ABSTRACT
T lymphocytes and gamma interferon (IFN-γ) are known mediators of immune resistance to Toxoplasma gondii infection, but whether B cells also play an important role is not clear. We have investigated this issue using B-cell-deficient (μMT) mice. If vaccinated with attenuated T. gondii tachyzoites, μMT mice are susceptible to a challenge intraperitoneal infection with highly virulent tachyzoites that similarly vaccinated B-cell-sufficient mice resist. Susceptibility is evidenced by increased numbers of parasites at the challenge infection site and by extensive mortality. The susceptibility of B-cell-deficient mice does not appear to be caused by deficient T-cell functions or diminished capacity of vaccinated and challenged B-cell-deficient mice to produce IFN-γ. Administration of Toxoplasma-immune serum, but not nonimmune serum, to vaccinated B-cell-deficient mice significantly prolongs their survival after challenge with virulent tachyzoites. Vaccinated mice lacking Fc receptors or the fifth component of complement resist a challenge infection, suggesting that neither Fc-receptor-dependent phagocytosis of antibody-coated tachyzoites nor antibody-dependent cellular cytotoxicity nor antibody-and-complement-dependent lysis of tachyzoites is a crucial mechanism of resistance. However, Toxoplasma-immune serum effectively inhibits the infection of host cells by tachyzoites in vitro. Together, the results support the hypothesis that B cells are required for vaccination-induced resistance to virulent tachyzoites in order to produce antibodies and that antibodies may function protectively in vivo by blocking infection of host cells by tachyzoites.
The protozoan parasiteToxoplasma gondii is a significant cause of morbidity and mortality in AIDS patients and congenitally infected individuals (19, 21). Because T. gondii infections can be devastating to those who are immunodeficient or immunologically immature, there is an obvious motive to understand the mechanisms of resistance to this parasite that are normally employed by immunocompetent individuals. To this end, a number of studies have employed infection of laboratory mice as a model system with which to dissect immunological mechanisms of resistance toToxoplasma. Thus, it has been found that acquired immunity to Toxoplasma, i.e., the ability to keep chronic infection in check and the capacity to resist challenge with virulent tachyzoites after vaccination, depends on T lymphocytes, particularly CD8+ T cells (8, 9, 24, 26, 34), and gamma interferon (IFN-γ) (9, 32, 33). Whether B cells and antibodies are important components of acquired resistance to T. gondii is much less clear, however.
There is little doubt that T. gondii infection elicits a specific antibody response. The Sabin-Feldman dye test (29), used clinically to diagnose infection with Toxoplasma, is simply an assay for serum antibodies which lyse tachyzoites in a complement-dependent manner (30). Both human and murine infections elicit not only immunoglobulin G (IgG) and IgM antibodies in serum but also IgA in secretions (3, 18, 20, 23). Still, it is not known whether any of these antibodies normally play a protective role against Toxoplasma. To address this question, Frenkel and Taylor (7) examined mice that were treated with anti-immunoglobulin from birth (μ-suppressed mice) and therefore that had no B cells. If such mice were infected with T. gondiiand protected with chemotherapy to allow immunity to develop, the mice lived longer than T-cell-deficient mice after chemotherapy was stopped but still eventually died. However, if given immune serum, many of the mice survived. The authors concluded that antibodies may be able to provide some protection against chronic T. gondii infection. A number of passive immunization studies have been performed to determine the role of antibodies in immunity to Toxoplasma. For example, immune lymphoid cells or immune sera partially protected guinea pigs against a challenge with RH tachyzoites (27). Other studies in which mice were challenged with parasites after being given immune sera have produced mixed results regarding the protective potential of Toxoplasma-specific antibodies (6, 15, 17, 25). Additional studies have shown that monoclonal antibodies against various Toxoplasma antigens have the potential to protect unimmunized mice against a challenge with moderately virulent parasites and, to a lesser extent, with highly virulent parasites (12, 31). Such passive immunization experiments may reveal whether an anti-Toxoplasma antibody is capable of providing protection in the absence of an already developed cell-mediated immunity but do not address whether antibodies generated in the normal course of infection are required for protection of chronically infected mice or vaccinated and subsequently challenged mice.
