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

SAMHD1 Promotes the Antiretroviral Adaptive Immune Response in Mice Exposed to Lipopolysaccharide

SAMHD1 is a potent HIV-1 restriction factor that blocks reverse transcription in monocytes, dendritic cells and resting CD4⁺ T cells by decreasing intracellular dNTP pools. However, SAMHD1 may diminish innate immune sensing and Ag presentation, resulting in a weaker adaptive immune response. To date, the role of SAMHD1 on antiretroviral immunity remains unclear, as mouse SAMHD1 had no impact on murine retrovirus replication in prior in vivo studies. Here, we show that SAMHD1 significantly inhibits acute Friend retrovirus infection in mice. Pretreatment with LPS, a significant driver of inflammation during HIV-1 infection, further unmasked a role for SAMHD1 in influencing immune responses. LPS treatment in vivo doubled the intracellular dNTP levels in immune compartments of SAMHD1 knockout but not wild-type mice. SAMHD1 knockout mice exhibited higher plasma infectious viremia and proviral DNA loads than wild-type mice at 7 d postinfection (dpi), and proviral loads inversely correlated with a stronger CD8⁺ T cell response. SAMHD1 deficiency was also associated with weaker NK, CD4⁺ T and CD8⁺ T cell responses by 14 dpi and weaker neutralizing Ab responses by 28 dpi. Intriguingly, SAMHD1 influenced these cell-mediated immune (14 dpi) and neutralizing Ab (28 dpi) responses in male but not female mice. Our findings formally demonstrate SAMHD1 as an antiretroviral factor in vivo that could promote adaptive immune responses in a sex-dependent manner. The requirement for LPS to unravel the SAMHD1 immunological phenotype suggests that comorbidities associated with a "leaky" gut barrier may influence the antiviral function of SAMHD1 in vivo.

B-Cell Control of Regulatory T Cells in Friend Virus Infection

B lymphocytes have well-established effector roles during viral infections, including production of antibodies and functioning as antigen-presenting cells for CD4+ and CD8+ T cells. B cells have also been shown to regulate immune responses and induce regulatory T cells (Tregs). In the Friend virus (FV) model, Tregs are known to inhibit effector CD8+ T-cell responses and contribute to virus persistence. Recent work has uncovered a role for B cells in the induction and activation of Tregs during FV infection. In addition to inducing Tregs, B cell antibody production and antigen-presenting cell activity is a target of Treg suppression. This review focuses on the dynamic interactions between B cells and Tregs during FV infection.

Systemic Expression of a Viral RdRP Protects against Retrovirus Infection and Disease

The innate immune system is normally programmed for immediate but transient upregulation in response to invading pathogens, and interferon (IFN)-stimulated gene (ISG) activation is a central feature. In contrast, chronic innate immune system activation is typically associated with autoimmunity and a broad array of autoinflammatory diseases that include the interferonopathies. Here, we studied retroviral susceptibility in a transgenic mouse model with lifelong innate immune system hyperactivation. The mice transgenically express low levels of a picornaviral RNA-dependent RNA polymerase (RdRP), which synthesizes double-stranded RNAs that are sensed by melanoma differentiation-associated protein 5 (MDA5) to trigger constitutive upregulation of many ISGs. However, in striking counterpoint to the paradigm established by numerous human and murine examples of ISG hyperactivation, including constitutive MDA5 activation, they lack autoinflammatory sequelae. RdRP-transgenic mice (RdRP mice) resist infection and disease caused by several pathogenic RNA and DNA viruses. However, retroviruses are sensed through other mechanisms, persist in the host, and have distinctive replication and immunity-evading properties. We infected RdRP mice and wild-type (WT) mice with various doses of a pathogenic retrovirus (Friend virus) and assessed immune parameters and disease at 1, 4, and 8 weeks. Compared to WT mice, RdRP mice had significantly reduced splenomegaly, viral loads, and infection of multiple target cell types in the spleen and the bone marrow. During chronic infection, RdRP mice had 2.35 ± 0.66 log10 lower circulating viral RNA than WT. Protection required ongoing type I IFN signaling. The results show that the reconfigured RdRP mouse innate immune system substantially reduced retroviral replication, set point, and pathogenesis.IMPORTANCE Immune control of retroviruses is notoriously difficult, a fundamental problem that has been most clinically consequential with the HIV-1 pandemic. As humans expand further into previously uninhabited areas, the likelihood of new zoonotic retroviral exposures increases. The role of the innate immune system, including ISGs, in controlling retroviral infections is currently an area of intensive study. This work provides evidence that a primed innate immune system is an effective defense against retroviral pathogenesis, resulting in reduced viral replication and burden of disease outcomes. RdRP mice also had considerably lower Friend retrovirus (FV) viremia. The results could have implications for harnessing ISG responses to reduce transmission or control pathogenesis of human retroviral pathogens.

