Synthesis of IL-13 by human B lymphocytes: regulation and role in IgE production.

BACKGROUND: Our laboratory has demonstrated previously that human tonsillar B lymphocytes express IL-13 mRNA
OBJECTIVE: We sought to investigate IL-13 production by human B cells and the association between B cell-derived IL-13 and IgE secretion.
METHODS: Human B lymphocytes were isolated from tonsils and purified by means of rosetting with sheep RBCs or positive or negative selection with magnetic beads. They were stimulated with anti-CD40 antibodies with or without recombinant IL-4. Total mRNA was extracted, and IL-13 mRNA was measured by means of standard RT-PCR or by means of real-time PCR with commercially available primers. B cells were cultured with or without IL-13 neutralizing antibodies, and C epsilon transcripts and supernatant IgE levels were measured.
RESULTS: IL-13 mRNA was detected in human B lymphocytes stimulated with anti-CD40 antibodies and IL-4 or IL-2 but not in unstimulated B cells. Real-time PCR demonstrated a 10- to 15-fold increase in IL-13 mRNA, maximizing at 36 hours. IL-13 protein was detected from B lymphocytes on day 3 and accumulated through day 7. The synthesis of IL-13 required both CD40 and IL-4 stimulation. The presence of IL-13 was confirmed by means of intracellular staining of cultured B lymphocytes and antigen-stimulated nasal biopsy specimens from atopic individuals. Addition of IL-13 neutralizing antibodies to purified B-cell cultures inhibited IgE production by up to 80% and diminished IgE (C epsilon) transcripts by 50%.
CONCLUSION: Human B lymphocytes express IL-13 mRNA after ligation of CD40 and the addition of cytokines. Human B lymphocytes produce significant IL-13, and neutralization of IL-13 impairs IgE synthesis. IL-13 might be an important autocrine growth factor for IgE-producing B lymphocytes.