Cellular immune responses to nine Mycobacterium tuberculosis vaccine candidates following intranasal vaccination

Abstract

Background: The identification of Mycobacterium tuberculosis vaccines that elicit a protective immune response in the lungs is important for the development of an effective vaccine against tuberculosis.

Methods and principal findings: In this study, a comparison of intranasal (i.n.) and subcutaneous (s.c.) vaccination with the BCG vaccine demonstrated that a single moderate dose delivered intranasally induced a stronger and sustained M. tuberculosis-specific T-cell response in lung parenchyma and cervical lymph nodes of BALB/c mice than vaccine delivered subcutaneously. Both BCG and a multicomponent subunit vaccine composed of nine M. tuberculosis recombinant proteins induced strong antigen-specific T-cell responses in various local and peripheral immune compartments. Among the nine recombinant proteins evaluated, the alanine proline rich antigen (Apa, Rv1860) was highly antigenic following i.n. BCG and immunogenic after vaccination with a combination of the nine recombinant antigens. The Apa-induced responses included induction of both type 1 and type 2 cytokines in the lungs as evaluated by ELISPOT and a multiplexed microsphere-based cytokine immunoassay. Of importance, i.n. subunit vaccination with Apa imparted significant protection in the lungs and spleen of mice against M. tuberculosis challenge. Despite observed differences in the frequencies and location of specific cytokine secreting T cells both BCG vaccination routes afforded comparable levels of protection in our study.

Conclusion and significance: Overall, our findings support consideration and further evaluation of an intranasally targeted Apa-based vaccine to prevent tuberculosis.

Figure 1. Kinetics of T-cell responses induced by intranasal or subcutaneous BCG vaccination.

The frequencies of M. tuberculosis whole cell lysate (WCL) specific IFN-γ, IL-2 and IL-4 cytokine secreting cells were enumerated in lungs, cervical lymph nodes (CLN), spleen and inguinal lymph nodes (ILN) of BALB/c mice at 3, 6 and 12 weeks post BCG or diluent (control) vaccination by ELISPOT assay. The frequencies are expressed as spot forming units (SFUs)/million cells of organ after in vitro stimulation of 1×10⁵ cells/well from each organ with WCL for 36–40 h in the presence of BM-DCs at the ratio of 5∶1 organ cells/DC. The results are calculated as means ± standard deviation of three to six determinations of pooled cells from four mice after subtracting the SFUs from respective unstimulated cultures. Data presented are representative of three similar experiments. Increased frequencies of WCL-specific IFN-γ secreting cells in the lungs of intranasal BCG vaccinated mice were confirmed by evaluation of individual mice responses at 12 weeks in one of the three experiments. Significant differences among BCG vaccinated groups determined by ANOVA are shown. *: p<0.05; **: p<0.01; ***: p<0.001 ****: p<0.0001 versus s.c. BCG and #: p<0.05; ##: p<0.01; ###: p<0.001 ####: p<0.0001 versus i.n. BCG.

Figure 2. Kinetics of NO response induced by intranasal or subcutaneous BCG vaccination.

A. The levels of nitrite released after in vitro stimulation with M. tuberculosis whole cell lysate (WCL) were estimated in lung and spleen cell culture supernatants at 3, 6 and 12 weeks post vaccination of BALB/c mice by Griess assay. The levels are expressed as µM nitrite/million cells of the organ after 96 h of stimulation with WCL. B. Nitrite levels produced by WCL-stimulated exudate cells isolated from peritoneal cavity (PC) and thoracic cavity (TC) at 12 weeks. The results are presented as means ± standard deviation of three WCL-stimulated independent culture supernatants assayed in duplicate after subtracting the background nitrite levels from respective unstimulated cultures. The nitrite levels of unstimulated cultures were always less than 5 µM. Significant differences determined by ANOVA are shown. ****: p<0.0001 versus s.c. BCG and #: p<0.05 versus i.n. BCG.

Figure 3. The distribution of T-cell responses in different immune sites after intranasal BCG vaccination.

