The histology of co-carcinogenesis in mouse skin

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Antagonism of croton oil inflammation by topical emu oil in CD-1 mice

Emu oil is derived from the emu (Dromaius novaehollandiae), which originated in Australia, and has been reported to have anti-inflammatory properties. Inflammation was induced in anesthetized CD-1 mice by applying 50 microL of 2% croton oil to the inner surface of the left ear. After 2 h, the area was treated with 5 microL of emu, fish, flaxseed, olive, or liquified chicken fat, or left untreated. Animals were euthanized at 6 h postapplication of different oils, and earplugs (EP) and plasma samples were collected. Inflammation was evaluated by change in earlobe thickness, increase in weight of EP tissue (compared to the untreated ear), and induction in cytokines interleukin (IL)-1alpha and tumor necrosis factor-alpha (TNF-alpha) in EP homogenates. Although reductions relative to control (croton oil) were noted for all treatments, auricular thickness and EP weights were significantly reduced (-72 and -71%, respectively) only in the emu oil-treated group. IL-1alpha levels in homogenates of auricular tissue were significantly reduced in the fish oil (-57%) and emu oil (-70%) groups relative to the control group. The cytokine TNF-alpha from auricular homogenates was significantly reduced in the olive oil (-52%) and emu oil (-60%) treatment groups relative to the control group. Plasma cytokine levels were not changed by croton oil treatment. Although auricular thickness and weight were significantly correlated with each other (r = 0.780, P < 0.003), auricular thickness but not weight was significantly correlated with cytokine IL-alpha (r = 0.750, P < 0.006) and TNF-alpha (r = 0.690, P < 0.02). These studies indicate that topical emu oil has anti-inflammatory properties in the CD-1 mouse that are associated with decreased auricular thickness and weight, and with the cytokines IL-1alpha and TNF-alpha.

Anti-inflammatory sesquiterpene lactones from Lourteigia ballotaefolia

Three sesquiterpene lactones were isolated from Lourteigia ballotaefolia (H. B. K.). 9beta-hydroxy-atripliciolide-8- O-tiglate ( 1) was isolated for the first time from this plant and was previously reported in Conocliniopsis prasiifolia (DC) K. et R., 9beta-hydroxy-atripliciolide-8- O-(5'-acetoxytiglate) ( 2) had been already reported in this species. The minor component, 9beta-(tigloyloxy)-atripliciolide, is a new compound. The anti-inflammatory activity of compounds 1 and 2 was evaluated using the croton oil ear test in mice.

Topical anti-inflammatory activity of extracts and compounds from Thymus broussonettii

The topical anti-inflammatory activity of four extracts from Thymus broussonetii Boiss (Labiatae) leaves, a herbal drug used in Moroccan traditional medicine, has been studied using the croton oil ear test in mice. A bioassay-oriented fractionation revealed that the pharmacological activity is mainly in the chloroform extract. Fractionation and analysis of this extract allowed the identification of ursolic acid and oleanolic acid as the main anti-inflammatory principles. Some flavonoids (luteolin, eriodictyol, thymonin) and glycosides (luteolin-7-O-glucoside, luteolin-3'-O-glucuronide, eriodictyol-7-O-glucoside) were also isolated from the methanol extract.

The involvement of 5-HT3 and 5-HT4 receptors in two models of gastrointestinal transit in mice

Our aim was to study the involvement of 5-hydroxytryptamine (5-HT)(3) and 5-HT(4) receptors in two models of gastrointestinal transit (GIT) in mice: the 5-hydroxytryptophan (5-HTP)-induced diarrhea and intestinal inflammation produced by an irritant agent, croton oil (CO). 5-HTP (10 mg/kg) produced diarrhea that was significantly inhibited after pretreatment with ondansetron (5-HT(3) antagonist) or RS 39604 (5-HT(4) antagonist) (1-5 mg/kg). The GIT speed was increased after CO and 5-HTP administration. 5-HT(3-4) antagonists decreased GIT after 5-HTP-treatment but not after CO-treatment. Our results show that 5-HT(3) and 5-HT(4) receptors are involved in 5-HTP-induced diarrhea. This may be the reason why 5-HT(3-4) antagonists could be useful in the treatment of carcinoid syndrome diarrhea. 5-HT(3-4) antagonists were not effective in the modifications of GIT; nevertheless, they could be useful in the treatment of inflammatory bowel diseases because some symptoms as abdominal pain, discomfort or abnormal bowel function are modulated via 5-HT(3).

