MOLECULAR EVOLUTION OF THE HUMAN IMMUNOGLOBULIN-E RESPONSE - HIGH-INCIDENCE OF SHARED MUTATIONS AND CLONAL RELATEDNESS AMONG EPSILON VH5 TRANSCRIPTS FROM 3 UNRELATED PATIENTS WITH ATOPIC-DERMATITIS

We have analyzed the nucleotide sequences of 19 epsilon V(H)5 transcripts derived from in vivo isotype switched peripheral blood B cells of three patients with atopic dermatitis. Comparison with the patients' own germline V(H)5 gene segments revealed that the epsilon transcripts were derived from both functional members of the human V(H)5 gene family and harbored numerous somatic mutations (range 5-36 per V(H)5 gene). In two patients, we detected clonally related but diverged transcripts, permitting the construction of a genealogical tree in one patient. We observed a high proportion of shared silent (S) and replacement (R) mutations among epsilon V(H)5 sequences derived from all three individuals, even among transcripts descending from the two different germline V(H)5 gene segments. A remarkably high number of these mutations is shared with previously reported V(H)5 genes encoding antibodies with defined specificities. The shared S mutations, and likely a fraction of the R mutations, appear to mark preferential sites (''hot spots'') of somatic hypermutations in human V(H)5 genes. The distribution of R and S mutations over complementarity determining region and framework regions in the majority of V(H) regions deviated from that characteristic of antigen-driven immune response. We hypothesize that the V regions of immunoglobulin E-bearing B cells have accumulated ''selectively neutral'' mutations over extended periods of clonal expansion, resulting in unusual R/S ratios. We propose that the molecular characteristics of the epsilon V(H) regions in atopic dermatitis may be representative of antigens that recurrently or chronically stimulate the immune system.