Macrophage inhibiting activity in serum and central lymph of Listeria-immune mice

Comparison of widely used Listeria monocytogenes strains EGD, 10403S, and EGD-e highlights genomic variations underlying differences in pathogenicity

For nearly 3 decades, listeriologists and immunologists have used mainly three strains of the same serovar (1/2a) to analyze the virulence of the bacterial pathogen Listeria monocytogenes. The genomes of two of these strains, EGD-e and 10403S, were released in 2001 and 2008, respectively. Here we report the genome sequence of the third reference strain, EGD, and extensive genomic and phenotypic comparisons of the three strains. Strikingly, EGD-e is genetically highly distinct from EGD (29,016 single nucleotide polymorphisms [SNPs]) and 10403S (30,296 SNPs), and is more related to serovar 1/2c than 1/2a strains. We also found that while EGD and 10403S strains are genetically very close (317 SNPs), EGD has a point mutation in the transcriptional regulator PrfA (PrfA*), leading to constitutive expression of several major virulence genes. We generated an EGD-e PrfA* mutant and showed that EGD behaves like this strain in vitro, with slower growth in broth and higher invasiveness in human cells than those of EGD-e and 10403S. In contrast, bacterial counts in blood, liver, and spleen during infection in mice revealed that EGD and 10403S are less virulent than EGD-e, which is itself less virulent than EGD-e PrfA*. Thus, constitutive expression of PrfA-regulated virulence genes does not appear to provide a significant advantage to the EGD strain during infection in vivo, highlighting the fact that in vitro invasion assays are not sufficient for evaluating the pathogenic potential of L. monocytogenes strains. Together, our results pave the way for deciphering unexplained differences or discrepancies in experiments using different L. monocytogenes strains.

Over the past 3 decades, Listeria has become a model organism for host-pathogen interactions, leading to critical discoveries in a broad range of fields, including bacterial gene regulation, cell biology, and bacterial pathophysiology. Scientists studying Listeria use primarily three pathogenic strains: EGD, EGD-e, and 10403S. Despite many studies on EGD, it is the only one of the three strains whose genome has not been sequenced. Here we report the sequence of its genome and a series of important genomic and phenotypic differences between the three strains, in particular, a critical mutation in EGD’s PrfA, the main regulator of Listeria virulence. Our results show that the three strains display differences which may play an important role in the virulence differences observed between the strains. Our findings will be of critical relevance to listeriologists and immunologists who have used or may use Listeria as a tool to study the pathophysiology of listeriosis and immune responses.

Impaired resistance to Mycobacterium tuberculosis infection after selective in vivo depletion of L3T4+ and Lyt-2+ T cells

The resistance of mice against Mycobacterium tuberculosis infection after selective in vivo depletion of L3T4+ and Lyt-2+ T cells was studied. Thymectomized mice were treated with rat monoclonal antibodies against the L3T4 or Lyt-2 molecule to selectively eliminate the respective T-cell subset. In both L3T4+ and Lyt-2+ T-cell-depleted mice, resistance against subsequent infection with M. tuberculosis was markedly impaired compared with that in untreated controls, with L3T4+ T-cell-depleted mice showing more pronounced effects. Simultaneous depletion of L3T4+ and Lyt-2+ T cells did not further exacerbate infection. These findings suggest that both L3T4+ and Lyt-2+ T cells are involved in the acquisition of resistance against tuberculosis.

Modulation of natural killer activity in mice following infection with Listeria monocytogenes

Mice that received a sublethal, intraperitoneal dose of viable Listeria monocytogenes, virulent strain 10403, exhibited a systemic increase in natural killer (NK) activity. The kinetics of the response differed with respect to the various effector cell populations analyzed. Resident peritoneal cells and peripheral blood leukocytes demonstrated high NK activity on Days 3, 7, and 10. Peak spleen and bone marrow NK activity was observed on Day 3, returning to normal levels by Day 7. In contrast, peritoneal exudate cells, elicited with proteose peptone, expressed enhanced NK activity for 60 days following infection with viable Listeria. Augmented NK activity was detected with all cell types as early as 12 hr after infection. The intraperitoneal injection of nonviable antigenic preparations derived from L. monocytogenes, strain 10403, resulted in the enhancement of peritoneal and splenic NK activity. In contrast, mice that received an intraperitoneal injection of avirulent Listeria, strain 19113, failed to express enhanced levels of NK activity. The genetic trait of anti-listerial resistance which is associated with non-H-2 linked genes was of no importance with respect to enhanced NK activity. Listeria-resistant C57BL/6J and Listeria-susceptible DBA/2J mice both produced systemic augmentation of NK activity following infection. NK activity was not abrogated by macrophage depletion or by treatment with anti-Thy 1.2 serum plus complement. These results confirm the potent immunostimulatory capacity of virulent Listeria for NK activity and provide further insight into the kinetics of this response in various lymphoid compartments. The protracted augmentation of NK activity of elicited peritoneal exudate cells as compared to nonelicited peritoneal cells in Listeria-primed mice suggests that the influx of inflammatory cells may provide NK-enriched and/or accessory populations for immunopotentiation of NK activity in inflammatory sites.

Antibacterial defence mechanisms

Pathogenic bacteria fall into two groups with regard to their fate within phagocytes: extracellular bacteria are promptly killed after phagocytosis and facultative intracellular bacteria are resistant to intracellular killing unless macrophages are activated. Extracellular bacteria cause purulent infections, and facultative intracellular bacteria granulomatous ones. Humoral immune mechanisms (antibody, complement) deal mainly with extracellular bacteria, while cellular immune mechanisms (T cells, macrophages) deal with facultative intracellular bacteria. The specific and nonspecific factors and their interactions are discussed with respect to their role in the buildup of an effective antibacterial defence.

The role of macrophages in acquired cell-mediated immunity to Toxoplasma gondii

Mice immunized by primary infection with an avirulent strain of Toxoplasma gondii were protected against challenge infection with a highly virulent strain, even though macrophages were eliminated either by dextran sulfate or by carbon ink. This findings differs strikingly from previous results obtained in similar experiments with other intracellular pathogens such as Listeria monocytogenes and Salmonella typhimurium. It is therefore concluded that the macrophage population, which plays an essential role in the cell-mediated immunity to reinfection with L. monocytogenes and S. typhimurium, is not of primary importance in cell-mediated immunity to reinfection with T. gondii.

T lymphocyte-macrophage interactions in cellular antibacterial immunity

Acquired resistance to facultative intracellular bacteria depends on a bicellular mechanism whereby specific T lymphocytes activate macrophages for enhanced bacteriocidal capacity. In vivo, protection is paralleled by delayed-type hypersensitivity. In vitro correlates are specific T lymphocyte proliferation and interleukin induction. Macrophage activation results from complex cell interactions involving both T lymphocytes and macrophages. Although such interactions are not yet fully understood, it appears likely that interleukin-facilitated collaboration between Lyt 1 and Lyt 123 T lymphocytes is required. Most probably, H2-restricted interactions between antigen-presenting mononuclear phagocytes and Lyt 1 T lymphocytes induce secretion of interleukins which further recruit additional Lyt 1 T lymphocytes from the Lyt 123 T lymphocyte set. In this way, the pool of Lyt 1 T lymphocytes capable of attracting and activating macrophages at the site of bacterial and implantation via lymphokines (macrophage activating factor, migration inhibition factor) could be markedly enhanced.