The human lung is complex. More than 40 different types of cells comprise the human lung, including epithelial cells, nerve cells, hormone-producing cells, interstitial connective cells, blood cells, and more. Together, these cells assemble to form the complex tissue architecture of the lung from blood vessels, to alveolar structures, to the in vivo branching airway. Modeling the complete cellular diversity, dynamic molecular interactions, and structural assembly of the lung in a two-dimensional cell culture plate is simply impossible.
Lung organoids allow researchers to more accurately model the three-dimensional architecture and in vivo physiology of the human lung. Researchers use lung organoids to better understand natural lung development, as well as lung malfunction during respiratory diseases such as cystic fibrosis, asthma, and COPD. Researchers also use lung organoids to identify the molecular consequences of environmental assaults, such as air pollution or smoking. The lung is intricately involved in the disease symptoms observed with respiratory viral infections including H1N1, seasonal influenza, and most recently, SARS-CoV-2, making lung organoids the ideal model to explore the acute and long-term effects of these viruses on respiratory function.
Lung organoids may be grown from immortalized pulmonary cell lines or primary cell lines, but such cell lines cannot produce the plethora of cell types that exist in the in vivo lung. In contrast, human induced pluripotent stem cells (hiPSCs) have the potential to differentiate into any cell type. hiPSCs also offer the possibility of deriving patient-specific lung organoids. In such cases, lung organoids could be derived from a patient with a specific disease to understand the disease’s pathology and to explore the effect of various therapeutic options.
Differentiating hiPSCs depends on providing the ideal culture conditions for the desired cell type. But with so many types of cells in the lung, how can researchers provide all the optimal cell-type-specific conditions to foster differentiating hiPSCs into organoids that truly represent the human lung? A new serum-free, multi-stage culture system from MilliporeSigma, called the 3dGRO Human Lung Organoid Culture System, provides specialized cell culture media for each step of the lung organoid differentiation process.
Scientists differentiate their hiPSCs into definitive endoderm cells using a 4-day induction medium. Additives in the induction medium than direct definitive endoderm cells toward an anterior foregut endoderm fate. At this stage, scientists may choose to cryopreserve their AFE cells to resume at a later date, or continue the differentiation process. Researchers use 3dGRO Lung Organoid Branching medium to differentiate their AFE cells into branching lung bud organoids. To further mature these cells into branching and alveolar lung organoids, researchers simply culture the cells using the 3dGRO Lung Organoid Maturation medium.
The 3dGRO Human Lung Organoid Culture System generates large numbers of lung organoids within just 70 days. Lung organoids generated through this process express markers indicative of multiple cell types found in the mature lung and its airways, including SFTPB and SFTPC (surfactant protein B and surfactant protein C) found in type II alveolar epithelial (ATII) cells, MUC5AC (airway goblet cells), EpCAM, Sox9 and Nkx2.1 (pulmonary endoderm), Acetyl-α-Tubulin (ciliated cells), and the mesenchymal marker vimentin. They also express the key protein receptor needed for SARS-CoV-2 entry, angiotensin-converting enzyme 2 (ACE2), and TMPRSS2, a serine protease that enhances the ability of SARS-CoV-2 to infect cells. The range of cell-type-specific markers found on lung organoid cells derived using the 3dGRO Human Lung Organoid Culture System hints at the potential of these cells more closely model the in vivo human lung.