IL-15 is a recently described cytokine which resembles IL-2 in its biologic activities, stimulating T cell and NK cell proliferation and activation as well as enhancing B cell expansion and Ab production. Unlike IL-2, IL-15 is not produced by lymphocytes, but instead (at least among cells of the immune system) appears to be synthesized primarily by monocyte/macrophages. We have examined the induction of IL-15 in murine macrophages (by semiquantitative reverse transcriptase-PCR and bioassay) in response to a variety of different macrophage-activating stimuli and compared the regulation of IL-15 production to that of IL-12 and TNF-alpha. Optimal induction of IL-15, in each of the macrophages populations tested, was found to require both priming (IFN-gamma) and triggering (LPS, mycobacteria, or Toxoplasma gondii) stimuli. When compared with IL-12 mRNA synthesis by the same macrophages, IL-15.mRNA production was more resistant to inhibition by the down-regulatory cytokines IL-14, IL-13, and TGF-beta. Moreover, IL-10, which is inhibitory for most other monokines, increased levels of IL-15 mRNA found after stimulation. These data establish IL-15 as a product of the macrophage/monocyte lineage, which is up-regulated on activation. IL-15 could thus play an important role in the initiation of immune responses by microbial agents.

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