Ashraful Haque

Complete genome sequence of a multiple drug resistant Salmonella enterica serovar Typhi CT18.

Salmonella enterica serovar Typhi (S. typhi) is the aetiological agent of typhoid fever, a serious invasive bacterial disease of humans with an annual global burden of approximately 16 million cases, leading to 600,000 fatalities. Many S. enterica serovars actively invade the mucosal surface of the intestine but are normally contained in healthy individuals by the local immune defence mechanisms. However, S. typhi has evolved the ability to spread to the deeper tissues of humans, including liver, spleen and bone marrow. Here we have sequenced the 4,809,037-base pair (bp) genome of a S. typhi (CT18) that is resistant to multiple drugs, revealing the presence of hundreds of insertions and deletions compared with the Escherichia coli genome, ranging in size from single genes to large islands. Notably, the genome sequence identifies over two hundred pseudogenes, several corresponding to genes that are known to contribute to virulence in Salmonella typhimurium. This genetic degradation may contribute to the human-restricted host range for S. typhi. CT18 harbours a 218,150-bp multiple-drug-resistance incH1 plasmid (pHCM1), and a 106,516-bp cryptic plasmid (pHCM2), which shows recent common ancestry with a virulence plasmid of Yersinia pestis.

Immune-mediated mechanisms of parasite tissue sequestration during experimental cerebral malaria

Cerebral malaria is a severe complication of malaria. Sequestration of parasitized RBCs in brain microvasculature is associated with disease pathogenesis, but our understanding of this process is incomplete. In this study, we examined parasite tissue sequestration in an experimental model of cerebral malaria (ECM). We show that a rapid increase in parasite biomass is strongly associated with the induction of ECM, mediated by IFN-γ and lymphotoxin α, whereas TNF and IL-10 limit this process. Crucially, we discovered that host CD4+ and CD8+ T cells promote parasite accumulation in vital organs, including the brain. Modulation of CD4+ T cell responses by helminth coinfection amplified CD4+ T cell-mediated parasite sequestration, whereas vaccination could generate CD4+ T cells that reduced parasite biomass and prevented ECM. These findings provide novel insights into immune-mediated mechanisms of ECM pathogenesis and highlight the potential of T cells to both prevent and promote infectious diseases.

Granzyme B Expression by CD8+ T Cells Is Required for the Development of Experimental Cerebral Malaria

Parasite burden predicts disease severity in malaria and risk of death in cerebral malaria patients. In murine experimental cerebral malaria (ECM), parasite burden and CD8+ T cells promote disease by mechanisms that are not fully understood. We found that the majority of brain-recruited CD8+ T cells expressed granzyme B (GzmB). Furthermore, gzmB−/− mice harbored reduced parasite numbers in the brain as a consequence of enhanced antiparasitic CD4+ T cell responses and were protected from ECM. We showed in these ECM-resistant mice that adoptively transferred, Ag-specific CD8+ T cells migrated to the brain, but did not induce ECM until a critical Ag threshold was reached. ECM induction was exquisitely dependent on Ag-specific CD8+ T cell-derived perforin and GzmB, but not IFN-γ. In wild-type mice, full activation of brain-recruited CD8+ T cells also depended on a critical number of parasites in this tissue, which in turn, was sustained by these tissue-recruited cells. Thus, an interdependent relationship between parasite burden and CD8+ T cells dictates the onset of perforin/GzmB-mediated ECM.

A Critical Role for Neutrophils in Resistance to Experimental Infection with Burkholderia pseudomallei

Inhalation is an important route of infection with Burkholderia pseudomallei, the causative agent of melioidosis. In resistant C57BL/6 mice, activated neutrophils are rapidly recruited to the lungs after intranasal B. pseudomallei infection. Prevention of this response by use of the anti-Gr-1+ cell-depleting monoclonal antibody RB6-8C5 severely exacerbated disease, resulting in an acute lethal infection associated with a 1000-fold increase in lung bacterial loads within 4 days. C57BL/6 interferon (IFN)-gamma(-/-) mice were also acutely susceptible to pulmonary B. pseudomallei infection, dying within 3 days of challenge; this suggests that IFN-gamma is essential for control in the lungs and precedes the protective role of neutrophils in resistance. In neutrophil-depleted mice, lung concentrations of tumor necrosis factor (TNF)-alpha, IFN-gamma, and interleukin-6 were decreased by up to 98%. Natural killer cells were the principle source of IFN-gamma, and monocytes were the principle source of TNF-alpha, suggesting that neutrophils play an important indirect role in the generation of the early cytokine environment in the lungs.

