Langerhans’ cells (LC) are the dendritic cells (DC) of the epidermis and, as sentinels of the immune system, act as a bridge between the innate and adaptive immune responses. When LC, and other DC, recognise an antigen or pathogen they mature and are stimulated to migrate to the lymph nodes, where they orchestrate immune responses. Pathogen derived toll-like receptor (TLR) ligands, and chemical allergens, are recognised as being potentially harmful and stimulate LC to mobilise and mature. Cytokine signals, including tumour necrosis factor (TNF)-α, interleukin (IL)-1β, and IL-18, all induce LC migration and are required for initiating LC mobilisation in response to certain contact allergens. Subsequently, chemokines promote the migration and localisation of LC within the draining lymph nodes. Chemokines are also involved in shaping the adaptive immune response by promoting differential T cell activation, such as T helper (Th)1 or Th2 responses, which are involved in immunity against different pathogens, and also in the development of different types of chemical allergy. The hypothesis is that LC phenotype (activation, migration and chemokine production), is dependent on the nature of the challenge ligand. The murine LC-like cell line XS106 was used to investigate the response of LC following stimulation with TLR ligands and chemical allergens. In addition, LC migration in response to these stimuli was investigated in vivo and the role of TNF-α was examined using mice deficient in either one of the two TNF-α receptors; TNF-R1 or TNF-R2.XS106 cells and freshly isolated LC were associated with a selective type 2 immune response, as determined by preferential expression of type 2 associated chemokines. Furthermore, XS106 cells responded to type 2, but not to type 1, associated TLR ligands. In contrast, all of the TLR ligands tested induced the migration of LC from the epidermis in vivo. Similarly, chemical allergens failed to induce a maximal response of XS106 cells, but did induce the migration of LC in vivo. There were differences in LC migration between the two mouse strains tested, with C57/BL6 strain mice being less responsive to administration of TNF-α and the contact allergen oxazolone compared with BALB/c strain mice. However, C57/BL6 and BALB/c strain mice responded similarly after exposure to the contact allergen 2,4-dinitrochlorobenzene (DNCB). Furthermore, DNCB was able to induce LC migration in mice deficient in TNF-R2, the TNF-α receptor expressed by LC.Collectively, these data suggest a paradigm in which keratinocytes and LC in the epidermis have distinct roles in promoting type 1 and type 2 immune responses, respectively. Therefore, LC may not be activated directly by certain TLR ligands or chemical allergens that are associated with type 1 responses. Consequently the migration of LC in vivo after encounter with these stimuli may be secondary to interaction with keratinocytes, or with other skin resident cells. Together, LC and keratinocytes allow the epidermis to respond to a range of pathogens, in addition to developing the necessary type 1 and type 2 responses. Chemical allergens may have divergent cytokine signalling requirements for the induction of LC migration as, unlike other contact allergens (and other stimuli such as irritant and ultraviolet [UV]B exposure), DNCB may induce LC migration independently of TNF-α.

Dendritic cells (DCs), a hematopoietic cell type belonging to the sub-group of cells called antigen presenting cells (APCs), inhabit a central role in innate and adaptive immunity. Although the DC family is very heterogeneous, all members share unique features. Most importantly, DCs can stimulate an immune response. This is due to the cells’ ability to capture and process antigens and to maturate in the presence of danger signals presented by pathogens. Maturation in turn results in the migration of DCs from the tissue they reside in to the draining lymph nodes, as well as in the subsequent presentation of the acquired antigens to T cells. In the skin, which is one of the most immunogeneic organs, DCs are present in sizable numbers in both the epidermis and the dermis. This study focused on two types of DCs: epidermal Langerhans cells (LCs) and dermal DCs (DDCs). While much is understood about LCs, far less is known about the role that DDCs play in skin immunity. Therefore one purpose of this study was to characterize DDCs and to compare their phenotype and functions to that of LCs. This study used two different methods to characterize human skin resident immune cells with regard to their number and distribution. First, a stable analytical immunohistochemistry-based method was developed and applied to a substantial number of healthy skin donors. This enabled a quantitative analysis of skin DC types and skin resident T cells at different anatomical locations in situ. A novel method to count dermal cell populations in situ was developed that resulted in the first published quantification of APCs, DDCs, as well as T cells in human dermis. Second, the traditional form of the emigration assay, which selectively enriches vital cells capable of ex vivo emigration from the skin, was upgraded toward a stable analytical method to separate epidermal LCs from DDCs. In this way, both skin DC types became accessible in sufficient numbers to allow for a comparison of phenotypes and functions in vitro. The resulting phenotypic observations clearly showed that both, LCs and DDCs are not fully mature after their emigration ex vivo and that both can be transformed into a phenotypically more mature state by treating them with inflammatory cytokines. What’s more, LCs are also functionally in an immature state after their emigration. They efficiently took up antigen, showed a low capacity to trans-migrate in response to chemokines, and demonstrated a low capacity to stimulate allogeneic T cells in a mixed leukocyte reaction (MLR). For the first time this study observed all these main APC functions not only for LCs but additionally for DDCs. As these observations were made in relation to LCs of the same donor, it could be concluded that DDCs are functionally more mature than LCs after emigration. DDCs showed a lower antigen uptake capacity than LCs but were superior in terms of their migratory and stimulatory capacity. However, treatment with cytokines could skew LC functions toward functional capacities observed for DDCs, i.e., it decreased LCs’ Ag uptake and increased their migratory and stimulatory capacity, whereas the cytokine treatment did not alter DDCs’ functional capacities. After improving immuno-histochemistry and the emigration assay using healthy skin samples, these newly developed techniques were implemented in clinical trials to observe the number, distribution and migratory capacity of skin DCs and T cells in patients undergoing allogeneic hematopoietic cell transplantation (aHSCT). Such a study is of importance because the turnover of DCs and T cells is closely associated with the occurrence of acute graft-versus-host disease (aGvHD), the major cause of morbidity and mortality after aHSCT. Due to the study design used, this study concisely demonstrate that at the onset of aGvHD, different DC types accumulate along with effector T cells in skin lesions of aGvHD but not in uninvolved skin of the same patient. These results suggest that in addition to donor T cells LCs and DDCs play a role during the early phase of cutaneous aGvHD directly within the site of inflammation. The view of many authors that DC depletion in the transplant recipient, especially in target organs, is a promising approach for aGvHD prophylaxis and therapy is further underscored by these results. One targeting strategy to inhibit GvHD by eliminating recipient DCs may be the use of DC specific monoclonal antibodies. Alemtuzumab (anti-CD52) is a monoclonal antibody and has proven effective in preventing aGvHD after aHSCT. It may, despite depleting donor T cells, also work by targeting recipient DCs. To determine whether the last mechanism of action is significant, a second clinical study investigated the effects of intravenous alemtuzumab on DCs by comparing the number of these cells in skin and blood of patients before and after a 4-week course of alemtuzumab treatment. The result was that although skin DCs weakly express the target antigen CD52 the number of these cells was not consistently reduced by alemtuzumab. In contrast, circulating blood DCs have a stronger CD52 expression and were significantly reduced by the treatment. In conclusion, this work provides new insights into the phenotypical and functional characteristics of human skin DCs, as well as into the fate of these cell types during aHSCT. The investigation of the APC system during aGvHD as carried out here will help to understand the process of aGvHD in more detail. All these efforts may hopefully support the development of new approaches for therapy and prevention of this major limitation of aHSCT and may help to improve this only curative therapy for several life-threatening diseases.