Recent advances towards tuberculosis control: vaccines and biomarkers

Of all infectious diseases, tuberculosis (TB) remains one of the most important causes of morbidity and mortality. Recent advances in understanding the biology of Mycobacterium tuberculosis (Mtb) infection and the immune response of the infected host have led to the development of several new vaccines, a number of which are already undergoing clinical trials. These include pre‐exposure prime vaccines, which could replace bacille Calmette–Guérin (BCG), and pre‐exposure booster vaccines given in addition to BCG. Infants are the target population of these two types of vaccines. In addition, several postexposure vaccines given during adolescence or adult life, in addition to BCG as a priming vaccine during infancy, are undergoing clinical testing. Therapeutic vaccines are currently being assessed for their potential to cure active TB as an adjunct to chemotherapy. BCG replacement vaccines are viable recombinant BCG or double‐deletion mutants of Mtb. All booster vaccines are composed of one or several antigens, either expressed by viral vectors or formulated with adjuvants. Therapeutic vaccines are killed mycobacterial preparations. Finally, multivariate biomarkers and biosignatures are being generated from high‐throughput data with the aim of providing better diagnostic tools to specifically determine TB progression. Here, we provide a technical overview of these recent developments as well of the relevant computational approaches and highlight the obstacles that still need to be overcome.     

Inflammation and tuberculosis: host‐directed therapies

Tuberculosis (TB) is an airborne infectious disease that kills almost two million individuals every year. Multidrug‐resistant (MDR) TB is caused by strains of Mycobacterium tuberculosis (M. tb) resistant to isoniazid and rifampin, the backbone of first‐line antitubercular treatment. MDR TB affects an estimated 500 000 new patients annually. Genetic analysis of drug‐resistant MDR‐TB showed that airborne transmission of undetected and untreated strains played a major role in disease outbreaks. The need for new TB vaccines and faster diagnostics, as well as the development of new drugs, has recently been highlighted. The major problem in terms of current TB research and clinical demands is the increasing number of cases of extensively drug‐resistant and ‘treatment‐refractory’ TB. An emerging scenario of adjunct host‐directed therapies is intended to target pulmonary TB where inflammatory processes can be deleterious and lead to immune exhaustion. ‘Target‐organ‐saving’ strategies may be warranted to prevent damage to infected tissues and achieve focused, clinically relevant and long‐lasting anti‐M. tb cellular immune responses. Candidates for such interventions may be biological agents or already approved drugs that can be ‘re‐purposed’ to interfere with biologically relevant cellular checkpoints. Here, we review current concepts of inflammation in TB disease and discuss candidate pathways for host‐directed therapies to achieve better clinical outcomes.     

Resistance to first-line tuberculosis drugs in three cities of Nigeria

Objectives To determine the levels of resistance to first‐line tuberculosis drugs in three cities in three geopolitical zones in Nigeria. Methods A total of 527 smear‐positive sputum samples from Abuja, Ibadan and Nnewi were cultured on BACTEC‐ MGIT 960. Drug susceptibility tests (DST) for streptomycin, isoniazid, rifampicin and ethambutol were performed on 428 culture‐positive samples on BACTEC‐MGIT960. Results Eight per cent of the specimens cultured were multi‐drug‐resistant Mycobacterium tuberculosis (MDR‐TB) with varying levels of resistance to individual and multiple first–line drugs. MDR was strongly associated with previous treatment: 5% of new and 19% of previously treated patients had MDR‐TB (OR 4.1 (95% CI 1.9–8.8), P = 0.001) and with young adult age: 63% of patients with and 38% without MDR‐TB were 25–34 years old (P = 0.01). HIV status was documented in 71%. There was no association between MDR‐TB and HIV coinfection (P = 0.9) and gender (P > 0.2 for both). Conclusions  MDR‐TB is an emerging problem in Nigeria. Developing good quality drug susceptibility test facilities, routine monitoring of drug susceptibility and improved health systems for the delivery of and adherence to first‐ and second‐line treatment are imperative to solve this problem.     

