C57BL mice, upon immunization with chicken ),-globulin conjugated with (4- hydroxy-3-nitrophenyl) acetyl (NP), a produced anti-NP antibodies having higher affinity for a cross-reactive analogue, (4-hydroxy-5-iodo-3-nitrophenyl) acetyl (NIP),

C57BL mice, upon immunization with chicken ),-globulin conjugated with (4hydroxy-3-nitrophenyl) acetyl (NP), a produced anti-NP antibodies having higher affinity for a cross-reactive analogue, (4-hydroxy-5-iodo-3-nitrophenyl) acetyl (NIP), than for NP itself (1). Furthermore, these "heteroclitic" antibodies share a set of common idiotypes, the predominant idiotype in primary B6 anti-NP antibodies (NP b idiotype) that is composed ofidiotypically related but nonidentical antibody molecules (2). Primary C57BL anti-NP antibodies expressing NP b idiotype bear predominantly ~kl-light chain (3, 4). Both heteroclieity and NP b idiotype were inherited in a simple Mendelian fashion and were controlled by Igh-V genes linked to the Igh-1 b allele of the heavy chain linkage group (1, 3, 4). However, Igh-lb-bearing SJL mice, which have a genetic defect in the ability to produce normal levels of ~1 chain (5), produce either extremely low or undetectable levels of NP b idiotype (3, 4). SJL mice on the other hand produce normal levels of X2 chain (6). The gene controlling 2~2-chain synthesis is demonstrated to be on the same chromosome that regulates 2tl-chain production (7). This suggests that the expression of a particular idiotype requires synthesis of a particular X chain. There is evidence that expression of X-chain-bearing molecules requires the presence of the appropriate Igh-V gene or vice versa (5). Furthermore, amino acid sequences between the Jkl chain variable region (V~tl) and V~z exhibit greater homology than between VX and the g chain variable region (Vr) (8). Only eight framework residue differences were observed between the available VX1 and V~2 sequences. Moreover, the strong homology of V)q and VX2 genes was demonstrated by DNA sequence analyses (9, 10). This raises the possibility that SJL mice that possess the appropriate Igh-Np b gene(s) (3) may produce X2-bearing anti-NP antibody molecules idiotypically related to NP b idiotypes. Studies of the idiotypes of X2-bearing anti-NP antibodies are, however, limited by the small quantity of such antibodies produced in SJL and C57BL/6 (B6) mice (3). The present study focused on the idiotype (NP-1 idiotype) of an IgM anti-NP