Utility of historical vehicle-control data in the interpretation of the local lymph node assay

Immunotoxicology: A Brief History

Immunotoxicological research and testing have evolved from early studies of anaphylaxis to the robust and diverse field of immunotoxicology as we know it today. Early studies connecting immune dysfunction with exposure to exogenous agents focused on adverse reactions to immunogenic agents present in vaccines. Over time, work done by immunologists and pathologists leads to descriptions of characteristics of immunogenic agents as well as mechanisms by which anaphylaxis occurs and an understanding of the concept of immunosuppression. These myriad achievements greatly improved public health and led the field of immunotoxicology, which addresses all aspects of adverse immunological responses following exposure to exogenous agents as well as the development of testing paradigms to understand immunological responses of designed agents such as drugs and biologics.

The local lymph node assay in practice: a current regulatory perspective

Following the formal acceptance of the local lymph node assay (LLNA) as an Organization for Economic Cooperation and Development (OECD) guideline in April 2002, the UK Health and Safety Executive (HSE) informed notifiers that this was now the method of choice for the assessment of skin sensitization potential under the EU notification scheme for new industrial chemicals (NONS). This paper summarizes the experience of the HSE for the 2-year period immediately following the issuing of this statement, during which 48 LLNA study reports were assessed for notification purposes. The issues discussed here include adherence to the OECD guideline, interpretation of results, and classification outcomes. Generally, notifying laboratories followed the OECD guideline successfully, with regard to the sex/ strain/numbers of mice used, the precise process used for measurement of cell proliferation, and the use of recommended vehicles and positive controls. Initially, use of the individual animal approach (measuring the cell proliferation in each animal rather than for a pooled dose group) highlighted problems caused by technical inexperience, but these were overcome by practice. Toxicity or irritation were found to be minor factors in dose selection; more important was the choice of vehicle to correctly maximize the test substance concentration, while maintaining appropriate application properties. Contrary to concerns that the LLNA would prove to be less sensitive or more sensitive than the traditionally used Guinea Pig Maximization Test (GPMT), the proportion of new substances classified as skin sensitizers was within the range observed in previous years. Although the sample size is relatively small, the experience of the HSE indicates that the LLNA is satisfactory for routine regulatory use.

The local lymph node assay (LLNA)

The murine local lymph node assay (LLNA) is a widely accepted method for assessing the skin sensitization potential of chemicals. Compared with other in vivo methods in guinea pig, the LLNA offers important advantages with respect to animal welfare, including a requirement for reduced animal numbers as well as reduced pain and trauma. In addition to hazard identification, the LLNA is used for determining the relative skin sensitizing potency of contact allergens as a pivotal contribution to the risk assessment process. The LLNA is the only in vivo method that has been subjected to a formal validation process. The original LLNA protocol is based on measurement of the proliferative activity of draining lymph node cells (LNC), as determined by incorporation of radiolabeled thymidine. Several variants to the original LLNA have been developed to eliminate the use of radioactive materials. One such alternative is considered here: the LLNA:BrdU-ELISA method, which uses 5-bromo-2-deoxyuridine (BrdU) in place of radiolabeled thymidine to measure LNC proliferation in draining nodes.

Perspectives on Non-Animal Alternatives for Assessing Sensitization Potential in Allergic Contact Dermatitis

Skin sensitization remains a major environmental and occupational health hazard. Animal models have been used as the gold standard method of choice for estimating chemical sensitization potential. However, a growing international drive and consensus for minimizing animal usage have prompted the development of in vitro methods to assess chemical sensitivity. In this paper, we examine existing approaches including in silico models, cell and tissue based assays for distinguishing between sensitizers and irritants. The in silico approaches that have been discussed include Quantitative Structure Activity Relationships (QSAR) and QSAR based expert models that correlate chemical molecular structure with biological activity and mechanism based read-across models that incorporate compound electrophilicity. The cell and tissue based assays rely on an assortment of mono and co-culture cell systems in conjunction with 3D skin models. Given the complexity of allergen induced immune responses, and the limited ability of existing systems to capture the entire gamut of cellular and molecular events associated with these responses, we also introduce a microfabricated platform that can capture all the key steps involved in allergic contact sensitivity. Finally, we describe the development of an integrated testing strategy comprised of two or three tier systems for evaluating sensitization potential of chemicals.

The local lymph node assay and skin sensitization testing

The mouse local lymph node assay (LLNA) is a method for the identification and characterization of skin sensitization hazards. In this context the method can be used both to identify contact allergens, and also determine the relative skin sensitizing potency as a basis for derivation of effective risk assessments.The assay is based on measurement of proliferative responses by draining lymph node cells induced following topical exposure of mice to test chemicals. Such responses are known to be causally and quantitatively associated with the acquisition of skin sensitization and therefore provide a relevant marker for characterization of contact allergic potential.The LLNA has been the subject of exhaustive evaluation and validation exercises and has been assigned Organization for Economic Cooperation and Development (OECD) test guideline 429. Herein we describe the conduct and interpretation of the LLNA.