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.

Type I interferon signaling is required for the APOBEC3/Rfv3-dependent neutralizing antibody response but not innate retrovirus restriction

Background

APOBEC3/Rfv3 restricts acute Friend retrovirus (FV) infection and promotes virus-specific neutralizing antibody (NAb) responses. Classical Rfv3 studies utilized FV stocks containing lactate-dehydrogenase elevating virus (LDV), a potent type I interferon inducer. Previously, we showed that APOBEC3 is required for the anti-FV activity of exogenous IFN-alpha treatment. Thus, type I interferon receptor (IFNAR) signaling may be required for the APOBEC3/Rfv3 response.

Results

To test if the APOBEC3/Rfv3 response is dependent on type I IFN signaling, we infected IFNAR knockout versus IFNAR/APOBEC3 double-knockout mice with FV/LDV or LDV-free FV, and evaluated acute FV infection and subsequent NAb titers. We show that LDV co-infection and type I IFN signaling are not required for innate APOBEC3-mediated restriction. By contrast, removal of LDV and/or type I IFN signaling abrogated the APOBEC3-dependent NAb response.

Conclusions

APOBEC3 can restrict retroviruses in a type I IFN-independent manner in vivo. By contrast, the ability of APOBEC3 to promote NAb responses is type I IFN-dependent. These findings reveal novel insights on the interplay between type I IFNs and APOBEC3 in vivo that may have implications for augmenting antiretroviral NAb responses.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-017-0349-2) contains supplementary material, which is available to authorized users.

Requirement for Fc effector mechanisms in the APOBEC3/Rfv3-dependent neutralizing antibody response

Antiretroviral neutralizing antibody (NAb) responses are often evaluated in the absence of Fc-dependent immune effectors. In murine Friend retrovirus infection, Apobec3/Rfv3 promotes a potent polyclonal NAb response. Here, we show that the Apobec3/Rfv3-dependent NAb response correlated with virus-specific IgG2 titers and that the in vivo neutralization potency of Apobec3/Rfv3-resistant antisera was dependent on activating Fcγ receptors but not complement. The data strengthen retroviral vaccine strategies aimed at eliciting NAbs that activate specific Fcγ receptors.

APOBEC3A functions as a restriction factor of human papillomavirus

Human papillomaviruses (HPVs) are small DNA viruses causally associated with benign warts and multiple cancers, including cervical and head-and-neck cancers. While the vast majority of people are exposed to HPV, most instances of infection are cleared naturally. However, the intrinsic host defense mechanisms that block the early establishment of HPV infections remain mysterious. Several antiviral cytidine deaminases of the human APOBEC3 (hA3) family have been identified as potent viral DNA mutators. While editing of HPV genomes in benign and premalignant cervical lesions has been demonstrated, it remains unclear whether hA3 proteins can directly inhibit HPV infection. Interestingly, recent studies revealed that HPV-positive cervical and head-and-neck cancers exhibited higher rates of hA3 mutation signatures than most HPV-negative cancers. Here, we report that hA3A and hA3B expression levels are highly upregulated in HPV-positive keratinocytes and cervical tissues in early stages of cancer progression, potentially through a mechanism involving the HPV E7 oncoprotein. HPV16 virions assembled in the presence of hA3A, but not in the presence of hA3B or hA3C, have significantly decreased infectivity compared to HPV virions assembled without hA3A or with a catalytically inactive mutant, hA3A/E72Q. Importantly, hA3A knockdown in human keratinocytes results in a significant increase in HPV infectivity. Collectively, our findings suggest that hA3A acts as a restriction factor against HPV infection, but the induction of this restriction mechanism by HPV may come at a cost to the host by promoting cancer mutagenesis.

