Multipotent stromal cells skew monocytes towards an anti-inflammatory interleukin-10-producing phenotype by production of interleukin-6

Multipotent stromal cells have immunomodulatory capacities and have been used in transplantation and autoimmune diseases. One of the effects of multipotent stromal cells involves the inhibition of dendritic cell differentiation. Since interleukin-6 and interleukin-10 are known to play a role in inhibiting immature dendritic cell differentiation, we hypothesized that these cytokines may also mediate the inhibitory effect of human multipotent stromal cells in immature dendritic cell differentiation. In order to test this hypothesis monocytes were cultured with interleukin-4 and granulocyte-monocyte colony-stimulating factor in the presence or absence of culture-expanded bone marrow-derived multipotent stromal cells. Neutralization and cytokine-depletion strategies were applied to reveal the cellular source and effect of interleukin-6 and interleukin-10. Addition of multipotent stromal cells to monocyte cultures significantly reduced the generation of immature dendritic cells (CD14⁻CD1a⁺) and resulted in the generation of CD14⁺CD1a⁻ cells that displayed a significantly reduced immunostimulatory effect. We found that culture supernatants of co-cultures of multipotent stromal cells and monocytes contained higher concentrations of interleukin-6 and interleukin-10. Multipotent stromal cells produced interleukin-6 and neutralizing this interleukin-6 reversed the inhibitory effect of the multipotent cells. Interleukin-10 was not produced by multipotent stromal cells, but exclusively by monocytes after exposure to multipotent stromal cell-produced interleukin-6. In conclusion, through constitutive production of interleukin-6, multipotent stromal cells prevent the differentiation of monocytes towards antigen-presenting immunogenic cells and skew differentiation towards an anti-inflammatory interleukin-10-producing cell type.