An attenuated strain of the facultative intracellular bacterium Francisella tularensis can escape the phagosome of monocytic cells

The facultative intracellular bacterium Francisella tularensis is a highly virulent and contagious organism, and little is known about its intracellular survival mechanisms. We studied the intracellular localization of the attenuated human vaccine strain, F. tularensis LVS, in adherent mouse peritoneal cells, in mouse macrophage-like cell line J774A.1, and in human macrophage cell line THP-1. Confocal microscopy of infected J774A.1 cells indicated that during the first hour of infection the bacteria colocalized with the late endosomal-lysosomal glycoprotein LAMP-1, but within 3 h this colocalization decreased significantly from approximately 60% to 30%. Transmission electron microscopy revealed that >90% of bacteria were not enclosed by a phagosomal membrane after 2 h of infection, and some bacteria were in vacuoles that were only partially surrounded by a limiting membrane. Similar findings were obtained with all three host cell types. Immunoelectron microscopy performed with an F. tularensis LVS-specific polyclonal rabbit antiserum showed that the antiserum stained a thick, evenly distributed capsule-like material in bacteria grown in broth. In contrast, intracellular F. tularensis LVS cells were only marginally stained with this antiserum. Instead, most of the immunoreactive material was diffusely localized in the phagosomes or was associated with the phagosomal membrane. Our findings indicate that F. tularensis LVS is able to escape from the phagosomes of macrophages via a mechanism that may involve degradation of the phagosomal membrane.     

Toll-like receptor 2-mediated signaling requirements for Francisella tularensis live vaccine strain infection of murine macrophages

Francisella tularensis, an aerobic, non-spore-forming, gram-negative coccobacillus, is the causative agent of tularemia. We reported previously that F. tularensis live vaccine strain (LVS) elicited strong, dose-dependent NF-kappaB reporter activity in Toll-like receptor 2 (TLR2)-expressing HEK293T cells and proinflammatory gene expression in primary murine macrophages. Herein, we report that F. tularensis LVS-induced murine macrophage proinflammatory cytokine gene and protein expression are overwhelmingly TLR2 dependent, as evidenced by the abrogated responses of TLR2(-/-) macrophages. F. tularensis LVS infection also increased expression of TLR2 both in vitro, in mouse macrophages, and in vivo, in livers from F. tularensis LVS-infected mice. Colocalization of intracellular F. tularensis LVS, TLR2, and MyD88 was visualized by confocal microscopy. Signaling was abrogated if the F. tularensis LVS organisms were heat or formalin killed or treated with chloramphenicol, indicating that the TLR2 agonist activity is dependent on new bacterial protein synthesis. F. tularensis LVS replicates in macrophages; however, bacterial replication was not required for TLR2 signaling because LVSDeltaguaA, an F. tularensis LVS guanine auxotroph that fails to replicate in the absence of exogenous guanine, activated NF-kappaB in TLR2-transfected HEK293T cells and induced cytokine expression in wild-type macrophages comparably to wild-type F. tularensis LVS. Collectively, these data indicate that the primary macrophage response to F. tularensis LVS is overwhelmingly TLR2 dependent, requires de novo bacterial protein synthesis, and is independent of intracellular F. tularensis replication.     

Interaction of B cells with intracellular pathogen Francisella tularensis

Immunity to Francisella tularensis is largely mediated by T lymphocytes but an important role of B lymphocytes in early stage of infection was previously uncovered. We wanted to find out if F. tularensis is able to infect B cells and/or influence them by direct contact. To investigate this possibility we infected B cell lines from mouse (A20) or humans (Ramos RA-1), or primary mouse spleen cells, with F. tularensis LVS and F. tularensis FSC200 in vitro. In all cases, we detected bacteria on the cell surface and inside the B cells using transmission electron microscopy. More than 20% cells were infected by microbes after 24h. The number of bacteria, determined by CFU, increased about 1 log during 24h. Infection with live bacteria led to apoptosis of Ramos cells and mouse CD19(+) spleen cells. Approximately 30% of cells were apoptotic after 24h and 70% after 48 h, independently of the F. tularensis strain, while only 10% of non-infected cell were apoptotic at either time point. Apoptosis was confirmed by Western blot using anti-PARP antibodies. Thus, this study demonstrates unique phenomenon - namely, the ability of the intracellular pathogen F. tularensis to invade and induce apoptosis in B cells.     

