Construction and characterization of attenuated recombinant B. anthracis and Y. pestis vaccines

We constructed the following B. anthracis and Y. pestis vaccine candidates by using LVS ΔcapB17 and Lm ΔactA ΔinlB prfA25 as vaccine vectors (Table S1): (1) rLVS ΔcapB/Ba expressing the shuttle plasmid-encoded fusion protein of B. anthracis antigens LF amino-terminal PA binding domain [LFn, 255 aa]26,27 and the PA extended carboxy-terminal host cell receptor-binding domain [PAc, 183 aa]12 separated by a flexible linker GGSG (LFnPAc) and driven by the F. tularensis bfr promoter (upstream of FTT_1441)28; (2) rLVS ΔcapB/Yp expressing the fusion protein of Y. pestis F1 and LcrV (F1V), separated by the GGSG linker and driven by the F. novicida omp promoter (upstream of FTN_1451)29,30; (3) rLm ΔactA ΔinlB prfA/ActAN-Ba and rLm ΔactA ΔinlB prfA/LLOss-Ba expressing the secreted forms of the fusion protein of B. anthracis LFnPAc downstream of the Lm actA promoter, ligated in frame with the actA-encoded N-terminal 100 amino acid of ActA (ActAN-BaLFnPAc), or downstream of the Lm hly promoter, ligated in frame with the hly-encoded Listeriolycin O (LLO) signal sequence (LLOss-BaLFnPAc)24; and (4) rLm ΔactA ΔinlB prfA/ActAN-Yp and rLm ΔactA ΔinlB prfA/LLOss-Yp expressing secreted forms of ActAN-YpF1V and LLOss-YpF1V, respectively.

Evaluation of the LVS ΔcapB-vectored vaccines showed that the 51 kDa fusion protein of B. anthracis, LFnPAc, is expressed by rLVS ΔcapB/Ba grown on agar (Fig. 1a, left panel, lane 2), but not by the LVS ΔcapB vector control (lane 1), as detected by monoclonal antibody to B. anthracis PA protein, which also detected the 83-kDa PA protein (lane 3). Similarly, the 55 kDa fusion protein of Y. pestis F1V is expressed by rLVS ΔcapB/Yp grown on agar (Fig. 1a, right panel, lanes 2–5), as detected by goat polyclonal antibody to Y. pestis LcrV, and has a molecular mass similar to the F1-LcrV monomer protein (lane 6). F. tularensis Bfr was detected as a 17-kDa protein31 from the LVS ΔcapB vector as well as from rLVS ΔcapB/Ba (Fig. 1a, left panel, lanes 1 and 2) and rLVS ΔcapB/Yp (Fig. 1a, right panel, lanes 2–5). The expression of the fusion proteins of B. anthracis and Y. pestis was also detected in human macrophage-like THP-1 cells infected with rLVS ΔcapB/Ba as double bands (Fig. 1b, lane 4) and rLVS ΔcapB/Yp as a single band (Fig. 1b, lanes 5 & 6), respectively. Although the rLVS ΔcapB vaccine strains showed somewhat delayed growth kinetics in broth (Fig. S2a), they grew similarly to the parental LVS ΔcapB strain in infected THP-1 cells (data not shown) and in mouse macrophage-like J774A.1 cells (Fig. S3a).

