The microbial community that colonizes all living organisms is gaining appreciation for its contributions to both physiologic and pathogenic processes. The virome, a subset of the overall microbiome, large and diverse, including viruses that persistently inhabit host cells, endogenous viral elements genomically or epigenomically integrated into cells, and viruses that infect the other (bacterial, protozoan, fungal, and archaeal) microbiome phylla. These viruses live in the organism for its life, and therefore are to be considered part of the aging process experienced by the organism. This review considers the impact of the persistent latent virome on immune aging. Specific attention will be devoted to the role of herpesviruses, and within them, the cytomegalovirus, as the key modulators of immune aging.
Human cytomegalovirus (HCMV) resides latently in hematopoietic cells of the bone marrow. Although viral genomes can be found in CD14+ monocytes and CD34+ progenitor cells, the primary reservoir for latent cytomegalovirus is unknown. We analyzed human hematopoietic subpopulations infected in vitro with a recombinant virus that expresses a green fluorescent protein marker gene. Although many hematopoietic cell subsets were infected in vitro, CD14+ monocytes and various CD34+ subpopulations were infected with the greatest efficiency. We have developed an in vitro system in which to study HCMV infection and latency in CD34+ cells cultured with irradiated stromal cells. Marker gene expression was substantially reduced by 4 days postinfection, and infectious virus was not made during the culture period. However, viral DNA sequences were maintained in infected CD34+ cells for >20 days in culture, and, importantly, virus replication could be reactivated by coculture with human fibroblasts. Using an HCMV gene array, we examined HCMV gene expression in CD34+ cells. The pattern of viral gene expression was distinct from that observed during productive or nonproductive infections. Some of these expressed viral genes may function in latency and are targets for further analysis. Altered gene expression in hematopoietic progenitors may be indicative of the nature and outcome of HCMV infection.
We report that UL133-UL138 (UL133/8), a transcriptional unit within the ULb' region (ULb') of the human cytomegalovirus (HCMV) genome, and UL97, a viral protein kinase encoded by HCMV, play epistatic roles in facilitating progression of the viral lytic cycle. In studies with HCMV strain TB40/E, pharmacological blockade or genetic ablation of UL97 significantly reduced the levels of mRNA and protein for IE2 and viral early and early-late genes during a second wave of viral gene expression that commenced at between 24 and 48 h postinfection. These effects were accompanied by significant defects in viral DNA synthesis and viral replication. Interestingly, deletion of UL133/8 likewise caused significant defects in viral DNA synthesis, viral gene expression, and viral replication, which were not exacerbated upon UL97 inhibition. When UL133/8 was restored to HCMV laboratory strain AD169, which otherwise lacks the locus, the resulting recombinant virus replicated similarly to the parental virus. However, during UL97 inhibitor treatment, the virus in which UL133/8 was restored showed significantly exacerbated defects in viral DNA synthesis, viral gene expression, and production of infectious progeny virus, thus recapitulating the differences between wild-type TB40/E and its UL133/8-null derivative. Phenotypic evaluation of mutants null for specific open reading frames within UL133/8 revealed a role for UL135 in promoting viral gene expression, viral DNA synthesis, and viral replication, which depended on UL97. Taken together, our findings suggest that UL97 and UL135 play interdependent roles in promoting the progression of a second phase of the viral lytic cycle and that these roles are crucial for efficient viral replication.
Clinical trials have shown oncolytic adenoviruses to be tumor selective with minimal toxicity toward normal tissue. The virus ONYX-015, in which the gene encoding the early region 1B 55-kDa (E1B-55K) protein is deleted, has been most effective when used in combination with either chemotherapy or radiation therapy. Therefore, improving the oncolytic nature of tumor-selective adenoviruses remains an important objective for improving this form of cancer therapy. Cells infected during the G(1) phase of the cell cycle with the E1B-55K deletion mutant virus exhibit a reduced rate of viral late protein synthesis, produce fewer viral progeny, and are less efficiently killed than cells infected during the S phase. Here we demonstrate that the G(1) restriction imposed on the E1B-55K deletion mutant virus is due to the viral oncogene encoded by open reading frame 1 of early region 4 (E4orf1). E4orf1 has been reported to signal through the phosphatidylinositol 3'-kinase pathway leading to the activation of Akt, mTOR, and p70 S6K. Evidence presented here shows that E4orf1 may also induce the phosphorylation of Akt and p70 S6K in a manner that depends on Rac1 and its guanine nucleotide exchange factor Tiam1. Accordingly, agents that have been reported to disrupt the Tiam1-Rac1 interaction or to prevent phosphorylation of the ribosomal S6 kinase partially alleviated the E4orf1 restriction to late viral protein synthesis and enhanced tumor cell killing by the E1B-55K mutant virus. These results demonstrate that E4orf1 limits the oncolytic nature of a conditionally replicating adenovirus such as ONYX-015. The therapeutic value of similar oncolytic adenoviruses may be improved by abrogating E4orf1 function.
Human cytomegalovirus (HCMV) exists indefinitely in infected individuals by a yet poorly characterized latent infection in hematopoietic cells. We previously demonstrated a requirement for the putative UL138 open reading frame (ORF) in promoting a latent infection in CD34(+) hematopoietic progenitor cells (HPCs) infected in vitro. In our present study, we have identified two coterminal transcripts of 2.7 and 3.6 kb and a 21-kilodalton (kDa) protein (pUL138) that are derived from the UL138 locus with early-late gene kinetics during productive infection. The UL138 transcripts and protein are detected in both fibroblasts and HPCs. A recombinant virus, FIX-UL138(STOP), that synthesizes the UL138 transcripts but not the protein exhibited a partial loss-of-latency phenotype in HPCs, similar to the phenotype observed for the UL138-null recombinant virus. This finding suggests that the UL138 protein is required for latency, but it does not exclude the possibility that the UL138 transcripts or other ORFs also contribute to latency. The mechanisms by which pUL138 contributes to latency remain unknown. While the 86- and 72-kDa immediate-early proteins were not detected in HPCs infected with HCMV in vitro, pUL138 did not function directly to suppress expression from the major immediate-early promoter in reporter assays. Interestingly, pUL138 localizes to the Golgi apparatus in infected cells but is not incorporated into virus particles. The localization of pUL138 to the Golgi apparatus suggests that pUL138 contributes to HCMV latency by a novel mechanism. pUL138 is the first HCMV protein demonstrated to promote an infection with the hallmarks of latency in CD34(+) HPCs.