Contact dermatitis I. Pathophysiology of contact sensitivity

ARTICLE

Contact dermatitis (CD) is one of the most common skin diseases, with a great socio-economic impact. As the outermost barrier of the human body, the skin is the first to encounter chemical and physical factors from the environment. According to the pathophysiological mechanisms involved, two main types of CD may be distinguished: allergic contact dermatitis (ACD) and irritant contact dermatitis (ICD). Allergic contact dermatitis is a T cell-mediated inflammatory reaction occurring at the site of challenge with a contact allergen in sensitized individuals. It is characterized by redness, papules and vesicles, followed by scaling and dry skin (cf. part II). 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 sensitivity (CS) or contact hypersensitivity (CHS). ACD and CS (CHS) are thus considered as synonymous and define a hapten-specific T cell-mediated skin inflammation. They represent a form of delayed type hypersensitivity.

CS is considered to be a classical Th1-type immune response in the skin. The skin and the draining lymph nodes play a central role in the induction and triggering of a CS reaction. At the cellular level CS involves three cell types: the dendritic antigen-presenting cell, hapten-specific T cells and the hapten.

The pathophysiology of CS consists of two distinct phases (Fig. 1):

1. The sensitization phase (also referred to as afferent phase or induction phase of CS). It occurs at the first contact of skin with the hapten and leads to the generation of hapten-specific T cells in the lymph nodes and their migration to the skin. Hapten is taken up by Langerhans cells (LC), which migrate from the epidermis to the draining lymph nodes, where hapten-specific memory T lymphocytes develop in the para-cortical area. The sensitization step lasts 8 to 15 days in man, 5 to 7 days in the mouse. This first step has no clinical consequence.

2. The elicitation phase, also known as efferent phase or challenge phase of CS. Challenge with the same hapten leads, in a few hours, to the appearance of contact dermatitis. Upon subsequent contact of the skin with the hapten, specific T lymphocytes are activated in the dermis, and trigger the inflammatory process responsible for the cutaneous lesions. This efferent phase of CS takes 72 hrs in man, and 24 to 48 hrs in the mouse. The inflammatory reaction persists over several days and progressively decreases upon physiological down-regulating mechanisms. In the next chapters, we shall detail the pathophysiological events occurring during these two steps, and in the last part, we shall present recent data on the effector and regulatory T cell populations involved in the development of the CS reaction.

Haptens. Contact allergens

The origin and nature of the compounds able to induce a CS reaction are very diverse, but they share some common features: contact allergens are low molecular weight chemicals named haptens, that are not immunogenic by themselves. They need to bind to epidermal proteins, which act as carrier proteins [1] to form the hapten-carrier complex that finally acts as immunogen. Almost all haptens are electrophilic substances, which can bind covalently with the nucleophilic residues of cutaneous proteins [2].

Haptens often derive from chemicals, named prohaptens, which require an additional metabolization step in vivo in the epidermis to be converted into electrophilic compounds able to bind to nucleophilic residues. This is the case of urushiol (poison ivy), of para-phenylenediamine and of photosensitizers, which must be activated by UV-light in order to bind to epidermal proteins [3]. Metals do not bind covalently to cutaneous proteins but form complexes with these proteins through weaker bonds. Some metal salts also undergo chemical conversion in the skin, as hexavalent chromium salts, which in the epidermis are turned into trivalent chromium, the highly reactive form capable of binding to cutaneous proteins [4]. Evidence that conversion of the parent compound to a reactive metabolite was necessary for the development of CS was recently demonstrated for the polyaromatic hydrocarbon (PAH) dimethylbenz(a)anthracene (DMBA). CS to DMBA only occurred in strains of mice that could metabolize the compound, and inhibitors of PAH metabolism reduced the magnitude of the reaction. Furthermore, among the PAHs, only those that could induce aryl hydrocarbon hydroxylase, the rate-limiting enzyme in the PAH metabolic pathway, were immunogenic [5]. The implications of these experiments are that at least for some contact allergens, the metabolic status of the host is a key determinant for individual susceptibility to the development of allergic contact dermatitis.

