We have characterized circular DNA in mouse splenocytes treated with the mitogen lipopolysaccharide (LPS) and various cytokines, including transforming growth factor β (TGF-β) and interleukin 4 (IL-4). Using probes of immunoglobulin heavy chain constant genes (CH), excision products of class switch recombination were identified. The majority of the clones contained the 3′ portion of the switch μ (Sμ) region and the 5′ portion of other switch regions. Some clones contained 3′-Sγ sequences instead of 3′-Sμ. This indicates that isotype switching may occur not only from Cμ, but also from one of the Cγ genes to other CH genes further downstream. In the presence of LPS, the cytokine TGF-β enhanced the detection of 5′-Sα-positive clones, while the lymphokine IL-4 enhanced 5′-Sγ1 positives. The data support the notion that TGF-β and IL-4 can direct isotype-specific class switching.
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