Research to Viral Respiratory Infections Needs a Realistic Model

Model refinement drives a transition from 2D to 3D and air-liquid interface culture

Submerged monolayer cultures derived from human respiratory tissues, such as normal human bronchial epithelial cells have been used widely for respiratory research and toxicology studies. The mammalian respiratory system consists of numerous specialized cell types, however, and such cultures cannot reproduce the differentiated phenotype of in vivo airway epithelial tissues. This has prompted the development of culture methods that generate well-differentiated 3D human airway organoids from, for example, human cell induced pluripotent stem cells. But while these organoids can be maintained under submerged conditions in long-term culture their spherical form and lack of an air interface limits their value when it comes to studying respiratory infections.

An alternative is to culture primary airway cells on microporous membrane scaffolds at the air-liquid interface (ALI) ^{(1, 3–7)}. These differentiated, organotypic airway models based on air-liquid interface culture have several advantages compared to simpler models such as submerged or organoid cultures, including:

• They mimic in vivo structures, such as polarized epithelial cells, and metabolic functions more closely.
• They can be dosed in a more relevant manner. 
• Primary cells that undergo cellular differentiation can reproduce an in vivo–like transcriptional profile similar to that of the human airway epithelium. This is important since infection by human respiratory pathogens, such as Bordetella pertussis, influenzae, and coronavirus, is highly dependent on the differentiation state of airway epithelium. 
  
  
  

One important factor in the development of methods to prevent and treat respiratory infectious diseases is the availability of reliable and predictive model systems.

Organotypic ALI airway culture models for a range of applications

Cell models including ALI have been prepared from a wide range of primary cells and cell lines from nasal, bronchial, and alveolar epithelium^{(5, 6)}. There has been a significant increase in the use of polarized epithelial cells grown on permeable membranes⁽⁵⁾ and the physiological relevance of the in vitro models can also be increased by co-culturing⁽⁷⁾. Modelling can achieve a high level of mimicry. For example, human respiratory cells can be grown in air-liquid interface as organotypic airway cultures with polarized epithelial cells to resemble the human airway both morphologically (basal, ciliated, and secretory cells) and functionally (beating cilia and mucus secretion)⁽⁸⁾.

Applications for cell models with ALI range from the study of respiratory pathogens to diseases such as asthma, cystic fibrosis, lung cancer, and chronic obstructive pulmonary disease (COPD)⁽⁷⁾.

ALI models can also be used to screen for therapeutically relevant agents and their effectiveness on virus inactivation⁽⁷⁾. For example, SARS-CoV-2 could be isolated from bronchial lavage fluid of patients suffering from COVID-19 disease and studied using ALI models of primary human bronchial epithelial cells. As a result, polarized ALI cultures proved their value as a high–throughput tool for rapidly gaining information about infection, replication, and pathogenesis of the virus.

In the next article in this series, we will look at a specific example of how introducing an air-liquid interface can transform the value of cell cultures to generate relevant data for respiratory infections. 

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