HEAVY CLASS SWITCHING
The first isotype of immunoglobulin secreted during the primary humoral response to an antigen is IgM. In order for genes of the other isotypes of Ig to be expressed, there is a second DNA rearrangement which occurs in B lymphocytes following antigenic stimulation. This rearrangement is termed heavy class switching (or isotype switching). This process occurs in Blymphocytes which are present within the germinal centers of secondary lymphoid organs.  The process is Th cell dependent and requires the synthesis of Th cell cytokines.

In this DNA rearrangement, a previously rearranged V_HD_HJ_H recombines with a different C_H gene segment. The precise mechanism is not yet clear, but it is known that there are specific switch sites located 2-3 kb upstream from each C_H gene segment with the exception of C d.

The switch site is composed of multiple copies of a short highly repetitive sequence. It has been hypothesized that there are switch site specific recombinases which recombine the DNA between two switch sites leading to the looping out and excision of the sequences between the two switch sites. These circular excision products have been identified in B lymphocytes undergoing heavy class switching.

Rearranged heavy chain genes in the mouse:

5'--L-V-D-J--s--Cmu----Cdelta--s-Cgamma3--s--Cgamma 1--s--Cgamma 2b--s--Cgamma2a--s--Cepsilon--s--Calpha--3'

s = switch site

Certain cytokines have been shown to regulate heavy class switching. For instance, IL-4 (an important T_h2 lymphocyte cytokine) has been shown to trigger the switch from Cm to Cg1 and the switch from Cg1 to Ce. In other words, when the Ig isotype switches from IgM to IgE there is a two-step rearrangement.

 Once functional rearrangements have occurred, Ig expression is regulated in large part by differential post-transcriptional processing of Ig primary transcripts.

*Poly A polymerase adds a poly A tail at a poly A adenylation site
*introns are excised
*exons are connected by RNA splicing

Now you have a mature mRNA which is ready to leave the nucleus and enter the cytoplasm so that it may be engaged by a ribosome for translation.

Heavy chain C gene segments are organized as a series of coding exons and non-coding introns. Each exon corresponds to a C _H domain.
 
For example: Cm contains:

C_H1 C_H2 C_H3 C_H4  exons

Production of different mRNAs from the same primary transcript

The secreted Ig and membrane-bound Ig differ in the amino acid sequences (in the carboxyl terminal domains) of the heavy chain.

Secreted form - has a terminal hydrophilic sequence of about 20 amino acids.
An S exon codes for this hydrophilic piece.

Membrane form - has an extracellular hydrophilic sequence followed by a hydrophobic transmembrane sequence, followed by a short hydrophilic cytoplasmic segment at the terminus (entire length ~41 amino acids).

M1 and M2 exons code for this 41 amino acid terminal sequence.

The primary transcript of a rearranged mu (IgM) heavy chain gene contains 2 polyadenylation sites [indicated by*]

5'--L-VDJ--CH1---CH2---CH3---CH4-S *---M1---M2*-3'

If cleavage of primary transcript and addition of poly A tail occurs at site 1, M1 and M2 are lost, leaving S, producing an mRNA encoding the secreted form of Ig,

If cleavage and polyadenylation occurs at site 2, mRNA encoding the membrane form of Ig is produced.
In mature [but naive] B cells only the membrane form is synthesized.
In plasma cells you have predominantly the secreted form.

Also due to differential RNA processing of a primary transcipt.

An Ig primary transcript contains both the Cm and Cd genes in very close proximity, with no switch region in between.

5'--L-VDJ--Cm--Cd--3'

Therefore, there are actually four possible poly A sites.

If sites 3 or 4 are used RNA splicing will remove the Cm exons and produce mRNA for membrane or secreted form of the IgD heavy chain.

A mature B cell expresses both IgM and IgD on its membrane so processing by both pathways must occur simultaneously.

H and L chains are encoded on different chromosomes and the mRNAs are translated on separate polyribosomes of RER (rough endoplasmic reticulum). Leader sequence guides the chains into the lumen of the RER where it (leader sequence) is removed. Assembly of 2L and 2H chains occurs as the chains pass through the RER.

The assembled immunoglobulin now leaves the RER in membrane-bound vesicles which transport it to the golgi apparatus. In the golgi, further post-translational modifications (such as glycosylation) occur and then the "completed" immunoglobulin is packaged into membrane-bound vesicles which then transport the Ig to the plasma membrane.

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