Enhanced immunoglobulin somatic hypermutation in human APOBEC3 transgenic mice

Abstract

Abstract Eliciting a broadly neutralizing antibody (bNAb) response remains a major hurdle in HIV-1 vaccine development, as a majority of these bNAbs are heavily mutated. Immunoglobulin (Ig) somatic hypermutation (SHM) is primarily driven by AID. However, our previous data in Friend retrovirus (FV) infected mice revealed that an evolutionarily-related deaminase, mouse APOBEC3 (mA3), may complement AID in mutating virus-specific Abs (Halemano et al, PNAS 2014). AID mutates primarily in an RC context (R=purine), whereas the A3s mutate in a YC context (Y=pyrimidine). Of note, mice encode only one A3 gene, but humans encode seven. To date, it remains unclear if the human APOBEC3 (hA3) deaminases can directly mutate Ig genes. To study the role of hA3s in Ig SHM, bacterial artificial chromosome (BAC)-transgenic mice expressing the 7 hA3s (hA3-Tg) were crossed onto an mA3 KO background. All hA3s were expressed in hA3-Tg B cells during FV infection. hA3-Tg (n=13) and mA3 KO (n=12) mice were vaccinated with live-attenuated FV and analyzed at 28 d. Surprisingly, the hA3-Tg mice have an impaired plasma NAb response (p=0.0003) despite similar virus-specific binding IgG titers (p>0.1). Illumina sequencing of bone marrow IgG transcripts showed increased deamination mutations (C to T or G to A) in hA3-Tg versus mA3 KO mice (p=0.0043). Interestingly, this increase was accounted for by AID-type (RC), but not hA3-type (YC), mutations. Our findings reveal that hA3 can increase IgG SHM rates following vaccination, but the initial impact on the vaccine NAb response may be detrimental. Studies are ongoing to monitor the development of virus-specific antibodies in hA3-Tg mice, as harmful mutations may be more prominent during early time points post-vaccination.