Soluble ORF2 protein enhances HEV replication and induces long-lasting antibody response and protective immunity in vivo

BACKGROUND AND AIMS

The HEV is a small positive-sense RNA virus that encodes a cytoplasmic form of the capsid protein (ORF2c), essential for virion structure, and a secreted glycosylated form (ORF2s) that accumulates at high titer in serum and can mask neutralizing epitopes. We explored the contribution of ORF2s to HEV replication and its role in generating antibodies against ORF2 in a nonhuman primate model.

APPROACH AND RESULTS

We used a recombinant HEV genotype 3 variant that does not express ORF2s due to the introduction of stop codons (ORF2s mut ). Rhesus macaques (RMs) were given intrahepatic injections of infectious wildtype HEV (ORF2s wt ) RNA or a variant lacking ORF2s expression (ORF2s mut ). The replication of the ORF2s mut virus was delayed by ~2 weeks compared with ORF2s wt , and peak titers were nearly tenfold lower. Reversions of the 3 mutations that blocked ORF2s expression were not detected in the ORF2s mut genomes, indicating genetic stability. However, serum antibodies against ORF2 were transiently detected in RMs infected with ORF2s mut , whereas they were long-lasting in RMs infected with ORF2s wt . Moreover, RMs infected with ORF2s mut were more susceptible to reinfection, as evidenced by the viral RNA detected in fecal samples and the expansion of HEV-specific CD8 + T cells.

CONCLUSIONS

These findings indicate that ORF2s may be dispensable for viral replication in vivo but is required for long-lived antibody-mediated responses that protect against HEV re-exposure.

Contribution of soluble ORF2 viral protein to viral replication and host immune response during acute Hepatitis E virus infection

The hepatitis E virus (HEV) is a small, positive-stranded RNA virus that is a major cause of acute viral hepatitis globally. Acute HEV infection is typically asymptomatic and resolves within 8–10 weeks. HEV encodes 2 forms of capsid protein. A cytoplasmic form (ORF2c) is essential for virion structure. A secreted glycosylated form (ORF2s) accumulates at high titer in serum and can mask anti-ORF2 neutralizing antibodies. Here, we explored the contribution of ORF2s to HEV replicative fitness in vivo, and its role in generating anti-ORF2 antibodies (Abs). Rhesus Macaques (RM) were challenged by direct hepatic injection of infectious ORF2s+ and ORF2s− RNA. The replication of an HEV mutant lacking ORF2s expression was delayed by ~2 weeks when compared with wildtype virus and peak titers were nearly 10-fold lower for ORF2s−. No reversion of the 3 ORF2s silencing mutations was detected in the ORF2s− genomes, indicating genetic stability. The delay in replication and lower peak titer was unexpected as the viruses replicate similarly in cell culture. In addition, our data demonstrated that ORF2s has a significant and unexpected impact on generation of antibodies. Specifically, serum anti-ORF2 antibodies were only transiently detected in ORF2s− infected RM. As expected, anti-ORF2 titers were high and sustained in ORF2s+ infected RM. Furthermore, anti-ORF2 Ab response primed by ORF2s− infection differed in protection against reinfection when compared to ORF2s+. The ORF2s− challenged animals were re-infected upon second exposure to HEV infection. These findings indicate ORF2s may be dispensable for viral replication in vivo but is required for long-lived antibody response to mediate protection against re-exposure.

An Optimized High-Throughput Neutralization Assay for Hepatitis E Virus (HEV) Involving Detection of Secreted Porf2

