Use of superparamagnetic microbeads in tracking subretinal injections

The purpose of these studies was to develop a method to track intraocular injections.

Retinal pigment epithelial (RPE) cells, purified from adult mouse eyes, were incubated with superparamagnetic microbeads (Dynabeads, 4.5 microm) coated with bovine serum albumin to verify that they could phagocytose the microbeads. For in vivo tracking studies, mice were anesthetized and a small incision was made at the pars plana and 2 mul of microbeads (around 10(5) microbeads) or RPE that had taken up the microbeads were injected into the subretinal space (SRS). Mice were sacrificed at various times after injection. The eyes were enucleated, fixed in formalin, and embedded in paraffin. Sections were stained with H&E, visualized by light microscopy. Some eyes were digested with collagenase and inflammatory cells determined by flow cytometry.

Cultured adult RPE phagocytosed the magnetic microbeads. One day after injection into the SRS, a retinal detachment was observed at the injection point and free microbeads were easily detected at this site. One week later, the host RPE cells had phagocytized the microbeads and the retina had reattached. No inflammatory response was detected in the eyes that were injected with microbeads in the SRS at any time examined. Histology showed normal morphology of all retinal layers around the injection site. The microbeads remained in situ throughout the study.

Protein coated magnetic microbeads are non-inflammatory after injection into the SRS. Host RPE cells phagocytized the microbeads and the retina maintained a healthy morphology after reattachment. This technique proved not only to be a good training tool to determine the precise location of injection, but also provided a noninflammatory method for long term marking of delivery into the SRS. However, the microbeads can not be used as a tracer of injected RPE because the microbeads were readily transferred to the endogenous RPE.