No abstract

Interleukin-12 (IL-12) is a pivotal cytokine that has dramatic effects on cell-mediated immunity. It is now becoming increasingly recognized that IL-12 also strongly controls humoral immunity. We have investigated the mechanism by which IL-12 induces alterations in antibody isotype expression by determining the influence of IL-12 on in vitro immunoglobulin (Ig) production in polyclonally activated murine spleen cell cultures. Cells exposed to IL-12 plus lipopolysaccharide or anti-CD40 monoclonal antibody showed dramatically elevated IgG2a and suppressed IgG1 production compared to cells cultured in the absence of IL-12. IL-12 treatment of spleen cell cultures induced expression of gamma2a germ-line transcripts, consistent with initiation of switch recombination to IgG2a. In addition, exposure of limiting dilution cultures to IL-12 increased IgG2a+ cell precursor frequency. All of the above results were dependent on interferon-gamma (IFN-gamma). However, in the absence of IFN-gamma, IL-12 still had significant effects on Ig secretion. Specifically, IL-12 enhanced IgG1 and IgG2b anti-DNP antibody levels in mice containing specific disruptions in the IFN-gamma gene. Our results suggest that IL-12 induces T helper type 1 and natural killer cells to secrete large amounts of IFN-gamma which then causes B cells to switch to IgG2a and IgG3 production. In addition, IL-12 has direct or indirect effects on B cells that are independent of IFN-gamma. The IFN-gamma-independent effects may include enhancement of Ig expression by post-switched cells.

Protein antigens elicit humoral responses in mice that consist predominantly of IgG1 antibodies. We have now investigated the ability of IL-12, a cytokine reported to augment IgG2a anti-hapten responses through activation of Th1 cells, to alter antibody responses to hen eggwhite lysozyme (HEL). The normal response of BALB/c mice to HEL is highly restricted to IgG1 expression and therefore provides an excellent system for determining effects of cytokines on expression of other isotypes. Seven days after immunization, IL-12 treated mice demonstrated greatly elevated HEL-specific IgG2a antibody levels and suppressed IgG1 production, while PBS-treated control mice showed a typical IgG1-restricted response. On day 28, IL-12-treated mice showed heightened serum antibody levels of both isotypes. Delaying cytokine treatment until after the typical IgG1 anti-HEL response had already been established also led to significant elevation of serum IgG2a antibody levels. These effects correlated with increased IFN-gamma production; however, administration of IL-12 plus anti-IFN-gamma had little influence on IgG2a enhancement, although it did relieve the early IgG1 suppression. Furthermore, the differential effects of Il-12 on isotype expression did not correlate with time; in fact, IgG2a enhancement correlated with loss of IgG1 suppression. Our findings indicate that (i) IL-12 reproducibly induces large amounts of IgG2a HEL-specific antibodies in vivo; (ii) it can alter isotype profiles of both primary and secondary responses; and (iii) its effects on humoral immunity are not completely explained by induction of Th1 cell derived IFN-gamma.

We have found that IL-12 treatment of mice leads to long-lasting enhancement in production of most antibody isotypes in conventional B-cell responses. Initial recruitment of new B-cell clones into the response is mediated by IFN-gamma, but subsequent enhancement of Ig secretion appears to be IFN-gamma-independent. We have further found that activated B cells can directly bind IL-12. Taken together, our results suggest a two-step model for the role of IL-12 in enhancement of humoral immunity. Initially, IL-12 induces production of IFN-gamma from Th1 and NK cells. Enough cytokine can be produced from either cell type to then mediate gamma 2a heavy chain isotype switching as well as temporary suppression of IgG1 production. IL-12 further stimulates post-switched cells, including cells producing IgG1, to secrete greatly increased amounts of antibody. This step is not mediated by IFN-gamma but might be due to direct IL-12 binding to activated B lymphocytes. Depletion of B1 cells by IL-12 may further enhance antibody responsiveness since B1 cells are known to competitively inhibit Ig secretion by conventional B cells. The end result is that IL-12 causes a generalized upregulation in production of all antibodies and therefore acts as a strong adjuvant for humoral as well as cellular immunity.

No abstract