The COVID-19 outbreak, which began in late 2019 and is still spreading globally, is highly infectious and pathogenic. With the continuous spread of the epidemic, China initiated a level I response and the World Health Organization listed COVID-19 as a public health emergency of international concern[13]. Among COVID-19 patients, the proportion with severe and critical illnesses is as high as 5%-10% due to the rapid progression of the disease[14]. In addition, according to a retrospective study, the 28-d fatality rate of critically ill patients hospitalized in intensive care units is as high as 61.5%[15]. Therefore, early and effective identification and management of severe/critically ill COVID-19 patients is essential to reducing the mortality rate among infected people. The early clinical symptoms of critically ill COVID-19 patients are often atypical and easily overlooked. In addition, early critically ill COVID-19 patients are difficult to diagnose due to lack of effective biomarker. Finding new and effective biomarker is critical for early diagnosis and treatment of critically ill patients with COVID-19.
We performed an in-depth analysis of the RNA-SEQ dataset, GSE172114, using R software and bioinformatics to compare gene expression in critically ill and non-critically ill patient samples. Two hundred ten DEGs were identified, of which 186 were upregulated and 24 were downregulated.
We conducted enrichment analysis of GO and KEGG to explore the interaction between DEGs. Upregulated DEGs were mainly enriched in immunoglobulin activity regulation, ligand binding, nod-like receptor (NLR) activation, neutrophil activation, and complement activation, while downregulated DEGs were mainly enriched in cellular immunity, humoral immunity, and cell adhesion molecule signal transduction in immune responses.
Research involving COVID-19 has shown that the immune system plays a key role in determining the severity of the disease. SARS-COV-2 disrupts the normal immune response, leading to compromised immune systems and runaway inflammatory responses in severe and critically ill COVID-19 patients. These patients present with lymphocytopenia, lymphocyte activation and dysfunction, abnormal granulocytes and monocytes, high cytokine levels, and increased immunoglobulin G (IgG) and total antibodies[16].
Previous studies have shown that SARS-COV-2 infection activates multiple immune responses, which is consistent with the GO and KEGG enrichment analysis in this study. We speculate that neutrophil upregulation in COVID-19 patients is closely related to lymphocytopenia. Lymphocyte damage in COVID-19 patients easily leads to microbial infections, in which pathogenic microbial infections can directly induce neutrophils to accumulate in tissue sites. The activation and recruitment of neutrophils is promoted in the blood of patients, thus magnifying the inflammatory response[17]. Studies have shown that neutrophils and neutrophil-to-lymphocyte ratios are usually important indicators of severe and adverse clinical outcomes, and the neutrophil-to-lymphocyte ratios in severe patients are significantly higher than non-severe patients[18]. In contrast, it has also been reported that SARS-COV-2 directly activates NLRP3 inflammasomes, resulting in endogenous adjuvant activity that produces an appropriate adaptive immune response to the virus[19]. Damage-associated molecular patterns (DAMPs) released after activation of NLRP3 inflammasomes have a dual function: induce the necessary co-stimulatory activation of antigen-presenting cells (APCs) in a normal immune response; and decomposition and tissue regeneration. Excessive activation of NLRP3 inflammatory bodies promotes high concentration release of DAMPs and apoptosis, release of high mobility group 1 (HMGB1), activation of macrophages, neutrophil infiltration, cytokine overproduction (e.g., IL-1 β, IL-2, IL-6, IL-17, TNF -α, G-CSF, GM-CSF, IFN-γ, CXCL10, CCL2, and CCL3), and causes inflammation through various mechanisms[20, 21]. Activation of HMGB1 by IL-1β released by inflammasome signaling is a key late marker of sepsis and infection. HMGB1 leads to epithelial barrier failure, organ dysfunction, vascular leakage, and even death[22, 23]. Elevated HMGB1 is the central agent of excessive inflammatory responses and pathologic severity during viral infection[24, 25]. The level of NLRP3 activation is correlated with the severity of COVID-19 disease[26].
Additional studies have shown that COVID-19 is closely related to cellular and humoral responses[27]. It has been reported that during COVID-19 infection, particularly in severe cases, the total number of T cells, and CD4 + and CD8 + T cells declines significantly, and the surviving T cells fail. Total T cell number, CD8 + T cell number, and CD4 + T cell number < 800, 300, and 400/ L, respectively, are negatively correlated with survival rate[28, 29]. In addition, SARS-COV-2 inhibits T-cell-mediated immune response by downregulating MHC class I and II molecules by inhibiting antigen presentation, similar to other coronaviruses[30]. In contrast, the humoral immune response also plays an important role in COVID-19 infections, although antibodies may not be enough to neutralize the virus because SARS-COV-2 inhibits antigen presentation by downregulating MHC class I and II molecules, thus inhibiting T-cell-mediated immune responses[31]. These results suggest that SARS-COV-2 has strong adaptive immunosuppressive function.
To reduce bias, we first performed GSEA analysis on all detected genes. Gene enrichment analysis showed that SARS-COV-2 infection is still closely related to the activation of inflammatory responses. An effective immune response is essential to fight off viral infections; however, overactivation of the inflammatory immune response can lead to cytokine storm and subsequent immune failure. Further clinical evidence has shown that proinflammatory cytokines are elevated in severe COVID-19 patients, and the surge of these inflammatory molecules is also known as the cytokine storm in COVID-19[32,33].
