Molecular analysis of B‐cell differentiation in selective or partial IgA deficiency

SUMMARY Selective IgA deficiency is the most common form of primary immunodeficiency, the molecular basis of which is unknown. To investigate the cause of selective IgA deficiency, we examined what stage of B‐cell differentiation was blocked. DNA and RNA were extracted from three Japanese patients with selective IgA deficiency and three with a partial IgA deficiency. In selective IgA deficiency patients, I α germline transcript expression levels decreased and α circle transcripts were not detected. Stimulation with PMA and TGF‐ β 1 up‐regulated I α germline and α circle transcripts. In some patients, IgA secretion was induced by stimulation with anti‐CD40, IL‐4 and IL‐10. In partial IgA deficiency patients, I α germline, α circle transcripts and C α mature transcripts were detected in the absence of stimulation. Our findings suggest that the decreased expression level of I α germline transcripts before a class switch might be critical for the pathogenesis of some patients with selective IgA deficiency. However, in patients with a partial IgA deficiency, B‐cell differentiation might be disturbed after a class switch.