Membrane expression of IgG but not maturation to secretion requires DNA replication

In this work we have addressed two questions. Does switch recombination occur before membrane expression or only concomitantly with induction to high rate synthesis and secretion of IgG? Does interleukin-4 induce switch to IgG1 or maturation of already switched cells? To answer these questions we used the DNA synthesis inhibitor hydroxyurea (HU) to analyze the requirements for DNA replication in the induction of membrane expression or high rate secretion of all IgG subclasses by cultures of mouse spleen cells stimulated by lipopolysaccharide (LPS) and supplemented with IL-4, the absolute numbers of cells bearing membrane bound IgG was always decreased by HU, indicating that immunoglobulin class switching requires DNA replication. IL-4 did not augment the numbers of cells expressing any IgG isotype. In contrast, the number of high rate secretors of all IgG isotypes was not affected by HU, except in the case of IgG3 and IgG2b shortly after stimulation. Addition of IL-4 resulted in an increased number of secretory cells, and also this effect was resistant to HU. Thus, for any isotype, treatment with HU or IL-4 increased the frequency of secretory cells relative to the surface positive population. This data indicates that: 1) IL-4 is a broad range, non isotype-specific maturation factor for LPS-activated B cells; 2) induction to high rate secretion has a negative effect on proliferation; and 3) immunoglobulin class switch, but not induction to secretion of any immunoglobulin isotype, requires DNA replication, suggesting that switch recombination had to occur before expression of IgG in the membrane-bound form.