BLOOD-FREE MEDIUM FOR THE RAPID GROWTH OF PASTEURELLA TULARENSIS

FeoB-mediated uptake of iron by Francisella tularensis

Francisella tularensis, the bacterial cause of tularemia, infects the liver and replicates in hepatocytes in vivo and in vitro. However, the factors that govern adaptation of F. tularensis to the intrahepatocytic niche have not been identified. Using cDNA microarrays, we determined the transcriptional profile of the live vaccine strain (LVS) of F. tularensis grown in the FL83B murine hepatocytic cell line compared to that of F. tularensis cultured in broth. The fslC gene of the fsl operon was the most highly upregulated. Deletion of fslC eliminated the ability of the LVS to produce siderophore, which is involved in uptake of ferric iron, but it did not impair its growth in hepatocytes, A549 epithelial cells, or macrophages. Therefore, we sought an alternative means by which F. tularensis might obtain iron. Deletion of feoB, which encodes a putative ferrous iron transporter, retarded replication of the LVS in iron-restricted media, reduced its growth in hepatocytic and epithelial cells, and impaired its acquisition of iron. Survival of mice infected intradermally with a lethal dose of the LVS was slightly improved by deletion of fslC but was not altered by loss of feoB. However, the ΔfeoB mutant showed diminished ability to colonize the lungs, liver, and spleen of mice that received sublethal inocula. Thus, FeoB represents a previously unidentified mechanism for uptake of iron by F. tularensis. Moreover, failure to produce a mutant strain lacking both feoB and fslC suggests that FeoB and the proteins of the fsl operon are the only major means by which F. tularensis acquires iron.

Characterization of fig operon mutants of Francisella novicida U112

Francisella species secrete a polycarboxylate siderophore that resembles rhizoferrin to acquire ferric iron. Several of the Francisella siderophore synthesis genes are contained in a Fur-regulated operon (designated fig or fsl) comprised of at least seven ORFs including fur. Reverse transcriptase-PCR showed transcriptional linkage between figD and figE and between figE and figF. Mutations were constructed in four of these ORFs (figB, figC, figD, and figE) in Francisella novicida U112. All four of these new mutants and a F. novicida figA mutant grew at rates comparable to that of wild type under iron-replete conditions but growth of all five mutants was stunted in iron-limiting media. When ferric rhizoferrin was added to the iron-limited media, growth of the figA, figB, figC, and figD mutants was restored to levels similar to those obtained in iron-replete media. However, this exogenously added siderophore could not rescue the figE mutant. When Chrome Azurol S assays were used to measure siderophore production, the figA, figB, and figC mutants were markedly deficient in their ability to synthesize siderophore whereas the figD and figE mutants produced siderophore at levels equivalent to the wild-type parent strain.

Identification of Francisella tularensis genes affected by iron limitation

Cells of an attenuated live vaccine strain (LVS) of F. tularensis grown under iron-restricted conditions were found to contain increased quantities of several proteins relative to cells of this same strain grown under iron-replete conditions. Mass spectrometric analysis identified two of these proteins as IglC and PdpB, both of which are encoded by genes located in a previously identified pathogenicity island in F. tularensis LVS. Regions with homology to the consensus Fur box sequence were located immediately in front of the iglC and pdpB open reading frames (ORFs), and in silico analysis of the F. tularensis Schu4 genome detected a number of predicted 5' untranslated regions that contained putative Fur boxes. The putative Fur box preceding Francisella iron-regulated gene A (figA) had the highest degree of identity with the consensus Fur box sequence. DNA microarray analysis showed that nearly 80 of the genes in the F. tularensis LVS genome were up- or down-regulated at least twofold under iron-restricted growth conditions. When tested for possible siderophore production by means of the Chrome Azurol S assay, a wild-type F. novicida strain produced a large reaction zone whereas its figA mutant produced very little reactivity in this assay. In addition, a cross-feeding experiment demonstrated that this siderophore-like activity produced by the wild-type F. novicida strain could enhance the ability of the F. novicida figA mutant to grow under iron-restricted conditions. This study provides the first identification of iron-regulated genes in F. tularensis LVS and evidence for the production of a siderophore-like molecule by F. novicida.

