Integrating Signals from IFN-γ and IL-4 by B Cells: Positive and Negative Effects on CD40 Ligand-Induced Proliferation, Survival, and Division-Linked Isotype Switching to IgG1, IgE, and IgG2a

IL-4 and IFN-γ each have potent effects on B cell responses as well as strong mutual antagonism. Here we have examined the quantitative effects of these cytokines on CD40 ligand-induced B cell proliferation, cell survival, and division-linked isotype switching. Both IL-4 (strongly) and IFN-γ (weakly) enhanced the number of B cells found in culture by reducing the average time cells take to enter the first division cycle and by promoting B cell survival. When added in combination, the net effect of IL-4 and IFN-γ on time to division and survival was a response intermediate between that of the two cytokines alone, indicating a partial antagonism of IL-4 by IFN-γ. By modulating both time to division and cell survival, these small effects of IFN-γ are amplified and give rise to large reductions in cell number in the presence of IL-4. At higher concentrations, IFN-γ had minor inhibitory effects on IL-4-induced isotype switching to IgG1 and greater effects on IgE. A reciprocal relation was observed between the ability to inhibit IgE at late cell divisions vs induction of IgG2a. In contrast, IL-4 did not prevent switching to IgG2a induced by IFN-γ alone. Therefore, antagonism between IFN-γ and IL-4 is observed at multiple levels and over different concentration ranges, resulting in complex net outcomes. The evolutionary significance of this complexity is discussed.

Resistance to Antigen-Induced Airway Hyperresponsiveness Requires Endogenous Production of IL-12

We have demonstrated previously that susceptibility of murine strains to the development of allergic airway responses is associated with a type 2 cytokine pattern. In the present study, we examine the in vivo role of IL-12 in the immune response to allergen exposure in susceptible (A/J) and resistant (C3H/HeJ, C3H) strains of mice. OVA sensitization and challenge induced significant increases in airway reactivity in A/J mice as compared with their PBS-challenged controls, while no increases in airway reactivity were observed in OVA-challenged C3H mice. OVA exposure of A/J mice resulted in marked increases in the Th2 cytokines, IL-4 and IL-10, in the bronchoalveolar lavage fluid, whereas increases in IFN-γ were observed in C3H mice. Strikingly, anti-IL-12 mAb (1 mg/mouse) treatment resulted in threefold increases in airway reactivity in OVA-challenged resistant C3H mice, concomitant with significant increases in bronchoalveolar lavage levels of Th2 cytokines and decreases in IFN-γ. IL-12 depletion of C3H mice also suppressed OVA-specific serum IgG2a levels and increased both serum OVA-specific IgG1 and IgE levels. Blockade of endogenous IL-12 levels in susceptible A/J mice resulted in further augmentation of type 2 immune responses. These results demonstrate that endogenous production of IL-12 is essential for resistance to Ag-induced airway hyperresponsiveness, and furthermore, that dysregulation of IL-12 production may lead to the development of deleterious type 2 immune responses to inhaled allergens.