Studies on the anti-inflammatory activity of phytopharmaceuticals prepared from Arnica flowers

Phytopharmaceuticals prepared from flowerheads of Arnica montana of Spanish origin and of the new type "Arbo", which can be easily and economically cultivated, were studied for their capability to impair activation of the transcription factors NF-kappa B and NF-AT. Both proteins are responsible for the transcription of genes encoding various inflammatory mediators. Additionally, their influence on the release of the cytokines IL-1 and TNF-alpha were examined. The inhibitory activities correlate with their quantitative and qualitative content of sesquiterpene lactones (Sls). Moreover, it was shown that the inhibitory potency of 11 alpha,13-dihydrohelenalin derivatives being the main Sls in the Spanish flowers depend on their esterfication. Compounds with unsaturated acyl moieties, such as methacrylate and tiglinate, exhibited a stronger activity in the NF-kappa B EMSA as well as in the croton oil ear test in mice than the acetate derivative.

Direct effect of croton oil on intestinal epithelial cells and colonic smooth muscle cells

AIM

To investigate the direct effect of croton oil (CO) on human intestinal epithelial cell (HIEC) and guinea pig colonic smooth muscle cells in vitro.

METHODS

Growth curves of HIEC were drawn by MTT colorimetry. The dynamics of cell proliferation was analyzed with flow cytometry, and morphological changes were observed under light and electron microscopy after long-term (6 weeks) treatment with CO. Expression of cyclooxygenase-2 (COX-2) mRNA was detected by dot blot in HIEC treated with CO. Genes related to CO were screened by DD-PCR, and the direct effect of CO on the contractility of isolated guinea pig colonic smooth muscle cells was observed.

RESULTS

High concentration (20-40 mg x L(-1)) CO inhibited cell growth significantly (1, 3, 5, 7d OD sequence: (20 mg x L(-1)) 0.040+/-0.003, 0.081+/-0.012, 0.147+/-0.022,0.024+/-0.016; (40 mg x L(-1)) 0.033+/-0.044, 0.056+/-0.012, 0.104+/-0.010, 0.189+/-0.006; OD control 0.031+/-0.008, 0.096+/-0.012, 0.173+/-0.009, 0.300+/-0.016, P<0.01), which appeared to be related directly to the dosage. Compared with the control, the fraction number of cells in G1 phase decreased from 0.60 to 0.58, while that in S phase increased from 0.30 to 0.34 and DNA index also increased after 6 weeks of treatment with CO (the dosage was increased gradually from 4 to 40 mg x L(-1)). Light microscopic observation revealed that cells had karyomegaly, less plasma and karyoplasm lopsidedness. Electron microscopy also showed an increase in cell proliferation and in the quantity of abnormal nuclei with pathologic mitosis. Expression of COX-2 mRNA decreased significantly in HIEC treated with CO. Thirteen differential cDNA fragments were cloned from HIEC treated with CO, one of which was 100 percent homologous with human mitochondrial cytochrome C oxidase subunit II. The length of isolated guinea pig colonic smooth muscle cells was significantly shortened after treatment with CO (P<0.05).

CONCLUSION

At a high CO concentration (>20 mg x L(-1)), cell growth and proliferation are inhibited in a dosage-dependent manner. Increase in cell proliferation and in malignant conversion of the cellular phenotype is observed in cells cultured chronically with CO. COX-2 mRNA expression decreases significantly, while human mitochondrial cytochrome C oxidase subunit IImRNA expression increases significantly in HIEC treated with CO. CO also has a direct effect on the contractility of Guinea pig colonic smooth muscle cells.

Analysis of the temporal expression of chemokines and chemokine receptors during experimental granulomatous inflammation: role and expression of MIP-1alpha and MCP-1