T-cell co-operation in the mediation of acquired resistance to Listeria monocytogenes

Monoclonal antibodies were used to select T-cell subsets that mediate delayed-type hypersensitivity (DTH) and acquired cellular resistance (CRI) in rats infected with Listeria monocytogenes. The mediators of DTH were identified as W3/25+ OX8- T cells. The latter comprised a subset distinct from that which could protect recipient rats against a Listeria challenge. The protective T cells had a W3/25- OX8+ phenotype. The T-cell mediators of cellular resistance to infection (TCRI) failed to augment the expression of DTH; however, the mediators of DTH (TDTH) significantly enhanced the protective capacity of TCRI. This property of TDTH correlated with the ability of the cells to promote the focal deployment of TCRI and macrophages at sites of soluble Listeria antigen injection in skin, and in peritoneal exudates induced by killed L. monocytogenes. These findings illustrate the co-operative interaction of activated T cells in acquired resistance to L. monocytogenes, and imply that DTH has a purposeful role in the host defence against infection.

Stimulation of activated rat T cells in vitro by Listeria monocytogenes antigens

A soluble extract of Listeria monocytogenes bound firmly and in similar amounts to a variety of rat cells. Cells that bound this material differed in their capacity to stimulate the in vitro proliferation of lymphocytes obtained from the thoracic duct of Listeria-immune donors. The capacity of cells to serve as antigen-presenting cells in this system coincided or closely overlapped the expression on these cells of an Ia antigen-like structure. Three lines of evidence indicate that T cells respond to L. monocytogenes antigen: the responder cells are members of a nylon-wool nonadherent population that lacks readily detectable surface immunoglobulin; they express determinants recognized by the W3/25 monoclonal antibody (a surface marker of rat peripheral T cells); and they are stimulated optimally by L. monocytogenes antigen when the latter is displayed on cells that share a haplotype with the responder lymphocytes.

Stimulation of subpopulations of human lymphocytes by a vaccine strain of Francisella tularensis

When purified T lymphocytes from individuals vaccinated with a viable, attenuated strain of Francisella tularensis were incubated in vitro in the presence of heat-killed bacteria or a membrane preparation of the vaccine strain, they were stimulated to form blast cells and to synthesize deoxyribonucleic acid. The blast cells had the characteristics of T cells, being devoid of surface immunoglobulin and able to form rosettes with sheep erythrocytes. The stimulation occurred only when monocytes were present. A lymphocyte preparation enriched in B lymphocytes did not respond to the heat-killed bacteria or to the membrane preparation. In a stimulated mononuclear leukocyte preparation, about 70% of the blast cells formed rosettes with sheep erythrocytes, and 10 to 20% of them had surface immunoglobulin. The results show that there is an enlarged population of specifically committed T lymphocytes after tularemia vaccination. It is suggested that the lymphocyte stimulation test measures mainly T-lymphocyte reactivity when membranes or whole bacteria of F. tularensis LVS are used as antigen, and that the stimulation of human T lymphocytes by whole bacteria or bacterial membranes is completely monocyte or macrophage dependent. The present experimental procedure may provide a model for study of antigen-induced stimulation of human lymphocytes under controlled conditions. The technique used gave a reproducible, extremely purified preparation of T lymphocytes and a preparation of monocytes especially suitable for microcultures.

The mediator of cellular immunity. IX. The relationship between cellular hypersensitivity and acquired cellular resistance in rats infected with Listeria monocytogenes

Acquired resistance to the intracellular bacterial parasite, Listeria monocytogenes can be transferred to normal recipients by thoracic duct lymphocytes or peritoneal exudate cells obtained from rats infected with this organism; The appearance of protective cells in thoracic duct lymph coincides with the development in the donors of delayed-type hypersensitivity to Listeria antigens and accumulation in induced peritoneal exudates of cells which are responsive to these antigens in the migration inhibitory factor (MIF) assay. The cells in exudates that confer protection, and those that release MIF, arise at sites remote from their final destination. From their point of origin in the caudal lymph nodes of infected rats, cells with these properties are delivered to the thoracic duct and hence to the blood from where they are drawn into the peritoneal cavity in response to inflammation. The parallel observed in the appearance, increase and subsequent decline of protective lymphocytes and MIF-producing cells in exudates suggest that the two activities are mediated by a single line of T cells. However this may be, the development and deployment of the cells concerned encourages the belief that MIF has a meaningful role in the expression of cellular resistance to infection.

