Biomedical subjects

R J Dearman

Publications and source records attributed to R J Dearman.

At least 145 records · Page 8Linked to original sources

PubMed1992

The mechanisms and evaluation of chemically induced allergy.

Many chemicals are able to induce contact allergy, a delayed-type hypersensitivity reaction. Some of these have, in addition, the potential to cause respiratory allergy, an immediate hypersensitivity reaction effected by IgE antibody. The characteristics of immune responses provoked by chemical allergens determine the type of hypersensitivity reaction which will predominate. Recent evidence indicates that contact and respiratory allergens stimulate qualitatively different immune responses in mice consistent with the selective activation of T helper (TH) cell subpopulations. Contact allergens which lack the potential for respiratory sensitization preferentially activate TH1 cells which effect delayed-type hypersensitivity reactions. A soluble product of TH1 cells, interferon gamma (IFN-gamma), inhibits IgE antibody responses, and thereby the induction of respiratory sensitization. Conversely, chemical allergens which are known to cause respiratory hypersensitivity stimulate preferentially TH2-type responses. TH2 cells promote the development of respiratory allergy through the production of interleukin 4 (IL-4), a cytokine which is required for the initiation and maintenance of IgE responses. Although the molecular basis for the selective activation by contact and respiratory allergens of TH1 and TH2 cells, respectively, awaits clarification, these qualitative differences in immune response provide opportunities for the identification and evaluation of chemical sensitizers.

Animals↗

PubMed1992

Langerhans cells, antigen presentation, and the diversity of responses to chemical allergens.

Respiratory and contact chemical allergens provoke differential immune responses in mice, stimulating preferentially T helper-2 (TH2) and TH1 cells, respectively. In an attempt to discover whether such differences are effected at the level of antigen handling and presentation we have examined the effect of topical exposure to trimellitic anhydride (TMA), a respiratory allergen, and 2,4-dinitrochlorobenzene (DNCB), a contact allergen, on Langerhans cell (LC) MHC class II (Ia) expression. Neither chemical caused a significant change in LC size. As measured by analytical flow cytometry, exposure to DNCB resulted in a time-dependent increase in LC Ia expression that exceeded 160% of control values within 24 h. Exposure to concentrations of TMA that caused an equivalent activation of draining lymph nodes failed to affect Ia expression by LC. Application of sodium lauryl sulfate at concentrations that caused edema also failed to influence LC Ia. These data demonstrate that TMA and DNCB exert differential effects on epidermal LC, possibly indicative of differences in antigen handling.

Allergens↗

PubMed1992

Divergent immune responses to respiratory and contact chemical allergens: antibody elicited by phthalic anhydride and oxazolone.

In previous studies we observed that exposure of mice to the contact allergen 2,4-dinitrochlorobenzene (DNCB) and the respiratory allergen trimellitic anhydride (TMA) resulted in qualitatively different immune responses characteristic of selective Th1- and Th2-type T helper cell activation respectively. The purpose of the present investigation was to determine whether the effects recorded with DNCB and TMA are characteristic of immune responses to contact and respiratory chemical allergens in general. Experiments have been performed with phthalic anhydride, a known human respiratory sensitizer, and with oxazolone, a potent contact allergen. Under conditions of exposure where both chemicals elicited an IgG anti-hapten antibody response, only phthalic anhydride caused an increase in the serum concentration of IgE. Furthermore, like TMA, phthalic anhydride preferentially induced IgG2b rather than IgG2a antibody. In contrast, oxazolone, like DNCB, induced a markedly stronger IgG2a than IgG2b antibody response. These data provide confirmatory evidence that chemical respiratory allergens and chemical contact allergens elicit qualitatively different immune responses which reflect their clinical effects in man.

Allergens↗

PubMed1992

Differential ability of occupational chemical contact and respiratory allergens to cause immediate and delayed dermal hypersensitivity reactions in mice.

