The process of itch transduction begins in the skin, where activated immune cells and cutaneous cells release endogenous pruritogenic mediators. These mediators can stimulate specific receptors on unmyelinated C-fiber afferents and A-delta fibers in the skin, with their cell bodies residing in dorsal root ganglia (DRG). The nerve fibers then transmit the itch signal to the spinal cord, ultimately reaching the brain for central processing. Existing studies suggest that neuro-immune crosstalk contributes to itch sensitization. A variety of signaling molecules secreted by immune cells, e.g., leukotrienes, nerve growth factor, cytokines, chemokines, substance P, bradykinin, histamine, and proteases, play a crucial role in enhancing peripheral sensitization. These mediators not only induce phosphorylation of Transient receptor potential (TRP) and other ion channels, but also modulate their expression at the transcriptional and post-translational levels, further amplifying the peripheral sensitization process. Most studies have primarily focused on immune cells within the skin. Additionally, some studies have indicated the presence of immune cells in DRG under pain conditions. However, the investigation of immune cell infiltration into DRG and their functions in itch under allergic contact dermatitis (ACD) conditions remains an area that requires further study. Accordingly, this study aims to examine the presence of immune cells within DRG under ACD condition and to elucidate the functions of immune cell interactions with sensory neurons in the context of itch. Sensitization and repetitive challenge with toluene diisocyanate (TDI) to induced ACD mouse model and the co-culture model of immune cells and DRG neurons were employed in this study. The findings demonstrated the successful induction of ACD in the TDI-sensitized mice, as evidenced by the development of edema, elevated total serum IgE levels, and an observed itch response in the TDI-sensitized mice. These mice also exhibited signs of itch peripheral sensitization, with a higher percentage of DRG neurons responding to pruritogens and increased activation and expression of itch receptors. Furthermore, an increase in the number of dendritic cells (DCs) and mast cells within DRG during ACD was observed. Culturing DRG neurons with mast cells under hapten-sensitized condition led to a higher percentage of neurons responding to non-histaminergic pruritogen and activation of TRPA1 compared to neurons in monoculture. This indicates that mast cells might play a role in enhancing sensitivity to non-histaminergic pruritogen. The co-culture experiments between DRG neurons and DCs under allergic condition demonstrated that the presence of DCs led to an increased proportion of DRG neurons exhibiting response to pruritogens and elevated activation of itch-associated receptors, including TRPA1, TRPV1, H1R, and TRPV4. These findings suggest that DCs within the DRG may contribute to peripheral sensitization of itch by modulating the activity of receptors involved in itch signaling. Transcriptomic analysis of DCs within the DRG revealed their involvement in immune responses, inflammation, and itch, with increased expression of Ctss and Sgpp2. Further functional experiments targeting CTSS through inhibition in co-culture studies demonstrated a diminished response to pruritogens and agonists of TRPA1 and TRPV1, suggesting a potential role for CTSS in itch sensitization. The co-culture study findings further suggest that the neuroimmune modulation may be mediated by the S1P metabolism in DCs and signaling of S1P in DRG neurons. In summary, targeting DCs and further elucidating the non-histaminergic functions of mast cells within DRG could represent a promising novel therapeutic approach for itch treatment.