Haptens can be classified according to their sensitizing potential in man. Strong haptens, like oxazolone or dinitrofluorobenzene (DNFB), usually result in the development of a CS reaction in all individuals, and can be used to test for the presence of a normal cellular immunity. However, most contact allergens are "weak haptens", in that they will induce a CS reaction in only a small percentage of individuals i.e. around 5% for CS to metal salts and below 0.01% for CS to haptens like para-phenylenediamine or eugenol. The basis for such differences among haptens in their ability to generate immunogenic hapten-carrier complexes is still unknown.

Recent studies from several laboratories have shown that T cells recognize haptens as structural entities bound covalently or by complexation to peptides anchored in the grooves of major histocompatibility (MHC) class I and class II antigens [6]. Thus the contact allergen is a chemical but the antigen able to activate T cells is a haptenated peptide (Fig. 2).

Dendritic cells

Langerhans cells (LC) are epidermal MHC class II⁺ dendritic cells (DC) specialized in antigen presentation. LC form with their dendrites an epidermal cellular network which allows the capture of haptens that have penetrated the skin barrier (Fig. 3). The basic role of LC in the development of CS reactions has been clearly shown by two sets of experiments. On the one hand, animals painted with a hapten on cutaneous sites naturally or artificially depleted in LC are unable to mount a CS response [7, 8]. On the other hand, sensitization of naive mice can be achieved by injection of total epidermal cells, purified LC or cells from a DC line [9, 10], all haptenized in vitro, whereas injection of total epidermal cells depleted of LC is inefficient in inducing sensitization [11]. However, cells from the dendritic group of APC different from epidermal LC, especially dermal DC, could participate in the induction phase of CS [12, 13], inasmuch as some haptens are able to cross the epidermis.

Langerhans cells load the hapten in the epidermis and migrate to the draining lymph nodes

Numerous studies have suggested that activation of the naive hapten-specific T lymphocytes by LC having loaded the hapten in the epidermis, occurs in the draining lymph nodes (Fig. 1, steps 1 and 2). Initial observations have shown that induction of a CS reaction requires an intact draining lymphatic system [14], and that after a cutaneous application of the hapten fluorescein isothiocyanate (FITC), DC bearing the hapten and containing Birbeck granules accumulate in the draining lymph nodes [15]. Convincing evidence was provided by Kripke et al. in 1990; nude mice were sensitized by FITC on an allogeneic skin graft, and allogeneic DC bearing the hapten could be recovered from the draining lymph nodes of the mice. These cells contained Birbeck granules and were able to induce a sensitization to FITC after injection into naive mice of the same haplotype as the graft donor, which means that they were derived from hapten-painted skin grafts [16].

Maturation of LC during their migration from the epidermis to the draining lymph nodes

LC undergo morphological, phenotypic and functional modifications during their migration from the epidermis to the lymph nodes: in the epidermis, LC exhibit a dendritic shape and contain a large number of Birbeck granules (Fig. 4). When LC migrate into the dermis, they become round ("indeterminate cells"), and their shape changes again in the afferent lymph vessels, where they are named "veiled cells". Finally, in the paracortical area of the draining lymph nodes, LC are known as interdigitating cells, which is the form able to present antigen to the naive specific T lymphocytes [17, 18].

Mechanisms underlying DC migration to draining lymph nodes are still not well understood. Two epidermal cell cytokines, IL-1ß and TNF-alpha seem to be particularly important for its initiation [19, 20]. Migration is due to the differential expression of several families of adhesion molecules by DC, namely of E-cadherins [21], some ß1-integrins [22] and CD44 isoforms [23] and chemokines [24].

