The adult human skin harbors a variety of leukocytes providing immune surveillance and host defense, but knowledge about their ontogeny is scarce. In this study we investigated the number and phenotype of leukocytes in prenatal human skin (dermal dendritic cells (DDCs), macrophages, T cells (including FoxP3 + regulatory T cells), and mast cells) to unravel their derivation and to get a clue as to their putative function in utero . By flow cytometry and immunofluorescence, we found a distinction between CD206 + CD1c + CD11c + DDCs and CD206 + CD209 + CD1c − skin macrophages by 9 weeks estimated gestational age (EGA). T cells appear at the end of the first trimester, expressing CD3 intracytoplasmatically. During midgestation, CD3 + FoxP3 − and CD3 + FoxP3 + cells can exclusively be found in the dermis. Similarly, other leukocytes such as CD117 + (c-kit) mast cells were not identified before 12–14 weeks EGA and only slowly acquire a mature phenotype during gestation. Our data show at which time point during gestation antigen-presenting cells, T cells, and mast cells populate the human dermis and provide a step forward to a better understanding of the development of the human skin immune system.

Toll-like receptors (TLRs) initiate innate immune responses and direct subsequent adaptive immunity. They play a major role in cutaneous host defense against micro-organisms and in the pathophysiology of several inflammatory skin diseases. To understand the role of TLRs in the acquisition of immunological competence, we conducted a comprehensive study to evaluate TLR expression and function in the developing human skin before and after birth and compared it with adults. We found that prenatal skin already expresses the same spectrum of TLRs as adult skin. Strikingly, many TLRs were significantly higher expressed in prenatal (TLRs 1-5) and infant and child (TLRs 1 and 3) skin than in adult skin. Surprisingly, neither dendritic cell precursors in prenatal skin nor epidermal Langerhans cells and dermal dendritic cells in adult skin expressed TLRs 3 and 6, whereas the staining pattern and intensity of both TLRs in fetal basal keratinocytes was almost comparable to those of adults. Stimulation of primary human keratinocytes from fetal, neonatal and adult donors with selected TLR agonists revealed that the synthetic TLR3 ligand poly (I:C) specifically, mimicking viral double-stranded RNA, induced a significantly enhanced secretion of CXCL8/IL8, CXCL10/IP-10 and TNFα in fetal and neonatal keratinocytes compared with adult keratinocytes. This study demonstrates quantitative age-specific modifications in TLR expression and innate skin immune reactivity in response to TLR activation. Thus, antiviral innate immunity already in prenatal skin may contribute to protect the developing human body from viral infections in utero in a scenario where the adaptive immune system is not yet fully functional.

Atopic dermatitis arises primarily in early infancy. In these patients, corticosteroids are used especially with great caution because of their side effects. Calcineurin inhibitors such as pimecrolimus (PIM) could be useful, but safety concerns have been raised in particular because of the lack of knowledge about their effects on the developing skin immune system. This study was designed to investigate the impact of PIM and corticosteroids on epidermal cells (EC) in infants and newborn mice. We found that the percentage of unfractionated viable infant ECs was significantly decreased in the presence of beta-methasone-17-valerate (BMV) but not PIM. Exposure of unfractionated infant ECs to BMV but not to PIM and vehicle control caused a significant inhibition of the upregulation of CD86 molecules on Langerhans cells (LC). The release of cytokines by LCs and ECs, cultured in the presence of BMV and PIM, was not significantly reduced compared with controls. Topical corticosteroid but not PIM application onto newborn mice induced apoptosis in some LC precursors. Our data suggest that similar to the situation in adult skin, corticosteroids may impair LC maturation as well as viability of ECs in infants, effects not seen with PIM.

Despite intense efforts, the exact phenotype of the epidermal Langerhans cell (LC) precursors during human ontogeny has not been determined yet. These elusive precursors are believed to migrate into the embryonic skin and to express primitive surface markers, including CD36, but not typical LC markers such as CD1a, CD1c and CD207. The aim of this study was to further characterize the phenotype of LC precursors in human embryonic epidermis and to compare it with that of LCs in healthy adult skin. We found that epidermal leukocytes in first trimester human skin are negative for CD34 and heterogeneous with regard to the expression of CD1c, CD14 and CD36, thus contrasting the phenotypic uniformity of epidermal LCs in adult skin. These data indicate that LC precursors colonize the developing epidermis in an undifferentiated state, where they acquire the definitive LC marker profile with time. Using a human three-dimensional full-thickness skin model to mimic in vivo LC development, we found that FACS-sorted, CD207-cord bloodderived haematopoietic precursor cells resembling foetal LC precursors but not CD14 + CD16-blood monocytes integrate into skin equivalents, and without additional exogenous cytokines give rise to cells that morphologically and phenotypically resemble LCs. Overall, it appears that CD14 -haematopoietic precursors possess a much higher differentiation potential than CD14 + precursor cells.

RNA aptamer provides a novel approach for the induction of apoptosis by targeting the HPV16 E7 oncoprotein. PLoS One 8: e64781 Reyes-Reyes EM, Teng Y, Bates PJ (2010) A new paradigm for aptamer therapeutic AS1411 action: uptake by macropinocytosis and its stimulation by a nucleolin-dependent mechanism. Cancer Res 70:8617-29 Wittmann M, Doble R, Bachmann M et al. (2012) IL-27 regulates IL-18 binding protein in skin resident cells. PLoS One 7: e38751 Zhou J, Bobbin ML, Burnett JC et al. (2012) Current progress of RNA aptamer-based therapeutics.