1. Chemokine expression and function was monitored in an experimental model of granulomatous tissue formation after injection of croton oil in complete Freund's adjuvant (CO/CFA) into mouse dorsal air-pouches up to 28 days. 2. In the first week, mast cell degranulation and leukocyte influx (mononuclear cell, MNC, and polymorphonuclear cell, PMN) were associated with CXCR2, KC and macrophage inflammatory protein (MIP)-2 mRNA expression, as determined by TaqMan reverse transcriptase-polymerase chain reaction. KC ( approximately 400 pg x mg protein(-1), n=12) and MIP-2 (approximately 800 pg x mg protein(-1), n=12) proteins peaked at day 7, together with myeloperoxidase (MPO) activity. Highest MIP-1alpha (>1 ng x mg protein(-1), n=12) levels were measured at day 3. 3. After day 7, a gradual increase in CCR2 and CCR5 mRNA, monocyte chemoattractant protein (MCP)-1 mRNA and protein expression was measured. MCP-1 protein peaked at day 21 (approximately 150 pg x mg protein(-1), n=12) and was predominantly expressed by mast cells. A gradual increase in N-acetyl-beta-D-glucosaminidase (NAG) activity (maximal at 28 days) was also measured. 4. An antiserum against MIP-1alpha did not modify the inflammatory response measured at day 7 (except for a 50% reduction in MIP-1alpha levels), but provoked a significant increase in MPO, NAG and MCP-1 levels as measured at day 21 (n=6, P<0.05). An antiserum to MCP-1 reduced NAG activity at day 21 but increased MPO activity values (n=8, P<0.05). 5. In conclusion, we have shown that CO/CFA initiates a complex inflammatory reaction in which initial expression of MIP-1alpha serves a protective role whereas delayed expression of MCP-1 seems to have a genuine pro-inflammatory role.

Topical anti-inflammatory activity of a new germacrane derivative from Achillea pannonica

The topical anti-inflammatory activity of a germacrane derivative [1,4-dihydroxy-germacra-5E-10(14)-diene; DHGD] isolated from Achillea pannonica Scheele (Asteraceae) was investigated employing the Croton oil-induced dermatitis in the mouse ear. Its effects on the oedematous response and on leukocytes infiltration are described. The germacrane derivative significantly inhibited ear oedema in a dose-dependent manner, with an ID(50) of 0.40 micromol/cm(2). DHGD (0.75 micromol/cm(2)) provoked a global inhibition of the oedematous response (61 %) higher than that induced by an equimolar dose of indomethacin (43 %) within 24 hours; the reduction induced by hydrocortisone (0.10 micromol/cm(2)) was 68 %. The effect of DHGD (61 % inhibition) was higher than that of the equimolar dose of indomethacin (51 % inhibition) also on granulocytes recruitment at the site of inflammation. Hydrocortisone (0.10 micromol/cm(2)) reduced the cellular infiltrate by 44 %.

Tephrosia purpurea alleviates phorbol ester-induced tumor promotion response in murine skin

In recent years, considerable emphasis has been placed on identifying new cancer chemopreventive agents, which could be useful for the human population. Tephrosia purpurea has been shown to possess significant activity against hepatotoxicity, pharmacological and physiological disorders. Earlier we showed that Tephrosia purpurea inhibits benzoyl peroxide-mediated cutaneous oxidative stress and toxicity. In the present study, we therefore assessed the effect of Tephrosia purpurea on 12-O-tetradecanoyl phorbal-13-acetate (TPA; a well-known phorbol ester) induced cutaneous oxidative stress and toxicity in murine skin. The pre-treatment of Swiss albino mice with Tephrosia purpurea prior to application of croton oil (phorbol ester) resulted in a dose-dependent inhibition of cutaneous carcinogenesis. Skin tumor initiation was achieved by a single topical application of 7,12-dimethyl benz(a)anthracene (DMBA) (25 microg per animal per 0.2 ml acetone) to mice. Ten days later tumor promotion was started by twice weekly topical application of croton oil (0.5% per animal per 0.2 ml acetone, v /v). Topical application of Tephrosia purpurea 1 h prior to each application of croton oil (phorbol ester) resulted in a significant protection against cutaneous carcinogenesis in a dose-dependent manner. The animals pre-treated with Tephrosia purpurea showed a decrease in both tumor incidence and tumor yield as compared to the croton oil (phorbol ester)-treated control group. In addition, a significant reduction in TPA-mediated induction in cutaneous ornithine decarboxylase (ODC) activity and [3H]thymidine incorporation was also observed in animals pre-treated with a topical application of Tephrosia purpurea. The effect of topical application of Tephrosia purpurea on TPA-mediated depletion in the level of enzymatic and non-enzymatic molecules in skin was also evaluated and it was observed that topical application of Tephrosia purpurea prior to TPA resulted in the significant recovery of TPA-mediated depletion in the level of these molecules, namely glutathione, glutathione S-transferase, glutathione reductase and catalase. From these data we suggest that Tephrosia purpurea can abrogate the tumor-promoting effect of croton oil (phorbol ester) in murine skin.

