Researchers have identified how SARS-CoV-2 hijacks lung cells to drive COVID-19 severity. They have identified the cellular tropism and transcriptome consequences of SARS-CoV-2 by infecting human lung tissue and using single-cell ribonucleic acid sequencing (scRNA-seq) to rebuild the transcriptional program in "infection pseudotime" for distinct lung cell types.
(Note: Tropism refers to the way in which different viruses/pathogens have evolved to preferentially target specific host species, specific tissue, or specific cell types)
They used sliced lung tissue obtained from surgical specimens or organ donor individuals into thick sections and used them for tissue culture analysis. They exposed the tissues to the SARS-CoV-2 USA-WA1 2020 strain at 1.0 multiplicity of infection (MOI) for two hours before allowing the SARS-CoV-2 infection to continue for two to three days.
(Note: The MOI represents the ratio of the numbers of virus particles to the numbers of the host cells in a given infection medium)
What they found -
The analysis showed that SARS-CoV-2 preferentially infects active interstitial macrophages (IMs), which can amass hundreds of SARS-CoV-2 RNA molecules, comprising >60% of the cell transcriptome and producing dense viral RNA bodies. Infected alveolar macrophages (AMs) exhibit no severe reactions, with spike (S) protein-dependent viral entrance into AMs utilizing angiotensin-converting enzyme 2 (ACE2) and the cluster of differentiation CD169 and IM entry via CD209.
The study found that COVID-19 pneumonia infection and takeover cause an early antiviral cell response specific to activated interstitial macrophages, resulting in a powerful immunological and fibrotic signaling center. Inflammasome activation is uncommon and only detectable late in a-IM infection. Blocking antibodies against CD169 and CD209 prevented entrance into IMs and AMs. The study also highlighted IMs as the most vulnerable lung target, with initial emphasis on inflammation and fibrosis. Two unique molecular lineages of macrophage targets react differently to SARS-CoV-2, influencing etiology and treatments.
Model of initiation, transition, and pathogenesis of COVID-19 and the viral lifecycle in AMs and IMs. (a–d) Model of COVID-19 initiation in the human lung and transition from viral pneumonia to lethal COVID-19 ARDS.
(a) SARS-CoV-2 virion dissemination and arrival in the alveoli. Luminal AM encounter virions shed from the upper respiratory tract that enter the lung. AMs can express low to moderate numbers of viral RNA molecules and can propagate the infection but “contain” the viral RNA from taking over the total transcriptome and show only a very limited host cell inflammatory response to viral infection.
(b) Replication and epithelial injury. SARS-CoV-2 virions enter AT2 cells through ACE2, its canonical receptor, and “replicate” to high viral RNA levels, producing infectious virions and initiating viral pneumonia.
(c) a-IM takeover and inflammation signaling. SARS-CoV-2 virions spread to the interstitial space through either transepithelial release of virions by AT2 cells or injury of the epithelial barrier, and enter a-IMs. Infected a-IMs can express very high levels of viral RNA that dominate (“take over”) the host transcriptome and can propagate the infection. Viral takeover triggers induction of the chemokines and cytokines shown, forming a focus of inflammatory and fibrotic signaling.
(d) Endothelial breach and immune infiltration. The a-IM inflammatory cytokine IL6 targets structural cells of the alveolus causing epithelial and endothelial breakdown, and the inflammatory cytokines recruit the indicated immune cells from the interstitium or bloodstream, which flood and infiltrate the alveolus causing COVID-19 ARDS. Local inflammatory molecules are amplified by circulating immune cells, and reciprocally can spread through the bloodstream to cause systemic symptoms of cytokine storm.
(e) Comparison of the SARS-CoV-2 viral lifecycle in AMs and IMs. Although both can produce infectious virions, note differences in viral entry receptors (AMs can use ACE2 and CD169/SIGLEC1, whereas IMs use CD209); viral RNA transcription of dsRNA intermediates (greater in AMs); replication of full-length genomic RNA (greater in IMs); viral takeover, formation of RNA bodies, and induction of a robust host cell inflammatory response (only in IMs), and cell destruction/death (only in IMs).
