Air−Liquid Interface Cultures of the Healthy and Diseased Human Respiratory Tract: Promises, Challenges, and Future Directions

Abstract

Air−liquid interface (ALI) culture models currently represent a valid instrument to recreate the typical aspects of the respiratory tract in vitro in both healthy and diseased state. They can help reducing the number of animal experiments, and hence support the 3R principle. This review discusses ALI cultures and co‐cultures derived from immortalized as well as primary cells, which are used to study the most common disorders of the respiratory tract, in terms of both pathophysiology and drug screening. The article displays ALI models used to simulate inflammatory lung diseases such as chronic obstructive pulmonary disease (COPD), asthma, cystic fibrosis, lung cancer, and viral infections. It also focuses on ALI cultures described in literature studying respiratory viruses such as SARS‐CoV‐2 causing the global Covid‐19 pandemic at the time of writing this review. Additionally, commercially available models of ALI cultures are presented. Ultimately, the aim of this review is to provide a detailed overview of ALI models currently available and to critically discuss them in the context of the most prevalent diseases of the respiratory tract.

Organoid Models of SARS-CoV-2 Infection: What Have We Learned about COVID-19?

“…Early expression studies either did not detect or detected low levels [87] or did not report [85] ACE2 protein expression in respiratory tissues, despite robust protein expression reported in other human tissues such as the intestine analyzed in parallel [85,87]. More recent studies have confirmed ACE2 protein expression and SARS-CoV-2 infections in respiratory epithelia, identifying the differentiated ciliated cells as the dominant cell type infected [21,29,32,88,89] (reviewed extensively elsewhere, e.g., [90][91][92]).…”

“…Organoids 2022, 1, FOR PEER REVIEW 7 [87] or did not report [85] ACE2 protein expression in respiratory tissues, despite robust protein expression reported in other human tissues such as the intestine analyzed in parallel [85,87]. More recent studies have confirmed ACE2 protein expression and SARS-CoV-2 infections in respiratory epithelia, identifying the differentiated ciliated cells as the dominant cell type infected [21,29,32,88,89] (reviewed extensively elsewhere, e.g., [90][91][92]). With the urgency to discover and evaluate therapies of severe respiratory damage in COVID-19, researchers have sought to establish human alveolar models of SARS-CoV-2 infection (Figure 4).…”

“…With the urgency to discover and evaluate therapies of severe respiratory damage in COVID-19, researchers have sought to establish human alveolar models of SARS-CoV-2 infection (Figure 4). Unlike primary upper respiratory cells with well-established and characterized ALI culture systems [91], establishing similar cultures of primary alveolar cells proved more difficult. The main cell types of the alveoli sacs are the type I and type II pneumocytes (AT1 and AT2, respectively).…”

Air-Liquid-Interface Differentiated Human Nose Epithelium: A Robust Primary Tissue Culture Model of SARS-CoV-2 Infection

“…To circumvent these caveats, we revisit a well-established and characterized upper respiratory epithelium model [ 30 , 31 , 32 ]. The human nasal epithelium (HNE) is considered the first site of SARS-CoV-2 infection and expresses high levels of ACE2 [ 33 ], the cell surface receptor for SARS-CoV-2 [ 16 , 17 ], and is thus potentially the ideal tissue for testing prevention of virus entry into the body.…”

Can pulmonary RNA delivery improve our pandemic preparedness?

“…Therefore, liquid formulations are generally developed, particularly for in vitro assessment. To mimic the nebulization process in vitro, the ALICE Cloud exposure system can be used to directly nebulize drug formulations onto cell culture wells [ 49 ]. As rodents are obligate nose breathers [ 50 ], inhalation exposure is in principle also possible in preclinical experiments.…”