Transforming growth factor-β: Activation by neuraminidase and role in highly pathogenic H5N1 influenza pathogenesis

Transforming growth factor-beta (tgf-β),a multifunctional cytokine regulating several immunologic processes,is expressed by virtually all cells as a biologically inactive molecule termed latent tgf-β (ltgf-β). we have previously shown that tgf-β activity increases during influenza virus infection in mice and suggested that the neuraminidase (na) protein mediates this activation. in the current study,we determined the mechanism of activation of ltgf-β by na from the influenza virus a/gray teal/australia/2/1979 by mobility shift and enzyme inhibition assays. we also investigated whether exogenous tgf-β administered via a replication-deficient adenovirus vector provides protection from h5n1 influenza pathogenesis and whether depletion of tgf-β during virus infection increases morbidity in mice. we found that both the influenza and bacterial na activate ltgf-β by removing sialic acid motifs from ltgf-β,each na being specific for the sialic acid linkages cleaved. further,na likely activates ltgf-β primarily via its enzymatic activity,but proteases might also play a role in this process. several influenza a virus subtypes (h1n1,h1n2,h3n2,h5n9,h6n1,and h7n3) except the highly pathogenic h5n1 strains activated ltgf-β in vitro and in vivo. addition of exogenous tgf-β to h5n1 influenza virus-infected mice delayed mortality and reduced viral titers whereas neutralization of tgf-β during h5n1 and pandemic 2009 h1n1 infection increased morbidity. together,these data show that microbe-associated nas can directly activate ltgf-β and that tgf-β plays a pivotal role protecting the host from influenza pathogenesis.