Fine mapping of the friend retrovirus resistance gene, Rfv3, on mouse chromosome 15

Mouse APOBEC3 Restriction of Retroviruses

Apolipoprotein B mRNA editing enzyme, catalytic peptide 3 (APOBEC3) proteins are critical host proteins that counteract and prevent the replication of retroviruses. Unlike the genome of humans and other species, the mouse genome encodes a single Apobec3 gene, which has undergone positive selection, as reflected by the allelic variants found in different inbred mouse strains. This positive selection was likely due to infection by various mouse retroviruses, which have persisted in their hosts for millions of years. While mouse retroviruses are inhibited by APOBEC3, they nonetheless still remain infectious, likely due to the actions of different viral proteins that counteract this host factor. The study of viruses in their natural hosts provides important insight into their co-evolution.

Mapping Viral Susceptibility Loci in Mice

Genetic alleles that contribute to enhanced susceptibility or resistance to viral infections and virally induced diseases have often been first identified in mice before humans due to the significant advantages of the murine system for genetic studies. Herein we review multiple discoveries that have revealed significant insights into virus-host interactions, all made using genetic mapping tools in mice. Factors that have been identified include innate and adaptive immunity genes that contribute to host defense against pathogenic viruses such as herpes viruses, flaviviruses, retroviruses, and coronaviruses. Understanding the genetic mechanisms that affect infectious disease outcomes will aid the development of personalized treatment and preventive strategies for pathogenic infections.

Friend retrovirus studies reveal complex interactions between intrinsic, innate and adaptive immunity

Approximately 4.4% of the human genome is comprised of endogenous retroviral sequences, a record of an evolutionary battle between man and retroviruses. Much of what we know about viral immunity comes from studies using mouse models. Experiments using the Friend virus (FV) model have been particularly informative in defining highly complex anti-retroviral mechanisms of the intrinsic, innate and adaptive arms of immunity. FV studies have unraveled fundamental principles about how the immune system controls both acute and chronic viral infections. They led to a more complete understanding of retroviral immunity that begins with cellular sensing, production of type I interferons, and the induction of intrinsic restriction factors. Novel mechanisms have been revealed, which demonstrate that these earliest responses affect not only virus replication, but also subsequent innate and adaptive immunity. This review on FV immunity not only surveys the complex host responses to a retroviral infection from acute infection to chronicity, but also highlights the many feedback mechanisms that regulate and counter-regulate the various arms of the immune system. In addition, the discovery of molecular mechanisms of immunity in this model have led to therapeutic interventions with implications for HIV cure and vaccine development.

Antibody-mediated immune control of a retrovirus does not require the microbiota

Commensal microbes are often required to control viral infection by facilitating host immune defenses. However, we found that this does not hold true for retroviral infection. We report that retrovirus-resistant mice control the pathogen with virus-neutralizing antibodies independently of commensal microbiota. This is in contrast to orthomyxoviruses and arenaviruses, where resistance is ablated in animals depleted of microbiota. Clearly, when it comes to antiviral immunity, the role of the microbiota cannot be generalized.

Humoral immunity in the Friend retrovirus infection model

Major conceptual roadblocks impede the development of an HIV-1 vaccine that can stimulate a potent neutralizing antibody response. Animal models that support HIV-1 replication and allow for host genetic manipulation would be an ideal platform for testing various immunological hypotheses, but progress on this research front has been slow and disappointing. In contrast, many valuable concepts emerged from more than 50 years of studying the Friend retrovirus model. This was recently exemplified by the identification of an innate restriction gene, Apobec3, that could promote the retrovirus-specific neutralizing antibody response. Here we review both classical and recent data on humoral immunity against Friend retrovirus infection, and highlight the potential of this model for unraveling novel aspects of the retrovirus-specific antibody response that may guide HIV-1 vaccine development efforts.

Innate immune sensing of retroviral infection via Toll-like receptor 7 occurs upon viral entry

Innate immune sensors are required for induction of pathogen-specific immune responses. Retroviruses are notorious for their ability to evade immune defenses and establish long-term persistence in susceptible hosts. However, some infected animals are able to develop efficient virus-specific immune responses, and thus can be employed for identification of critical innate virus-sensing mechanisms. With mice from two inbred strains that control retroviruses via adaptive immune mechanisms, we found that of all steps in viral replication, the ability to enter the host cell was sufficient to induce antivirus humoral immune responses. Virus sensing occurred in endosomes via a MyD88-Toll-like receptor 7-dependent mechanism and stimulated virus-neutralizing immunity independently of type I interferons. Thus, efficient adaptive immunity to retroviruses is induced in vivo by innate sensing of the early stages of retroviral infection.

