HPAI H7N7 and H5N1 viruses are both capable of causing severe disease and death following human infection, and similar molecular determinants of pathogenicity between these viruses suggest a shared mechanism of action following virus infection [3
]. However, while previous studies have shown that numerous cell types present in the human respiratory tract support H5N1 virus replication and contribute toward a detrimental “cytokine storm” in some infected hosts [20
], it was unknown if H7 viruses share this ability. Here, we provide extensive side-by-side data using two cell types found in the respiratory tract to more accurately compare the activation of human host responses following infection with avian influenza viruses of multiple subtypes and lineages. We found that HPAI H7 viruses from both North American and Eurasian lineages exhibited a delayed and diminished induction of innate immune responses in human cells compared with LPAI H7N2, HPAI H5N1, or human H3N2 viruses.
Differential cytokine production in human respiratory cells following infection with avian and human viruses that exhibit similar infectivity and replication kinetics has been shown previously [20
]. HPAI H5N1 viruses from 1997 have been shown to elicit hypercytokinemia following virus infection compared with human viruses [20
], whereas an attenuated IFN response was observed following infection of human respiratory cells with H5N1 viruses isolated from 2004 [20
]. In a separate study, infection of human monocyte-derived macrophages and dendritic cells with the reconstructed 1918 H1N1 virus or a 2009 H1N1 pandemic virus revealed diminished cytokine production and reduced induction of antiviral genes, demonstrating the ability of pandemic influenza viruses to efficiently attenuate host innate immune responses [37
]. As we found that HPAI H7 viruses from multiple lineages similarly escaped induction of host interferons and receptors following infection, the pandemic potential of this virus subtype should not be overlooked.
The relationships between IFN signaling, cytokine production, virus replication, and overall pathogenesis in a host following influenza virus infection are complex and not fully understood [27
]. The dramatic upregulation of both type I and type III IFNs following NY/107 virus infection is striking in that these classes of IFN employ different receptors (IFN-αβR for type I, IFN-λR for type III) [39
]. Both type I and type III IFNs use the JAK/STAT signaling pathway, suggesting a common virus-induced event during initial signaling events [39
]. Further study is needed to ascertain if the heightened expression of IFN related genes following NY/107 virus infection contributed to the elevated levels of nucleosomes detected in culture supernatant from infected cells, or if the attenuated IFN response displayed by HPAI H7 viruses prevented induction of necrosis in this system [30
]. Despite this marked difference in IFN production, all viruses tested were equally sensitive to the antiviral effects of IFNβ as demonstrated by a delay in viral replication following pretreatment (data not shown).
Delayed transcription in HPAI H7–infected cells of IRF7 and RIG-I, genes that contribute to the induction and augmentation of the IFN response [41
], has been demonstrated following infection with select H5N1 viruses and may contribute to the poor initiation of the IFN response observed with these H7 viruses [20
]. The reduction of host antiviral responses early after infection with HPAI H7 influenza viruses could additionally allow for the growth advantage of these viruses observed in 2 cell types [43
]. Interestingly, the HPAI H7N7 and H7N3 viruses tested here, all of which poorly elicited cytokine and chemokine responses early following infection, markedly differed in their virulence in both humans and mammalian models [16
]. The efficient replication of the HPAI H7N7 virus NL/219 may in part be attributed to the E627K substitution in PB2, the only virus in this study to contain this substitution; recent work has demonstrated the necessity of 627K for the optimal replication of this virus in a human lung cell line [35
]. Infection with LPAI H7N2 viruses, which possess an HA receptor binding specificity resembling that of human viruses (), resulted in production of cytokines and chemokines similar to the H3N2 virus [7
]. Despite this similarity, titers of NY/107 virus were 2–3 logs lower than the H3N2 virus 12–24 h p.i. in Calu-3 cells, suggesting differing mechanisms between these viruses to overcome host responses. Further study characterizing the role of the NS1 protein of H7 viruses, in addition to examining differences between these viruses in the early activation of signaling pathways associated with cytokine production, will help elucidate those molecular determinants that contribute to differential induction of the early innate immune response following H7 virus infection.
The severity of HPAI H5N1 infection in humans is in part attributed to a high viral load and heightened production of proinflammatory cytokines [9
]. In accord with other studies, this work suggests that the severe disease observed following infection with some HPAI viruses may in part be caused not by cytokine dysregulation but rather a suppression of the early innate immune response [45
]. Future investigation of innate and adaptive immune responses to H7 viruses throughout the course of infection, especially in in vivo models, is warranted. Furthermore, given the ocular tropism displayed associated with H7 viruses, study of host responses within ocular cell types would further improve our understanding of this virus subtype. These studies allow for a better understanding of not only human infection with viruses within the H7 subtype but of the complex host response following infection with HPAI viruses.