To get at the question of whether antibodies or B cells are necessary for resistance to Toxoplasma infection, we have studied mice which have no B cells (μMT mice) as a result of a targeted mutation (14). Our results clearly demonstrate that B cells are required for resistance to T. gondii in a model in which mice are vaccinated with avirulent tachyzoites and later challenged with highly virulent tachyzoites. Our findings suggest that the role of B cells is to produce antibodies that block the infection of host cells by tachyzoites.
MATERIALS AND METHODS
Mice.Adult (>8-week-old) male and female B-cell-deficient mice with a targeted mutation in a transmembrane exon of the immunoglobulin μ chain gene (μMT mice [14]) were used. Mice were verified to be B cell deficient by flow cytometric analysis of peripheral blood lymphocytes. In addition, sera from B-cell-deficient mice were found to be devoid of T. gondii-specific antibodies when tested by enzyme-linked immunosorbent assay (ELISA). Also, fecal extracts from infected B-cell-deficient mice were found to be negative when tested for antibodies able to bind to tachyzoites in a flow cytometric assay (5). Thus, the B-cell deficiency of μMT mice was confirmed by their inability to produce antibodies specific for T. gondii in sera or intestinal secretions. In initial experiments, B-cell-deficient mice with a 129×B6 mixed genetic background were used. In later experiments, mice that were fully backcrossed to the C57BL/6J strain (B6) were used. Essentially the same results were obtained with both strains of mice. B6 mice were used as controls. B-cell-deficient JHD/JHD mice (4) having the BALB/c genetic background were also used, along with BALB/cByJ controls.
Mice with a targeted mutation in the gene for the γ chain common to FcɛR and to FcγRI and FcγRIII (36) and mice with a targeted mutation in the gene for FcγRII (35) were also used. B6129F2 mice were used as controls for Fc receptor-deficient mice, which were not fully backcrossed. In addition, C5-deficient B10.D2/oSnJ and DBA/2J mice were used along with major histocompatibility complex-matched BALB/cByJ controls. Mice were bred at the Trudeau Institute from founders obtained from the Jackson Laboratory.
Mice were fed laboratory chow and were given acidified water ad libitum. Mice at the Trudeau Institute are free of known common viral pathogens of mice as evidenced by periodic screening of sera from sentinel mice, performed by the University of Missouri Research Animal Diagnostic and Investigative Laboratory, Columbia, Mo.
Parasites and immunizations.Mice were immunized by intraperitoneal (i.p.) injections of 2 × 104 ts-4 strain tachyzoites (29). Mice were challenged by i.p. injection of 2 × 103 RH strain tachyzoites. Tachyzoites were maintained in cultures of Hs68 human fibroblasts (ATCC CRL 1635) in HEPES-buffered RPMI 1640 medium supplemented with heat-inactivated fetal bovine serum (FBS; 10%),l-glutamine, and penicillin-streptomycin at 33 (ts-4) or 37°C (RH) in a humidified 5% CO2 atmosphere. To test whether immune serum inhibits the infection of cultured cells, ts-4 tachyzoites were cultured in immune or normal (nonimmune) serum (10% [vol/vol]) for 10 min, washed, and added to monolayers of Hs68 fibroblasts cultured on coverslips in supplemented RPMI 1640 medium (10% FBS and antibiotics) containing 10% immune or normal serum. Cells and tachyzoites were cultured at 37°C and 5% CO2for progressive lengths of time, washed, and stained for microscopy.
In some in vitro experiments, RH strain tachyzoites engineered to constitutively express green fluorescent protein (GFP) were used. These tachyzoites were a kind gift from John Boothroyd, Stanford University. To test whether immune serum inhibits the ability of these parasites to infect cells, the tachyzoites were added to Hs68 fibroblast cultures containing 10% immune or normal serum. Cells were trypsinized at various later times and analyzed by flow cytometry.
Spleen cell cultures.Spleen cell suspensions were prepared by crushing spleens through stainless steel mesh and suspending cells in HEPES-buffered Hanks' balanced salt solution. Erythrocytes were removed by hypo-osmotic lysis from cells pelleted by centrifugation. Washed leukocytes were counted and suspended in HEPES-buffered RPMI 1640 containing 10% FBS, 50 μM 2-mercaptoethanol, and antibiotics. Spleen cells were placed in 96-well flat-bottom plates (6 × 105/well) in triplicate wells with either concanavalin A (5-μg/ml final concentration), ts-4 tachyzoites (1 × 105 per well), or medium alone in a total volume of 200 μl per well. Supernatants were harvested after 72 h and stored frozen at −20°C until assayed for cytokine content.
