Background: Pancreatic cancer is a lethal disease due to its rapid growth, invasion, and systemic metastasis. In the recent years, compelling evidence has emerged that the complex crosstalk between stromal cells and cancer cells in the tumor microenvironment may regulate tumor growth, invasion and metastasis. The tumor microenvironment of pancreatic cancer consists mainly of a dense desmoplastic stroma rich in collagen fibers, extracellular matrix proteins, fibroblasts, and infiltrating inflammatory cells. Macrophages that infiltrate and interact with cancer cells are called tumor-associated macrophages (TAMs) which properties are not fully understood. Four members of the folate receptor (FR) family (FRα, FRβ, FRγ, and FRδ) that bind folic acid with high affinity are known. Our evidence suggests that the expression of FRβ is limited to activated macrophages in human rheumatoid arthritis synovium and human glioblastoma. Furthermore, we previously reported that the administration of a recombinant immunotoxin against FRβ in rat C6 glioma implanted in nude mice resulted in significant depletion of TAMs and blood vessels, and tumor growth suppression. These results suggest that FRβ+ macrophages in the tumor microenvironment play crucial roles in tumor angiogenesis and growth. However, the role of FRβ+ TAMs in pancreatic cancer is unclear. The aims of this study were to investigate the distribution and degree of FRβ+ TAM infiltration and the association of these FRβ+ TAMs with tumor angiogenesis, hematogenous metastasis, and prognosis in human pancreatic cancer.

Methods: Tumor samples were obtained from 76 patients with pancreatic cancer. None of these patients had received any preoperative chemotherapy or radiotherapy. Both FRβ+ and tumor-infiltrating (CD68+) macrophages were examined in each tumor specimen by immunohistochemical and immunofluorescence staining using a newly developed antihuman FRβ monoclonal antibody and CD68 antibody.

Results: 1) FRβ+ macrophages showed a heterogeneous distribution with foci of high density in pancreatic cancer. In contrast, they were rarely observed in noncancerous pancreatic tissues. Compared with CD68+ macrophages, a larger population of FRβ+ macrophages existed in the invasive front. Although the numbers of CD68+ and FRβ+ macrophages were both higher in the invasive front compared to the central portion of pancreatic tumors, the proportion of FRβ+ macrophages to CD68+ macrophages in the invasive front (71.8%) was significantly higher than that in the central portion (44.6%). 2) We evaluated the expression of VEGF in tumor-infiltrating macrophages by double immunofluorescence staining. The majority of FRβ+ macrophages showed distinct VEGF expression. By contrast, lower numbers of CD68+ macrophages showed VEGF expression. The mean proportion of FRβ+ macrophages with VEGF expression (70.1%) was significantly higher than that of CD68+ macrophages with VEGF expression (40.5%). 3) A high number of FRβ+ macrophages showed a positive association with high MVD, high incidence of hematogenous metastasis and a poor prognosis in pancreatic cancer patients.

Conclusions: Our findings revealed for the first time that FRβ+ macrophages are a novel subset of TAMs in pancreatic cancer and may play an important role in the tumor microenvironment in association with systemic metastasis through the interaction with tumor cells and vessels. Therefore, FRβ+ macrophages may be promising a targeted therapy for pancreatic cancer.

Citation Format: Sonshin Takao, Qiang Ding, Hiroshi Kurahara, Taku Nagai, Koki Maeda, Yumi Miyazaki, Kousei Maemura, Hiroyuki Shinchi, Shoji Natsugoe, Takami Matsuyama. Distribution and significance of folate receptor β-expressing macrophages in pancreatic cancer. [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Progress and Challenges; Jun 18-21, 2012; Lake Tahoe, NV. Philadelphia (PA): AACR; Cancer Res 2012;72(12 Suppl):Abstract nr A86.