Interactions between monocytes, mesenchymal stem cells, and implants evaluated using flow cytometry and gene expression.

Monocytes and mesenchymal stem cells (MSC) are evident at the implants during early healing. However, when coexisting, their interactions at different implants have not been determined. This study uses an in vitro system, consisting of monoculture and direct co-cultures of monocytes and MSC on screw-shaped machined and oxidized titanium implants in combination with scanning electron microscopy, enzyme-linked immunosorbent assay, flow cytometry, cell sorting, and quantitative polymerase chain reaction. The cell-specific adhesion and gene expression of monocytes and MSC was determined. After 24 hr, the coexistence of monocytes and MSC in co-culture led to equal proportions of adherent monocytes and MSC, irrespective of the implant type. In contrast, higher number of adherent monocytes than MSC was found on the oxidized implant in monoculture. Quantitative polymerase chain reaction analysis of fluorescent activated cell sorting-sorted cells revealed up-regulation of interleukin-1beta, in monocytes, and interleukin-1beta and C-X-C chemokine receptor type 4, in MSC, when the cell types coexisted compared with monocultures. Further, in co-culture, the expression of bone morphogenetic protein-2, stromal cell-derived factor 1, and integrin-β1 was enhanced in the implant-adherent MSC, but not monocytes. It is concluded that during the first 24 hr in an in vitro static condition, the effect of co-culture of monocytes and MSC was more prominent than the effect of the implant surface properties. The results indicate that the coexistence of monocytes and MSC on an implant alters the adhesion and expression of some genes compared with when each cell type existed alone. Further, the results show that the gene expression of major growth and recruitment factors is mainly enhanced in the implant-adherent MSC in contrast to implant-adherent monocytes in co-culture.