IFN-γ assay.Levels of IFN-γ in sera, peritoneal lavage fluids, and spleen cell culture supernatants were measured by ELISA (13).
Flow cytometry.Spleen cells were prepared as described above. To obtain peripheral blood leukocytes, a few drops of blood were collected and erythrocytes were lysed hypo-osmotically. Leukocytes were pelleted in a microcentrifuge, washed, and incubated with antibodies to detect both CD45RB (16A; phycoerythrin [PE] conjugated; Pharmingen) and CD4 [GK1.5; fluorescein isothiocyanate (FITC)-conjugated F(ab′)2; prepared at this institute] or CD8 [ATCC TIB210; FITC-conjugated F(ab′)2; prepared at this institute]. Additional samples were stained with anti-CD19 (1D3, PE conjugated; Pharmingen) and anti-mouse IgG plus IgM plus IgA [FITC-conjugated F(ab′); Southern Biotechnology Associates]. Lymphocytes were identified by forward scatter-side scatter characteristics.
Statistics.Unless otherwise stated, differences in means between groups were analyzed by Student's t test. Survival data were analyzed by the Mann-Whitney U test.
RESULTS
Vaccinated B-cell-deficient mice are susceptible to a challenge infection with virulent tachyzoites.Tachyzoites of the temperature-sensitive ts-4 strain of Toxoplasma have been widely used to immunize mice for analysis of mechanisms of immunity to a later challenge with highly virulent Toxoplasma. Virtually all unimmunized mice will die from a challenge with as few as 10 highly virulent RH tachyzoites, the parent strain of ts-4 (28), but ts-4-immune immunocompetent mice can resist a challenge with several orders of magnitude more RH tachyzoites. Therefore, to investigate whether B cells are required for vaccination-induced immunity to T. gondii, we immunized B-cell-deficient mice with ts-4 tachyzoites and challenged them at later times with an i.p. injection of RH tachyzoites. Results shown in Fig.1a are representative of five such experiments using B-cell-deficient μMT mice and B6 controls and show clearly that ts-4-immunized B-cell-deficient μMT mice are highly susceptible to RH challenge, whereas B-cell-sufficient B6 mice are not.
Susceptibility of ts-4-vaccinated B-cell-deficient mice to challenge with virulent tachyzoites. (a) Groups of five μMT and B6 mice were vaccinated with ts-4 tachyzoites and challenged with RH tachyzoites 6 weeks later. The median survival time of vaccinated μMT mice (8 days; range, 7 to 18 days) was significantly (P < 0.05) shorter than that of vaccinated B6 mice (>100 days). Data are representative of five such experiments. In all experiments, the survival of vaccinated B-cell-sufficient mice was significantly longer than that of vaccinated μMT mice. In some experiments, the survival of vaccinated μMT mice was statistically longer than that of naive controls, although the difference was quantitatively small. (b) Groups of male and female BALB/c background JHD/JHD mice (n = 22) and B-cell-sufficient BALB/cByJ (n = 9) mice were vaccinated with ts-4 tachyzoites and challenged 52 days later, together with nonimmune JHD/JHD controls (n = 6), with virulent RH tachyzoites. The median survival time of vaccinated JHD/JHD mice (11 days; range, 7 to 24 days) was significantly (P < 0.01) shorter than that of vaccinated BALB/c controls (>98 days) but significantly (P < 0.01) longer than that of naive JHD/JHD controls (7.5 days; range, 7 to 11 days).
B6 mice (H-2b) are classified as generally susceptible to infection with T. gondii, developing more brain cysts and dying sooner after infection with cysts, for example, than more-resistant H-2d strains such as BALB/c (1, 2, 22, 38). To determine whether the susceptibility to challenge infection in μMT mice was unique to mice having the B6 genetic background, we examined B-cell-deficient JHD mice (4) having the more intrinsically resistant BALB/c background. They, too, are susceptible to an RH challenge after vaccination with ts-4 tachyzoites (Fig. 1b).
