Allelic differences in the V_HOx-1 gene explain the absence of a B cell clonal dominance in the primary response of C57BL/6 mice to phthalate

A highly conserved Id (CRI_xmp-1) associated with the murine (BALB/c) humoral immune response to the hapten phthalate (Xmp) is conspicuously absent in C57BL/6 mice. The absence of this Id in C57BL/6 mice is shown here to be due to the absence of the appropriate V_H gene (V_HOx-1) usage in the Xmp response. To determine whether the failure to utilize this V_H was due to an active suppression or to the lack of the requisite V_H gene in the available repertoire, V_HOx-1 gene-specific primers were used to amplify the germ-line V_HOx-1 gene from genomic DNA from BALB/c and C57BL/6 mice. The germ-line coding sequence of the C57BL/6 allele of the V_HOx-7 gene is 99% similar to the germ-line coding sequence of the BALB/c allele. Amplification of cDNA made from splenic RNA from C57BL/6 mice confirmed that this gene is expressed. There are four nucleotide differences that lead to three amino acid changes in the predicted protein sequence. Each change is either in or immediately adjacent to a complementarity-determining region (CDR). Two of these changes are unique to the C57BL/6 allele and are not shared with CRI_xmp-1-expressing strains. These two changes are predicted to alter the Xmp binding capabilities of the C57BL/6 allelic form of this V_H gene, thereby explaining the absence of the Xmp-1 clonotype, which is dominant in the primary Xmp immune response of most other strains of mice.