IgE regulation and lymphokine patterns in aging humans.

IgE production declines with age, and allergic symptoms tend to improve. Aging therefore represents an in vivo model to study IgE regulation. We compared IgE production in older (> or = 60 years old) and young (15 to 30 years old) nonatopic individuals. Addition of exogenous interleukin-4 (IL-4) to mononuclear cells from older and young subjects induced equivalent amounts of IgE, indicating that IL-4 responsiveness is preserved in aging. After surface receptor stimulation with concanavalin A, IL-4 production by mononuclear cells from older subjects was approximately 50% as compared with the young, whereas interferon-gamma (IFN-gamma) production was reduced threefold (p = 0.008). By contrast, stimulation with phorbol esters and ionophore, which bypass surface receptor signaling, induced comparable amounts of IL-4 and IFN-gamma in older and young subjects. These data point to an impairment in T-cell membrane signal transduction in older individuals. This hypothesis was directly confirmed by showing that Ca+2 fluxes after CD3 crosslinking were significantly (p = 0.014) decreased in the older population. Our findings altogether suggest that an age-dependent T-cell activation defect may result in decreased availability of IL-4 and in the waning of IgE responses.

Deletional switch recombination occurs in interleukin-4-induced isotype switching to IgE expression by human B cells.

There is controversy as to whether deletional rearrangement occurs between the IgM and IgE switch regions (S mu and S epsilon, respectively) during switching to the IgE isotype. We have addressed the issue by stimulating normal human B cells, sorted for lack of expression of surface IgE, to produce IgE by infection with Epstein-Barr virus (EBV) in the presence of interleukin 4 (IL-4). Genomic DNA was amplified for S mu/S epsilon switch junction fragments by utilizing the nested-primer polymerase chain reaction. Switch junction fragments were amplified from B cells infected with EBV in the presence of IL-4 but not from B cells infected with EBV alone. The DNA sequence of these "switch fragments" revealed direct joining of S mu to S epsilon in each case. The recombination sites within S mu were clustered within 900 base pairs at the 5' end of the switch region, suggesting that there are "hot spots" for recombination within S mu. The S epsilon recombination sites were scattered throughout the S epsilon region. These findings indicate that IL-4-induced isotype switching to IgE production in human B cells is accompanied by DNA rearrangements with joining of S mu to S epsilon.

Hydrocortisone and IL-4 induce IgE isotype switching in human B cells.

Induction of IgE synthesis in human B cells requires two signals. The first signal is delivered by the cytokine IL-4. The second signal activates B cells and is delivered by T cells, EBV infection, or engagement of the B cell-specific Ag CD40. Hydrocortisone (HC) has recently been shown to synergize with IL-4 to induce IgE synthesis in CD5+ chronic lymphocytic leukemia B cells. We show herein that a combination of HC and rIL-4 induces IgE synthesis in highly purified normal peripheral blood B cells. HC and IL-4 acted directly on B cells, because T cells and monocytes were not required for IgE synthesis. IgE induction was shown to occur in surface IgE- B cells isolated by cell sorting. These results suggest that IgE synthesis results from isotype switching, rather than from expansion of a precommitted B cell population. Furthermore, IgE synthesis was induced in sorted CD5- B cells, indicating that the ability to produce IgE in response to HC and IL-4 is not constrained by CD5 expression. Endogenous IL-6 was critical for induction of IgE synthesis by HC and IL-4, because an anti-IL-6 antibody strongly inhibited IgE production. These data suggest that hormones may play an important role in the regulation of IgE synthesis.