Interleukin-15 receptor alpha (IL-15R alpha) is a pleiotropically expressed molecule that chaperones and trans-presents IL-15 to NK and T cells. To investigate whether IL-15R alpha presented by different cells perform distinct physiological functions, we have generated four lines of mice lacking IL-15R alpha in various cell types. We find that IL-15R alpha expression on macrophages but not dendritic cells (DCs) supports the early transition of antigen specific effector CD8(+) T cells to memory cells. After memory CD8(+) T cell differentiation, IL-15R alpha expression on DCs selectively supports central memory CD8(+) T cells, whereas IL-15R alpha expression on macrophages supports both central and effector memory CD8(+) T cells. By contrast, mice lacking IL-15R alpha on macrophages, DCs, or both, exhibit equivalent defects in NK cell homeostasis and activation. These studies define unique roles for macrophage expression of IL-15R alpha and show that NK cells rely upon distinct IL-15R alpha dependent IL-15 signals than memory CD8(+) T cells. Moreover, they demonstrate the diversity, specification, and geographic restriction of cytokine signals.

Control of microbial infection requires regulated induction of NF-κB-dependent proinflammatory cytokines such as IL-12 and TNF-α. Activation of this important transcription factor is driven by phosphorylation-dependent degradation of the inhibitory IκB molecule, an event which enables NF-κB translocation from the cytoplasm to the nucleus. In this study, we show that intracellular infection of macrophages with the protozoan parasite Toxoplasma gondii induces rapid IκB phosphorylation and degradation. Nevertheless, NF-κB failed to translocate to the nucleus, enabling the parasite to invade cells without triggering proinflammatory cytokine induction. Infected cells subsequently subjected to LPS triggering were severely crippled in IL-12 and TNF-α production, a result of tachyzoite-induced blockade of NF-κB nuclear translocation. Our results are the first to demonstrate the ability of an intracellular protozoan to actively interfere with the NF-κB activation pathway in macrophages, an activity that may enable parasite survival within the host.

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Infection of mouse macrophages by Toxoplasma gondii renders the cells resistant to proinflammatory effects of LPS triggering. In this study, we show that cell invasion is accompanied by rapid and sustained activation of host STAT3. Activation of STAT3 did not occur with soluble T. gondii extracts or heat-killed tachyzoites, demonstrating a requirement for live parasites. Parasite-induced STAT3 phosphorylation and suppression of LPS-triggered TNF-α and IL-12 was intact in IL-10-deficient macrophages, ruling out a role for this anti-inflammatory cytokine in the suppressive effects of T. gondii. Most importantly, Toxoplasma could not effectively suppress LPS-triggered TNF-α and IL-12 synthesis in STAT3-deficient macrophages. These results demonstrate that T. gondii exploits host STAT3 to prevent LPS-triggered IL-12 and TNF-α production, revealing for the first time a molecular mechanism underlying the parasite’s suppressive effect on macrophage proinflammatory cytokine production.

The organized lymphoid tissues of the intestine likely play an important role in the balance between tolerance harmless mucosal Ags and commensal bacteria and immunity to mucosal pathogens. We examined the phenotype and function of plasmacytoid dendritic cells (pDCs) from murine Peyer’s patches (PPs). When stimulated with CpG-enriched oligodeoxynucleotides in vitro, PPs and spleen pDCs made equivalent levels of IL-12, yet PP pDCs were incapable of producing significant levels of type I IFNs. Three regulatory factors associated with mucosal tissues, PGE2, IL-10, and TGFβ, inhibited the ability of spleen pDCs to produce type I IFN in a dose-dependent fashion. These studies suggest that mucosal factors may regulate the production of type I IFN as well as IL-12 by pDCs. In the intestine, this may be beneficial in preventing harmful innate and adaptive immune responses to commensal microorganisms.