Activation-Induced Cytidine Deaminase Expression in Human B Cell Precursors Is Essential for Central B Cell Tolerance

Abstract

SUMMARY Activation-induced cytidine deaminase (AID), the enzyme mediating class switch recombination (CSR) and somatic hypermutation (SHM) of immunoglobulin genes, is essential for the removal of developing autoreactive B cells. How AID mediates central B-cell tolerance remains unknown. We report that AID enzymes were produced in a discrete population of immature B cells that expressed recombination-activating gene 2 (RAG2), suggesting that they undergo secondary recombination to edit autoreactive antibodies. However, most AID+ immature B cells lacked anti-apoptotic MCL-1 and were deleted by apoptosis. AID inhibition using lentiviral-encoded short hairpin (sh)RNA in B cells developing in humanized mice resulted in a failure to remove autoreactive clones. Hence, B-cell intrinsic AID expression mediates central B-cell tolerance potentially through its RAG-coupled genotoxic activity in self-reactive immature B cells.

Decreased somatic hypermutation induces an impaired peripheral B cell tolerance checkpoint

“…single IgG + memory B cells isolated from asymptomatic AID +/-subjects, a feature correlating with decreased AID expression in their B cells upon activation (28). These data are also in agreement with the identification of increased microhomology sequences in switched regions of AID +/-memory B cells, a characteristic associated with less efficient CSR through decreased AID activity (56).…”

“…Indeed, Aicda-KO mice and AID-deficient patients as well as asymptomatic AID +/-individuals display an impaired central B cell tolerance, resulting in the production of large numbers of autoreactive B cells that migrate from the bone marrow into the periphery (8,28,32). However, we previously reported that autoreactive clones can be prevented from colonizing the mature naive B cell compartment when Tregs are functional, despite the impaired removal of developing autoreactive immature B cells in the bone marrow (16).…”

“…However, we previously reported that autoreactive clones can be prevented from colonizing the mature naive B cell compartment when Tregs are functional, despite the impaired removal of developing autoreactive immature B cells in the bone marrow (16). In addition, AD-AID patients show only a defective peripheral B cell tolerance checkpoint, since their central B cell tolerance was found to be functional, thereby excluding a contribution from the bone marrow to peripheral selection impediments in subjects with AICDA gene mutation(s) (28). For those reasons, we sought to determine which factors control the peripheral B cell tolerance checkpoint that is defective when SHM is impaired.…”

Mutations, kataegis and translocations in B cells: understanding AID promiscuous activity

“…Yet, the overlap between AID off-targets in mouse and human B cells is less than 50% 17 . Interestingly, a recent report showed that AID is also expressed in self-reactive bone marrow B cells, and proposed that, when combined with RAGs, AID genotoxic activity might help remove autoreactive clones from the B cell compartment 44 . The idea is supported on the observation that B cells treated with shRNAs against AID fail to undergo central tolerance when they recombine self-specificities in humanized mice 44 .…”

“…Interestingly, a recent report showed that AID is also expressed in self-reactive bone marrow B cells, and proposed that, when combined with RAGs, AID genotoxic activity might help remove autoreactive clones from the B cell compartment 44 . The idea is supported on the observation that B cells treated with shRNAs against AID fail to undergo central tolerance when they recombine self-specificities in humanized mice 44 . We note that under this scenario the precise genes where off-targeting damage occurs become irrelevant, so long as the damage induces apoptosis.…”