Trimellitic anhydride (TMA) is known to cause occupational respiratory allergy associated with the presence of specific IgE antibody. Other chemicals, such as 2,4-dinitrochlorobenzene (DNCB), while exhibiting a clear potential for contact sensitization, apparently lack the ability to induce respiratory allergy in man. It has been shown previously that although both chemicals are immunogenic in mice, each provoking contact sensitization, exposure only to TMA results in an IgE antibody response. In the present study, to examine further the characteristics of human allergens, we have compared the ability of TMA and DNCB to elicit immediate and delayed cutaneous hypersensitivity reactions in mice. Topical exposure to both chemicals resulted in delayed (24 h) hypersensitivity. However, only TMA induced, in addition, an immediate (1 h) dermal reaction following local challenge. Serum from TMA-immune mice, but not from untreated mice or mice sensitized with DNCB, was able to transfer immediate hypersensitivity to naive recipients. The kinetics of passive sensitization with TMA-immune serum, together with the fact that immediate hypersensitivity to DNCB could be induced with monoclonal IgE anti-dinitrophenol (DNP) antibody, suggests that the immediate dermal responses caused by TMA are effected by hapten-specific IgE. These data demonstrate that different classes of occupational chemical allergen exhibit a variable potential to elicit immediate and delayed dermal hypersensitivity reactions in mice, and provide a novel approach to the classification and characterization of human allergens.

Administration, Topical↗

PubMed1992

Influence of topical exposure to chemical allergens on murine Langerhans cells. Comparison of 2,4-dinitrochlorobenzene with trimellitic anhydride.

Chemical allergens differ with respect to the type of hypersensitivity reactions they preferentially elicit. Some chemicals, such as trimellitic anhydride (TMA), have the potential to induce both contact and respiratory hypersensitivity. Other chemicals cause only contact allergy. For example, 2,4-dinitrochlorobenzene (DNCB), a potent contact allergen, appears not to induce respiratory sensitization. In previous studies we have shown that topical exposure of mice to TMA and DNCB, under conditions of equivalent immunogenicity with respect to draining lymph node activation and contact sensitization, caused qualitatively different antibody responses. While the chemicals provoked IgG anti-hapten antibody responses of equivalent magnitude, only TMA induced an IgE response, and DNCB caused a significantly stronger IgG2a response. These data are consistent with the preferential activation by DNCB and TMA of Th1 and Th2 cells respectively. The purpose of the present study was to examine whether the qualitative differences in immune responses stimulated by these chemicals is reflected by variable affects on Langerhans cells (LC) in situ. Mice were exposed to concentrations of DNCB (1%) and TMA (50%) which caused equivalent levels of contact sensitization. Under these conditions topical exposure to DNCB, but not to TMA, or to vehicle alone, resulted in increased expression by LC of Ia antigen. Similar treatment with an irritant concentration (20%) of sodium dodecyl sulphate failed to influence Ia expression by LC. These data indicate that, at concentrations which induce similar levels of skin sensitization, not all contact allergens cause rapid changes in LC Ia expression, and that the qualitative differences in immune responses elicited by chemical allergens DNCB and TMA is associated with variable effects on LC.

Administration, Topical↗

PubMed1991

Investigation of lymph node cell proliferation as a possible immunological correlate of contact sensitizing potential.

In an attempt to define an immunological correlate of contact allergic potential it was investigated whether the magnitude of the lymphocyte proliferative response provoked following primary exposure to skin allergens influences the degree to which sensitization is induced. Previous studies have provided circumstantial evidence that there is a causal relationship between proliferation and sensitization. Thus, inhibition of the primary proliferative response to a sensitizing chemical, induced by prior exposure to an unrelated allergen, is associated with a significant depression of contact hypersensitivity. The results of comparative studies performed using a series of antigenically cross-reactive dinitrobenzene derivatives, 2,4-dinitrofluorobenzene, 2,4-dinitrochlorobenzene and 2,4-dinitrothiocyanobenzene are now reported. Evidence is presented that there is a direct correlation between cumulative proliferative activity in the draining lymph nodes and the severity of elicitation reactions. The tentative conclusion which can be drawn is that measurement of lymphocyte proliferative activity may form the basis of a method for comparative evaluation of the contact allergic potential of chemicals.

Animals↗

PubMed1991

Inhalation exposure of mice to trimellitic anhydride induces both IgG and IgE anti-hapten antibody.

