FoxP3 Expression in Macrophages, Cancer, and B Cells-Is It Real?

During the last decade, B regulatory cells are appreciated to have a central role in preventing autoimmunity and maintaining self-tolerance. They are characterized by expressing different phenotypic markers and the production of either IL-10 or TGF-β or both. The recent recognition of Fas ligand expressing B regulatory cells as "killer" cells established their role in maintaining viral persistence by preventing effective antiviral immune responses. The forkhead lineage-transcription factor (FoxP3) was considered for many years to be a highly specific intracellular regulatory marker of CD4+CD25+ T regulatory cells. The possibility of FoxP3 being expressed in B regulatory cells was suggested in many studies. Though controversial, FoxP3 expression was also reported in macrophages and cancer cells. Aiming to avoid artifact staining, many researchers required the usage of FoxP3 messenger RNA (mRNA) and PCR in order to prove a true expression of FoxP3 in these different cells. In addition, most studies' report on that FoxP3 expression in all abovementioned cells is related to their status of activation since naïve (non-activated cells) were found poorly FoxP3 expressing. In this review, we present the existing data on FoxP3 expression in non-T-regulatory cells, but we suggest that further studies are needed to better establish this concept.