Persistent Friend virus replication and disease in Apobec3-deficient mice expressing functional B-cell-activating factor receptor

Rfv3 is an autosomal dominant gene that influences the recovery of resistant mice from Friend retrovirus (FV) infection by limiting viremia and promoting a more potent neutralizing antibody response. We previously reported that Rfv3 is encoded by Apobec3, an innate retrovirus restriction factor. However, it was recently suggested that the Rfv3 susceptible phenotype of high viremia at 28 days postinfection (dpi) was more dominantly controlled by the B-cell-activating factor receptor (BAFF-R), a gene that is linked to but located outside the genetically mapped region containing Rfv3. Although one prototypical Rfv3 susceptible mouse strain, A/WySn, indeed contains a dysfunctional BAFF-R, two other Rfv3 susceptible strains, BALB/c and A.BY, express functional BAFF-R genes, determined on the basis of genotyping and B-cell immunophenotyping. Furthermore, transcomplementation studies in (C57BL/6 [B6] × BALB/c)F(1) and (B6 × A.BY)F(1) mice revealed that the B6 Apobec3 gene significantly influences recovery from FV viremia, cellular infection, and disease at 28 dpi. Finally, the Rfv3 phenotypes of prototypic B6, A.BY, A/WySn, and BALB/c mouse strains correlate with reported Apobec3 mRNA expression levels. Overall, these findings argue against the generality of BAFF-R polymorphisms as a dominant mechanism to explain the Rfv3 recovery phenotype and further strengthen the evidence that Apobec3 encodes Rfv3.

Adaptive evolution of Mus Apobec3 includes retroviral insertion and positive selection at two clusters of residues flanking the substrate groove

Mouse APOBEC3 (mA3) is a cytidine deaminase with antiviral activity. mA3 is linked to the Rfv3 virus resistance factor, a gene responsible for recovery from infection by Friend murine leukemia virus, and mA3 allelic variants differ in their ability to restrict mouse mammary tumor virus. We sequenced mA3 genes from 38 inbred strains and wild mouse species, and compared the mouse sequence and predicted structure with human APOBEC3G (hA3G). An inserted sequence was identified in the virus restrictive C57BL strain allele that disrupts a splice donor site. This insertion represents the long terminal repeat of the xenotropic mouse gammaretrovirus, and was acquired in Eurasian mice that harbor xenotropic retrovirus. This viral regulatory sequence does not alter splicing but is associated with elevated mA3 expression levels in spleens of laboratory and wild-derived mice. Analysis of Mus mA3 coding sequences produced evidence of positive selection and identified 10 codons with very high posterior probabilities of having evolved under positive selection. Six of these codons lie in two clusters in the N-terminal catalytically active cytidine deaminase domain (CDA), and 5 of those 6 codons are polymorphic in Rfv3 virus restrictive and nonrestrictive mice and align with hA3G CDA codons that are critical for deaminase activity. Homology models of mA3 indicate that the two selected codon clusters specify residues that are opposite each other along the predicted CDA active site groove, and that one cluster corresponds to an hAPOBEC substrate recognition loop. Substitutions at these clustered mA3 codons alter antiviral activity. This analysis suggests that mA3 has been under positive selection throughout Mus evolution, and identified an inserted retroviral regulatory sequence associated with enhanced expression in virus resistant mice and specific residues that modulate antiviral activity.

APOBEC3 (mA3) is a cytidine deaminase with antiretroviral activity. Genetic variants of mA3 are associated with the restriction factor Rfv3 (recovery from Friend leukemia virus) and with resistance to mouse mammary tumor virus. We sequenced mA3 from laboratory strains and wild mouse species to examine its evolution. We discovered that the mA3 allele in virus resistant mice is disrupted by insertion of the regulatory sequences of a mouse leukemia virus, and this insertion is associated with enhanced mA3 expression. We also subjected the Mus mA3 protein coding sequences to statistical analysis to determine if specific sites are subject to strong positive selection, that is, show an increased number of amino acid replacement mutations. We identified 10 such sites, most of which distinguish the mA3 genes of Rfv3 virus restrictive and nonrestrictive mice. Six of these sites are in two clusters that, in human APOBEC3G, are important for function. We generated a structural model of mA3, positioned these clusters opposite each other along the putative mA3 active site groove, and demonstrated that substitutions at these sites alter antiviral activity. Thus, mA3 has been involved in genetic conflicts throughout mouse evolution, and we identify an inserted regulatory sequence and two codon clusters that contribute to mA3 antiviral function.