Figure 1 Expression of heterologous fusion proteins of B. anthracis and Y. pestis by rLVS ΔcapB and rLm ΔactA ΔinlB prfA vaccines grown on agar and in infected macrophage-like cells. (a) Expression of B. anthracis (left) and Y. pestis (right) fusion proteins by rLVS ΔcapB grown on agar. Single colonies of chocolate agar grown rLVS ΔcapB/Ba and rLVS ΔcapB/Yp (4 clones) were lysed in SDS sample buffer and lysates analyzed by Western blotting using a mixture of antibody to B. anthracis PA and to F. tularensis Bfr (left panel) or antibody to Y. pestis LcrV protein followed by antibody to Bfr (right panel). Left panel, lane 1, LVS ΔcapB vector; lane 2, rLVS ΔcapB/Ba; lane 3, PA protein control; lane 4, protein mass standards. Right panel, lane 1, protein mass standards; lanes 2–5, rLVS ΔcapB/Yp; lane 6, monomer of F1-LcrV (F1V) protein control. (b) Expression of fusion proteins by rLVS ΔcapB in infected human macrophage-like cells. Monocytic THP-1 cells seeded on 24-well plates and differentiated in the presence of PMA were left uninfected or infected with LVS ΔcapB, rLVS ΔcapB/Ba or rLVS ΔcapB/Yp; cells were lysed at 24 h post infection, and cell lysates analyzed by Western blotting using a mixture of antibody to B. anthracis PA and to Y. pestis LcrV. Lanes 1 & 7, two different protein standards; lane 2, uninfected control; lane 3, LVS ΔcapB; lane 4, rLVS ΔcapB/Ba; lanes 5 and 6, two clones of rLVS ΔcapB/Yp vaccines; lane 8, B. anthracis PA and degraded proteins; lane 9, Y. pestis F1-LcrV monomer protein and degraded proteins. (c) Expression and secretion of heterologous fusion proteins by rLm vaccines in broth. Culture filtrates of Lm vector or rLm vaccines were analyzed by Western blotting using antibody to B. anthracis PA (left panel) or to Y. pestis LcrV (right panel). Left panel, lane 1, protein mass standards; lane 2, Lm vector; lane 3, rLm/ActAN-Ba; lane 4, rLm/LLOss-Ba. Right panel, lane 1, protein mass standards; lane 2, Lm vector; lanes 3 & 4, two clones of rLm/ActAN-Yp; lanes 5 & 6, two clones of rLm/LLOss-Yp. (d) Expression of heterologous fusion proteins by rLm vaccines in infected mouse macrophage-like cells. Monolayers of J774A.1 cells were not infected or infected with a stationary culture of rLm vaccines similarly as described above in the legend to b . Lysates were subjected to Western blotting analysis using antibody to B. anthracis PA (left) or to Y. pestis LcrV (right). Left panel, lane 1, protein standards; lane 2, uninfected control; lane 3, rLm/ActAN-Ba; lane 4, rLm/LLOss-Ba; lane 5, PA protein. Right panel, lane 1, protein standards; lane 2, rLm/ActAN-Yp; lane 3, rLm/LLOss-Yp; lane 4, F1-LcrV protein control. (a–d) On the left border of each panel are listed the masses of protein standards; on the right border are listed the proteins of interest. Each blot was processed by using the Bio-Rad imaging system (ChemiDoc XRS) and Quantity One software, which allows the overlap of a white-light image, for visualization of the protein standards (a, left panel lane 4 and right panel lane 1; b–d, lane 1), and a chemiluminescent image, for visualization of the antibody-labeled protein bands. The full-length blots in panels b–d are shown in the Supplementary Information (Fig. S1). Full size image

Lm-vectored vaccines were assessed for expression and secretion of B. anthracis and Y. pestis fusion proteins after growth in broth and in infected mouse macrophage-live J774A.1 cells by evaluating culture filtrates by Western blotting using antibodies specific to B. anthracis PA or to Y. pestis LcrV. As shown in Fig. 1c, the antibody to PA or LcrV detected major protein bands of 59-kDa ActAN-BaLFnPAc (ActAN-Ba) (Fig. 1c, left panel, lane 3), 51-kDa LLOss-BaLFnPAc (LLOss-Ba) (lane 4), 63-kDa ActAN-YpF1V (ActAN-Yp) (Fig. 1c, right panel, lanes 3 & 4), and 56-kDa LLOss-YpF1V (LLOss-Yp) (lanes 5 & 6), expressed and secreted by rLm ΔactA ΔinlB prfA/ActAN-Ba, rLm ΔactA ΔinlB prfA/LLOss-Ba, rLm ΔactA ΔinlB prfA/ActAN-Yp, and rLm ΔactA ΔinlB prfA/LLOss-Yp, respectively, as predicted, but not by the Lm vector controls (Fig. 1c, left and right panels, lane 2). The expression of the fusion proteins of B. anthracis and Y. pestis was also detected from J774A.1 cells infected with rLm ΔactA ΔinlB prfA/ActAN-Ba (Fig. 1d, left panel, lane 3) or rLm ΔactA ΔinlB prfA/LLOss-Ba (lane 4) or with rLm ΔactA ΔinlB prfA/ActAN-Yp (Fig. 1d, right panel, lane 2) or rLm ΔactA ΔinlB prfA/LLOss-Yp (lane 3), respectively. The rLm vaccines strains grew similarly to the parental Lm ΔactA ΔinlB prfA strain in broth (Fig. S2b) and in infected J774A.1 cells (Fig. S3b). Because rLm ΔactA ΔinlB prfA/LLOss-Ba and rLm ΔactA ΔinlB prfA/LLOss-Yp expressed the B. anthracis and Y. pestis fusion proteins more abundantly in both broth and in macrophages than rLm ΔactA ΔinlB prfA/ActAN-Ba and rLm ΔactA ΔinlB prfA/ActAN-Yp vaccines, respectively, we chose to use rLm ΔactA ΔinlB prfA/LLOss-Ba and rLm ΔactA ΔinlB prfA/LLOss-Yp in the following studies where they are hereafter referred to as rLm/Ba and rLm/Yp, respectively.

Initial in vivo studies examined dissemination, clearance, and plasmid stability of the newly constructed B. anthracis and Y. pestis vaccines. Our results showed that rLVS ΔcapB/Ba grew and disseminated similarly to the parental LVS ΔcapB, while rLVS ΔcapB/Yp showed delayed growth and dissemination (Figs S4 and S5). The shuttle plasmid-encoded antigen expression cassettes in rLVS ΔcapB/Ba and rLVS ΔcapB/Yp were stably maintained in mouse liver, spleen, local skin (after i.d. administration), and lung (after i.n. administration) up to 14 days post vaccination (data not shown); The rLm/Ba and rLm/Yp vaccines also showed systemic dissemination, similar to the parental Lm vector, and all were cleared by Day 7 post vaccination (Fig. S6).

