Comparison of innate immune responses to pathogenic and putative non-pathogenic hantaviruses in vitro

Abstract

Hantaviruses are human pathogens that cause hemorrhagic fever with renal syndrome or hantavirus cardiopulmonary syndrome. The mechanisms accounting for the differences in virulence between pathogenic and non-pathogenic hantaviruses are not well known. We have examined the pathogenesis of different hantavirus groups by comparing the innate immune responses induced in the host cell following infection by pathogenic (Sin Nombre, Hantaan, and Seoul virus) and putative non-pathogenic (Prospect Hill, Tula, and Thottapalayam virus) hantaviruses. Pathogenic hantaviruses were found to replicate more efficiently in interferon-competent A549 cells than putative non-pathogenic hantaviruses. The former also suppressed the expression of the interferon-β and myxovirus resistance protein genes, while the transcription level of both genes increased rapidly within 24 h post-infection in the latter. In addition, the induction level of interferon correlated with the activation level of interferon regulatory factor-3. Taken together, these results suggest that the observed differences are correlated with viral pathogenesis and further indicate that pathogenic and putative non-pathogenic hantaviruses differ in terms of early interferon induction via activation of the interferon regulatory factor-3 in infected host cells.

Introduction

Hantavirus is a genus of rodent-borne, lipid-enveloped, negative-sense, single-stranded RNA viruses belonging to the family Bunyaviridae. Viruses of this family are characterized by a trisegmented genome, designated according to size as the large (L), medium (M), and small (S) segments, respectively, which encode four proteins: the RNA-dependent RNA polymerase, two envelope glycoproteins (Gn and Gc), and the nucleocapsid protein (N) (Elliott et al., 1991). Hantaviruses are broadly classified into Old World and New World categories based on geographic distribution. The transmission of hantaviruses to humans occurs via inhalation of contaminated aerosolized excreta from rodents (Nuzum et al., 1988). Pathogenic hantaviruses cause hemorrhagic fever with renal syndrome (HFRS) in Eurasia (Old World hantavirus) and hantavirus cardiopulmonary syndrome (HCPS) in the Americas (New World hantavirus). However, not all hantaviruses are human pathogens; for example, there is no known association between the Prospect Hill, Tula, and Thottapalayam virus, and human disease (Carey et al., 1971, Lee et al., 1982b, Lee et al., 1985, Plyusnin et al., 1994, Song et al., 2007, Vapalahti et al., 1996).

Virus infection elicits a cascade of rapid innate immune responses in the host that are aimed at blocking viral replication through the activation of an early defense mechanism. Activation of the innate immune responses generally leads to the induction of type-I Interferon (IFN-α/β) as one of the first lines of defense against viral infection (Bonjardim, 2005, Conzelmann, 2005). However, IFN-α/β is not only synthesized in response to viral infection, but also induced at low levels in the absence of viral infection (Taniguchi and Takaoka, 2001). The regulation of IFN responses may differ among virus species, cell lines, signaling pathways, and integrin receptors.

The expression of IFN-β requires the activation of transcription factors, such as IFN regulatory factor (IRF)-3 and nuclear factor kappa B (NF-κB), both of which play a major role in the induction of IFN by viruses in the cytoplasm (Bonjardim, 2005, Haller et al., 2006, Hiscott, 2007). Viral infection triggers the phosphorylation and dimerization of IRF-3 and its subsequent translocation to the nucleus where it initiates IFN-β gene expression. Secreted IFN-β binds to the type-I IFN receptors (IFNARs) and initiates activation of the IFN signaling pathway. The expression of IFN-stimulated genes (ISGs), including 2′,5′ oligoadenylate synthetases (2′,5′-OAS), protein kinase R (PKR), and myxovirus resistance protein (Mx), is activated via Janus kinase/signal transducers and activators of the transcription (JAK-STAT) pathway (Borden et al., 2007, Randall and Goodbourn, 2008).

In the study reported here, we have compared innate immune responses to the pathogenic hantaviruses (PaV), including Sin Nombre virus (SNV), Hantaan virus (HTNV), and Seoul virus (SEOV), and the putative non-pathogenic hantaviruses (NPaV), including Prospect Hill virus (PHV), Tula virus (TULV), and Thottapalayam virus (TPMV) (Table 1). Our results provide interesting insights into the differences in human virulence among hantavirus strains and demonstrate that infection with NPaV may induce a strong innate immune response through the activation of such effectors as IFN-β and the MxA gene during the early post-infection period.