On the phenotypic level, LC lose several surface markers during their migration, whereas others are either induced or upregulated. It is often difficult to distinguish between phenotypic changes induced by hapten uptake and those that occur during DC migration and maturation induced by any other stimuli. Peripheral blood monocyte-derived DC which preserve for a longer time an "immature" phenotype have been used lately for studying hapten-induced modifications [25]. Up-regulation of MHC class II molecules has been the most extensively studied as a marker of DC activation. In physiological conditions, LC are the only cells in the epidermis which constitutively express major histocompatibility (MHC) class II molecules, and expression of these molecules is strongly up-regulated by haptens and not by irritants [25-27]. In the first 3 hrs following application of a hapten, expression of MHC class II molecules first decreases [28], whereas their intracellular level increases, which probably reflects endocytosis triggered by hapten binding [29]. However, 24 hrs after application of the hapten, cell surface expression of MHC class II molecules is strongly increased [30], and this is in keeping with changes observed in messenger RNA expression of these molecules, whose level increases from the sixth hour following application of the hapten to reach its maximum level at 18 hrs [31]. Other surface molecules that can be up-regulated by hapten uptake are ICAM-1 (CD54) and B7-2 (CD86) but application of irritants could also induce their expression at a lower or even at an equal level [25, 27]. Conversely, the CD1a molecules, which are a specific marker of human LC in the epidermis, disappear from the cell surface during migration to the draining lymph nodes. Finally, some intracellular markers are also altered, such as ATPase activity which strongly decreases after loading of the hapten by LC [32].

The morphological and phenotypical changes of LC during migration from the epidermis to the draining lymph nodes parallel functional modifications which account for the central role of LC in the presentation of the hapten to specific T lymphocytes during the CS reaction. Thus, as other immature DC, LC appear to be very efficient at antigen processing and presentation, and the balance between these two functional properties would be altered during migration. More precisely, LC are particularly efficient at antigen processing in the epidermis, where they pick up haptens, while antigen processing becomes less efficient upon migration to the draining lymph nodes. Conversely, antigen presenting capacities of LC that have reached the draining lymph nodes are strongly increased, which allows activation of naive T lymphocytes in the paracortical areas [18, 33-35].

Hapten-specific T lymphocytes

Haptenated peptides are presented by LC to the naive specific T lymphocytes in the paracortical area of regional lymph nodes draining the cutaneous site of hapten application (Fig. 1, step 3). Precise mechanisms of hapten presentation by LC had remained largely undefined until recently, but important progress in the more general understanding of processing and presentation of classical protein antigens by specialized antigen-presenting cells has opened the way to numerous studies dealing with this question.

Hapten determinants for T cells

In the case of classical protein antigens, antigen recognition by specific T lymphocytes occurs by interaction of the T cell receptor (TcR) expressed by CD8⁺ or CD4⁺ T cells with processed antigenic peptides associated with MHC class I or class II molecules, respectively. In the case of haptens, numerous studies have now shown that T lymphocytes usually recognize hapten-modified peptides in the groove of MHC molecules (Fig. 2). Most results were obtained with the strong hapten TNP (trinitrophenyl). For MHC class I- as well as for MHC class II-restricted determinants, T cells react with MHC-associated TNP-peptides and not with covalently modified MHC molecules. Thus, the TcR interacts mainly with the hapten TNP and parts of the MHC molecule. The immunodominant TNP-epitopes are generally largely independent of the carrier's amino acid sequence, and the carrier peptide would serve essentially to anchor and position the hapten on the MHC surface [6, 36].

