Commensal Myeloid Crosstalk in Neonatal Skin Regulates Long-term Cutaneous Type 17 Inflammation

Early life immune interactions help shape longer-term skin health and homeostasis. Commensal microbes facilitate neonatal skin accumulation of innate and adaptive T cells, thereby promoting fundamental needs such as immune tolerance to commensals and wound healing. Comparatively little is known about commensal-myeloid cell crosstalk in neonatal skin and the functional consequences of these interactions. Using CyTOF and gnotobiotic mouse models we observed a population of classical monocytes uniquely enriched in the skin of microbially replete neonates. Corroborative studies revealed that skin monocytes rapidly accumulate between D1 and D3 of life, after which their numbers gradually decline. This early monocyte wave was prevented in antibiotic-treated SPF pups as well as in Myd88^-/- but not IL1R1^-/- mice, suggesting a key role for tonic TLR signaling in their accumulation. To dissect the functional relevance of these cells in cutaneous biology, we developed an antibody-based regimen to temporarily deplete monocytes in the first two weeks of life (NeoΔmono). scRNA sequencing revealed a heightened type 17 signature in D15 NeoΔmono skin T cells. Flow cytometry assays confirmed sustained elevation of IL-17A production by these cells through adulthood. This reflected a heightened response to commensals as IL-17 production was significantly reduced in antibiotic-treated NeoΔmono mice. While there was no visible skin pathology in NeoΔmono mice under homeostatic conditions, imiquimod treatment of the ears in adulthood led to significantly increased ear swelling and neutrophils. Taken together, our data demonstrate a previously unappreciated, commensal-dependent regulatory imprinting function of cutaneous classical monocytes in the early life window.