Somatic Mutation in V_H Complementarity-Determining Region 2 and Framework Region 2: Differential Effects on Antigen Binding and Ig Secretion

The extent to which somatic mutation impairs the Ig complementarity-determining region (CDR) and framework region (FRW) structure/function is not clear. Previously, we found that the V_H CDR2 of the murine T15 Ab is highly sensitive to mutation; 56% (26 of 46) of Abs mutated in vitro had reduced or no Ag binding capability, and 9% were secretion impaired. Here we test whether the T15 V_H CDR2 structure is unique by mutating the V_H CDR2 of the anti-PC-protein murine Ab, PCG1-1. PCC1-1 V_H is encoded by the M141 gene and is unrelated in sequence or structure to that of T15 V_H1. The majority (54%, 20 of 37) of PCG1-1 mutants carrying one to five mutations in V_H CDR2 had reduced or abolished Ag binding, while 10% were secretion impaired. Taken together, mutational analysis of the V_H1 and V_H M141 genes demonstrates that impaired binding and may be common outcomes of CDR2 somatic mutation. We also tested the tolerance of the V_H FRW2 of T15 to mutation, expecting this sequence-conserved region to be highly sensitive to alterations. However, FRW2 accommodated many nonconservative changes, and only 12% (3 of 25) of secreted mutants had impaired Ag binding. Moreover, mutations in FRW2 caused secretion defects in 24% (8 of 33), a frequency twice that of V_H CDR2 mutants. A total of 16 unique secretion mutants have now been identified. These findings suggest that B cell losses from somatic mutation may be extensive and due to varied causes not all related to Ag binding.