Gammaherpesvirus Small Noncoding RNAs Are Bifunctional Elements That Regulate Infection and Contribute to Virulence In Vivo

Many viruses express noncoding RNAs (ncRNAs). The gammaherpesviruses (gamma HVs), including Epstein-Barr virus, Kaposi's sarcoma-associated herpesvirus, and murine gamma HV68, each contain multiple ncRNA genes, including microRNAs (miRNAs). While these ncRNAs can regulate multiple host and viral processes in vitro, the genetic contribution of these RNAs to infection and pathogenesis remains largely unknown. To study the functional contribution of these RNAs to gamma HV infection, we have used gamma HV68, a small-animal model of gamma HV pathogenesis. gamma HV68 encodes eight small hybrid ncRNAs that contain both tRNA-like elements and functional miRNAs. These genes are transcribed by RNA polymerase III and are referred to as the gamma HV68 TMERs (tRNA-miRNA-encoded RNAs). To determine the total concerted genetic contribution of these ncRNAs to gamma HV acute infection and pathogenesis, we generated and characterized a recombinant gamma HV68 strain devoid of all eight TMERs. TMER-deficient gamma HV68 has wild-type levels of lytic replication in vitro and normal establishment of latency in B cells early following acute infection in vivo. In contrast, during acute infection of immunodeficient mice, TMER-deficient gamma HV68 has reduced virulence in a model of viral pneumonia, despite having an enhanced frequency of virus-infected cells. Strikingly, expression of a single viral tRNA-like molecule, in the absence of all other virus-encoded TMERs and miRNAs, reverses both attenuation in virulence and enhanced frequency of infected cells. These data show that gamma HV ncRNAs play critical roles in acute infection and virulence in immunocompromised hosts and identify these RNAs as a new potential target to modulate gamma HV-induced infection and pathogenesis. IMPORTANCE The gammaherpesviruses (gamma HVs) are a subfamily of viruses associated with chronic inflammatory diseases and cancer, particularly in immunocompromised individuals. These viruses uniformly encode multiple types of noncoding RNAs (ncRNAs) that are not translated into proteins. It remains unclear how virus-expressed ncRNAs influence the course and outcome of infection in vivo. Here, we generated a mouse gamma HV that lacks the expression of multiple ncRNAs. Notably, this mutant virus is critically impaired in the ability to cause disease in immunocompromised hosts yet shows a paradoxical increase in infected cells early during infection in these hosts. While the original mouse virus encodes multiple ncRNAs, the expression of a single domain of one ncRNA can partially reverse the defects of the mutant virus. These studies demonstrate that gamma HV ncRNAs can directly contribute to virus-induced disease in vivo and that these RNAs may be multifunctional, allowing the opportunity to specifically interfere with different functional domains of these RNAs.