Cell sorting microbeads as novel contrast agent for magnetic resonance imaging

The success of several cell-based therapies and prevalent use of magnetic resonance imaging (MRI) in the clinic has fueled the development of contrast agents for specific cell tracking applications. Safe and efficient labeling of non-phagocytic cell types such as T cells nonetheless remains challenging. We developed a one-stop shop approach where the T cell sorting agent also labels the cells which can subsequently be depicted using non-invasive MRI. We compared the MR signal effects of magnetic-assisted cell sorting microbeads (CD25) to the current preclinical gold standard, ferumoxytol. We investigated in vitro labeling efficiency of regulatory T cells (Tregs) with MRI and histopathologic confirmation. Thereafter, Tregs and T cells were labeled with CD25 microbeads in vitro and delivered via intravenous injection. Liver MRIs pre- and 24 h post-injection were performed to determine in vivo tracking feasibility. We show that CD25 microbeads exhibit T2 signal decay properties similar to other iron oxide contrast agents. CD25 microbeads are readily internalized by Tregs and can be detected by non-invasive MRI with dose dependent T2 signal suppression. Systemically injected labeled Tregs can be detected in the liver 24 h post-injection, contrary to T cell control. Our CD25 microbead-based labeling method is an effective tool for Treg tagging, yielding detectable MR signal change in cell phantoms and in vivo. This novel cellular tracking method will be key in tracking the fate of Tregs in inflammatory pathologies and solid organ transplantation.