Herpes simplex virus 1 ICP34.5 acts to maintain latency in human and mouse neurons

Herpes simplex virus 1 (HSV-1) establishes latent infections in sensory neurons, from which HSV sporadically reactivates, often due to external stress and other stimuli. Latency and reactivation are studied using in vivo models in a variety of hosts, as well as in vitro models including primary mouse neurons, and neurons derived from human pluripotent stem cells (iPSCs). The interferon (IFN)-based neuronal innate immune response is critical in controlling HSV-1 replication and HSV-1 counters these responses, in part, through infected-cell protein 34.5 (ICP34.5). ICP34.5 also promotes neurovirulence by preventing host translational shutoff and interfering with host cell autophagy through its interaction with the autophagy regulator Beclin 1. Here we demonstrate in human iPSC-derived neurons (iNeurons) that ICP34.5 unexpectedly suppresses spontaneous reactivation and thereby is critical for maintenance of HSV-1 latency. Furthermore, our results suggest that both sustaining host translation and the interaction of ICP34.5 with Beclin 1 are important for maintaining latency in iNeurons. Experiments using primary mouse neurons show that ICP34.5 may be essential for maintaining latency but in an IRF3/7-dependent manner. In wild type mouse neurons, ICP34.5-null and ΔPP1⍺ viruses exhibited little spontaneous reactivation and had defects in induced reactivation suggesting that countering PKR-mediated responses is the key activity of ICP34.5 for enhancement of reactivation. These results highlight the value of studying HSV-1 latency and reactivation in different models and we explore explanations for how ICP34.5 may differentially impact latency in the two systems studied herein.