The production of immunoglobulin E (IgE) is tightly regulated. This is evidenced by the fact that it comprises less than 0.0001% of serum Ig, and aberrant production causes atopic conditions, including allergy, rhinitis, and anaphylaxis. Interleu
IntroductionIn normal individuals, the concentration of serum immunoglobulin E (IgE) is approximately 100 ng/mL, 10 4 to 10 5 times lower than IgG and IgA, making it the least abundant serum Ig. 1 However, aberrant production of IgE, often resulting from dysregulated Th2 or deficient Th1 responses, is associated with numerous diseases, including atopy, allergy, asthma, eczema/ dermatitis, and parasitic infections. 2 Furthermore, extraordinarily high levels of IgE are present in patients with hyper-IgE syndrome (HIES), Ommen syndrome, Wiskott-Aldrich syndrome, Comel-Netherton syndrome, and immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome, diseases also associated with eczema, allergy, and atopy. 2,3 Thus, to maintain IgE at normal (nonpathogenic) levels, its production must be strictly regulated. Identification of modulators of IgE production is therefore critical for understanding the pathogenesis of, and designing novel therapeutics for, conditions such as allergy and atopy.It is well established that interleukin-4 (IL-4) induces IgE production by human and murine B cells. 4,5 Human B cells also produce IgE when stimulated with IL-13. 2,6,7 Several factors, such as interferon-␥ (IFN-␥), IFN-␣, transforming growth factor-, IL-10, and prostaglandins, have been identified that antagonize IL-4-induced IgE production by human and murine B cells. 2,4,5,[8][9][10] Recent studies have also revealed that IL-21 inhibits IL-4-induced IgE secretion by murine B cells. [11][12][13][14][15] The increased levels of serum IgE in IL-21R-or IL-21-deficient mice 11,13 most probably results from the ability of IL-21 to directly inhibit isotype switching to IgE by their B cells. 12 Despite these findings, the effects of IL-21 on IgE secretion by human B cells remain incompletely characterized. On one hand, IL-21 inhibited IL-4-induced IgE secretion in cultures of total peripheral blood mononuclear cells (PBMCs) 16,17 by inducing apoptosis of B cells committed to producing IgE and/or inducing IFN-␥ production by non-B cells in the PBMCs. 14,17 On the other hand, IL-21 had no effect on IgE secretion by human B cells stimulated with anti-CD40 monoclonal antibody (mAb), 18,19 yet it did enhance IgE secretion induced by anti-CD40 mAb and IL-4. [16][17][18]20 Because of conflicting reports on the role of IL-21 in switching to IgE by human B cells, and potential differences in regulating IgE by human and murine B cells, we examined the effect of IL-21 on IgE secretion by human naive B cells. IL-21 induced IgE secretion by CD40L-stimulated naive B cells. Furthermore, addition of both IL-4 and IL-21 to CD40L-stimulated human B cells resulted in potent synergy, inducing 10-to 100-fold higher levels of IgE than those induced by IL-4 or IL-21 alone. One explanation for the int...