Herpes simplex virus (HSV) causes genital and oral lesions and is a life-long infection. HSV persists by retreating into ganglia (nerve-cell bodies) and lying dormant until reactivation whereupon the virus uses nerve fibers to relocate for replication and transmission. Interestingly, there is little nerve damage during HSV infection, even though the virus is continually cycling between latency and reactivation in these cells. This phenomenon is something that the Corey Lab at Fred Hutch is working to understand. In a paper published in Journal of Experimental Medicine the team lead by Drs. Zhu and Peng worked on understanding the interplay between HSV infection and nerve health. They identified an epithelial cell-derived cytokine, IL-17c, which is produced during HSV infection by keratinocytes, epidermal cells making up the outer layer of skin. The accumulation of IL-17c during infection spurred the group to try and follow the production and downstream effects of the cytokine.
To start, genital skin biopsies were stained for nerve fibers in symptomatic, asymptomatic reactivation, and control samples. Their analysis suggested that peripheral nerve fibers in the reactivation samples had longer, more dense nerve fibers compared to control samples (see figure). Nerve fibers from symptomatic samples also had longer lengths compared to control cells. To assess the role of keratinocytes in influencing nerve fibers during HSV infection, cells were isolated and analyzed for their expression of cytokine, chemokine and growth factor genes. Of the 300 genes sampled, HSV infected samples had an increase in the expression of IL-17c among other genes. To test the hypothesis that HSV infection could induce IL-17c, human primary keratinocytes were infected in vitro and the expression of IL-17c was measured, resulting in early expression. To further test the effect of IL-17c on nerve growth the scientists blocked IL-17c signaling by using an antibody against its receptor (IL-17RA), thus blocking downstream signaling. The presence of antibody did not have any effect on HSV replication, suggesting that IL-17c does not act as an antiviral. This finding led the scientists to ask what cell types IL-17c might be acting on if not directly on the virus. They hypothesized that neuronal cells were a possibility based on the presence of the IL-17c receptor in those cells and suggest that keratinocyte-derived IL-17c could lead to signaling on nerve fibers in the dermal area during reactivation.
To further test the possibility that IL-17c effects neuronal fibers, in vitro studies were performed to determine whether exogenous recombinant IL-17c effected neuron growth. Neuron-like cells had significantly longer and more neurites when grown in the presence of cytokine compared to control-treated cells and this was similar to the effects observed in the presence of known growth factors. By adding a IL-17c-blocking antibody to the mix the team saw reduced neurotropic effects. A similar assay was performed on isolated human sensory neurons from fetal dorsal root ganglia and resulted in faster-growing, longer and more branched neurites. As a whole, this data suggests that IL-17c acts to promote neurite growth and branching. Based on the previous findings in epithelial cells that IL-17c may have anti-apoptotic effects, the group looked at whether apoptosis was also prevented in the IL-17c treated neurons. Mouse primary cortical neurons were pre-incubated with IL-17c and then infected with HSV. In these cultures, there was a reduction of the apoptotic markers caspase 3 and apoptotic enzyme as well as apoptosis of neurons. IL-17c blocking antibody reduced this effect. This blocking suggests that IL-17c provides a survival signal to neurons during HSV infection. In summary, this work demonstrated that IL-17c might acts as a HSV-induced signal to protect and promote neuronal growth and branching. This could provide new routes for HSV to travel to new skin cells to infect, thus allowing for recurrence. When asked what direction further research will go Dr. Peng said, “The findings in this paper warrant further exploration on how to optimize exogenous IL-17c as a prospective neurotrophic cytokine and/or growth repair factor for peripheral nerve regeneration/repair. Induction of IL-17c by reactivating HSV-2 in humans is a natural repair and regeneration pathway for peripheral nerves that may if more completely understood have implications for treatment for and/or preventing peripheral neuropathies such as chemotherapy induced peripheral neuropathy (CIPN) and diabetes induced poly-neuropathy (DIPN)”.
Peng T, Chanthaphavong RS, Sun S,Trigilio JA, Phasouk K, Jin L,Layton ED, Li AZ, Correnti CE, De van der Schueren W, Vazquez J, O'Day DR, Glass IA, Knipe DM, Wald A, Corey L, Zhu J. 2017. Keratinocytes produce IL-17c to protect peripheral nervous systems during human HSV-2 reactivation. J Exp Med.
Funding was provided by the National Institutes of Health.