Development of human plasmacytoid dendritic cells depends on the combined action of the basic helix‐loop‐helix factor E2‐2 and the Ets factor Spi‐B

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Abstract
Plasmacytoid dendritic cells (pDC) are central players in the innate and adaptive immune response against viral infections. The molecular mechanism that underlies pDC development from progenitor cells is only beginning to be elucidated. Previously, we reported that the Ets factor Spi‐B and the inhibitors of DNA binding protein 2 (Id2) or Id3, which antagonize E‐protein activity, are crucially involved in promoting or impairing pDC development, respectively. Here we show that the basic helix‐loop‐helix protein E2‐2 is predominantly expressed in pDC, but not in their progenitor cells or conventional DC. Forced expression of E2‐2 in progenitor cells stimulated pDC development. Conversely, inhibition of E2‐2 expression by RNA interference impaired the generation of pDC suggesting a key role of E2‐2 in development of these cells. Notably, Spi‐B was unable to overcome the Id2 enforced block in pDC development and moreover Spi‐B transduced pDC expressed reduced Id2 levels. This might indicate that Spi‐B contributes to pDC development by promoting E2‐2 activity. Consistent with notion, simultaneous overexpression of E2‐2 and Spi‐B in progenitor cells further stimulated pDC development. Together our results provide additional insight into the transcriptional network controlling pDC development as evidenced by the joint venture of E2‐2 and Spi‐B.