Vaccine immunogenicity and protective immunity against pulmonary anthrax

To examine the protective immunity of the LVS ΔcapB- and Lm-vectored B. anthracis vaccines, we sham-immunized the mice, or immunized them twice with AVA (BEI NR-2642, BioThrax) subcutaneously (s.q., its standard route of immunization) in a 100-μL volume after the AVA vaccine was diluted (0.25 ml: 0.75 ml) in sterile PBS32, or immunized homologously with rLVS ΔcapB/Ba mucosally (i.n.) or systemically (i.d.), or heterologously primed with rLVS ΔcapB/Ba (i.n. or i.d.) and subsequently boosted with rLm/Ba [i.n. or i.m.; i.m. is the most immunogenic systemic route for Lm33], as indicated in Fig. 2a. All mice were bled at Week 7, one week prior to challenge; challenged at Week 8 with 205,000 B. anthracis Ames spores (~5 LD 50 ; the pre-determined LD 50 for B. anthracis Ames spores was ~40,000 CFU); and monitored for survival for three weeks. Homologous priming-boosting with rLVS ΔcapB/Ba i.n. or i.d. and heterologous priming-boosting with rLVS ΔcapB/Ba – rLm/Ba, i.n./i.n. or i.d./i.m induced elevated PA-, LF- and HI-LVS-specific serum antibodies (Fig. 2b, leftmost panel); the LF- and HI-LVS antibody titers were significantly higher than those of sham- and AVA-immunized mice (Fig. 2b, leftmost and rightmost panels); these live-vectored vaccines tended to induce more LF- and PA-specific IgG2a than IgG1 antibody, representing stimulation of Th1 and Th2 immune responses, respectively (Fig. 2b, middle and rightmost panels). AVA vaccination induced high antibody titers, dominated by IgG1, specific to PA, the major component of AVA vaccine, but not to LF and HI-LVS (Fig. 2b). Systemic homologous priming-boosting with rLVS ΔcapB/Ba (Group D) and heterologous priming-boosting with rLVS ΔcapB/Ba – rLm/Ba i.n./i.n. (Group E) and i.d./i.m (Group F) showed significant protection (P < 0.001–0.01 vs. sham-immunized mice) (Fig. 2c). The licensed AVA vaccine did not induce significant protection compared with sham-immunized mice, and heterologous priming-boosting with rLVS ΔcapB/Ba – rLm/Ba systemically was significantly more protective than the AVA vaccine (P = 0.02) (Fig. 2c). Interestingly, mean survival time three weeks post-challenge correlated with pre-challenge serum antibody to LF but not to PA (Fig. 2d).

Figure 2 Two immunizations by homologous priming-boosting with rLVS ΔcapB/Ba or by heterologous priming-boosting with rLVS ΔcapB/Ba - rLm ΔactA ΔinlB prfA/Ba induce humoral immune responses and protective immunity against respiratory challenge with virulent B. anthracis Ames spores. (a) Experiment schedule. Mice (n = 8/group) were immunized homologously twice, 4 weeks apart, with PBS intranasally (i.n.) (Sham), AVA (0.025 ml) subcutaneously (s.q.), or 106 CFU rLVS ΔcapB/Ba (rLVS/Ba) i.n. or intradermally (i.d.), or heterologously with 106 CFU rLVS ΔcapB/Ba i.n. or i.d. and rLm/Ba i.n. or intramuscularly (i.m.), 4 weeks apart; bled at Week 7; challenged at week 8 with B. anthracis Ames spores (205,000/mouse, ~5 LD 50 ); and monitored for survival for three weeks, as indicated. (b) Serum antibody after vaccination. Sera were assayed for antibody endpoint titer to B. anthracis PA and LF antigens and to F. tularensis heat-inactivated LVS (HI-LVS). Values are mean + SEM of serum antibody endpoint titer for n = 8 mice per group. Differences among individual groups were evaluated by Two-way ANOVA with Tukey’s correction. Values significantly different from the Sham group are marked with asterisks over the comparison groups. *P < 0.05; **P < 0.01; ***P < 0.001; ****P < 0.0001. (c) Survival after vaccination and challenge. The survival curve of each vaccinated group is compared with that of Group A (Sham) or Group B (AVA) by the log-rank test (Mantel-cox); P values for individual vaccine groups significantly different from the Sham or AVA group are marked with asterisks and “§”, respectively, color-coded to the color of the vaccine symbol; **P < 0.01; ***P < 0.001 vs. Sham group; §P < 0.05 vs. AVA group. (d) Correlation between serum antibody and mean survival time. Linear regression was used to obtain values for the slope and intercept and the correlation coefficient (R2) between pre-challenge serum antibody and post-challenge mean survival time at 21 days post-challenge. Two-tailed P values were calculated for the correlation. Full size image

