Preprint Club
A cross-institutional Journal Club Initiative

Keywords

• Skin immunity

• Langerhans cells

• Monocyte differentiation

• Bone Marrow Transplant

Main Findings

Langerhans cells (LC) are mononuclear phagocytes of the epidermis characterized by expression of Langerin and EpCAM. Despite being a tissue resident macrophage population, they adopt dendritic cell like functions such as migration out of the skin to present antigen to T cells. Embryonic derived-LCs (eLC) develop from macrophage precursors and proliferate in the skin throughout life independently of the adult circulation. Numbers of LC are sparse at birth and they proliferate within the first period of life, however, it remains to be clarified which signals shape eLC function and identity.

In a mouse model of GVHD, eLCs are destructed through T cell dependent mechanisms after bone marrow transplantation and epidermal monocytes are replenishing the LC compartment (monocyte-derived LC (mLCs)), being transcriptionally indistinguishable from eLCs. However, it remained unclear how fate decision of monocytes is made either into the direction of macrophages or mLCs. Appios, Daview, Sirvent et al. performed single-cell RNA sequencing (scRNA-seq) on sorted CD11b+MHCII+ cells of the GVHD mice. By clustering and trajectory analysis they observed a highly heterogeneous monocyte compartment. They identified MDP-derived monocytes to be the most likely progenitors of mLCs, which was also validated by in vitro cultures and flow cytometry.

Next, the authors characterized the keratinocyte compartment by scRNA-seq, flow cytometry and immunofluorescence stainings and could show that follicular keratinocytes attract MDP-derived monocytes to the hair follicles. Notch signalling via the ligands Jag1 and Jag2 within the hair follicle niche causes loss of the transcription factor Zeb2 and upregulation of Id2 and Ahr, thereby promoting the development of monocytic precursors into mLCs. These findings were validated by in vitro mLC differentiation in presence or absence of Notch ligand Jag1 and Ahr agonist FICZ and subsequent bulk RNA-seq.

Finally, the authors perform bulk RNA-seq on sorted LC in a human cohort of newborns, infants and children to unravel the natural development of eLCs. They find that the natural development of eLCs transcriptionally resembles the development of monocytes to mLCs in mice previously described in the paper.

Limitations

• There are some inconsistencies in Figure labeling or their referencing in the text (specifically Fig 5F and Fig 4J)

• The data about the metabolic adaptation of resident mLC (Fig 3) seems a bit disconnected from the rest of the story, therefore it could perhaps be moved to the supplementary.

• In contrast to other figures derived from scRNA-seq data in the manuscript, Fig 3 lacks validation data and the conclusions made are speculative. The authors should make clear that the data analysed with COMPASS in Fig 3B shows the relative preference of pathways based on the transcriptional profile, which does not ensure that Mrc1+ mac are not equally adapting to their tissue environment.

• These hypotheses about metabolic adaptation could be validated in vitro by differentiating mLC in the presence of certain energy sources and metabolites in the culture medium as well as metabolomics experiments.

• Even though the most important subsets of the bulk RNA data from in vitro cultures (Fig 6) were analyzed in triplicates, for some subsets there is only one or two replicates, respectively. Including more replicates here would increase the reliability of the data. At least, it should be clearly indicated if certain samples were omitted by which criteria they were excluded from the study.

• In the manuscript it was nicely shown that Notch signaling and particularly Jag1 play an essential role in mLC differentiation in vitro. It would be interesting to additionally investigate Notch signaling pathways and Jag1 in vivo, especially as other Notch ligands as DL4 seem to have quite contrary functions. Therefore, we would suggest to block Notch signaling in vivo, either by treating mice with inhibitors or by generating mice deficient for specific components of the Notch signaling cascade.

Significance/Novelty

The preprint sheds light on previously unknown signalling pathways of differentiation of monocytes into LCs. The article assesses mainly mLC development in the context of bone marrow transplantation in mice, but also shows some unique insights into physiological eLC development in children. The findings of this article are not directly applicable to treat human patients but are rather of interest to fundamental researchers.

Credit

Reviewed by Javier Marchena-Hurtado and Katja Knapp as part of a cross-institutional journal club between the Vanderbilt University Medical Center (VUMC), the Max-Delbrück Center Berlin, the Medical University of Vienna and other life science institutes in Vienna.

The author declares no conflict of interests in relation to their involvement in the review.