Croton zehntneri essential oil: effects on behavioral models related to depression and anxiety

Croton zehntneri (Cz), a popular plant used to treat "nervous disturbance", contains a complex mixture of compounds, including substances exhibiting central nervous system activity. The effects of Cz essential oil administration (p.o.) on the rat's central nervous system were studied in behavioral models used to evaluate anxiety and antidepressive drugs. The results showed that administration of Cz essential oil: 1) increased the immobility duration measured in the forced swimming test as compared to control group (control = 89.8 +/- 45.8; 1 microl = 153.0 +/- 48.7; 3 microl = 157.4 +/- 45.3; 10 microl = 145.3 +/- 51.0); 2) reduced the locomotion frequency observed in the open field (control = 62.5 +/- 22.7; 3 microl = 38.0 +/- 13.5; 10 microl = 39.2 +/- 22.2); 3) had no effect on the experimental group (1 microl) observed in open field; 4) had no effect on animals tested in social interactions, plus-maze and holeboard tests. These data suggest that Cz oil produced central depressor effects in rats without any anxiety alterations. These results may explain the popular use of this plant in Brazilian folk medicine for treating "nervous disturbances".

Chemical activation of innate and specific immunity in contact dermatitis

Recent reports have suggested that chemical-induced allergic contact dermatitis may not be a traditional type IV hypersensitivity, in part due to the dual irritant and antigenic properties of sensitizing chemicals. In order to investigate the contribution of these properties to the molecular and cellular mechanism underlying allergic contact dermatitis, we evaluated oxazolone-induced changes in cell populations and cytokine production in the dermis of transgenic mice with impaired innate immunity (the FcgammaR subunit knockout mouse), and absent specific immunity (the athymic mouse), and the appropriate B6,129F2 and C57BL/6 control mice. Oxazolone and croton oil were applied in a single sensitizing dose, or in sensitizing and challenge doses, and the dermal response was evaluated by immunohistochemistry. In the wild type mice, with or without sensitization to oxazolone or croton oil, we observed mixed Th1/Th2 cytokine production and both CD4+ and CD8+ T lymphocytes; however, the neutrophil was the predominant cell in the dermis, even 72 h after final chemical application. Athymic mice displayed a similar neutrophil response with moderate Th1/Th2 cytokine production, and FcgammaR subunit knockout mice exhibited very mild dermatitis when treated with either oxazolone or croton oil. These results provide support for the hypothesis that allergic contact dermatitis is not a classic delayed type hypersensitivity, demonstrate the importance of the interaction between the irritant and antigenic properties of sensitizing chemicals in the development of allergic contact dermatitis, and suggest that the irritant effect of chemicals may be mediated through the cutaneous innate immune system.

Effects of mu-opioid receptor agonists on intestinal secretion and permeability during acute intestinal inflammation in mice

We evaluated and compared the effects of mu-opioid receptor agonists on mucosal fluid transport and permeability, during acute intestinal inflammation. We hypothesized that inflammation would sensitize mu-opioid receptors in the submucosal plexus and/or enterocytes enhancing the effects of mu-opioid receptor agonists. Inflammation was induced by intragastric administration of croton oil, whereas controls received saline. Fluid transport was assessed by enteropooling, and intestinal permeability by blood-to-lumen passage of [51Cr] etylenediaminetetraacetate ([51Cr] EDTA). Intestinal inflammation induced a significant increase in enteropooling (1.9 times) and permeability (2.5 times). In saline- and croton oil-treated animals, mu-opioid receptor agonists produced dose-related inhibitions of enteropooling and intestinal permeability. During inflammation, the potency of morphine increased 4.8 and 3.7 times, inhibiting enteropooling and intestinal permeability, respectively; the potencies of fentanyl and PL017 similarly increased by approximately three (enteropooling) and two times (permeability) in croton oil animals. All effects were reversed by naloxone and naloxone methiodide. The results show that inflammation increases the inhibitory potency of mu-opioid receptor agonists on secretion and permeability, suggesting a sensitization of peripheral mu-opioid receptors.

