Immunoglobulin gene usage and immunohistochemical characteristics of humanmonoclonal antibodies to the mitochondrial autoantigens of primary biliarycirrhosis induced in the XenoMouse

The immunodominant antimitochondrial antibody (AMA) response in primary bil iary cirrhosis (PBC) is directed against the E2 component of pyruvate dehyd rogenase (PDC-E2). The nature of the clonal selection process is unclear, a nd to address this issue, we took advantage of a transgenic technology, Xen oMouse, that contains 80% of the human immunoglobulin (Ig) variable gene re pertoire and can produce high-affinity human antibodies to virtually any im munogen without evidence of clonal bias. We immunized mice with PDC-E2 to o btain 13 HmAbs, including 4 IgG(2) and 9 IgM isotypes. Immunoglobulin gene analysis was unique and demonstrated a clonal bias; the immunoglobulin,gene usage was considerably different from other antibody responses analyzed in XenoMouse systems. Four of the 13 mAbs recognized the inner lipoyl domain of PDC-E2, 2 of 13 recognized the entire PDC-E2 molecule, 4 of 13 recognize d PDC-E2 and OGDC-E2, 1 of 13 recognized OGDC only, 1 recognized BCOADC-E2 only, and I recognized an unidentified 100-kd mitochondrial protein. Immuno histochemical staining using these HmAbs produced mitochondrial staining of septal bile ducts in both PBC and control livers. la gene analysis showed that 7 of 13 HmAbs used the V(H)3 and 4 of 13 used VH4 gene repertoire, res pectively. Three of 7 V(H)3 antibodies used the same Ig VH3-21 gene family found in human AMA from patients with PBC. The CDRs of these autoantibodies were slightly mutated when compared with the sequences present within the Ig germline genes. In conclusion, the XenoMouse not only recapitulates the unique specificity and restriction of PBC patients, but indicates that the autoantibodies are derived from a restricted clonal selection process. Such data suggest that the original immunogen leads to somatic mutation without subsequent development of determinant spreading.