Dr. Dorothy M. Ainsworth
The epithelial cells lining the horse’s respiratory tract form an interface between the horse’s external environment and its internal milieu. As such, the epithelial layer represents the first point of contact for inhaled substances—airborne allergens, organic dust particles, noxious fumes or viral (Equine Herpes Virus 1, Equine Influenza), bacterial (Strep, Pasteurella) and fungal (Aspergillus, Coccidioides) organisms. Interestingly, the airway epithelium is not merely a physical barrier to these inhaled particulates but actively participates in lung defense and inflammatory responses. In epithelial cell cultures derived from human airways, inflammatory and chemotactic mediators (proteins that induce white blood cells to enter the airways), antimicrobial peptides and mucus are actively produced and are thus capable of either ameliorating or ramping-up the pulmonary defense system (Figure 1 below). Furthermore, epithelial cells contain receptors that engage cell wall components of infectious organisms, enabling viruses, bacteria and fungi to gain entrance into the epithelial cells and cause systemic disease.
Figure 1: Ciliated epithelial cells as well as Goblet cells (that contain mucus—strands) line the lower respiratory tract, serving as a first line of defense against inhaled particulates. Epithelial cells are able to remove particulates via ciliary movement (hair-like structures on cell surface), entrap particles via release of mucus and alter immune responses by secretion of chemokines like interleukin-8 or endogenous antimicrobial peptides like β-defensin-1.
Currently, little is known about how the equine airway epithelium responds to inhaled particulates and whether these epithelial-derived peptides can be up-regulated or manipulated. For example, one can easily imagine the advantages of enhancing epithelial expression of antimicrobial proteins (defensins) during a herd outbreak of an infectious disease on a farm or racetrack. Furthermore, there is evidence from epithelial cell culture studies that synthesis of these antimicrobial peptides can be augmented by isoleucine (an essential amino acid). Thus, the potential of administering inhaled isoleucine (as an immunomodulator) during an out break is extremely attractive (Ganz 2002; Fehlbaum et al., 2000). Such a strategy would eliminate the need to administer painful injections, would reduce the incidence of drug-induced colitis and would lessen the chance for microbial drug resistance to develop on that farm or in that hospital environment. Likewise, given that excessive mucus production is associated with poor race performance (Holcombe et al., 2006; Allen et al., 2006), one could also imagine that by attenuating its production (and thus reducing airway plugging) one could improve athleticism and decrease horse wastage. Similarly—knowing that release of free radicals or enzymes from white blood cells (e.g. neutrophils) that have emigrated to the lung can cause more tissue damage than the original inciting agent—it would be advantageous to attenuate their influx into the airways (especially in horses with inflammatory airway disease or even recurrent airway obstruction).