The role of CD4+ and CD8+ T cells in contact hypersensitivity and allergic contact dermatitis

ARTICLE

Auteur(s) : Pierre SAINT-MEZARD², Frédéric BÉRARD², Bertrand DUBOIS¹, Dominique KAISERLIAN¹, Jean-François NICOLAS²

¹ Inserm U 404, 69007 Lyon
² Inserm U 503, 69007 Lyon, Unité Immunologie Clinique et Allergologie, Dufourt 5F, CHU Lyon-Sud, 69495 Pierre-Bénite Cedex, France

Article accepted on 14/3/2004

Contact hypersensitivity (CHS) is a DTH reaction, i.e. a skin inflammatory reaction due to the activation of antigen-specific T cells. In contrast to classical DTH, which needs intradermal injection of exogenous protein, initiation of CHS is generated by topical application on the epidermis of sensitizing chemical products (nickel, chrome, DNFB, TNCB, oxazolone...). CHS is one form of DTH reaction and as such was considered to be CD4+ T cell-mediated. Since cutaneous infiltrates in human allergic contact dermatitis (ACD) show a clear preponderance of CD4+ T cells, it is not surprising that this T cell subset has most often be held responsible for mediating CHS and ACD. However, it has become clear that both CD4+ and CD8+ T cells can act as effector cells in both DTH and CHS reactions [1-3]. 
Studies from the last 10 years have emphasized that CD8+ T cells were the main effector cells of CHS while CD4+ T cells behave as down-regulatory cells [4-6]. It is noteworthy that there is still some controversy as to whether CD8+ T cells are effector cells of CHS in all strains of mice and for all types of haptens [7]. Similarly, the precise contribution of CD4 and CD8 T cells in human ACD is still not known [8]. 
CD8+ T cells are now known to mediate DTH responses in allergic contact dermatitis, drug eruptions, asthma, and autoimmune diseases [3]. This inflammatory effector capability of CD8+ cytotoxic T cells was previously poorly recognized, but there is now considerable evidence that these diseases may be mediated by CD8+ DTH cells. The difference between CD8+ T cells and CD4+ T cells mediating DTH may relate to the molecular mechanisms by which antigens are processed and presented to the T cells. Antigens external to the cell are phagocytosed and processed for presentation on MHC class II molecules (eg, HLA-DR) to CD4+ T cells. In contrast, internal cytoplasmic antigens are processed by the endogenous pathway for presentation on MHC class I molecules (eg, HLA-A, -B, and -C) to CD8+ T cells. External allergens can also enter the endogenous pathway to be presented to CD8+ T cells. These include many contact sensitizers, chemical and protein respiratory allergens, viral antigens, metabolic products of drugs, and autoantigens. Haptens are also able to directly interact with peptides which are already in the groove of MHC class II and class I molecules [4]. Thus CD8+ and CD4+ T cells could be activated in the lymph nodes by skin APC expressing haptenated peptides.

Pathophysiology of CHS – general scheme

Knowledge of the pathophysiology of ACD is derived chiefly from animal models in which the skin inflammation induced by hapten painting of the skin is referred to as contact hypersensitivity (CHS). Two temporally and spacially dissociated phases are usually necessary to achieve optimal CHS reaction: the sensitization and the elicitation phase (Fig. 1). We describe here the well-accepted pathophysiological pathways of CHS and ACD.
The sensitization phase (also known as afferent phase) occurs at the first skin contact with a hapten and leads to the priming and expansion of hapten specific T cells in lymph nodes. Topically applied hapten is taken up by skin DC, especially Langerhans cells (LC), which migrate from the epidermis to the para-cortical area of draining lymph nodes, where they present haptenated peptide/MHC molecule complexes to hapten-specific T cell precursors. Specific T cells emigrate from the lymph nodes and enter the blood through the thoractic duct and recirculate to tissues including the skin.
The elicitation phase occurs a few hours after a subsequent challenge of the skin with the same hapten. Hapten is uptaken by skin APCs, particularly skin DC and keratinocytes which present haptenated peptides to specific T cells which patrol in the skin. Activated T cells produce type 1 cytokines (γIFN), activate skin resident cells which produce cytokines and chemokines allowing the recruitment of the polymorphous cellular infiltrate characteristic of CHS. This efferent phase of CHS takes 72 hours in humans and 24 to 48 hours in the mouse. The inflammatory reaction persists over several days and progressively decreases upon physiological down-regulating mechanisms which could also be explained by the disappearance of the hapten.

