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* Department of Biotechnology, Faculty of Engineering, Okayama University, Tsushima-Naka, Okayama, Japan; and
Department of Surgery and Immunology, Mayo Clinic, Rochester, MN 55905
The quasi-monoclonal mouse has limited B cell diversity, whose
major (
80%) B cell Ag receptors are comprised of the knockin
VH 17.2.25 (VHT)-encoded H chain and the
1 or
2 L chain, thereby being specific for
4-hydroxy-3-nitrophenylacetyl. The p-nitrophenylacetyl
(pNP) was found to be a low affinity analog of nitrophenylacetyl. We
examined affinity maturation of anti-pNP IgG by analyzing mAbs
obtained from quasi-monoclonal mice that were immunized with this low
affinity Ag. The results are: 1) Although VHT/
1 and
VHT/
2 IgM were equally produced, VHT/
2
IgG almost exclusively underwent affinity maturation toward pNP. 2) A
common mutation in complementarity-determining region 3 of
VHT (T313A) mainly contributed to generating the
specificity for pNP. 3) Because mutated VHT-encoded
-chains could form
1-bearing IgG in Chinese hamster ovary cells,
apparent absence of VHT/
1 anti-pNP IgG may not be
due to the incompatibility between the
-chains and the
1-chain,
but may be explained by the fact that VHT/
1 B cells
showed 50- to 100-fold lower affinity for pNP than VHT/
2
B cells. 4) Interestingly, a pNP-specific IgM mAb that shared common
mutations including T313A with high affinity anti-pNP IgG was
isolated, suggesting that a part of hypermutation coupled with positive
selection can occur before isotype switching. Thus, even weak B cell
receptor engagement can elicit an IgM response, whereas only B cells
that received signals stronger than a threshold may be committed to an
affinity maturation process.
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