The precise features of generation of hapten determinants at the cell surface of antigen-presenting cells are not clear as yet. Most haptens interact with proteins in an indiscriminate way, and they are theoretically able to modify numerous surface proteins, including MHC molecules, or cytoplasmic proteins after diffusion through cellular membranes. It is likely that haptenated proteins would be endocytosed, processed and presented by MHC class II molecules like other antigens. Conversely, liposoluble haptens can penetrate the cytosol of dendritic cells, bind to cytoplasmic proteins and follow thereafter the endogenous pathway which results in MHC class I-restricted antigen presentation. Finally, for class I as well as for class II molecules, haptenization of fixed cells has shown that MHC-associated peptides may also be directly modified at the cell surface [6, 37]. The contribution of hapten processing to the priming of hapten-specific T cells has not yet been firmly established. However, two kinds of T cell clones recognizing respectively processing-dependent and processing-independent epitopes have been raised from peripheral blood of nickel-allergic [38] and penicillin-allergic [39] patients.

Effector T cells of contact sensitivity

Unlike classical delayed-type hypersensitivity (DTH) to protein or cellular antigens which is mediated primarily by MHC class II-restricted CD4⁺ T cells [40], the T cell response to haptens in CS appears more complex. From all the data available on the role of T cell subsets in CS, it appears that both CD4⁺ and CD8⁺ hapten-specific T cells could mediate the skin inflammatory reaction. However, recent studies have clearly demonstrated that hapten-specific CD8⁺ T cells could mediate the CS reaction in the absence of CD4⁺ T cells.

In vivo murine models have been used in which the role of CD4⁺ and CD8⁺ T cells in CS has been examined using either the abrogation of the respective population (by depleting antibodies or in genetically deficient animals) or by injecting T cells obtained from sensitized donors (adoptive transfer studies) or hapten-treated DC (immunization studies) into naive recipient mice. Adoptive transfer studies have first highlighted the fundamental differences existing between DTH to protein and cellular antigens and CS, the former being transferable into MHC class II-matched recipients and the latter requiring class I-matched recipients [41]. That CD8⁺ T cells are necessary and sufficient for the expression of CS to DNFB was later confirmed by T cell depletion studies, adoptive transfer experiments and using mice genetically deficient in MHC class I and class II molecules [42, 43]. Contact sensitivity to haptens other than DNFB, such as oxazolone [44], DMBA [5] and TNP [45, 46], is mediated by class I-restricted CD8⁺ T cells. However, all these studies have been carried out using strong haptens not usually encountered in the human environment and some strain- and species-specific, as well as hapten-specific differences in the phenotype of the effector populations are likely to exist [47, 48].

All these studies point to the complexity of the T cell response to haptens and emphasize the difference between the mechanisms involved in classical DTH versus CS. One reason for the complexity of the T cell response to haptens may be linked to their property to be presented as haptenated peptides by both MHC class I and class II molecules. For a given hapten, both hapten-specific class I-restricted CD8⁺ T cells and class II-restricted CD4⁺ T cells can be activated. The contribution of hapten presentation by MHC class I and class II molecules in the pathophysiology of CS was tested using mice genetically deficient in MHC class I or class II molecules [43, 49]. Results showed that DC could activate CD8⁺ effector cells of CS through hapten presentation by MHC class I molecules and CD4⁺ regulatory T cells through hapten presentation by MHC class II molecules (Fig. 5). Thus a functional dichotomy exists between class I and class II presentation of haptenated peptides by DC, confirming the fundamental differences between CS and classical DTH.

Taken together, these studies demonstrate that hapten presentation by MHC class I molecules can prime naive CD8⁺ effector T cells in the absence of T cell help. The precise nature of the CD8⁺ T cells and the mechanisms by which they induce the inflammation in CS has not yet been determined. CD8⁺ T cells could mediate CS responses through cytotoxic activity or through the release of type 1 inflammatory cytokines. It has been recently shown in in vitro experiments that the effector CD8⁺ T cell population primed by hapten sensitization in contact sensitivity is distinguished by a type 1 pattern of cytokine production (production of IFN-gamma) [44] (Fig. 6). IL-12, produced by dendritic cells is a dominant factor in directing a type 1 phenotype development of naive T cells [50]. It is likely that IL-12 is the cytokine responsible for the priming of type 1 CD8⁺ T cells in ACD. Cytokine production does not exclude however the contribution of cytotoxic mechanisms to tissue injury.

