טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentEdry )Kaduri( Efrat
SubjectSpontaneous and Induced Class Switch Recombination in
Developing B Lymphocytes
DepartmentDepartment of Medicine
Supervisor Professor Doron Melamed
Full Thesis textFull thesis text - English Version


Abstract

In B-lymphocytes, IgM receptors drive development and construction of a diverse naive repertoire. The great diversity of antigen receptors is achieved by somatic genomic alterations. B lymphocytes utilize three known genomic modifications, which function throughout B cell development and maturation in construction and shaping of the antibody repertoire. During early stages of development in the bone marrow, B cells undergo unique V(D)J recombination events to form the variable region exon of the B cell antigen receptor (BCR). In the periphery, upon BCR engagement, mature B cells modify their receptor by additional genomic alterations: class switch recombination (CSR) and somatic hypermutation (SHM). CSR directs antibody production towards the synthesis of effector antibodies: IgG, IgA and IgE, whereas the introduction of SHM into the Ig variable region genes aims to increase antibody diversity and to select high affinity clones. Studies suggest that CSR induction is limited to mature activated B cells in the germinal center. Recently, we have shown that CSR spontaneously occur in B lymphopoiesis. In addition we showed that developing B lymphocytes respond to CpG stimulation, undergo proliferation and secrete IgM autoantidodies. However, it is unknown whether these cells can undergo CSR as well. The major aim of this study is to determine the extent of both spontaneous and induced CSR during B cell development and the mechanism and regulation of this process. Here we show that developing B cells in the BM are competent to undergo induced CSR in response to mitogenic stimulation and secret IgG Abs. We demonstrate that both induced and spontaneous CSR occur in all stages of B cell development and generates aberrant joining of the switch junctions as revealed by:

  1. increased load of somatic mutations around the CSR breakpoints,
  2. reduced sequence overlaps at the junctions, and,
  3. excessive deletion of the switch region. These findings were more profound in spontaneous CSR. In addition we found that incidence of both spontaneous and induced CSR is increased in cells carrying VDJ rearrangements. Hence, our results reveal major differences between spontaneous and induced CSR in developing B cells and the CSR induced in mature B cells upon activation, suggesting a different type of regulation and/or expression of enzymes that facilitate CSR.