IMPORTANCE

Human papillomaviruses (HPVs) are highly prevalent and potent human pathogens that cause >5% of all human cancers, including cervical and head-and-neck cancers. While the majority of people become infected with HPV, only 10 to 20% of infections are established as persistent infections. This suggests the existence of intrinsic host defense mechanisms that inhibit viral persistence. Using a robust method to produce infectious HPV virions, we demonstrate that hA3A, but not hA3B or hA3C, can significantly inhibit HPV infectivity. Moreover, hA3A and hA3B were coordinately induced in HPV-positive clinical specimens during cancer progression, likely through an HPV E7 oncoprotein-dependent mechanism. Interestingly, HPV-positive cervical and head-and-neck cancer specimens were recently shown to harbor significant amounts of hA3 mutation signatures. Our findings raise the intriguing possibility that the induction of this host restriction mechanism by HPV may also trigger hA3A- and hA3B-induced cancer mutagenesis.

Reassessment of murine APOBEC1 as a retrovirus restriction factor in vivo

APOBEC1 is a cytidine deaminase involved in cholesterol metabolism that has been linked to retrovirus restriction, analogous to the evolutionarily-related APOBEC3 proteins. In particular, murine APOBEC1 was shown to inhibit Friend retrovirus (FV) in vitro, generating high levels of C-to-T and G-to-A mutations. These observations raised the possibility that FV infection might be altered in APOBEC1-null mice. To examine this question directly, we infected wild-type and APOBEC1-null mice with FV complex and evaluated acute infection levels. Surprisingly, APOBEC1-null mice exhibited similar cellular infection levels and plasma viremia relative to wild-type mice. Moreover, next-generation sequencing analyses revealed that in contrast to APOBEC3, APOBEC1 did not enhance retroviral C-to-T and G-to-A mutational frequencies in genomic DNA. Thus, APOBEC1 neither inhibited nor significantly drove the molecular evolution of FV in vivo. Our findings reinforce that not all retrovirus restriction factors characterized as potent in vitro may be functionally relevant in vivo.

Tetherin promotes the innate and adaptive cell-mediated immune response against retrovirus infection in vivo

Tetherin/BST-2 is a host restriction factor that could directly inhibit retroviral particle release by tethering nascent virions to the plasma membrane. However, the immunological impact of Tetherin during retrovirus infection remains unknown. We now show that Tetherin influences antiretroviral cell-mediated immune responses. In contrast to the direct antiviral effects of Tetherin, which are dependent on cell surface expression, the immunomodulatory effects are linked to the endocytosis of the molecule. Mice encoding endocytosis-competent C57BL/6 Tetherin exhibited lower viremia and pathology at 7 d postinfection with Friend retrovirus (FV) compared with mice encoding endocytosis-defective NZW/LacJ Tetherin. Notably, antiretroviral protection correlated with stronger NK cell responses. In addition, Friend retrovirus infection levels were significantly lower in wild-type C57BL/6 mice than in Tetherin knockout mice at 2 wk postinfection, and antiretroviral protection correlated with stronger NK cell and virus-specific CD8+ T cell responses. The results demonstrate that Tetherin acts as a modulator of the cell-mediated immune response against retrovirus infection in vivo.

