First AID (Activation-induced Cytidine Deaminase) Is Needed to Produce High Affinity Isotype-switched Antibodies

Ronda Bransteitter,Jessica L Sneeden,Sariah Allen,Phuong Pham,Myron F Goodman

doi:10.1074/jbc.r600006200

Ronda Bransteitter, Jessica L Sneeden + Show 3 more

Open Access

https://doi.org/10.1074/jbc.r600006200

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Abstract

An answer to the question "why don't you die when I sneeze?" resides in the ability of higher organisms to mount an aggressive defense in response to an invasion by foreign substances called antigens. In part this defense mechanism is accomplished through the synthesis of high affinity antibody (Ab) 2 proteins that effectively defuse the attack. Initially, lymphoid B cells rearrange immunoglobulin (Ig) genes using V(D)J recombination to generate a diverse repertoire of low affinity Abs. Subsequently these Ig genes undergo somatic hypermutation (SHM) and class switch recombination (CSR) to generate high affinity Ab molecules of different isotypes. Activation-induced cytidine deaminase (AID) is required to initiate both CSR and SHM. The current understanding of immunological diversification can be attributed to the contributions made by cellular and molecular immunologists with biochemical enzymology recently entering the arena. This minireview discusses advances in the understanding ofAb diversification from a biochemical perspective emphasizing the enzymatic properties of AID in initiating SHM and CSR and exploring the role of downstream mutation fixation events mediated by mismatch repair (MMR), basic excision repair (BER), and error-prone DNA polymerases (EP pols).

Highlights

Activated germinal center B cells mutate and rearrange their Ig genes to produce high affinity Abs of different isotypes using two tightly regulated mechanisms, somatic hypermutation (SHM) and class switch recombination (CSR)
Activation-induced cytidine deaminase (AID) acting on ssDNA substrates explains why transcription is required for both SHM and CSR and how transcription rates affect the rate of SHM mutations
A reduction in processivity could result in fewer mutations and might help to explain why B cells expressing only S38A AID mutants have diminished rates of CSR and SHM

Summary

Defining Features of SHM and CSR

Activated germinal center B cells mutate and rearrange their Ig genes to produce high affinity Abs of different isotypes using two tightly regulated mechanisms, SHM and CSR. Antibody; Ig, immunoglobulin; V, variable region; C, constant region; S, switch region; AID, activation-induced DNA cytidine deaminase; SHM, somatic hypermutation; CSR, class-switch recombination; EP pol, error-prone DNA polymerase; PKA, protein kinase A; iE, intronic enhancer; MAR, matrix attachment region; RPA, human single-stranded binding protein replication protein A; UDG, uracil glycosylase; MMR, mismatch repair; BER, base excision repair; PCNA, proliferating nuclear cell antigen; dsDNA, double-stranded DNA; ssDNA, single-stranded DNA. AID is both necessary and sufficient to induce SHM and CSR in B cells, hybridoma B cells, and fibroblasts [10, 11], and expression of AID in Escherichia coli induces hypermutation on a target chromosomal gene [12] It was not clear if AID was acting on dsDNA, DNA/RNA hybrids, secondary DNA structures, or RNA. Similar phenomenon are seen on artificial substrates incubated with AID in vitro indicating that these mutational events observed in vivo are attributed to the specificity of AID and its random binding to DNA [14, 16]

AID Is Targeted to Ig Loci during Transcription

AID Expression and Activity in B Cells Is Tightly Regulated

Findings

Biochemical Perspective