Protective CD4 T cells specific for M. tuberculosis (Mtb) are maintained in the lungs during active Mtb infection. Similar to memory CD4 T cells, persistence of these Mtb-specific cells requires intrinsic expression of Bcl6 and ICOS.

Immune responses are tightly regulated to ensure efficient pathogen clearance while avoiding tissue damage. Here we report that SET domain bifurcated 2 (Setdb2) was the only protein lysine methyltransferase induced during influenza virus infection. Setdb2 expression depended on type-I interferon signaling and it repressed the expression of the neutrophil attractant Cxcl1 and other NF-κB target genes. This coincided with Setdb2 occupancy at the Cxcl1 promoter, which in the absence of Setdb2 displayed reduced H3K9 tri-methylation. Setdb2 hypomorphic gene-trap mice exhibited increased neutrophil infiltration in sterile lung inflammation and were less sensitive to bacterial superinfection upon influenza virus infection. This suggests that a Setdb2-mediated regulatory crosstalk between the type-I interferon and NF-κB pathways represents an important mechanism for virus-induced susceptibility to bacterial superinfection.

The regulation of host-pathogen interactions during Mycobacterium tuberculosis (Mtb) infection remains unresolved. MicroRNAs (miRNAs) are important regulators of the immune system, and so we used a systems biology approach to construct an miRNA regulatory network activated in macrophages during Mtb infection. Our network comprises 77 putative miRNAs that are associated with temporal gene expression signatures in macrophages early after Mtb infection. In this study, we demonstrate a dual role for one of these regulators, miR-155. On the one hand, miR-155 maintains the survival of Mtbinfected macrophages, thereby providing a niche favoring bacterial replication; on the other hand, miR-155 promotes the survival and function of Mtb-specific T cells, enabling an effective adaptive immune response. MiR-155-induced cell survival is mediated through the SH2 domain-containing inositol 5-phosphatase 1 (SHIP1)/protein kinase B (Akt) pathway. Thus, dual regulation of the same cell survival pathway in innate and adaptive immune cells leads to vastly different outcomes with respect to bacterial containment.

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The ability of Acanthamoeba to feed on Gram-negative bacteria, as well as to harbour potential pathogens, such as Legionella pneumophila, Coxiella burnetii, Pseudomonas aeruginosa, Vibrio cholerae, Helicobacter pylori, Listeria monocytogenes and Mycobacterium avium, suggest that both amoebae and bacteria are involved in complex interactions, which may play important roles in the environment and in human health. In this study, Acanthamoeba castellanii (a keratitis isolate belonging to the T4 genotype) was used and its interactions with Escherichia coli (strain K1, a cerebrospinal fluid isolate from a meningitis patient, O18 : K1 : H7, and a K-12 laboratory strain, HB101) were studied. The invasive K1 isolate exhibited a significantly higher association with A. castellanii than the non-invasive K-12 isolate. Similarly, K1 showed significantly increased invasion and/or uptake by A. castellanii in gentamicin protection assays than the non-invasive K-12. Using several mutants derived from K1, it was observed that outer-membrane protein A (OmpA) and LPS were crucial bacterial determinants responsible for E. coli K1 interactions with A. castellanii. Once inside the cell, E. coli K1 remained viable and multiplied within A. castellanii, while E. coli K-12 was killed. Again, OmpA and LPS were crucial for E. coli K1 intracellular survival in A. castellanii. In conclusion, these findings suggest that E. coli K1 interactions with A. castellanii are carefully regulated by the virulence of E. coli.

The majority of the keratitis-causing Acanthamoeba isolates are genotype T4. In an attempt to determine whether predominance of T4 isolates in Acanthamoeba keratitis is due to greater virulence or greater prevalence of this genotype, Acanthamoeba genotypes were determined for 13 keratitis isolates and 12 environmental isolates from Iran. Among 13 clinical isolates, eight (61 . 5 %) belonged to T4, two (15 . 3 %) belonged to T3 and three (23 %) belonged to the T2 genotype. In contrast, the majority of 12 environmental isolates tested in the present study belonged to T2 (7/12, 58 . 3 %), followed by 4/12 T4 isolates (33 . 3 %). In addition, the genotypes of six new Acanthamoeba isolates from UK keratitis cases were determined. Of these, five (83 . 3 %) belonged to T4 and one was T3 (16 . 6 %), supporting the expected high frequency of T4 in Acanthamoeba keratitis. In total, the genotypes of 24 Acanthamoeba keratitis isolates from the UK and Iran were determined. Of these, 17 belonged to T4 (70 . 8 %), three belonged to T2 (12 . 5 %), three belonged to T3 (12 . 5 %) and one belonged to T11 (4 . 1 %), confirming that T4 is the predominant genotype (÷ 2 ¼ 4 . 167; P ¼ 0 . 0412) in Acanthamoeba keratitis.