The development of cytokine storm is a potentially fatal immune state characterized by rapid proliferation and overactivation of T cells, macrophages, and natural killer cells, as well as excessive production of over 150 inflammatory cytokines and chemical mediators released by immune or non-immune cells[34]. Abnormal release of pro-inflammatory factors leads to apoptosis of pulmonary epithelial and endothelial cells, damage to pulmonary microvascular and alveolar epithelial cell barriers, and leads to vascular leakage, alveolar edema, and hypoxia in viral infections[35]. Clinical studies have shown that the severity of COVID-19 is positively correlated with the level of inflammatory cytokines, including IL-1β, IL-2, IL-6, IL-7, IL-10,IL-17, IL-18, and other cytokines in plasma/serum[36]. Among patients infected with SARS coronavirus or Middle East respiratory syndrome (MERS), serum levels of cytokines, such as IL-6, IFN-α/β and CXCL10, in patients with severe infections were also significantly higher than those in patients with mild infections[37]. An inflammatory cytokine storm is an important factor leading to severe infection or death in H7N9, SARS-COV-2, MERS-CoV, and other viral infections, and the mechanism may be that the overexpression of inflammatory cytokines and chemokines leads to acute lung injury and ARDS[37–39].
In conclusion, cytokine storm in the pathogenesis of severe COVID-19 pneumonia deserves attention, and excessive activation of inflammatory immune response can lead to the persistence of cytokine storm. Although current therapies are based on inhibiting the onset and development of cytokine storms, the efficacy of current therapies is not ideal, and more research is needed to explore the key inflammatory pathways triggered by SARS-COV-2.
In addition, 10 candidate genes (IL10, MMP9, RETN, RSAD2, IFI44, IFI44L, IFI6, IFIT3, IFIT1, and PLSCR1) were screened through the PPI network. Further, a MCODE plug-in was used to identify the gene cluster module method and clusters to four modules. The highest score was obtained by intersection of the Venn diagram and 10 candidate genes identified by PPI to obtain 7 core genes (RSAD2, IFI44, IFI44L, IFI6, IFIT3, IFIT1, and PLSCR1). By analyzing the area under the ROC curve of the 7 core genotypes, it was found that the AUC area of PLSCR1 was the largest, representing the best sensitivity and specificity. Further analysis of the PLSCR1 gene revealed that human phospholipid scramblase 1 (PLSCR1) is an upregulated gene in patients with severe coronavirus infection, and further revealed that PLCSR1 is a calcium-dependent, non-specific enzyme involved in rapid phospholipid reallocation[40,41]. Studies have shown that PLCSR1 is abnormally expressed during interferon (IFN) therapy and viral infections. Negative regulation of human cytomegalovirus (HCMV) replication by inhibiting transcription of major immediate early proteins (MIEPs) and early promoters of the virus plays an important role in the regulation of inflammatory immune responses and IFN-dependent antiviral responses[42]. PLSCR1 is transcriptionally-regulated by IFN through pathways involved in the activation of PKC-δ, JNK, and STAT1[43]. PLSCR1 also regulates TLR9 signaling and subsequent production of IFN in plasmacytoid dendritic cells[44]. The analysis showed that PLSCR1 is expressed in both neutrophils and monocytes, which induced neutrophil chemotaxis and adhesion, which was consistent with the inflammatory response of COVID-19 patients in our previous GO, KEGG, and GSEA analyses. A detailed analysis of the effects of viral proteins on calcium signaling showed that elevated levels of virus-associated cytoplasmic calcium lead to viral protein expression, and calcium (Ca2+) acts as a cellular second messenger and regulates a variety of cellular functions[45]. A variety of viruses control the host cell Ca2+ signal cascade by controlling the host cell Ca2+ processing mechanism. Viruses can also induce changes in the cytoplasmic Ca2+ concentration and activate Ca2+-dependent enzymes, resulting in maladjustment of the host cell signal cascade and highly affected cellular calcium dynamics, thus driving virus infection and pathogenesis[46]. It has been reported that Ca2+ is required for fusion of MERS-COV, SARS-COV, and rubella virus with host cells, suggesting that Ca2+ plays a key role in infection of these viruses[47–49]. Interestingly, based on structural homology, bioinformatics, and meta-analysis, Ca2+ may be an important regulator of SARS-CoV-2 entry into host cells[46]. In addition, recent studies have shown that PLSCR1 negatively regulates viral replication by inhibiting multiple viral transcriptional pathways, such as PLSCR1, can form a trimer complex with NP and the imported protein α family members, which inhibits β, a key mediator of the classical nuclear import pathway, from entering the complex, thus destroying the nuclear import of NP, inhibiting viral replication, and negatively regulating viral replication[50]. Furthermore, PLSCR1 can interact with HCV envelope proteins (E1 and E2) to promote HCV virus replication[51].
In summary, PLSCR1 encodes calcium-binding proteins that regulate cellular Ca2+ levels, thereby affecting a variety of cellular biological signal transductions, and also regulates viral replication through other pathways. Therefore, early recognition may be important for the early diagnosis and prognosis of critically ill patients with COVID-19. Thus, molecular typing of critically ill patients with COVID-19 can reveal diagnostic, prognostic, and therapeutic biomarker for critically ill patients with COVID-19, leading to early diagnosis and treatment. Nevertheless, biomarker need to be improved and prognostic models developed.