The appreciation that tissue-derived CD103+ DCs in mice, and BDCA3hi DCs in humans, appear to be functionally

and developmentally very closely related to CD8+ DCs, but do not express CD8, has recently lead to the proposal to define this lineage of DCs by their expression of XCR1 [5, 6], a chemokine receptor that is conserved between the different DC subsets and across the species. In Enzalutamide molecular weight addition to this proposed DC lineage, DCs expressing high levels of surface CD11b appear to be functionally biased toward promoting MHC class II-restricted CD4+ T-cell responses [7]. However, only a proportion of splenic CD11bhi DCs express CD4, and tissue-resident CD11bhi DCs are characterized by CD205 expression rather than CD4 [8]. Consequently, the cohort of CD11bhi DCs appears considerably more heterogeneous compared with the relatively well-defined CD8+/XCR1+ lineage [4, 9]. This view is supported by the diverse range of transcription factors and molecules that have been implicated in the development of CD11bhi DCs [10]. Interestingly, it

was recently shown that differential JQ1 ic50 requirement for Notch 2 receptor signaling defines two distinct lineages within the CD11bhi DC population [11]. The Notch 2 receptor signaling-dependent CD11bhi DC population is characterized by high-level expression of ESAM, an immunoglobulin superfamily molecule previously associated with neutrophil extravasation [12], and ESAMhi CD11bhi DC have been described as potent inducers of CD4+

T-cell priming [11]. Conversely, ESAMlo CD11bhi DCs develop independently Methane monooxygenase of Notch 2 receptor signaling and have a gene expression signature resembling that of monocytes [11]. However, exactly how ESAMhi and ESAMlo CD11bhi DCs diverge during development and what factors control Notch 2 receptor signaling in CD11bhi DCs remains obscure. In this issue of the European Journal of Immunology, Beijer et al. [13] have described an unexpected role for vitamin A in promoting the development of these newly described ESAMhi CD11bhi DCs within the spleen. Vitamin A, or retinol, is acquired through dietary intake and stored predominantly within the liver before release into the circulation. Upon conversion of circulating vitamin A into its active metabolite retinoic acid (RA) by retinaldehyde dehydrogenase (Raldh), RA acts as a transcriptional regulator, binding retinoic acid receptors (RAR), and retinoic X receptors (RXR) that are located in the nucleus. The binding of RA to RAR/RXR heterodimers facilitates the recruitment of coactivators and the formation of transcriptional complexes that dock onto RA response elements within the regulatory regions of target genes, which in turn initiates transcription [14]. Vitamin A has long been appreciated for its essential role in host immunity, and more recently has gained considerable attention as a major player in controlling intestinal immunity [15].