Apicomplexan parasites are the cause of numerous important human diseases including malaria and AIDS-associated opportunistic infections. Drug treatment for these diseases is not satisfactory and is threatened by resistance. The discovery of the apicoplast, a chloroplast-like organelle, presents drug targets unique to these parasites. The apicoplast-localized fatty acid synthesis (FAS II) pathway, a metabolic process fundamentally divergent from the analogous FAS I pathway in humans, represents one such target. However, the specific biological roles of apicoplast FAS II remain elusive. Furthermore, the parasite genome encodes additional and potentially redundant pathways for the synthesis of fatty acids. We have constructed a conditional null mutant of acyl carrier protein, a central component of the FAS II pathway in Toxoplasma gondii. Loss of FAS II severely compromises parasite growth in culture. We show FAS II to be required for the activation of pyruvate dehydrogenase, an important source of the metabolic precursor acetyl-CoA. Interestingly, acyl carrier protein knockout also leads to defects in apicoplast biogenesis and a consequent loss of the organelle. Most importantly, in vivo knockdown of apicoplast FAS II in a mouse model results in cure from a lethal challenge infection. In conclusion, our study demonstrates a direct link between apicoplast FAS II functions and parasite survival and pathogenesis. Our genetic model also offers a platform to dissect the integration of the apicoplast into parasite metabolism, especially its postulated interaction with the mitochondrion. apicomplexa ͉ fatty acid biosynthesis ͉ lipoic acid ͉ Plasmodium ͉ plastid

There is increasing awareness that helminth infections can ameliorate proinflammatory conditions. In part, this is due to their inherent ability to induce Th2 and, perhaps, regulatory T cell responses. However, recent evidence indicates that helminths also have direct anti-inflammatory effects on innate immune responses. In this study, we address this issue and show that soluble molecules from the eggs of the helminth parasite Schistosoma mansoni (SEA) suppress LPS-induced activation of immature murine dendritic cells, including MHC class II, costimulatory molecule expression, and IL-12 production. SEA-augmented LPS-induced production of IL-10 is in part responsible for the observed reduction in LPS-induced IL-12 production. However, analyses of IL-10−/− DC revealed distinct IL-10-independent suppressive effects of SEA. IL-10-independent mechanisms are evident in the suppression of TLR ligand-induced MAPK and NF-κB signaling pathways. Microarray analyses demonstrate that SEA alone uniquely alters the expression of a small subset of genes that are not up-regulated during conventional TLR-induced DC maturation. In contrast, the effects of SEA on TLR ligand-induced DC activation were striking: when mixed with LPS, SEA significantly affects the expression of >100 LPS-regulated genes. These findings indicate that SEA exerts potent anti-inflammatory effects by directly regulating the ability of DC to respond to TLR ligands.

The production of interleukin-12 (IL-12) is critical to the development of innate and adaptive immune responses required for the control of intracellular pathogens. Many microbial products signal through Toll-like receptors (TLR) and activate NF-κB family members that are required for the production of IL-12. Recent studies suggest that components of the TLR pathway are required for the production of IL-12 in response to the parasite Toxoplasma gondii; however, the production of IL-12 in response to this parasite is independent of NF-κB activation. The adaptor molecule TRAF6 is involved in TLR signaling pathways and associates with serine/threonine kinases involved in the activation of both NF-κB and mitogen-activated protein kinase (MAPK). To elucidate the intracellular signaling pathways involved in the production of IL-12 in response to soluble toxoplasma antigen (STAg), wild-type and TRAF6−/− mice were inoculated with STAg, and the production of IL-12(p40) was determined. TRAF6−/− mice failed to produce IL-12(p40) in response to STAg, and TRAF6−/− macrophages stimulated with STAg also failed to produce IL-12(p40). Studies using Western blot analysis of wild-type and TRAF6−/− macrophages revealed that stimulation with STAg resulted in the rapid TRAF6-dependent phosphorylation of p38 and extracellular signal-related kinase, which differentially regulated the production of IL-12(p40). The studies presented here demonstrate for the first time that the production of IL-12(p40) in response to toxoplasma is dependent upon TRAF6 and p38 MAPK

There is growing evidence that plasmacytoid dendritic cells (pDC) are involved in the innate recognition of various microbes. However, the precise consequences of pathogen recognition on pDC activation and function are incompletely understood. Using a novel transgenic mouse model that facilitates the isolation of highly pure pDC populations, we found that influenza virus PR/8, a TLR7 ligand, and CpG 1826 oligonucleotide, a TLR9 ligand, induced surprisingly divergent activation programs in these cells. pDC stimulated with PR/8 produced large amounts of type I IFNs, and CpG 1826-stimulated pDC expressed higher levels of costimulatory molecules and proinflammatory cytokines and induced stronger proliferation of T cells. Transcriptome analysis uncovered the differential regulation in pDC of 178 and 1577 genes by PR/8 and CpG 1826, respectively. These differences may relate to the activation of discrete signaling pathways, as evidenced by distinct ERK1/2 and p38 MAPK phosphorylation kinetics. Finally, pDC isolated ex vivo during PR/8 infection or after i.v. CpG 1826 injection resembled their in vitro counterparts, corroborating that these cells can adopt specialized phenotypes in vivo. Thus, pDC display remarkable functional flexibility, which emphasizes their versatile functions in antimicrobial immunity and inflammatory processes.

There was an error published in J. Cell Sci. 119, 4565-4573.In Fig. 7C, the 1 mM and 5 mM bars were incorrectly labelled. The corrected figure and legend are shown below. The authors apologise for this error.