Spectrotype analysis of human ABO antibodies: evidence for different clonal heterogeneity of IgM, IgG, and IgA antibody populations

Clonal characteristics of ABO antibodies in 18 paired samples of serum and breast milk were analyzed by isoelectric focusing and affinity immunoblotting. Anti-A and/or -B (anti-A/B) IgM showed a uniform, polyclonal spectrotype with more than 20 bands and only minimal interindividual differences. In contrast, IgG spectrotypes were oligoclonal ( < 12 bands) and individually distinct. IgA of serum as well as of breast milk showed oligoclonal motifs of up to 15 bands, but with more interindividual variance in intensity than IgM. The bands did not appear at an identical pH in milk and serum samples. The uniformity of IgM spectrotypes could be due to an absence of somatic mutations and thus reflect the use of unmutated germline genes. The greater clonal heterogeneity of IgG as compared to IgA indicates a difference in isotype-switch regulation. Somatic hypermutation may be less active during the switch from IgM to IgA than during the switch to IgG, or the latter switch may be accompanied by a repertoire shift. Alternatively, the anti-A/B IgA and IgG antibody populations could be derived from different clonal precursors.