Regulation of germline transcription in the immunoglobulin heavy chain locus
2004
During an immune response, activated B cells develop into high rate
immunoglobulin (Ig) secreting plasma cells. They also switch from
production of IgM to IgG, IgA or IgE. Specific isotypes of antibodies
have functional and structural features that make them particularly well
suited to defend against different types of pathogens. IgM is changed to
another isotype via class switch recombination (CSR) - a non-homologous
DNA recombination process. CSR is at least in part regulated at the
transcriptional level. The external stimuli (cytokines or cell-to-cell
contact) induce CSR to a particular constant heavy chain (CH) gene via
their ability to modulate germline (GL) transcription of a given CH gene.
The GL transcription is driven by GL promoters located upstream of each
CH region. The GL promoters contain multiple transcription-factor binding
sites and are tightly regulated during B cell differentiation. In
addition, several enhancer elements with developmentally regulated
activities are located in the IgH locus. The mouse 3' enhancers span a
region of more than thirty kilobases and consist of four DNAse
hypersensitivity sites - HS3a, HSI,2, HS3b and HS4. One possible function
of the 3' enhancers could be regulation of CSR. We aimed to clarify the
role of different IgH locus regulatory elements and transcription factors
in regulation of GL transcription. Much evidence indicated that T
cell-dependent activation of B lymphocytes is generated through the
interaction of CD40 (on B cells) with CD40 ligand (on activated T cells).
Therefore, using agonistic rat anti-mouse CD40 monoclonal antibody, we
investigated the role of CD40 signalling in CSR. We found that
stimulation of murine cells through CD40 induced GL y 1, y2b and low
levels of e transcripts. However, cells were expressing only IgG2b, but
not IgG1 or IgE. CD40, but not lipopolysaccharide (LPS), could induce GL
y I transcripts, although both CD40 and LPS signalling involve NF-kB
family proteins. We showed that LPS and CD40 stimulation induce different
NF-kB complexes binding to GL y I promoter. he increased levels of IgE in
atopic individuals could be associated with upregulation of GL e
transcription. The transcription factor STAT6 is an important activator
of the GL e promoter, induced by interleukin 4 (IL-4). We investigated
whether increased levels of IgE in allergic individuals may be associated
with alterations in the level or activation of STAT6. Our results
demonstrated for the first time that upon IL-4 signalling, STAT6
activation and basal GL E promoter activity differ in B cells from
different individuals. Although we did not find any association between
STAT6 activation and allergy, we do not exclude the possibility that
stronger activation of this transcription factor is associated with the
atopic phenotype. Next, we investigated regulation of GL transcription by
the IgH locus 3' enhancers. Previous studies have shown that the activity
of the HS1,2 enhancer was induced in B cells approximately at the same
differentiation stage when GL transcription is…
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