Clonal analysis of the BALB/c secondary B cell repertoire specific for a self-antigen, cytochrome c.

mAb to rat cytochrome c (cyt c), totaling 556, were produced by individual clones of secondary B lymphocytes from nine groups of five BALB/c mice each in vitro using the splenic focus culture system. Inasmuch as rat and mouse cyt c are identical, these B cells can be considered specific for a self-antigen. The mAb were categorized into specificity groups based on their reactivities with a panel of seven cyts c that differ at two to six amino acid residues. The number of distinct specificities for the native protein was restricted to fewer than 20. Different groups of mice expressed the same specificities at comparable frequencies, including a single dominant one, and the total number of secondary cyt c-specific B cells was constant among groups of mice. This suggests that the acquisition of the secondary B cell specificity repertoire for this self-antigen is regulated. However, it is indeed possible that each specificity group may comprise a number of distinct mAb molecules that have arisen stochastically. Specificities expressed by as few as 1% of the total mAb were observed. Thus, it is likely that the identified specificities reflect the secondary B cell specificity repertoire for rat cyt c. The dominant specificity expressed by 50% of the mAb was characterized by elimination of antigen recognition as a result of replacement of aspartic acid by glutamic acid at position 62. Minor specificities expressed by 19% of the mAb were characterized by more subtle affects of an amino acid change at position 62 and/or an amino acid substitution from rat cyt c at position 60. Antibodies in other specificity groups reacted with epitopes in the region of residues 44 and 47. Whereas substitutions at positions 44, 47, 60, and 62 eliminated recognition by most of the mAb, changes at position 92 and at 103 also appeared to affect the binding of some mAb in the region around residues 60 and 62. The amino acid residues implicated in the recognition by murine mAb of murine cyt c have been shown previously to be involved in the epitopes of foreign mammalian cyt c. Therefore, self-tolerance cannot fully explain the restriction of the epitopes to these regions on foreign mammalian cyt c.