Drug‐resistant tuberculosis: Past,present, future

In a population of Mycobacterium tuberculosis, random chromosomal mutation that results in genetic resistance to anti‐tuberculosis (TB) drugs occurs at a relatively low frequency. Anti‐TB drugs impose selection pressure so that mycobacterial mutants gradually outnumber susceptible bacilli and emerge as the dominant strains. Resistance to two or more anti‐TB drugs represents cumulative results of sequential mutation. The fourth report on global anti‐TB drug resistance provides the latest data on the extent of such problem in the world. The median prevalence of multi‐drug‐resistant TB (MDR‐TB) in new TB cases was 1.6%, and in previously treated TB cases 11.7%. Of the half a million MDR‐TB cases estimated to have emerged in 2006, 50% were in China and India. The optimal duration of any given combination of anti‐TB drugs for treatment of MDR‐ and extensively drug‐resistant TB (XDR‐TB) has not been defined in controlled clinical trials. Standardized treatment may be feasible for MDR‐TB patients not previously treated with second‐line drugs, but a different strategy needs to be applied in the treatment of MDR‐TB patients who have received second‐line drugs before. Unfortunately, the reliability of drug susceptibility testing of most second‐line anti‐TB drugs is still questionable. Drug‐resistant TB is not necessarily less virulent. Findings from modelling exercise warned that if MDR‐TB case detection and treatment rates increase to the World Health Organization target of 70%, without simultaneously increasing MDR‐TB cure rates, XDR‐TB prevalence could increase exponentially. Prevention of development of drug resistance must be accorded the top priority in the era of MDR‐/XDR‐TB.     

Global Impact of Multidrug-Resistant Pulmonary Tuberculosis Among HIV-Infected and Other Immunocompromised Hosts: Epidemiology,Diagnosis, and Strategies for Management

Multidrug-resistant (MDR) tuberculosis (TB), or TB caused by strains of Mycobacterium tuberculosis resistant to at least isoniazid and rifampicin, represents a major threat to global TB control. Comprising more than 5% of all TB cases annually worldwide, these cases require treatment duration of 2 years on average with expensive and toxic second-line anti-TB drugs. Cure rates are far lower and mortality far higher than for drug-susceptible TB, particularly if patients are coinfected with HIV. Use of rapid diagnostic tools and assessment of risk factors for MDR TB, as well as rapid initiation of MDR TB treatment as recommended by the World Health Organization, including use of appropriate empiric regimens as necessary, is essential to achieving good outcomes from treatment. Rapid initiation of antiretroviral therapy (ART) for those with HIV coinfection, as well as strategic management of overlapping side effects from ART and first and second-line drugs for treating MDR TB to maintain patients on treatment are critical to patient survival and achieving good treatment outcomes. Employing sensible infection control practices in the context of diagnosis and treatment is essential to reducing transmission of MDR TB strains among patient populations and healthcare personnel.     

Guanosine triphosphatases as novel therapeutic targets in tuberculosis

Tuberculosis (TB) is an infectious disease caused by the aerobic microbe Mycobacterium tuberculosis H₃₇Rv. Despite the availability of the Bacille Calmette–Guérin (BCG) vaccine and directly observed treatment, short-course (DOTS), TB is a leading cause of death and affects a third of the world's population. The most important factor associated with disease severity is the development of antibiotic-resistant strains, including multidrug-resistant (MDR)-TB and extensively drug-resistant (XDR)-TB. In order to understand disease pathogenesis, it is necessary to delineate the specific features of M. tuberculosis that enable it to evade the host defense system and contribute to its virulence. Here, we have reviewed the various characteristics, such as cell wall components, virulence genes, and the role of small guanosine triphosphatases (GTPases) in the pathogenesis of TB. GTPases are known to play a crucial role in the survival and pathogenesis of various pathogens. The key role of these proteins involves interference in phagosome maturation arrest, enabling pathogens to survive by escaping from lysozymes and toxic free radicals. This observation provides a new avenue for the development of anti-TB drugs.     

The roles of latency-associated antigens in tuberculosis vaccines

Tuberculosis (TB) is a re-emerging disease and is caused by Mycobacterium tuberculosis (M. tuberculosis). TB is currently one of the leading causes of morbidity and mortality, worldwide. The only available vaccine against TB infection, Bacillus Calmette–Guérin (BCG), fails to adequately protect against reactivation of latent infections in adults. Furthermore, recently developed subunit vaccines, which are in various stages of clinical trials, are all prophylactic vaccines based on proteins expressed in replicating stage of M. tuberculosis and they are not preventive of reactivation of latent TB infection. Thus, an appropriate subunit post-exposure vaccine needs to be developed to control all forms of TB infection. To produce a multi-stage subunit vaccine, scientists should combine the early secreted M. tuberculosis antigens with latency antigens. For this purpose, some latency proteins are known which could be important antigens in the production of specific humoral and cellular immune responses in latent M. tuberculosis infected individuals. Several studies have evaluated the immunogenicity of these proteins in improving the TB vaccines. The present study is a comprehensive review of several studies on the role of the latency antigens in the development of TB vaccines. Overall, the studies indicate that the latency-associated antigens including the resuscitation-promoting factors, the Dormancy of survival regulon (DosR) proteins and the starvation stimulant proteins are potential candidates for the development of subunit vaccines against TB infection.     

Management of difficult multidrug‐resistant tuberculosis and extensively drug‐resistant tuberculosis: Update 2012

Multidrug‐resistant (MDR) tuberculosis (TB) denotes bacillary resistance to at least isoniazid and rifampicin. Extensively drug‐resistant (XDR) TB is MDR‐TB with additional bacillary resistance to any fluoroquinolone and at least one second‐line injectable drugs. Rooted in inadequate TB treatment and compounded by a vicious circle of diagnostic delay and improper treatment, MDR‐TB/XDR‐TB has become a global epidemic that is fuelled by poverty, human immunodeficiency virus (HIV) and neglect of airborne infection control. The majority of MDR‐TB cases in some settings with high prevalence of MDR‐TB are due to transmission of drug‐resistant bacillary strains to previously untreated patients. Global efforts in controlling MDR‐TB/XDR‐TB can no longer focus solely on high‐risk patients. It is difficult and costly to treat MDR‐TB/XDR‐TB. Without timely implementation of preventive and management strategies, difficult MDR‐TB/XDR‐TB can cripple global TB control efforts. Preventive strategies include prompt diagnosis with adequate TB treatment using the directly observed therapy, short‐course (DOTS) strategy and drug‐resistance programmes, airborne infection control, preventive treatment of TB/HIV, and optimal use of antiretroviral therapy. Management strategies for established cases of difficult MDR‐TB/XDR‐TB rely on harnessing existing drugs (notably newer generation fluoroquinolones, high‐dose isoniazid, linezolid and pyrazinamide with in vitro activity) in the best combinations and dosing schedules, together with adjunctive surgery in carefully selected cases. Immunotherapy may also have a role in the future. New diagnostics, drugs and vaccines are required to meet the challenge, but science alone is insufficient. Difficult MDR‐TB/XDR‐TB cannot be tackled without achieving high cure rates with quality DOTS and beyond, and concurrently addressing poverty and HIV.     

Drug‐resistant tuberculosis: An update on disease burden,diagnosis and treatment

The emergence of antimicrobial resistance against Mycobacterium tuberculosis , the leading cause of mortality due to a single microbial pathogen worldwide, represents a growing threat to public health and economic growth. The global burden of multidrug‐resistant tuberculosis (MDR‐TB) has recently increased by an annual rate of more than 20%. According to the World Health Organization approximately only half of all patients treated for MDR‐TB achieved a successful outcome. For many years, patients with drug‐resistant tuberculosis (TB) have received standardized treatment regimens, thereby accelerating the development of MDR‐TB through drug‐specific resistance amplification. Comprehensive drug susceptibility testing (phenotypic and/or genotypic) is necessary to inform physicians about the best drugs to treat individual patients with tailor‐made treatment regimens. Phenotypic drug resistance can now often, but with variable sensitivity, be predicted by molecular drug susceptibility testing based on whole genome sequencing, which in the future could become an affordable method for the guidance of treatment decisions, especially in high‐burden/resource‐limited settings. More recently, MDR‐TB treatment outcomes have dramatically improved with the use of bedaquiline‐based regimens. Ongoing clinical trials with novel and repurposed drugs will potentially further improve cure‐rates, and may substantially decrease the duration of MDR‐TB treatment necessary to achieve relapse‐free cure.     

Recombinant and synthetic peptides to identify Mycobacterium tuberculosis antigens and epitopes of diagnostic and vaccine relevance

The failures of Bacillus Calmette Guerin (BCG) as a vaccine and purified protein derivative as a diagnostic reagent in controlling the worldwide prevalence of tuberculosis (TB) have accelerated the research to identify Mycobacterium tuberculosis-specific antigens that could be useful as new vaccines and diagnostic reagents against TB. In the recent years, the comparative analyses of M. tuberculosis genome with the genomes of other mycobacteria have led to the identification of several genomic regions of M. tuberculosis that are deleted in BCG and other mycobacteria. These deleted regions (RDs) are predicted to encode over 100 proteins. If found immunologically reactive, the proteins encoded by M. tuberculosis-specific RDs could be useful in the specific diagnosis of TB and developing new vaccines. Among the approaches available for immunological characterization of the predicted M. tuberculosis-specific proteins are the evaluations of recombinant proteins and/or overlapping synthetic peptides, covering the sequence of each protein, for antibody and/or Th1 cell reactivity. These approaches have resulted into the identification of several antigenic proteins of M. tuberculosis encoded by genes located in RD1 with potentials in specific diagnosis of TB in low endemic areas and/or development of new vaccines, e.g. ORF14, ESAT6, CFP10, PE, PPE proteins, etc. In addition, prediction programs to identify peptides that could bind several HLA molecules, and presented to T-cells in a promiscuous manner, have been developed. These programs have been used, on a limited scale, to identify the promiscuous peptides encoded by the genes spanning the M. tuberculosis-specific sequence. The promiscuous antigens/peptides recognized by T-cells in cell mediated immunity assays may have potentials in developing peptide-based vaccines and diagnostic reagents against TB.     

Tuberculosis vaccines: Opportunities and challenges

Tuberculosis (TB) is a serious disease around the world. Bacillus Calmette–Guérin (BCG) is the only TB vaccine licensed for use in human beings, and is effective in protecting infants and children against severe miliary and meningeal TB. However, BCG's protective efficacy is variable in adults. Novel TB vaccine candidates being developed include whole‐cell vaccines (recombinant BCG (rBCG), attenuated Mycobacterium tuberculosis , killed M. tuberculosis or Mycobacterium vaccae ), adjuvanted protein subunit vaccines, viral vector‐delivered subunit vaccines, plasmid DNA vaccines, RNA‐based vaccines etc. At least 12 novel TB vaccine candidates are now in clinical trials, including killed M. vaccae , rBCG ΔureC::hly , adjuvanted fusion proteins M72 and H56 and viral vectored MVA85A. Unfortunately, in TB, there are no correlates of vaccine‐induced protection, although cell‐mediated immune responses such as interferon‐gamma (IFN‐γ) production are widely used to assess vaccine's immunogenicity. Recent studies suggested that central memory T cells and local secreted IgA correlated with protection against TB disease. Clinical TB vaccine efficacy trials should invest in identifying correlates of protection, and evaluate new TB biomarkers emerging from human and animal studies. Accumulating new knowledge on M. tuberculosis antigens and immune profiles correlating with protection or disease risk will be of great help in designing next generation of TB vaccines.     

Perspectives for personalized therapy for patients with multidrug‐resistant tuberculosis

According to the World Health Organization (WHO), tuberculosis is the leading cause of death attributed to a single microbial pathogen worldwide. In addition to the large number of patients affected by tuberculosis, the emergence of Mycobacterium tuberculosis drug‐resistance is complicating tuberculosis control in many high‐burden countries. During the past 5 years, the global number of patients identified with multidrug‐resistant tuberculosis (MDR‐TB), defined as bacillary resistance at least against rifampicin and isoniazid, the two most active drugs in a treatment regimen, has increased by more than 20% annually. Today we experience a historical peak in the number of patients affected by MDR‐TB. The management of MDR‐TB is characterized by delayed diagnosis, uncertainty of the extent of bacillary drug‐resistance, imprecise standardized drug regimens and dosages, very long duration of therapy and high frequency of adverse events which all translate into a poor prognosis for many of the affected patients. Major scientific and technological advances in recent years provide new perspectives through treatment regimens tailor‐made to individual needs. Where available, such personalized treatment has major implications on the treatment outcomes of patients with MDR‐TB. The challenge now is to bring these adances to those patients that need them most.     

Optimal models to evaluate the protective efficacy of tuberculosis vaccines.

BCG has been used widely as a vaccine to prevent tuberculosis (TB) for 80 years, yet there is still considerable controversy about its efficacy. Many experimental variables have obscured the true efficacy of BCG. The absence of appropriate animal models for the study of protective efficacy and the lack of in vitro correlates of protective immunity have impeded progress. Laboratory animal studies, which have contributed to understanding the pathogenesis, heritability of resistance and immunology of TB, have failed to identify the immunological pathways necessary for protective immunity. In recent years, cattle and deer, which are naturally susceptible to TB, have been used to study protective immunity in vaccinated animals, challenged with virulent bacteria. A deer TB infection model has been developed that can measure protection against TB infection or the development of disease. Data from this model show that, providing live BCG is administered in a short interval prime-boost protocol, significant protection against infection and disease can be obtained. Single dose vaccine provides suboptimal protection that attenuates pathology but does not prevent infection. Low dose BCG vaccine (10(4)cfu), administered in a prime-boost protocol, sufficient to prevent infection, does not cause conversion to delayed type hypersensitivity or produce unacceptable side-effects. Immune memory for protection against infection persists at optimal levels for at least 12 months post vaccination. Used optimally, BCG produces good levels of protection against TB and improved protocols or its use should be explored, before attempts are made to replace it with new-generation vaccines. It is now possible to integrate the fundamental information obtained from laboratory animals with studies of functional immune protection in target host species. Justification for the use of TB vaccines for domestic livestock under field conditions, must be underpinned by scientific evidence that they provide acceptable levels of protection long term.     

MDR- and XDR-TB among suspected drug-resistant TB patients in a tertiary care hospital in India

Aim: To study the anti‐tubercular drug resistance pattern among suspected cases of drug‐resistant TB. Materials and Methods: First and second line drug susceptibility data were retrospectively analysed for all suspected cases of drug‐resistant tuberculosis (TB), presenting to the Pulmonary Medicine department of tertiary care hospital in South India from 2003 to 2007. Results and Discussion: Out of 177 cases of suspected drug‐resistant TB, 103 (58.2%) cases were multi‐drug‐resistant tuberculosis (MDR‐TB). Out of 75 cases of MDR‐TB for whom second‐line drug susceptibility test was performed, 45 (60.0%) cases met the criteria of extensively drug–resistant (XDR) TB, which is very high when compared with existing worldwide data on XDR‐TB (6.6% cases of MDR‐TB). In comparison with non‐MDR‐TB cases, MDR and XDR‐TB cases had a history of significantly higher duration of anti‐TB treatment (ATT) and significantly higher exposure to multiple ATT regimens. Past exposure to second‐line anti‐TB drugs was significantly high in XDR‐TB cases than in MDR‐TB and non MDR‐TB cases. Conclusion: This study highlights the high burden of XDR‐ and MDR‐TB among TB patients coming to tertiary care hospitals in India. Please cite this paper as: James P, Gupta R, Christopher DJ, Thankagunam B and Veeraraghavan B. MDR and XDR‐TB among suspected drug resistant TB patients in a tertiary care hospital in India. Clin Respir J 2011; 5: 19–25.     