Activation of hapten-specific T lymphocytes is followed by a rapid expansion of these cells, which then migrate from the lymph nodes to the whole organism first by efferent lymph vessels and by general blood circulation thereafter (Fig. 1, step 4). Hapten-specific T lymphocytes which have been stimulated by hapten-presenting LC express a skin homing receptor, named CLA (cutaneous lymphocyte-associated antigen) at their surface, and are able to migrate from the post-capillary venules in the dermis [51]. All the cellular elements necessary to the development of a CS reaction upon challenge are then present in the blood and lymphoid organs.

Expression of the contact sensitivity reaction

The sensitization step leads to the generation of hapten-specific memory T lymphocytes. In sensitized individuals, the second and subsequent epicutaneous contacts with the hapten leads to a typical cutaneous inflammation involving the dermis and the epidermis in humans and involving mainly the dermis in mice.

The relative contribution of the different epidermal and dermal cell types in the activation of effector T cells is still a matter of debate. It is currently thought that LC elicit the CS response by presenting Ag to trafficking Ag-specific T cells within the skin. Epidermal LC could load the hapten and migrate to the dermis where they can activate memory CLA⁺ specific T lymphocytes (Fig. 1, step 5), leading to T cell activation and cytokine production, especially of IL-1, IL-2, IFN-gamma and TNF-alpha (Fig. 1, step 6). The cutaneous lymphocyte antigen (CLA, HECA-452 antigen) is supposed to represent a skin-homing receptor for human memory T cells allowing a selective, trans-endothelial migration of memory/effector T cells in vitro by interaction with E-selectin on endothelial cell layers after activation with proinflammatory cytokines [52]. Whereas epidermal Langerhans cells (LC) are thought to be the principal APC for initiation of CS responses, their role as APC in the effector phase of CS is still unclear. Recent studies demonstrating that corticosteroid- and UV-induced LC depletion paralleled an increase in the CS reaction suggested that resident LC are not the relevant APC in the effector phase of CS and that they may even provide down-regulatory, rather than stimulatory, signals [13]. Other cell types including dermal DC and keratinocytes have been postulated to be, at least in part, responsible for the generation of T cell activation.

The trafficking pathways of hapten-specific T cells activated upon hapten presentation is still not defined precisely. It has been determined by the use of a limiting dilution procedure that, between 1 in 100 and 1 in 3,000 of the T cells present in the lesions of urushiol dermatitis, are specific for the inducing allergen, therefore only a minute fraction of the infiltrating T cells are hapten-specific [53]. Activated effector T cells produce inflammatory cytokines characteristic of Th1 cells in humans and mice [44, 54, 55]. Hapten-specific T cell activation is followed by activation of other cell types, particularly keratinocytes and endothelial cells. Keratinocytes are also activated directly by hapten application. Activated keratinocytes produce pro-inflammatory cytokines, chemokines and express surface ICAM-1 and MHC class II molecules, which can be important in T cell migration to epidermis as well as in hapten presentation. Haptenated Ia⁺ keratinocytes cannot prime for CS when injected into naive mice, suggesting they cannot efficiently present haptenated peptides to effector T cells. However, they could induce hapten-specific unresponsiveness which is in favour of a regulatory role of keratinocytes in CS [56].

Endothelial cell activation is a critical step for the recruitment of inflammatory cells, which will lead to the establishment of contact dermatitis lesions. Activated endothelial cells modify the expression of adhesion molecules (selectins, integrins, chemokines and their receptors) in the post-capillary venules, which allows leucocyte migration from the blood vessel to the dermis (Fig. 1, step 7). Recruitment of inflammatory cells occurs in several steps of rolling and adhesion to endothelial cells, which finally end in the leucocyte migration through the vascular endothelium into the dermis [57]. Once in the dermis, leucocytes migrate to the superficial dermis and to the epidermis, and induce histological changes typical of contact dermatitis, namely epidermal oedema leading to the exocytosis of T lymphocytes in the epidermis and the development of vesicles.

