Molecular analysis of IgA deficiency. Evidence for impaired switching to IgA.

The most common form of primary immunodeficiency is IgA deficiency (IgAD). However, the molecular basis of this disease remains elusive. Therefore, to address this issue we made a systematic analysis of the molecular events leading to IgA production. B lymphocytes that produce IgA have undergone somatic rearrangement that joins the switch (S) mu to S alpha region with deletion of the intervening sequences. Examination of the resulting S mu/S alpha junctions in unstimulated PBMC from IgAD patients by nested primer PCR revealed a significant decrease in the number of the S mu/S alpha fragments. To obtain the antisense primers to generate the S mu/S alpha fragments, we sequenced the human S alpha 1 and the downstream region extending to the C alpha 1 locus. Similar to previously reported switch sequences, we also found the S alpha 1 to be predominantly composed of pentameric repeats GAGCT and GGGCT. The decrease in the number of S mu/S alpha fragments is consistent with a profound decrease in the C alpha membrane mRNA expression in unstimulated PBMC, as well as in the C alpha mRNA levels and IgA production in PWM-stimulated PBMC. Sequence analysis of the switch junctions from IgA-producing cell lines, control donors, and an IgAD patient showed direct joining in 8 of 9 cases examined. TGF-beta 1, previously shown to be the switch factor for human and mouse IgA, was also examined. No difference in the TGF-beta 1 mRNA levels in unstimulated PBMC between the control subjects and the IgAD patients were detected. Our findings indicate that the failure to switch to IgA-producing B lymphocytes, or an impaired survival of such cells, may be an important molecular mechanism in IgAD.