Local delivery of lipid-based nanoparticles containing microbial nucleic acid for osteoimmunomodulation

Osteoimmunomodulation is a strategy to promote bone regeneration in implants by modifying the immune environment. CpG-containing oligonucleotides type C (CpG ODN C) and Polyinosinic:polycytidylic acid (Poly[I:C]) are analogs of microbial nucleic acids that have been studied for various immunotherapeutic applications. This research investigates the potential of CpG ODN C and Poly(I:C) as an osteoimmunomodulatory agent for bone regenerative purposes. We encapsulated each nucleic acid in a lipid-based nanoparticle to facilitate the delivery into intracellular pathogen recognition receptors in immune cells. The lipid-based nanoparticles were ±250 nm in size with a negative charge (-36 to -40 mV) and an encapsulation efficiency of ±60 %. Lipid-based nanoparticles containing nucleic acids, Lip/CpG ODN C and Lip/Poly(I:C), increased the production of TNF, IL-6, and IL-10 by primary human macrophages compared to free-form nucleic acids. Conditioned medium from macrophages treated with CpG ODN C (10 µg/ml) and Lip/CpG ODN C (0.1, 1, and 10 µg/ml) promoted osteoblast differentiation of human mesenchymal stromal cells by 2.6-fold and 3-fold, respectively; no effect was seen for Lip/Poly(I:C). Bone implants were prepared, consisting of a biphasic calcium phosphate scaffold, bone morphogenetic protein (BMP) 2, and lipid-based nanoparticles suspended in gelatin methacryloyl (GelMA) hydrogel. Implants were evaluated for de novo bone formation in an extra-skeletal implantation model in rabbits for 5 weeks. Based on the particles suspended in GelMA, six groups of implants were prepared: Lip/CpG ODN C, Lip/Poly(I:C), Lip (empty), CpG ODN C, Poly(I:C), and a control group consisting of empty GelMA. After 5 weeks, healthy bone tissue formed in all of the implants with active osteoblast and osteoclast activity, however, the amount of new bone volume and scaffold degradation were similar for all implants. We suggest that the working concentrations of the nucleic acids employed were inadequate to induce a relevant inflammatory response. Additionally, the dosage of BMP-2 used may potentially mask the immune-stimulatory effect. Lip/CpG ODN C holds potential as a bioactive agent for osteoimmunomodulation, although further in vivo demonstration should corroborate the current in vitro findings.