Mycoplasma-mediated alterations of in vitro generation and functions of human dendritic cells

While tumor cell-derived factors have been demonstrated to hamper the in vitro differentiation and maturation of dendritic cells (DCs) from hematopoietic stem cells, their effects on DC differentiation from CD14+ plastic-adherent monocytic precursors have been controversial. To address this issue, we examined the effects of the culture supernatants from six tumor cell lines on in vitro DC differentiation and maturation from monocytes. Two tumor cell supernatants, MDA468 and 293T, were found to be able to affect the in vitro differentiation of DCs from monocytic precursors, leading to the generation of a distinct type of DC with markedly reduced expression of DC-SIGN, downregulation of CD11c, HLA-DR and CD1a, and upregulation of CD123, HLA-ABC, CD80, CD40, CD86, CD54, CD83, CD25 and CCR7. Functionally, these DCs exhibited reduced phagocytosis and enhanced allostimulatory capacity. Further investigation demonstrated that the changes in DC phenotype and functions were due to the presence of mycoplasmas in these two cell lines; eradication of mycoplasmas completely abolished the observed effects, and importantly, pure mycoplasmas in the absence of tumor cell supernatants were able to produce the same effects. Since mycoplasmas are common contamination agents in routine tissue culture, our results caution that many reported effects of DCs in culture warrant re-evaluation. The distinct effects of mycoplasmas on DC differentiation described in this report could potentially benefit future development of DC-based vaccination and therapeutic applications.