Immunoglobulin somatic hypermutation by APOBEC3/Rfv3 during retroviral infection

Somatic hypermutation (SHM) is an integral process in the development of high-affinity antibodies that are important for recovery from viral infections and vaccine-induced protection. Ig SHM occurs predominantly in germinal centers (GC) via the enzymatic activity of activation-induced deaminase (AID). In contrast, the evolutionarily related apolipoprotein B mRNA-editing enzyme, catalytic polypeptide 3 (APOBEC3) proteins are known to restrict retroviruses, including HIV-1. We previously reported that mouse APOBEC3 encodes Recovery from Friend virus 3 (Rfv3), a classical resistance gene in mice that promotes the neutralizing antibody response against retrovirus infection. We now show that APOBEC3/Rfv3 complements AID in driving Ig SHM during retrovirus infection. Analysis of antibody sequences from retrovirus-specific hybridomas and GC B cells from infected mice revealed Ig heavy-chain V genes with significantly increased C-to-T and G-to-A transitions in wild-type as compared with APOBEC3-defective mice. The context of the mutations was consistent with APOBEC3 but not AID mutational activity. These findings help explain the role of APOBEC3/Rfv3 in promoting the neutralizing antibody responses essential for recovery from retroviral infection and highlight APOBEC3-mediated deamination as a previously unidentified mechanism for antibody diversification in vivo.

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.

Ribonuclease L is not critical for innate restriction and adaptive immunity against Friend retrovirus infection

Ribonuclease L (RNase L) is a type I interferon regulated factor that can significantly inhibit retroviruses in vitro and may activate cytoplasmic sensing pathways to augment adaptive immunity. However, the antiretroviral activity of RNase L remains to be validated in vivo. We investigated the role of RNaseL in counteracting Friend retrovirus (FV) infection relative to a well-described restriction factor, Apobec3. C57BL/6 wild-type (WT) and RNaseL knock-out (KO) mice exhibited similar acute FV infection levels despite significant transcriptional induction of oligoadenylate synthetase 1, which produces activators of RNase L. Apobec3 KO mice showed higher FV infection levels relative to WT mice, but deletion of RNaseL in Apobec3 KO mice did not augment FV infection. Moreover, RNaseL did not influence FV-specific IgG responses and recovery from viremia by 28 days post-infection. The results suggest that RNase L is not an evolutionarily-conserved host defense mechanism to counteract retroviruses in vivo.

Fv1 restriction and retrovirus vaccine immunity in Apobec3-deficient 129P2 mice

Understanding the host genetics of the immune response in retrovirus infection models could provide insights for basic HIV vaccine discovery. In Friend retrovirus (FV) infection of mice, Fv1 differentially inhibits N-tropic versus B-tropic FV infection by mediating a capsid-dependent post-entry block, Fv2 susceptibility governs splenomegaly induction, and Rfv3 resistance primes a stronger neutralizing antibody response due to more potent Apobec3 activity. Apobec3 polymorphisms in inbred mouse strains correlate with Rfv3 resistance and susceptibility, with one unresolved exception. The 129/OlaHsd (129P2) mouse strain is Fv2 and Rfv3 susceptible based on genotyping, but infection of 129P2 mice with B-tropic FV resulted in strong neutralizing antibody responses and no splenomegaly. Here we confirm that 129P2 mice are Fv1^nr/nr, explaining its resistance to B-tropic FV. Infection of 129P2 mice with NB-tropic FV, which can efficiently infect mice independent of Fv1 genotype, resulted in severe splenomegaly, high levels of viremia and weak neutralizing antibody responses regardless of Apobec3 status. Notably, high-dose B-tropic FV infection of 129P2 Apobec3-deficient mice induced significant adaptive immune responses and conferred high levels of protection following challenge with pathogenic NB-tropic FV. This immunological protection complemented previous studies that N-tropic FV can act as a live-attenuated vaccine in Fv1^b/b mice. Altogether, the results obtained in 129P2 mice strengthen the conclusion that Rfv3 is encoded by Apobec3, and highlight Fv1 incompatibility as a retroviral vaccine paradigm in mice. Due to its susceptibility to disease that allows for pathogenic challenge studies, B-tropic FV infection of 129P2 mice may be a useful model to study the immunological pathways induced by retroviral capsid restriction.

