Review: Influenza virus in pigs

Early responses of natural killer cells in pigs experimentally infected with 2009 pandemic H1N1 influenza A virus

Natural killer (NK) cells are important players in the innate immune response against influenza A virus and the activating receptor NKp46, which binds hemagglutinin on the surface of infected cells, has been assigned a role in this context. As pigs are natural hosts for influenza A viruses and pigs possess both NKp46⁻ and NKp46⁺ NK cells, they represent a good animal model for studying the role of the NKp46 receptor during influenza. We explored the role of NK cells in piglets experimentally infected with 2009 pandemic H1N1 influenza virus by flow cytometric analyses of cells isolated from blood and lung tissue and by immunostaining of lung tissue sections. The number of NKp46⁺ NK cells was reduced while NKp46⁻ NK cells remained unaltered in the blood 1–3 days after infection. In the lungs, the intensity of NKp46 expression on NK cells was increased during the first 3 days, and areas where influenza virus nucleoprotein was detected were associated with increased numbers of NKp46⁺ NK cells when compared to uninfected areas. NKp46⁺ NK cells in the lung were neither found to be infected with influenza virus nor to be undergoing apoptosis. The binding of porcine NKp46 to influenza virus infected cells was verified in an in vitro assay. These data support the involvement of porcine NKp46⁺ NK cells in the local immune response against influenza virus.

Innate immune response to a H3N2 subtype swine influenza virus in newborn porcine trachea cells, alveolar macrophages, and precision-cut lung slices

Viral respiratory diseases remain of major importance in swine breeding units. Swine influenza virus (SIV) is one of the main known contributors to infectious respiratory diseases. The innate immune response to swine influenza viruses has been assessed in many previous studies. However most of these studies were carried out in a single-cell population or directly in the live animal, in all its complexity. In the current study we report the use of a trachea epithelial cell line (newborn pig trachea cells – NPTr) in comparison with alveolar macrophages and lung slices for the characterization of innate immune response to an infection by a European SIV of the H3N2 subtype. The expression pattern of transcripts involved in the recognition of the virus, interferon type I and III responses, and the host-response regulation were assessed by quantitative PCR in response to infection. Some significant differences were observed between the three systems, notably in the expression of type III interferon mRNA. Then, results show a clear induction of JAK/STAT and MAPK signaling pathways in infected NPTr cells. Conversely, PI3K/Akt signaling pathways was not activated. The inhibition of the JAK/STAT pathway clearly reduced interferon type I and III responses and the induction of SOCS1 at the transcript level in infected NPTr cells. Similarly, the inhibition of MAPK pathway reduced viral replication and interferon response. All together, these results contribute to an increased understanding of the innate immune response to H3N2 SIV and may help identify strategies to effectively control SIV infection.

In vitro and ex vivo analyses of co-infections with swine influenza and porcine reproductive and respiratory syndrome viruses

Viral respiratory diseases remain problematic in swine. Among viruses, porcine reproductive and respiratory syndrome virus (PRRSV) and swine influenza virus (SIV), alone or in combination, are the two main known contributors to lung infectious diseases. Previous studies demonstrated that experimental dual infections of pigs with PRRSV followed by SIV can cause more severe disease than the single viral infections. However, our understanding of the impact of one virus on the other at the molecular level is still extremely limited. Thus, the aim of the current study was to determine the influence of dual infections, compared to single infections, in porcine alveolar macrophages (PAMs) and precision cut lung slices (PCLS). PAMs were isolated and PCLS were acquired from the lungs of healthy 8-week-old pigs. Then, PRRSV (ATCC VR-2385) and a local SIV strain of H1N1 subtype (A/Sw/Saskatchewan/18789/02) were applied simultaneously or with 3 h apart on PAMs and PCLS for a total of 18 h. Immuno-staining for both viruses and beta-tubulin, real-time quantitative PCR and ELISA assays targeting various genes (pathogen recognition receptors, interferons (IFN) type I, cytokines, and IFN-inducible genes) and proteins were performed to analyze the cell and the tissue responses. Interference caused by the first virus on replication of the second virus was observed, though limited. On the host side, a synergistic effect between PRRSV and SIV co-infections was observed for some transcripts such as TLR3, RIG-I, and IFNβ in PCLS. The PRRSV infection 3 h prior to SIV infection reduced the response to SIV while the SIV infection prior to PRRSV infection had limited impact on the second infection. This study is the first to show an impact of PRRSV/SIV co-infection and superinfections in the cellular and tissue immune response at the molecular level. It opens the door to further research in this exciting and intriguing field.