37 Histopathology of Irritant Contact Dermatitis

Home , Dyskeratosis, Hydropic swell, Spongiosis

345

Carolyn M. Willis

Contents the skin [1], and with a histopathology largely indistinguishable from that of the majority of chronic in- 37.1 Introduction . . . 345 flammatory dermatoses. 37.2 Acute Irritant Contact Dermatitis . . . 346 When considering the histopathology of ICD, it is 37.2.1 Epidermal Features . . . 346 important to bear in mind that all of the following pa- 37.2.2 Dermal Features . . . 348 rameters will influence the histopathological changes 37.2.3 Leukocyte Infiltration . . . 349 observed under the light microscope: 37.3 Chronic Irritant Contact Dermatitis . . . 350 37.3.1 Epidermal Features . . . 350 37.3.2 Dermal Features . . . 350 1. Chemical nature and concentration of irritant References . . . 350 chemical In addition to the physicochemical properties of an irritant, which have a direct bearing on the nature of the cellular damage inflicted, concentration effects are also profound. At sufficiently high concentration, 37.1 Introduction many irritants will cause overt tissue necrosis. Lower concentrations produce more subtle changes, partic- Irritant contact dermatitis is a heterogeneous inflam- ularly in the epidermis. matory condition, both clinically and histopathologi- cally. Arising primarily from contact with chemicals, 2. Mode and duration of exposure the inflammation may be acute or chronic in nature, The circumstances of irritant exposure, such as single, depending upon the irritation potential of the sub- occlusive patch testing or repetitive open testing, and stance and the circumstances of exposure. Chemicals the length of time the chemical is in contact with the such as acids, alkalis, and detergents will, at high con- skin, will all influence the severity and nature of re- centration, cause sufficient damage to the skin to in- sponse, and hence the histological picture. duce inflammation after single exposure, while more marginal irritants require repeated exposure to over- 3. Time of tissue sampling come the skin’s innate restorative capacity sufficiently The time at which the tissue sample is taken relative to induce an inflammatory response. to the course of the inflammation is clearly of signifi- The clinical spectrum of acute ICD ranges from a cance. Early onset of chronic ICD, for example, may mild reaction with transient erythema or chapping involve little more than stratum corneum disruption, through to a more florid dermatitis with edema, ve- while a site of healing acute ICD would be character- siculation, bullae formation, exudation, and necrosis ized primarily by epidermal proliferation. [1]. Histopathological features vary accordingly, but, importantly, for mild-to-moderate reactions at least, 4. Individual susceptibility they show a degree of irritant dependency, reflect- One of the idiosyncrasies of the development of ICD, ing the different mechanisms of action of structurally which is particularly apparent in experimental situa- varying chemicals on the cellular components of the tions, is the often very differing severities of reaction skin [2]. Chronic ICD, in contrast, is somewhat more exhibited by individuals under the exact same condi- uniform in appearance, being characterized clinically tions of exposure. In studies carried out in this de- by erythema, dryness, chapping, and thickening of partment, for example, patch testing with the cationic 346 Carolyn M. Willis

