Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of coronavirus disease 2019 (COVID-19), has caused more than 623 million infections along with more than 6.56 million deaths, as reported by the World Health Organization. as of December 1, 2021. It is estimated that approximately 80% of patients with COVID-19 had asymptomatic, moderate, or mild symptoms, while 20% experienced severe illness and death. However, the mechanisms and molecular events of how SARS-CoV-2 infection can lead to severe pneumonia are still unknown.
Study: Metabolic modeling of single bronchoalveolar macrophages reveals regulators of hyperinflammation in COVID-19. Image credit: sathon keeratikunchorn/Shutterstock
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Several studies have highlighted that the dysfunctional immune response plays an important role in the severe symptoms of COVID-19. Patients with severe COVID-19 were observed to display abnormal peripheral immune activities along with a cytokine storm resulting from the upregulation of chemokines and proinflammatory cytokines as well as calprotectin. Additionally, elevated levels of monocyte-derived hyperinflammatory macrophages have been discovered in the bronchoalveolar lavage fluid (BALF) of severe COVID-19 patients. In addition, highly expanded clonal CD8+ T cells were observed in moderate patients.
Furthermore, cellular metabolism is known to be important for the proper functioning of immune cells in both healthy and diseased conditions. Many recent clinical studies have indicated that COVID-19 causes altered plasma metabolites. A drop in nutrients in the blood and a dysregulation of metabolic components have been observed in patients with severe COVID-19.
In addition, higher levels of T cells expressing voltage-dependent anion channel 1 were observed in the peripheral blood of patients with acute COVID-19. However, the immunometabolic regulation of leukocytes in the lungs, which is the most affected organ, and its association with disease severity and immune function remain unclear.
A new study to be published in iScience aimed to analyze immunometabolism BALF cells from patients with mild to severe COVID-19 using computational algorithms that quantified metabolic fluxes and metabolic pathways.
About the study
The study involved single-cell RNA sequencing (scRNA-seq) collection of BALF from patients with severe and mild COVID-19 as well as healthy controls, followed by cell clustering and annotation. Comparison of metabolic pathway activities between various subsets of cells was carried out by data imputation and normalization along with calculation of metabolic pathway activity. After that, differential gene expression analysis, correlation analysis, trajectory analysis, single-cell flux balance analysis, visualization of gene expressions and the calculation of the score of certain metabolic pathways.
Plasma metabolome data were collected from patients with COVID-19 and healthy controls, following stimulation of primary macrophages by SARS-CoV-2 or the TLR7/8 agonist resiquimod (R848). Phagocytosis and fatty acid uptake assay measured macrophages and fatty acids. Preparation of SARS-CoV-2 viral stocks was carried out in Vero-E6 cells. Human monocyte-derived macrophages were obtained from human peripheral blood mononuclear cells (PBMC) and stimulated by SARS-CoV-2. Finally, RNA isolation, real-time PCR, and enzyme-linked immunosorbent assay (ELISA) were performed.
Results of the study
The results reported 9 major cell populations including neutrophils, macrophages, plasmacytoid dendritic cells (pDC), myeloid dendritic cells (mDC), natural killer (NK) cells, plasma cells, B and T lymphocytes, as well as epithelial cells of the immune cells. of BALF from patients with COVID-19. BALF cells were found to be segregated into mild, healthy, and severe based on 1526 metabolic gene expression levels. In addition, most metabolic pathways such as oxidative phosphorylation, citrate cycle (TCA cycle) and glycolysis were downregulated in BALF mDCs, B cells, pDCs, NK cells, macrophages and T cells. of mild patients with COVID-19. However, most of these pathways were downregulated for severe patients compared to mild patients.
71 of 85 metabolic pathways were upregulated in BALF macrophages from mild COVID-19 patients. However, only 8 pathways were reported to be up-regulated in BALF macrophages from severe COVID-19 patients, while 65 were down-regulated. Only fatty acid biosynthesis pathways and glycolysis pathways and their genes were observed to be upregulated in severe COVID-19 patients, while most of the metabolic pathways and their genes were downregulated. observed that they were upregulated in mild patients with COVID-19.
Of the 294 detectable metabolic fluxes, 223 were reduced in BALF macrophages from severe COVID-19 patients compared to mild patients. Furthermore, of the 18 macrophage clusters discovered, 8 comprised macrophages from mild patients with COVID-19 and 9 macrophages from severe patients. CD14+ lung-derived macrophage (BALF-MCs) from mild patients were observed to show elevated levels of metabolism-related gene expression and metabolic pathway activity compared with those from severe patients. Furthermore, metabolic activity increased along the CD14 + monocytes from PBMC (PBMC-MCs) to BALF-MCs pathway for mild patients.
In addition, genes encoding chemokines and proinflammatory cytokines were higher in severe patients, while genes for endocytosis and antigen presentation were downregulated. The expression of enzymes involved in pyruvate and glutamate metabolism was lower in severe COVID-19 patients, resulting in elevated chemokines and proinflammatory cytokines. Macrophages with inhibited glycolysis showed reduced levels of these chemokines and cytokines. In addition, lipid metabolism was also lower for severe COVID-19 patients.
The nuclear receptor peroxisome proliferator-activated receptor gamma (PPARγ) signaling pathway that plays an important role in lipid metabolism and maintenance of metabolic homeostasis was also inhibited in macrophages derived from patients with COVID-19 serious Inhibition of the PPARγ signaling pathway was also associated with increased production of inflammatory cytokines and chemokines. Finally, treatment with rosiglitazone, an FDA-approved antidiabetic drug, was reported to activate the PPARγ pathway in macrophages. This led to a reduction in the production of pro-inflammatory cytokines and an increase in metabolic gene expression.
Therefore, the current study demonstrated that the metabolic imbalance of bronchoalveolar macrophages could lead to hyperinflammation in severe patients with COVID-19. Regulation of metabolism using drugs such as rosiglitazone may help prevent cytokine storms in these patients. In addition, the study also identified the dysregulation of immunometabolism as an important part of the pathogenesis of COVID-19, which could be used to develop new COVID-19 therapeutics.
limitations
The current study has certain limitations. First, the computational algorithms used in the study did not take post-translational modulation and post-transcriptional regulation into account. Second, all of the study’s main findings were conducted in vitro. Third, the impact of rosiglitazone on PPARγ needs to be confirmed using PPARγ genetically deleted cells. Fourth, the viral biomass objective function (VBOF) was not added to the study to determine the metabolic requirement of the host virus. Finally, only human macrophages from healthy donors were used to test the effect of rosiglitazone, while its effect has yet to be tested in cases of severe COVID-19 patients with type 2 diabetes.