Fig. 1.

Establishment of immunity during epidermal development. Early in embryonic development, the epidermis is specified as a single layer of progenitors that stratify to give rise to the differentiated superbasal layers. The outermost layer of the epidermis is known as the stratum corneum and provides a watertight barrier. LCs and DETCs colonize prior to birth and rely on local survival factors (IL-34 and IL-7/IL-15, respectively) produced by adjacent keratinocytes. Within moments of birth, the skin is colonized by commensal microbes, which inhabit the surface of the skin and invaginations like hair follicles. Commensal colonization induces CCL20-mediated migration of commensal-specific CCR6⁺ Tregs into neonatal skin.