Tularaemia in the rat. I. The cellular basis on host resistance to infection

Rats infected with the live vaccine strain (LVS) of Francisella tularensis develop in vivo and in vitro evidence of cellular hypersensitivity and a concomitant state of cellular resistance to infection. They key role of sensitized lymphocytes in cellular resistance was domonstrated in transfer experiments. Using this technique, it was shown that thoracic duct lymphocytes from Francisella immune donors conferred specific antimicrobial resistance on normal recipients, whereas antiserum afforded no protection whatsoever. Further evidence for the participation of sensitized lymphocytes in the host's defence emerged from experiments in which a comparative analysis was made of the immunogenic properties of living and heat-killed LVS organisms. Rats stimulated with the living parasite developed cellular hypersensitivity and specific antibodies. Throacic duct lymphocytes obtained from such animals could immunize adoptively. By comparison, rats stimulated with a substantially larger number of dead organisms failed to develop cellular hypersensitivity and their lymphocytes were devoid of protective activity. Dead organisms, however, provoked an antibody response similar to that observed in infected rats. The development of cellular hypersensitivity in Francisella-infected rats is associated with enhanced resistance to Listeria monocytogenes. This finding accords with the results of similar studies of infection immunity to other intracellular parasites, and implies that the expression of cellular resistance to F. tularensis is a cooperative venture involving specifically sensitized lymphocytes and non-specific inflammatory cells, presumably macrophages.

Vaccine potencies of the live vaccine strain of Francisella tularensis and isolated bacterial components

The live vaccine strain of Francisella tularensis rapidly lost virulence (to mice) when grown under controlled conditions in the Porton mobile enclosed chemostat but protective activity was lost much less rapidly. In contrast, bacteria maintained their virulence and vaccine potency during batch culture in shaken flasks. Attempts to identify virulence and protective factors of F.tularensis were unsuccessful. The protective activities in mice of killed suspensions of F.tularensis, bacterial fractions and extracts of live bacteria were compared with the activity of live bacteria; all the non-living potential vaccines were ineffective.

The mediator of cellular immunity. VII. Localization of sensitized lymphocytes in inflammatory exudates

Peritoneal exudates induced in rats infected with Listeria monocytogenes contain sensitized lymphocytes which can protect normal recipients against a Listeria challenge. The protective cells arise in lymphoid tissue remote from the peritoneal cavity. Those formed in the caudal lymph nodes of subcutaneously infected rats are delivered to the thoracic duct and hence to the blood from where they are drawn into exudates. Immunoblasts are the most immature members of this protective cell population and they alone among the cells in central lymph localize in exudates induced by killed bacteria. They do so in substantial numbers, but only during the early postinduction period. The "homing" of immunoblasts to inflammatory foci seems to be determined by a general property of the cells rather than their immunological commitment; however, the intense inflammation induced by organisms to which an animal has been specifically sensitized is accompanied by an exuberant influx of immunoblasts into lesions. Sensitized lymphocytes that extravasate in the inflamed peritoneal may generate more of their own kind, but some give rise to small lymphocytes. The latter also have protective properties and, with time, comprise an increasing portion of the protective cell population. The results imply that the tissue disposition of sensitized lymphocytes in the body is determined by a complementary relationship between blood-borne immunoblasts and vascular endothelium in inflamed tissue. The results also provide a plausible explanation for the concentration of sensitized lymphocytes at sites of microbial implantation where they alone would be expected to collaborate with monocyte-derived macrophages in the control of infection.

Properties of aldolase from Francisella tularensis

The aldolase of Francisella tularensis resembles Class II aldolases in its requirement for divalent ions and its inactivation by metal chelating agents. Cysteine and other reducing agents stimulated the activity of the enzyme.