Subsequently, we explored the efficacy of three homologous (rLVS ΔcapB/Ba or rLm/Ba) or heterologous (one rLVS ΔcapB/Ba prime + two rLm/Ba boosts) immunizations, both mucosally (i.n.) and systemically (i.d. for rLVS ΔcapB/Ba and i.m. for rLm/Ba), and compared them with that of sham or AVA immunization (s.q.), and with one rLVS ΔcapB/Ba prime + one rLm/Ba boost vaccination, as depicted in Fig. 3a. The immunized animals were bled, subsequently challenged with 371,000 B. anthracis Ames spores (~10 LD 50 ), and monitored for 3 weeks (Fig. 3a). As shown on the left side of Fig. 3b, mice homologously primed-boosted with rLVS ΔcapB/Ba i.n. (Group C) or i.d. (Group D) or with rLm/Ba i.n. (Group E) or i.m. (Groups F) produced significantly greater amounts of B. anthracis PA and/or LF antigen-specific serum IgG antibody, dominated by subtype IgG2a, than sham-immunized mice (Fig. 3b, top left two panels), consistent with the results from Experiment I (Fig. 2a and b) and additional experiments (Fig. S7a–d, Group C). Of note, as in the previous experiment, mice immunized with AVA produced PA-specific antibody, but did not produce LF-specific serum antibody. Upon challenge, mice homologously primed-boosted with rLVS ΔcapB/Ba or rLm/Ba, systemically or mucosally, had greater survival than sham- and AVA-immunized mice; the survival of mice immunized systemically with rLVS ΔcapB/Ba and rLm/Ba and mucosally with rLm/Ba was significantly greater than that of the sham-immunized mice (Fig. 3b, bottom left panel).

Figure 3 Three immunizations by homologous priming-boosting with rLVS ΔcapB/Ba or by heterologous priming-boosting with rLVS ΔcapB/Ba – rLm ΔactA ΔinlB prfA/Ba induce high-level antibody responses and potent protective immunity against virulent B. anthracis respiratory challenge. (a) Experiment schedule. Mice (n = 8/group) were immunized two or three times homologously with 106 CFU rLVS ΔcapB/Ba (rLVS/Ba) or rLm ΔactA ΔinlB prfA/Ba (rLm/Ba) or heterologously first with rLVS/Ba and subsequently with rLm/Ba, as indicated. Controls were sham-immunized with PBS i.d. or with AVA s.q. three times. All mice were bled at week 11; challenged at week 12 with B. anthracis Ames spores (371,000 CFU, ~10 LD 50 ); and monitored for survival for 3 weeks post-challenge. (b) Serum antibody prior to challenge and survival post challenge. Top panels. Sera were assayed for IgG or IgG subtypes IgG1 and IgG2a to B. anthracis PA and LF proteins, as indicated, after homologous (left two panels) and heterologous (right two panels) prime-boost vaccination. Values are mean + SEM of serum antibody endpoint titer for n = 8 per group. Differences in serum endpoint titer among individual groups were analyzed by two-way ANOVA with Tukey’s corrections. *P < 0.05; **P < 0.01; ****P < 0.0001 vs. Sham group. Bottom panels. The survival curve of each vaccinated group, color-coded as indicated in panel b, after homologous (left) and heterologous (right) prime-boost vaccination and challenge is compared with that of the Sham group by the log-rank test (Mantel-cox); P values for vaccine groups that are significantly different from the Sham group are marked with one or more asterisks color-coded to the color of the vaccine symbol. *P < 0.05 and **P < 0.01. (c) Correlation between serum antibody and mean survival time. The correlation coefficient (R2) and one-tailed P values were obtained as described in legend to Fig. 2d. Full size image

As shown on the right side of Fig. 3b, mice heterologously primed-boosted with rLVS ΔcapB/Ba - rLm/Ba, mucosally (i.n./i.n.) (Group G & H) or systemically (i.d./i.m.) (Group I & J) also produced significantly greater amounts of B. anthracis PA and LF antigen-specific serum IgG antibody than sham-immunized mice, dominated by subtype IgG2a to PA and LF (Fig. 3b, top right two panels; Fig. S7b–e, Groups E and F), and elevated levels of serum antibodies that neutralized anthrax toxin (assayed in mouse macrophage cell line J774A.1), as did AVA-immunized mice (Fig. S7f, Groups B, E, and F). After challenge, these heterologously primed-boosted mice (Groups G, H, I, J) showed significantly increased survival compared with the sham-immunized mice (P < 0.05 or P < 0.01), whether boosted only once (Groups G and I) or twice (Groups H and J) with rLm/Ba (Fig. 3b, bottom, right panel). In contrast, survival of mice immunized with the AVA vaccine was not significantly different from that of sham-immunized mice (P = 0.3) (Fig. 3b, bottom panels). These results indicate that both systemic and mucosal homologous priming-boosting with rLVS ΔcapB/Ba or rLm/Ba and both systemic and mucosal heterologous priming-boosting with rLVS ΔcapB/Ba - rLm/Ba induce strong protective immunity against lethal respiratory challenge with B. anthracis spores. As in the previous challenge experiment, mean survival time 3 weeks post-challenge correlated with pre-challenge serum antibody to LF but not to PA (Fig. 3c) or to toxin neutralizing antibody (Fig. S8).

