Inflammasomes are cytosolic multi-protein complexes that detect infection or cellular damage and activate the Caspase-1 (CASP1) protease. The NAIP5/NLRC4 inflammasome detects bacterial flagellin and is essential for resistance to the flagellated intracellular bacterium Legionella pneumophila . The effectors required downstream of NAIP5/NLRC4 to restrict bacterial replication remain unclear. Upon NAIP5/NLRC4 activation, CASP1 cleaves and activates the pore-forming protein Gasdermin-D (GSDMD) and the effector caspase-7 (CASP7). However, Casp1 –/– (and Casp1/11 –/– ) mice are only partially susceptible to L . pneumophila and do not phenocopy Nlrc4 –/– mice, because NAIP5/NLRC4 also activates CASP8 for restriction of L . pneumophila infection. Here we show that CASP8 promotes the activation of CASP7 and that Casp7/1/11 –/– and Casp8/1/11 –/– mice recapitulate the full susceptibility of Nlrc4 –/– mice. Gsdmd –/– mice exhibit only mild susceptibility to L . pneumophila , but Gsdmd –/– Casp7 –/– mice are as susceptible as the Nlrc4 –/– mice. These results demonstrate that GSDMD and CASP7 are the key substrates downstream of NAIP5/NLRC4/CASP1/8 required for resistance to L . pneumophila .

Leishmaniasis is caused by protozoan parasites of the genus In mammalians, these parasites survive and replicate in macrophages and parasite elimination by macrophages is critical for host resistance. Endosomal Toll-like receptors (TLRs) have been shown to be crucial for resistance to For example, mice in the resistant C57BL/6 genetic background that are triple-deficient for TLR3, -7, and -9 () are highly susceptible to infection. mice are as susceptible as mice deficient in MyD88 or UNC93B1, a chaperone required for appropriate localization of endosomal TLRs, but the mechanisms are unknown. Here we found that macrophages infected with undergo autophagy, which effectively accounted for restriction of parasite replication. Signaling via endosomal TLRs was required for autophagy because macrophages deficient for TLR3, -7, and 9, UNC93B1, or MyD88 failed to undergo-induced autophagy. We also confirmed that ,, and cells were highly permissive to replication. Accordingly, shRNA-mediated suppression of Atg5, an E3 ubiquitin ligase essential for autophagosome elongation, in macrophages impaired the restriction of replication in C57BL/6, but did not affect parasite replication in or macrophages. Rapamycin treatment reduced inflammatory lesions formed in the ears of-infected C57BL/6 and mice, indicating that autophagy operates downstream of TLR signaling and is relevant for disease development Collectively, our results indicate that autophagy contributes to macrophage resistance to replication, and mechanistically explain the previously described endosomal TLR-mediated resistance to infection.

