Submitted April 10, 2008
Accepted October 6, 2008
A functional folate receptor is induced during macrophage activation and can be used to target drugs to activated macrophages
Wei Xia, Andrew R Hilgenbrink, Eric L Matteson, Michael B Lockwood, Ji-Xin Cheng, and Philip S Low*
Department of Chemistry, Purdue University, West Lafayette, IN, United States
Division of Rheumatology, Mayo Clinic College of Medicine, Rochester, MN, United States
Department of Rheumatology, Clarian Arnett Health, Lafayette, IN, United States
Weldon School of Biomedical Engineering, Purdue University, West Lafayette, IN, United States
* Corresponding author; email: plow{at}purdue.edu.
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 either saline, thioglycollate, zymosan, heat-killed or live bacteria, and cell surface markers that co-express with FR were identified. Virtually no F4/80+ peritoneal macrophages from saline-injected mice expressed FR, while numerous macrophages from mice injected with each inflammatory stimulus expressed FR. Examination of cell differentiation antigens that are upregulated 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 TNF-
and reactive oxygen species (ROS), and production of ROS correlated linearly with expression of FR. FR was also demonstrated to mediate binding and internalization of folate-linked molecules, and synovial macrophages collected from arthritic patients were found to bind 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 non-activated macrophages. This trial has been registered at www.clinicaltrials.gov as NCT00588393.
