Effects of IL-12 on in vivo cytokine gene expression and Ig isotype selection

The effects of murine rIL-12 on cytokine gene expression and Ig secretion were studied in vivo. In untreated mice IL-12 enhanced IFN-γ and IL-10 gene expression and protein secretion, reduced base line IL-3 and IL-4 gene expression, and increased serum IgG2a concentration. In mice that had been injected with goat anti-mouse IgD antibody (GαMδ) to induce increases in IL-3, IL-4, and IL-10 gene expression and serum IgE, IgG1, IgG2a, and IgG3 concentrations, the simultaneous injection of IL-12 enhanced IFN-γ and IL- 10 gene expression and suppressed IL-3 and IL-4 gene expression and serum IgG and IgE responses. Anti-IFN-γ mAb neutralized most, but not all, IFN-γ produced by mice treated with GαMδ and IL-12. Anti-IFN-γ mAb enhanced IL- 3 and IL-4 gene expression, did not affect IL-10 or IFN-γ gene expression, and increased serum IgG1, IgG2a, and IgG3 levels, but had relatively little effect on serum IgE in these mice. In contrast to its effects in GαMδ- treated mice. IL-12 failed to inhibit the IgE response to GαMε antibody, which stimulates mIgE⁺ B cells to secrete IgE. These observations demonstrate that: 1) IL-12 may limit its own effects by inducing the production of a cytokine (IL-10) that downregulates both IL-12 production and IL-12-induced IFN-γ production; 2) IL-12 inhibits the production of at least one cytokine, IL-3, that is not generally regarded to be strictly Th1- or Th2-associated; 3) IL-12 inhibits switching to IgE secretion to a greater extent than it inhibits switching to other Ig isotypes; and 4) the in vivo effects of IL-12 are, to a large extent, IFN-γ-dependent.