CHS to strong haptens

The vast majority of available data on CHS have been obtained with strong sensitizers which have unique chemical and immunological properties:
– they represent a minority of the chemicals among the thousands which are able to induce ACD in humans.
– they are endowed with potent proinflammatory properties, known as irritancy, due to the toxicity of the chemical. This toxicity provides a danger signal for the skin innate immune system, leading to: i) production of inflammatory cytokines (IL-1, TNF) and chemokines by skin cells, ii) activation of skin DC which can initiate their maturation process and emigrate to draining lymph nodes.
– they can induce a primary ACD, i.e. a hapten-specific immune reaction, following a single skin contact, which has the same pathophysiology as the classical CHS reaction obtained with two hapten skin paintings [9]. That CHS occurs after a single exposure to haptens could be explained by the persistence of haptens in the skin for several days after painting, allowing the recruitment of specific T cells at the site of skin sensitization.
Thus, strong haptens, through their toxicity, represent a danger signal able to potently activate innate immunity which in turn allows the development of a robust and rapid hapten-specific immunity. Alternatively, the most frequently encountered haptens, classified as moderate, weak or very weak, are much less irritant than strong haptens and may not have the same ability to activate innate immune cells. Consequently, the resulting ACD may operate through slightly different mechanisms.

CD8+ T cells are effector cells while CD4+ T cells behave as regulatory cells

The respective contribution of CD4+ and CD8+ T cells in CHS has been examined using different strategies: i) in vivo depletion of normal mice with anti-CD4 and anti-CD8 mAbs; ii) transfer of CD4+ or CD8+ T cells from sensitized mice into Rag°/° mice; iii) use of MHC class I°/° (CD8+ T cell-deficient) or MHC class II°/° (CD4+ T cell-deficient) mice; iv) transfer of the ability to mediate a CHS reaction by injection of purified primed CD4+ and/or CD8+ T cells in naïve recipients; v) transfer of haptenated DC from MHC class I°/° or MHC class II°/° mice to induce CHS in naïve recipients. Mice genetically deficient in the CD4 or CD8 molecule (CD4°/° or CD8°/°) do not represent models of CD4+ or CD8+ T cell deficiency and results obtained with these mice will be discussed later (see “CD4-deficient mice”).
Adoptive transfer experiments first highlighted that DTH to protein was transferable into MHC class II-matched recipients whereas transfer of CHS required class I-matched recipients [10]. Using in vivo depletion of CD4+ and CD8+ T cell subsets, Gocinski et al. were the first to suggest that CD8+ T cells could mediate the CHS response to DNFB and other strong haptens [11]. They further showed that CD4+ T cells were endowed with down-regulatory activity, since the CHS reaction was enhanced following in vivo depletion in CD4+ T cells.
Bour et al. used another approach to study the contribution of CD4+ and CD8+ T cell subsets. They studied CHS in MHC class I and MHC class II KO mice which are deficient in CD8+ and CD4+ T cells, respectively [12-14]. Indeed, CD4+ T cells develop during the ontogeny by interaction with thymic APCs expressing MHC class II molecules. In the absence of such MHC class II molecules the CD4+ T cell pool cannot differentiate and CD8+ T cells compose most of the circulating mature T cells. Likewise, due to lack of positive selection of CD8+ T cells in MHC class I-deficient mice, CD4+ T cells constitute the vast majority of mature peripheral T cells. Application of a CHS reaction to these mice showed surprising results. Class I°/° mice did not develop any CHS response to DNFB, indicating that CD8+ T cells were mandatory for the development of the pathology. Since these I°/° mice have normal numbers and functions of CD4+ T cells and can mount a classical DTH reaction to alloantigens [12-14], these data demonstrated that CD4+ T cells do not mediate the CHS reaction to DNFB. On the other hand, class II°/° mice developed an enhanced CHS reaction, with chronic skin inflammation. Moreover, in vivo depletion of CD8+ T cells in MHC class II°/° mice resulted in a complete abrogation of the CHS. More importantly, development of hapten-specific CD8+ effectors could occur in the absence of CD4 help. Indeed, there is no need of CD4+ T cells for the priming of CD8+ T cells in II°/° mice and the presence of CD4+ T cells has a negative effect on the intensity of the CD8+ T cell-mediated CHS response [12, 15]. Thus, other important information from these studies was the characterization of MHC class II-restricted CD4+ T cells as down-regulatory cells of CHS.
Most of the data summarized above have been obtained with DNFB in C57BL/6 (H2b) [12, 16] and in BALB/C (H2d) [17] mice. Similar results have been obtained for other haptens such as oxazolone [17, 18], DMBA [19] and TNP [15, 20]. Thus, these findings indicated that a functional dichotomy exists between CD8+ T cells and CD4+ T cells which behave as effector cells and regulatory cells, respectively, in CHS to strong haptens. No data are available yet for moderate to weak haptens, mainly because of the lack of experimental murine model for these haptens.

