Our study provided the first in vivo evidence that MBL may be a risk factor during pdmH1N1 and H9N2/G1 infection by upregulating proinflammatory response.

Although smokers have increased susceptibility and severity of seasonal influenza virus infection, there is no report about the risk of 2009 pandemic H1N1 (pdmH1N1) or avian H9N2 (H9N2/G1) virus infection in smokers. In our study, we used mouse model to investigate the effect of cigarette smoke on pdmH1N1 or H9N2 virus infection. Mice were exposed to cigarette smoke for 21 days and then infected with pdmH1N1 or H9N2 virus. Control mice were exposed to air in parallel. We found that cigarette smoke exposure alone significantly upregulated the lung inflammation. Such prior cigarette smoke exposure significantly reduced the disease severity of subsequent pdmH1N1 or H9N2 virus infection. For pdmH1N1 infection, cigarette smoke exposed mice had significantly lower mortality than the control mice, possibly due to the significantly decreased production of inflammatory cytokines and chemokines. Similarly, after H9N2 infection, cigarette smoke exposed mice displayed significantly less weight loss, which might be attributed to lower cytokines and chemokines production, less macrophages, neutrophils, CD4+ and CD8+ T cells infiltration and reduced lung damage compared to the control mice. To further investigate the underlying mechanism, we used nicotine to mimic the effect of cigarette smoke both in vitro and in vivo. Pre-treating the primary human macrophages with nicotine for 72 h significantly decreased their expression of cytokines and chemokines after pdmH1N1 or H9N2 infection. The mice subcutaneously and continuously treated with nicotine displayed significantly less weight loss and lower inflammatory response than the control mice upon pdmH1N1 or H9N2 infection. Moreover, α7 nicotinic acetylcholine receptor knockout mice had more body weight loss than wild-type mice after cigarette smoke exposure and H9N2 infection. Our study provided the first evidence that the pathogenicity of both pdmH1N1 and H9N2 viruses was alleviated in cigarette smoke exposed mice, which might partially be attributed to the immunosuppressive effect of nicotine.

Intravenous immunoglobulin has long been used in treating autoimmune diseases, although mechanisms remain uncertain. Activating Fcγ receptors are receptors of IgG and reported to be essential in intravenous immunoglobulin (IVIG) therapy. Therefore, we hypothesized natural killer (NK) cells, which express abundant activating Fcγ receptors, are the potential cellular target. In experimental autoimmune encephalomyelitis (EAE), we demonstrated that IgG suppressed disease development in intact, but not in NK cell depleted mice. Adoptive transfer of IgG-treated NK cell could protect mice against EAE, and suppressed interferon γ and interleukin 17 production. The percentage of CD4+Foxp3+ regulatory T cells was significantly increased. The increase of regulatory T cells was also observed in IgG-treated EAE mice but not in NK cell depleted mice. In vitro experiments confirmed that IgG-treated NK cells enhanced regulatory T cell induction from naïve CD4+ T cells. Interestingly, cells from draining lymph nodes produced more interleukin 2 after the adoptive transfer of IgG-treated NK cells. We neutralized interleukin 2 and the induction of CD4+Foxp3+ T cells by IgG-treated NK cells was significantly reduced. To our knowledge, we identified for the first time the critical role of NK cells in the mechanism of IgG-induced induction of Treg cells in treatment of autoimmunity.

Pandemic H1N1 (pdmH1N1) influenza A virus caused massive outbreak in 2009 and currently circulates in the community worldwide. Recent studies on this pandemic revealed that most of the deaths were resulted from secondary bacterial pneumonia caused by common bacteria found in upper respiratory tract, such as Streptococcus pneumoniae. Previous studies have shown that mannose binding lectin (MBL) exhibits anti-viral and anti-bacterial effects in vitro and in vivo. However, the immunomodulatory role of MBL during pdmH1N1/ S. pneumoniae co-infection has not been reported before. In this study, we aim to compare the severity of pdmH1N1/S. pneumoniae co-infection with pdmH1N1 infection alone, and to elucidate the functional implication of MBL during co-infection. Our in vitro preliminary data showed that recombinant MBL could successfully bind to immobilized pdmH1N1 in a dose-dependent manner. However, recombinant MBL was found to be incapable of binding immobilized S. pneumoniae. WT and MBL KO mice were infected with a sublethal dose of pdmH1N1 and S. pneumoniae. Co-infection resulted in a more severe disease phenotype and mortality in mice. However, no significant difference in weight loss and survival rate was observed between WT mice and MBL KO mice. Taken together, these data suggest that MBL may play a more vital role in modulating the first pdmH1N1 infection before subsequent bacterial infection and may attenuate the severity of the co-infection.