Primary immune response in skin and skin-associated lymphoid tissue of interleukin-4 transgenic mice

The interleukin-4 transgenic mice investigated here exhibit a ubiquitous expression of interleukin-4 in all organs, including the skin. In this study, the induction phase of oxazolone-induced local primary contact hypersensitivity and croton oil-induced irritant contact dermatitis in transgenic and wild-type mice was analysed. Compared to wild-type mice, the transgenic mice showed a decreased activation of the skin-draining lymph nodes but a strong hyperreactivity in the skin after topical sensitisation. In contrast to this, both the transgenic and the wild-type mice developed a strong and comparable inflammatory skin reaction after topical irritation. A striking increased expression level of tumour necrosis factor-alpha and macrophage inflammatory protein-2 genes were found in the skin of the transgenic mice during primary local contact hypersensitivity, while both the transgenic and the wild-type mice developed comparable expression levels of these cytokines during irritant contact dermatitis. Compared to wild-type mice, a strongly enhanced expression level of interleukin-6 transcripts derived from epidermal antigen presenting cells were detected in the skin of IL-4 transgenic mice, whereas in the skin-draining lymph nodes of transgenic mice significantly lower levels were detected. We conclude that the migration of epidermal antigen-presenting cells towards the skin-draining lymph nodes is reduced in transgenic mice, which could be due to the different cytokine balance in these mice strains. The atypical irritant-like reaction observed in transgenic mice after topical sensitisation is a phenomenon comparable to atopic diseases and therefore this transgenic strain might be a helpful model for investigating the immunopathophysiological features of these diseases.

Effects of cinnarizine on different experimentally induced oedemas

Experiments with male mice (28-32 g) of the CFLP strain showed that cinnarizine in doses of 2.5, 5.0 or 10.0 mg kg-1 significantly inhibited the extent of ear oedema induced by croton oil, capsaicin or dithranol, in a dose-dependent manner. In rats of the Wistar strain, oedema was induced in the hind paw by subplantar injection of carrageenin, and simultaneously by the application of croton oil to the inner surface of the ear. Preliminary cinnarizine treatment (5, 10 or 20 mg kg-1) inhibited the development of both types of oedema, to a statistically significant extent, in a dose-dependent manner.

Effects of protein kinase C activators and staurosporine on protein kinase activity, cell survival, and proliferation in Tetrahymena thermophila

Autocrine factors prevent cell death in the ciliate Tetrahymena thermophila, a unicellular eukaryote, in a chemically defined medium. At certain growth conditions these factors are released at a sufficient concentration by > 500 cells ml-1 to support cell survival and proliferation. The protein kinase C activators phorbol 12-myristate 13-acetate (PMA) or 1-oleyl 2-acetate glycerol (OAG) when added to 250 cells ml-1 supported cell survival and proliferation. In the presence of the serine and threonine kinase inhibitor staurosporine the cells died both at 250 cells ml-1 in cultures supplemented with either PMA or OAG, or at 2,500 cells ml-1. At 500 cells ml-1 PMA induced the in vivo phosphorylation of at least six proteins. The myelin basic protein fragment 4-14 was phosphorylated in vitro in crude extracts of a culture of 250,000 cells ml-1. Both the in vivo and the in vitro phosphorylation were inhibited by staurosporine.

Effects of indole-3-acetic acid on croton oil- and arachidonic acid-induced mouse ear edema

The indole-3-acetic acid (IAA) is a plant growth hormone (auxin) being considered as a tryptophan metabolite in animals. The main purpose of this work was to verify IAA's topical anti-inflammatory action using croton oil- or arachidonic acid-induced mouse ear edema, in comparison to known anti-inflammatory agents. IAA antioxidant activity was also verified by measuring the inhibition of brain homogenate lipid peroxidation with the thiobarbituric acid reactive substances (TBARS) test. IAA inhibited the action of both croton oil-and arachidonic acid-induced edema in a dose-dependent manner (4.0 mumoles IAA inhibited 75.8% in croton oil and 82.5% in arachidonic acid induced ear edema). Both IAA (5.3 mM) and indomethacin (8.0 mM) inhibited TBARS formation. Data suggest that IAA exhibits antiinflammatory effect possibly by its anti-oxidant activity.

Effects of the 6 beta-7 beta-epoxide of 12-O-tetradecanoyl-phorbol-13-acetate on radiation transformation in vitro

Experiments were performed to compare the abilities of 12-O-tetradecanoyl-phorbol-12-acetate (TPA), croton oil and TPA-6 beta-7 beta-epoxide to enhance radiation transformation in vitro. Both croton oil and TPA-6 beta-7 beta-epoxide were more powerful than TPA in the ability to promote radiation transformation in vitro.