Priming of specific CD8+ and CD4+ in lymphoid organs during the sensitization phase of CHS

CD8+ type 1 cells and CD4+ type 2 cells

Priming of naïve T cell precursors occurs in the draining lymph nodes in a few days following hapten skin painting. The optimal time between hapten painting and T cell priming is 5 days in murine models. At that time T cells recovered from lymph nodes are endowed with potent proliferative activities [16, 21]. Analysis of cytokine production by CD4 and CD8 T cell subsets after in vitro restimulation by haptenated APCs have shown that CD8+ T cells produce type 1 cytokines, mostly γIFN, while CD4+ T cells produce type 2 cytokines, including IL-4, IL-5 and IL-10 [18]. These results were subsequently confirmed using ELISPOT assays to demonstrate high numbers of γIFN-producing CD8+ T cells [22] and IL-4- producing CD4+ T cells in the lymph nodes of hapten-sensitized mice [18]. Analysis of the number of DNFB-specific CD8+ T cells using an γIFN ELISPOT assay showed an average of 50 CD8+ T cell precursors/10⁵ lymph node cells at day 5 post sensitization, a number which is similar to what is found in other antigen-specific immune responses [22].

MHC restriction of hapten-specific T cells

Investigators from different groups provided evidence that hapten presentation to T cell in CHS was MHC restricted and thus similar to the presentation of protein antigen-derived peptides in classical DTH [15, 20]. Immunization of mice with hapten-pulsed DC recovered from the epidermis or derived from bone-marrow precursors is able to prime for specific T cells which proliferate to DNFB in secondary proliferative responses. Immunization procedures using DC recovered from MHC class I or MHC class II-deficient mice confirmed the opposite functional effects of the CD8 and CD4 T cell pools [16]. In these experiments, MHC class I-expressing DC (either from normal mice or from MHC I+/II mice) induced the priming of CD8+ T cells in the lymph nodes (assessed by specific proliferation) and the CHS reaction upon subsequent challenge. Conversely, immunization by DC lacking MHC class I molecules (recovered from MHC class I- deficient mice) was inefficient at inducing a CHS reaction but could prime for CD4+ T cells. Indeed, the CD4+ T cells purified from the lymph nodes of such mice were hapten-specific, as assessed in secondary proliferative responses [16].

These results were confirmed by a recent study in non genetically modified mice, using bone-marrow-derived DC which were pulsed with trinitrophenyl (TNP)-derivatized peptides and administered intradermally to generate a CHS reaction. Two types of peptides that have affinity for either MHC class I or class II peptides were used. Martin et al. showed that the class I binding peptides induced CHS responses similar to that obtained with epicutaneous TNP application. In contrast, DC pulsed with class II binding peptides did not sensitize for optimal CHS [23]. On this basis, Cavani et al. speculated that the ability of chemical haptens to drive CD8+ T cell activation may be associated with their capacity to interact directly with peptides bound within MHC-I or MHC-II to create immunogenic trimolecular complexes [5].

