Characterization of an IL-12 p40/p35 Truncated Fusion Protein That Can Inhibit the Action of IL-12

Interleukin-12 (IL-12), a potent inducer of interferon gamma (IFN gamma), is a heterodimeric protein consisting of p40 and p35 subunits whose expression is regulated independently. IL-12 is part of a cytokine family (currently consisting of IL-12, IL-23, IL-27, and IL-35) that can have profoundly different immunologic effects, despite sharing subunits. In constructing a single-chain fusion of p40 and p35, we discovered an insert corresponding to an intron in the gene encoding the p35 subunit that would result in a truncated form of p35 if translated. To test its possible role, we constructed, expressed, and analyzed fusions of p40 with the full-length or the truncated form of p35. The fusion protein containing the truncated p35 did not stimulate the proliferation of the IL-12-responsive cell line CTLL-2 nor did it induce IFN gamma or the chemokine IFN gamma-inducible protein 10 (IP-10, CXCL10) or monokine induced by IFN gamma (MIG, CXCL9) from spleen cells. In striking contrast, the full-length IL-12 p40/p35 fusion induced robust responses in both assays. Moreover, the truncated IL-12 fusion protein inhibited the action of the full-length IL-12 p40/p35 fusion in the proliferation assay and also blocked the induction of IFN gamma. These findings raise the possibility that alternative splicing may provide an additional regulatory mechanism for IL-12.