Herpes simplex virus types 1 and 2 (HSV-1 and HSV-2) are major human pathogens. HSV establishes latency in the nervous system and reactivates to cause recurrent disease, resulting in transmission of progeny virions to naïve individuals. Though HSV-1 and HSV-2 share similar structure and genes, they have distinctive recurrence profiles. Generally, HSV-1 reactivation is associated with disease 'above the waist' and HSV-2 reactivation is associated with disease 'below the waist'. This phenomenon was described decades ago but still remains unexplained.

The mechanism of HSV latent infection in the peripheral nervous system (PNS) has been extensively investigated, especially with in sensory neurons. Another component of the peripheral nervous system (PNS), autonomic neurons, were also known to be infected with HSV productively and latently, but largely ignored because of the assumption that there is no difference in the pathogenesis of HSV in the neurons and that both HSV-1 and HSV-2 behave in the same way in different types of neurons.

However, autonomic neurons differ in physiological function compared to sensory neurons. Activation factors of autonomic neurons, such as emotional stress, trauma and hormonal fluctuation, are also known HSV reactivation triggering factors. Therefore, I hypothesized that autonomic neurons innervating the site of HSV infection are responsible the different reactivation frequencies of HSV-1 and HSV-2 after peripheral invasion.

In this report, the role of autonomic neurons in HSV pathogenesis were examined using the female guinea pig reactivation model. Major findings of this report are that 1) parasympathetic ganglia innervating the ocular region support latent infection of HSV-1 selectively, thus contributing the more frequent HSV-1 reactivation, 2) mixed autonomic ganglia in the genital area support HSV-2 latent infection selectively, and 3) sympathetic neurons in the genital region supported productive and latent infection of HSV-1 and HSV-2 differently.

All of the results in this report indicate that autonomic neurons play a distinctive role in HSV pathogenesis compared to the sensory neurons and are responsible for the different reactivation frequencies of HSV-1 and HSV-2. This report raises the importance of autonomic neurons in HSV pathogenesis and challenges the paradigm of HSV pathogenesis.