MicroRNA and the innate immune response toinfluenza A virus infection in pigs

Influenza A virus infections are a major public health concern. Many million cases of diseaseassociated with influenza A virus occur every year during seasonal epidemics, and especially vulnerable populations such as the elderly, pregnant women, young children, and individual swith underlying conditions such as diabetes and patients of autoimmune diseases are at higher risk of severe complications from influenza A virus infection.

However, in otherwise healthy individuals, influenza A virus infection is relatively short-lived, commonly being cleared within one to two weeks. Influenza A virus causes respiratory infection, primarily infecting the respiratory epithelial cells. In the time span from influenza A virus infects until specific antibodies and cytotoxic T lymphocytes arrive at the site of infection, innate immunity is highly important for restricting viral spread and facilitating development of a tailored adaptive immune response.

Upon infection, the influenza A virus is recognized by innate viral pathogen sensors which initiate the induction of a balanced pro- and anti-inflammatory cytokine response as well as the hallmark interferon response, inducing an ‘antiviral state’ in the infected cell as well as neighboring cells. As with numerous other cellular processes, the innate host response is modulated by microRNAs, a class of short non-coding RNAs important for the regulation of translation of protein-coding gene transcripts.

Comprehensive assessment of the transcriptional host response to influenza A virus infection requires the joint expression profiling of protein-coding gene and microRNA expression. Paper 1 is a review which emphasizes the importance of the pig in the study of influenza Avirus infections. Pigs are themselves natural hosts for influenza A virus, and our close relationship with this species poses an ever present risk of emergence of zoonotic influenza Avirus strains.

The porcine response to influenza A virus infection greatly mirrors human conditions, and the pig thus represents an important animal model with great translational value for the study of human influenza A virus infection. Paper 2 presents results demonstrating the temporal dynamics of microRNA expression in circulating leukocytes from pigs after influenza A virus challenge, and emphasizes the need for control of the timeparameter in suitable animal models for the evaluation of the biomarker potential of circulating microRNAs.

Differential microRNA expression in circulating leukocytes peaks two weeks after challenge, suggesting that microRNAs may influence susceptibility to secondary infections. The study likewise shows that the expression profile of protein-coding genes in porcine circulating leukocytes mirrors what is seen in humans after natural or experimental influenza A virus infection.

Paper 3 examines the local innate immune andmicroRNA response in the lungs of pigs after influenza A virus challenge. In contrast to observations in circulating leukocytes, differential microRNA expression peaks three day after challenge, suggesting that pulmonary microRNA expression may be aimed at modulating the rapid transcriptional pro-inflammatory response which peaks already one day after challenge.

Paper 4 compares the local lung microRNA expression in vaccinated and unvaccinated pigs after influenza A virus challenge. Vaccinated and unvaccinated pigsdisplayed significantly different clinical signs, with a more severe course of disease observed in unvaccinated pigs presenting. This difference in disease severity was reflected in the pulmonary transcriptional innate host response of protein-coding genes and microRNA during infection.

Target analysis of the differentially expressed microRNA between the two groups of pigs indicated the involvement of microRNAs in host innate and adaptive immune responses, apoptosis, and lung regeneration.