CD8+ T cell priming does not require CD4+ T cell help

Classically, optimal activation of naïve CD8+ T cells requires signals received by CD4+ T cells, and referred to as CD4+ T cell help. Two models have been proposed which provide a general framework for the role of CD4+ T cells in mediating help for CTLs. In the “three-cell” model, help is provided to CTLs by CD4+ T cells that recognize Ag on the same APC. In the sequential “two-cell” model CD4+ T cells first interact with APCs, which in turn activate naive CTLs. This CD4+ T cell help involves the CD40-CD40L interaction. Indeed, more than IL-2 synthesis, expression of CD40-L by activated CD4+ T cells may be one necessary and sufficient factor to fulfill the helper function [24].
In CHS to strong haptens, CD8+ T cell activation in the lymph nodes does not require CD4+ T cell help and involvement of the CD40/CD40-L is unlikely, since CD40-L-deficient mice mount a normal CHS to DNFB [25]. Indeed, immunization of mice which are deficient in CD4+ T cells either by in vivo treatment with monoclonal antibodies or because they lack the MHC class II-restricted CD4+ T cell compartment (MHC class II KO mice) develop a strong CHS response to haptens. Similarly, the immunization studies performed with either MHC I+/II^- dendritic cells [16] or with I+/II+ DC haptenized with MHC class I binding peptides [23] have confirmed that CHS could develop without the activation of MHC class II-restricted CD4+ T cell pool.
Other studies on viral-induced DTH responses have indicated that activation of naive CD8+ T cells for the generation of MHC class I-restricted immune responses can occur in the absence of T cell help [26, 27]. Recently, it was demonstrated that the main parameter which dictates the requirement or the absence of requirement of CD4 help was the number of CTL precursors which could be activated at time of priming [28, 29]. Indeed, CTL responses induced by cross-priming can be converted from CD4-dependent to CD4-independent by increasing the frequency of CTL precursors. In the absence of CD4 T cells, high numbers of CTL precursors were able to expand and become effector CTLs [29].The ability of high frequencies of CD8 T cells to override help was not due to their ability to signal CD40 via expression of CD154. These findings suggest that when precursor frequencies are high, priming of CD8 T cell responses may not require CD4 T cell help.
Another explanation for the development of CD8+ effector cells is that antigens which have the intrinsic ability to induce DC maturation bypass the need for CD4 help via CD40 activation [30]. Indeed, mice depleted in CD4+ T cells can be primed for CTL responses by transfer of LPS-activated, antigen-pulsed DC.
In CHS, DC maturation induced by haptens with strong inflammatory capacities may bypass the need for CD4 help via CD40/CD40-L interaction and may be sufficient to trigger specific CTL responses with a high precursor frequency.