To further characterize the difference between vaccinated μMT mice and B6 mice in response to an RH challenge, their peritoneal lavage fluids were examined for the presence of parasites and tissues were examined histologically. Figure 2 shows representative cytospins of cells and parasites in peritoneal lavages obtained from groups of four vaccinated μMT mice, vaccinated B6 mice, and unvaccinated μMT mice at times after RH challenge at which μMT mice were becoming moribund. Unvaccinated mice had massive numbers of tachyzoites (>107/ml of lavage fluid). Vaccinated B6 mice had almost no detectable tachyzoites (<104/ml), whereas individual vaccinated μMT mice had 105 to 106/ml, which was a significantly higher value than that for vaccinated B6 mice (P < 0.05) but less than that for unvaccinated controls (P < 0.05). Thus, vaccination rendered μMT mice able to control parasite proliferation at the site of infection better than unvaccinated mice but not as well as vaccinated B-cell-sufficient mice.
Cytospins of peritoneal lavage fluids from vaccinated μMT and B6 mice challenged with virulent RH tachyzoites. (a) A moribund μMT mouse vaccinated 60 days prior to challenge with RH tachyzoites. Lavage fluids were harvested on day 10 after challenge. (b) Apparently healthy identically treated B6 mouse. (c) A moribund unvaccinated control μMT mouse 7 days after RH challenge. Arrows, tachyzoites. Magnification, ×400.
Histological analysis of lungs, livers, and brains from challenged mice as they were becoming moribund revealed that all μMT mice had significant pathology in one or more organs. In one mouse there was a marked multifocal meningoencephalitis throughout the brain. Inflammatory foci were accompanied by necrotic cellular debris, occasional degenerating neurons, and areas of malacia. In three other mice, liver inflammation was multifocal and associated with blood vessels. Mixed populations of mononuclear and polymorphonuclear inflammatory cells adhered to the endothelial surfaces and filled perivascular spaces. In some areas, vessel walls were obscured by the infiltrate and the response appeared to constitute a true vasculitis. There was multifocal thrombus formation in affected vessels which was associated with liver infarcts characterized by coagulation necrosis. In one animal there was evidence of a possible systemic effect on coagulation, with a prominent thrombus present in a pulmonary vessel. In vaccinated B6 mice there were no significant brain lesions. There was liver inflammation, but it was milder than that in μMT mice and without infarcts.
Vaccinated B-cell-deficient mice are not deficient in IFN-γ production in response to T. gondii in vivo.Because IFN-γ is required for resistance to challenge with RH tachyzoites in ts-4-immunized mice (9), we examined whether there was deficient production of IFN-γ in vaccinated μMT mice. ts-4-immunized and unimmunized μMT and B6 mice were challenged with RH tachyzoites and their sera were collected and their peritoneal cavities were flushed 7 days after challenge (experiment 1) or 9 to 10 days later, as mice were becoming moribund (experiment 2). Fluids were analyzed for IFN-γ by ELISA (Table 1). In neither compartment was the level of IFN-γ less in immunized μMT mice than in immunized B6 mice, and was in fact significantly greater in μMT mice except in peritoneal lavages in experiment 1. It was also found that the percentage of CD8+ peripheral blood T cells from ts-4-vaccinated μMT mice expressing relatively lower levels of the CD45RB marker (indicative of prior antigen exposure) did not differ significantly 50 days after vaccination from the corresponding percentage of the population in vaccinated B-cell-sufficient mice (Fig.3), or at any of several times during the first 5 weeks after vaccination (P. C. Sayles and L. L. Johnson, unpublished data). Similarly, the total numbers of CD45RBloCD8+ cells in the two groups were not different. Thus activation of the critical CD8+ T-cell population in peripheral blood did not appear to be impaired in B-cell-deficient mice.
In vivo production of IFN-γ by vaccinated and naive μMT and B6 mice challenged with virulent RH tachyzoites
Activated peripheral blood T cells (PBL) in ts-4-vaccinated μMT and B6 mice. Peripheral blood cells taken from unvaccinated mice and vaccinated mice 50 days after immunization with 2 × 104 ts-4 tachyzoites were examined by flow cytometry. Cells were stained with CD4- or CD8-specific FITC-conjugated F(ab′)2 antibodies and PE-conjugated anti-CD45RB. Bars show the percentages of CD45RBlo cells within the CD4+ and CD8+ lymphocyte populations (determined by forward scatter-side scatter characteristics). Asterisks, significant (P < 0.05) differences from corresponding populations in naive mice.
