Microbubbles as used for contrast-enhanced ultrasound affect the migration of human primary leukocytes

Abstract

Poly n-butylcyanoacrylate (PBCA) microbubbles are used for a wide variety of biomedical applications, such as targeted delivery of drugs or as contrast agents for ultrasound imaging of the liver. Microbubbles are generally considered as non-toxic and immunologically inert. Up to now, possible effects of PBCA microbubbles on immune cells are unclear. Here, we studied the effects of the microspheres on human primary leukocytes with a focus on cell migration and differentiation. Microbubbles, which had been labeled with rhodamine for fluorescent detection, did not cause cytotoxic effects upon incubation with primary human leukocytes. Interestingly, microbubbles significantly inhibited the migration of lymphocytes and monocytes, whereas the migration of granulocytes and mature macrophages remained unaffected. Flow cytometric studies were performed to elucidate the cellular uptake of the microbubbles by immune cells, and demonstrated that microbubbles were primarily cleared by monocytes in a dose-dependent manner. This internalization was energy-dependent, since it was blocked upon incubation at 4 °C. Realtime-PCR revealed that microbubbles did not affect inflammatory mediator gene expression by human primary macrophages, even at up to seven days of incubation. Our findings demonstrate a cell-specific inhibition of migration of peripheral blood mononuclear cell subsets by the micro-sized constructs, indicating potential immunological side-effects of PBCA microbubbles in patients with inflammatory liver diseases.