Elicitation phase of CHS is due to the recruitment and activation of CD8+ CTLs

Since the main function of CD8+ T cells is cytotoxicity, the observation that CHS was mediated by CD8+ cells raised the possibility that cytotoxicity was mandatory for expression of CHS. CD8+ CTLs are effector cells of the immune defence system again viruses and tumors [31] and exert their lytic functions through 2 main independant mechanisms [32]. The secretory pathway involves the release of perforin and granzymes from cytolytic granules. The non-secretory pathway involves interaction of the Fas-L upregulated during T cell activation, with the apoptosis-inducing Fas molecule on the target cell.
Preliminary experiments using mice deficient in either perforin or Fas-L were disappointing since perforin-KO mice and Fas-L (gld) – deficient mice developed a normal CHS reaction to DNFB and contained CD8+ CTLs able to kill haptenated targets. However, Fas-L- and perforine-double deficient mice could not develop CHS suggesting that cytotoxicity was necessary for the development of the pathologic process and that one cytotoxic pathway could compensate the absence of the other one [22]. This hypothesis was confirmed by extensive in vitro studies which demonstrated that primed CD8+ T cells from perforine- or Fas-L-KO mice could kill haptenized targets whereas no cytotoxicity was observed using CD8+ T cells from double deficient mice. Finally, the observation that perforine-KO CTLs could not kill Fas-deficient target cells confirmed that both cytotoxic pathways were involved in the anti-hapten CTL activity [22].
The involvement of cytotoxic CTLs in the development of CHS was analyzed during the elicitation phase by following the migration of CD8+ T cells. H Akiba et al. could demonstrate that CD8+ T cells could infiltrate the challenged skin as early as 9 hours after skin painting and that this migration was associated with γIFN production and induction of apoptosis in skin epidermal cells [17]. Double staining experiments using MHC class II antibodies and TUNEL staining showed that keratinocytes were the main target of CTLs. Thus, CD8+ T cells are endowed with in vivo cytotoxic activity and keratinocytes behave as antigen-presenting cells during the elicitation phase of CHS. The contribution of other cell types in the activation of hapten-specific CTLs remains however unknown. Since MHC class I molecules are expressed on all cells, it is highly probable that haptens which rapidly diffuse through the epidermis could be expressed as haptenated peptides by different skin cell types. In this respect Biderman et al. reported that mastocyte activation and chemokine production was needed for the recruitment of neutrophils which are necessary for development of CHS [33]. Since neutrophils enter the skin after CD8+ T cells, it is possible that mast cell activation could be secondary to CTL activation by presentation of haptenated peptides by mast cells.
Apoptosis is involved in several skin pathologies and is not restricted to CHS. In ACD, several reports have emphasized the existence of apoptotic processes involving the epidermis [4]. More recently, Akdis et al. have demonstrated that skin lesions of atopic dermatitis were associated with the occurrence of massive apoptosis of epidermal cells [34]. Although the contribution of CTLs in the pathophysiology of AD is not known precisely, these data emphasize that epidermal cell apoptosis is a common feature of eczematous dermatoses and suggest that anti-apoptic drugs could be new therapeutic tools [35].

CD4+ T cells down-regulate the CHS reaction

Regulatory cells are key actors in maintaining peripheral tolerance and controlling inflammatory responses. They are endowed with the capacity to inhibit the development of a potentially dangerous immune response as has been described in many models. The current complexity is in part due to the diversity of models used, which have enabled the identification of a regulatory component in almost every T cell subset and which have brought into evidence many much-debated regulatory mechanisms [5, 36, 37]. Three main regulatory CD4+ T cell subsets have been studied: i) Tr1 cell clones which produce high amounts of the immunosuppressive cytokine IL-10; ii) Type 2 CD4+ T cells which polarize T cells towards a type 2 phenotype and antagonize the type 1 bias characteristic of CHS; iii) and naturally occurring CD4+CD25+ T cells.
From the studies described above, CD4+ T lymphocytes behave as down-regulatory cells and most likely regulate both the sensitization and elicitation phases of CHS (Fig. 2).
Within secondary lymphoid organs, following sensitization, CD4+ cells limit the size of the CD8+ effector cell pool [14] or modify their functional properties. The number of specific CD8+ T cells, determined by IFNγ ELISOPT assay, is much higher in CD4+ T cell deficient mice than in normal mice, suggesting that CD4+ T cells control the development of the CD8+ T cell pool [38].
After migrating to the challenge site, these cells probably contribute to the control of inflammation and its resolution [17]. Indeed, in the absence of CD4+ T cells, mice develop a more pronounced and persistent inflammation [11, 12, 18, 39]. It is noteworthy that CD4+ T cells are recruited in challenged skin hours after recruitment of CD8+ T cells [17], suggesting that their entry into the skin is responsible for the inactivation of CD8+ CTL activity, thereby limiting the development of the inflammatory pathogenic process.
Limited information is currently available regarding whether a particular subset of regulatory cells is involved in the regulation of CHS. Nickel specific Tr1 cells (producing high amounts of IL-10) have been cloned from skin lesions of ACD patients suggesting that this subset of regulatory cells might contribute to the regulation of the efferent phase of contact sensitivity [8]. Indirect evidence for the implication of CD4+CD25+ cells comes from the observation that IL-2-IgG2b fusion protein inhibited CHS associated with an increase of the size of the CD4+CD25+ T cell compartment [40]. Our own data, in the model of CHS to DNFB support a role for CD4+CD25+ regulatory T cells in the control of the response and in the establishment of oral tolerance [36].