Down-regulation of hapten-specific cutaneous inflammation

The mechanisms underlying the down-regulation of acute or chronic inflammatory reactions are still not well understood. CS is a subacute, self-limited skin inflammatory reaction. The skin inflammation of a positive patch test in a patient allergic to a hapten will last from 3 to 6 days and fades spontaneously. In murine models of CS, the inflammation peaks at 24-48 hrs and decreases by day 5-7. The rapid decrease of the skin inflammation has been thought to be due to clearance of the hapten from the skin. However, studies using the hapten Rhodamin B have shown that it could be found in the epidermis ten days after the skin painting. At that time, Rhodamin B-labeled epidermal LC continued to emigrate regularly from the epidermis and thus could potentially continue to present haptens to specific T cells [58]. Although haptens may not all behave like Rhodamin B, i.e. remain in the epidermis for a long time, these observations suggest that mechanisms other than simple elimination from the skin may account for the down-regulation of the CS reaction. The inhibition of the inflammatory response might be due to anergy or active suppression. It might require the interaction of several cellular types (keratinocytes, dendritic cells or other antigen-presenting cells and lymphocytes) and be mediated by humoral (anti-inflammatory cytokines) or cytotoxic mecanisms (lysis of effector cells).

Murine models of CS have again been used for the characterization of the cellular and humoral factors responsible for the down-regulation of hapten-induced cutaneous inflammation. Results from several laboratories have clearly shown that CS was down-regulated by CD4⁺ T cells, since (1) mice depleted of class II-restricted CD4⁺ cells [42, 43, 46] develop an enhanced and prolonged CS response as compared to normal non-treated controls (Fig. 7); (2) adoptive transfer of hapten-specific CD4⁺ T cells from wild type mice into previously sensitized MHC I⁺II^- mice, deficient in endogenous CD4⁺ T cells abolishes the CS reaction [46]; (3) injection of hapten-treated I^-II⁺ DC into sensitized wild type mice inhibits the development of the inflammatory reaction by priming a regulatory CD4⁺ T cell population [49]. This regulatory CD4⁺ T cell population produced type 2 cytokines (IL-4 and IL-10) [44] (Fig. 6). As for the effector cells, there might be however some strain- and species-specific and hapten-specific differences in the phenotype of the regulatory populations. Furthermore, there could be differences in the regulatory populations generated as a result of a simple epicutaneous application [43, 44, 46, 49] and as a result of a special treatment aimed at inducing tolerance, such as low dose tolerance [59], oral tolerance [60, 61] and UV-induced tolerance [62]. In the latter models CD4⁺ as well as CD8⁺ T cells have been found to down-regulate the CS reaction. These results highlight the diversity of the populations that might be involved in the down-regulation of an inflammatory response, as well as the complexity of the CS reaction.

Two type 2 cytokines have been extensively studied as suppressors of the contact sensitivity reaction: IL-4 and IL-10 (Fig. 6). IL-10 is a potent immunosuppressive cytokine via the inhibition of antigen-presenting cell functions. IL-10 has been shown to down-regulate CS [62-64] as well as most of the classical DTH reactions [65]. IL-10-treated mice develop a diminished inflammation which is specific to the hapten tested [62-64]. In vivo application of IL-10 by intradermal injection prior to epicutaneous application of TNCB induced antigen-specific tolerance and impeded the induction of pro-inflammatory cytokines [66]. The role of endogenously produced IL-10 in the regulation of inflammatory and immune reactions in the skin was tested in mice with targeted disruptions of the IL-10 (IL-10 KO) gene [66]. IL-10 KO mice mounted an exaggerated CS response to oxazolone, increased in both magnitude and duration as compared with normal mice. Based on all these studies, IL-10 should be considered as a natural suppressant of CS. Initially described as a product of Th2 cells able to block the IFN-gamma production by Th1 cells [68], IL-10 was subsequently found to be produced by several cell types, including keratinocytes. The down-regulating effect of IL-10 could be due to the inhibition of the functional maturation of DC, since IL-10-treated DC induced hapten-specific tolerance in recipient mice [69].

