A new site-directed transgenic rheumatoid factor mouse model demonstrates extrafollicular class switch and plasmablast formation

Abstract

The AM14 rheumatoid factor (RF) transgenic (Tg) mouse has been valuable for studying how self-reactive B cells are regulated beyond central tolerance, because they remain ignorant in normal mice. AM14 B-cell activation can be studied on autoimmune-prone strains or by inducing activation with IgG2a anti-chromatin antibodies (Abs). Despite the utility of conventional Ig-Tg mice, site-directed Ig-Tg (sd-Tg) mice provide a more physiological model for B-cell responses, allowing class switch and somatic hypermutation. We report here the creation of an AM14 sd-Tg mouse and describe its phenotype on both normal and autoimmune-prone backgrounds. AM14 sd-Tg B cells develop normally but remain unactivated in the BALB/c background, even after significant aging. In contrast, in the autoimmune-prone strain MRL/lpr, AM14 sd-Tg B cells become activated and secrete large amounts of IgG RF Ab into the serum. Class-switched Ab-forming cells were found in the spleen and bone marrow. IgG RF plasmablasts were also observed in extrafollicular clusters in the spleens of aged AM14 sd-Tg MRL/lpr mice. Class switch and Ab secretion were observed additionally in AM14 sd-Tg BALB/c B cells activated in vivo using IgG2a anti-chromatin Abs. Development of IgG auto-Abs is a hallmark of severe autoimmunity and is related to pathogenesis. Using the AM14 sd-Tg, we now show that switched auto-Ab-forming cells develop robustly outside germinal centers, further confirming the extrafollicular expression of activation induced cytidine deaminase (AID). This model will allow more physiological studies of B-cell biology in the future, including memory responses marked by class switch.

Keywords::

• Plasmablast
• isotype switch
• lupus
• extrafollicular response

Acknowledgments

We thank Tim Nottoli and the Yale Animal Genomics Core of the Yale Comprehensive Cancer Center for their expert work in creating the site-directed Tg mice. We also thank the staff of the Yale Animal Resources Center for outstanding animal care. We thank Drs Qing Chen and Martin Weigert for providing the vectors from which the AM14-targeting vector was constructed.

Declaration of interest: This work was supported by NIH grants R01-AI073722, P01-AI/AR36529, and CA-016359. The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.