The cell biology of microbial infections: coming of age

Immunogenicity of recombinant attenuated Salmonella enterica serovar Typhimurium vaccine strains carrying a gene that encodes Eimeria tenella antigen SO7

Recombinant attenuated Salmonella vaccines against avian coccidiosis were developed to deliver Eimeria species antigens to the lymphoid tissues of chickens via the type 3 secretion system (T3SS) and the type 2 secretion system (T2SS) of Salmonella. For antigen delivery via the T3SS, the Eimeria tenella gene encoding sporozoite antigen SO7 was cloned downstream of the translocation domain of the Salmonella enterica serovar Typhimurium sopE gene in the parental pYA3868 and pYA3870 vectors to generate pYA4156 and pYA4157. Newly constructed T3SS vectors were introduced into host strain chi8879 (Delta phoP233 Delta sptP1033::xylE Delta asdA16), an attenuated derivative of the highly virulent UK-1 strain. The SopE-SO7 fusion protein was secreted by the T3SS of Salmonella. The vector pYA4184 was constructed for delivery of the SO7 antigen via the T2SS. The SO7 protein was toxic to Salmonella when larger amounts were synthesized; thus, the synthesis of this protein was placed under the control of the lacI repressor gene, whose expression in turn was dependent on the amount of available arabinose in the medium. The pYA4184 vector was introduced into host strain chi9242 (Delta phoP233 Delta asdA16 Delta araBAD23 Delta relA198::araC P(BAD) lacI TT [TT is the T4ipIII transcription terminator]). In addition to SO7, for immunization and challenge studies we used the EAMZ250 antigen of Eimeria acervulina, which was previously shown to confer partial protection against E. acervulina challenge when it was delivered via the T3SS. Immunization of chickens with a combination of the SO7 and EAMZ250 antigens delivered via the T3SS induced superior protection against challenge by E. acervulina. In contrast, chickens immunized with SO7 that was delivered via the T2SS of Salmonella were better protected from challenge by E. tenella.

Involvement of caspase-3, lipid peroxidation and TNF-alpha in causing apoptosis of macrophages by coordinately expressed Salmonella phenotype under stress conditions

Invasive Salmonella has been reported to induce apoptosis of macrophages as a part of its infection process, which may allow it to avoid detection by the innate immune system. However, the bacterial components capable of inducing apoptosis, particularly under the environments offered by the host have not been fully identified. Therefore, in the present study, attempts were made to evaluate the apoptotic potential of Salmonella enterica serovar Typhi (S. typhi) outer membrane protein expressed under stress conditions like iron, oxidative and anaerobic simulating the in vivo situations encountered by the pathogen. Analysis of data revealed that a coordinately expressed 69kDa outer membrane protein (OMP) expressed with enhanced intensity under iron, oxidative and anaerobic stress conditions caused apoptotic cell death in 51% of macrophages, whereas OMPs of S. typhi extracted under normal conditions accounted for apoptotic cell death in only 31% of macrophages. A significantly enhanced activity of caspase-3 was observed during macrophage-apoptosis induced by this protein. A significant increase in the extent of lipid peroxidation (levels of oxidant) and decrease in the activities of antioxidants was also observed which correlated with the increased generation of tumor necrosis factor-alpha, interleukine-1alpha and interleukine-6. These results suggest that caspase-3 and tumor necrosis factor-alpha in conjunction with other cytokines may induce apoptotic cell death through the up-regulation of oxidants and down-regulation of antioxidants. These findings may be relevant for the better understanding of the disease pathophysiology and for the future developments of diagnostic and preventive strategies during the host-pathogen interactions.

Manipulation of innate immunity by bacterial pathogens

The past decade has witnessed tremendous growth in two related fields: innate immunity and microbial pathogenesis. Many pathogens have evolved mechanisms to infect their hosts in the face of a fully functional innate immune system, and there are numerous examples by which pathogens avoid recognition and/or suppress inflammation. In this review, I suggest that pathogens not only survive the innate immune response, but use it to promote their pathogenesis.

Bacterial toxins and the immune system: show me the in vivo targets

Microorganisms that cause persistent infection often exhibit specific adaptations that allow them to avoid the adaptive immune response. Recently, several bacterial toxins have been shown in vitro to disrupt immune cell functions. However, it remains to be established whether these activities are relevant during infection and whether these toxins have specifically evolved to disrupt the adaptive immune system.

Salmonella-induced macrophage death: multiple mechanisms, different outcomes

The facultative intracellular pathogen Salmonella enterica triggers programmed cell death in macrophages. The close examination of this phenomenon has revealed an unusually complex picture involving diverse mechanisms that lead to different types of programmed cell death. It appears that the outcome of the interaction of salmonella with macrophages depends on the relative contribution of two type III protein secretion systems, in conjunction with the stimulation of innate immunity outputs through conserved determinants collectively known as 'pathogen-associated molecular patterns' (PAMPs). These interactions result in a breakdown of the balance between survival and pro-apoptotic cellular pathways, which eventually leads to macrophage cell death. The relative significance for the infection process of the different types of macrophage cell death triggered by salmonella remains to be established.