Innate retroviral restriction by Apobec3 promotes antibody affinity maturation in vivo

Apobec3/Rfv3 is an innate immune factor that promotes the neutralizing Ab response against Friend retrovirus (FV) in infected mice. Based on its evolutionary relationship to activation-induced deaminase, Apobec3 might directly influence Ab class switching and affinity maturation independently of viral infection. Alternatively, the antiviral activity of Apobec3 may indirectly influence neutralizing Ab responses by reducing early FV-induced pathology in critical immune compartments. To distinguish between these possibilities, we immunized wild-type and Apobec3-deficient C57BL/6 (B6) mice with (4-hydroxy-3-nitrophenyl) acetyl (NP) hapten and evaluated the binding affinity of the resultant NP-specific Abs. These studies revealed similar affinity maturation of NP-specific IgG1 Abs between wild-type and Apobec3-deficient mice in the absence of FV infection. In contrast, hapten-specific Ab affinity maturation was significantly compromised in Apobec3-deficient mice infected with FV. In highly susceptible (B6 x A.BY)F(1) mice, the B6 Apobec3 gene protected multiple cell types in the bone marrow and spleen from acute FV infection, including erythroid, B, T, and myeloid cells. In addition, B6 Apobec3 deficiency was associated with elevated Ig levels, but decreased induction of splenic germinal center B cells and plasmablasts during acute FV infection. These data suggest that Apobec3 indirectly influences FV-specific neutralizing Ab responses by reducing virus-induced immune dysfunction. These findings raise the possibility that enabling Apobec3 activity during acute infection with human pathogenic retroviruses, such as HIV-1, may similarly facilitate stronger virus-specific neutralizing Ab responses.

Persistence of viremia and production of neutralizing antibodies differentially regulated by polymorphic APOBEC3 and BAFF-R loci in friend virus-infected mice

Several host genes control retroviral replication and pathogenesis through the regulation of immune responses to viral antigens. The Rfv3 gene influences the persistence of viremia and production of virus-neutralizing antibodies in mice infected with Friend mouse retrovirus complex (FV). This locus has been mapped within a narrow segment of mouse chromosome 15 harboring the APOBEC3 and BAFF-R loci, both of which show functional polymorphisms among different strains of mice. The exon 5-lacking product of the APOBEC3 allele expressed in FV-resistant C57BL/6 (B6) mice directly restricts viral replication, and mice lacking the B6-derived APOBEC3 exhibit exaggerated pathology and reduced production of neutralizing antibodies. However, the mechanisms by which the polymorphisms at the APOBEC3 locus affect the production of neutralizing antibodies remain unclear. Here we show that the APOBEC3 genotypes do not directly affect the B-cell repertoire, and mice lacking B6-derived APOBEC3 still produce FV-neutralizing antibodies in the presence of primed T helper cells. Instead, higher viral loads at a very early stage of FV infection caused by either a lack of the B6-derived APOBEC3 or a lack of the wild-type BAFF-R resulted in slower production of neutralizing antibodies. Indeed, B cells were hyperactivated soon after infection in the APOBEC3- or BAFF-R-deficient mice. In contrast to mice deficient in the B6-derived APOBEC3, which cleared viremia by 4 weeks after FV infection, mice lacking the functional BAFF-R allele exhibited sustained viremia, indicating that the polymorphisms at the BAFF-R locus may better explain the Rfv3-defining phenotype of persistent viremia.

Retroviral immunology: lessons from a mouse model

Friend virus (FV) is a murine retrovirus that causes acute disease leading to lethal erythroleukemia in most strains of mice. Strains of mice that mount strong and rapid immune responses can recover from acute infection, but nevertheless develop life-long chronic infections. The study of this infection has revealed the types of immune responses required for both recovery from the acute phase and the control of the chronic phase of infection. This knowledge has led to vaccines and therapeutics to prevent and treat infections and associated disease states. The FV model has provided insights into immunological mechanisms found to be relevant to human infections with retroviruses such as HIV-1 and HTLV-1.

