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Cord blood B cells are mature in their capacity to switch to IgE‐producing cells in response to interleukin‐4 in vitro

Neonatal B cells have been considered immature because of their impaired capacity to produce immunoglobulins in response to polyclonal activators in vitro. Here we demonstrate that cord blood mononuclear cells (MNC) produce normal levels of IgE in vitro when cultured in the presence of interleukin‐4 (IL‐4), indicating that the B cells are mature in their capacity to switch to IgE‐producing cells. However, in contrast to adult peripheral blood T cells, cord blood T cells failed to produce detectable levels of IL‐4 upon activation by phytohaemagglutinin (PHA) concanavalin A (Con A) or combinations of PHA and the phorbol ester TPA. Interferon‐gamma (IFN‐γ) production by cord blood T cells following activation by Con A or PHA was also strongly reduced. However, high levels of IFN‐γ, significantly higher than those produced by adult T cells, were synthesized in response to combinations of PHA and TPA, indicating that IFN‐γ production by cord blood T cells is not intrinsically defective. In contrast, cord blood T cells produced levels of IL‐2 that were significantly higher than those obtained by adult T cells tested in parallel. Collectively, our data indicate that the minimal levels of IgE production measured in cord blood (< 1 U/ml) are not due to immaturity of the cord blood B cells, but may be associated with the failure of cord blood T cells to produce detectable levels of IL‐4, which has been shown to be responsible for induction of IgE synthesis both in vitro and in vivo.