Spleen cells from vaccinated and nonimmune μMT and B6 mice were also characterized for expression of markers of activation and for the capacity to produce IFN-γ when cultured with tachyzoites in vitro. Table 2 shows representative results from one of three such experiments, in which similar results were obtained. Although the percentage of the CD45RBloCD8+ population in vaccinated μMT mice was not significantly different from that in vaccinated B6 mice, the absolute numbers of these cells was far less in the vaccinated μMT mice, because the total number of spleen cells was much smaller in these mice. In addition, although the amount of IFN-γ produced by a fixed number of vaccinated μMT mouse spleen cells was significantly greater than that produced by vaccinated B6 mouse spleen cells, the estimated capacity of IFN-γ production by the entire spleen of vaccinated B6 mice was greater than that of vaccinated μMT mice, again owing to the great difference between the groups in total spleen cellularity.
IFN-γ production in vitro by spleen cells from vaccinated μMT and B6 micea
Toxoplasma-immune serum enhances resistance to RH challenge in vaccinated B-cell-deficient mice.The foregoing results establish that B cells are required by immunized mice to resist an RH challenge but not to produce IFN-γ or to activate CD8+ T cells. Moreover, several T-cell-dependent functions appear to be normal in μMT mice in our experience, including resistance to primary and secondary intravenous Listeria monocytogenes infection and rejection of allogeneic skin grafts (Sayles and Johnson, unpublished data). These findings suggest that the defect in μMT mice, with respect to resistance toToxoplasma, may lie in their inability to produce antibodies, rather than resulting from an indirect effect of B-cell deficiency on T-cell-dependent immunity. Therefore, we hypothesized that providing immunized μMT mice with Toxoplasma-immune serum would improve their resistance to RH challenge. Results presented in Fig. 4 show thatToxoplasma-immune serum significantly lengthened the survival time of vaccinated μMT mice beyond that of nonimmune μMT mice or vaccinated μMT mice given nonimmune serum.
Survival of ts-4-vaccinated μMT mice given immune or nonimmune serum. Groups of 5 μMT mice were vaccinated with ts-4 and challenged with RH tachyzoites 37 days later, along with unimmunized controls. Vaccinated mice were given 0.5 ml ofToxoplasma-immune serum (log10 IgG2a titer = 4.72 by ELISA) or nonimmune serum i.p. on days −1, 0, 2, 4, 7, and 10 relative to RH challenge. Survival of mice given immune serum (median survival time, 27 days; range, 18 to 41 days) was significantly (P < 0.05) prolonged relative to that of mice given nonimmune serum (median survival time, 13 days; range, 4 to 14 days).
Vaccinated mice lacking Fc receptors or deficient in C5 are resistant to RH challenge.There are several possible mechanisms by which antibodies could function protectively in vaccinated mice, including facilitation of Fc receptor-dependent uptake of parasites by phagocytic cells or killing by cytotoxic cells (ADCC), lysis of extracellular tachyzoites in a complement-dependent manner, and direct blocking of tachyzoite entry into host cells. To examine the first two of these possibilities, mice lacking Fc receptors or mice lacking the fifth component of complement were immunized with ts-4 tachyzoites and challenged with RH tachyzoites. Both kinds of mice were adequately protected against the RH challenge (Fig.5 and 6), which argues that neither Fc receptor-mediated phagocytosis or ADCC nor antibody-and-complement-dependent lysis of tachyzoites is a likely mechanism by which antibodies function protectively in ts-4-immunized, RH-challenged mice.
Survival of vaccinated FcR−/−mice challenged with virulent tachyzoites. Groups of five male mice with a targeted mutation in the gene for the γ chain common to FcɛR and FcγRIII (FcR γ−/−) or with a targeted mutation in the gene for FcγRII, along with nonmutant B6129F2 controls, were vaccinated with ts-4 tachyzoites and, together with unimmunized controls, were challenged with RH tachyzoites 49 days later. Vaccinated FcR−/− mice survived (median survival time of each group, >56 days) significantly (P < 0.05) longer than unvaccinated controls (median survival time, 8 days; range, 8 to 9 days) and as long as vaccinated nonmutant controls (median survival time, >56 days).
