Systemic lupus erythematosus (SLE) is characterized by high-avidity IgG antinuclear antibodies (ANAs) that are almost certainly products of T cell–dependent immune responses. Whether critical amino acids in the third complementarity-determining region (CDR3) of the ANA originate from V(D)J recombination or somatic hypermutation (SHM) is not known. We studied a mouse model of SLE in which all somatic mutations within ANA V regions, including those in CDR3, could be unequivocally identified. Mutation reversion analyses revealed that ANA arose predominantly from nonautoreactive B cells that diversified immunoglobulin genes via SHM. The resolution afforded by this model allowed us to demonstrate that one ANA clone was generated by SHM after a VH gene replacement event. Mutations producing arginine substitutions were frequent and arose largely (66%) from base changes in just two codons: AGC and AGT. These codons are abundant in the repertoires of mouse and human V genes. Our findings reveal the predominant role of SHM in the development of ANA and underscore the importance of self-tolerance checkpoints at the postmutational stage of B cell differentiation.

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.

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.

A spontaneous, autoreactive autoantibody called SN5–18 (IgG2b, κ) binds to a complex of H2A/H2B/dsDNA in chromatin, but erroneously appears to bind dsDNA when the Ab is used in a form that is not highly purified. Because of this finding, we evaluated the antigenic specificity of a prototypic anti-dsDNA Ab, 3H9/Vκ4, now used widely in transgenic studies of tolerance and autoimmunity. We found that the purified mAb 3H9/Vκ4 binds chromatin and specifically a complex of H2A/H2B/dsDNA, but not dsDNA in solid phase or in solution. When used in the form of culture supernatant or as a standard protein G preparation, mAb 3H9/Vκ4 appears to bind dsDNA, apparently due to nuclear proteins in the preparation that assemble on target DNA. Because of the reported role of VHCDR3 Arg residues in dsDNA binding and the near identity of the SN5–18 sequence to other dsDNA-specific Ab, we tested the contributions of two VHCDR3 Arg residues in SN5–18 to chromatin specificity. We found that both these Arg residues at positions 104 and 106 were required for detectable chromatin binding. These results indicate a role for VHCDR3 Arg residues in chromatin specificity of lupus-derived autoantibodies. Further, they provide an explanation for a possible discrepancy in the form of tolerance observed in different anti-DNA Ig transgene models.