Accelerated Systemic Autoimmunity in the Absence of Somatic Hypermutation in 564Igi: A Mouse Model of Systemic Lupus with Knocked-In Heavy and Light Chain Genes.

Gabrielle McDonald,Thereza Imanishi-Kari,Erik Selsing,Reiko Shinkura,Henry H. Wortis,Carlos O. Medina,John F. Kearney,Tasuku Honjo,Monika Pilichowska

doi:10.3389/fimmu.2017.01094

Abstract

564Igi mice have knocked-in immunoglobulin (Ig) heavy (H) and light (L) chain genes that encode an autoantibody recognizing RNA. Previously, we showed that these mice produce pathogenic IgG autoantibodies when activation-induced deaminase (AID) is expressed in pre-B and immature B cells but not when it is expressed only in mature B cells. AID has two functions; it is necessary for somatic hypermutation (SHM) and class switch recombination (CSR). To determine the role of each of these functions in the generation of pathogenic autoantibodies, we generated 564Igi mice that carry a mutant AID-encoding gene, Aicda (AicdaG23S), which is capable of promoting CSR but not SHM. We found that 564Igi AicdaG23S mice secreted class-switched antibodies (Abs) at levels approximately equal to 564Igi mice. However, compared to 564Igi mice, 564Igi AicdaG23S mice had increased pathogenic IgG Abs and severe systemic lupus erythematosus-like disease, including, glomerulonephritis, and early death. We suggest that in 564Igi mice SHM by AID changes Ig receptors away from self reactivity, thereby mitigating the production of autoantibody, providing a novel mechanism of tolerance.

Highlights

Various systemic autoimmune diseases exhibit a high frequency of antibodies (Abs) that recognize nucleic acid antigens, suggesting that nucleic acids may have properties that promote the breaking of B-cell tolerance
As these anti-RNA IgG Abs are pathogenic, they resemble those found in systemic lupus erythematosus (SLE) patients having RNA-specific Abs, indicating that the 564Igi mouse is an excellent model for unraveling the developmental mechanisms that give rise to autoantibodies that recognize nucleic acids, thereby providing insight into potential new targets for intervention in disease progression
There is no difference in the number of splenic or bone marrow (BM) B cells found in 564Igi and 564Igi AicdaG23S mice (Figures 1A–D)

Summary

INTRODUCTION

Various systemic autoimmune diseases exhibit a high frequency of antibodies (Abs) that recognize nucleic acid antigens, suggesting that nucleic acids may have properties that promote the breaking of B-cell tolerance. The production of these anti-RNA Abs is toll-like receptor 7 (TLR7) and TLR8 dependent [2, 3] As these anti-RNA IgG Abs are pathogenic, they resemble those found in SLE patients having RNA-specific Abs, indicating that the 564Igi mouse is an excellent model for unraveling the developmental mechanisms that give rise to autoantibodies that recognize nucleic acids, thereby providing insight into potential new targets for intervention in disease progression. TLR7 and TLR8, which recognize RNA, could potentially be stimulated by this internalized self-antigen [32], leading to the expression of Aicda, followed by premature CSR and the production of pathogenic IgG autoantibodies. In order to test this hypothesis, we developed a novel mouse 564Igi model with a knock-in mutation (G23S) in the Aicda gene (designated 564Igi AicdaG23S) This mutation leads to deficient SHM activity but has no apparent effect on CSR [35] and, thereby, separates the two AID functions. We used this mouse model to definitively determine the specific roles of SHM in central B cell tolerance

RESULTS

B Cells of 564Igi and 564Igi AicdaG23S Mice

B Cells in 564Igi AicdaG23S GCs Actively

DISCUSSION

EXPERIMENTAL PROCEDURES

ETHICS STATEMENT