Class switch recombination in selective IgA-deficient subjects

Selective IgA deficiency is a common immunodeficiency in Caucasians, but the molecular basis of the disorder remains elusive. To address this issue we examined the molecular events leading to IgA production. Naive IgD positive B cells were purified from four donors with IgA deficiency and four control donors, all Caucasians. Stimulation of B cells from IgA-deficient donors with the cytokines transforming growth factor (TGF)-β, interferon (IFN)-γ or interleukin (IL)-10 in the presence of anti-CD40 antibodies showed reduced expression of both activation-induced cytidine deaminase (AID) and α germline transcripts (GLT) compared to controls. It was possible, however, to induce AID and α GLT when stimulating the cells with anti-CD40 antibody and TGF-β in the combination with IL-10. Moreover, in anti-CD40 antibody-stimulated cultures, addition of IL-10 or IL-10 + TGF-β in combination, induced IgA production, albeit lower than found in B cells from controls. The B cells from the IgA-deficient subjects were less effective in differentiating into CD138 ⁺ X-box binding protein 1 (XBP-1) ⁺ plasma cells when stimulated with TGF-β, IFN-γ or IL-10. Interestingly, when adding IL-4 to TGF-β alone or in combination with IL-10, the immunoglobulin production in B cells from IgA-deficient donors was comparable with those of normal controls. These data show that in healthy subjects in vitro IgA production can be up-regulated by addition of IL-10 to CD40-stimulated B cells, whereas a similar B cell differentiation does not occur in IgA-deficient subjects. Addition of IL-4, however, reverts this abnormality.