Noninfectious Retrovirus Particles Drive the Apobec3/Rfv3 Dependent Neutralizing Antibody Response

Abstract

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.

Requirement for Fc Effector Mechanisms in the APOBEC3/Rfv3-Dependent Neutralizing Antibody Response

“…At 7 dpi, the percentages of infected bone marrow cells were evaluated by flow cytometry using a monoclonal antibody (MAb) against the FV glyco-gag protein (Fig. 1A) (19). Compared to control sera from uninfected mice, mA3 ϩ/s antisera conferred significant protection, whereas mA3 Ϫ/s antisera did not (Fig.…”

“…2A). To test if infected mA3 ϩ/s mice produce higher titers of FV-specific IgG2 antibodies, we quantified virus-specific titers of the different isotypes and IgG subclasses by endpoint titration enzyme-linked immunosorbent assay (ELISA) (19). mA3 ϩ/s mice produced higher levels of virus-specific IgG2 against native virions than mA3 Ϫ/s mice but similar IgG1, IgG3, and IgM titers (Fig.…”

“…Thus, mA3 may directly enhance IgG2 class switching. Alternatively, the increased number of infectious virions in mA3 Ϫ/s mice (19,27) may lead to deregulated class switching. Further studies are needed to determine which Fc␥RIV-expressing cells are required for in vivo antibody neutralization.…”

Les exosomes

“…Par ailleurs, les virus défectifs interférents bloquent la réplication des virus infectieux, notamment en réquisitionnant la polymé-rase virale et d'autres facteurs pour leur propre réplication [3]. En outre, des particules rétrovirales non infectieuses contenant des génomes mutants générés par l'action de déoxycytidines déaminases -facteurs de restriction comme par exemple APOBEC3 -ont la capacité d'induire une réponse neutralisante, ce qui contribue au contrôle de la charge virale in vivo [16]. La majorité des virus codent pour des protéines qui bloquent la réponse antivirale dans les cellules qu'ils infectent (Figure 1).…”

Incorporation of Mouse APOBEC3 into Murine Leukemia Virus Virions Decreases the Activity and Fidelity of Reverse Transcriptase

“…Although exogenous murine leukemia viruses (MuLVs) are relatively insensitive to the actions of mA3 (2-7), several studies have reported partial inhibition of exogenous MuLVs after incorporation of mA3 (2,3,6,(8)(9)(10)(11). Furthermore, the finding that Rfv3, a resistance gene for Friend erythroleukemia, encodes mA3 and is responsible for a decreased infectious titer of the Friend MuLV (Fr-MuLV) (11)(12)(13) strongly suggests that mA3 inhibits the replication of exogenous MuLVs in vivo. Exogenous ecotropic MuLVs, such as the FrMuLV and Moloney MuLV (Mo-MuLV), are inhibited through mechanisms that do not appear to involve cytidine deamination (2,9,10).…”