The WHO released the Global Influenza Prevention and Control Strategy (2019–2030), and the importance of influenza prevention and control has attracted global attention[1, 11]. In this study, the global influenza epidemic characteristics and trends were analyzed using descriptive epidemiology in conjunction with the GBD database from 1990 to 2019. The Joinpoint regression trend analysis shows that the overall trend of influenza DALYs rate and mortality rate is decreasing, which can be attributed to the following reasons: firstly, with the wider influenza vaccination and public acceptance of the vaccine, the number of influenza infections can be reduced, which can reduce the speed and severity of the disease; secondly, increased awareness of personal hygiene, effective dissemination of public health information, and better preventive measures during the influenza season can help reduce infection rates; thirdly, the availability of more rapid influenza tests, more effective antiviral therapeutic drugs on the market, and improvements in care and treatment regimens; fourth, improvements in disease surveillance systems globally have facilitated earlier identification and respond to influenza epidemics; and fifth, increased international collaboration in influenza virus surveillance and vaccine production has helped predict changes in influenza virus strains in advance, leading to the manufacture of more effective seasonal influenza vaccines. Our findings can raise awareness among doctors and patients, improve the health care system, and provide a basis for effective measures to prevent and control influenza.
Next, based on a large amount of publicly available genetic data, we explored causal relationships between 731 immune cells and influenza. To the best of our knowledge, this is the first MR analysis to explore causal relationships between multiple immune phenotypes and influenza. In this study, we found that HLA-DR on the monocyte cell group CD14 + CD16- has been shown to be associated with an increased risk of influenza. HLA-DR is an important Major Histocompatibility Complex (MHC) class II molecule commonly known as Human Leukocyte Antigen in humans[32, 55]. HLA-DR surface molecules play an important role in the immune response, particularly in assisting in the activation of specific immune responses[56]. CD14 + CD16- monocytes are a typical class of inflammatory response monocytes that mainly phagocytose pathogens and present antigens in the immune response[57]. CD14 + CD16- monocytes with high HLA-DR expression play a crucial role in the development of influenza. In influenza infection, these monocytes may be involved in the role of antigen presentation. When influenza virus infects the organism, CD14 + CD16- monocytes phagocytose viral particles, process and present peptides of the virus on their HLA-DR molecules[58, 59]. These peptides are presented to CD4 + T cells, inducing activation and differentiation of the latter. These cells also produce and release a variety of pro-inflammatory factors, such as TNF-α and IL-1, which contribute to the immune system response and cause local and systemic inflammatory responses[60].
ScRNA-seq, a new technique for transcriptome sequencing of isolated single cells, is now being used to analyse virus-host interactions at the single-cell level. We verified at the single-cell level that the immune cells identified by UVMR HLA-DR on CD14 + CD16- monocyte belong to the CMs subpopulation. And the expression level of CMs was higher in monocyte subpopulations from influenza patients compared to healthy controls. MR analysis of key genes in the cellular subpopulation identified 7 genes as causally associated with influenza, and 3 genes were identified as druggable genes by drug prediction, namely VIM, CTSA and CSF3R. Among them, the high binding activity of CSF3R and RUXOLITINIB by molecular docking demonstrated the strong potential of the CSF3R gene as a drug target.
The protein encoded by the VIM gene is vimentin, a structural protein belonging to the intermediate fibronectin family. This protein supports the structural integrity of cells and is involved in a variety of cellular functions such as cell attachment, cell migration and signaling[61]. Although the VIM gene is not an immune gene that directly interacts with influenza viruses, some studies suggest that intermediate fibre proteins may play a role during viral infection, especially during the invasion and replication phases of the virus. For example, VIM may act as a non-specific viral attachment site during viral entry into cells, or during viral replication and assembly[62]. Some viruses are able to modulate host cell backbone proteins to aid in their replication and propagation[63]. The CTSA gene encodes galactose neuraminidase 1 (Cathepsin A), a protective protein of the lysosomal enzyme class that is implicated in a variety of biochemical processes[64]. It has been suggested that lysosomes may play a role in the life cycle of viral infections, where influenza virus replication may require lysosomal function. After the influenza virus invades a cell, its RNA must be released from the inner vesicles that encapsulate them in order to begin the process of viral replication, and the acidification process of this inner vesicle is associated with lysosomal function[65]. On the other hand, lysosomal proteins, such as galactose neuraminidase 1, may also be involved in regulating the immune response of host cells to viral invasion. For example, they may influence the production of inflammatory mediators or activate other cellular pathways involved in viral defense[66].
CSF3R encodes a receptor whose primary function is to regulate the production of white blood cells (granulocytes)[67]. This receptor is essential for the growth and differentiation of granulocytes (mainly neutrophils), which are one of the body's main white blood cells and play an important role in warding off infections[68]. During infection, neutrophils act as a front-line defense for the body's immune system, and they respond rapidly to help fight the invasion of the influenza virus through the regulation of the CSF3R receptor. Several studies have suggested that differences in the immune system response to the virus in different individuals may be associated with variations in specific genes, which may include the CSF3R gene[69, 70]. CSF3R is associated with may indirectly affect the course, severity, and duration of influenza infection[71, 72]. Gene variants may affect an individual's susceptibility to viral infections as well as the ability to recover. Thus, individual differences in the CSF3R gene may affect their response to influenza[73, 74]. However, the onset and progression of influenza is multifactorial, involving host genetic background, viral strain specificity, and other genetic and environmental factors. Genetic variation is only one part of the equation, and these interactions and effects are often very complex. In addition, influenza virus-host cell interactions are complex and often involve the synergistic action of multiple genes and metabolic pathways. Therefore, identifying the roles of the aforementioned genes in influenza infection will require studies that include virology, molecular biology and host genetics to more fully understand the potential functions of these genes in the influenza virus life cycle.
In summary, this is the first time that descriptive epidemiology has been used to analyze the epidemiological characteristics and trends of influenza based on the GBD database. This study is also the first to identify potential causal associations between HLA-DR on CD14 + CD16- monocyte and influenza by UVMR. Gene expression of different cell types in blood samples from influenza patients was also analyzed and annotated using scRNA-seq, and potential drug targets for influenza were initially identified from genetic insights. However, there are some limitations to this study. First, only global influenza DALYs rates and mortality rates were analyzed, and subgroup analyses of trends in influenza DALYs rates and mortality rates for different age groups were not conducted. In addition, the influenza data information for this study was derived from sources only up to 2019, and updated stream GBD information is not yet available. Additionally, the applicability of the study's findings is limited due to its focus on predominantly European-descended individuals. To extend these results to people of different ethnic backgrounds, further research and thorough validation are needed to confirm their universal relevance. Despite meticulous attempts to eradicate bias, MR analysis is still susceptible to confounding by unobserved variables or pleiotropy, which may distort the outcomes. Also, while enrichment analysis provides meaningful insights, it is not without its confines, as it depends on predetermined groups of genes or biological pathways that may not fully capture the entire spectrum of potential biological processes or their interplay. Precision in molecular docking analysis is contingent on the high quality of the protein structures and ligands involved. Although this method is instrumental in pinpointing likely drug candidates, it cannot ensure their success in a clinical environment. It is therefore imperative that further laboratory research and clinical trials be conducted to substantiate the medical viability of these proposed targets. Acknowledging and addressing these limitations will pave the way for future research to improve the understanding of influenza and its potential treatments, and will help to create a more holistic perspective and advance relevant research in the field in a meaningful way.