Abstract

Osteopontin (OPN) is involved in exacerbating various inflammatory diseases. A severe pulmonary inflammation is frequently found in lethal influenza A virus (IAV) infection. However, the function of OPN against the infection was poorly understood. Here, we demonstrate an importance of OPN on immune response and disease severity after IAV infection. We found that the expression level of OPN was increased in mice infected with IAV. The OPN knockout (KO) mice exhibited a severe pathological phenotype and the survival rate decreased after the lethal IAV infection, compared to the wild type mice, while the survival rate increased in OPN transgenic (Tg) mice. The population of natural killer (NK) cells significantly decreased in OPN KO mice at day 5 after the infection, whereas, it increased in OPN Tg mice. These results suggest that OPN plays an important role in host defense against IAV infection through the regulation of NK cell population.

Highlights

► Osteopontin (OPN) is involved in the exacerbation of various inflammatory diseases. ► OPN KO mice exhibit severe phenotype against a lethal influenza A virus challenge. ► OPN Tg mice survive for long periods against a lethal influenza A virus challenge. ► OPN upregulates NK cell population after influenza A virus infection. ► OPN is crucial to activate early immune responses to influenza A virus infection.

Introduction

Influenza A viruses (IAVs) are well known to induce highly contagious respiratory infections in humans and several animal species. Aberrant production of inflammatory cytokines, such as TNF-α (tumor necrosis factor-α), IL-6 (interleukin-6), and IFNγ (interferon-γ), is frequently found in the course of lethal infections of IAV, and this is thought to be a prominent phenomenon linked to the viral pathogenicity [1], [2], [3], [4]. However, the inflammatory cytokines are also important in functioning to eliminate the virus through activation of the immune system [5], [6], [7], [8]. Therefore, simply repressing these cytokines would not always be efficient to rescue a patient from lethal infection with IAV [9]. Nevertheless, an analysis of the functions of the various inflammatory cytokines in infectious diseases caused by IAV infections would provide important information for the development of effective prevention and treatment regimes against the infectious disease.

Osteopontin (OPN), a secreted phospho-glycoprotein, is a multi-functional protein, known to be involved in biomineralization, tissue remodeling, and inflammation [10], [11], [12]. Expression of OPN is involved in the deterioration of various inflammatory diseases [13], [14], [15]. For instance, osteopontin deficiency reduced joint swelling in the mouse model for rheumatoid arthritis [16], [17], and treatment with anti-OPN antibody protects a joint from the experimental rheumatoid arthritis. In addition, in acute inflammatory liver injury caused by concanavalin A (Con-A) injection, OPN plays a crucial role in the development of the hepatitis [18], [19]. Despite knowledge of physiological significances of OPN to exacerbate inflammation, the function of OPN on IAV infection has been poorly understood.

Here, we report on the protective role of OPN against IAV infections. The expression levels of OPN increased in mice infected with IAV and the increase correlates with the virulence of the IAV strains in the mice. The OPN knockout (OPN KO) mice exhibited the more severe clinical phenotype against lethal doses of infections with IAV. In addition, lengthening of survival times and a more moderate clinical phenotype against IAV infections were observed in OPN transgenic (OPN Tg) mice. In parallel with the severity of the IAV infection, the numbers of natural killer (NK) cells decreased in the OPN KO mice during the course of the viral infection, and increased in the OPN Tg mice. These results suggest that OPN is involved in the elimination of the IAV infection at the early course of the infection through activation of innate immunity.

Section snippets

Mice

Male, specific pathogen-free C57BL/6N mice were purchased from Clea Japan Inc (Tokyo, Japan). OPN deficient mice [20], [21] were backcrossed 11 generations with C57BL/6N, and then used for this study. Information about OPN transgenic mice is described elsewhere [22]. All animal experiments were performed in accordance with the guidelines of the Bioscience Committee of Hokkaido University and were approved by the Animal Care and Use Committee of Hokkaido University.

Viruses and virus titers

Influenza A viruses (IAVs),

The expression level of OPN is increased in the lung of mice infected with the IAV, correlated to the severity of the pathological phenotype

To understand functions of OPN on infection with IAV, we initially monitored the expression level of OPN during the course of the IAV infection. The C57/BL6N mice were infected intranasally with the PR8 or Aichi strain of IAV at 103 pfu, and then the expression levels of OPN in the plasma and lung collected from the infected mice were measured by ELISA. As shown in Fig. 1A, the body weight loss of the PR8 infected mice was more severe than that of the Aichi infected mice, in agreement with the

Discussion

The present study demonstrated the physiological significance of OPN in the activation of the immunoresponse to eliminate IAV for the infection. OPN is known to be involved in the worsening of various inflammatory diseases, such as rheumatoid arthritis and concanavalin A-induced hepatitis [16], [17], [18], [19]. Our data indicate that the expression level of OPN in the lungs is remarkably increased in PR8 infected mice in comparison with that of Aichi infected mice, and that the OPN level is

References (35)

References

https://doi.org/10.1016/j.bbrc.2011.11.100