To investigate T-cell mediated immune responses induced by rLVS ΔcapB and rLm vaccines, we immunized mice, observed them for signs of discomfort or weight loss, and assayed their lung and spleen cells for cytokine secretion and intracellular cytokine staining in response to in vitro stimulation with B. anthracis and F. tularensis antigens (Figs 4a and S9). After vaccination with rLVS ΔcapB/Ba i.n., rLm/Ba i.n. or i.m., or AVA s.q., mice did not show signs of significant discomfort (e.g. ruffled fur) or weight loss (data not shown). As expected, mice immunized with the rLVS ΔcapB/Ba and rLm/Ba anthrax vaccines developed T-cell mediated immune responses, dominated by Th1 responses (Figs 4 and 5). Lung and spleen cells of mice immunized with rLVS ΔcapB/Ba i.n. twice (Group C) or once (Group D) or heterologously primed-boosted with rLVS ΔcapB/Ba - rLm/Ba i.n./i.n. (Group E) or i.n./i.m. (Group F) secreted much higher levels of IFN-γ and IL-4 in response to LF and HI-LVS than sham- (statistically significant) or AVA-immunized mice (Fig. 4b). In response to LF and HI-LVS, these mice had elevated levels of lung and spleen CD4+ T cells expressing IFN-γ, TNF-α, IL-2, and IL-17A compared with sham- and AVA-immunized mice and greater frequencies of polyfunctional CD4+ T cells expressing IFN-γ, TNF-α, IL-2, and/or IL-17 than sham-immunized mice (Figs. 4c, 5); the heterologously immunized mice had especially high levels of cytokine-expressing CD4+ T cells (Fig. 4c, leftmost and rightmost upper and lower panels), and modestly increased levels of CD8+ T-cells secreting IFN-γ in the lung when boosted i.n. and in the spleen when boosted i.m. (Fig. 4d). In response to PA, CD4+ T-cells from mice heterologously primed-boosted by the i.n. route also had increased cytokine-producing cells in the lung and to a much less extent in the spleen; not unexpectedly, when the booster was instead administered systemically (i.m.), there were increased cytokine positive CD4+ T-cells in the spleen (Fig. 4c, middle upper and lower panels). With a few minor exceptions, the AVA vaccine induced very poor cell-mediated immune responses (Fig. 4b–d). Mice immunized i.d. once with rLVS ΔcapB/Ba or heterologously primed-boosted with rLVS ΔcapB/Ba i.d. – rLm/Ba i.m. also showed enhanced IFN-γ secretion by lung and spleen cells and increased frequencies of lung and spleen CD4+ T cells expressing IFN-γ, TNF-α, IL-2 and/or IL-17A in response to LF and HI-LVS compared with sham-immunized mice and mice immunized with AVA (data not shown). These results indicate that both homologous priming-boosting with rLVS ΔcapB/Ba and heterologous priming-boosting with rLVS ΔcapB/Ba – rLm/Ba vaccines induce F. tularensis and B. anthracis antigen-specific Th1-type cytokine secretion and polyfunctional CD4+ T cells.

Figure 4 Homologous prime-boost vaccination with rLVS ΔcapB/Ba or heterologous prime-boost vaccination with rLVS ΔcapB/Ba - rLm ΔactA ΔinlB prfA/Ba induces antigen-specific cell-mediated immune responses. (a) Experiment schedule. Mice (n = 4/group) were immunized homologously with PBS i.n. (Sham), AVA s.q., or 106 CFU rLVS ΔcapB/Ba (rLVS/Ba) i.n., or heterologously with rLVS/Ba i.n. followed by 106 CFU rLm ΔactA ΔinlB prfA/Ba (rLm/Ba) i.n. or i.m. as indicated. At week 7, all mice were bled, euthanized, and their lung and spleen cells assayed for cytokine secretion and intracellular cytokine staining. (b) Antigen-specific cytokine secretion. Single cell suspensions of lung and spleen cells were stimulated with B. anthracis PA or LF proteins or F. tularensis heat-inactivated LVS (HI-LVS) for 3 days, as indicated, and cell supernatants assayed for interferon gamma (IFN-γ) (left two panels) and IL-4 (right two panels) by ELISA. Shown are the amounts of IFN-γ and IL-4 in the culture supernate in response to PA, LF, and HI-LVS. (c,d) Cytokine-expressing CD4+ (c) and CD8+ (d) T cells. Lung and spleen cells were stimulated with LF, PA, or HI-LVS as indicated at the top of the panels (c) or on the horizontal axis (d) and assayed by intracellular cytokine staining for CD4+ (c) and CD8+ (d) T cells expressing IFN-γ, TNF-α, IL-2, and/or IL-17A. Shown are the frequencies of CD4+ T cells expressing IFN-γ, TNF-α, IL-2, and/or IL-17A (c) and CD8+ T cells expressing IFN-γ (d). Values in b–d are means + SEM. Differences among individual groups were evaluated by Two-way ANOVA with Tukey’s correction. Values significantly different from the Sham group are marked with asterisk(s) over brackets above the comparison groups; *P < 0.05; **P < 0.01; ***P < 0.001; ****P < 0.0001. Results shown are representative of three similar experiments. Full size image

