Introduction:

Nociceptors are sensory peripheral pseudo-unipolar neurons with split axons. Upon sensing painful noxious stimuli, nociceptor ‘central’ axonal processes conduct pain information to the central nervous system, while their ‘peripheral’ axonal processes innervate the skin and terminate as free intraepidermal nerve fibers (IENF) that secret the neuropeptide calcitonin gene-related peptide (CGRP). IENF physically interact with antigen-presenting Langerhans cells (LCs), and we previously showed that CGRP inhibits LCs infection by mucosal viruses, e.g., herpes simplex virus (HSV). Importantly, maintenance of the IENF network is a balance between axonal guidance vs. degeneration, and LCs depletion in mouse skin decreases IENF density. The neuroimmune mechanisms by which LCs modulate nociceptive innervation, especially in humans, remain elusive.

Methods:

We developed two neuroimmune experimental models: 1) non-polarized murine nociceptors co-cultured with human monocyte-derived LCs (MDLCs); 2) humanized ‘mucosa-on-chip’ microfluidic model, in which nociceptor soma are seeded in one chamber; polarize their axons across micro-channels into a distal chamber; their IENF are co-cultured with genital epithelial cells and MDLCs.

Results:

In steady-state, untreated MDLCs induce significant nociceptor axonal outgrowth. In contrast, CGRP-pretreated MDLCs markedly abrogate outgrowth, which associates with MDLCs increased expression of the axonal guidance factor semaphorin 4A that repels axons, and with MDLCs decreased secretion of the neurotrophic factor nerve growth factor that attracts axons. During HSV infection, HSV-infected MDLCs relay the virus to nociceptor axons, resulting in inhibition of axonal outgrowth, axonal degeneration, and reduction in CGRP content.

Conclusions:

Our studies show that in addition to exerting sentinel functions, LCs fine-tune homeostatic nociceptor axonal attraction vs. repulsion via both physical neuroimmune synapses and soluble factors, thereby actively participating in nociceptor innervation. Our studies further reveal host vs. pathogen protective mechanisms, whereby nociceptor axons degenerate to protect them from HSV infection, while HSV disrupts CGRP to counteract its anti-viral roles.