Direct stimulation of macrophages by IL-12 and IL-18--a bridge too far?

Area of Science:

• Immunology
• Cell Biology
• Molecular Biology

Background:

• A novel autocrine macrophage activation pathway involving interleukin-12 (IL-12), interleukin-18 (IL-18), and interferon-gamma (IFN-gamma) has been proposed.
• The macrophage isolation technique used in prior studies may have yielded impure cell populations, casting doubt on the proposed autocrine mechanism.

Purpose of the Study:

• To investigate the validity of the proposed autocrine macrophage activation pathway.
• To determine the role of lymphoid cell contamination in macrophage activation responses to IL-12 and IL-18.

Main Methods:

• Utilized macrophage-like cells (Raw 264.7) and assessed nitric oxide production.
• Experimentally introduced minor lymphoid cell contamination into pure macrophage-like cell cultures.
• Stimulated both pure and contaminated cultures with IL-12 and IL-18.

Main Results:

• Minor contamination of macrophage-like cells with lymphoid cells resulted in significant nitric oxide production upon stimulation with IL-12 and IL-18.
• Neither pure macrophage-like cells nor pure lymphoid cells alone secreted substantial nitric oxide when stimulated with IL-12 and/or IL-18.
• The observed nitric oxide production was dependent on the presence of both cell types and cytokine stimulation.

Conclusions:

• The previously suggested autocrine feedback loop for macrophage activation is likely incorrect.
• The phenomenon appears to be paracrine, with lymphoid cells stimulated by IL-12 and IL-18 secreting IFN-gamma, which then activates macrophages.
• Accurate assessment of immune cell function requires highly pure cell populations to avoid misinterpretation of results.