Figure 5 Heterologous prime-boost vaccination with rLVS ΔcapB/Ba - rLm ΔactA ΔinlB prfA/Ba induces elevated frequencies of LF- and PA-specific polyfunctional lung and spleen CD4+ T-cells producing IFN-γ, IL-2, TNF-α, and IL-17. As described in Fig. 4a and indicated by the colors and letters at the bottom of the figure, mice (n = 4/group) were sham-immunized or immunized with various vaccines. Lung (left panels) and spleen (right panels) cells were stimulated with LF (a,b), PA (c,d), HI-LVS (e,f), or PMA (g,h) and assayed by intracellular cytokine staining for 15 possible combinations of CD4+ T-cells expressing IFN-γ, TNF-α, IL-2, and/or IL-17A. Values are means + SEM and differences among individual groups were analyzed by Two-way ANOVA with Tukey’s multiple comparisons test (Prism). Values significantly different from the Sham group are marked with asterisks over the comparison groups. In panels e, f and g, the Sham group is indicated by a short bar with an open end and the comparison groups are indicated by a short vertical line across the short bar. *P < 0.05; **P < 0.01; ***P < 0.001; ****P < 0.0001. Results shown are representative of three similar experiments. Full size image

Vaccine immunogenicity and protective immunity against pneumonic plague

To evaluate the protective efficacy of rLVS ΔcapB- and rLm-vectored Y. pestis vaccines administered by the mucosal and systemic routes, we sham-immunized mice, or immunized them once with the unlicensed EV76 vaccine s.q. (its standard route of administration), or twice homologously with rLVS ΔcapB/Yp i.n. or i.d. or heterologously primed-boosted with rLVS ΔcapB/Yp (i.n. or i.d.) - rLm/Yp (i.n. or i.m., respectively), as indicated in Fig. 6a. All mice were bled at Week 8; challenged at Week 9 with 1900 CFU Y. pestis CO92 strain (~8 LD 50 ; the pre-determined LD 50 for Y. pestis CO92 was ~250 CFU), and monitored for signs of illness, weight change, and survival for three weeks. Mice immunized with rLVS ΔcapB/Yp i.d or with rLm/Yp i.m. did not show signs of significant discomfort (e.g. ruffled fur) or weight loss (data not shown); in one experiment, mice immunized with rLVS ΔcapB/Yp i.n. showed transient mild weight loss (~5%) but rapidly recovered such that their weights matched that of sham-immunized controls by at least Day 6 post–vaccination (data not shown). Homologous priming-boosting with rLVS ΔcapB/Yp i.n. or i.d. (Groups C, D) or heterologous priming-boosting with rLVS ΔcapB/Yp – rLm/Yp i.n. or i.m. (Groups E, F) induced significantly elevated serum antibody titers to LcrV protein and to F1/LcrV fusion protein, balanced between IgG1 and IgG2a, compared with sham-immunization (Fig. 6b) (Fig. S10a,b, upper panel, Groups B, E, & F). The majority of the IgG antibody induced by LVS ΔcapB- and Lm-vectored vaccines was directed to LcrV. In contrast, mice immunized with EV76 produced very little antibody to LcrV (small amounts of IgG1 and IgG2a were evident) but substantial amounts of IgG to F1 and F1-LcrV (Fig. 6b).

Figure 6 Two immunizations by homologous priming-boosting with rLVS ΔcapB/Yp or heterologous priming-boosting with rLVS ΔcapB - rLm ΔactA ΔinlB prfA/Yp induces humoral immune responses and protective immunity against respiratory challenge with virulent Y. pestis. (a) Experiment schedule. Mice (n = 8/group) were immunized homologously twice with PBS (Sham) or 106 CFU LVS ΔcapB/Yp (rLVS/Yp) or once with 106 CFU EV76, or heterologously first with 106 CFU rLVS ΔcapB/Yp and then with 106 CFU rLm ΔactA ΔinlB prfA/Yp (rLm/Yp). All mice were bled at Week 8; challenged at week 9 with virulent Y. pestis (CO92) (1,900 CFU/mouse, ~8 LD 50 ); and monitored for survival for three weeks, as indicated. (b) Serum antibody after vaccination. Sera were assayed for IgG antibody specific to F1, LcrV, or the monomer of F1-LcrV proteins (leftmost panel) and subtypes IgG1 and IgG2a antibodies specific to LcrV or F1-LcrV (middle and rightmost panels, resp.). Data are mean + SEM of serum antibody endpoint titer for n = 8 per group. Differences among individual groups were analyzed by two-way ANOVA with Tukey’s correction. ****P < 0.0001 vs. Sham. (c) Survival after challenge. The survival curve of each vaccinated group is compared with the Sham group (Group A) by the log-rank test (Mantel-cox); P values that are significantly different from the Sham group are marked with one or more asterisks color-coded to the color of the vaccine symbol. There were no changes in percent survival of mice in any group after day 14 post challenge until the end of the experiment. *P < 0.05; **P < 0.01; ***P < 0.001; ****P < 0.0001. (d) Correlation between serum antibody and mean survival time. The correlation coefficient (R2) and one-tailed P values were obtained as described in legend to Fig. 2D. Full size image