Evaluation of irritancy and sensitization potential of metalworking fluid mixtures and components

There are approximately 1.2 million workers exposed to metalworking fluids (MWF), which are used to reduce the heat and friction associated with industrial machining and grinding operations. Irritancy and sensitization potential of 9 National Toxicology Program (NTP) nominated MWFs (TRIM 229, TRIM VX, TRIM SC210, CIMTECH 310, CIMPERIAL 1070, CIMSTAR 3800, SYNTILO 1023, SUPEREDGE 6768, and CLEAREDGE 6584) were examined in a combined local lymph node assay (LLNA). BALB/c mice were dermally exposed to each MWF at concentrations up to 50%. Significant irritation was observed after dermal exposure to all MWFs except CIMTECH 310 and SYNTILO 1023. Of the 9 MWFs, 6 induced greater than a 3-fold increase in lymphocyte proliferation and 7 tested positive in the irritancy assay. TRIM VX yielded the lowest EC3 value (6.9%) with respect to lymphocyte proliferation. Chemical components of TRIM VX identified using HPLC were screened for sensitization potential using structural activity relationship (SAR) modeling and the LLNA. TOPKAT predicted triethanolamine (TEA) as a sensitizer while Derek for Windows predicted only 4-chloro-3-methylphenol (CMP) to be positive for sensitization. When tested in the LLNA only CMP (EC3 = 11.6%) and oleic acid (OA) (EC3 = 29.7%) were identified as sensitizers. Exposure to all tested TRIM VX components resulted in statistically significant irritation. An additive proliferative response was observed when mixtures of the two identified sensitizing TRIM VX components, OA and CMP, were tested in the LLNA. This is one explanation of why the EC3 value of TRIM VX, with respect to lymphocyte proliferation, is lower than those assigned to its sensitizing components.

The skin sensitization potential of resorcinol: experience with the local lymph node assay

Resorcinol is a simple aromatic chemical (1,3-benzenediol) that has found widespread use, particularly as a coupler in hair dyes. Clinical experience clearly shows that resorcinol is a (albeit uncommon) skin sensitizer. By contrast, predictive methods, both animal and human, have previously failed to identify resorcinol as such. Here, we describe the outcome of a recent local lymph node assay performed in accordance with Organisation for Economic Co-operation and Development guideline 429, which correctly identified resorcinol as a skin sensitizer. Clear evidence of a dose response was apparent, and an EC3 value of approximately 6% was calculated. This suggests that the skin-sensitizing potency of resorcinol is approximately 2 orders of magnitude lower than that of p-phenylenediamine but similar to that of hexyl cinnamic aldehyde. These data show the importance of adherence to test guidelines and aligns the clinical experience with resorcinol with that obtained in predictive animal methods.

The use of ethanol:diethylphthalate as a vehicle for the local lymph node assay

The murine local lymph node assay (LLNA) is used for the prospective identification of contact allergens. This method is not only accepted as a stand-alone test for the identification of contact allergenic hazard but also used increasingly for the measurement of the relative potency of skin-sensitizing chemicals as an integral component of the risk assessment process. During the development and validation phases of the method, a list of standard vehicles for use in the LLNA was identified, among them being the vehicle most commonly used acetone/olive oil (4:1, AOO). We have now explored the performance in the LLNA of a non-standard vehicle, ethanol:diethyl phthalate (1:3, EtOH:DEP), that is used frequently to evaluate dermal effects of fragrance materials in both human and experimental studies. Current investigations have demonstrated that EtOH:DEP induces similar levels of background proliferative responses in lymph nodes comparable with the standard vehicle AOO. Moreover, expected levels of activity are observed when EtOH:DEP is used to deliver a known contact allergen in the LLNA. The conclusion drawn is that EtOH:DEP provides a suitable vehicle for use in the LLNA and that the approach described provides a basis for future evaluation of novel vehicles.

Information derived from sensitization test methods: test sensitivity, false positives and false negatives

Predictive toxicology tests for the prospective identification of skin-sensitizing chemicals are well known and have been used for many years. However, of these, only the local lymph node assay (LLNA) has actually undergone formal independent assessment to determine the accuracy of the predictions, particularly with respect to the likelihood of false positives and false negatives. Often, efforts to increase the sensitivity of a test (reducing false negatives) tend to increase the number of false positives. In this short review, these issues are discussed in particular relation to the 3 predictive tests available in regulatory toxicology, the guinea-pig maximization test, the occluded patch test of Buehler and the LLNA. A key perspective is that no predictive test is without limitations; having a good appreciation of these limitations is necessary for making the best use of the information derived from these methods.

Potency and risk assessment of a skin-sensitizing disperse dye using the local lymph node assay

Disperse Blue 106 is an acknowledged skin-sensitization hazard. However, information about the relative sensitization potency of this chemical is lacking, and to provide this information was the purpose of the investigations described here. The approach taken was to measure dose-response relationships for C.I. Disperse Blue in the local lymph node assay, a method for the assessment of skin-sensitization potential in which activity is measured as a function of lymphocyte proliferative responses induced in draining lymph nodes. From these data, it was possible to derive EC3 values (such being the estimated concentration of chemical required to elicit a 3-fold increase in proliferation) that have been shown previously to reflect the relative sensitizing potency of contact allergens. These analyses revealed that Disperse Blue 106 had a relatively low EC3 value (0.01%), comparable to that measured concurrently for 2,4-dinitrochlorobenzene, a potent contact allergen. Collectively, these data reveal that Disperse Blue 106 represents a significant skin-sensitization hazard, and, in combination with information on dye migration and percutaneous penetration from various types of fabric and use conditions provide a basis for the development of effective and accurate risk assessments.

Local lymph node assays

The murine local lymph node assay (LLNA) allows identification of chemicals that have the potential to cause skin sensitization and allergic contact dermatitis. In this test, contact allergens are identified as a function of events occurring during the induction phase of skin sensitization; specifically, stimulation of proliferative responses in draining lymph nodes measured as a function of radiolabeled thymidine incorporation in situ. Two methods are presented. In one, lymph nodes are pooled on an experimental group basis, while in the other, lymph nodes of individual animal are pooled, which enables statistical analyses. These methods have been developed for contact sensitization hazard assessment and provide a robust and reliable method as an alternative to guinea pig tests which measure skin sensitizing potential as a function of challenge-induced skin reactions in previously sensitized animals. It is also possible to use LLNA data for measurement of relative potency in terms of risk assessment.