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Review
. 2021 May 7:8:644678.
doi: 10.3389/fmed.2021.644678. eCollection 2021.

Human-Based Advanced in vitro Approaches to Investigate Lung Fibrosis and Pulmonary Effects of COVID-19

Affiliations
Review

Human-Based Advanced in vitro Approaches to Investigate Lung Fibrosis and Pulmonary Effects of COVID-19

Mirjam Kiener et al. Front Med (Lausanne). .

Abstract

The coronavirus disease 2019 (COVID-19) pandemic has caused considerable socio-economic burden, which fueled the development of treatment strategies and vaccines at an unprecedented speed. However, our knowledge on disease recovery is sparse and concerns about long-term pulmonary impairments are increasing. Causing a broad spectrum of symptoms, COVID-19 can manifest as acute respiratory distress syndrome (ARDS) in the most severely affected patients. Notably, pulmonary infection with Severe Acute Respiratory Syndrome coronavirus 2 (SARS-CoV-2), the causing agent of COVID-19, induces diffuse alveolar damage (DAD) followed by fibrotic remodeling and persistent reduced oxygenation in some patients. It is currently not known whether tissue scaring fully resolves or progresses to interstitial pulmonary fibrosis. The most aggressive form of pulmonary fibrosis is idiopathic pulmonary fibrosis (IPF). IPF is a fatal disease that progressively destroys alveolar architecture by uncontrolled fibroblast proliferation and the deposition of collagen and extracellular matrix (ECM) proteins. It is assumed that micro-injuries to the alveolar epithelium may be induced by inhalation of micro-particles, pathophysiological mechanical stress or viral infections, which can result in abnormal wound healing response. However, the exact underlying causes and molecular mechanisms of lung fibrosis are poorly understood due to the limited availability of clinically relevant models. Recently, the emergence of SARS-CoV-2 with the urgent need to investigate its pathogenesis and address drug options, has led to the broad application of in vivo and in vitro models to study lung diseases. In particular, advanced in vitro models including precision-cut lung slices (PCLS), lung organoids, 3D in vitro tissues and lung-on-chip (LOC) models have been successfully employed for drug screens. In order to gain a deeper understanding of SARS-CoV-2 infection and ultimately alveolar tissue regeneration, it will be crucial to optimize the available models for SARS-CoV-2 infection in multicellular systems that recapitulate tissue regeneration and fibrotic remodeling. Current evidence for SARS-CoV-2 mediated pulmonary fibrosis and a selection of classical and novel lung models will be discussed in this review.

Keywords: COVID-19; SARS-CoV-2; alveolar regeneration; in vitro lung models; interstitial pulmonary fibrosis; lung-on-chip; organoids; precision-cut lung slices.

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Conflict of interest statement

NR and NH are employed by AlveoliX AG. OG and TG are shareholder and in the scientific board of AlveoliX AG. MK and MK-dJ are collaborators of AlveoliX AG. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Figures

Figure 1
Figure 1
SARS-CoV-2 infection in the respiratory tract. (A) In the pseudostratified epithelium of the airways, secretory goblet, and club cells produce mucus, which is transported by ciliated cells to clear trapped particles and protect the lung from micro-injuries and infection. Basal cells reside at the lamina propria and comprise progenitor cells. The composition and frequency of the individual cell types is variable among the distinct anatomical sites in the nose, trachea, bronchi, and bronchioles. (B) The alveolar epithelium is specialized for gas exchange with flattened ATI cells forming an ultra-thin (~2 m) epithelial-endothelial barrier allowing oxygen and CO2 diffusion. Cuboidal ATII cells are considered as progenitor cells of ATI cells and fulfill vital functions by the production of pulmonary surfactant (PS), which lowers surface tension and prevents alveolar collapse. Lung fibroblasts are essential to maintain the ATII stem cell niche. Resident alveolar macrophages (AM) and immune cells defend the epithelium from infection. (C) SARS-CoV-2 initially infects the airway epithelium. The virus can efficiently replicate in ciliated and secretory cells resulting in the shedding of high viral titers and mild to moderate COVID-19 symptoms. (D) The respiratory epithelium exhibits differential susceptibility to SARS-CoV-2 infection. In correlation with ACE2 expression, SARS-CoV-2 infection is most efficient in the upper airways, particularly in the nasal epithelium. Infectivity gradually decreases toward the alveoli. However, when SARS-CoV-2 reaches the alveoli it can result in severe manifestation of COVID-19. (E) Upon reaching the alveoli, SARS-CoV-2 infects alveolar epithelial cells and endothelial cells and causes viral pneumonia. Cytopathic effects of SARS-CoV-2 are evident as syncytial and apoptotic alveolar epithelial cells resulting in the breakdown of pulmonary surfactant and barrier integrity. In some patients, alveolar damage culminates in life-threatening microvascular activation and an imbalanced immune response. Tissue regeneration takes place already during acute COVID-19 as indicated by fibrin deposition, ATII cell hyperplasia and alveolar wall thickening. Moreover, severely ill COVID-19 patients exhibit radiological signs of fibrosis even months after recovery indicative for the induction of COVID-19-associated fibrosis. ACE2, angiotensin-converting enzyme 2; ATI cell, type I alveolar epithelial cell; ATII cell, type II alveolar epithelial cell; COVID-19, coronavirus disease 2019; End. cell, endothelial cell; AM, alveolar macrophage; PS, pulmonary surfactant; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.
Figure 2
Figure 2
Comparison of in vivo and 3D in vitro lung models for COVID-19 and fibrosis research. General aspects of experimental animal models and advanced in vitro models including PCLS, iPSC-derived organoids, mature organoids, 3D in vitro tissues, and LOC are rated based on similarities to human physiology (physiological biomechanics, cell heterogeneity, cell differentiation, long-term model, and 3D microenvironment), genetic manipulation (modifications), the possibility for mechanistic investigations (pathways), and throughput capabilities (throughput). Their applicability to model the diseased state of the lung has been evaluated separately for COVID-19 and IPF. COVID-19, coronavirus disease 2019; IPF, idiopathic pulmonary fibrosis; iPSC, induced pluripotent stem cells; LOC, lung-on-chip; environment, microenvironment; PCLS, precision-cut lung slices; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.

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