A 3-D airway epithelial cell and macrophage co-culture system to study Rhodococcus equi infection.

The researchers developed a 3-D culture of equine bronchial epithelial cells (BECs) and used it to study how mucus produced by these cells affects the ability of macrophages to engulf and destroy the bacterium Rhodococcus equi. They found that presence of mucus reduced the number of bacteria engulfed by the macrophages, but it was also associated with increased production of an immune response mediator, TNFα, by macrophages infected with R. equi.

Development of 3-D Cell Culture System

• The researchers developed a 3-D culture of equine bronchial epithelial cells (BECs), cells that line the bronchi (airway tubes in the lungs).
• This culture successfully differentiated into cells with cilia (small, hair-like structures) and cells that produce mucus, thus replicating the features of bronchial epithelium in living organisms.

Study Of Macrophage- Bacteria Interactions With and Without Mucus

• These BECs were combined with macrophages obtained from adult horses and then exposed to the bacterium Rhodococcus equi to replicate infection conditions.
• Both light and electron microscopy were used to evaluate the interactions between the macrophages and bacteria.
• The researchers found that in the presence of mucus, both the number of bacteria-infected macrophages and the number of intracellular bacteria within the macrophages were lesser.
• This suggests that mucus impairs the phagocytic activity, i.e., the ability of macrophages to engulf and destroy bacteria.

The Role of BECs and Mucus in Immune Response

• They also found that presence of BECs promoted production of TNFα, a molecule that mediates immune response, in macrophages infected with R. equi.
• They observed an increase in expression of EGF-R, a molecule involved in mucus production, and a decrease in TLR-2, a molecule implicated in recognition of R. equi by host cells.
• These observations indicate that BECs and their products, such as mucus, may play a key role in immune response.

Results With Foal Cells

• Interestingly, a different response was observed when macrophages and BECs from foals (young horses) were used in the same experiments.
• A large number of foal macrophages were loaded with the bacteria, indicating a difference in interactions between the bacteria and macrophages between adults and young horses.

Potential of In Vitro System

• The 3-D cell culture system used in this study demonstrates significant potential for further research to understand the mechanisms of host response to bacterial infections.
• The varying results between adult horse cells and foal cells suggest a possible difference in the way the host immune system in the airways of adult and young horses responds to R. equi infection.