A novel B cell-mediated transport of IgE-immune complexes to the follicle of the spleen.

• Ag administered i.v. to mice along with specific IgE or IgG2a induces higher Ab- and CD4(+) T cell responses than Ag administered alone. The IgE effect is completely dependent on the low-affinity receptor for IgE, CD23, whereas the IgG2a effect depends on activating FcgammaRs. In vitro studies suggest that IgE/Ag is presented more efficiently than Ag alone to CD4(+) T cells by CD23(+) B cells and that IgG2a/Ag is presented by FcgammaR(+) dendritic cells (DCs). In this study, we investigate in vivo the early events leading to IgE- and IgG2a-mediated enhancement of immune responses. OVA administered i.v. in PBS in combination with specific IgE binds circulating B cells after 5 min and is found in B cell follicles bound to follicular B cells (CD23(high)) after 30 min. This novel B cell-dependent route of entry is specific for IgE because IgG2a-Ag complexes were trapped in the marginal zone. OVA-specific CD4(+) T cells were found at the T-B border in the T cell zones 12 h after immunization both with IgE/OVA or IgG2a/OVA and proliferated vigorously after 3 days. The findings suggest that IgE- and IgG2a-immune complexes are efficient stimulators of early CD4(+) T cell responses and that Ag bound to IgE has a specific route for transportation into follicles.

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