Role of primary human alveolar epithelial cells in host defense against Francisella tularensis infection

Francisella tularensis, an intracellular pathogen, is highly virulent when inhaled. Alveolar epithelial type I (ATI) and type II (ATII) cells line the majority of the alveolar surface and respond to inhaled pathogenic bacteria via cytokine secretion. We hypothesized that these cells contribute to the lung innate immune response to F. tularensis. Results demonstrated that the live vaccine strain (LVS) contacted ATI and ATII cells by 2 h following intranasal inoculation of mice. In culture, primary human ATI or ATII cells, grown on transwell filters, were stimulated on the apical (AP) surface with virulent F. tularensis Schu 4 or LVS. Basolateral (BL) conditioned medium (CM), collected 6 and 24 h later, was added to the BL surfaces of transwell cultures of primary human pulmonary microvasculature endothelial cells (HPMEC) prior to the addition of polymorphonuclear leukocytes (PMNs) or dendritic cells (DCs) to the AP surface. HPMEC responded to S4- or LVS-stimulated ATII, but not ATI, CM as evidenced by PMN and DC migration. Analysis of the AP and BL ATII CM revealed that both F. tularensis strains induced various levels of a variety of cytokines via NF-kappaB activation. ATII cells pretreated with an NF-kappaB inhibitor prior to F. tularensis stimulation substantially decreased interleukin-8 secretion, which did not occur through Toll-like receptor 2, 2/6, 4, or 5 stimulation. These data indicate a crucial role for ATII cells in the innate immune response to F. tularensis.     

Delineation of the molecular mechanisms of Francisella tularensis-induced apoptosis in murine macrophages

Francisella tularensis is a facultative intracellular bacterium capable of inducing apoptosis in murine macrophages. Here we analyzed the pathway leading to apoptosis in the murine macrophage-like cell line J774A.1 after infection with F. tularensis strain LVS (named LVS for live vaccine strain). We obtained evidence that the infection affected the mitochondria of the macrophages, since it induced release of the mitochondrial molecule cytochrome c into the cytosol and changed the potential over the mitochondrial membrane. Moreover, activation of caspase 9 and the executioner caspase 3 was also observed in the LVS-infected J774A.1 macrophages. The activated caspase 3 degraded poly(ADP-ribose) polymerase (PARP). All of these events were observed within 9 to 12 h after the initiation of infection, and maximum degradation of a synthetic caspase 3 substrate occurred at 18 h. The internucleosomal fragmentation and PARP degradation resulting from activation of this apoptotic pathway was prevented by the caspase 3 inhibitor Z-DEVD-fmk. No involvement of caspase 1, caspase 8, Bcl-2, or Bid was observed. Thus, the F. tularensis infection induces macrophage apoptosis through a pathway partly resembling the intrinsic apoptotic pathway.     