(Note: Tropism refers to the way in which different viruses/pathogens have evolved to preferentially target specific host species, specific tissue, or specific cell types)
They used sliced lung tissue obtained from surgical specimens or organ donor individuals into thick sections and used them for tissue culture analysis. They exposed the tissues to the SARS-CoV-2 USA-WA1 2020 strain at 1.0 multiplicity of infection (MOI) for two hours before allowing the SARS-CoV-2 infection to continue for two to three days.
(Note: The MOI represents the ratio of the numbers of virus particles to the numbers of the host cells in a given infection medium)
What they found -
The analysis showed that SARS-CoV-2 preferentially infects active interstitial macrophages (IMs), which can amass hundreds of SARS-CoV-2 RNA molecules, comprising >60% of the cell transcriptome and producing dense viral RNA bodies. Infected alveolar macrophages (AMs) exhibit no severe reactions, with spike (S) protein-dependent viral entrance into AMs utilizing angiotensin-converting enzyme 2 (ACE2) and the cluster of differentiation CD169 and IM entry via CD209.
The study found that COVID-19 pneumonia infection and takeover cause an early antiviral cell response specific to activated interstitial macrophages, resulting in a powerful immunological and fibrotic signaling center. Inflammasome activation is uncommon and only detectable late in a-IM infection. Blocking antibodies against CD169 and CD209 prevented entrance into IMs and AMs. The study also highlighted IMs as the most vulnerable lung target, with initial emphasis on inflammation and fibrosis. Two unique molecular lineages of macrophage targets react differently to SARS-CoV-2, influencing etiology and treatments.
Model of initiation, transition, and pathogenesis of COVID-19 and the viral lifecycle in AMs and IMs. (a–d) Model of COVID-19 initiation in the human lung and transition from viral pneumonia to lethal COVID-19 ARDS.
(a) SARS-CoV-2 virion dissemination and arrival in the alveoli. Luminal AM encounter virions shed from the upper respiratory tract that enter the lung. AMs can express low to moderate numbers of viral RNA molecules and can propagate the infection but “contain” the viral RNA from taking over the total transcriptome and show only a very limited host cell inflammatory response to viral infection.
(b) Replication and epithelial injury. SARS-CoV-2 virions enter AT2 cells through ACE2, its canonical receptor, and “replicate” to high viral RNA levels, producing infectious virions and initiating viral pneumonia.
(c) a-IM takeover and inflammation signaling. SARS-CoV-2 virions spread to the interstitial space through either transepithelial release of virions by AT2 cells or injury of the epithelial barrier, and enter a-IMs. Infected a-IMs can express very high levels of viral RNA that dominate (“take over”) the host transcriptome and can propagate the infection. Viral takeover triggers induction of the chemokines and cytokines shown, forming a focus of inflammatory and fibrotic signaling.
(d) Endothelial breach and immune infiltration. The a-IM inflammatory cytokine IL6 targets structural cells of the alveolus causing epithelial and endothelial breakdown, and the inflammatory cytokines recruit the indicated immune cells from the interstitium or bloodstream, which flood and infiltrate the alveolus causing COVID-19 ARDS. Local inflammatory molecules are amplified by circulating immune cells, and reciprocally can spread through the bloodstream to cause systemic symptoms of cytokine storm.
(e) Comparison of the SARS-CoV-2 viral lifecycle in AMs and IMs. Although both can produce infectious virions, note differences in viral entry receptors (AMs can use ACE2 and CD169/SIGLEC1, whereas IMs use CD209); viral RNA transcription of dsRNA intermediates (greater in AMs); replication of full-length genomic RNA (greater in IMs); viral takeover, formation of RNA bodies, and induction of a robust host cell inflammatory response (only in IMs), and cell destruction/death (only in IMs).
