Primary anti‐(4‐hydroxy‐3‐nitrophenyl)acetyl (NP) antibodies from C57BL/6 mice were idiotypically defined by a rabbit anti‐idiotypic antiserum. The idiotypic marker detected by this antiserum (the NPb idiotype) requires for its expression a specific heavy (H) chain‐lambda (Λ) light (L) chain combination as shown in chain reassociation experiments. The idiotypic binding reaction is inhibitable by hapten. BALB/c and CBA mice also produce large amounts of Λ‐bearing anti‐NP antibodies, but these do not express the NPb idiotype as defined by the rabbit antiserum. Since the NPb idiotype could be reconstituted by reassociating C57 BL/6 anti‐NP antibody H chains with either HOPC 1, or BALB/c or CBA anti‐NP antibody Λ chains, the absence of the NPb idiotype in BALB/c and CBA antibodies implies that these animals express different sets of H chain variable (V) regions in the anti‐NP response. SJL mice, in contrast to other mouse strains with the Ig‐1b allotype, express only traces of NPb idiotype in their anti‐NP antibodies, the L chains of which are almost exclusively of the χ type. Knowing that SJL mice possess a regulatory gene responsible for the low level of Λ (Λ10) chains in their immunoglobulins, we have analyzed the progeny of (SJL × BALB/C)F1, (SJL × BALB/C)F1 × BALB/C and (SJL × BALB/C)F1 × SJL for the expression of Λ‐bearing anti‐NP antibodies and NPb idiotype. In accord with previous work (SJL × BALB/c)F1 mice produced substantial amounts of anti‐NP antibodies with the NPb idiotype. Backcross of these mice to BALB/c revealed linkage of the NPb idiotype to the Ig‐1b allotype, in agreement with previous data. Backcross of the F1 animals to SJL demonstrated control of the NPb idiotype by the gene regulating Λ chain expression in serum immunoglobulin. Chain reassociation experiments showed that in SJL mice, the regulation against Λ chain expression prevents the selection in the anti‐NP response of those H chains that together with Λ chains constitute the NPb idiotype. Apparently, these H chains are unable to contribute to a binding site with specificity for the NP hapten when combined with χ chains. Copyright © 1979 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim
