Progress and hindrances in tuberculosis vaccine development

Generating prophylactic immunity against arboviruses in vertebrates and invertebrates

The World Health Organization recently declared a global initiative to control arboviral diseases. These are mainly caused by pathogenic flaviviruses (such as dengue, yellow fever and Zika viruses) and alphaviruses (such as chikungunya and Venezuelan equine encephalitis viruses). Vaccines represent key interventions for these viruses, with licensed human and/or veterinary vaccines being available for several members of both genera. However, a hurdle for the licensing of new vaccines is the epidemic nature of many arboviruses, which presents logistical challenges for phase III efficacy trials. Furthermore, our ability to predict or measure the post-vaccination immune responses that are sufficient for subclinical outcomes post-infection is limited. Given that arboviruses are also subject to control by the immune system of their insect vectors, several approaches are now emerging that aim to augment antiviral immunity in mosquitoes, including Wolbachia infection, transgenic mosquitoes, insect-specific viruses and paratransgenesis. In this Review, we discuss recent advances, current challenges and future prospects in exploiting both vertebrate and invertebrate immune systems for the control of flaviviral and alphaviral diseases.

Mycobacterium tuberculosis PE25/PPE41 protein complex induces activation and maturation of dendritic cells and drives Th2-biased immune responses

Mycobacterium tuberculosis evades innate host immune responses by parasitizing macrophages and causes significant morbidity and mortality around the world. A mycobacterial antigen that can activate dendritic cells (DCs) and elicit effective host innate immune responses will be vital to the development of an effective TB vaccine. The M. tuberculosis genes PE25/PPE41 encode proteins which have been associated with evasion of the host immune response. We constructed a PE25/PPE41 complex gene via splicing by overlapping extension and expressed it successfully in E. coli. We investigated whether this protein complex could interact with DCs to induce effective host immune responses. The PE25/PPE41 protein complex induced maturation of isolated mouse DCs in vitro, increasing expression of cell surface markers (CD80, CD86 and MHC-II), thereby promoting Th2 polarization via secretion of pro-inflammatory cytokines IL-4 and IL-10. In addition, PE25/PPE41 protein complex-activated DCs induced proliferation of mouse CD4(+) and CD8(+) T cells, and a strong humoral response in immunized mice. The sera of five TB patients were also highly reactive to this antigen. These findings suggest that interaction of the PE25/PPE41 protein complex with DCs may be of great immunological significance.

PPE57 induces activation of macrophages and drives Th1-type immune responses through TLR2

Proline-glutamic acid (PE) and proline-proline-glutamic acid (PPE) are related proteins exclusive to Mycobacteria that play diverse roles in modulating critical innate immune pathways. In this study, we observed that the PPE57 protein is associated with the cell wall and is exposed on the cell surface. PPE57 enhances Mycobacterium spp. entering into macrophages and plays a role in macrophage phagocytosis. To explore the underlying mechanism, we demonstrated that PPE57 is able to recognise Toll-like receptor 2 (TLR2) and further induce macrophage activation by augmenting the expression of several cell surface molecules (CD40, CD80, CD86 and MHC class II) and pro-inflammatory cytokines (TNF-α, IL-6 and IL-12p40) within macrophages. These molecules are involved in the mitogen-activated protein kinase (MAPK) and nuclear factor κB (NF-κB) signalling pathways. We demonstrated that PPE57 effectively polarises T cells to secrete interferon (IFN)-γ and IL-2 and to up-regulate CXCR3 expression in vivo and in vitro, suggesting that this protein may contribute to Th1 polarisation during the immune response. Moreover, recombinant Bacillus Calmette-Guérin (BCG) over-expressing PPE57 could provide better protective efficacy against Mycobacterium tuberculosis challenge compared with BCG. Taken together, our data provides several pieces of evidence that PPE57 may regulate innate and adaptive immunity by interacting with TLR2. These findings indicate that PPE57 protein is a potential antigen for the rational design of an efficient vaccine against M. tuberculosis.

KEY MESSAGES

PPE57 is located on the cell surface and enhances mycobacterium entry into macrophage. PPE57 interacts directly with TLR2 on macrophages. PPE57 plays a key role in the activation of macrophages in a TLR2-dependent manner. PPE57 induces a Th1 immune response via TLR2-mediated macrophage functions. Recombinant BCG over-expressing PPE57 could improve protective efficacy against M. tuberculosis.

Primary application of PPE68 of Mycobacterium tuberculosis

PPE68 protein is absent from BCG and the attenuated strains of Mycobacterium tuberculosis (MTB). In this study, the shuttle plasmid pBudCE4.1/PPE68/OriM was constructed and transformed into BCG to obtain PPE68 recombination BCG (PPE68-rBCG), and BALB/c mice were immunized with PPE68-rBCG to evaluate the immunological characterization of PPE68-rBCG. The level of lgG2a, IFN-γ, IL-12 and IL-4 in serum of immunized mice were detected, the proliferation response of spleen lymphocyte were measured, the frequency of CD4(+), CD8(+) and CD4(+)/CD8(+) were determined, and the spleen and lung tissue were prepared for pathological analysis. PPE68-rBCG was constructed successfully and could induce powerful Th1 immune response in mice. Besides, we took the purified recombination PPE68 (rPPE68) protein as diagnostic antigen to detect pulmonary tuberculosis patients (n=252) and extrapulmonary tuberculosis patients (n=66). We also used anti-PPE68 polyclonal antibody as coating antibody to detect specific antigen in the same serum samples. Our data provide an experimental basis for potential application of rPPE68 in the diagnosis of tuberculosis, especially for extrapulmonary tuberculosis.

Multidrug-resistant and extensively drug-resistant tuberculosis: The new face of an old disease

PURPOSE

To provide an overview of tuberculosis (TB), especially the persistent threat of multidrug-resistant (MDR) and extensively drug-resistant (XDR), and the role of the nurse practitioner (NP) in diagnosis, treatment, and public health surveillance.

DATA SOURCES

Public health sources such as the World Health Organization and the Centers for Disease Control as well as current literature.

CONCLUSIONS

One hundred twenty-five years after the discovery of Mycobacterium tuberculosis, the disease remains a persistent threat and a leading cause of death worldwide. Medication adherence and prevention are critical to successfully treating and ultimately eradicating this killer disease. Healthcare providers need to be knowledgeable in the detection and diagnosis of TB and to understand that they assume responsibility for public health by monitoring treatment adherence and/or appropriate referral.

IMPLICATIONS FOR PRACTICE

The persistent and increasing threat of MDR-TB and XDR-TB is a significant public health threat. Healthcare providers need to be knowledgeable and vigilant in diagnosing and treating this disease.

Induction of granulysin and perforin cytolytic mediator expression in 10-week-old infants vaccinated with BCG at birth

Background. While vaccination at birth with Mycobacterium bovis Bacilli Calmette-Guérin (BCG) protects against severe childhood tuberculosis, there is no consensus as to which components of the BCG-induced immune response mediate this protection. However, granulysin and perforin, found in the granules of cytotoxic T lymphocytes and Natural Killer (NK) cells, can kill intracellular mycobacteria and are implicated in protection against Mycobacterium tuberculosis. Methods. We compared the cellular expression of granulysin and perforin cytolytic molecules in cord blood and peripheral blood from 10-week-old infants vaccinated at birth with either Japanese or Danish BCG, administered either intradermally or percutaneously. Results. In cord blood, only CD56⁺ NK cells expressed granulysin and perforin constitutively. These cytolytic mediators were upregulated in CD4⁺ and CD8⁺ cord blood cells by ex vivo stimulation with BCG but not with PPD. Following BCG vaccination of neonates, both BCG and PPD induced increased expression of granulysin and perforin by CD4⁺ and CD8⁺ T cells. There was no difference in expression of cytolytic molecules according to vaccination route or strain. Conclusions. Constitutive expression of perforin and granulysin by cord blood NK-cells likely provides innate immunity, while BCG vaccination-induced expression of these cytolytic mediators may contribute towards protection of the neonate against tuberculosis.

Regulatory T cells induced by Mycobacterium chelonae sensitization influence murine responses to bacille Calmette-Guerin

The efficacy of live Mycobacterium bovis BCG as a tuberculosis vaccine is highly varied globally. Differential sensitization to environmental mycobacteria prior to BCG vaccination may prime immune effects leading to this variation, but the precise immune mechanisms and cell types involved in this phenomenon are unknown. We hypothesized that pre-vaccination sensitization to environmental mycobacteria induces mycobacterium-specific Tregs that suppress responses to BCG. This was investigated by testing Treg responses following priming of BALB/c mice by i.p. immunization with heat-killed CHE. Such mice produced higher levels of IL-10 before and after intranasal, live BCG administration and had fewer lung inflammatory cells post-BCG, relative to nonsensitized mice. In CHE-sensitized mice, the percentage of splenic CD4+CD25+ cells expressing Foxp3 amongst total lymphocytes was not elevated significantly, but these cells limited nonspecific proliferation of CD4+CD25⁻ effector cells upon coculture and promoted higher expression levels of CD103 and Foxp3 in response to BCG antigen stimulation than CD4+CD25+ cells from nonsensitized mice. In adoptive transfer experiments, naïve, WT mice receiving CD4+CD25+ cells from CHE-sensitized mice and then given live BCG intranasally had significantly elevated lung IL-10 levels, reduced frequencies of lung IL-2-producing cells, and lower lymphocyte numbers in the BAL. Therefore, CHE sensitization induced CD4+CD25+ Tregs with functional, suppressive activity on BCG responses in vitro and in vivo. Treg induction could therefore be one mechanism underlying how environmental mycobacteria priming modulates host responses to the BCG vaccine.

Recombinant Mycobacterium bovis BCG expressing the chimeric protein of antigen 85B and ESAT-6 enhances the Th1 cell-mediated response

The chimeric protein that relies on the T-cell epitopes of antigen 85B (Ag85B) and the 6-kDa early secreted antigen target (ESAT-6) has been demonstrated to augment the Th1 immune response. In this study, we developed a recombinant Mycobacterium bovis BCG (rBCG) strain that secretes the chimeric protein of Ag85B and ESAT-6 (rBCG-A(N)-E-A(C)). Immunization with this rBCG strain induced stronger antigen-specific gamma interferon (IFN-gamma) activities, as determined by an enzyme-linked immunospot assay, and higher levels of antigen-specific CD4(+) and CD8(+) T-cell responses than those in the control groups immunized with either rBCG expressing the Ag85B-ESAT-6 fusion protein (rBCG-A-E) or BCG. Likewise, rBCG-A(N)-E-A(C) significantly increased the level of production of the major Th1 cytokines IFN-gamma and tumor necrosis factor alpha in splenocyte cultures to levels comparable to those elicited by control BCG. Moreover, the antigen-specific immunoglobulin 2c (IgG2c)/IgG1 ratio for mice immunized with rBCG-A(N)-E-A(C) was also much higher than the ratios for the other immunized groups. Together, these results indicate that this rBCG-A(N)-E-A(C) strain enhances the Th1 cell-mediated response and may serve as a potential vaccine against M. tuberculosis.

Modelling the effects of pre-exposure and post-exposure vaccines in tuberculosis control

Epidemic control strategies alter the spread of the disease in the host population. In this paper, we describe and discuss mathematical models that can be used to explore the potential of pre-exposure and post-exposure vaccines currently under development in the control of tuberculosis. A model with bacille Calmette-Guerin (BCG) vaccination for the susceptibles and treatment for the infectives is first presented. The epidemic thresholds known as the basic reproduction numbers and equilibria for the models are determined and stabilities are investigated. The reproduction numbers for the models are compared to assess the impact of the vaccines currently under development. The centre manifold theory is used to show the existence of backward bifurcation when the associated reproduction number is less than unity and that the unique endemic equilibrium is locally asymptotically stable when the associated reproduction number is greater than unity. From the study we conclude that the pre-exposure vaccine currently under development coupled with chemoprophylaxis for the latently infected and treatment of infectives is more effective when compared to the post-exposure vaccine currently under development for the latently infected coupled with treatment of the infectives.

Adjuvant modulation of the cytokine balance in Mycobacterium tuberculosis subunit vaccines; immunity, pathology and protection

It is known that protection against tuberculosis is mediated primarily by T helper type 1 (Th1) cells but the influence of the Th1/Th2 balance of a vaccination response on the subsequent protection and pathology during infection has not been studied in detail. We designed a panel of Ag85B-ESAT-6 subunit vaccines based on adjuvants with different Th1/Th2-promoting activities and studied cellular responses, bacterial replication and pathology in the lungs of mice infected with Mycobacterium tuberculosis. All vaccines induced cell-mediated and humoral responses but with markedly different interferon-gamma : interleukin-5 (IFN-gamma : IL-5) and immunoglobulin G1 (IgG1) : IgG2 ratios. The vaccines promoted different levels of control of bacterial replication with the most efficient protection being exerted by cationic liposomes containing monophosphoryl lipid A and low to completely absent immunity with conventional aluminium. The level of protection correlated with the amount of IFN-gamma produced in response to the vaccine whereas there was no inverse correlation with the level of IL-5. Characterizing a protective response was an accelerated recruitment of IL-17 and IFN-gamma-producing lymphocytes resulting in the early formation of granulomas containing clustered inducible nitric oxide synthase-activated macrophages. In comparison, non-protected mice exhibited a different inflammatory infiltrate rich in neutrophil granulocytes. This study indicates that the adjuvant component of a tuberculosis vaccine may be crucial in determining the kinetics by which effective granulomas, pivotal in controlling bacterial growth, are formed.

Sphingosine 1-phosphate promotes antigen processing and presentation to CD4+ T cells in Mycobacterium tuberculosis-infected monocytes

Sphingosine 1-phosphate (S1P) has recently been described to induce antimycobacterial activity. The present study analyses the role played by S1P in antigen presentation of monocytes and in the next activation of Mycobacterium tuberculosis (MTB)-specific CD4+ T cell response. Results reported herein show that S1P stimulation of MTB-infected monocytes (i) inhibits intracellular mycobacterial growth, (ii) enhances phagolysosome maturation and the transit of mycobacteria in MHC class II compartments, (iii) increases the frequency of MTB-specific CD4+CD69+ T cells, expressing the inflammatory homing receptor CCR5, derived from tuberculosis patients and PPD+, BCG naïve, healthy subjects, and (iv) induces IFN-gamma production in CD4+CD69+CCR5+ T cells derived from PPD+ healthy individuals, only. Altogether, these results show that S1P promotes antigen processing and presentation in monocytes, increases the frequency of MTB-specific CD4+ T cells and can regulate IFN-gamma production by antigen specific CD4+ T cells in the course of active disease.