Purified lipopolysaccharide from Francisella tularensis live vaccine strain (LVS) induces protective immunity against LVS infection that requires B cells and gamma interferon

Previous results have demonstrated that nonspecific protective immunity against lethal Francisella tularensis live vaccine strain (LVS) or Listeria monocytogenes infection can be stimulated either by sublethal infection with bacteria or by treatment with bacterial DNA given 3 days before lethal challenge. Here we characterize the ability of purified lipopolysaccharide (LPS) from F. tularensis LVS to stimulate similar early protective immunity. Treatment of mice with surprisingly small amounts of LVS LPS resulted in very strong and long-lived protection against lethal LVS challenge within 2 to 3 days. Despite this strong protective response, LPS purified from F. tularensis LVS did not activate murine B cells for proliferation or polyclonal immunoglobulin secretion, nor did it activate murine splenocytes for secretion of interleukin-4 (IL-4), IL-6, IL-12, or gamma interferon (IFN-gamma). Immunization of mice with purified LVS LPS induced a weak specific anti-LPS immunoglobulin M (IgM) response and very little IgG; however, infection of mice with LVS bacteria resulted in vigorous IgM and IgG, particularly IgG2a, anti-LPS antibody responses. Studies using various immunodeficient mouse strains, including LPS-hyporesponsive C3H/HeJ mice, muMT(-) (B-cell-deficient) knockout mice, and IFN-gamma-deficient mice, demonstrated that the mechanism of protection does not involve recognition through the Lps(n) gene product; nonetheless, protection was dependent on B cells as well as IFN-gamma.     

Susceptibility of immunodeficient mice to aerosol and systemic infection with virulent strains of Francisella tularensis

Previous studies have shown that IFN-gamma, TNF-alpha and NOS-2, but not B cells, are crucial for host defense against primary systemic infection with the attenuated live vaccine strain (LVS) of Francisella tularensis. In this study, we examined the importance of these and additional immune components in host resistance against infection with virulent strains of F. tularensis initiated by systemic and airborne routes. Wild-type (WT) mice and mice deficient in IFN-gamma, TNFR1R2, NOS-2, or B cells were equally susceptible to low dose ( approximately 10 colony forming units) aerosol or intradermal challenge with virulent type B F. tularensis, and succumbed to the infection between days 6 and 8 post-inoculation. Quantitative bacteriology showed that IFN-gamma-/- and B cell-/- mice consistently harbored up to one log(10) more bacteria in their lungs, spleens and livers than WT mice at day 5 post aerosol exposure. Surprisingly, however, compared to other strains of KO mice and WT control mice, IFN-gamma-/- mice showed only mild liver damage as assessed by histopathology and liver function tests. Additional experiments established that even mice with broad immunodeficiency (SCID, neutropenic, splenectomized or thymectomized mice and mice treated with corticosteroid) were no more susceptible to aerosol-initiated infection with virulent type B or type A F. tularensis than immunosufficient control mice. Combined, our results indicate that, unlike LVS, normal type A and type B F. tularensis strains are so extremely virulent that even immunocompetent mice are virtually defenseless to low dose aerosol and intradermal challenges with them.     

Expression of IglC is necessary for intracellular growth and induction of apoptosis in murine macrophages by Francisella tularensis

Francisella tularensis is a facultative intracellular bacterium capable of inducing apoptosis in murine macrophages. In a previous study, an iglC null mutant of F. tularensis live vaccine strain LVS was generated by allelic replacement and in the current study this iglC mutant was successfully complemented in trans. We characterized the capacity of this iglC mutant and the complemented strain to induce macrophage apoptosis. The iglC mutant did not induce apoptosis in the infected cells. In contrast, the complemented iglC strain was able to multiply in the murine macrophage-like cell line J774A.1 and induced apoptosis similar to that of the wild-type strain. It is the first successful example of complementation in trans of a F. tularensis mutant strain and more importantly this work provides direct evidence that the intracellular growth ability is essential for F. tularensis to induce macrophage apoptosis.     

In vivo modulation of the murine immune response to Francisella tularensis LVS by administration of anticytokine antibodies.