The direct effect of IL-4 in the CS reaction has remained controversial; it has been found to be down-regulatory [70], to have no effect [67], or to be pro-inflammatory [71]. However, data showing that IL-4 down-regulates CS prevail [70, 72]. Furthermore it has been shown that IL-4 is an essential environmental factor, directing the differentiation of naive Th0 T cells to a Th2 phenotype [73-75], thus shifting them from the Th1 phenotype characterizing the CS response.

Two main types of CD may be distinguished: allergic contact dermatitis (ACD) and irritant contact dermatitis (ICD). Allergic contact dermatitis is a T cell-mediated inflammatory reaction occurring at the site of challenge with a contact allergen in sensitized individuals.

Haptens are electrophilic substances, which can bind covalently with the nucleophilic residues of cutaneous proteins.

T cells recognize haptens as structural entities bound covalently or by complexation to peptides anchored in the grooves of major histocompatibility (MHC) class I and class II antigens.

Langerhans cells (LC) are epidermal MHC class II⁺ dendritic cells (DC) specialized in antigen presentation. LC form with their dendrites an epidermal cellular network which allows the capture of haptens that have penetrated the skin barrier

Activation of the naive hapten-specific T lymphocytes by LC having loaded the hapten in the epidermis, occurs in the draining lymph nodes.

LC undergo morphological, phenotypic and functional modifications during their migration from the epidermis to the lymph nodes.

LC are the only cells in the epidermis which constitutively express major histocompatibility (MHC) class II molecules, and expression of these molecules is strongly up-regulated by haptens and not by irritants.

Other surface molecules that can be up-regulated by hapten uptake are ICAM-1 (CD54) and B7-2 (CD86).

LC are particularly efficient at antigen processing in the epidermis, where they pick up haptens, while antigen processing becomes less efficient upon migration to the draining lymph nodes. Conversely, antigen presenting capacities of LC that have reached the draining lymph nodes are strongly increased, which allows activation of naive T lymphocytes in the paracortical areas.

In the case of classical protein antigens, antigen recognition by specific T lymphocytes occurs by interaction of the T cell receptor (TcR) expressed by CD8⁺ or CD4⁺ T cells with processed antigenic peptides associated with MHC class I or class II molecules, respectively. In the case of haptens, T lymphocytes usually recognize hapten-modified peptides in the groove of MHC molecules.

Haptenization of fixed cells has shown that MHC-associated peptides may also be directly modified at the cell surface. The contribution of hapten processing to the priming of hapten-specific T cells has not been firmly established.

Unlike classical delayed-type hypersensitivity (DTH) to protein or cellular antigens which is mediated primarily by MHC class II-restricted CD4⁺ T cells, the T cell response to haptens in CS appears more complex.

Both CD4⁺ and CD8⁺ hapten-specific T cells could mediate the skin inflammatory reaction. Recent studies have clearly demonstrated that hapten-specific CD8⁺ T cells could mediate the CS reaction in the absence of CD4⁺ T cells.

For a given hapten, both hapten-specific class I-restricted CD8⁺ T cells and class II-restricted CD4⁺ T cells can be activated.

DC activate CD8⁺ effector cells of CS through hapten presentation by MHC class I molecules and CD4⁺ regulatory T cells through hapten presentation by MHC class II molecules.

A functional dichotomy exists between class I and class II presentation of haptenated peptides by DC, confirming the fundamental differences between CS and classical DTH.

