Following the generation of double-stranded breaks in switch regions, general cellular mechanisms for repairing these breaks lead to the nonhomologous recombination between switch regions that results in class switching (see Fig. 10.21, fourth and fifth panels). The ends to be joined are brought together by the alignment of repetitive sequences common to the different switch regions, and rejoining of the DNA ends then leads to excision of all DNA between the two switch regions and the formation of a chimeric region at the junction. Loss of AID completely blocks class switching, but deficiency of UNG in both mice and humans severely impairs class switching, suggesting sequential actions of AID and UNG in generating DNA breaks. Joining of DNA ends is probably mediated by classic nonhomologous end joining (as in V(D) J recombination) as well as by a poorly understood alternative end-joining pathway. Class switching is sometimes impaired in the disease ataxia telangiectasia, which is caused by mutations in the DNA-PKcs-family kinase ATM, a known DNA repair protein. The role of ATM in class switching is not yet entirely clear, however.
10-12 Cytokines made by TFH cells direct the choice of isotype for class switching in T-dependent antibody responses.
Now that we understand the general mechanisms that control DNA rearrangements of class switching, we are ready to explain how a particular heavy-chain is selected during an immune response. It is the choice of antibody isotype that ultimately determines the effector function of antibodies, and we will see that this choice is largely controlled by the cytokines that are produced by TFH cells in the germinal center reaction.
As discussed above, interactions between germinal center B cells and TFH cells are essential for class switching to occur. The required interactions occur through the interplay of CD40 on B cells with CD40 ligand on activated helper T cells. Genetic deficiency of CD40 ligand greatly reduces class switching and causes abnormally high levels of plasma IgM, a condition known as hyper IgM syndrome. People with this defect lack antibodies of classes other than IgM and exhibit severe humoral immunodeficiency, manifested as repeated infections with common bacterial pathogens. Much of the IgM in hyper IgM syndromes may be induced by thymus-independent antigens on the pathogens that chronically infect these patients. Nevertheless, people with CD40 ligand deficiency can make IgM antibodies in response to thymus-dependent antigens, which indicates that in the B-cell response, CD40L–CD40 interac tions are most important in enabling a sustained response that includes class switching and affinity maturation, rather than in the initial activation of B cells.