Rheological comparison of sputum and reconstituted airway epithelium mucus

Abstract

Pulmonary mucus serves as a crucial protective barrier in the respiratory tract, defending against pathogens and contributing to effective clearance mechanisms. In Muco Obstructive Pulmonary Diseases (MOPD), abnormal rheological properties lead to highly viscous mucus, fostering chronic infections and exacerbations. While prior research has linked mucus viscoelasticity to its mucin content, the variability in MOPD patients implies the involvement of other factors. To isolate these effects, mucus produced by epithelia reconstituted in vitro serves as a powerful versatile model for mucin research. This study characterises the rheology of mucus collected from Air-Liquid Interface (ALI) cultures and compares it to sputum samples from MOPD patients, demonstrating that macrorheology with cone-plate geometries is a reproducible method for analysing small mucus quantities from ALI cultures. While sputum samples exhibit similarities in rigidity with ALI mucus, they also display structural differences and variations in their response to substantial deformations. The study highlights the importance of understanding mucus behaviour under large deformations, emphasising the role of ALI cultures as a controlled environment for conducting detailed studies.

Fig. 1

Strain sweep of a CF sputum sample. The solid line is the elastic modulus (G′) and the dashed line is the viscous modulus (G″). The critical strain (γ_c) is retrieved when G′ and G″ cross.

Fig. 2

Rheological parameters G*, tan δ, σ_c and γ_c measured from ALI mucus samples from healthy donors (N = 43) and CF donors (N = 7); as well as sputum collected by auto drainage from CF and NCFB patients (N = 81) and induced sputum from healthy individuals (N = 11) and CF patients (N = 18). Statistical significance is marked as follows: ***p < 0.001, **p < 0.01, *p < 0.05 compared to healthy ALI mucus.

Fig. 3

Correlation between σ_c and G* for the 81 CF and NCFB sputum samples (blue circles) and the 50 healthy and CF ALI mucus samples (red triangles), m indicates the slopes of the fits.

Fig. 4

Illustration depicting the cross-linked structure of in vitro mucus, primarily composed of mucins and extracellular DNA. Additionally, in vivo mucus typically incorporates bacteria that can form biofilms, along with immune response cells capable of releasing polymeric materials such as actine polymerised neutrophil extracellular traps (NETs) and Charcot-Leyden crystals.