IFN-α treatment inhibits acute Friend retrovirus replication primarily through the antiviral effector molecule Apobec3

Therapeutic administration of IFN-α in clinical trials significantly reduced HIV-1 plasma viral load and human T-lymphotropic virus type I proviral load in infected patients. The mechanism may involve the concerted action of multiple antiretroviral effectors collectively known as "restriction factors," which could vary in relative importance according to the magnitude of transcriptional induction. However, direct genetic approaches to identify the relevant IFN-α restriction factors will not be feasible in humans in vivo. Meanwhile, mice encode an analogous set of restriction factor genes and could be used to obtain insights on how IFN-α could inhibit retroviruses in vivo. As expected, IFN-α treatment of mice significantly upregulated the transcription of multiple restriction factors including Tetherin/BST2, SAMHD1, Viperin, ISG15, OAS1, and IFITM3. However, a dominant antiretroviral factor, Apobec3, was only minimally induced. To determine whether Apobec3 was necessary for direct IFN-α antiretroviral action in vivo, wild-type and Apobec3-deficient mice were infected with Friend retrovirus, then treated with IFN-α. Treatment of infected wild-type mice with IFN-α significantly reduced acute plasma viral load 28-fold, splenic proviral load 5-fold, bone marrow proviral load 14-fold, and infected bone marrow cells 7-fold, but no inhibition was observed in Apobec3-deficient mice. These findings reveal that IFN-α inhibits acute Friend retrovirus infection primarily through the antiviral effector Apobec3 in vivo, demonstrate that transcriptional induction levels did not predict the mechanism of IFN-α-mediated control, and highlight the potential of the human APOBEC3 proteins as therapeutic targets against pathogenic retrovirus infections.

APOBEC3 versus Retroviruses, Immunity versus Invasion: Clash of the Titans

Since the identification of APOBEC3G (A3G) as a potent restriction factor of HIV-1, a tremendous amount of effort has led to a broadened understanding of both A3G and the APOBEC3 (A3) family to which it belongs. In spite of the fine-tuned viral counterattack to A3 activity, in the form of the HIV-1 Vif protein, enthusiasm for leveraging the Vif : A3G axis as a point of clinical intervention remains high. In an impressive explosion of information over the last decade, additional A3 family members have been identified as antiviral proteins, mechanistic details of the restrictive capacity of these proteins have been elucidated, structure-function studies have revealed important molecular details of the Vif : A3G interaction, and clinical cohorts have been scrutinized for correlations between A3 expression and function and viral pathogenesis. In the last year, novel and unexpected findings regarding the role of A3G in immunity have refocused efforts on exploring the potential of harnessing the natural power of this immune defense. These most recent reports allude to functions of the A3 proteins that extend beyond their well-characterized designation as restriction factors. The emerging story implicates the A3 family as not only defense proteins, but also as participants in the broader innate immune response.

[Molecular evolution of physiologically functioning anti-retroviral APOBEC3 deaminases]

Recent in vivo findings clearly indicate that mammalian cytidine deaminase APOBEC3 can function as a physiological restriction factor to retrotransposons and infectious retroviruses. However, some retroviruses, including primate lentiviruses, have evolved to counter their natural host's APOBEC3. To survive this arms race, primates seem to have acquired multiple copies of APOBEC3 genes. Surprisingly, however, during the process of the diversification of rodent species, as well as the human race, some ancestral individuals acquired genetic variants that reduced the protein levels of APOBEC3 expression, and these variants currently show unexpectedly wide geographic distributions. These data suggest that in the absence of a heavy burden of infectious retroviruses, high-level expression of APOBEC3 cytidine deaminase might be costly to the integrity of the host genome.