CD8+ T Cells from a Novel T Cell Receptor Transgenic Mouse Induce Liver-Stage Immunity That Can Be Boosted by Blood-Stage Infection in Rodent Malaria

To follow the fate of CD8+ T cells responsive to Plasmodium berghei ANKA (PbA) infection, we generated an MHC I-restricted TCR transgenic mouse line against this pathogen. T cells from this line, termed PbT-I T cells, were able to respond to blood-stage infection by PbA and two other rodent malaria species, P. yoelii XNL and P. chabaudi AS. These PbT-I T cells were also able to respond to sporozoites and to protect mice from liver-stage infection. Examination of the requirements for priming after intravenous administration of irradiated sporozoites, an effective vaccination approach, showed that the spleen rather than the liver was the main site of priming and that responses depended on CD8α+ dendritic cells. Importantly, sequential exposure to irradiated sporozoites followed two days later by blood-stage infection led to augmented PbT-I T cell expansion. These findings indicate that PbT-I T cells are a highly versatile tool for studying multiple stages and species of rodent malaria and suggest that cross-stage reactive CD8+ T cells may be utilized in liver-stage vaccine design to enable boosting by blood-stage infections.

Cutting Edge: Conventional Dendritic Cells Are the Critical APC Required for the Induction of Experimental Cerebral Malaria

Cerebral malaria (CM) is a serious complication of Plasmodium falciparum infection, causing significant morbidity and mortality among young children and nonimmune adults in the developing world. Although previous work on experimental CM has identified T cells as key mediators of pathology, the APCs and subsets therein required to initiate immunopathology remain unknown. In this study, we show that conventional dendritic cells but not plasmacytoid dendritic cells are required for the induction of malaria parasite-specific CD4+ T cell responses and subsequent experimental CM. These data have important implications for the development of malaria vaccines and the therapeutic management of CM.

CD4+ Natural Regulatory T Cells Prevent Experimental Cerebral Malaria via CTLA-4 When Expanded In Vivo

Studies in malaria patients indicate that higher frequencies of peripheral blood CD4+ Foxp3+ CD25+ regulatory T (Treg) cells correlate with increased blood parasitemia. This observation implies that Treg cells impair pathogen clearance and thus may be detrimental to the host during infection. In C57BL/6 mice infected with Plasmodium berghei ANKA, depletion of Foxp3+ cells did not improve parasite control or disease outcome. In contrast, elevating frequencies of natural Treg cells in vivo using IL-2/anti-IL-2 complexes resulted in complete protection against severe disease. This protection was entirely dependent upon Foxp3+ cells and resulted in lower parasite biomass, impaired antigen-specific CD4+ T and CD8+ T cell responses that would normally promote parasite tissue sequestration in this model, and reduced recruitment of conventional T cells to the brain. Furthermore, Foxp3+ cell-mediated protection was dependent upon CTLA-4 but not IL-10. These data show that T cell-mediated parasite tissue sequestration can be reduced by regulatory T cells in a mouse model of malaria, thereby limiting malaria-induced immune pathology.

Multinucleated Giant Cell Formation and Apoptosis in Infected Host Cells Is Mediated by Burkholderia pseudomallei Type III Secretion Protein BipB

