Junctional diversity of H and L chains allows the coexpression of two mutually exclusive idiotopes (IdI104 and IdI558).

The molecular basis for the unexpected coexpression of the individual Id (IdI)558 and IdI104 Id by anti-alpha(1-3) DEX antibody (Ab) (126.33 and 414.2) derived from the MPW wild mouse strain has been investigated by the comparison of the structures of their VH and V lambda 1 chain regions with those of two other MPW-derived Ab (262.9 and 16.3) expressing either IdI558 or IdI104 Id. Our data show that 262.9 and 16.3 Ab display identical V lambda 1 and very similar VH regions when compared with BALB/c anti-alpha (1-3) dextran Ab expressing IdI104 or IdI558, respectively. The two Ab (414.2 and 126.33) that express both IdI104 and IdI558 Id display two main features. First, their VH CDR3 are different from those found in IdI104 or IdI558 expressing anti-alpha(1-3) dextran Ab. Second, their V lambda 1 are identical to those from BALB/c origin except for the presence of an additional residue, a phenylalanine at position 95A of CDR3. This additional residue is encoded by the V lambda 1 gene segment and results from a hitherto undescribed V lambda 1-J lambda 1 junction. The alteration of the length of the V lambda 1 CDR3 loop, in conjunction with particular residues within VH CDR3, allows the coexpression of two Id that were found to be mutually exclusive in laboratory mice.