Human nucleus pulposus cells significantly enhanced biological properties in a coculture system with direct cell-to-cell contact with autologous mesenchymal stem cells.

Activated nucleus pulposus (NP) cells can be reinserted into the disc to inhibit intervertebral disc degeneration. Experimental studies in animals showed that using a coculture system with direct cell-to-cell contact with mesenchymal stem cells (MSCs) significantly upregulated the biological activity of NP cells. The purpose of this study is to determine whether this activation of NP cells by autologous MSCs is applicable to human cells in vitro. Human NP tissue was obtained from surgical specimens and MSCs from bone marrow of 10 subjects. Six-well culture plates and inserts were used for culture; 1.0x10(4) NP cells were seeded onto each insert and incubated alone, in standard coculture with 1.0x10(4) MSCs, or cocultured with direct cell-to-cell contact. NP cell proliferation, DNA synthesis, and proteoglycan (PG) synthesis were evaluated. Chromosome abnormalities in the activated NP cells and tumorigenesis of the cells were evaluated in an additional 10 patients by microscopic examination for segmented cells and histological assessment of activated cells transplanted into nude mice. Cell proliferation, DNA synthesis, and PG synthesis were significantly upregulated. The positive effects of the coculture system with direct cell-to-cell contact seen in animal studies were also confirmed in human cells. Chromosome abnormalities and tumorigenesis were not observed in the activated NP cells. In conclusion, a coculture system with direct cell-to-cell contact demonstrated a significant positive effect, enhancing the biological properties of human NP cells, as it did in animal models. These results should prove useful for conducting trials leading to the clinical use of activated NP cell transplantation.