Preprint Club
A cross-institutional Journal Club Initiative

Keywords

• barrier immunology

• myeloid cell heterogeneity

• host-microbiota interactions

Main Findings

In this preprint, the authors investigate the immune adaptations during the early life of new born mice. An initial characterization using CyTOF reveals a detailed unbiased map of the immune landscape of the developing skin. The authors observe an increase in monocytes in the skin of mice from day 1 to day 15 and reason to determine the effects of monocyte depletion on the skin immune system during this period. To determine if monocyte accumulation in the early life skin requires the microbiota or signalling through Myd88, the author analyse germ-free mice, antibiotic treated mice and total body Myd88-/- mice. All of these mice fail show an accumulation of monocytes during the early life, suggesting an instructive role for monocytes accumulation. The authors then reason to investigate the effects of monocyte depletion on the developing skin using multiple antibody-mediated deletion strategies to successfully deplete skin monocytes during the early life (anti-Gr1, anti-CCR2). Monocyte-specificity is validated using anti-Ly6G antibodies. Following the successful deletion of monocytes in the skin of mice during early life, the authors characterize the immune landscape of the skin using scRNA-Seq. Their analysis reveals an unexpected elevation in IL-17 producing T cell subsets that affects ab T cells and gd T cells. To determine is the elevated IL-17 response following monocyte depletion is driven by the skin microbiota, the authors use their monocyte-deletion  strategy in conjunction with topical antibiotic application on the skin. Their results demonstrate that the local skin microbiota is needed to promote IL-17 producing T cells in the absence of early life skin monocytes.

The authors then employ the use of the imiquimod-driven psoriasis model to determine if mice lacking monocytes during the early life are more susceptible to the development of chronic skin inflammation. In line with their observations of elevated IL-17 producing T cells in monocyte-depleted mice, the authors observe an exacerbated degree of skin pathology in mice lacking early life monocytes. This confirms their observations that early life skin monocytes restrain IL-17 production in the skin (and the skin-draining lymph nodes). Using scRNA-Seq data the authors determine the expression of gene related to IL-1 signalling and observe that monocytes and macrophages in the early life skin express high levels of Il1r1n and Il1r2 allowing them to sequester bioactive IL-1 in the skin to prevent the activation of T cells via IL-1. They experimentally confirmed this hypothesis using the application of IL-1RA into mice depleted of skin monocytes or by genetically abrogating IL-1R signalling in CD4+ T cells. In both settings the authors observe that monocyte-depletion fails to promote IL-17 production in T cells, suggesting that early life monocytes confer innate immune tolerance through sequestering of bioactive IL-1, to prevent IL-17 production by T cells.

Limitations

• A central question remaining after their observation is the role of fungal commensals in the skin. Is their selected antibiotic treatment affecting only bacteria? Would the antibiotic treatment used by the authors permit the blooming of skin-resident fungal commensals?

• In line with the question above, a description of the microbiota in the presence of absence of early life monocytes could give hints a potential drives of IL-1 following colonization of the skin.

• It would be helpful to determine if the expression of Il1rnand Il1r2 is dependent on T cells (IL-17 producing T cells or Tregs). A possible involvement of microbiota in the expression of these genes would be very supportive too.

• The authors observations raise the question for differences in monocyte ontogeny. Determining if fetal monocytes and adult monocytes infiltrating the skin differ in their gene expression profile could explain why the observed suppression of IL-17 applies to a fairly small window of time during early life. Could sustaining fetal-derived monocytes/macrophages possibly via IL-34 or CSF1 modify this early life window?

• A kinetic of Il1rn and Il1r2 expression in skin monocytes and the different macrophage subsets the authors define via scRNA-Seq would be helpful to strengthen their points. Would it be possible to visualize Il1rn-expressing monocytes of the first 4 weeks of life in the skin? It could be interesting to see if these cells co-localize with T cells in the skin.

• Does the mode of delivery matter for the observations in the early life skin? Vaginal vs caesarean delivery could be compared or discussed in this case.

• Is there are role for IL-17 in suppressing the IL-1 sequestering functions of monocytes in the early skin?

• Does the absence of Treg-mediated tolerance via cDCs contribute to the authors observations on early life monocytes? Could early life monocytes compensate for Treg defects in the skin?

Significance/Novelty

What is the novelty of the preprint for the field specific?

The authors delineate a previously unknown role for monocytes in the early life immune responses towards commensal microbes of the skin. Their findings suggest that monocytes and macrophages are functionally distinct in the early life skin when compared to the “adult” skin. Early life skin monocytes and macrophages actively contribute to the suppression of inflammatory IL-17 responses in the skin by sequestering or dampening IL-1 signalling, a key trigger of chronic inflammatory skin diseases. These findings are another important puzzle piece towards a better understanding of how monocyte and macrophage function adapts within tissues and sustains immune homeostasis. The authors’ work provokes many new questions and inspires new approaches to fully delineate how early life innate immune tolerance in the skin operates.

How does the result of the preprint matter for general immunologists and/or patients?

This work reveals a previously underappreciated mechanism on how early life innate immune cells suppress a microbiota-driven inflammatory response in the skin. The identification of several genes and immune cells subsets involved in this pathway make this report an attractive inspiration for future investigation of pharmaceuticals and potentially commensal microbes to prevent the development of chronic skin inflammation.

Credit

Reviewed by Arthur Mortha (University of Toronto, Department of Immunology) as part of a cross-institutional journal club between the Icahn School of Medicine at Mount Sinai, the University of Oxford, the Karolinska Institute and the University of Toronto.

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