CD8⁺ T cells could mediate CS responses through cytotoxic activity or through the release of type 1 inflammatory cytokines.

The sensitization step leads to the generation of hapten-specific memory T lymphocytes. In sensitized individuals, the second and subsequent epicutaneous contacts with the hapten leads to a typical cutaneous inflammation involving the dermis and the epidermis.

LC elicit the CS response by presenting Ag to trafficking Ag-specific T cells within the skin leading to T cell activation and cytokines production.

Activated effector T cells produce inflammatory cytokines characteristic of Th1 cells.

Hapten-specific T cell activation is followed by activation of other cell types, particularly keratinocytes and endothelial cells. Keratinocytes are also activated directly by hapten application. Activated keratinocytes produce pro-inflammatory cytokines.

Endothelial cell activation is a critical step for the recruitment of inflammatory cells, which will lead to the establishment of contact dermatitis lesions.

Recruitment of inflammatory cells occurs in several steps of rolling and adhesion to endothelial cells, which finally end in leucocyte migration through the vascular endothelium into the dermis. Leucocytes migrate to the superficial dermis and to the epidermis, and induce histological changes typical of contact dermatitis, namely epidermal oedema leading to the exocytosis of T lymphocytes in the epidermis and the development of vesicles.

CS is a subacute, self-limited skin inflammatory reaction.

Mechanisms other than simple elimination from the skin may account for the down-regulation of the CS reaction. The inhibition of the inflammatory response might be due to the interaction of several cellular types (keratinocytes, dendritic cells or other antigen-presenting cells and lymphocytes) and be mediated by humoral (anti-inflammatory cytokines) or cytotoxic mecanisms (lysis of effector cells).

Murine models of CS have shown that CS was down-regulated by CD4⁺ T cells.

Two type 2 cytokines have been extensively studied as suppressors of the contact sensitivity reaction: IL-4 and IL-10.

IL-10 has been shown to down-regulate CS as well as most of the classical DTH reactions.

Initially described as a product of Th2 cells able to block the IFN-gamma production by Th1 cells, IL-10 was subsequently found to be produced by several cell types, including keratinocytes.

The direct effect IL-4 in the CS reaction has remained controversial.

Hapten-specific cutaneous inflammation can be viewed as the result of the activation of two T cell populations endowed with opposite functions: effector and regulatory. The duration and the severity of the cutaneous inflammation would be related to the activation and to the sizes of the respective compartments of effector and regulatory T cells, with, at the two endpoints, absence of inflammatory reaction in sensitized individuals and chronic contact dermatitis.

ACD - allergic contact dermatitis

APC - antigen presenting cell

CD - contact dermatitis

CHS - contact hypersensitivity

CS - contact sensitivity

CLA - cutaneous lymphocyte antigen

DC - dendritic cell

DMBA - dimethylbenz(a)anthracene

DNFB - 2,4-dinitrofluorobenzene

DTH - delayed type hypersensitivity

FITC - fluorescein isothiocyanate

ICD - irritant contact dermatitis

LC - Langerhans cell

MHC - major histocompatibility complex

PAH - polyaromatic hydrocarbon

TcR - T cell receptor

TNP - trinitrophenyl

CONCLUSION

To summarize the data presented, hapten-specific cutaneous inflammation can be viewed as the result of the activation of two T cell populations endowed with opposite functions: effector and regulatory. The duration and the severity of the cutaneous inflammation would be related to the activation and to the sizes of the respective compartments of effector and regulatory T cells, with, at the two endpoints, absence of inflammatory reaction in sensitized individuals despite further contact with the hapten (tolerance) and chronic contact dermatitis. This approach to the pathophysiology of contact dermatitis might explain the presence of hapten-specific memory T cells in the peripheral blood of patients with negative patch tests, totally devoid of past or present clinical manifestation (53, 54).

Acknowledgements

We are indebted to Bioderma Laboratory for technical assistance and financial support.

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