Survival of vaccinated C5-deficient mice challenged with virulent tachyzoites. Groups of five C5-deficient male mice (B10.D2o and DBA/2J) and C5-sufficient B6 controls were vaccinated with ts-4 and challenged with RH tachyzoites 48 days later, together with unvaccinated controls. Vaccinated C5-deficient mice survived significantly longer than unvaccinated controls (median survival time, 8 days; range, 8 to >50 days) and as long as vaccinated C5-sufficient controls (median survival time, >50 days).
Toxoplasma-immune serum inhibits infection of cultured cells by tachyzoites.To determine whetherToxoplasma-specific antibodies directly affect the ability of tachyzoites to enter host cells, tachyzoites were incubated in serum from mice immunized against Toxoplasma or from unimmunized mice and then added to monolayers of Hs68 human fibroblasts on coverslips. Mouse serum (either immune or nonimmune) was present in the culture medium at 10% final concentration. Coverslips were removed at progressive intervals and washed, and the cells were stained and examined microscopically for the presence of tachyzoites. Results from one of three such experiments are shown in Fig.7, where it may be seen that immune serum almost completely inhibited tachyzoite entry into host cells but that nonimmune serum did not.
Immune serum-mediated inhibition of infection of cultured fibroblasts by ts-4 tachyzoites. (a) Immune serum at 45 h of incubation. (b) Nonimmune serum at 45 h of incubation. Arrows, intracellular and extracellular tachyzoites. Magnification, ×400.
To more quantitatively assess the capacity of immune serum to inhibit the entry of tachyzoites into cells, cultures of Hs68 fibroblasts were incubated with RH tachyzoites expressing GFP that had been treated with heat-inactivated (56°C, 30 min) or non-heat-inactivated immune or nonimmune serum from B6 mice or μMT mice. Cells were detached from culture dishes at progressive times after infection by treatment with trypsin and analyzed by flow cytometry to determine the proportion of cells harboring parasites. Analysis gates were placed around fibroblasts, which were easily distinguished from free parasites by forward-scatter-side scatter characteristics, and the percentages of infected cells were determined. Representative histograms are shown in Fig.8. Both heat-inactivated and untreated immune B6 serum drastically reduced the proportion of Hs68 cells that became infected with GFP-expressing tachyzoites (Fig. 8a to d). In contrast, serum from ts-4-vaccinated μMT mice did not inhibit infection of fibroblasts (Fig. 8e and f). Similar results were obtained at time points from 3 to 30 h after infection. Visual inspection of cytospin preparations of infected cells indicated that almost all of the cell-associated parasites were inside the cells rather than stuck to the surface. The higher mean fluorescence intensity of parasite-infected cells in the presence of nonimmune sera (Fig. 8a, c, and e) is consistent with intracellular multiplication of parasites. Together, the evidence supports the contention that immunoglobulins are the key infection-inhibiting components of immune sera.
Inhibition of infection of cultured fibroblasts by serum from immune B6 mice but not by serum from vaccinated μMT mice. Numbers in the upper right of each panel denote the percentages of infected (fluorescent) cells and the mean fluorescence intensity of the infected population.
DISCUSSION
Our findings show that B cells are required for immunity to an intraperitoneal challenge with virulent Toxoplasma parasites in mice vaccinated with attenuated tachyzoites. A requirement for B cells, in addition to cell-mediated immunity, has also been reported for mice infected with Plasmodium chabaudi (37) and Schistosoma mansoni (11). In several of the experiments performed in this study (e.g., Fig. 1b), B-cell-deficient ts-4-vaccinated mice challenged with RH tachyzoites survived significantly longer than unimmunized mice. This finding suggests that, even in the complete absence of B cells, some amount of host resistance, presumably T cell mediated, is developed but that it is insufficient to completely protect the mice. We have also observed that unimmunized mice given repeated injections ofToxoplasma-immune serum have very little resistance to challenge with RH tachyzoites (Sayles and Johnson, unpublished data). Thus, mice apparently require both immune T cells and antibody-producing B cells for effective resistance to an i.p. challenge with virulent tachyzoites.
