Downstream class switching leads to IgE antibody production by B lymphocytes lacking IgM switch regions.
Ig heavy chain (IgH) class-switch recombination (CSR) replaces the IgH C mu constant region exons with one of several sets of downstream IgH constant region exons (e.g., C gamma, C epsilon, or C alpha), which affects switching from IgM to another IgH class (e.g., IgG, IgE, or IgA). Activation-induced cytidine deaminase (AID) initiates CSR by promoting DNA double-strand breaks (DSBs) within switch (S) regions flanking the donor C mu (S mu) and a downstream acceptor C(H) (e.g., S gamma, S epsilon, S alpha) that are then joined to complete CSR. DSBs generated in S mu frequently are joined within S mu to form internal switch region deletions (ISD). AID-induced ISD and mutations have been considered rare in downstream S regions, suggesting that AID targeting to these S regions requires its prior recruitment to S mu. We have now assayed for CSR and ISD in B cells lacking S mu (S mu(-/-) B cells). In S mu(-/-) B cells activated for CSR to IgG1 and IgE, CSR to IgG1 was greatly reduced; but, surprisingly, CSR to IgE occurred at nearly normal levels. Moreover, normal B cells had substantial S gamma1 ISD and increased mutations in and near S gamma1, and levels of both were greatly increased in S mu(-/-) B cells. Finally, S mu(-/-) B cells underwent downstream CSR between S gamma1 and S epsilon on alleles that lacked S mu CSR to these sequences. Our findings show that AID targets downstream S regions independently of CSR with Smu and implicate an alternative pathway for IgE class switching that involves generation and joining of DSBs within two different downstream S regions before S mu joining.
Department of Genetics, Harvard Medical School, Boston, MA 02115, USA.
Tingting
T
Zhang
Andrew
A
Franklin
Cristian
C
Boboila
Amy
A
McQuay
Michael P
MP
Gallagher
John P
JP
Manis
Ahmed Amine
AA
Khamlichi
Frederick W
FW
Alt
D000818:Animals; D001402:B-Lymphocytes / Q000276:immunology*; D015139:Blotting, Southern; D003564:Cytidine Deaminase / Q000378:metabolism; D004252:DNA Mutational Analysis; D017931:DNA Primers / Q000235:genetics; D005434:Flow Cytometry; D006825:Hybridomas / Q000276:immunology / Q000378:metabolism; D017578:Immunoglobulin Class Switching / Q000276:immunology*; D007073:Immunoglobulin E / Q000096:biosynthesis* / Q000276:immunology; D007143:Immunoglobulin Heavy Chains / Q000235:genetics* / Q000276:immunology; D007075:Immunoglobulin M / Q000235:genetics; D007134:Immunoglobulin Switch Region / Q000235:genetics; D051379:Mice; D018345:Mice, Knockout; D000098830:AICDA (Activation-Induced Cytidine Deaminase)
D017931:DNA Primers; D007143:Immunoglobulin Heavy Chains; D007075:Immunoglobulin M; D007073:Immunoglobulin E; D000098830:AICDA (Activation-Induced Cytidine Deaminase); D003564:Cytidine Deaminase