A single nucleotide polymorphism in tetherin promotes retrovirus restriction in vivo

Tetherin is a membrane protein of unusual topology expressed from rodents to humans that accumulates enveloped virus particles on the surface of infected cells. However, whether this ‘tethering’ activity promotes or restricts retroviral spread during acute retrovirus infection in vivo is controversial. We report here the identification of a single nucleotide polymorphism in the Tetherin gene of NZW/LacJ (NZW) mice that mutated the canonical ATG start site to GTG. Translation of NZW Tetherin from downstream ATGs deleted a conserved dual-tyrosine endosomal sorting motif, resulting in higher cell surface expression and more potent inhibition of Friend retrovirus release compared to C57BL/6 (B6) Tetherin in vitro. Analysis of (B6×NZW)F₁ hybrid mice revealed that increased Tetherin cell surface expression in NZW mice is a recessive trait in vivo. Using a classical genetic backcrossing approach, NZW Tetherin expression strongly correlated with decreased Friend retrovirus replication and pathogenesis. However, the protective effect of NZW Tetherin was not observed in the context of B6 Apobec3/Rfv3 resistance. These findings identify the first functional Tetherin polymorphism within a mammalian host, demonstrate that Tetherin cell surface expression is a key parameter for retroviral restriction, and suggest the existence of a restriction factor hierarchy to counteract pathogenic retrovirus infections in vivo.

Significant portions of the human and mouse genomes are comprised of retroviral sequences, revealing the long history of conflict between mammalian hosts and retroviruses that led to the evolution of host restriction factors. Nucleotide mutations in restriction factor genes provide a glimpse of this ongoing evolutionary process, but studies that directly probe the impact of restriction factor mutations during retrovirus infection are limited. In this study, we identified a single nucleotide mutation in the Tetherin host restriction gene that resulted in retention of Tetherin on the cell surface. In cell culture, Tetherin accumulates virions on the infected cell surface and prevents virion release, but some studies suggested that Tetherin might facilitate cell-to-cell virus spread. Our studies reveal that the Tetherin polymorphism inhibits retrovirus replication and disease. Thus, increased Tetherin cell surface expression enhanced the antiretroviral function of Tetherin. These results could have important implications in harnessing the biology of Tetherin for controlling pathogenic retroviruses such as HIV-1.

Two genetic determinants acquired late in Mus evolution regulate the inclusion of exon 5, which alters mouse APOBEC3 translation efficiency

Mouse apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like editing complex 3 (mA3), an intracellular antiviral factor, has 2 allelic variations that are linked with different susceptibilities to beta- and gammaretrovirus infections among various mouse strains. In virus-resistant C57BL/6 (B6) mice, mA3 transcripts are more abundant than those in susceptible BALB/c mice both in the spleen and bone marrow. These strains of mice also express mA3 transcripts with different splicing patterns: B6 mice preferentially express exon 5-deficient (Δ5) mA3 mRNA, while BALB/c mice produce exon 5-containing full-length mA3 mRNA as the major transcript. Although the protein product of the Δ5 mRNA exerts stronger antiretroviral activities than the full-length protein, how exon 5 affects mA3 antiviral activity, as well as the genetic mechanisms regulating exon 5 inclusion into the mA3 transcripts, remains largely uncharacterized. Here we show that mA3 exon 5 is indeed a functional element that influences protein synthesis at a post-transcriptional level. We further employed in vitro splicing assays using genomic DNA clones to identify two critical polymorphisms affecting the inclusion of exon 5 into mA3 transcripts: the number of TCCT repeats upstream of exon 5 and the single nucleotide polymorphism within exon 5 located 12 bases upstream of the exon 5/intron 5 boundary. Distribution of the above polymorphisms among different Mus species indicates that the inclusion of exon 5 into mA3 mRNA is a relatively recent event in the evolution of mice. The widespread geographic distribution of this exon 5-including genetic variant suggests that in some Mus populations the cost of maintaining an effective but mutagenic enzyme may outweigh its antiviral function.