The development of antibody responses resulting from inhalation exposure to chemical allergens has been studied in mice. Inhalation exposure of BALB/c mice to atmospheres containing approximately 5 mg/m3 of the respiratory allergen trimellitic anhydride (TMA) resulted in the appearance of both serum IgG and IgE anti-hapten antibody. IgE anti-TMA was first detectable 2-3 weeks following the initiation of exposure and was still present at 6 weeks. Under the same conditions of exposure, 2,4-dinitrochlorobenzene (DNCB), a contact allergen which apparently lacks the capacity for respiratory sensitization, failed to elicit detectable amounts of anti-dinitrophenol (DNP) antibody. Exposure to increased concentrations of atmospheric DNCB (15 mg/m3) did, however, result in an IgG anti-DNP response but not in IgE antibody. These data demonstrate firstly, that atmospheres containing low molecular weight respiratory allergens can initiate specific IgE responses in mice, and secondly, that inhaled chemicals may differ in their ability to induce IgE antibody.

Administration, Inhalation↗

PubMed1991

Differential stimulation of immune function by respiratory and contact chemical allergens.

The nature of immune responses induced following topical exposure to 2,4-dinitrochlorobenzene (DNCB), a potent contact allergen which lacks the capacity to cause respiratory sensitization, and trimellitic anhydride (TMA), a respiratory allergen with comparatively weak skin-sensitizing potential, have been investigated. Exposure of BALB/c strain mice to concentrations of TMA and DNCB which resulted in equivalent levels of activation (cell proliferation) in lymph nodes draining the site of application (50% TMA and 1% DNCB) induced comparable levels of contact sensitization and IgG anti-hapten antibody production. However, under these conditions, exposure only to TMA resulted in an elevation of serum IgE. Furthermore, while TMA induced IgG2b rather than IgG2a antibody the reverse pattern was observed with DNCB. These data demonstrate that TMA and DNCB elicit qualitatively different immune responses which are consistent with their potential to cause respiratory and contact allergy, respectively. The possibility that the responses induced by these chemicals reflect a differential stimulation of T-helper cell subsets (Th1 and Th2) is discussed.

Allergens↗

PubMed1990

Inhalation exposure to respiratory sensitising chemicals down-regulates guinea pig IgE and pulmonary responses.

Inhalation exposure of the guinea pig to the respiratory sensitising chemicals trimellitic anhydride (TMA) or diphenylmethane di-isocyanate down-regulated the IgE antibody response to subsequent systemic challenge with alum-precipitated hapten-protein conjugate. The suppression was isotype and hapten specific, challenge with unrelated hapten-protein conjugate resulting in high-titre IgE and IgGl antibody responses. Preliminary results indicate that the down-regulation of the IgE response to TMA markedly reduced the capacity of the animals to undergo a bronchoconstriction response following challenge with an aerosol of TMA, suggesting that the initial route of exposure may have profound effects upon the development of occupational respiratory allergy.

Administration, Inhalation↗

PubMed1988

Lymphokine-activated killer cells from normal and lymphoma subjects are cytotoxic for cells coated with antibody derivatives displaying human Fc gamma.

Lymphokine-activated killer (LAK) cells were successfully generated in all cases from blood mononuclear cells obtained from six patients with lymphoma. The LAK cells from three of these patients and from five normal adult donors were tested for their effector abilities in antibody-dependent cellular cytotoxicity (ADCC) against guinea pig leukemic lymphocytes coated with various antiidiotype antibodies. Cells from all the donors behaved similarly. Mouse monoclonal antibodies of IgG1, IgG2a, and IgG2b isotypes invoked no ADCC. However, substantial ADCC was invoked by the chimeric antibody FabFc, in which Fab'gamma from mouse antiidiotype is thioether-bonded to human normal Fc gamma. Similar results were obtained on testing LAK cells from a normal donor against uncultured human lymphoma targets coated with native or chimeric antiidiotype. The ADCC invoked by the mouse-human chimeric antibodies appears to depend on the human Fc gamma they display and not on the univalency of the derivatives used. The findings imply that LAK technology could usefully augment serotherapy that uses antibody derivatives displaying human Fc gamma.

Adult↗

PubMedDate not supplied

Selective stimulation of cutaneous interleukin 6 expression by skin allergens.

