Transcriptional Profile in Allergic Contact Dermatitis to Methylisothiazolinone

This study aimed to investigate the signaling pathways induced by MI in the skin test of individuals with MI-sensitized ACD, comparing them to healthy controls. Individuals initially testing positive only for MI were recruited at the Contact Dermatitis Clinic at Hospital das Clínicas of São Paulo, re-exposed to MI or saline, and biopsied 48 hours after exposure. Negative controls for the patch test were also exposed to MI and saline for comparison. Histopathological and transcriptomic (RNAseq) analyses identified two groups among the ACD patients: Group A (GA), with more pronounced histopathological features such as spongiosis and microvesicles, and Group B (GB), which showed milder reactions and absence of spongiosis. In GA, a total of 1588 genes were upregulated and 2090 downregulated in response to MI, compared to GB. These differentially expressed genes (DEGs) were associated with inflammation and neurological signaling, such as IL-24, IL-9, IL-13, and NTRK1, while IL-37 and IL-18 were downregulated. Similarly, GA compared to the MI-negative ACD group showed 1169 upregulated and 321 downregulated genes. DEG validation in GA through qPCR confirmed increased expression of NTRK1, IL-9, IL-6, IL-13, and CXCL8, with a reduction in IL-18. Protein analysis of IL-24 and IL-9 revealed higher expression in the dermal layer of GA. These results indicate distinct transcriptional profiles in MI-sensitized individuals, despite positive patch tests in all cases. The observed heterogeneity, not previously described in ACD, points to new potential therapeutic targets, such as IL-9, IL-37, and NTRK1. Furthermore, the study suggests that inflammatory and neurological cytokines, such as IL-9 and NTRK1, play key roles in the pathogenesis of ACD.