Pseudorabies virus (PrV) and swine influenza virus (SIV) both initiate their infection in the respiratory mucosa. Crossing the mucus layer which covers the respiratory mucosa is a crucial step for the mucosal invasion of PrV and SIV. In this thesis, interactions of PrV and SIV with porcine respiratory mucus were investigated, and the association between the viral behavior in mucus and the viral pathogenesis was discussed.
In the first part, it is demonstrated whether and how PrV invades the host via mucus barrier. We first set up a virus tracking model using single particle tracking. The diffusion of PrV in mucus was determined and compared with that of negatively, positively and neutrally charged nanoparticles. It was shown that PrV was almost completely obstructed, with 96% of viral particles being immobilized in the porcine respiratory mucus. The negatively and positively charged particles were similarly trapped, in contrast to the neutral particles that moved freely. This suggests that immobilization of PrV was at least partly due to charge-charge interactions between its surface proteins and mucus.
Since PrV had difficulties in crossing the mucus barrier, we further identified which component mediates the interactions, by using an explant model. We found that MUC5AC was a dominant mucin expressed in the apical epithelium. The content of MUC5AC on the apical epithelium was inversely related to the attachment and infection of PrV to/in porcine trachea explants, suggesting an important role of MUC5AC in blocking PrV to reach the epithelium. The MUC5AC present above the epithelium was able to block more than 50% of virus infection, which further confirmed a strong inhibition of respiratory mucus against PrV infection. However, the mucus barrier could be overcome by PrV at low temperature (4 oC), determined by a virus-mucus binding system, a virus in-capsule-mucus penetration system and the explant model. We found that, compared to 37 oC, less viral particles were bound to the respiratory mucus at 4 oC, which resulted in deeper viral penetration in mucus, leading to a higher percentage of PrV that overcame the mucus of the explants and caused infection eventually. These results may explain winter seasonality of PrV infection.
In the second part, single particle tracking and the virus in-capsule-mucus penetration system were applied to track SIV H1N1 in porcine respiratory mucus. Results showed that 70% of SIV particles were entrapped, while the rest diffused freely in mucus. In addition, SIV was partially able to penetrate through the respiratory mucus over time. Moreover, both the microscopic diffusion and macroscopic penetration were enhanced by the addition of exogenous neuraminidase, while they were in contrast diminished by the use of a neuraminidase inhibitor, indicating that neuraminidase helps SIV to move through the porcine respiratory mucus.
In summary, PrV was highly hindered in porcine respiratory mucus. Thus, it may depend on mucus defects or physiological changes which for example may be caused by low temperature to invade the mucus barrier. On the other hand, SIV has evolved to produce neuraminidase which is able to release SIV particles which may be bound to mucins, thereby enabling the virus to move through the respiratory mucus. The in-depth investigation of virus-mucus interaction may provide novel insights into the study of prophylactic treatment for swine influenza and Aujeszky’s diseas