Epidermal cells, both keratinocytes and Langerhans cells, are able to synthesize and secrete a variety of cytokines, many of which influence or are essential for the induction of skin sensitization and the elicitation of local inflammatory reactions. It has been proposed that it may prove possible to distinguish between contact allergens and skin irritants as a function of differential induction or upregulation of epidermal cytokine expression. In the present study we have addressed this by examination of the local cutaneous production of interleukin 6 (IL-6) following topical exposure of mice to oxazolone, a potent contact allergen, or to benzalkonium chloride (BZC), a skin irritant that is considered not to have a significant potential to cause skin sensitization. Both oxazolone and BZC could induce the production of IL-6 as measured by enzyme-linked immunosorbent assay in homogenates prepared from treated skin. However, when these chemicals were applied at concentrations that resulted in equivalent cutaneous inflammatory responses, based on induced oedema, only oxazolone provoked the production in skin of IL-6. Moreover, under these conditions exposure only to oxazolone resulted in the secretion by draining lymph node cells of measurable concentrations of this cytokine. These data suggest that the ability of oxazolone to stimulate local IL-6 production is not secondary simply to the induction of local inflammatory responses. As such, the results support the possibility that skin allergens and skin irritants may stimulate variable patterns of epidermal cytokine production.

Adjuvants, Immunologic↗

PubMedDate not supplied

The mouse IgE test for the identification of potential chemical respiratory allergens: considerations of stability and controls.

The mouse IgE test is a novel approach to the predictive identification of chemicals that have the potential to cause sensitization of the respiratory tract. The method is based upon measurement of induced changes in the serum concentration of IgE in BALB/c strain mice following topical exposure to the test chemical. The investigations described here were undertaken to examine the stability of the assay, to evaluate the utility of trimellitic anhydride (TMA) and 2,4-dinitrochlorobenzene (DNCB) as, respectively, positive and negative controls for use in the test and to consider criteria for the classification of chemicals as potential respiratory allergens based upon changes induced in serum IgE concentration. On the basis of fifteen independent experiments it was found that, while there was some intra- and inter-test variability with respect to absolute concentrations of IgE measured in the sera of individual mice, the relative pattern of reactivity observed following treatment of mice with TMA, DNCB or with vehicle (4:1 acetone:olive oil) alone remained consistent. In each experiment treatment with TMA provoked a very substantial increase in serum IgE relative to vehicle controls. In no instance did exposure of mice to DNCB cause an increase in the concentration of serum IgE, and in some instances treatment with this chemical resulted in reduced IgE levels. In a parallel series of experiments it was found that serum IgE concentrations in vehicle-treated mice were comparable with those found in the serum of untreated animals. It is concluded that the differential ability of chemicals to induce changes in the concentration of serum IgE in BALB/c mice is a stable phenomenon. It is recommended that, in practice, TMA and DNCB should be incorporated in mouse IgE tests as, respectively, positive and negative controls. Finally, it is proposed that activity in the test is assessed as a function of induced changes in serum IgE levels relative to concurrent vehicle controls, rather than by reference to historical normal range values.

Allergens↗

PubMedDate not supplied

Evaluation of the sensitizing potential of eugenol and isoeugenol in mice and guinea pigs.

The sensitizing properties of the fragrances eugenol and isoeugenol have been investigated experimentally. The potential of these materials to induce sensitization of the respiratory tract was examined using the mouse IgE test, a novel but as yet unvalidated method for the predictive identification of chemical respiratory allergens. Comparisons were made with the activity of eugenol and isoeugenol in two predictive tests for contact sensitization potential: the murine local lymph node assay and the guinea pig maximization test. Both chemicals elicited positive responses in these tests, isoeugenol exhibiting a greater potential for contact sensitization than eugenol. Isoeugenol was negative at all concentrations examined in the mouse IgE test. In the same assay, exposure to eugenol was associated with a statistically significant increase in serum IgE concentrations when initial application concentrations of 2.5% were used. However, at higher test concentrations eugenol was negative in the mouse IgE test. It is concluded that neither eugenol nor isoeugenol have a significant potential to cause sensitization of the respiratory tract, a conclusion that is apparently consistent with the lack of evidence for occupational respiratory allergy associated with exposure to these chemicals. The evidence for isoeugenol lacking respiratory sensitization activity is particularly strong and it is proposed that this chemical may be of value as a negative control in the development and validation of new predictive test methods for the identification of chemical respiratory allergens.

Allergens↗

PubMedDate not supplied

Classification of chemical allergens according to cytokine secretion profiles of murine lymph node cells.

