Analysis of somatic hypermutations in the IgM switch region in human B cells

J Allergy Clin Immunol. 2014 Aug;134(2):411-9. doi: 10.1016/j.jaci.2014.02.043. Epub 2014 May 15.


Background: The molecular mechanism of class-switch recombination (CSR) in human subjects has not been fully elucidated. The CSR-induced mutations occurring in the switch region of the IgM gene (Smu-SHMs) in in vitro CSR-activated and in vivo switched B cells have been analyzed in mice but not in human subjects.

Objective: We sought to better characterize the molecular mechanism of CSR in human subjects.

Methods: Smu-SHMs were analyzed in vitro and in vivo by using healthy control subjects and patients with molecularly defined CSR defects.

Results: We found that Smu-SHMs can be induced in vitro by means of CSR activation in human subjects. We also found large amounts of Smu-SHMs in in vivo class-switched memory B cells, smaller (although significant) amounts in unswitched memory B cells, and very low amounts in naive B cells. In class-switched memory B cells a high frequency of Smu-SHMs was found throughout the Smu. In unswitched memory B cells, the Smu-SHM frequency was significantly decreased in the 5' part of the Smu. The difference between switched and unswitched B cells suggests that the extension of somatic hypermutation (SHM) to the 5' upstream region of the Smu might be associated with the effective induction of CSR. The analysis of the pattern of mutations within and outside the WRCY/RGYW (W, A/T; R, A/G; and Y, C/T) motifs, as well as the Smu-SHMs, in CD27(+) B cells from CD40 ligand (CD40L)-, activation-induced cytidine deaminase (AID)-, and uracil-DNA glycosylase (UNG)-deficient patients revealed the dependence of Smu-SHM on CD40L, AID, UNG, and the mismatch repair system in human subjects.

Conclusion: CD40L-, AID-, UNG-, and mismatch repair system-dependent Smu-SHMs and extension to the 5' region of Smu are necessary to accomplish effective CSR in human subjects.

Keywords: Activation-induced cytidine deaminase; antibody maturation; class-switch recombination; immunoglobulin switch region; somatic hypermutation; uracil-DNA glycosylase.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • 5' Flanking Region
  • Adult
  • Amino Acid Motifs
  • B-Lymphocytes / immunology
  • CD40 Ligand / deficiency
  • CD40 Ligand / genetics
  • CD40 Ligand / immunology*
  • Child
  • Cytidine Deaminase / deficiency
  • Cytidine Deaminase / genetics
  • Cytidine Deaminase / immunology*
  • Gene Expression Regulation
  • Humans
  • Immunoglobulin Class Switching*
  • Immunoglobulin M / genetics*
  • Immunoglobulin M / immunology
  • Immunologic Memory
  • Infant
  • Male
  • Molecular Sequence Data
  • Somatic Hypermutation, Immunoglobulin*
  • Uracil-DNA Glycosidase / deficiency
  • Uracil-DNA Glycosidase / genetics
  • Uracil-DNA Glycosidase / immunology*


  • Immunoglobulin M
  • CD40 Ligand
  • Uracil-DNA Glycosidase
  • AICDA (activation-induced cytidine deaminase)
  • Cytidine Deaminase