HAdVs are considered the common viral agent of pediatric severe pneumonia, with HAdV-7 in particular causing more severe clinical consequences, which is often associated with the high replication competence of HAdV-7 in human lung[3], and prolonged virus shedding and persistence of HAdV-7 in targeting cell. Some of which possible mechanisms have been previously reported. for example, adenovirus's own protein (e.g. E1A) can suppress of type I interferon signaling. However, the underlying pathophysiological mechanisms of HAdV-7 infection are complicated and remain unclear. Viral infection usually results in changes in the expression of host genes following replication. Thus, host cell transcriptomes can reflect the changes in specific genes and pathways that occur during infection [15, 22]. WGCNA may be an effective method of mining valuable data to analyze complicated genetic networks in HAdV-7 infection. One of the advantages of WGCNA is its high reliability and biological significance because the analysis focuses on the association between co-expression modules and infection traits [4].In this study, using gene expression data obtained by RNA-seq from HAdV-7-infected and mock-infected A549 cells, a total of 12 co-expression modules were constructed using WGCNA. Among them, the blue, tan, and brown modules were positively correlated with HAdV-7-infected traits at 24, 48, and 72 hpi, respectively. Genes in the same module are considered functionally related to each other. Functional enrichment analysis indicated that genes in the blue module were primarily enriched in cell cycle regulation and DNA replication, consistent with previous studies noting that adenovirus infection has proceeded far into the early phase at 24 h and expression changes favor its DNA replication, with about 50% of those genes with known functions involved in cell cycle control [43]. Genes in the tan module were significantly associated with cell metabolism. At this time, the virus has gained control of the cellular metabolic machinery to create conditions in which the viral genome can replicate efficiently [44]. Genes in the brown module were mainly enriched in mitochondrial respiratory chain functions, protein translation, and apoptosis. During this period, considerable energy and proteins are needed to complete virus assembly. Moreover, cell death is thought to be imminent, facilitating efficient release and spread of the virion progeny. DEGs with high intramodular connectivity were identified as hub genes for HAdV-7 infection. Furthermore, concordant genes among hub genes from the blue, tan, and brown modules and DEGs in the GSE68004 dataset were identified using a Venn diagram (Fig. 3-5C). Among them, SOCS3, OASL, ISG15, and IFIT1 were closely involved in the process of adenovirus infection and were thus regarded as candidate genes. During viral invasion, the host rapidly establishes several defensive mechanisms by eliciting the innate immune response. Conventionally, IFN-I is the major component of the innate immune system against viral infection via induction of various ISGs [20, 26]. OASL, ISG15, and IFIT1 are ISGs and are involved in confrontation between adenovirus and host cells (Fig. 8). OASL codes for an important ISG and plays different roles in DNA and RNA viruses by inhibiting cGAS-mediated IFN production or enhancing RIG-I-mediated IFN induction, respectively [5, 19]. Several studies have found that OASL inhibits the replication of some RNA viruses, such as vesicular stomatitis virus (VSV), Sendai virus (SeV), and respiratory syncytial virus (RSV) [5, 48, 49]. Unlike RNA viruses, however, much less is known about the effects of OASL in the context of DNA virus infection. Human OASL and mouse Oasl2 are reported to promote replication of certain DNA viruses, including herpes simplex virus (HSV), mouse cytomegalovirus (MCMV), and adenovirus [10]. Therefore, OASL may serve as a biomarker to improve discrimination between DNA and RNA virus infection. As one of the most highly up-regulated genes during viral infections, ISG15 acts as both an effector and a signaling molecule in various phases of the innate immune response [8]. ISG15 is involved in many antiviral-signaling pathways, both intracellular and extracellular, and activates various immune cells and promotes the production of many antiviral cytokines to facilitate viral clearance [8, 26]. Several studies have reported on ISG15-deficient patients and found that human ISG15 is redundant for antiviral immunity and negatively regulates IFN-I immunity[42]. Although the multiple biological functions of ISG15, including as a biomarker of antiviral treatment [14], offer promise for intervention in disease progression, several important questions remain to be answered in future research. The expression of IFIT1 is strongly induced by IFN-I, double-stranded RNAs, and viral infection[28], and increasing evidence has demonstrated that IFIT1 has antiviral activity during both DNA and RNA virus infection, mainly by intervening translation by differentially recognizing the 5’ terminus of target RNA [30], but it could also inhibit the interferon signal pathway[7]. As one of the most studied members of the SOCS family, SOCS3 can be stimulated by JAK/STAT signals to regulate the proinflammatory response via negatively regulating cytokine receptors [21]. There is considerable evidence that multiple viruses can up-regulate SOCS3 expression and dampen host antiviral responses to promote viral replication through various immune evasion strategies [13]. For example, influenza A virus [25], HSV [38], and RSV [47] can suppresses IFN-I production and response by stimulating SOCS3 expression [2], consistent with our data showing that SOCS3 was up-regulated after adenovirus infection. SOCS3, as viral virulence factor, may also have great therapeutic potential. For example, SOCS3 expression could be manipulated to restore antiviral immune responses. In addition, SOCS3 antagonists have shown antiviral effects on a broad range of viruses in cell and animal models [13, 16, 24].
Altogether, SOCS3, OASL, ISG15 and IFIT1 are not only involved in confrontation between adenovirus and host cells together with other molecules (mainly disturbing type I interferon signaling), but also have good a correlation in the expression levels. We therefore speculate that HAdV-7 could inhibit interferon production through multiple targets after infecting host cells, which partly explain the possible mechanism of more severe disease and significant morbidity. Although the present study is the first to investigate co-expression gene networks associated with HAdV-7 infection using WGCNA, our research has several limitations. For instance, we only used a single cell line rather than human specimens, thus, we select the data of children whole blood from a public database for validation of hub genes. On the other hand, we did not further study the exact mechanism of the identified key genes in HAdV-7 infection. Further studies are required to evaluate and confirm the involvement of the candidate genes in different races and samples.