IFN-γ production by CD27⁺ NK cells exacerbates Listeria monocytogenes infection in mice by inhibiting granulocyte mobilization.

Natural killer (NK) cells are key components of the immune system involved in several immune reactions, including the clearance of intracellular pathogens. When activated, NK cells rapidly secrete particular cytokines that activate innate immunity and facilitate development of adaptive responses. Conflicting reports on the role of NK cells during infection by Listeria monocytogenes can be found in the literature. Here, we demonstrate that during lethal infection by L. monocytogenes, activation of NK cells via the costimulatory molecule CD27 leads to excessive IFN-γ production. This impairs innate anti-bacterial host defenses by inducing downregulation of CXCR2 on granulocytes and consequently inhibiting their recruitment to the sites of infection. The use of antibodies to block CD27 signaling or to deplete IFN-γ was sufficient to rescue mice from lethal challenge by L. monocytogenes. Our findings contribute to a better understanding of the importance of CD27 signaling in activation of NK cells and should provide new ways of interfering with infections.

Pathogenicity of different PR8 influenza A virus variants in mice is determined by both viral and host factors.

Experimental mouse models were used to compare virulence and reproduction rate of three mouse-adapted variants of the PR8 influenza A virus strain. We observed large differences in pathogenicity in two mouse strains. The PR8M variant was lethal in DBA/2J mice but not in C57BL/6J mice, whereas PR8F and hvPR8 variants were lethal in both mouse strains. High lethality of PR8M in DBA/2J correlated with high viral load at early time points after infection and spread of the virus into alveolar regions. Also, higher viral loads and mortality in mice infected with PR8F resulted in a higher number of infiltrating leukocytes. 3D-protein structure predictions of the HA indicated amino acid sequence alterations which may render the HA cleavage site in PR8F more accessible to host proteases. Infection of C57BL/6J mice with a re-assorted PR8 virus revealed that the HA gene is the main determinant of virulence of the PR8F variant.

Gene expression changes in the host response between resistant and susceptible inbred mouse strains after influenza A infection.

Inbred mouse strains exhibit differences in susceptibility to influenza A infections. However, the molecular mechanisms underlying these differences are unknown. Therefore, we infected a highly susceptible mouse strain (DBA/2J) and a resistant strain (C57BL/6J) with influenza A H1N1 (PR8) and performed genome-wide expression analysis. We found genes expressed in lung epithelium that were specifically down-regulated in DBA/2J mice, whereas a cluster of genes on chromosome 3 was only down-regulated in C57BL/6J. In both mouse strains, chemokines, cytokines and interferon-response genes were up-regulated, indicating that the main innate immune defense pathways were activated. However, many immune response genes were up-regulated in DBA/2J much stronger than in C57BL/6J, and several immune response genes were exclusively regulated in DBA/2J. Thus, susceptible DBA/2J mice showed a hyper-inflammatory response. This response is similar to infections with highly pathogenic influenza virus and may serve as a paradigm for a hyper-inflammatory host response to influenza A virus.

Tumor invasion of Salmonella enterica serovar Typhimurium is accompanied by strong hemorrhage promoted by TNF-alpha.

BACKGROUND: Several facultative anaerobic bacteria with potential therapeutic abilities are known to preferentially colonize solid tumors after systemic administration. How they efficiently find and invade the tumors is still unclear. However, this is an important issue to be clarified when bacteria should be tailored for application in cancer therapy. METHODOLOGY/PRINCIPAL FINDINGS: We describe the initial events of colonization of an ectopic transplantable tumor by Salmonella enterica serovar Typhimurium. Initially, after intravenous administration, bacteria were found in blood, spleen, and liver. Low numbers were also detected in tumors associated with blood vessels as could be observed by immunohistochemistry. A rapid increase of TNF-alpha in blood was observed at that time, in addition to other pro-inflammatory cytokines. This induced a tremendous influx of blood into the tumors by vascular disruption that could be visualized in H&E stainings and quantified by hemoglobin measurements of tumor homogenate. Most likely, together with the blood, bacteria were flushed into the tumor. In addition, blood influx was followed by necrosis formation, bacterial growth, and infiltration of neutrophilic granulocytes. Depletion of TNF-alpha retarded blood influx and delayed bacterial tumor-colonization. CONCLUSION: Our findings emphasize similarities between Gram-negative tumor-colonizing bacteria and tumor vascular disrupting agents and show the involvement of TNF-alpha in the initial phase of tumor-colonization by bacteria.

Somatic hypermutation in peritoneal B1b cells.

Murine B1 cells have been shown to be able to switch to IgA in vitro. In agreement, we could demonstrate in the peritoneum of mice the presence of IgA producing B1 cells. Interestingly, enzyme-linked immunospot assays of lipopolysaccharide stimulated cultures revealed that only the B1b cell subpopulation contained high numbers of such cells while IgA producing B cells were rare amongst the B2 and B1a cell populations. This was confirmed by RT-PCR on sorted peritoneal B cell subpopulations. In addition, the variable regions associated with IgA of peritoneal B1b cells displayed extensive variation due to somatic hypermutation. In contrast, mutations were found only at low frequencies in VH regions associated with IgM of both B1 cell populations. Thus, peritoneal B1b cells display many similarities to B2 cells. This finding is consistent with the idea of a layered immune system in which peritoneal B1a and splenic follicular B2 cells appear at the two extremes and peritoneal B1b and B2 cells represent intermediates.

Listeria monocytogenes desensitizes immune cells to subsequent Ca2+ signaling via listeriolysin O-induced depletion of intracellular Ca2+ stores.

Listeriolysin O (LLO), the pore-forming toxin of Listeria monocytogenes, is a prototype of the cholesterol-dependent cytolysins (CDCs) secreted by several pathogenic and nonpathogenic gram-positive bacteria. In addition to mediating the escape of the bacterium into the cytosol, this toxin is generally believed to be a central player in host-pathogen interactions during L. monocytogenes infection. LLO triggers the influx of Ca(2+) into host cells as well as the release of Ca(2+) from intracellular stores. Thus, many of the cellular responses induced by LLO are related to calcium signaling. Interestingly, in this study, we report that prolonged exposure to LLO desensitizes cells to Ca(2+) mobilization upon subsequent stimulations with LLO. Cells preexposed to LLO-positive L. monocytogenes but not to the LLO-deficient Deltahly mutant were found to be highly refractory to Ca(2+) induction in response to receptor-mediated stimulation. Such cells also exhibited diminished Ca(2+) signals in response to stimulation with LLO and thapsigargin. The presented results suggest that this phenomenon is due to the depletion of intracellular Ca(2+) stores. The ability of LLO to desensitize immune cells provides a significant hint about the possible role played by CDCs in the evasion of the immune system by bacterial pathogens.