Cheng, Ji‐Xin

Abstract Folate receptor (FR) expressing macrophages have been shown to play an integral role in a number of inflammatory diseases. (1) In this report, we characterize these FR+ macrophages and further investigate whether monocytes, the precursors to these macrophages, also express FR. We demonstrate that FR expression is observed on a subset of highly activated peritoneal and synovial macrophages, but not on other infiltrating immune cells or resident macrophages. We further find that a subset of human peripheral blood monocytes also expresses FR, albeit at a lower level than activated macrophages. Surface antigen characterization of the FR+ monocytes suggests markers characteristic of an inflammatory phenotype (CD14hiCD16), while the resident monocytes (CD14loCD16+) express few or no FR. Preliminary clinical data also suggest that the numbers of peripheral FR+ monocytes can be reduced by a folate-targeted immunotherapy. These results suggest that activated macrophages and inflammatory monocytes express a FR that can be targeted with folate-linked drugs without promoting drug uptake by non-activated macrophages and resident monocytes. 1. Xia W, Hilgenbrink AR, et al. Blood. 2008 Oct 24. [Epub ahead of print] This work was supported in part by a grant from Endocyte, Inc.

Abstract Previous work has demonstrated that a subset of macrophages expresses a folate receptor (FR) that can mediate internalization of folate-linked molecules, including imaging and therapeutic agents. To characterize this subset, macrophages were collected from peritoneal cavities of mice injected with saline, thioglycolate, zymosan, heat-killed or live bacteria, and cell-surface markers that coexpress with FR were identified. Virtually no F4/80+ peritoneal macrophages from saline-injected mice expressed FR, whereas numerous macrophages from mice injected with each inflammatory stimulus expressed FR. Examination of cell differentiation antigens that are up-regulated in FR+ macrophages revealed markers characteristic of an activated state (CD80, CD86, Ly-6C/G), whereas macrophages lacking these activation markers expressed few or no FR. FR+ macrophages also produced tumor necrosis factor-α (TNF-α) and reactive oxygen species, and production of reactive oxygen species correlated linearly with expression of FR. Synovial macrophages collected from arthritic patients were found to bind and internalize folate-linked dyes. Moreover, a folate-linked radioimaging agent was shown to image inflamed joints of rheumatoid arthritic patients. These results suggest that FR constitutes a marker for macrophage activation and that FR+ macrophages can be targeted with folate-linked drugs without promoting drug uptake by nonactivated macrophages. This trial was registered at www.clinicaltrials.gov as #NCT00588393.

Quantitation of circulating tumor cells (CTCs) constitutes an emerging tool for the diagnosis and staging of cancer, assessment of response to therapy, and evaluation of residual disease after surgery. Unfortunately, no existing technology has the sensitivity to measure the low numbers of tumor cells (<1 CTC per ml of whole blood) that characterize minimal levels of disease. We present a method, intravital flow cytometry, that noninvasively counts rare CTCs in vivo as they flow through the peripheral vasculature. The method involves i.v. injection of a tumor-specific fluorescent ligand followed by multiphoton fluorescence imaging of superficial blood vessels to quantitate the flowing CTCs. Studies in mice with metastatic tumors demonstrate that CTCs can be quantitated weeks before metastatic disease is detected by other means. Analysis of whole blood samples from cancer patients further establishes that human CTCs can be selectively labeled and quantitated when present at ≈2 CTCs per ml, opening opportunities for earlier assessment of metastatic disease.