Here we have assessed the role of a type III translocator protein, BipB, in the cell biology and virulence of Burkholderia pseudomallei. Genetic inactivation of bipB reduced multinucleated giant cell formation, cell-to-cell spreading of bacteria, and induction of apoptosis of J774A.1 macrophages. The bipB mutant was also significantly attenuated following intranasal challenge of BALB/c mice, whereas virulence was fully restored by complementation with a functional bipB gene. Burkholderia pseudomallei, the etiological agent of melioidosis in humans and animals, is a gram-negative bacterium. Melioidosis is endemic in southeast Asia and tropical Australia and has been reported sporadically elsewhere (6). Currently, there is no vaccine against melioidosis. Uniquely among intracellular bacterial pathogens, B. pseudomallei induces host cell fusion leading to multinucleated giant cell (MNGC) formation in tissue culture models of infection (14). This novel phenotype may be relevant to pathogenesis, since granuloma formation and generation of MNGC are also found in tissues of humans with melioidosis (23). In addition to inducing MNGC formation, B. pseudomallei is able to spread from cell to cell and induce apoptotic death in infected host cells (14). The molecular mechanisms of these pathogenic characteristics have not been elucidated. Analysis of the B. pseudomallei genome and several other studies have demonstrated the presence of a type III secretion system (TTSS) (for reviews, see references 3, 12, 17, 20, and 22). A knockout mutant of B. pseudomallei lacking a functional bipD gene, a homologue of Salmonella enterica serovar Typhimurium sipD, on the TTSS3/bsa cluster of TTSS exhibited reduced replication in murine macrophage-like cells (20), was significantly attenuated in BALB/c mice and gave partial protection against subsequent challenge with wild-type B. pseudomallei (19). These data correlated with the recent report that the TTSS3/bsa cluster is required for the pathogenicity of B. pseudomallei (21). In addition to BipD, B. pseudomallei BipB and BipC (46 and 30% amino acid identity to Salmonella SipB and SipC, respectively) have been identified in the TTSS3/bsa cluster (3). Here, we report on the role of BipB in the pathogenesis of infection with B. pseudomallei. With Salmonella organisms, purified SipB integrates into artificial membranes and induces liposome fusion (10), and it is required for inducing apoptosis in murine macrophages (11). By analogy with SipB, therefore, we investigated the role of BipB for MNGC formation, cell-to-cell spreading, and induction of apoptosis in infected host cells. We also examined the virulence of a B. pseudomallei bipB mutant in a murine model of melioidosis.

HDAC inhibitors in parasitic diseases

Parasitic diseases cause significant global morbidity and mortality, particularly in underdeveloped regions of the world. Malaria alone causes ∼800 000 deaths each year, with children and pregnant women being at highest risk. There is no licensed vaccine available for any human parasitic disease and drug resistance is compromising the efficacy of many available anti‐parasitic drugs. This is driving drug discovery research on new agents with novel modes of action. Histone deacetylase (HDAC) inhibitors are being investigated as drugs for a range of diseases, including cancers and infectious diseases such as HIV/AIDS, and several parasitic diseases. This review focuses on the current state of knowledge of HDAC inhibitors targeted to the major human parasitic diseases malaria, schistosomiasis, trypanosomiasis, toxoplasmosis and leishmaniasis. Insights are provided into the unique challenges that will need to be considered if HDAC inhibitors are to be progressed towards clinical development as potential new anti‐parasitic drugs.

Type I interferons suppress CD4+ T-cell-dependent parasite control during blood-stage Plasmodium infection

During blood‐stage Plasmodium infection, large‐scale invasion of RBCs often occurs before the generation of cellular immune responses. In Plasmodium berghei ANKA (PbA)‐infected C57BL/6 mice, CD4+ T cells controlled parasite numbers poorly, instead providing early help to pathogenic CD8+ T cells. Expression analysis revealed that the transcriptional signature of CD4+ T cells from PbA‐infected mice was dominated by type I IFN (IFN‐I) and IFN‐γ‐signalling pathway‐related genes. A role for IFN‐I during blood‐stage Plasmodium infection had yet to be established. Here, we observed IFN‐α protein production in the spleen of PbA‐infected C57BL/6 mice over the first 2 days of infection. Mice deficient in IFN‐I signalling had reduced parasite burdens, and displayed none of the fatal neurological symptoms associated with PbA infection. IFN‐I substantially inhibited CD4+ T‐bet+ T‐cell‐derived IFN‐γ production, and prevented this emerging Th1 response from controlling parasites. Experiments using BM chimeric mice revealed that IFN‐I signalled predominantly via radio‐sensitive, haematopoietic cells, but did not suppress CD4+ T cells via direct signalling to this cell type. Finally, we found that IFN‐I suppressed IFN‐γ production, and hampered efficient control of parasitaemia in mice infected with non‐lethal Plasmodium chabaudi. Thus, we have elucidated a novel regulatory pathway in primary blood‐stage Plasmodium infection that suppresses CD4+ T‐cell‐mediated parasite control.