[Modifications of gene expression by tumor promoters]

The modifications of gene expression by tumor promoters were analyzed in vitro and in vivo. The results of slot blot hybridizations showed that tumor promoter TPA induced c-fos and c-myc expressions in mouse fibroblast cell line BALB/3T3 and rat liver, decreased the levels of Rb RNA in BALB/3T3 cell line and of alpha 1-I3 RNA in rat liver. It was also demonstrated that tumor promoter phenobarbital influenced c-fos and c-myc expressions and decreased alpha 1I3 mRNA level in rat liver during a long term experiment. Phenobarbital was found to have no effect on c-fos and c-myc expressions in rat liver during a short experiment. Tumor promoters induced the expressions of c-fos and c-myc which were positively-related to cancer formation and inhibited the expressions of Rb and alpha 1-I3 which were negatively-related to cancer formation. This implied that tumor promotion played an important role in cancer development and tumor promoters exerted their effects selectively according to the attributes of different genes.

Croton zehntneri: possible central nervous system effects of the essential oil in rodents

The effects of essential oil of Croton zehntneri (Euphorbiaceae), orally administered were studied on behavioral parameters using rats and mice. The oil suspension did not modify pentobarbital induced-hypnosis, stereotypic behavior, catalepsy and amphetamine-induced hypermotility. The open-field behaviors were decreased and the minimal convulsant dose of pentylenetetrazole was increased.

In vivo effects of interleukin-10 on contact hypersensitivity and delayed-type hypersensitivity reactions

Interleukin (IL) 10 is a recently discovered cytokine, originally isolated from T-helper 2 (Th2) cells, which inhibits cytokine production of T-helper 1 (Th1) cells. Because Th1 cells appear to be of importance during the contact hypersensitivity reaction (CHS) we hypothesized that IL-10 might modulate the outcome of CHS in vivo. Intraperitoneal injection of murine recombinant IL-10 (1000 ng) into naive mice 24, 72, or 120 h before sensitization by epicutaneous application of 2,4-dinitrofluorobenzene (DNFB) did not affect ear swelling when ears were challenged 5 d later. However, intraperitoneal injection of IL-10 into already sensitized mice 24 h before challenge resulted in a significant suppression of the ear swelling response, suggesting that under the conditions employed IL-10 is able to block the effector phase, but not the induction phase of CHS in vivo. The suppression could be reversed by the concurrent injection of an IL-10 antibody. Moreover, heat inactivation of native IL-10 resulted in loss of the inhibitory capacity. When mice were sensitized by subcutaneous injection of trinitrophenyl-coupled spleen cells (DTH) instead of epicutaneous application of the hapten (CHS), intraperitoneally-injected IL-10 suppressed the effector phase, but also the induction phase of DTH. IL-10 did not inhibit the toxic ear-swelling response induced by topical application of two irritants tested (croton oil or benzalkonium chloride). The capacity of IL-10 to suppress the effector phase of CHS and DTH supports an important role for this cytokine in the downregulation of type IV immune reactions in vivo. The finding that IL-10 suppresses the induction of DTH, but not of CHS, further suggests that CHS and DTH are related but distinct immune reactions.

Differential patterns of epidermal leukocyte infiltration in patch test reactions to structurally unrelated chemical irritants

In previous studies, we showed that a number of aspects of the histopathology of irritant contact dermatitis are profoundly influenced by the chemical nature of the irritant applied. We report here that this phenomenon also extends to the infiltration of leukocytes into the epidermis. Healthy volunteers were patch tested with the following irritants and their appropriate controls: benzalkonium chloride, sodium lauryl sulphate, croton oil, dithranol, nonanoic acid, and propylene glycol. After visually grading the intensity of the resulting inflammation, biopsies were removed and the major phenotypic classes of leukocytes identified immunocytochemically. Dermal and epidermal cell densities were determined, and the expression of several activation/proliferation antigens studied. We found a similar pattern of cellular infiltration in the dermis of all irritant groups; the densities of most of the cell types rising in line with the intensity of inflammation. Within the epidermis, however, there were marked differences in the patterns of cellular infiltration between the irritant groups, leading to poorer correlations between leukocyte density and visual grading. The greatest disparity occurred between croton oil and nonanoic acid biopsies, the former being characterized by the influx of large numbers of leukocytes, the latter showing remarkably little exocytosis. Infiltration of neutrophils occurred to varying degrees with all irritants, but a disproportionately large number were present in sodium lauryl sulphate biopsies. All control groups showed a rise in CD4+ cells, with distilled water also producing increases in CD11c+ cells and neutrophils. A selective influx of CD25+ cells occurred in the epidermis of both irritant and control groups. Our observations further highlight the heterogeneous nature of irritant contact dermatitis, and confirm previous findings that visually negative control patch tests show marked cellular reactivity.