Langerhans cell (LC) proliferation mediates neonatal development, homeostasis, and inflammation-associated expansion of the epidermal LC network

Most tissues develop from stem cells and precursors that undergo differentiation as their proliferative potential decreases. Mature differentiated cells rarely proliferate and are replaced at the end of their life by new cells derived from precursors. Langerhans cells (LCs) of the epidermis, although of myeloid origin, were shown to renew in tissues independently from the bone marrow, suggesting the existence of a dermal or epidermal progenitor. We investigated the mechanisms involved in LC development and homeostasis. We observed that a single wave of LC precursors was recruited in the epidermis of mice around embryonic day 18 and acquired a dendritic morphology, major histocompatibility complex II, CD11c, and langerin expression immediately after birth. Langerin+ cells then undergo a massive burst of proliferation between postnatal day 2 (P2) and P7, expanding their numbers by 10–20-fold. After the first week of life, we observed low-level proliferation of langerin+ cells within the epidermis. However, in a mouse model of atopic dermatitis (AD), a keratinocyte signal triggered increased epidermal LC proliferation. Similar findings were observed in epidermis from human patients with AD. Therefore, proliferation of differentiated resident cells represents an alternative pathway for development in the newborn, homeostasis, and expansion in adults of selected myeloid cell populations such as LCs. This mechanism may be relevant in locations where leukocyte trafficking is limited

Lack of IL-2 in PPAR-alpha deficient mice triggers allergic contact dermatitis by affecting regulatory T cells

The aim of the present work was to decipher the cellular basis of the immuno-regulatory role of peroxisome proliferator-activated receptor (PPAR)-alpha in cutaneous hypersensitivity reactions. After challenge with a contact allergen, we observed augmented hypersensitivity reactions with increased numbers of activated T-lymphocytes in the skin of PPAR-alpha(-/-) mice. Furthermore, following antigen challenge, the percentages of regulatory T-cells (Tregs) in the blood, draining lymph nodes and skin were decreased in these mice. PPAR-alpha deficiency impaired the production of IL-2 in lymph nodes, whereas TGF-beta levels remained unchanged. Injection of IL-2 into PPAR-alpha(-/-) mice restored the Treg population in the skin draining lymph nodes of allergen challenged mice. In vivo induction of Tregs from wild type CD4(+) CD25(-) T-cells was impaired when adoptively transferred into PPAR-alpha(-/-) mice as compared with transfer into wild type mice, and reversed by injection of IL-2 into PPAR-alpha(-/-) mice. Furthermore, the suppressive capacity of PPAR-alpha(-/-) Tregs was impaired when compared to wild type Tregs in vitro and in co-adoptive transfer experiments. Finally, injection of IL-2 to PPAR-alpha(-/-) mice decreased skin inflammation to a level similar to wild type mice. In conclusion, the pro-inflammatory skin phenotype of PPAR-alpha(-/-) mice is due to lack of IL-2-mediated Treg induction in these mice

Langerhans cells are critical in the development of atopic dermatitis-like inflammation and symptoms in mice

Genetic or vitamin D3-induced overexpression of thymic stromal lymphopoietin (TSLP) by keratinocytes results in an atopic dermatitis (AD)-like inflammatory phenotype in mice echoing the discovery of high TSLP expression in epidermis from AD patients. Although skin dendritic cells (DC) are suspected to be involved in AD, direct evidence of a pathogenetic role for skin DC in TSLP-induced skin inflammation has not yet been demonstrated. In a mouse model of AD, i.e. mice treated with the low-calcemic vitamin D3 analogue, MC903, we show that epidermal Langerhans cells (LC)-depleted mice treated with MC903 do neither develop AD-like inflammation nor increased serum IgE as compared to vitamin D3 analogue-treated control mice. Accordingly, we show that, in mice treated with MC903 or in K14-TSLP transgenic mice, expression of maturation markers by LC is increased whereas maturation of dermal DC is not altered. Moreover, only LC are responsible for the polarization of naïve CD4(+) T cells to a Th2 phenotype, i.e. decrease in interferon-gamma and increase in interleukin (IL)-13 production by CD4(+) T cells. This effect of LC on T-lymphocytes does not require OX40-L/CD134 and is mediated by a concomitant down-regulation of IL-12 and CD70. Although it was previously stated that TSLP up-regulates the production of thymus and activation-regulated chemokine (TARC)/chemokine (C-C motif) ligand 17 (CCL17) and macrophage-derived chemokine (MDC)/CCL22 by human LC in vitro, our work shows that production of these Th2- cell attracting chemokines is increased only in keratinocytes in response to TSLP overexpression. These results demonstrate that LC are required for the development of AD in mouse models of AD involving epidermal TSLP overexpression

Peroxisome Proliferator-Activated Receptor-α Activation Inhibits Langerhans Cell Function

International audienceAbstract Epidermal Langerhans cells (LC) play a pivotal role in initiating and maintaining primary immune responses in the skin. In the present study, we asked whether peroxisome proliferator-activated receptor-α (PPARα) activation modulates LC function. Our results show that PPARα is expressed in immature LC and is down-regulated in mature LC suggesting that an early decrease of PPARα expression in LC may allow them to mature after contact with an Ag. We further show that pharmacologic PPARα activation inhibits LC maturation, migratory capacity, cytokine expression, and the ability to drive T cell proliferation. Moreover, PPARα activation inhibits NF-κB but not stress-activated protein kinase/JNK, p38MAPK, and ERK1/2. In conclusion, PPARα activation by endogenous ligands may provide a molecular signal that allows LC to remain in an immature state within the epidermis for extended periods of time despite minor environmental stimuli

Peroxisome proliferator-activated receptor-alpha activation inhibits Langerhans cell function.

Epidermal Langerhans cells (LC) play a pivotal role in initiating and maintaining primary immune responses in the skin. In the present study, we asked whether peroxisome proliferator-activated receptor-alpha (PPARalpha) activation modulates LC function. Our results show that PPARalpha is expressed in immature LC and is down-regulated in mature LC suggesting that an early decrease of PPARalpha expression in LC may allow them to mature after contact with an Ag. We further show that pharmacologic PPARalpha activation inhibits LC maturation, migratory capacity, cytokine expression, and the ability to drive T cell proliferation. Moreover, PPARalpha activation inhibits NF-kappaB but not stress-activated protein kinase/JNK, p38MAPK, and ERK1/2. In conclusion, PPARalpha activation by endogenous ligands may provide a molecular signal that allows LC to remain in an immature state within the epidermis for extended periods of time despite minor environmental stimuli

Peroxisome proliferator-activated receptor-alpha activation inhibits Langerhans cell function

Epidermal Langerhans cells (LC) play a pivotal role in initiating and maintaining primary immune responses in the skin. In the present study, we asked whether peroxisome proliferator-activated receptor-alpha (PPARalpha) activation modulates LC function. Our results show that PPARalpha is expressed in immature LC and is down-regulated in mature LC suggesting that an early decrease of PPARalpha expression in LC may allow them to mature after contact with an Ag. We further show that pharmacologic PPARalpha activation inhibits LC maturation, migratory capacity, cytokine expression, and the ability to drive T cell proliferation. Moreover, PPARalpha activation inhibits NF-kappaB but not stress-activated protein kinase/JNK, p38MAPK, and ERK1/2. In conclusion, PPARalpha activation by endogenous ligands may provide a molecular signal that allows LC to remain in an immature state within the epidermis for extended periods of time despite minor environmental stimuli