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, PCG1-1 V-H is encoded by the M141 gene and is unrelated in sequence or structure to that of T15 V(H)1, 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(H)1 and V-H M141 genes demonstrates that impaired binding and secretion 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.