Characteristic cytokine secretion profiles, consistent with the selective activation of discrete functional subpopulations of T helper (Th) cells, have been demonstrated following repeated topical exposure of mice to chemical contact or respiratory allergens. Draining lymph node cells (LNC) derived from animals treated with the respiratory allergen trimellitic anhydride (TMA; 10%) expressed high levels of the Th2 cytokines interleukins 4 and 10, but little of the Th1 cell product interferon gamma. Under conditions of exposure of equivalent immunogenicity with respect to LNC proliferation, the contact allergen 2,4-dinitrochlorobenzene (DNCB) provoked the converse pattern of cytokine secretion. The purpose of the present investigations was to examine dose-response relationships with respect to cytokine production with a wider range of chemical allergens. In each case, cytokine secretion patterns were compared with those observed with LNC prepared from animals exposed concurrently to TMA or DNCB. Despite some inter-experimental variation in the absolute amounts of cytokines produced, DNCB- and TMA-activated LNC invariably expressed Th1- and Th2-type patterns, respectively. At all concentrations tested, the contact allergens isoeugenol and formaldehyde stimulated a Th1-type cytokine secretion profile, whereas a Th2-type pattern was induced following exposure to the chemical respiratory allergens cyanuric chloride and diphenylmethane diisocyanate. These data demonstrate that divergent cytokine secretion profiles characterize immune responses to different classes of chemical allergen and suggest that it may be possible, in a single integrated assay, to identify and classify chemical allergens as a function of induced cytokine production patterns.

Allergens↗

PubMedDate not supplied

Analysis of cytokine mRNA expression following repeated exposure of mice to chemical contact and respiratory allergens.

It has been shown previously that cytokine secretion patterns characteristic of the activation of discrete responses by functional subsets of T cells of type 1 and type 2, respectively, are elicited following topical exposure of BALB/c strain mice to chemical contact and respiratory allergens. In order to investigate if the differences in protein profiles are paralleled by changes in steady-state mRNA levels we have now investigated cultured draining lymph node cell (LNC) cytokine mRNA expression profiles by reverse transcriptase-polymerase chain reaction (RT-PCR) under conditions where divergent cytokine secretion is observed. Mice were exposed topically by repeated application of the respiratory allergen trimellitic anhydride (TMA) or of the contact allergen 2,4-dinitrochlorobenzene (DNCB). An elevation in the expression of mRNA for interleukin 4 (IL-4) and interleukin 10 (IL-10) by LNC from both TMA- and DNCB-treated animals was observed within 6 h of culture, reaching maximal levels after 72 h. Relative mRNA levels for both of these type 2 cytokines were considerably higher in cultured cells derived from TMA-exposed mice, compared with those from DNCB-treated animals. Transient low levels of the type 1 cytokine interferon y (IFN-gamma) were observed in response to treatment with TMA, whereas a substantial upregulation of IFN-gamma gene expression was seen from 24 h onwards in cultured LNC derived from DNCB-exposed mice. Changes in cytokine mRNA in allergen-activated LNC preceded protein production, although the kinetic profiles were similar. These data suggest that the divergent cytokine secretion profiles exhibited by mice treated by repeated topical exposure to contact and respiratory allergens are controlled primarily at the level of transcription. The RT-PCR methodology described herein may be more sensitive for the detection of cytokines expressed in low copy number, such as IL-4, where previously it has been found necessary to stimulate LNC with mitogen to elicit measurable levels of protein secretion. However, this technique was not found to offer major practical advantages when compared with protein detection methods (enzyme-linked immunosorbent assays) for the routine predictive characterization of chemicals as a function of cytokine 'fingerprinting'.

Allergens↗

PubMedDate not supplied

Temporal stability of local lymph node assay responses to hexyl cinnamic aldehyde.

The local lymph node assay is an alternative method for the prospective identification of chemicals that have the potential to cause skin sensitization. Activity in the assay is measured as a function of proliferative responses by draining lymph node cells induced by topical exposure of mice to the test chemical. Positive responses are defined as those where a test chemical, at one or more application concentrations, is able to induce a stimulation index of 3 or greater compared with concurrent vehicle-treated control values. Although the method has been evaluated extensively, the stability over time of responses induced in the local lymph node assay has not previously been addressed formally. It was the purpose of the investigations described here to consider this issue and, to this end, responses provoked in the local lymph node assay by hexyl cinnamic aldehyde (HCA)--a skin sensitizing chemical of moderate potency--were assessed in five separate experiments conducted in a single laboratory over a 10-month period. In each case, HCA elicited a positive response. Although some significant inter-experimental variation was recorded, this was attributable entirely to the stimulation by HCA of slightly more vigorous responses in one of the five experiments. When the results of this experiment were excluded from the data set, significant variations were lost. Finally, for each experiment an EC3 value was derived, this being the estimated concentration of test chemical required to induce a stimulation index of 3. Similar EC3 values were derived in each experiment. These data demonstrate the relative stability over time of activity in the local lymph node assay.