Latent tuberculosis infection: Opportunities and challenges

Diagnosing and treating latent tuberculosis (TB) infection (LTBI) is recognized by the World Health Organization as an important strategy to accelerate the decline in global TB and achieve TB elimination. Even among low‐TB burden countries that have achieved high rates of detection and successful treatment for active TB, a number of barriers have prevented implementing or expanding LTBI treatment programmes. Of those infected with TB, relatively few will develop active disease and the current diagnostic tests have a low predictive value. LTBI treatment using isoniazid (INH) has low completion rates due to the long duration of therapy and poor tolerability. Both patients and physicians often perceive the risk of toxicity to be greater than the risk of reactivation TB. As a result, LTBI treatment has had a limited or negligible role outside of countries with high resources and low burden of disease. New tools have emerged including the interferon‐gamma release assays that more accurately diagnose LTBI, particularly in people vaccinated with Bacillus Calmette–Guerin (BCG). Shorter, better tolerated treatment using rifamycins are proving safe and effective alternatives to INH. While still imperfect, TB prevention using these new diagnostic and treatment tools appear cost effective in modelling studies in the United States and have the potential to improve TB prevention efforts globally. Continued research to understand the host–organism interactions within the spectrum of LTBI is needed to develop better tools. Until then, overcoming the barriers and optimizing our current tools is essential for progressing toward TB elimination.     

Novel tuberculosis vaccination strategies based on understanding the immune response

Abstract. Kaufmann SHE (Max Planck Institute for Infection Biology, Berlin, Germany). Novel tuberculosis vaccination strategies based on understanding the immune response (Foresight). J Intern Med 2010; 267 : 337–353. The current tuberculosis (TB) vaccine bacillus Calmette–Guérin (BCG) fails to protect against adult pulmonary TB. Yet, its capacity to control miliary TB in newborn infants forms the basis for development of novel vaccine candidates. These either exploit genetic modification of BCG to create a viable replacement vaccine or use BCG to prime the immune response followed by boost with a novel subunit vaccine. This could ultimately result in a combination vaccination schedule comprising a prime with a live BCG replacement followed by a subunit vaccine boost. Ultimately, vaccination strategies that achieve sterile eradication of, or prevent infection with, tubercle bacilli would be an ambitious highly promising goal.     

NATCON 2022 Panel discussion: “TB vaccination - Going Forward”

Vaccination is important tuberculosis (TB) preventive strategy that is essential to achieve the goals of the End TB strategy. The BCG vaccination at birth offers protection against TB in young children but not in adolescents and adults. New TB vaccines are the need of the hour. The TB vaccine development pipeline in the past years is encouraging with newer TB vaccines in clinical trials in humans. The focus of the newer TB vaccine is the prevention of infection, disease, and recurrence of TB disease. Therapeutic vaccines focus on better treatment outcomes and prevention of TB recurrence. BCG revaccination is of current interest. Novel, safe, and efficient TB vaccines that prevent TB infection and disease if introduced in 2025 could drastically reduce the rate of TB incidence. However, the development of an effective vaccine for TB is challenging. Engagement of stakeholders, mobilizing funding, and advocacy could accelerate the newer TB vaccine development process.     

Management of tuberculosis and latent tuberculosis infection in human immunodeficiency virus‐infected persons

The syndemic of human immunodeficiency virus (HIV)/tuberculosis (TB) co‐infection has grown as a result of the considerable sociogeographic overlaps between the two epidemics. The situation is particularly worrisome in countries with high or intermediate TB burden against the background of a variable HIV epidemic state. Early diagnosis of TB disease in an HIV‐infected person is paramount but suffers from lack of sensitive and specific diagnostic tools. Enhanced symptom screening is currently advocated, and the wide application of affordable molecular diagnostics is urgently needed. Treatment of TB/HIV co‐infection involves the concurrent use of standard antiretrovirals and antimycobacterials during which harmful drug interaction may occur. The pharmacokinetic interaction between rifamycin and antiretrovirals is a case in point, requiring dosage adjustment and preferential use of rifabutin, if available. Early initiation of antiretroviral therapy is indicated, preferably at 2 weeks after starting TB treatment for patients with a CD4 of <50 cells/μL. Development of TB‐immune reconstitution inflammatory syndrome (TB‐IRIS) is however more frequent with early antiretroviral therapy. The diagnosis of TB‐IRIS is another clinical challenge, and cautious use of corticosteroids is suggested to improve clinical outcome. As a preventive measure against active TB disease, the screening for latent TB infection should be widely practiced, followed by at least 6–9 months of isoniazid treatment. To date tuberculin skin test remains the only diagnostic tool in high TB burden countries. The role of alternative tests, for example, interferon‐γ release assay, would need to be better defined for clinical application.