular infiltrate. CONCLUSIONS Although HNSCC tumor cells do not express FR, HNSCC tumors contain a significant population of FR-β-expressing macrophages. Folate conjugated fluorescent dye is able to specifically target and label tumor xenografts to permit macroscopic fluorescence imaging due to FR-β expression on the infiltrating inflammatory cells.