detergent, benzalkonium chloride, at a concentration Table 1. Epidermal changes observed after single, occlusive of 1%, resulted in reactions severe enough to cause patch testing with selected irritants blistering of the skin in some individuals, but pro- Combined human and animal data [2, 4–18]. duced little or no visible skin damage in others [3]. Irritant Epidermal features The histopathology of such reactions is therefore cor- respondingly variable. Sodium lauryl sulphate Parakeratosis, spongiosis, 5. Species vesiculation, nuclear/intracyto- The histopathology of irritant reactions induced in plasmic vacuolation, necrosis, animals can differ from that observed in man. This is hydropic swelling, epidermal/ particularly true for the leukocyte inflammatory infil- dermal separation, exocytosis trate in acute ICD. Benzalkonium chloride Necrosis, spongiosis, exocytosis, nuclear/intracytoplasmic In the following sections, consideration will be vacuolation, hydropic swelling given to the epidermal and dermal cellular changes Dithranol Hydropic swelling, spongiosis, which occur in both acute and chronic ICD, includ- intracytoplasmic vacuolation, ing analysis of the distribution and phenotype of re- necrosis, parakeratosis sponding white blood cells. Nonanoic acid Dyskeratosis, spongiosis, nuclear/intracytoplasmic vacuolation, parakeratosis 37.2 Acute Irritant Contact Croton oil Spongiosis, vesiculation, Dermatitis exocytosis, nuclear/intracytoplasmic vacuolation, hydropic 37.2.1 Epidermal Features swelling, parakeratosis Dinitrochloro-benzene Necrosis, epidermal/dermal Much of our understanding of the microscopical fea- separation, spongiosis, nuclear/ tures of the epidermis in acute ICD has come, not intracytoplasmic vacuolation from clinical material, but from skin sites experimen- Sodium hydroxide Epidermal/dermal separation, tally subjected to single, occlusive patch tests con- spongiosis, necrosis, nuclear/in- taining known irritant substances. Table 1 provides tracytoplasmic vacuolation a summary of the predominant histopathological features induced by selected chemicals tested in this Hydrochloric acid Spongiosis, intracytoplas- way [2, 4–18]. Damage is seen to occur at all levels mic vacuolation, necrosis of the epidermis, from the stratum granulosum down Potassium dichromate Intracytoplasmic vacuolation, to the dermo-epidermal junction. Although there is spongiosis, necrosis, epider- little doubt that the majority of these irritants have mal/dermal separation the capacity to cause overwhelming cellular destruc- Toluene Acantholysis, pyknosis, spon- tion if applied at high concentration, under ethically giosis, bullae, necrosis acceptable patch test conditions, the most commonly Trichloroethylene Acantholysis, spongiosis, observed morphological changes are intracellular nuclear vacuolation, necrosis vacuolation and nuclear pyknosis, the extent to which Acetone Acantholysis, spongiosis, nuclear/ these occur being both irritant and concentration de- intracytoplasmic vacuolation pendent. Spongiosis is also widely described, but is again variable in magnitude between irritants and the tion into the epidermis, making them largely indistin- concentration at which they are applied. In the main, guishable from those of ACD. however, spongiosis is much less marked in ICD than In a study comparing the cellular responses to six in allergic contact dermatitis (ACD), although there different irritants after 48-h patch testing, the use of are exceptions, most notably in the reactions to croton plastic embedding media, which facilitates high reso- oil, a mixture of chemicals including 12-O-tetradec- lution light microscopy, permitted the visualization anoylphorbol-13-acetate, where extensive spongiosis of subtle changes to keratinocytes which highlight and vesiculation frequently arise (Fig. 1). Exocytosis, the irritant-dependent nature of ICD [2]. In reactions also, is generally less pronounced in ICD, although, judged visually to be mild to moderate, the anionic again, reactions to croton oil show extensive infiltra- detergent, sodium lauryl sulphate, for example, in- 37 Histopathology of Irritant Contact Dermatitis 347

Fig. 1. Toluidine blue-stained 1-µm plastic section of skin Fig. 2. Skin biopsy taken from an individual patch tested for taken from a healthy individual patch tested for 48 h with cro- 48 h with the anionic detergent, SLS (4%). Marked parakerato- ton oil (0.08%). Extensive spongiosis and exocytosis of pre- sis in the epidermis is evident, a characteristic feature of reac- dominantly mononuclear cells are induced in the epidermis, tions to this irritant and one that is indicative of an increased making the reaction largely indistinguishable from that of density of proliferating keratinocytes (haematoxylin and eosin acute ACD (original magnification ×200) stained paraffin section; original magnification ×400)

Fig. 3. Frozen sections immunoperoxidase labeled with a monoclonal antibody to the Ki-67 (proliferation associated nuclear) antigen. (A) The baseline density of dividing keratinocytes in a biopsy of normal volar forearm skin. (B) The marked increase in the number of proliferating keratinocytes seen in the same individual after 48-h patch testing with SLS (4%) (original magnifications 00)×2

Fig. 4. 48-h human patch test reaction to the 12C long-chain fatty acid, nonanoic acid (80%), showing by light (A; toluidine blue- stained 1 µm plastic section; original magnification ×200) and electron microscopy B( ; original magnification ×4000), the tongues of dyskeratotic keratinocytes extending downwards from the stratum granulosum into the stratum spinosum, commonly induced by this irritant duced primarily parakeratosis, a feature indicative of in vitro studies [20–23]. Of significance to our under- enhanced keratinocyte proliferation (Fig. 2). Confir- standing of the pathogenesis of ICD is the fact that not mation that this is indeed a significant physiological all detergents exert this effect after 48 h; the cationic response to SLS at this time point was subsequently detergent, benzalkonium chloride, tested in parallel obtained immunocytochemically using an antibody on the same individuals, caused mild spongiosis and against the Ki-67 antigen (Fig. 3A, B) [19], and is exocytosis with focal regions of necrosis in lesions of consistent with data derived from other in vivo and similar visual intensity, but no parakeratosis [2]. 348 Carolyn M. Willis

Fig. 5. Toluidine blue-stained 1 µm plastic section of a 48-h dithranol (0.02%) treated human patch test site, showing markedly swollen, palely staining keratinocytes in the stratum granulosum and upper stratum spinosum (A; original magnification ×200). By electron microscopy (B; original magnification ×4000), mitochondrial clustering around the nucleus is apparent, with finely dispersed filaments and ribosomes within the cytoplasm