Acrolein↗

PubMedDate not supplied

Cytokine endpoints for the local lymph node assay: consideration of interferon-gamma and interleukin 12.

The murine local lymph node assay (LLNA) is a method for the prospective identification of contact allergens. Skin sensitization potential is assessed as a function of induced proliferative responses in lymph nodes draining the site of topical exposure measured in situ by incorporation of radiolabelled thymidine ([3H]thymidine). The results of previous investigations have demonstrated that the analysis of [3H]thymidine incorporation represents a robust and reliable endpoint for the LLNA for the assessment of skin sensitizing activity for strong and moderate allergens and, in addition, many weaker sensitizers. The aim of the current experiments was to explore the utility of the production of the cytokines interferon-gamma (IFN-gamma) and interleukin 12 (IL-12) by draining lymph node cells (LNC) as alternative readouts for the LLNA. Animals were exposed to a range of skin sensitizers at two application concentrations. The first of these was chosen on the basis of results from previous investigations to stimulate a strong proliferative response (tenfold or greater increase in proliferation compared with concurrent vehicle controls). The second concentration of test material in each case was the amount of chemical estimated to be necessary mathematically for elicitation of a stimulation index of 3 (EC3 value); the induction of a threefold or greater increase in proliferation is the current criterion for a positive response in the LLNA. In addition, analyses were conducted with para-aminobenzoic acid (PABA), a non-sensitizing chemical shown previously not to induce LLNA responses. Secretion of IFN-gamma and the p40 subunit of IL-12 by draining LNC was measured by cytokine-specific enzyme-linked immunosorbent assay. In parallel experiments, LNC activity was assessed as a function of [3H]thymidine incorporation in situ. All the chemical allergens tested provoked robust proliferative responses, with the stimulation indices recorded at both test concentrations reflecting only small changes in activity compared with previously recorded data. Exposure to vehicle (4:1 acetone:olive oil, AOO) alone resulted in detectable, although variable, expression of both IFN-gamma and IL-12. Treatment with chemical allergen in each case caused a marked increase in IFN-gamma secretion, with particularly vigorous production of cytokine being stimulated following exposure to oxazolone or hexyl cinnamic aldehyde. In contrast, application of chemical allergens was not generally associated with elevated IL-12 p40 secretion. Exposure of mice to PABA did not result in increased IFN-gamma or IL-12 production compared with vehicle-treated controls. In general, however, cytokine secretion did not correlate closely with the induction of LNC proliferation. These data indicate that expression by allergen-activated LNC of IFN-gamma or IL-12 does not provide a reliable or sufficiently sensitive endpoint for the LLNA compared with [3H]thymidine incorporation in situ.

Allergens↗

PubMedDate not supplied

Local lymph node assay responses to paraphenylenediamine: intra- and inter-laboratory evaluations.

The murine local lymph node assay (LLNA) is a method for the prospective identification of skin sensitizing chemicals. Proliferative responses induced in lymph nodes draining the site of topical application of the test chemical are measured and those chemicals that induce a stimulation index of three or more compared with concurrent vehicle-treated controls are considered to have the potential to cause skin sensitization. Dose-response data from the LLNA may be used to derive an estimate of relative skin sensitizing potency, based upon derivation of the concentration of chemical required to cause a stimulation index of 3 (EC3 value) as calculated by linear interpolation. The purpose of the present investigations was to examine the stability of LLNA responses and the consistency of derived EC3 values induced by the contact allergen paraphenylenediamine (PPD). Analyses were conducted once a month over a 4-month period in each of two independent laboratories. In all assays, and in both laboratories, PPD elicited a positive response. Although some minor differences in responses between and within laboratories were observed, the derived EC3 values were generally very consistent. In Laboratory 1, EC3 values varied between 0.06 and 0.09% PPD, whereas in Laboratory 2 the range was 0.09-0.20%. These EC3 values are consistent with clinical experience of this material insofar as it is a common and relatively potent cause of allergic contact dermatitis in humans. Taken together, these data confirm the stability of LLNA responses both with time and between laboratories and provide additional support for the use of derived EC3 values in the assessment of relative skin sensitizing potency.

Allergens↗