Interleukin‐12 can directly induce T‐helper 1 responses in interferon‐γ (IFN‐γ) receptor‐deficient mice, but requires IFN‐γ signalling to downregulate T‐helper 2 responses

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Abstract
An in vivo model of pulmonary granuloma formation around embolized schistosome eggs was investigated as an environment in which to analyse a role for interleukin-12 (IL-12) in the differentiation of T-helper 1 (Th1) and Th2 subsets. Specifically, mice deficient for the interferon-γ receptor (IFN-γR−/–) were used to determine the role for IL-12 in the absence of IFN-γ-mediated signalling. We show that recombinant IL-12 administered to IFN-γR−/– mice caused the up-regulation of mRNA for IFN-γ in lung tissue, and the secretion of abundant IFN-γ by in vitro-cultured lymph node cells in response to egg antigens. This indicates that IL-12 can act independently of IFN-γ to induce the development of Th1 cells. Administration of rIL-12 to wild-type mice markedly reduced the secretion of Th2-associated cytokines, IL-4 and IL-5. However, these cytokines were not dramatically reduced in IFN-γR−/– mice treated with IL-12. We conclude that inhibition of these cytokines by IL-12 is primarily dependent upon effective IFN-γ signalling, although abrogation of T-cell derived IL-10 appeared to be dependent upon IL-12. We also show that increases in mRNA for the β2 subunit of the IL-12 receptor and the p40 subunit of IL-12 after rIL-12 treatment were lower in IFN-γR−/– mice, compared to wild-type mice, indicating that their expression was primarily dependent upon IFN-γ with only a minor role for IL-12.