bThe relationship established between Leishmania infantum and the vertebrate host can lead to a self-healing infection or to the manifestation of visceral leishmaniasis, a chronic systemic infection associated with high rates of mortality. We hypothesized that regulatory cytokines, such as interleukin-27 (IL-27), play a role in susceptibility to L. infantum infection. IL-27 is a heterodimeric cytokine composed of IL-27p28 and EBi3 subunits which, when combined, bind to IL-27R, leading to STAT-1 and -3 activation, playing a role in the regulation of the immune response. We observed in this work that IL-27 regulates the Th1/Th17 profiles in a mouse model of visceral leishmaniasis (VL) caused by L. infantum. We showed here that the pathogen recognition by endosomal Toll-like receptors triggers a type I interferon (IFN) response, which acts through the type I IFN receptor and interferon regulatory factor 1 to induce IL-27 production by macrophages. Furthermore, IL-27 plays a major regulatory role in vivo, because Ebi3 ؊/؊ mice can efficiently control parasite replication despite reduced levels of IFN-␥ compared to wild-type mice. On the other hand, the absence of Ebi3 leads to exacerbated IL-17A production in the infected organs as well as in a coculture system, suggesting a direct regulatory action of IL-27 during L. infantum infection. As a consequence of exacerbated IL-17A in Ebi3 ؊/؊ mice, a greater neutrophil influx was observed in the target organs, playing a role in parasite control. Thus, this work unveiled the molecular steps of IL-27 production after L. infantum infection and demonstrated its regulatory role in the IL-17A-neutrophil axis. Visceral leishmaniasis (VL), or kala-azar, is a systemic chronic disease with high mortality rates if not treated. It is caused by the parasites Leishmania infantum and L. donovani, and it is estimated that 300,000 new cases and 20,000 deaths occur annually (http://www.who.int/mediacentre/factsheets/fs375/en/). After dermal inoculation by the sandfly vector, the parasite disseminates to the liver, spleen, bone marrow, and lymph nodes of susceptible hosts, causing symptoms such as hepatosplenomegaly, lymphadenopathy, anemia, constant fever, and immunosuppression (1). Even though it is considered one of the six most important parasitic diseases affecting humans, the immunobiology of VL is not completely understood and novel therapeutic approaches are desired.The immune response against Leishmania spp. is critically mediated by gamma interferon (IFN-␥)-producing Th1 cells, which activate macrophages to produce leishmanicidal compounds, such as nitric oxide (NO) (2). Together with IFN-␥, the inflammatory cytokine interleukin-17A (IL-17A) also mediates protection against L. infantum (3, 4). On the other hand, Th2 cytokines are involved in susceptibility to leishmaniasis (2, 5). Moreover, the regulatory cytokines IL-10, IL-21, and IL-27 are produced in the bone marrow of VL patients, suggesting their association with the disease (6).The cytokine IL-27 is a dimer composed of p28...

Inflammasomes are multimeric protein complexes that initiate inflammatory cascades. Their activation is a hallmark of many infectious or inflammatory diseases. Their composition and activity are specified by proinflammatory stimuli. For example, the NLRP3 inflammasome is activated in response to cell damage and K efflux, whereas the AIM2 inflammasome is activated in response to cytosolic DNA. We used Legionella pneumophila, an intracellular bacterial pathogen that activates multiple inflammasomes, to elucidate the molecular mechanisms regulating inflammasome activation during infection. Upon infection, the AIM2 inflammasome engaged caspase-1 to induce pore formation in the cell membrane, which then caused K-efflux-mediated activation of NLRP3. Thus, the AIM2 inflammasome amplifies signals of infection, triggering noncanonical activation of NLRP3. During infection, AIM2 and caspase-11 induced membrane damage, which was sufficient and essential for activating the NLRP3 inflammasome. Our data reveal that different inflammasomes regulate one another's activity to ensure an effective immune response to infection.

Neospora caninum is an apicomplexan parasite responsible for major economic losses due to abortions in cattle. Innate immune responses are crucial for host resistance against the infection, however the molecules involved in parasite recognition are still poorly understood. Nod2 is a cytosolic receptor that recognizes several pathogens and its role during N. caninum infection has not yet been described. In that sense, we evaluated the role of Nod2 in host response against this parasite. We found that infection of macrophages induced increased expression of Nod2, which colocalized with the parasites’ vacuoles. Nod2-deficient macrophages showed an impaired induction of pro-inflammatory cytokines, increased production of modulatory molecules, and failure to restrict parasite replication. In vivo, Nod2-knockout mice showed a reduction of MAPK phosphorylation and proinflammatory cytokines, followed by decreased inflammation in target organs and increment in parasite burden. Surprisingly, these mice were partially resistant to lethal doses of tachyzoites. In addition, these phenomena were not observed in Rip2−/− mice. In conclusion, our study indicates that Nod2-dependent responses account for N. caninum elimination. On the other hand, the inflammatory milieu induced by this innate receptor provoked pathogenesis and death in severe experimental neosporosis.