Hepatitis E virus (HEV) is a common cause of acute hepatitis worldwide. Current methods for evaluating the neutralizing activity of HEV-specific antibodies include immunofluorescence focus assays (IFAs) and real-time PCR, which are insensitive and operationally complicated. Here, we developed a high-throughput neutralization assay by measuring secreted pORF2 levels using an HEV antigen enzyme-linked immunosorbent assay (ELISA) kit based on the highly replicating HEV genotype (gt) 3 strain Kernow. We evaluated the neutralizing activity of HEV-specific antibodies and the sera of vaccinated individuals (n = 15) by traditional IFA and the novel assay simultaneously. A linear regression analysis shows that there is a high degree of correlation between the two assays. Furthermore, the anti-HEV IgG levels exhibited moderate correlation with the neutralizing titers of the sera of vaccinated individuals, indicating that immunization with gt 1 can protect against gt 3 Kernow infection. We then determined specificity of the novel assay and the potential threshold of neutralizing capacity using anti-HEV IgG positive sera (n = 27) and anti-HEV IgG negative sera (n = 23). The neutralizing capacity of anti-HEV IgG positive sera was significantly stronger than that of anti-HEV IgG negative. In addition, ROC curve analysis shows that the potential threshold of neutralizing capacity of sera was 8.07, and the sensitivity and specificity of the novel assay was 88.6% and 100%, respectively. Our results suggest that the neutralization assay using the antigen ELISA kit could be a useful tool for HEV clinical research.

Hepatitis E virus persists in the presence of a type III interferon response

The RIG-I-like RNA helicase (RLR)-mediated interferon (IFN) response plays a pivotal role in the hepatic antiviral immunity. The hepatitis A virus (HAV) and the hepatitis C virus (HCV) counter this response by encoding a viral protease that cleaves the mitochondria antiviral signaling protein (MAVS), a common signaling adaptor for RLRs. However, a third hepatotropic RNA virus, the hepatitis E virus (HEV), does not appear to encode a functional protease yet persists in infected cells. We investigated HEV-induced IFN responses in human hepatoma cells and primary human hepatocytes. HEV infection resulted in persistent virus replication despite poor spread. This was companied by a type III IFN response that upregulated multiple IFN-stimulated genes (ISGs), but type I IFNs were barely detected. Blocking type III IFN production or signaling resulted in reduced ISG expression and enhanced HEV replication. Unlike HAV and HCV, HEV did not cleave MAVS; MAVS protein size, mitochondrial localization, and function remained unaltered in HEV-replicating cells. Depletion of MAVS or MDA5, and to a less extent RIG-I, also diminished IFN production and increased HEV replication. Furthermore, persistent activation of the JAK/STAT signaling rendered infected cells refractory to exogenous IFN treatment, and depletion of MAVS or the receptor for type III IFNs restored the IFN responsiveness. Collectively, these results indicate that unlike other hepatotropic RNA viruses, HEV does not target MAVS and its persistence is associated with continuous production of type III IFNs.

HECT-E3 ligase ETC-1 regulates securin and cyclin B1 cytoplasmic abundance to promote timely anaphase during meiosis in C. elegans

The anaphase inhibitor securin plays a crucial role in regulating the timing of sister chromatid separation during mitosis. When sister chromatid pairs become bioriented, the E3 ligase anaphase promoting complex/cyclosome (APC/C) ubiquitylates securin for proteolysis, triggering sister chromatid separation. Securin is also implicated in regulating meiotic progression. Securin protein levels change sharply during cell cycle progression, enabling its timely action. To understand the mechanism underlying the tightly regulated dynamics of securin, we analyzed the subcellular localization of the securin IFY-1 during C. elegans development. IFY-1 was highly expressed in the cytoplasm of germ cells. The cytoplasmic level of IFY-1 declined immediately following meiosis I division and remained low during meiosis II and following mitoses. We identified a C. elegans homolog of another type of E3 ligase, UBE3C, designated ETC-1, as a regulator of the cytoplasmic IFY-1 level. RNAi-mediated depletion of ETC-1 stabilized IFY-1 and CYB-1 (cyclin B1) in post-meiosis I embryos. ETC-1 knockdown in a reduced APC function background caused an embryonic lethal phenotype. In vitro, ETC-1 ubiquitylates IFY-1 and CYB-1 in the presence of the E2 enzyme UBC-18, which functions in pharyngeal development. Genetic analysis revealed that UBC-18 plays a distinct role together with ETC-1 in regulating the cytoplasmic level of IFY-1 during meiosis. Our study reports a novel mechanism, mediated by ETC-1, that co-operates with APC/C to maintain the meiotic arrest required for proper cell cycle timing during reproduction.

