Cell analysis often requires the isolation of certain cell types. Various isolation methods have been applied to cell sorting, including fluorescence-activated cell sorting and magnetic-activated cell sorting. However, these conventional approaches involve exerting mechanical forces on the cells, thus risking cell damage. In this study we applied a novel isolation method called buoyancy-activated cell sorting, which involves using biotinylated
albumin microbubbles (
biotin-MBs) conjugated with
antibodies (i.e., targeted
biotin-MBs).
Albumin MBs are widely used as
contrast agents in ultrasound imaging due to their good biocompatibility and stability. For conjugating
antibodies,
biotin is conjugated onto the
albumin MB shell via covalent bonds and the biotinylated
antibodies are conjugated using an
avidin-
biotin system. The
albumin microbubbles had a mean diameter of 2 μm with a polydispersity index of 0.16. For cell separation, the MDA-MB-231 cells are incubated with the targeted
biotin-MBs conjugated with anti-CD44 for 10 min, centrifuged
at 10 g for 1 min, and then allowed 1 hour at 4 °C for separation. The results indicate that targeted
biotin-MBs conjugated with anti-CD44
antibodies can be used to separate MDA-MB-231
breast cancer cells; more than 90% of the cells were collected in the MB layer when the ratio of the MBs to cells was higher than 70:1. Furthermore, we found that the separating efficiency was higher for targeted
biotin-MBs than for targeted
avidin-incorporated
albumin MBs (
avidin-MBs), which is the most common way to make targeted
albumin MBs. We also demonstrated that the recovery rate of targeted
biotin-MBs was up to 88% and the sorting purity was higher than 84% for a a heterogenous cell population containing MDA-MB-231 cells (CD44(+)) and MDA-MB-453 cells (CD44-), which are classified as basal-like
breast cancer cells and
luminal breast cancer cells, respectively. Knowing that the CD44(+) is a commonly used cancer-stem-cell
biomarker, our targeted
biotin-MBs could be a potent tool to sort cancer stem cells from dissected
tumor tissue for use in preclinical experiments and clinical trials.