Granulomatous amoebic encephalitis due to Acanthamoeba castellanii is a serious human infection with fatal consequences, but it is not clear how the circulating amoebae interact with the blood-brain barrier and transmigrate into the central nervous system. We studied the effects of an Acanthamoeba encephalitis isolate belonging to the T1 genotype on human brain microvascular endothelial cells, which constitute the blood-brain barrier. Using an apoptosis-specific enzyme-linked immunosorbent assay, we showed that Acanthamoeba induces programmed cell death in brain microvascular endothelial cells. Next, we observed that Acanthamoeba specifically activates phosphatidylinositol 3-kinase. Acanthamoeba-mediated brain endothelial cell death was abolished using LY294002, a phosphatidylinositol 3-kinase inhibitor. These results were further confirmed using brain microvascular endothelial cells expressing dominant negative forms of phosphatidylinositol 3-kinase. This is the first demonstration that Acanthamoeba-mediated brain microvascular endothelial cell death is dependent on phosphatidylinositol 3-kinase.Acanthamoeba spp. are opportunistic protozoan parasites that are widely distributed throughout the environment (12, 18). The genus Acanthamoeba consists of both pathogenic and nonpathogenic species. Given the host susceptibility and correct environmental conditions, Acanthamoeba can cause granulomatous amoebic encephalitis (GAE), a fatal central nervous system (CNS) infection that occurs in immunocompromised patients (7-10, 11, 19). Several lines of evidence suggest that hematogenous spread is a prerequisite in Acanthamoeba encephalitis (19-21), but it is not clear how circulating amoebae cross the blood-brain barrier and gain access to the CNS to produce disease. We have demonstrated that pathogenic Acanthamoeba exhibits more than 60% binding to human brain microvascular endothelial cells (HBMEC), which constitute the blood-brain barrier (2). Acanthamoeba binding to HBMEC is mediated by a mannose-binding protein expressed on the surface of Acanthamoeba cells (2). Moreover, we showed that Acanthamoeba produces severe HBMEC cytotoxicity by secreting extracellular proteases, as well as using contact-dependent mechanisms such as phagocytosis (12), which may play an important role in blood-brain barrier perturbations. However, the host intracellular signaling pathways and the molecular mechanisms associated with Acanthamoeba-mediated HBMEC cytotoxicity have not been determined.Lipid second messengers, such as those derived from the polyphosphoinositide cycle, play a central role in many signaling networks. The majority of inositol lipids reside in membranes and serve as substrates for kinases, phosphatases, and phospholipases. Phosphatidylinositol 3-kinases (PI3Ks) are important signaling molecules that phosphorylate the 3Ј OH position of the inositol ring of phosphoinositides (PIs), generating the second messengers PI(3)P, PI(3,4)P 2 , and PI(3,4,5)P 3 (4, 17). These second messengers recruit the downstream effector molecu...

Summary Chronic tuberculosis in an immunocompetent host is a consequence of the delicately balanced growth of Mycobacterium tuberculosis (Mtb) in the face of host defense mechanisms. We identify an Mtb enzyme (TdmhMtb) that hydrolyzes the mycobacterial glycolipid trehalose dimycolate and plays a critical role in balancing the intracellular growth of the pathogen. TdmhMtb is induced under nutrient limiting conditions and remodels the Mtb envelope to increase nutrient influx, but concomitantly sensitizes Mtb to stresses encountered in the host. Consistent with this, a ΔtdmhMtb mutant is more resilient to stress and grows to higher levels than wild-type in immunocompetent mice. By contrast, mutant growth is retarded in MyD88−/− mice indicating that TdmhMtb provides a growth advantage to intracellular Mtb in an immunocompromised host. Thus, the effects and counter-effects of TdmhMtb play an important role in balancing intracellular growth of Mtb in a manner that is directly responsive to host innate immunity.