Human Influenza Virus Infections

Seasonal and pandemic influenza are the two faces of respiratory infections caused by influenza viruses in humans. As seasonal influenza occurs on an annual basis, the circulating virus strains are closely monitored and a yearly updated vaccination is provided, especially to identified risk populations. Nonetheless, influenza virus infection may result in pneumonia and acute respiratory failure, frequently complicated by bacterial coinfection. Pandemics are, in contrary, unexpected rare events related to the emergence of a reassorted human-pathogenic influenza A virus (IAV) strains that often causes increased morbidity and spreads extremely rapidly in the immunologically naive human population, with huge clinical and economic impact. Accordingly, particular efforts are made to advance our knowledge on the disease biology and pathology and recent studies have brought new insights into IAV adaptation mechanisms to the human host, as well as into the key players in disease pathogenesis on the host side. Current antiviral strategies are only efficient at the early stages of the disease and are challenged by the genomic instability of the virus, highlighting the need for novel antiviral therapies targeting the pulmonary host response to improve viral clearance, reduce the risk of bacterial coinfection, and prevent or attenuate acute lung injury. This review article summarizes our current knowledge on the molecular basis of influenza infection and disease progression, the key players in pathogenesis driving severe disease and progression to lung failure, as well as available and envisioned prevention and treatment strategies against influenza virus infection.

Infection with Influenzavirus A in a murine model induces epithelial bronchial lesions and distinct waves of innate immune-cell recruitment

Introduction

Inflammatory lesions after Influenza A viruses (IAV) are potential therapeutic target for which better understanding of post-infection immune mechanisms is required. Most studies to evaluate innate immune reactions induced by IAV are based on quantitative/functional methods and anatomical exploration is most often non-existent. We aimed to study pulmonary damage and macrophage recruitment using two-photon excitation microscopy (TPEM) after IAV infection.

Methods

We infected C57BL/6 CD11c+YFP mice with A/Puerto Ricco/8/34 H1N1. We performed immune cell analysis, including flow cytometry, cytokine concentration assays, and TPEM observations after staining with anti-F4/80 antibody coupled to BV421. We adapted live lung slice (LLS) method for ex-vivo intravital microscopy to analyze cell motility.

Results

TPEM provided complementary data to flow cytometry and cytokine assays by allowing observation of bronchial epithelium lesions and spreading of local infection. Addition of F4/80-BV421 staining allowed us to precisely determine timing of recruitment and pulmonary migration of macrophages. Ex-vivo LLS preserved cellular viability, allowing us to observe acceleration of macrophage motility.

Conclusion

After IAV infection, we were able to explore structural consequences and successive waves of innate immune cell recruitment. By combining microscopy, flow cytometry and chemokine measurements, we describe novel and precise scenario of innate immune response against IAV.

Decreased expression of surfactant Protein-C and CD74 in alveolar epithelial cells during influenza virus A(H1N1)pdm09 and H3N2 infection

The primary replication site of Influenza A virus (IAV) is type II alveolar epithelial cells (AECII), which are central to normal lung function and present important immune functions. Surfactant components are synthesized primarily by AECII, which play a crucial role in host defense against infection. The aim of this study was to analyze if the impact of influenza infection is differential between A(H1N1)pdm09 and A/Victoria/3/75 (H3N2) on costimulatory molecules and ProSP-C expression in AECII from BALB/c mice infected and A549 cell line infected with both strains. Pandemic A(H1N1)pdm09 and A/Victoria/3/75 (H3N2) were used to infect BALB/c mice and the A549 cell line. We evaluated the surface expression of co-stimulatory molecules (CD45/CD31/CD74/ProSP-C) in AECII and A549 cell lines. Our results showed a significant decrease in ProSP-C⁺ CD31^- CD45^- and CD74⁺ CD31^- CD45^- expression in AECII and A549 cell line with the virus strain A(H1N1)pdm09 versus A/Victoria/3/75 (H3N2) and controls (non-infection conditions). Our findings indicate that changes in the expression of ProSP-C in AECII and A549 cell lines in infection conditions could result in dysfunction leading to decreased lung compliance, increased work of breathing and increased susceptibility to injury.