Nociceptor nerves set the stage for skin immunity

Immunity to the microbiota promotes sensory neuron regeneration

Tissue immunity and responses to injury depend on the coordinated action and communication among physiological systems. Here, we show that, upon injury, adaptive responses to the microbiota directly promote sensory neuron regeneration. At homeostasis, tissue-resident commensal-specific T cells colocalize with sensory nerve fibers within the dermis, express a transcriptional program associated with neuronal interaction and repair, and promote axon growth and local nerve regeneration following injury. Mechanistically, our data reveal that the cytokine interleukin-17A (IL-17A) released by commensal-specific Th17 cells upon injury directly signals to sensory neurons via IL-17 receptor A, the transcription of which is specifically upregulated in injured neurons. Collectively, our work reveals that in the context of tissue damage, preemptive immunity to the microbiota can rapidly bridge biological systems by directly promoting neuronal repair, while also identifying IL-17A as a major determinant of this fundamental process.

Crosstalk Between Epithelial Cells, Neurons and Immune Mediators in HSV-1 Skin Infection

Herpes simplex virus type 1 (HSV-1) infection is highly prevalent in humans, with approximately two-thirds of the world population living with this virus. However, only a fraction of those carrying HSV-1, which elicits lifelong infections, are symptomatic. HSV-1 mainly causes lesions in the skin and mucosae but reaches the termini of sensory neurons innervating these tissues and travels in a retrograde manner to the neuron cell body where it establishes persistent infection and remains in a latent state until reactivated by different stimuli. When productive reactivations occur, the virus travels back along axons to the primary infection site, where new rounds of replication are initiated in the skin, in recurrent or secondary infections. During this process, new neuron infections occur. Noteworthy, the mechanisms underlying viral reactivations and the exit of latency are somewhat poorly understood and may be regulated by a crosstalk between the infected neurons and components of the immune system. Here, we review and discuss the immune responses that occur at the skin during primary and recurrent infections by HSV-1, as well as at the interphase of latently-infected neurons. Moreover, we discuss the implications of neuronal signals over the priming and migration of immune cells in the context of HSV-1 infection.

Role of the Neurokinin-1 Receptor in the Promotion of Corneal Epithelial Wound Healing by the Peptides FGLM-NH2 and SSSR in Neurotrophic Keratopathy

Purpose

Neurotrophic keratopathy is a corneal epitheliopathy induced by trigeminal denervation that can be treated with eyedrops containing the neuropeptide substance P (or the peptide FGLM-NH₂ derived therefrom) and insulin-like growth factor 1 (or the peptide SSSR derived therefrom). Here, we examine the mechanism by which substance P (or FGLM-NH₂) promotes corneal epithelial wound healing in a mouse model of neurotrophic keratopathy.

Methods

The left eye of mice subjected to trigeminal nerve axotomy in the right eye served as a model of neurotrophic keratopathy. Corneal epithelial wound healing was monitored by fluorescein staining and slit-lamp examination. The distribution of substance P, neurokinin-1 receptor (NK-1R), and phosphorylated Akt was examined by immunohistofluorescence analysis. Cytokine and chemokine concentrations in intraocular fluid were measured with a multiplex assay.

Results

Topical administration of FGLM-NH₂ and SSSR promoted corneal epithelial wound healing in the neurotrophic keratopathy model in a manner sensitive to the NK-1R antagonist L-733,060. Expression of substance P and NK-1R in the superficial layer of the corneal epithelium decreased and increased, respectively, in model mice compared with healthy mice. FGLM-NH₂ and SSSR treatment suppressed the production of interleukin-1α, macrophage inflammatory protein 1α (MIP-1α) and MIP-1β induced by corneal epithelial injury in the model mice. It also increased the amount of phosphorylated Akt in the corneal epithelium during wound healing in a manner sensitive to prior L-733,060 administration.

Conclusions

The substance P–NK-1R axis promotes corneal epithelial wound healing in a neurotrophic keratopathy model in association with upregulation of Akt signaling and attenuation of changes in the cytokine-chemokine network.