Differentiation-dependent expression of interferon gamma and toll-like receptor 9 in 3T3-F442A adipocytes

3T3-F442A and BFC-1 cells are widely used for studying adipocyte differentiation and metabolism. Macrophage markers were previously reported in these cell lines. We examined whether 3T3-F442A and BFC-1 would produce interferon-γ (IFN-γ), the expression of which is a matter of debate in cells other than T-lymphocytes and natural killer cells, like macrophages or dendritic. IFN-γ was absent from preadipocytes. However 3T3-F442A, but not BFC-1, presented a differentiation-dependent induction of IFN-γ mRNA and protein. Immunofluorescence studies showed that IFN-γ was located in mature adipocytes. IFN-γ was retrieved in the culture medium. Then, we examined the expression of other markers of T-lymphocytes or macrophages, like the CD3/T-cell receptor complex or Toll-like receptors (TLR) -2 and -9, in these cells. Transcripts for the three subunits of CD3 were undetectable whatever the differentiation stage. In contrast, TLR-2 and -9 genes were expressed differentially during the differentiation process. TLR-2 mRNA was induced early then decreased while TLR9 transcript appeared at later days and increased in parallel to IFN-γ. In contrast to what was expected from 3T3-F442A cells, IFN-γ was absent from adipocytes isolated either from subcutaneous or periepidydimal mouse adipose tissue. However, TLR-2 and -9 mRNAs were present in both adipose depots although at various levels. Hence, we detect the presence of two markers of innate immunity, TLR-2 and -9, in in vivo-derived adipocytes and we demonstrate that differentiated 3T3-F442A cells selectively express IFN-γ and TLR-9 in a manner that resembles what is occurring for natural killer dendritic cells.