CD4+ T cells may be effector cells of CHS in some experimental conditions

Although most recent studies have emphasized the major effector role of CD8+ T cells in CHS, it cannot be concluded that CD4+ T cells and other cells are unable to act as CHS effectors.

Particularity of some chemicals

As discussed above, there are no data on the effector cells of CHS to moderate, weak and very weak haptens. The main reason is because there are no reproducible animal models for these weak haptens. However, since weak haptens have very limited irritant properties and thus probably do not activate the innate immunity as do strong haptens, it is possible that the CHS response they induce could be mediated by both CD4 and CD8 T cells. It is also tempting to speculate that the number of specific T cell precursors will be lower and that CD4 T cell help would be required for optimal CHS responses.

Some chemicals, e.g. FITC and formaldehyde, seem to provoke preferential type 2 cytokine production by CD4+ T cells, which have been shown to mediate the CHS reaction [41]. However, experiments using either depletion of T cell subsets or mice deficient in CD4+ or CD8+ T cells are still lacking to sustain this hypothesis.

CD4-deficient mice

That CD4+ T cells were effectors of CHS was concluded from studies of Kondo et al. and Wang et al. showing that CHS to dinitrofluorobenzene (DNFB) and oxazolone was greatly impaired in CD4°/° mice, genetically deficient in the CD4 molecule [42, 43]. The reason for the discrepancy between these results and those reported by other investigators most likely reflects important functional differences between CD4-deficient and MHC class II-deficient mice. Indeed, although the CD4 gene has been knocked, cells exerting a helper cell activity can be recovered from the double negative, CD4-8- T cell subset, in CD4°/° mice. In this respect, it is noteworthy that efficient thymic maturation of helper T cells has been shown in these CD4°/° mice [44]. The CD4- helper T cells express αβ-TCR and are able to control Leishmania infections, to mediate antibody class switch and DTH reaction to KLH [44-46]. Thus, although CD4°/° mice do not have the CD4 molecule, they are able to mount MHC class II restricted reactions, suggesting that the CD4 molecule is not absolutely required for efficient recognition of antigens presented by MHC class II molecules on antigen presenting cells. To understand why CD4°/° mice cannot develop a normal CHS response to DNFB, we set up a series of experiments concerning hapten-specific CTL activity in this particular mice (P. Saint-Mezard, manuscript in preparation). We show that the absence of the CD4 molecule does not impair the priming of hapten-specific IFNγ-producing CD8+ T cells but dramatically reduces the development of cytotoxic activity. More importantly, deficient specific CTL activity could be restored by restimulating CD8+ T cells by class II-deficient APCs, suggesting that the development of specific cytotoxic activity in CD4°/° mice is blocked by an MHC class II restricted population. We hypothesize that the particular MHC class II-restricted population present in CD4°/° mice may assume a regulatory function toward the CHS reaction, as does the CD4+ T cell population in normal mice, and that this role may be exacerbated in this particular KO mouse. Collectively, the data from the CD4°/° mice support the concept that CD8+ T cells are effector cells of CHS and illustrate a role for the CD4 molecule in the development of a down-regulatory T cell population.

