Effect of Skin Sensitizers on Inducible Nitric Oxide Synthase Expression and Nitric Oxide Production in Skin Dendritic Cells: Role of Different Immunosuppressive Drugs

Interleukin-10 Reduces Neurogenic Inflammation and Pain Behavior in a Mouse Model of Type 2 Diabetes

Purpose

Neurogenic inflammation is a major component of chronic neuropathic pain. Previously, we established the db/db mouse as an animal model of painful diabetic neuropathy (PDN) of type 2 diabetes. In the current study, we investigate the roles of interleukin (IL)-10, an anti-inflammatory cytokine, in the development of neurogenic inflammation and pain behavior in db/db mouse.

Materials and Methods

We first studied IL-10 expression in lumbar dorsal root ganglion (LDRG) neurons of db/db mice using immunohistochemistry, immunoblots, and reverse transcription polymerase chain reaction during the period of pain behavior (from 8 to 16 wk of age). To determine if the reduced IL-10 expression mediates the mechanical allodynia in db/db mice, we administered recombinant mouse IL-10 or saline (control) intraperitoneally to control db/+ and db/db mice starting at 8 wk of age. IL-10 treatment was repeated every other day for 2 wk until the mice reached 10 wk of age.

Results

During the period of pain behavior, reduction of IL-10 protein and gene expression was detected in LDRG of db/db mice. Treatment with recombinant IL-10, from 8 to 10 wk of age, alleviates pain behaviors in db/db mice without affecting other diabetic parameters. In parallel, IL-10 treatment reduced the upregulation of nerve growth factor (NGF), inducible nitric oxide synthase (iNOS), tumor necrosis factor (TNF)-α, and high-affinity NGF receptor (Trk A) in LDRG, as well as the numbers of iNOS-positive Langerhans cells and CD-68-positive dermal dendritic cells in the hind-foot-pad skin in db/db mice.

Conclusion

Our findings suggest that the reduction in neuronal IL-10 increases inflammatory phenomena, ultimately contributing to PDN. These results suggest that the dysregulation of cytokine-mediated inflammation contributes to the development of PDN in db/db mice. Targeting this pathophysiologic mechanism could be an effective approach for treating PDN in type 2 diabetes.

Alternative skin sensitization prediction and risk assessment using proinflammatory biomarkers, interleukin-1 beta (IL-1β) and inducible nitric oxide synthase (iNOS)

As an alternative to animal tests for skin sensitization potency and risk assessment, cell viability and biomarkers related to skin sensitization were analyzed in THP-1 human monocytic leukemia cells. Cell viabilities of 90% (CV90) and 75% (CV75) were determined for 24 selected test chemicals. Further biomarkers related to skin sensitization were also determined under equivalent comparative conditions. In cell viability analyses, potent skin sensitizers exhibited high cytotoxicity, but non-sensitizers did not display this tendency. In biomarker analyses, interleukin-I beta (IL-1β), inducible nitric oxide synthase (iNOS), IL-1β+iNOS, and THP-1 IL-1β+Raw 264.7 IL-1β were found to be suitable for prediction of skin sensitization potency following classification as either skin sensitizers or non-sensitizers (accuracies of 91.7%, 87.5%, 83.3%, and 82.6%, respectively). A significant positive correlation was found between biomarkers and skin sensitization potency, with a correlation coefficient (R) of 0.7 or more (correlation coefficients of 0.77, 0.72, 0.7, and 0.84, respectively). Finally, the skin sensitization potency effective threefold concentration (EC) 3% was predicted using a biomarker equation, with resulting prediction rates (match rate with actual data) of 58.3%, 54.2%, 62.5%, and 60.9%, respectively. The prediction accuracy for the EC3 value obtained from animal data was calculated as 83.3%, 79.2%, 79.2%, and 73.9%, respectively. Thus, these biomarkers, IL-1β and iNOS, may be alternatively used to predict skin sensitization potency and risk assessment.

Development of an in vitro method to estimate the sensitization induction level of contact allergens

No standardized in vitro methods to assess potency of skin sensitizers are available. Recently, we standardized a procedure which combines the epidermal equivalent potency assay with assessment of IL-18 to provide a single test for identification and classification of skin sensitizers. This current study aimed to extend tested chemicals, and to provide a simple in vitro method for estimation of the expected sensitization induction level interpolating in vitro EC50 and IL-18 SI2 values to predict LLNA EC3 and/or human NOEL from standards curves generated using reference contact allergens. Reconstituted human epidermis was challenged with 14 chemicals not previously tested benzoquinone, chlorpromazine, chloramine T, benzyl salicylate, diethyl maleate, dihydroeugenol, 2,4-dichloronitrobenzene, benzyl cinnamate, imidazolidinyl urea, and limonene as contact sensitizers while benzyl alcohol, isopropanol, dimethyl isophthalate and 4-aminobenzoic acid as non-sensitizers in the LLNA. Where for benzyl salicylate and benzyl cinnamate no sensitization was observed in human predictive studies, positive responses to benzyl alcohol and dimethyl isophthalate were reported. The proposed method correlates better with human data, correctly predicting substances incorrectly classified by LLNA. With the exception of benzoquinone (interference with both MTT and IL-18 ELISA), and chloramine T (underestimated in the interpolation), a good estimation of LLNA EC3 and in vivo available human NOEL values was obtained.

Inducible nitric oxide synthase downmodulates contact hypersensitivity by suppressing dendritic cell migration and survival

Nitric oxide (NO) has several important roles in various physiological settings; one of the NO synthases, inducible NO synthase (iNOS), is induced by external stimulation of the skin. A prototypic example of external stimulation is hapten exposure, which induces the T-cell-mediated immune response known as contact hypersensitivity (CHS). We herein report on cutaneous dendritic cell (DC) function in the presence of an iNOS-specific inhibitor during the sensitization phase of CHS. First, we examined epidermal cell (EC) suspensions using flow cytometry with an iNOS antibody and confirmed that iNOS was expressed in the cytoplasm of Langerhans cells (LCs). We then studied the role of iNOS in CHS, and found that responses to DNFB were enhanced by the addition of an iNOS inhibitor during sensitization. Similarly, the iNOS inhibitor augmented FITC-induced migration of cutaneous DCs, including Langerin(+) LCs and Langerin(-) dermal DCs, to draining lymph nodes. Finally, we showed that iNOS inhibitor enhanced LC survival in vitro. We concluded that NO suppresses migration and survival of cutaneous DCs, resulting in a downmodulation of CHS.