Current concepts on Pseudomonas aeruginosa interaction with human airway epithelium

Abstract

Pseudomonas aeruginosa is a major, but opportunistic, respiratory pathogen, which rarely infects healthy individuals, mainly due to the barrier effect of the human airway epithelium (HAE). This review explores the interaction of P. aeruginosa with HAE and the progression of the infection. The basolateral part of the epithelium, which includes the basolateral membrane of the epithelial cells and the basement membrane, is inaccessible in normal tight epithelia with intact junctions. We highlight how P. aeruginosa exploits weaknesses in the HAE barrier to gain access to the basolateral part of the epithelium. This access is crucial to initiate respiratory infection and is mainly observed in the injured epithelium, in repairing or chronically remodeled epithelium, and during extrusion of senescent cells or cell multiplication during normal epithelium renewal. The subsequent adhesion of the bacteria and cytotoxic action of virulence factors, including the toxins delivered by the type 3 secretion system (T3SS), lead to retractions and cell death. Eventually, P. aeruginosa progressively reaches the basement membrane and propagates radially through the basal part of the epithelium to disseminate using twitching and flagellar motility.

Fig 1. Lung epithelia structure.

The lung is a complex organ composed of conducting airways and gas exchange zones. The conducting airways branch from the trachea to terminal bronchioles that end up in the alveoli. In airways, the lining epithelium provides a physical barrier between the external environment and the underlying parenchyma, whose integrity is maintained by intercellular junctions. The airway epithelium ensures the protection of the lung against inhaled particles, toxins, and pathogens through the mucociliary clearance and secretion of molecules with antibacterial, antioxidant, and antiprotease activity that act in an orchestrated way to protect the epithelium from lesion factors. The diversity of cells constituting the epithelial barrier is adapted to the epithelial functions. In the pseudostratified epithelium lining the trachea and bronchi, different specific cell types such as basal, goblet, and ciliated cells are found. Basal cells are progenitor cells involved in the epithelial renewal and the anchor of the epithelium to the basement membrane through hemidesmosomes. Goblet cells, within subepithelial glands, produce the respiratory mucus that entraps noxious particles and is moved towards the oropharyngeal junction by the coordinated beating of ciliated cells. Some rare secretory Club cells are also described. In the distal bronchioles, the number of basal cells decreases, and the goblet cells are progressively replaced by Club cells, mainly involved in the production of anti-inflammatory factors and surfactant proteins. In addition, other cell types were found more rarely: neuroendocrine cells, which serve as communicators between the immune and nervous system by secreting neuropeptides; Tuft cells, which have chemosensory, neuronal, and immunological functions; and pulmonary ionocytes. The alveoli are lined by type I and type II pneumocytes whose functions are completely different: Type I pneumocytes are involved in the O₂/CO₂ exchange through their thin cytoplasm, whereas type II pneumocytes secrete surfactant proteins and act as progenitor cells of the alveolar epithelium. Created with BioRender.com.

Fig 2. P. aeruginosa basolateral interaction and progression in the airway epithelium.

A. In the first step of infection, P. aeruginosa gains access to the basolateral part, exploiting various opportunities: 1. Injured epithelium: a. Mechanical lesion: removal of HAE cells and denudation of the basement membrane. Ex: endotracheal tube in VAP. b. Induced epithelial cell death: dying cells undergo retraction, tight junction disruption, and detachment from the adjacent cells and the basement membrane. The injury could be caused by pathogens (viruses, bacteria, etc.), chemical injury (pollutants, toxic compounds, etc.), or excessive inflammatory processes (excess of cytokines, proteases, oxidant stress, etc.). Ex: excess inflammation injuring HAE in CF and COPD. 2. Repair of the epithelium after injury/epithelium remodeling: a. After an injury, HAE undergoes a repair process: basal cell dedifferentiation, spread and migration, transitional squamous metaplasia or basal/mucous hyperplasia, and progressive differentiation. Cells display low differentiation levels, low polarity, and no functional tight junctions. Ex: repair in CF and COPD. b. Chronic and pathologically remodeled epithelium: squamous and goblet cells metaplasia, hyperplasia of surface goblet and basal cells. Cells display low differentiation levels, low polarity, and no functional tight junctions. Ex: remodeled epithelia in CF and COPD. 3. Nonpathological access in differentiated epithelium: transient disruption of tight junctions during: a. Extrusion of a senescent cell. b. Cell division. B. P. aeruginosa virulence factors induce airway damage, notably by T3SS toxin injection on the basolateral membranes of the cells. T3SS effectors (ExoS, ExoT, and ExoU) induce cell retraction and death, facilitating the subsequent access of bacteria to the adjacent or underlying cells and the basement membrane. C. Bacteria cross the epithelium by this paracellular route, gain access to the basal part, and progressively propagate radially through the epithelium to disseminate, using pili and flagella. Created with BioRender.com.

Fig 3. Airway receptors of P. aeruginosa.

Green membrane: P. aeruginosa; brown membrane: host cells. Created with BioRender.com.

Fig 4. aeruginosa main virulence factors in respiratory infections.

P. Created with BioRender.com.

Fig 5. Models of airway epithelia.

It is of the utmost importance to use relevant experimental models to assess the host–pathogen interactions driving P. aeruginosa infection, depending on the physiological relevance needed: Various models exist, from cell lines to primary cells and tissue explants/animal models, each with their advantages and disadvantages. All the models used in the references of the review are listed in S1 Table. Created with BioRender.com.