Systemic homologous priming-boosting with rLVS ΔcapB/Yp i.d. (Group D) or heterologous priming-boosting with rLVS ΔcapB/Yp – rLm/Yp (i.d./i.m.) (Group F) showed significant protection against challenge with Y. pestis CO92 compared with sham-immunized mice (P = 0.02 and P = 0.01, respectively), albeit less than EV76 (Fig. 6c). Mean survival time three weeks post-challenge was correlated with pre-challenge serum IgG antibody to F1 but not to LcrV (Fig. 6d).

Subsequently, we explored the efficacy of three homologous immunizations, both mucosally (i.n. for rLVS ΔcapB/Yp) and systemically (i.d. for rLVS ΔcapB/Yp and i.m. for rLm/Yp) and three heterologous immunizations systemically, as described in Fig. 7a. Controls were sham-immunized with PBS, administered the EV76 vaccine once s.q, or primed-boosted with rLVS ΔcapB/Yp – rLm/Yp once systemically. Mice were bled; subsequently challenged with 1800 CFU virulent Y. pestis CO92 (~7 LD 50 ); and monitored for survival for three weeks. Among the vaccines and administration regimens tested, three homologous priming-boosting immunizations with rLVS ΔcapB/Yp i.d. (Group D) induced significantly greater serum antibody titers to F1 antigen than sham-immunized mice, balanced between subtypes IgG1 and IgG2a (Fig. 7b, left panel), similar to mice vaccinated with EV76. Homologous priming-boosting with rLVS ΔcapB/Yp (i.n. or i.d) (Groups C, D) but not with rLm/Yp (i.m.) (Group E), and heterologous priming-boosting systemically (with one or two boosts) (Groups F, G) induced significantly elevated LcrV-specific IgG antibody compared with sham-immunized mice, balanced between subtypes IgG1 and IgG2a (Fig. 7b, right panel). Mice immunized with EV76 produced abundant IgG antibody to F1 antigen, but relatively little to LcrV (Fig. 7b). Mice homologously prime-boosted with rLVS ΔcapB/Yp i.n. or heterologously prime-boosted with rLVS ΔcapB/Yp – rLm/Yp (i.n./i.n. or i.n./i.m.) also developed T-cell mediated immunity, as evidenced by IFN-γ secretion by lung and spleen cells in response to F1 and/or LcrV (Fig. S10b, lower panels).

Figure 7 Three systemic immunizations by homologous priming-boosting with LVS ΔcapB/Yp induce high level antibody responses and potent protective immunity against respiratory challenge with virulent Y. pestis CO92 strain. (a) Experiment schedule. Mice (n = 8/group) were immunized homologously three times with 106 CFU rLVS ΔcapB/Yp (rLVS/Yp) or with 106 CFU rLm ΔactA ΔinlB prfA/Yp (rLm/Yp) or heterologously with rLVS ΔcapB/Yp and subsequently rLm/Yp. Mice vaccinated with PBS i.d. (Sham, Group A) or 106 CFU Y. pestis vaccine strain EV76 served as controls. All the mice were bled at Week 11; challenged with Y. pestis CO92 (1,800 CFU/mouse, ~7 LD 50 ) at Week 12; and monitored for survival for 3 weeks post-challenge, as indicated. (b) Serum antibody after vaccination. Sera were assayed for antibody endpoint titer of IgG or IgG subtypes IgG1 and IgG2a to Y. pestis F1 (left panel) and LcrV (right panel) proteins. Data are mean + SEM of serum antibody endpoint titer for n = 8 per group. Differences among individual groups were analyzed by two-way ANOVA with Tukey’s correction. ***P < 0.001; ****P < 0.0001 vs. Sham. (c) Survival after vaccination and challenge. The survival curve of each vaccinated group is compared with that of the Sham group by the log-rank test (Mantel-cox); P values for vaccine groups significantly different from the Sham group are marked with one or more asterisks color-coded to the color of the vaccine symbol. *P < 0.05; ***P < 0.001. (d) Correlation between serum antibody and mean survival time. The correlation coefficient (R2) and one-tailed P values were obtained as described in legend to Fig. 2D. Full size image

Mice immunized three times i.d. with rLVS ΔcapB/Yp (Group D) survived significantly longer than sham-immunized mice (P = 0.03); mice primed-boosted with rLVS ΔcapB/Yp - rLm/Yp (Group F) also survived longer than sham-immunized mice, although the difference was not statistically significant (Fig. 7c). As in the previous experiment shown in Fig. 6, the amount of serum antibody specific to F1, but not to LcrV, was highly correlated with the mean survival time three weeks post-challenge (P < 0.001, P < 0.0001 and P < 0.0001 for IgG, IgG1 and IgG2a, respectively) (Fig. 7d). These results indicate that systemic homologous priming-boosting with rLVS ΔcapB/Yp induces strong protective immunity against Y. pestis CO92 respiratory challenge and that vaccine efficacy is correlated with F1-specific antibody.