The role(s) of gamma interferon (IFN-gamma), tumor necrosis factor alpha (TNF-alpha), and interleukin-4 (IL-4) in establishment and maintenance of protective immunity to Francisella tularensis LVS in mice (C3H/HeN) was examined by selective removal of these cytokines in vivo with neutralizing antibodies. The 50% lethal dose (LD50) for mice infected intradermally with F. tularensis alone was 136,000 CFU; treatment of mice with anti-IFN-gamma or anti-TNF-alpha at the time of infection significantly reduced (P much less than 0.05) the LD50 to 2 and 5 CFU, respectively. Abrogation of protective immunity, however, was effective only when anti-IFN-gamma or anti-TNF-alpha was administered prior to day 3 postinfection. In contrast, the LD50 for mice treated with anti-IL-4 was repeatedly higher (555,000 CFU) than for controls; this difference, however, was not significant (P greater than 0.05). Thus, IL-4 may be detrimental, while IFN-gamma and TNF-alpha were clearly crucial to the establishment of protective immunity to F. tularensis during a primary infection. The importance of IFN-gamma and TNF-alpha during a secondary immune response to F. tularensis was also investigated. Spleen cells from immune mice passively transfer protective immunity to recipient mice in the absence of confounding antibody-mediated immunity. This passive transfer of immunity, however, was abrogated by treatment of recipient mice with anti-IFN-gamma or anti-TNF-alpha at the time of challenge infection. That anticytokines effectively abrogate protective immunity very early in the course of infection with F. tularensis suggests that T-cell-dependent activation of macrophages for microbicidal activity is unlikely. These T-cell-independent events early in the course of infection may suppress bacterial replication until a T-cell-dependent response ultimately clears the bacteria.     

Francisella tularensis LVS evades killing by human neutrophils via inhibition of the respiratory burst and phagosome escape

Francisella tularensis is a Gram-negative bacterium and the causative agent of tularemia. Recent data indicate that F. tularensis replicates inside macrophages, but its fate in other cell types, including human neutrophils, is unclear. We now show that F. tularensis live vaccine strain (LVS), opsonized with normal human serum, was rapidly ingested by neutrophils but was not eliminated. Moreover, evasion of intracellular killing can be explained, in part, by disruption of the respiratory burst. As judged by luminol-enhanced chemiluminescence and nitroblue tetrazolium staining, neutrophils infected with live F. tularensis did not generate reactive oxygen species. Confocal microscopy demonstrated that NADPH oxidase assembly was disrupted, and LVS phagosomes did not acquire gp91/p22(phox) or p47/p67(phox). At the same time, F. tularensis also impaired neutrophil activation by heterologous stimuli such as phorbol esters and opsonized zymosan particles. Later in infection, LVS escaped the phagosome, and live organisms persisted in the neutrophil cytosol for at least 12 h. To our knowledge, our data are the first demonstration of a facultative intracellular pathogen, which disrupts the oxidative burst and escapes the phagosome to evade elimination inside neutrophils, and as such, our data define a novel mechanism of virulence.     

Critical role for serum opsonins and complement receptors CR3 (CD11b/CD18) and CR4 (CD11c/CD18) in phagocytosis of Francisella tularensis by human dendritic cells (DC): uptake of Francisella leads to activation of immature DC and intracellular survival of the bacteria

Francisella tularensis is one of the most infectious human pathogens known. Although much has been learned about the immune response of mice using an attenuated live vaccine strain (LVS) derived from F. tularensis subspecies holarctica (Type B), little is known about the responses of human monocyte-derived immature dendritic cells (DC). Here, we show that optimal phagocytosis of LVS by DC is dependent on serum opsonization. We demonstrate that complement factor C3-derived opsonins and the major complement receptors expressed by DC, the integrins CR3 (CD11b/CD18) and CR4 (CD11c/CD18), play a critical role in this adhesion-mediated phagocytosis. LVS induced proinflammatory cytokine production and up-regulation of costimulatory surface proteins (CD40, CD86, and MHC Class II) on DC but resisted killing. Once taken up, LVS grew intracellularly, resulting in DC death. DC maturation and cytokine production were induced by direct contact/phagocytosis of LVS or interaction with soluble products of the bacteria, and enhanced activation was seen when LVS was pretreated with serum. Sonicated LVS and supernatants from LVS cultures were potent activators of DC, but LVS LPS failed to activate DC maturation or cytokine production. Serum-treated LVS rapidly induced (within 6 h) a number of cytokines including IL-10, a potent suppressor of macrophage functions and down-regulator of Th1-like responses and the Th1 response inducer IL-12. These results suggest that the simultaneous production of an activating (IL-12, IL-1beta, and TNF-alpha) and a suppressing (IL-10) cytokine profile could contribute to the immunopathogenesis of tularemia.     

