Herpes Simplex Virus type 1 lytic viral gene expression during the establishment and maintenance of latency

Abstract

Herpes Simplex Virus (HSV) type I causes cold sores but is also associated with severe outcomes such as encephalitis and blindness. The primary lytic HSV-1 infection in the skin and peripheral nervous system (PNS) is limited to around a week, but latent virus persists in neurons, from which it can reactivate periodically. A black and white view of lytic versus latent infection persists in the HSV literature. Somewhat paradoxically, there remains concern that ‘true’ latent infection cannot be assumed to occur in animal models within a month of primary infection. In this thesis, the ROSA26R/Cre mouse model was used that allows historic assessment of virus activity. In this model, β-galactosidase (β-gal) expression is switched on permanently in any cells that had experienced HSV-1-driven Cre recombinase expression. Further, placing the Cre gene under the control of various HSV-1 promoters allowed the number of cells that have experienced different types of viral activity to be determined, from entry of a virus genome to expression of lytic genes. This historical analysis found substantial lytic gene expression and spread of virus occurs in the PNS for at least five days beyond the peak of infectious virus load. This suggests that the period immediately after the bulk of the lytic infection is quelled remains highly dynamic. Further, there was continued accumulation in β-gal marked cells in mice infected with viruses that express Cre from the gB and infected cell protein (ICP) 6 promoters. Therefore, transient, likely low level, promoter activity does occur during latency, which can lead to protein production. This was not observed for the ICP0 promoter, indicating that expression from various lytic gene promoters differs during this time. More striking, when expression of Cre was directed by the promoter for ICP47, a viral gene that functions to inhibit adaptive immune responses, the number of β-gal-expressing neurons continued to rise sharply until day 20 after infection. Further, β-gal marked cells continued to accumulate throughout latency, suggesting that ICP47 may function during latency to facilitate evasion of the immune response, and potentially reactivation. However, attempts to overcome the effect of ICP47 expression by increasing antigen presentation on infected neurons did not have a substantial impact on the establishment or maintenance of latency. In summary, this thesis has provided insights into the dynamic interaction between viral lytic gene expression and the immune response during latency, challenging the traditional paradigm of an almost-quiescent form of HSV-1 latency. The results presented in this thesis further our understanding HSV-1 and α-herpesvirus latency, and with further research will hopefully lead to better therapeutic outcomes.