The lethality of highly pathogenic H5N1 avian influenza viruses (HPAI) is extraordinary. As of July 2011, more than 560 laboratory–confirmed human cases of H5N1 virus infection have been reported with a high fatality rate of approximately 59% (www.who.int/csr/disease/avian_influenza/
). This is considerably higher than that of the 1918 H1N1 influenza virus which caused the worst known influenza pandemic in history claiming at least 40 million lives. Concerns remain over the potential for a deadly HPAI pandemic due to the periodic emergence of HPAI cases and sporadic avian-to-human transmissions (Yuen et al., 1998
). The virulence of HPAI has been attributed in part to the over induction of inflammatory cytokines and chemokines resulting in subsequent lung damage, as shown in a variety of animal models (Baskin et al., 2009
; Maines et al., 2005
; Szretter et al., 2007
) and in human patients (de Jong et al., 2006
; Hien et al., 2004
). However, the molecular mechanisms causing this aberrant gene expression are largely unknown.
Cellular microRNAs were recently implicated in lethal infections of mice with a highly pathogenic 1918 pandemic H1N1 influenza virus (Li et al., 2010b
). By inducing increased mRNA degradation and translational inhibition of their cellular targets, cellular microRNAs are known to play key roles in crucial physiologic and pathologic processes, particularly those involving inflammatory responses (Davidson-Moncada et al., 2010
; O’Connell et al., 2010
). Notably, miR-223 and miR-10a were shown to increase the activity of NF-κB, a principle regulator of inflammatory responses (Fang et al., 2010
; Li et al., 2010a
). Upon activation, NF-κB represses the expression of Let-7 causing an increased expression of Il-6
, a direct target of Let-7 and key pro-inflammatory cytokine gene (Iliopoulos et al., 2009
). Other cellular microRNAs regulate the inflammatory responses more directly. Acting as a fine-tuner of the inflammatory response, miR-223 was able to regulate the expression patterns of a large number of target genes related to inflammation (Johnnidis et al., 2008
). Furthermore, over-expression of miR-21 resulted in elevated inflammation by repressing the expression of Il-12
(Lu et al., 2009
), a cytokine that plays a critical role in restraining antigen-induced airway inflammation (Gavett et al., 1995
). While the differential expression of cellular microRNAs was shown to contribute in part to the virulence of H1N1 pandemic influenza virus, it is not known if these cellular microRNAs also contribute to the virulence of other highly pathogenic influenza viruses, including HPAI.
To determine whether cellular microRNAs are associated with severe and fatal HPAI virus infection, we profiled 477 cellular microRNAs from lung tissues of Cynomolgus macaques infected by a human HPAI H5N1 virus (HPAI, most virulent), a reassortant H1N1 virus containing the hemagglutinin (HA) and neuraminidase (NA) surface proteins from the highly pathogenic 1918 pandemic strains (2:6) reassortant of intermediate virulence), and a human seasonal H1N1 virus strain (Tx/91, least virulent). We also attempted to identify a core group of microRNAs associated with high virulence across species and influenza subtypes by comparing the H5N1 data in macaques with data obtained from mice infected with the reconstructed 1918 pandemic (r1918) virus. This study sought to provide a better understanding of how microRNAs and their inversely regulated target genes affect cellular function during infections with highly pathogenic influenza viruses.