Francisella tularensis LVS grown in macrophages has reduced ability to stimulate the secretion of inflammatory cytokines by macrophages in vitro

The virulence of Francisella tularensis LVS is determined in part by its ability to invade and replicate within macrophages and stimulate the production of inflammatory cytokines. The present study determined the effects of growing F. tularensis in macrophages on its ability to stimulate cytokine secretion by macrophages. F. tularensis grown in Mueller-Hinton broth (FtB) stimulated the secretion of large amounts of TNF-alpha, IL-12p40, IL-6 and MCP-1/CCL2 when incubated with macrophages overnight. In contrast, F. tularensis released from infected macrophages (FtMac) stimulated very little secretion of these cytokines by primary cultures of murine peritoneal macrophages, human monocytes or macrophage cell lines. Stimulation of nitric oxide production by FtMac was also less than that elicited by FtB. FtMac killed with gentamicin or paraformaldehyde also stimulated low levels of cytokine secretion. FtMac recovered the ability to stimulate cytokine secretion after overnight culture in broth. Infection of macrophages with FtMac inhibited the cytokine response to subsequent stimulation with LPS from Escherichia coli but did not affect Fcgamma receptor-mediated phagocytosis. FtMac were ingested by macrophages at about half the rate of FtB, however, this did not account for the lower cytokine secretion. FtMac and FtB replicated at similar rates within macrophages. Finally, Mice infected with FtMac had a higher mortality rate than those infected with FtB. These results reveal that growth in macrophages causes a reversible phenotypic change in F. tularensis that is associated with decreased stimulation of cytokine secretion, inhibition of LPS-stimulated secretion of inflammatory cytokines by macrophages and increased lethality in mice.     

Toll-like receptor 2 is required for inflammatory responses to Francisella tularensis LVS

Francisella tularensis, a gram-negative bacterium, is the etiologic agent of tularemia and has recently been classified as a category A bioterrorism agent. Infections with F. tularensis result in an inflammatory response that plays an important role in the pathogenesis of the disease; however, the cellular mechanisms mediating this response have not been completely elucidated. In the present study, we determined the role of Toll-like receptors (TLRs) in mediating inflammatory responses to F. tularensis LVS, and the role of NF-kappaB in regulating these responses. Stimulation of bone marrow-derived dendritic cells from C57BL/6 wild-type (wt) and TLR4-/- but not TLR2-/- mice, with live F. tularensis LVS elicited a dose-dependent increase in the production of tumor necrosis factor alpha. F. tularensis LVS also induced in a dose-dependent manner an up-regulation in the expression of the costimulatory molecules CD80 and CD86 and of CD40 and the major histocompatibility complex class II molecules on dendritic cells from wt and TLR4-/- but not TLR2-/- mice. TLR6, not TLR1, was shown to be involved in mediating the inflammatory response to F. tularensis LVS, indicating that the functional heterodimer is TLR2/TLR6. Stimulation of dendritic cells with F. tularensis resulted in the activation of NF-kappaB, which resulted in a differential effect on the production of pro- and anti-inflammatory cytokines. Taken together, our results demonstrate the role of TLR2/TLR6 in the host's inflammatory response to F. tularensis LVS in vitro and the regulatory function of NF-kappaB in modulating the inflammatory response.     