Abstract 2165: Sprouty loss leads to aberrant regulation of receptor tyrosine kinase signaling pathways and accelerates mammary tumorigenesis

The SPROUTY (SPRY) proteins, key negative regulators of receptor tyrosine kinase (RTK) signaling, control developmental processes such as organ branching and maintain cellular homeostasis. The SPRY proteins inhibit critical signaling pathways that are downstream of RTKs, including the Ras-MAPK, PI3K-AKT, and the PLCα-PKC pathway. These pathways are highly deregulated in various cancers including HER2 overexpressing and PIK3CA mutant breast cancers. Importantly, the SPRY genes are significantly down-regulated in many cancers including 90% of human breast cancer cases, suggesting that they may function as putative tumor suppressors. To understand the role of Spry in normal mammary development, we examined the mammary glands of our Spry1 mutant mice at various developmental stages. In virgin Spry1-/- mice we observed a trend towards more branching of developing breast ducts. Upon pregnancy there was striking overgrowth of the mammary epithelium in Spry1+/− mice (Spry1-/- mice are sterile), suggesting that Spry regulates mammary development. To investigate the role of Spry in breast cancer we explored the consequence of Spry loss in vitro and in vivo. We found that Spry deficient cells possess accelerated cell growth, enhanced cell survival, and increased tumorigenic potential. In addition these cells were hypersensitive to a number of growth factors including EGF, showing enhanced and prolonged activation of the MAPK and PI3K-AKT pathways. To examine the role of Spry loss in promoting mammary tumorigenesis in vivo, we crossed Spry1 mutant mice with mice in which we can induce HER2/neu or PIK3CA expression. We found that Spry1 loss cooperates with oncogenic activation of HER2/neu or PIK3CA to increase cell proliferation and accelerate mammary hyperplasia. Breast tissues from the Spry1-/- mice with oncogenic activation of HER2/neu exhibited a trend towards increased levels of activated Mapk and Akt compared to their Spry1+/+ counterparts. These Spry1-/- breast tissues also accumulated significantly more DNA damage. Finally, microarray gene expression analysis of these tissues identified that genes differentially expressed in Spry1-/- breasts are involved in cell cycle regulation and the DNA damage response. Collectively, this data suggests that Spry functions as a tumor suppressor in breast cancer opposing HER2/neu and PIK3CA by suppressing MAPK and AKT activation to prevent DNA damage. Our work demonstrates that the misregulation of critical signaling pathways, through loss of Spry, leads to increased cell growth, enhanced cell survival, and accelerated tumorigenesis.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2165. doi:1538-7445.AM2012-2165

Sprouty proteins inhibit receptor-mediated activation of phosphatidylinositol-specific phospholipase C

PLCγ03B3 binds Spry1 and Spry2. Overexpression of Spry decreased PLCγ03B3 activity and IP₃ and DAG production, whereas Spry-deficient cells yielded more IP₃. Spry overexpression inhibited T-cell receptor signaling and Spry1 null T-cells hyperproliferated with TCR ligation. Through action of PLCγ03B3, Spry may influence signaling through multiple receptors.

Sprouty (Spry) proteins are negative regulators of receptor tyrosine kinase signaling; however, their exact mechanism of action remains incompletely understood. We identified phosphatidylinositol-specific phospholipase C (PLC)-γ as a partner of the Spry1 and Spry2 proteins. Spry–PLCγ interaction was dependent on the Src homology 2 domain of PLCγ and a conserved N-terminal tyrosine residue in Spry1 and Spry2. Overexpression of Spry1 and Spry2 was associated with decreased PLCγ phosphorylation and decreased PLCγ activity as measured by production of inositol (1,4,5)-triphosphate (IP₃) and diacylglycerol, whereas cells deficient for Spry1 or Spry1, -2, and -4 showed increased production of IP₃ at baseline and further increased in response to growth factor signals. Overexpression of Spry 1 or Spry2 or small-interfering RNA-mediated knockdown of PLCγ1 or PLCγ2 abrogated the activity of a calcium-dependent reporter gene, suggesting that Spry inhibited calcium-mediated signaling downstream of PLCγ. Furthermore, Spry overexpression in T-cells, which are highly dependent on PLCγ activity and calcium signaling, blocked T-cell receptor-mediated calcium release. Accordingly, cultured T-cells from Spry1 gene knockout mice showed increased proliferation in response to T-cell receptor stimulation. These data highlight an important action of Spry, which may allow these proteins to influence signaling through multiple receptors.