Molecular evolution of the human immunoglobulin E response: High incidence of shared mutations and clonal relatedness among ∈ V_H5 transcripts from three unrelated patients with atopic dermatitis

We have analyzed the nucleotide sequences of 19 ∈ V_H5 transcripts derived from in vivo isotype switched peripheral blood B cells of three patients with atopic dermatitis. Comparison with the patients' own germline V_H5 gene segments revealed that the ∈ transcripts were derived from both functional members of the human V_H5 gene family and harbored numerous somatic mutations (range 5-36 per V_H5 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 ∈ V_H5 sequences derived from all three individuals, even among transcripts descending from the two different germline V_H5 gene segments. A remarkably high number of these mutations is shared with previously reported V_H5 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_H5 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 ∈ V_H regions in atopic dermatitis may be representative of antigens that recurrently or chronically stimulate the immune system.