Genetic correlates of protection against HIV infection: the ally within

Repeated exposure to HIV does not necessarily result in infection and HIV infection does not inevitably lead to the development of the AIDS. Multiple immunological and genetic features can confer resistance to HIV acquisition and progression at different steps in viral infection; a full understanding of these mechanisms could result in the development of novel therapeutic and vaccine approaches for HIV infection. In this review, we focus on the genetic mechanisms associated with resistance to HIV infection and to the progression to AIDS.

Mouse APOBEC3 restricts friend leukemia virus infection and pathogenesis in vivo

Several members of the apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like complex 3 (APOBEC3) family in primates act as potent inhibitors of retroviral replication. However, lentiviruses have evolved mechanisms to specifically evade host APOBEC3. Likewise, murine leukemia viruses (MuLV) exclude mouse APOBEC3 from the virions and cleave virion-incorporated APOBEC3. Although the betaretrovirus mouse mammary tumor virus has been shown to be susceptible to mouse APOBEC3, it is not known if APOBEC3 has a physiological role in restricting more widely distributed and long-coevolved mouse gammaretroviruses. The pathogenicity of Friend MuLV (F-MuLV) is influenced by several host genes: some directly restrict the cell entry or integration of the virus, while others influence the host immune responses. Among the latter, the Rfv3 gene has been mapped to chromosome 15 in the vicinity of the APOBEC3 locus. Here we have shown that polymorphisms at the mouse APOBEC3 locus indeed influence F-MuLV replication and pathogenesis: the APOBEC3 alleles of F-MuLV-resistant C57BL/6 and -susceptible BALB/c mice differ in their sequences and expression levels in the hematopoietic tissues and in their abilities to restrict F-MuLV replication both in vitro and in vivo. Furthermore, upon infection with the pathogenic Friend virus complex, (BALB/c x C57BL/6)F(1) mice displayed an exacerbated erythroid cell proliferation when the mice carried a targeted disruption of the C57BL/6-derived APOBEC3 allele. These results indicate, for the first time, that mouse APOBEC3 is a physiologically functioning restriction factor to mouse gammaretroviruses.

Apobec3 encodes Rfv3, a gene influencing neutralizing antibody control of retrovirus infection

Recovery from Friend virus 3 (Rfv3) is a single autosomal gene encoding a resistance trait that influences retroviral neutralizing antibody responses and viremia. Despite extensive research for 30 years, the molecular identity of Rfv3 has remained elusive. Here, we demonstrate that Rfv3 is encoded by Apobec3. Apobec3 maps to the same chromosome region as Rfv3 and has broad inhibitory activity against retroviruses, including HIV. Not only did genetic inactivation of Apobec3 convert Rfv3-resistant mice to a susceptible phenotype, but Apobec3 was also found to be naturally disabled by aberrant messenger RNA splicing in Rfv3-susceptible strains. The link between Apobec3 and neutralizing antibody responses highlights an Apobec3-dependent mechanism of host protection that might extend to HIV and other human retroviral infections.

Species-specific restriction of apobec3-mediated hypermutation

Apobec proteins are a family of cellular cytidine deaminases, among which several members have been shown to have potent antiviral properties. This antiviral activity is associated with the ability to cause hypermutation of retroviral cDNA. However, recent research has indicated that Apobec proteins are also able to inhibit retroviruses by other mechanisms that are independent of their deaminase activity. We have compared the antiviral activities of human and murine Apobec3 (A3) proteins, and we have found that, consistent with previous reports, human immunodeficiency virus (HIV) is able to resist human A3G but is sensitive to murine A3, whereas murine leukemia virus (MLV) is relatively resistant to murine A3 (mA3) but sensitive to human A3G. In contrast to previous studies, we observed that mA3 is packaged efficiently into MLV particles. The C-terminal cytidine deaminase domain (CDD2) is required for packaging of mA3 into MLV particles, and packaging did not depend on the MLV viral RNA. However, mA3 packed into MLV particles failed to cause hypermutation of viral DNA, indicating that its deaminase activity is blocked or inhibited. hA3G also caused significantly less hypermutation of MLV than of HIV DNA. Both mA3 and the splice variant mA3Delta5 exhibited some residual antiviral activity against MLV and caused a reduction in the ability of MLV particles to generate reverse transcription products. These results suggest that MLV has evolved specific mechanisms to block the ability of Apobec proteins to mediate deaminase-dependent hypermutation.