The live vaccine strain of Francisella tularensis replicates in human and murine macrophages but induces only the human cells to secrete proinflammatory cytokines

Francisella tularensis is the highly infectious agent of tularemia, a disease that can prove fatal in humans. An attenuated live vaccine strain (LVS) of this bacterium is avirulent in man but produces lethal illness in mice. As a step toward understanding the species specificity of the LVS, we compared its interactions with murine and human leukocytes. The bacterium replicated within murine bone marrow-derived macrophages (muBMDM), human monocyte-derived macrophages (huMDM), and freshly isolated human monocytes. However, the murine and human phagocytes differed in their ability to secrete proinflammatory cytokines in response to the LVS. The huMDM released large amounts of CXC chemokine ligand 8 (CXCL8) and CC chemokine ligand 2 when incubated with live or killed LVS organisms, and live bacteria also elicited production of interleukin-1beta (IL-1beta). Furthermore, human monocytes secreted CXCL8, IL-1beta, and tumor necrosis factor alpha in response to various bacterial preparations. In contrast, muBMDM produced little to no proinflammatory cytokines or chemokines when treated with any preparations of the LVS. Clearly, human and murine macrophages support growth of this bacterium. However, the greater proinflammatory response of human leukocytes to F. tularensis LVS may contribute to the avirulence of this strain in the human host.     

Construction and characterization of an attenuated purine auxotroph in a Francisella tularensis live vaccine strain

Francisella tularensis is a facultative intracellular pathogen and is the etiological agent of tularemia. It is capable of escaping from the phagosome, replicating to high numbers in the cytosol, and inducing apoptosis in macrophages of a variety of hosts. F. tularensis has received significant attention recently due to its potential use as a bioweapon. Currently, there is no licensed vaccine against F. tularensis, although a partially protective live vaccine strain (LVS) that is attenuated in humans but remains fully virulent for mice was previously developed. An F. tularensis LVS mutant deleted in the purMCD purine biosynthetic locus was constructed and partially characterized by using an allelic exchange strategy. The F. tularensis LVS delta purMCD mutant was auxotrophic for purines when grown in defined medium and exhibited significant attenuation in virulence when assayed in murine macrophages in vitro or in BALB/c mice. Growth and virulence defects were complemented by the addition of the purine precursor hypoxanthine or by introduction of purMCDN in trans. The F. tularensis LVS delta purMCD mutant escaped from the phagosome but failed to replicate in the cytosol or induce apoptotic and cytopathic responses in infected cells. Importantly, mice vaccinated with a low dose of the F. tularensis LVS delta purMCD mutant were fully protected against subsequent lethal challenge with the LVS parental strain. Collectively, these results suggest that F. tularensis mutants deleted in the purMCD biosynthetic locus exhibit characteristics that may warrant further investigation of their use as potential live vaccine candidates.     

Biochemical and immunological properties of ribonucleic acid-rich extracts from Francisella tularensis.

Ribonucleic acid (RNA)-rich extracts derived from the attenuated strain of Francisella tularensis (strain LVS) protected Swiss mice against lethal challenge with F. tularensis strain 425 but not against strain SCHU S4. No killed preparation, including an RNA-rich extract from SCHU S4 itself, offered protection against strain SCHU S4 in contrast to the high level of protection offered against this strain by vaccination with live strain LVS. The protective activity observed against strain 425 was sensitive to ribonuclease but not to Pronase. Protective activity is not a general property of bacterial RNA, since RNA-rich extracts from Staphylococcus aureus offered no protection against tularemia, although disc gel electrophoresis showed similar kinds and amounts of RNA in preparations form F. tularensis and S. aureus. Furthermore, inability to localize activity to a specific region in sucrose gradients suggests a structural rather than an informational role for the RNA in such extracts. RNA-rich extracts from F. tularensis but not from S. aureus were efficient inducers of F. tularensis opsonins in mouse serum, suggesting one mechanism by which such extracts confer protection.     

