Transforming Growth Factor-β: Activation by Neuraminidase and Role in Highly Pathogenic H5N1 Influenza Pathogenesis

Transforming growth factor-beta (TGF-beta), a multifunctional cytokine regulating several immunologic processes, is expressed by virtually all cells as a biologically inactive molecule termed latent TGF-beta (LTGF-beta). We have previously shown that TGF-beta 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-beta 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-beta administered via a replication-deficient adenovirus vector provides protection from H5N1 influenza pathogenesis and whether depletion of TGF-beta during virus infection increases morbidity in mice. We found that both the influenza and bacterial NA activate LTGF-beta by removing sialic acid motifs from LTGF-beta, each NA being specific for the sialic acid linkages cleaved. Further, NA likely activates LTGF-beta 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-beta in vitro and in vivo. Addition of exogenous TGF-beta to H5N1 influenza virus-infected mice delayed mortality and reduced viral titers whereas neutralization of TGF-beta during H5N1 and pandemic 2009 H1N1 infection increased morbidity. Together, these data show that microbe-associated NAs can directly activate LTGF-beta and that TGF-beta plays a pivotal role protecting the host from influenza pathogenesis.