Potent protective immunity against pneumonic tularemia

In previous studies, we have shown that heterologous priming-boosting with LVS ΔcapB or rLVS ΔcapB overexpressing IglC as the prime vaccine and rLm expressing F. tularensis IglC (rLm/iglC) as the booster vaccine induces potent protective immunity in mice against virulent F. tularensis Schu S4 respiratory challenge18. We have also shown that immunization with rLVS ΔcapB/iglABC is highly safe and induces greater protective immunity than the parental LVS ΔcapB vector against F. tularensis Schu S4 respiratory challenge19. However, the efficacy of homologous priming-boosting with this vaccine has not been investigated. To evaluate the efficacy of homologous priming-boosting with rLVS ΔcapB/iglABC, both systemically and mucosally, we immunized BALB/c mice once or twice i.d. or i.n. with this vaccine, as indicated in Fig. 8a. Control mice were sham-immunized with PBS, immunized i.d. with the unlicensed LVS vaccine (which is highly lethal by the i.n. route), or i.d. with the LVS ΔcapB vector. Mice were challenged with 10 CFU F. tularensis Schu S4 (~10 LD 50 ) at Week 10 and monitored for survival for 3 weeks. All immunized mice survived significantly longer than the sham-immunized mice (P = 0.03 for LVS ΔcapB and P = 0.001 or 0.0001 for all other groups vs. sham-immunized mice). Mice immunized i.d. once (Group D) or twice (Group E) or i.n. twice (Group F) with rLVS ΔcapB/iglABC survived significantly longer than mice immunized once with the LVS ΔcapB vector (P = 0.006, P = 0.0001, and P = 0.0005, respectively); mice immunized i.d. or i.n. twice with rLVS ΔcapB/iglABC (Groups E and F) had survival times equivalent to or greater than that of mice immunized with LVS (Group B); differences were not statistically significant (Fig. 8b).

Figure 8 Two immunizations by homologous priming-boosting with rLVS ΔcapB/iglABC induces strong protection against respiratory challenge with the virulent F. tularensis Schu S4 strain. (a) Schedule - Experiment VI. Mice (n = 8/group) were immunized once with PBS (Sham), 104 CFU LVS, 106 CFU LVS ΔcapB vector, or 106 CFU rLVS ΔcapB/iglABC; or twice with 106 CFU rLVS ΔcapB/iglABC; challenged i.n. with F. tularensis Schu S4 (10 CFU, ~10 LD 50 ) at Week 10; and monitored for signs of illness, weight change, and death for 3 weeks, as indicated. (b) Survival after vaccination and challenge – Experiment VI. The survival curve of each vaccinated group is compared with that of the Sham (Group A) or LVS ΔcapB vector (Group C) group by the log-rank test (Mantel-cox); P values for vaccine groups that are significantly different from the Sham or LVS ΔcapB vector control group are marked with asterisk(s) and multiple “§”, respectively, color-coded to the color of the vaccine symbol. *P < 0.05; ***P < 0.001; and ****P < 0.0001 vs. Group A (Sham); §§P < 0.01; §§§P < 0.001 vs. Group C (vector control). (c) Schedule - Experiment VII. Mice (n = 8/group) were immunized once with PBS (Sham), 104 CFU LVS, or 106 CFU rLVS ΔcapB/iglABC three times at Weeks 0, 4, and 8, or twice at Weeks 4 and 8, bled at Week 13, and challenged i.n. with 2 CFU (2 LD 50 ) or 6 CFU (6 LD 50 ) F. tularensis Schu S4 at Week 14, and monitored for 3 weeks, as indicated. (d) Survival after vaccination and challenge and serum antibody pre-challenge – Experiment VII. Upper panels – Survival. The survival curve after challenge with 2 LD 50 (left panel) or 6 LD 60 (right panel) of each vaccinated group is compared with that of either the sham- or LVS-immunized group by the log-rank test (Mantel-cox); P values that are significantly different from the control group are marked with asterisk(s) color-coded to the color of the vaccine symbol. ***P < 0.001 vs. Group A (Sham); *P < 0.05 vs. Group B (LVS). Lower panel- Serum antibody after vaccination. Sera were assayed for IgG and subtypes IgG1 and IgG2a specific to heat-inactivated LVS. Data are mean + SEM of serum antibody endpoint titer for n = 8 per group. Differences among individual groups were compared by two-way ANOVA with Tukey’s correction. Values that are significantly different between two groups are marked with asterisk(s) over an open horizontal line crossing above the two groups. As indicated by the asterisks above the Sham group, its titers were significantly different from all other groups. *P < 0.05; **P < 0.01; and ****P < 0.0001. Full size image