Host genetic factors that control immune responses to retrovirus infections

Several host genes control retroviral replication and pathogenesis. These include genes that directly affect the replication of retroviruses in target cells and those that control the host immune responses to the viral antigens. Host genetic factors that affect retroviral replication and immune responses to the viral antigens have been best studied in mouse models of Friend leukemia virus (FV) infection. Several genes located within the major histocompatibility complex (MHC), along with a separate gene not linked to the MHC, influence the host immune responses to FV antigens. The latter, the Rfv3, regulates the production of virus-neutralizing antibodies, and thus affects the duration of viremia. T-cell responses to the viral epitopes are controlled by MHC class I and class II genotypes, and both CD8(+) and CD4(+) T-cells are required for spontaneous immune resistance to FV infection. When CD4(+) T-helper cells are efficiently primed with a viral epitope, however, CD8(+) T-cells are not required for immune protection against FV infection, while B cells are absolutely required. There are individuals who possess human immunodeficiency virus type 1 (HIV-1)-reactive IgA antibodies in their mucosal secretions and show strong T-cell responses to HIV-1 antigens, even though they are negative for HIV-1 genome and HIV-1-reactive serum IgG. These HIV-1-exposed but uninfected individuals rarely possess resistance-associated alleles at known AIDS-restricting loci such as CCR5Delta32. Recent genetic analyses have indicated that a large proportion of such exposed but uninfected individuals may share a common genetic background.

Replication of beta- and gammaretroviruses is restricted in I/LnJ mice via the same genetic mechanism

Mice of the I/LnJ inbred strain are unique in their ability to mount a robust and sustained humoral immune response capable of neutralizing infection with a betaretrovirus, mouse mammary tumor virus (MMTV). Virus-neutralizing antibodies (Abs) coat MMTV virions secreted by infected cells, preventing virus spread and hence the formation of mammary tumors. To investigate whether I/LnJ mice resist infection with other retroviruses besides MMTV, the animals were infected with murine leukemia virus (MuLV), a gammaretrovirus. MuLV-infected I/LnJ mice produced virus-neutralizing Abs that block virus transmission and virally induced disease. Generation of virus-neutralizing Abs required gamma interferon but was independent of interleukin-12. This unique mechanism of retrovirus resistance is governed by a single recessive gene, virus infectivity controller 1 (vic1), mapped to chromosome 17. In addition to controlling the antivirus humoral immune response, vic1 is also required for an antiviral cytotoxic response. Both types of responses were maintained in mice of the susceptible genetic background but congenic for the I/LnJ vic1 locus. Although the vic1-mediated resistance to MuLV resembles the mechanism of retroviral recovery controlled by the resistance to Friend virus 3 (rfv3) gene, the rfv3 gene has been mapped to chromosome 15 and confers resistance to MuLV but not to MMTV. Thus, we have identified a unique virus resistance mechanism that controls immunity against two distinct retroviruses.

Identification of quantitative trait loci for susceptibility to mouse adenovirus type 1

Adult SJL/J mice are highly susceptible to mouse adenovirus type 1 (MAV-1) infections, whereas other inbred strains, including BALB/cJ, are resistant (K. R. Spindler, L. Fang, M. L. Moore, C. C. Brown, G. N. Hirsch, and A. K. Kajon, J. Virol. 75:12039-12046, 2001). Using congenic mouse strains, we showed that the H-2(s) haplotype of SJL/J mice is not associated with susceptibility to MAV-1. Susceptibility of MAV-1-infected (BALB/cJ x SJL/J)F(1) mice was intermediate between that of SJL/J mice and that of BALB/cJ mice, indicating that susceptibility is a genetically controlled quantitative trait. We mapped genetic loci involved in mouse susceptibility to MAV-1 by analysis of 192 backcross progeny in a genome scan with 65 simple sequence length polymorphic markers. A major quantitative trait locus (QTL) was detected on chromosome 15 (Chr 15) with a highly significant logarithm of odds score of 21. The locus on Chr 15 alone accounts for 40% of the total trait variance between susceptible and resistant strains. QTL modeling of the data indicated that there are a number of other QTLs with small effects that together with the major QTL on Chr 15 account for 54% of the trait variance. Identification of the major QTL is the first step in characterizing host genes involved in susceptibility to MAV-1.