Fig. 6. Immunoperoxidase labeling of frozen sections with a monoclonal antibody against HLA-DR, which localizes the vast majority of leukocytes present in the skin. Large numbers of HLA-DR+ cells are seen in the epidermis of a mild 48 h patch test reaction to croton oil (0.08%) (A; original magnification ×200). In contrast, a reaction to nonanoic acid (80%) of the same intensity, in the same individual, shows very little exocytosis (B; original magnification ×200)

Other alterations to keratinocytes which exhibit 37.2.2 Dermal Features a strong irritant-dependency include dyskeratosis, which, in the irritant series described above, was Dermal changes are also influenced by the factors set induced almost exclusively by the 12-C long-chain out above, although common to most acute irritant fatty acid, nonanoic acid (Fig. 4A, B) [2], hydropic reactions are disruption and/or degeneration of col- swelling, which was a feature particularly common lagen [5, 8, 9, 11]. Edema is generally less pronounced to dithranol-induced irritation (Fig. 5A, B) [2], and than that seen in ACD, but can be quite significant acantholysis which occurs mainly, although not ex- with some irritants and where severe irritation has clusively, with cantharidin and chlorinated organic been induced. Degranulation of mast cells has been solvents [18, 24]. described following DMSO application in man [25], Although we now have a reasonably good under- but the extent to which this happens with other ir- standing of the histology of experimentally-induced ritants is unclear. Dilatation of blood vessels and lym- acute ICD as it peaks at around 48 h, time course phatics also occurs, although again to lesser extent studies, particularly in man, are still very much lack- than in ACD [18]. A notable exception to this is seen ing. Despite the variations seen between irritants early in the reactions to solvents, where profound effects on in the reactions, it is likely that a greater degree of on vasculature occur [26]. commonality will exist during the eventual healing In a more recent study of TNCB-induced irritation process. Increased keratinocyte proliferation, for in- in mice, evidence was presented of a significant con- stance, will almost certainly occur at some point prior tribution made by platelets to the pathogenesis of ICD, to the full restoration of barrier function in all clini- these having been found to adhere to the venular en- cally apparent reactions, irrespective of the chemical dothelium at an early stage in the response, preceding nature of the initiating agent. and influencing subsequent dermal edema [27]. 37 Histopathology of Irritant Contact Dermatitis 349

Fig. 7. An example of chronic ICD of the palm of the hand, Fig. 8. Toluidine blue-stained 1 µm plastic section of chronic showing acanthosis, elongation of rete ridges, spongiosis, and ICD of the palm of the hand, illustrating the perivascular exocytosis, with a marked cellular infiltrate in the upper dermis mononuclear infiltrate in the upper dermis (original magnifi- (toluidine blue-stained 1 µm plastic section, original magnification ×200) cation ×100)

37.2.3 Leukocyte Infiltration virtually indistinguishable from ACD (Fig. 6A) [29]. Nonanoic acid, on the other hand, gives rise to very During the development of ICD, as in the majority little epidermal infiltration, even in those reactions of acute inflammatory dermatoses, leukocytes are judged clinically to be moderate (Fig. 6B) [29]. attracted into both the dermis and the epidermis; Although there is evidence of quantitative variations the composition and density of the cellular infiltrate in the density of infiltrating cells in human ICD related again vary according to the circumstances of induc- to the irritant applied, qualitatively the infiltrates ap- tion. Experimental induction of irritation in rodents, pear to be very similar. Immunophenotypic analysis of in particular the guinea pig, results in a significant the mononuclear cell infiltrate by a number of groups, influx of neutrophils, irrespective of the intensity of has repeatedly revealed that, as in ACD, CD4+ T lym- response [10, 28]. In humans, this tends not to be the phocytes are generally in the majority, with an accom- case. Neutrophils generally only infiltrate in substan- panying admixture of CD8+ cells, macrophages, and tial numbers where overt necrosis has been induced CD1a+ cells [29, 30–32]. B cells, natural killer cells, or where infection has occurred [29, 18]. and follicular dendritic cells are absent or rare. Studies Mild to moderate responses in human subjects are by Brasch et al. (1992) showed that the majority of T characterized by the influx of predominantly mono- cells in both ICD and ACD express the CD45RO anti- nuclear cells, the numbers seen in the dermis and the gen and are therefore of memory phenotype, while sig- extent to which they infiltrate up into the epidermis nificant numbers bear interleukin-2 receptor α chains being influenced by the intensity of reaction and the (CD25) [33]. Most infiltrating cells are HLA-DR+, chemical applied. Croton oil, for example, attracts with just under a half bearing the transferrin recep- many cells into the application site, with significant tor (CD71); around 5% are actively dividing, as deter- exocytosis apparent after 48 h, producing reactions mined by the expression of the Ki-67 antigen [33]. 350 Carolyn M. Willis

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