Primary porcine respiratory epithelial cell models used in the PIGSs project to study host–pathobiont interactions in the respiratory tract: ALI culture.

The Tracheobronchial Epithelium in pigs. 

The epithelial lining of the tracheobronchial region of the porcine respiratory tract mainly consists of three cell types: ciliated cells, secretory cells (primarily mucus-secreting goblet cells) and basal cells.

 

The tracheobronchial epithelium acts as a protective barrier, defending against a variety of inhaled insults such as toxins, pollutants and pathogens. Barrier function is maintained primarily as a result of three key characteristics of the epithelium:

  1. Tight junction proteins maintain the integrity of the epithelium and are critical to its function as a physical barrier.
  2. Secreted mucus traps particulate matter and pathogens, which are subsequently transported out of the airway.
  3. Secreted protective mediators such as antimicrobial peptides and inflammatory mediators form a chemical and immunological barrier against pathogens and toxicants. 

 

There is a growing need for physiologically relevant models of the respiratory epithelium, but recreating these complex functions in vitro is a considerable challenge. In the PIGSs project, Air-Liquid-Interface (ALI) cultures from primary porcine tracheal and bronchial epithelial cells are developed as in vitro culture tools that closely mimics the in vivo situation in pigs. 

 

Modeling the Respiratory Epithelium In Vitro

A variety of cell types can be used to model the respiratory system, including immortalized respiratory cell lines and primary cells from animals. Although the use of immortalized cell lines and primary cells from animals is common, data generated using these models are not directly applicable to the porcine system. Submerged culture of primary porcine bronchial epithelial cells is possible; however, cells in this system fail to undergo mucociliary differentiation. In order to recapitulate the pseudostratified mucociliary phenotype observed in vivo, primary porcine bronchial epithelial cells must be cultured at the air-liquid interface (ALI).

 

The defining feature of ALI culture is that the basal surface of the cells is in contact with liquid culture medium, whereas the apical surface is exposed to air. A common approach is to seed cells onto the permeable membrane of a cell culture insert, which is initially supplied with culture medium to both the apical and basal compartments. Once confluence is reached, the cells are subjected to ‘air-lift’, where the medium is supplied only to the basal chamber. This configuration mimics the conditions found in the human airway and drives differentiation towards a mucociliary phenotype.

 

The University of Veterinary Medicine Hannover (TiHo), partner in the PIGSs project, is in charge of developing and maintaining these ALI cultures. 

 

Figure: Primary porcine bronchial epithelial cells under air–liquid-interface (ALI) conditions after 3 weeks of differentiation

 

More information about this in vitro model can be found in the following publication: 

Vötsch D, Willenborg M, Weldearegay YB and Valentin-Weigand P (2018) Streptococcus suis – The “Two Faces” of a Pathobiont in the Porcine Respiratory Tract. Front. Microbiol. 9:480. doi: 10.3389/fmicb.2018.00480 


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