IFN-γ is a pleiotropic lymphokine that influences the isotypes of immunoglobulin secreted by B cells. IFN-γ inhibits the secretion of IgG3, IgG2b, IgG1, and IgE, and enhances the secretion of IgG2a. We have examined the mechanism of IFN-γ-mediated enhancement of IgG2a secretion in sorted populations of B cells and find that IFN-γ reproducibly stimulates a twofold increase in the precursor frequency of IgG2a-secreting cells in the sIgG2a+ population. Additionally, we find that IFN-γ does not induce an increase in the clone size of IgG2a-secreting cells. IFN-γ stimulates a twofold increase in the precursor frequency of IgG2a-secreting cells from sIgG- and unsorted B cells which can be attributed to an increase in IgG2a secretion from IgG2a-committed cells in these populations. Hence, under the culture conditions utilized in these studies, IFN-γ enhances IgG2a secretion from IgG2a-committed cells and does not induce a class switch.
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