INTERLEUKIN-12 PROFOUNDLY UP-REGULATES THE SYNTHESIS OF ANTIGEN-SPECIFIC COMPLEMENT-FIXING IGG2A, IGG2B AND IGG3 ANTIBODY SUBCLASSES IN-VIVO

The influence of the cytokine interleukin-12 (IL-12) on humoral immune responses was studied in vivo. CBA/J mice immunized with protein anti gens (keyhole limpet hemocyanin, phospholipase A(2)) adsorbed to alumi num hydroxide (Alum) develop a Th2-like immune response characterized by the production of large amounts of IgG1 as well as some IgE but lit tle IgG2a, IgG2b and IgG3 antibodies. IL-12 is a cytokine that promote s the development and the activation of Th1 cells. Th1 cells are invol ved in the induction of cellular immunity, which is characterized by l ow or absent antibody production. On the other hand, some Thl-like imm une responses are associated with a strong antibody production of the IgG2a, IgG2b and IgG3, subclasses. Thus, we investigated whether treat ment with IL-12 would down-regulate the humoral immune response or sti mulate antibody production of the IgG2a, IgG2b and IgG3 subclasses. We observed that: 1) administration of IL-12 to mice together with prote in antigens adsorbed to Alum strongly enhanced the humoral immune resp onse by increasing the synthesis of antigen-specific antibodies of the IgG2a, IgG2b and IgG3 subclasses 10- to 1000-fold. The synthesis of I gG1 was not or only slightly (2-5-fold) enhanced, whereas that of the IgE isotype was suppressed. 2) These effects of IL-12 were observed wh en high (10 mu g, 100 mu g) or low doses (0.1 mu g) of antigen were us ed for immunization. 3) Titration of IL-12 in vitro revealed that IgG2 a is strongly up-regulated over a wide dose range of IL-12 (10 to 1000 ng/day). 4) The effects of IL-12 in vivo are at least partially inter feron (IFN)-gamma-dependent because an anti-IFN-gamma mAb in combinati on with IL-12 prevented most of the enhanced IgG2a production. 5) Mice receiving IL-12 showed a strong up-regulation of IFN-gamma but no inh ibition of IL-5 synthesis by spleen cells activated ex vivo with antig en. These results suggest that IL-12 is a potent adjuvant for enhancin g humoral immunity to protein antigens adsorbed to Alum, primarily by inducing the synthesis of the complement-fixing IgG subclasses 2a, 2b and 3.