Pathogenesis and Chemotherapy of Herpesvirus Infection

National Institute Of Allergy And Infectious Diseases

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Abstract

Herpesviruses, including herpes simplex (HSV) and varicella-zoster virus, establish lifelong latent infection in nerve cells. HSV type 1 can reactivate to cause cold sores, and HSV-2 can reactivate to cause genital herpes. Although these infections are usually mild, they can cause encephalitis and severe disease in newborn infants. In addition, HSV-2 is a risk factor for transmission and acquisition of HIV. HIV can cause opportunistic infections and is associated with increased rates of several cancers. Herpesviruses establish a latent infection in the neurons in the ganglia (a group of nerve cells outside the central nervous system). The mechanism by which these viruses establish latency is not understood. A better understanding of latency might result in novel ways to prevent latent infections or prevent the virus from reactivation once it establishes latency. HSV expresses a predominant viral RNA, termed the latency-associated transcript (LAT), when the virus is latent in nerve cells. Animal models, using viruses deleted for HSV LAT, indicate that LAT is important for regulating HSV reactivation. While the function of the HSV LAT in reactivation is uncertain, LAT has been postulated to increase the ability of the virus to establish or maintain latency, likely by increasing the survival of latently infected nerve cells. We have been studying other mechanisms whereby the LAT might affect latency or reactivation. We previously developed and reported transgenic mice that express the HSV type 2 LAT from its native promoter in trigeminal ganglia. In 2009 we performed in situ hybridization to detect HSV-2 LAT RNA in multiple tissues in the transgenic mice to determine which cells have detectable expression and accumulation of LAT. We identified LAT in multiple cell types in most tissues analyzed from these mice. While weak to moderate signals were seen in brain and spinal cord neurons, epithelial cells, and muscle cells, the strongest signals were detected in neurons from dorsal root and trigeminal ganglia. About 70-85% of neurons in these ganglia were LAT-positive with varying signal intensities, but surprisingly cells surrounding the neurons were LAT-negative. These data indicate that HSV-2 LAT promoter activity is not restricted to neurons, that cells surrounding the neurons have factors that may reduce expression of LAT or enhance its degradation, and that LAT accumulation in ganglionic neurons is likely regulated by cell-specific factors. Previous studies have shown that both the amount of viral DNA in the ganglia of animals latently infected with HSV and the number of CD8+ T lymphocytes in the ganglia influence the rate of reactivation of HSV. While LAT is important for efficient reactivation and establishment of latency, it is uncertain how LAT affects either the amount of viral DNA in the ganglia or the number of CD8+ T lymphocytes infiltrating the ganglia. In 2009 we infected mice with HSV-2 viruses that were deficient in expression of LAT (LAT-deficient HSV-2) or in which the LAT gene was restored (LAT-restored HSV-2). As expected the LAT-deficient HSV-2 had a reduced rate of reactivation;however, there was not a higher number of CD8+ T lymphocytes in the ganglia of mice infected with the LAT-deficient HSV-2 compared with the LAT-restored HSV-2. The rate of reactivation was lower for the LAT-deficient HSV-2 than LAT-restored HSV-2 even when the amount of viral DNA in the ganglia of animals latently infected with the two viruses was similar. Therefore, differences in reactivation for the LAT-deficient and LAT-restored HSV-2 were not solely dependent on the number of CD8+ T lymphocytes or the amount of latently infected viral DNA in the ganglia. Thus, LAT likely has additional functions important for reactivation.