CD4+ T cells may be effector cells in CHS to haptens when CD8+ T cells are deficient

CD4+ T cells could be effector cells in CHS to some haptens when the CD8+ T cell population is deficient. Evidence came from studies of Martin et al. [23]. They first showed that dendritic cells pulsed with TNP-derivatized peptides that have affinity for class II molecules could induce a low, albeit significant CHS reaction [23]. Next, they used C57BL/6 mice and class I°/° mice and studied the CHS to DNP and TNP. CHS to DNP was normal in C57BL/6 mice and absent in I°/° mice as previously reported [12]. TNP was able to induce a CHS response in C57BL/6 which was inhibited by in vivo depletion of CD8+ T cells using specific mAbs. Surprisingly, CD8+ T cell-deficient I°/° mice were able to develop a CHS reaction to TNP, which was similar to the TNP response in C57BL/6 mice in its kinetics (Martin et al., personal communication). These data show that, in the absence of CD8+ T cells, and for some but not all haptens, a CHS response can be mediated by CD4+ T cells.

Role of other cell types in CHS

Although the α β T cells are responsible for the hapten-specific CHS reaction, other lymphoid cell subsets have been shown to be implicated in the complex process of cellular activation required for optimal development of CHS [47, 48].
B-1 cells are activated in lyphoid organs during the sensitization phase and produce IgM antibodies. These antibodies diffuse in the skin and will bind the hapten immediately after the challenge, leading to complement activation which seems mandatory for the recruitment of effector T cells at the challenge site. NKT cells seem to be important for the activation of the B-1 cells through the production of a burst of IL-4. γδ T cells, known as dendritic epidermal T cells (DETC), are necessary for activation and function of specific αβ T cells once they are recruited into the challenged site.

CD8+ and CD4+ T cells in ACD

Most of the hypotheses on the precise mechanisms which may lead to human allergic contact dermatitis come from studies of the murine CHS reaction. Thus, the current pathophysiology of ACD postulates that CD8+ T cells are the main effector cells, while the CD4+ T cell pool comprises down-regulatory cells, able to block effector cell functions [5, 8].
T cells involved in ACD have been recovered from the blood or the skin of ACD patients. They are extremely heterogenous in their cytokine profile and function, with CD4+ and CD8+ T cells thought to play distinct roles in the development of the inflammatory response. For example, clinical studies examining long-term T cell clones generated from the peripheral blood and skin lesions of patients with allergic contact dermatitis (ACD) have yielded support for both CD4+ and CD8+ T cells as mediators of ACD in humans. Clones generated from lesions of patients with nickel-mediated contact dermatitis were CD4+ T cells, implicating these cells as effector cells in this pathology [49]. In a more recent study, expression of ACD to nickel correlates with the frequency of specific CD8+ T cells in the peripheral blood, which is high in allergic individuals. In contrast, the peripheral blood of both allergic and non allergic subjects shows comparable nickel-reactive CD4+ T cells responses [50]. Similarly, clinical studies of patients with contact allergies to classical haptens such as urushiol have demonstrated that most of the hapten-specific T cells isolated from the patient lesions were IFN-γ-producing CD8+ T cells.
Most, but not all, human hapten-specific CD8+ T cells display a type 1 cytokine profile, whereas CD4+ T cells isolated from ACD lesions show a more variable pattern of cytokine release, with a predominance of Th1 cells and a lower number of Th2 cells. Tr1 lymphocytes represent 7%-10% of nickel-specific T-cell clones isolated from ACD skin or the blood of allergic individuals, and their number is higher in the blood of non allergic subjects. In a manner dependent on IL-10, these Tr1 cells block the maturation of DCs and the release of IL-12, thus impairing the capacity of DCs to activate hapten-specific Th1 effector lymphocytes [8].

Conclusions

In summary, CHS reaction and ACD can be viewed as the result of activation of two distinct T cell subsets endowed with opposite functions: effector T cells and down-regulatory T cells. The severity and the duration of the skin inflammation appear directly related to the respective activation state and/or size of these two compartments. Thus, overwhelming regulation in sensitized individuals may lead to lack of inflammation (tolerance) despite repeated exposures to the hapten, while defects in regulatory cells may explain chronic contact dermatitis. Further studies will have to address the possibility of reversing an established ACD by either targetting the effector T cell population or by increasing the number or functionnal properties of regulatory T cells. n

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