Transfer of immunity against lethal murine Francisella infection by specific antibody depends on host gamma interferon and T cells.

Both serum and spleen cells from mice immune to Francisella tularensis transfer protection to naive recipients. Here we characterize the mechanism of protection induced by transfer of immune mouse serum (IMS). IMS obtained 4 weeks after intradermal infection with 10(3) bacteria of the live vaccine strain (LVS) contained high levels of immunoglobulin G2 (IgG2a) and IgM (end point titers, 1:16,600 and 1:7,200, respectively) and little IgG1, IgG2b, or IgG3. LVS-specific antibodies were detected 5 days after intradermal infection, and reached peak levels by 2 weeks postinfection. Only sera obtained 10 days or more after sublethal infection, when IgG titers peaked, transferred protection against a challenge of 100 50% lethal doses (LD50s). Purified high-titer IgG anti-LVS antibody but not IgM anti-LVS antibody was responsible for transfer of protection against an intraperitoneal challenge of up to 3,000 LD50s. IMS had no direct toxic effects on LVS and did not affect uptake or growth of bacteria in association with peritoneal cells. One day after LVS infection, liver, spleen, and lung tissue from mice treated with IMS contained 1 to 2 log units fewer bacteria than did tissue from mice treated with normal mouse serum or phosphate-buffered saline. Between 2 and 4 days after infection, however, bacterial growth rates in tissues were similar in both serum-protected mice and unprotected mice. Bacterial burdens in IMS-treated, LVS-infected mice declined in infected tissues after day 5, whereas control animals died. This lag phase suggested that development of a host response was involved in complete bacterial clearance. In fact, transfer of IMS into normal recipients that were simultaneously treated with anti-gamma interferon and challenged with LVS did not protect mice from death. Further, transfer of IMS into athymic nu/nu mice did not protect against LVS challenge; protection was, however, reconstituted by transfer of normal T cells into nu/nu mice. Thus, "passive" transfer of protection against LVS with specific antibody is not passive but depends on a host T-cell response to promote clearance of systemic infection and protection against lethal disease.     

A capsule-deficient mutant of Francisella tularensis LVS exhibits enhanced sensitivity to killing by serum but diminished sensitivity to killing by polymorphonuclear leukocytes

The live vaccine strain (LVS) of Francisella tularensis is killed by human polymorphonuclear leukocytes as a result of strictly oxygen-dependent mechanisms (S. L?fgren, A. T?rnvik, M. Thore, and J. Carlsson, Infect. Immun. 43:730-734, 1984). We now report that a capsule-deficient (Cap-) mutant of LVS survives in the leukocytes. In contrast to the encapsulated parent strain, the Cap- mutant was avirulent in mice and was susceptible to the bactericidal effect of nonimmune human serum. The mutant was killed by serum as a result of activation of the classical pathway of complement by naturally occurring immunoglobulin M. This killing by serum was mitigated by the presence of human polymorphonuclear leukocytes. After opsonization in complement component C5-deficient nonimmune serum, the Cap- mutant was ingested and survived in the leukocytes. Under these conditions, the parent strain was killed. The leukocytes responded to both the parent and the Cap- strain with a very low chemiluminescent response. Only the response to the parent strain was inhibited by superoxide dismutase. When the Cap- mutant was opsonized with immunoglobulin G, it induced a higher and superoxide dismutase-inhibitable chemiluminescent response and was killed by the leukocytes. In conclusion, the capsule of F. tularensis LVS seemed to protect this organism against the bactericidal effect of serum. When deprived of the capsule, the organism failed to induce an antimicrobial response in polymorphonuclear leukocytes and survived in the leukocytes. Survival in phagocytes is a key characteristic of intracellular parasites. The Cap- mutant of F. tularensis may become a useful tool in experiments to explain the differences between pathways of ingestion of intracellular parasites, evidenced by the death or survival of the parasite.