The distribution of M. tuberculosis whole cell lysate (WCL) and short term culture filtrate (STCF) specific T-cells in local and peripheral immune compartments of BALB/c mice was investigated at 30 weeks after intranasal BCG vaccination. The frequencies of IFN-γ and IL-4 secreting cells in lungs, spleen, cervical lymph nodes (CLN), peritoneal cavity (PC), bone marrow (BM), mesenteric lymph nodes (MLN), and inguinal lymph nodes (ILN) were enumerated by ELISPOT assay. The frequencies are expressed as spot forming units (SFUs)/million cells after in vitro stimulation of 1×10⁵ cells/well from each organ for 36–40 h in the presence of BM-DCs at the ratio of 5∶1 organ cells/DC. The results are calculated as means ± standard deviation of three to six determinations of pooled cells from four mice after subtracting the SFUs from respective unstimulated cultures.

Figure 4. T-cell responses induced by M. tuberculosis antigens in intranasally BCG or multicomponent subunit-vaccinated mice.

A. Representative of nine different M. tuberculosis recombinant antigens-specific IFN-γ spot forming units (SFUs) elicited in the lungs in vitro at 3 weeks (panel a) and 30 weeks (panel b) after BCG vaccination and 2 weeks after last dose of multicomponent subunit vaccination (panel c) of BALB/c mice. Multicomponent subunit vaccine (MSV) was comprised of a cocktail of nine different proteins in DDA-MPL adjuvant. The IFN-γ ELISPOT assay was developed after stimulation of 1×10⁵ or 0.5×10⁵ pooled lung cells from four vaccinated mice/well for 40 h with individual antigens or WCL in the presence of BM-DCs at the ratio of 5∶1 lung cells/DC. The WCL was not evaluated (ND) in case of MSV. (B, C and D) The comparative frequencies of nine antigen-specific IFN-γ, IL-2 and IL-4 cytokine secreting cells in lungs and spleen of mice at B. 3 weeks and C. 30 weeks after BCG or D. 2 weeks after multicomponent subunit vaccination. The assay was performed using 10 µg/ml of individual antigen or total combination (Combi.) for in vitro stimulation in the presence of BM-DCs. The results are presented as SFUs/million cells of organ and are calculated as means ± standard deviation of two to four determinations of pooled cells from four mice after subtracting the SFUs from respective unstimulated cultures. The data presented are representative of two similar experiments.

Figure 5. Protective efficacy of intranasal Apa/DDA-MPL subunit vaccine.

In two independent experiments (A and B) groups of BALB/c mice were vaccinated intranasally with three different doses of M. tuberculosis recombinant Apa or Ag85A in DDA-MPL and compared to unvaccinated naïve, adjuvant alone and BCG vaccinated controls. In experiment-A three subunit vaccine doses were instilled at two-week intervals while in experiment-B they were administered at four-week intervals. All groups were challenged by the intranasal route with virulent M. tuberculosis Erdman either eight weeks (experiment A) or twelve weeks (experiment B) after the first vaccination. Six weeks post-challenge, all mice were sacrificed and the bacterial burden (CFU's) was measured in the lungs and spleen. In both experiments data are presented as mean values from five mice per group and standard deviation of the means are indicated by error bars. Statistical comparisons among the groups were done by one-way ANOVA and Tukey's test. Significant differences are shown. **: p<0.01, *: p<0.05 with respect to (wrt) naïve controls, and ^!!: p<0.01, ^!: p<0.05 wrt DDA-MPL adjuvant controls. The significant differences (p<0.05) between the multiple groups were also confirmed by nonparametric Kruskal-Wallis test.

Figure 6. Recall responses induced by Apa subunit vaccination.

BALB/c mice were vaccinated with three different doses of Apa in DDA-MPL adjuvant at two week intervals and compared with Ag85A/DDA-MPL vaccinated and naïve (unvaccinated) and sham (adjuvant only) vaccinated control mice. Eight weeks after first dose (or four weeks after last dose), before M. tuberculosis challenge, mice were sacrificed and immunogen-specific T-cell responses were investigated in vitro in the lungs and spleen by ELISPOT assay. The frequencies IFN-γ, IL-2 and IL-4 cytokine-secreting cells are expressed as spot forming units (SFUs)/million cells of organ after in vitro restimulation of 1×10⁵ cells/well from each organ with either A. respective immunogen pulsed or B. M. bovis BCG Copenhagen infected BM-DCs (multiplicity of infection 1∶1) at the ratio of 5∶1 organ cells/DC for 40 h. The results are calculated as means ± standard deviation of triplicate determinations of pooled cells from four mice after subtracting the SFUs from respective unstimulated cultures.