Susceptibility to acutely leukemogenic Friend virus (FV) retrovirus infection varies among different mouse strains and is governed by several genetic factors, one of which is allelic variations at the mouse Apobec3 locus. FV-resistant C57BL/6 (B6) mice express higher amounts of Apobec3 transcripts than susceptible BALB/c mice. We previously showed that the differences in N-terminal amino acid sequences between B6 and BALB/c APOBEC3 proteins partly account for the distinct antiretroviral activities. In addition, B6 and BALB/c mice express major Apobec3 transcripts of different sizes: the exon 5-lacking and the full-length transcripts, respectively. Here we asked if exon 5 has any role in the antiviral activity of mouse APOBEC3 and found that the presence of this exon resulted in a profound decrease in the efficiency of protein synthesis without affecting the mRNA expression levels. We also identified two genomic polymorphisms that control the inclusion of exon 5 into the Apobec3 message: the number of TCCT repeats in intron 4 and a single nucleotide polymorphism within exon 5. The distribution of these functional polymorphisms among Mus species and wild mouse populations indicates that the exon 5 inclusion occurred recently in Mus evolution, and the full-length variant may have selective advantages in some mouse populations.

Multifaceted Antiviral Actions of Interferon-stimulated Gene Products

Interferons (IFNs) are extremely powerful cytokines for the host defence against viral infections. Binding of IFNs to their receptors activates the JAK/STAT signalling pathway with the Janus kinases JAK1, 2 and TYK2 and the signal transducer and activators of transcription (STAT) 1 and STAT2. Depending on the cellular setting, additional STATs (STAT3-6) and additional signalling pathways are activated. The actions of IFNs on infected cells and the surrounding tissue are mediated by the induction of several hundred IFN-stimulated genes (ISGs). Since the cloning of the first ISGs, considerable progress has been made in describing antiviral effector proteins and their many modes of action. Effector proteins individually target distinct steps in the viral life cycle, including blocking virus entry, inhibition of viral transcription and translation, modification of viral nucleic acids and proteins and, interference with virus assembly and budding. Novel pathways of viral inhibition are constantly being elucidated and, additionally, unanticipated functions of known antiviral effector proteins are discovered. Herein, we outline IFN-induced antiviral pathways and review recent developments in this fascinating area of research.

Noninfectious retrovirus particles drive the APOBEC3/Rfv3 dependent neutralizing antibody response

Members of the APOBEC3 family of deoxycytidine deaminases counteract a broad range of retroviruses in vitro through an indirect mechanism that requires virion incorporation and inhibition of reverse transcription and/or hypermutation of minus strand transcripts in the next target cell. The selective advantage to the host of this indirect restriction mechanism remains unclear, but valuable insights may be gained by studying APOBEC3 function in vivo. Apobec3 was previously shown to encode Rfv3, a classical resistance gene that controls the recovery of mice from pathogenic Friend retrovirus (FV) infection by promoting a more potent neutralizing antibody (NAb) response. The underlying mechanism does not involve a direct effect of Apobec3 on B cell function. Here we show that while Apobec3 decreased titers of infectious virus during acute FV infection, plasma viral RNA loads were maintained, indicating substantial release of noninfectious particles in vivo. The lack of plasma virion infectivity was associated with a significant post-entry block during early reverse transcription rather than G-to-A hypermutation. The Apobec3-dependent NAb response correlated with IgG binding titers against native, but not detergent-lysed virions. These findings indicate that innate Apobec3 restriction promotes NAb responses by maintaining high concentrations of virions with native B cell epitopes, but in the context of low virion infectivity. Finally, Apobec3 restriction was found to be saturable in vivo, since increasing FV inoculum doses resulted in decreased Apobec3 inhibition. By analogy, maximizing the release of noninfectious particles by modulating APOBEC3 expression may improve humoral immunity against pathogenic human retroviral infections.

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.