Genotypes at chromosome 22q12-13 are associated with HIV-1-exposed but uninfected status in Italians

OBJECTIVE

Despite multiple and repeated exposures to HIV-1, some individuals possess no detectable HIV genome and show T-cell memory responses to the viral antigens. HIV-1-reactive mucosal IgA detected in such uninfected individuals suggests their possible immune resistance against HIV. We tested if the above HIV-1-exposed but uninfected status was associated with genetic markers other than a homozygous deletion of the CCR5 gene.

METHODS

Based on our mapping in chromosome 15 of a gene controlling the production of neutralizing antibodies in a mouse retrovirus infection, we genotyped 42 HIV-1-exposed but uninfected Italians at polymorphic loci in the syntenic segment of human chromosome 22, and compared them with 49 HIV-1-infected and 47 uninfected healthy control individuals by a closed testing procedure.

RESULTS

A significant association was found between chromosome 22q12-13 genotypes and a putative dominant locus conferring anti-HIV-1 immune responses in the exposed but uninfected individuals. Distributions of linkage disequilibrium across chromosome 22 also differed between the exposed but uninfected and two other phenotypic groups.

CONCLUSIONS

The data indicated the presence of a new genetic factor associated with the HIV-1-exposed but uninfected status.

Unique resistance of I/LnJ mice to a retrovirus is due to sustained interferon gamma-dependent production of virus-neutralizing antibodies

Selection of immune escape variants impairs the ability of the immune system to sustain an efficient antiviral response and to control retroviral infections. Like other retroviruses, mouse mammary tumor virus (MMTV) is not efficiently eliminated by the immune system of susceptible mice. In contrast, MMTV-infected I/LnJ mice are capable of producing IgG2a virus-neutralizing antibodies, sustain this response throughout their life, and secrete antibody-coated virions into the milk, thereby preventing infection of their progeny. Antibodies were produced in response to several MMTV variants and were cross-reactive to them. Resistance to MMTV infection was recessive and was dependent on interferon (IFN)-gamma production, because I/LnJ mice with targeted deletion of the INF-gamma gene failed to produce any virus-neutralizing antibodies. These findings reveal a novel mechanism of resistance to retroviral infection that is based on a robust and sustained IFN-gamma-dependent humoral immune response.

Genetic control of neuroadapted sindbis virus replication in female mice maps to chromosome 2 and associates with paralysis and mortality

Neuroadapted Sindbis virus (NSV) infection of mice causes hindlimb paralysis and 100% mortality in the C57BL/6 mouse strain, while adults of the BALB/cBy mouse strain are resistant to fatal encephalomyelitis. Levels of viral RNA are higher in the brains of infected C57BL/6 mice than in BALB/cBy mice (D. C. Thach et al., J. Virol. 74:6156-6161, 2000). These phenotypic differences between the two strains allowed us to map genetic loci involved in mouse susceptibility to NSV and to find relationships between mortality, paralysis, and viral RNA levels. Analysis of percent mortality in H2-congenic and F(1) mice suggested that the H2 locus, sex linkage, and imprinting were not involved in determining susceptibility and that resistance was partially dominant over susceptibility. Segregation analysis using CXB recombinant inbred (RI) mice indicated that the percent mortality was multigenic. Interval mapping detected a suggestive quantitative trait locus (QTL) on chromosome 2 near marker D2Mit447. Analysis of paralysis in the RI mice detected the same suggestive QTL. Viral RNA level in F(1) mice was intermediate. Interval mapping using viral RNA levels in RI mice detected a significant QTL near marker D2Mit447 that explained 69% of the genetic variance. This QTL was confirmed in F2 mice and was designated as Nsv1. Viral RNA level, percent paralyzed, and percent mortality were linearly correlated (r = 0.8 to 0.9). These results indicate that mortality, paralysis, and viral RNA levels are related complex traits and that Nsv1 controls early viral load and determines the likelihood of paralysis and death.