CD8+ T cells and IFN-γ are known to be important components of resistance to Toxoplasma. Although μMT mice have numerically fewer splenic T cells than similarly vaccinated B6 mice (Table 2), the absence of B cells during infection does not seem to compromise the ability of vaccinated, challenged mice to activate the T cells they possess or to make substantial quantities of IFN-γ in serum or at the site of the challenge infection in the peritoneal cavity (Table 1). It should be noted that despite the deficiency of activated CD4+ and CD8+ T cells in the spleens of vaccinated μMT mice, we have found that these mice have numbers of activated (CD45RBlo) peripheral blood CD8+ T cells that are equivalent to those in vaccinated B6 mice (approximately 2.3 × 106/ml). Moreover, the estimated total number of activated peripheral blood CD8+ T cells in vaccinated μMT mice was greater than that in spleens of vaccinated B6 mice. This may explain why the levels of IFN-γ in peritoneal lavage fluids and sera of vaccinated μMT mice were not less than those of vaccinated B6 mice (Table 1), despite the deficiency of activated splenic T cells.
Our findings are most compatible with the hypothesis that the critical protective role of B cells is to produce anti-Toxoplasmaantibodies. We believe that antibodies directly block the ability of tachyzoites to infect host cells, rather than functioning as part of an Fc receptor-dependent or complement-dependent anti-Toxoplasma effector mechanism. Further evidence against a role for complement-dependent killing of tachyzoites was provided by the observation that heat inactivation of immune serum did not lessen its ability to prevent infection of fibroblasts in vitro (Fig. 8). It should be mentioned, however, that our in vivo experiments testing whether Fc receptors and C5 are essential components of protection in this model were performed using mice having genetic backgrounds different from those of μMT and B6 mice. We therefore view our conclusions regarding the lack of need for Fc receptors or C5 in vivo as provisional.
The hypothesis that Toxoplasma-specific antibodies are a source of protection in this vaccination-challenge model predicts that other mice with impaired capacity to produceToxoplasma-specific antibodies might also be poorly resistant to a challenge infection. Interestingly, we have found that mice with targeted mutations in genes for CD40 or CD40 ligand, which have B cells but which do not make isotype-switched antibodies (IgG or IgA) when vaccinated with ts-4 tachyzoites, are highly susceptible to a challenge infection with RH tachyzoites (P. C. Sayles, G. W. Gibson, and L. L. Johnson, submitted for publication). Similarly, mice inoculated with ME49 cysts as neonates have far lower titers ofToxoplasma-specific serum antibodies after reaching adulthood than do controls vaccinated as adults and die much sooner after an RH challenge (L. L. Johnson, unpublished observations). While T-cell defects may underlie the susceptibility of mice in both of these models, low titers of Toxoplasma-specific antibodies or their absence might also contribute to the observed high susceptibility to RH challenge.
Although a lack of B cells does not lead to a deficiency of IFN-γ production in sera or peritoneal lavages of infected mice relative to controls, it may, in fact, cause higher levels of this cytokine to be made locally (Table 1). Thus, we do not rule out the possibility that overproduction of cytokines associated with a Th1 response may contribute to the morbidity and mortality of the B-cell-deficient mice. Indeed, IFN-γ-dependent immunopathology of the intestine has been reported in B6 mice fed large numbers ofToxoplasma cysts (16) and interleukin 10 gene knockout mice acutely infected with an i.p. inoculation of parasites (10). Experiments are in progress to investigate whether an excessive Th1-type response contributes to the mortality of B-cell-deficient RH-challenged mice or whether tissue damage, in livers or lungs, for example, is a result of poorly controlled tachyzoite proliferation.
ACKNOWLEDGMENTS
This work was supported by USPHS grants AI-37090 and AI-42337 and by funds provided by the Trudeau Institute.
We thank Paula Lanthier for technical assistance and J. Wayne Conlan for evaluating the resistance to L. monocytogenes in μMT mice. We also thank John Boothroyd for providing GFP-expressing RH tachyzoites.
Notes
Editor: J. M. Mansfield
FOOTNOTES
- Received 28 June 1999.
- Returned for modification 5 August 1999.
- Accepted 29 November 1999.
- Copyright © 2000 American Society for Microbiology
