HPgV coinfection has been associated with reduced liver dise
HPgV coinfection has been associated with reduced liver disease in HPgV/HCV/HIV triply infected patients (Barbosa Ade et al., 2009, Berzsenyi et al., 2007, Berzsenyi et al., 2009, Berzsenyi et al., 2011). It is possible that HCV NS3 protease inhibitors could inhibit HPgV replication diminishing its protective effect in HPgV/HCV/HIV triply infected patients. Our data imply HCV protease inhibitors may not impair HPgV replication in such patients. However, we were unable to directly measure the effect of HCV protease inhibitors on HPgV replication because no suitable cell culture system for HPgV is available. The effect of HCV protease inhibitors on HPgV replication could be tested by measuring HPgV viral loads in HPgV/HCV/HIV coinfected people before and during treatment with HCV protease inhibitors.
We found that HPgV NS3/4AB inhibited the type I interferon response via cleaving MAVS, similar to the mechanism employed by the HCV protease (Fig. 6) (Li et al., 2005b). Inhibition of the type I interferon response would be expected to enhance HIV replication (Agy et al., 1995), however studies show that HIV replication is decreased in HPgV/HIV coinfected people (Williams et al., 2004, Xiang et al., 2001). Inhibiting the type I interferon response may also allow increased replication of HPgV (Lau et al., 1999). This discrepancy may be resolved by considering that increased HPgV viral levels could in turn enhance HIV-inhibitory effects mediated by the HPgV NS5A (Chang et al., 2007, Rydze et al., 2012, Xiang et al., 2008, Xiang et al., 2006, Xiang et al., 2009) and E2 proteins (Bhattarai et al., 2013, Jung et al., 2007, Koedel et al., 2011, McLinden et al., 2013, Mohr et al., 2010, Timmons et al., 2013, Xiang et al., 2012). Type I interferons also promote survival of activated T sodium butyrate synthesis (Gelman et al., 2004, Marrack et al., 1999) which are the primary host cell for HIV (Stevenson et al., 1990), and HPgV infection is associated with reduced expression of T cell activation markers in HIV infected people (Stapleton et al., 2012). By inhibiting the type I interferon response, HPgV NS3 could be reducing survival of activated T cells, reducing the number of HIV-permissive cells. This will need to be tested in further work.
TRIF was originally proposed to be cleaved by HCV NS3 in an in vitro translation system (Li et al., 2005a). However, Dansako et al. (2007) could not replicate this observation in cell culture. We were also unable to detect cleavage of TRIF by HCV or HPgV NS3 in three separate cell lines (Fig. 4). This implies that TRIF is not cleaved by HCV or HPgV NS3 in cell culture.
There are some limitations to our experimental methods. We use expression plasmids for transient transfections. These plasmids only contain specific genes from HPgV which have been HA tagged for detection, not the entire genome. This allows us to express specific proteins to isolate their effects, but it does not account for possible interactions with other HPgV proteins. In addition, the proteins are likely to be expressed at higher levels than those found during an infection. Also, the HPgV and HCV genes used represent only one isolate of each species, so viral genetic variation in these genes could affect the degree to which our results can be generalized. Finally, we used established cell lines, which are representative of but are not the primary cells where HPgV is thought to replicate. HPgV׳s known host cells are CD4+ and CD8+ T cells and B cells and possibly monocytes (George et al., 2006). Despite the limitations of these molecular techniques we have confirmed these observations in multiple cell lines or using multiple techniques such as measuring both interferon transcripts and CPE, suggesting that these data probably reflect the situation in primary cells.
Materials and methods
This work was funded in part by a President׳s Research Fund from Saint Louis University (S.G.) and by other institutional support though the Saint Louis University School of Medicine.