L-Myc expression by dendritic cells is required for optimal T-cell priming

The transcription factors c-Myc and N-Myc encoded by Myc and Mycn, respectively, regulate cellular growth(1) and are required for embryonic development(2,3). A third paralog, Mycl1, is dispensable for normal embryonic development but its normal biologic function has remained unclear(4). To examine the in vivo function of Mycl1, we generated an inactivating Mycl1(gfp) allele that also reports Mycl1 expression. We found that Mycl1 was selectively expressed in dendritic cells (DCs) of the immune system and controlled by IRF8, and that during DC development, Mycl1 expression was initiated in the common DC progenitor(5) (CDP) concurrent with reduction in c-Myc expression. Mature DCs lacked expression of c-Myc and N-Myc, but maintained L-Myc expression even in the presence of inflammatory signals, such as GM-CSF. All DC subsets developed in Mycl1-deficient mice, but several DC subsets, such as migratory CD103(+) cDCs in the lung and liver, were significantly reduced at steady state. Importantly, loss of L-Myc by DCs caused a significant decrease in the in vivo T-cell priming during infection by Listeria monocytogenes and vesicular stomatitis virus. The replacement of c-Myc by L-Myc in immature DCs may provide for Myc transcriptional activity in the setting of inflammation that is required for optimal T-cell priming(6).