The skin serves a dual function as an immunological barrier and a sensory interface between the body and environment. Protection against invading pathogens is accomplished by coordinated interactions between immune cells in the skin whose aberrant activity can provoke pathologic inflammation. Increasing evidence has demonstrated a unique role of pain sensing fibers, or nociceptors, in cutaneous immune responses. Briefly, Trpv1+ nociceptors were found to be required for IL-23-dependant production of IL-17 by dermal gamma delta and CD4 T-cells. Mice deficient in these nociceptors are more susceptible to C. albicans skin infection but have diminished pathology in an IMQ mouse model of Type-17 psoriasaform inflammation. The requirement of nociceptors for cutaneous immunity suggests that neuronal activation by itself may be sufficient to affect the balance between protective host defense and pathologic inflammation. To test the sufficiency of nociceptors for skin inflammation, we developed an optogenetic murine model which allows for selective activation of nociceptors with high temporal and spatial precision. We found that activation of Trpv1+ nerves is sufficient for skin inflammation involving production of IL-23, IL-6, and TNF? and infiltration of IL-17 producing T-cells. Interestingly, activation of distinct nociceptors expressing MrgprD induces expression of Type-2 but not Type-17 cytokines. Taken together, the sufficiency of nociceptors to modulate cutaneous immunity highlights nociceptors as a therapeutic target to enhance host defense or limit pathologic inflammation in the skin. We hypothesize that activation of Trpv1+ nociceptors is sufficient to activate a cascade of events starting with CGRP activation of cDC2 which in turn release cytokines that induce IL-17 production from T-cells. We anticipate that this Type-17 immune response will provide protection to epicutaneous S. aureus infection. Finally, we hypothesize that induction of Type-17 immunity is specific for Trpv1+ nociceptors whereas activation of MrgprD-expressing fibers will be sufficient for Type-2 immune responses. We will address these questions in three specific aims.
Aim 1 : Test the hypothesis that Trpv1+ nociceptor activation drives IL-23, IL-6 and TNF? secretion from cDC2. Trpv1-Ai32(Chr2-YFP) mice will be cutaneously photostimulated. Tissue will be analyzed by flow cytometry and qPCR for IL-23, IL-6 and TNF?. Finally, a series bone marrow chimeras will be generated allowing for the depletion of cDC2 to demonstrate functional non-redundancy.
Aim 2 : Test the hypothesis that Trpv1+ nociceptors are sufficient and required for host-protection against S. aureus infection. Mice with ablation of Trpv1+ nociceptors or photostimulated Trpv1-Ai32 mice will be epicutaneously infected with S. aureus. CFU and immune parameters for Type-17 inflammation will be analyzed by flow cytometry.
Aim 3 : Test the hypothesis that MrgprD+ nociceptors are sufficient for Type-2 inflammation. MrgprD-Ai32 mice will be optogenetically activated and assayed for Type-2 inflammation by flow cytometry and mRNA expression.
Emerging evidence has highlighted the contribution of cutaneous nociceptors to various immune-mediated processes ranging from host-defense to autoimmune inflammation. This project integrates optogenetic control of neuronal activity with immunological tools to determine the mechanisms by which cutaneous nociceptors drive inflammation and antimicrobial immunity in the skin. The sufficiency of nociceptors to drive cutaneous inflammation will enhances our basic understanding of skin biology and therefore is likely to aid in the rational design of novel therapies.
