Natural infection of guinea pigs exposed to patients with highly drug-resistant tuberculosis

Rifampicin drug resistance and host immunity in tuberculosis: more than meets the eye

Tuberculosis (TB) is the leading cause of death due to an infectious agent, with more than 1.5 million deaths attributed to TB annually worldwide. The global dissemination of drug resistance across Mycobacterium tuberculosis (Mtb) strains, causative of TB, resulted in an estimated 450 000 cases of drug-resistant (DR) TB in 2021. Dysregulated immune responses have been observed in patients with multidrug resistant (MDR) TB, but the effects of drug resistance acquisition and impact on host immunity remain obscure. In this review, we compile studies that span aspects of altered host-pathogen interactions and highlight research that explores how drug resistance and immunity might intersect. Understanding the immune processes differentially induced during DR TB would aid the development of rational therapeutics and vaccines for patients with MDR TB.

Controlled Human Infection Models To Accelerate Vaccine Development

The timelines for developing vaccines against infectious diseases are lengthy, and often vaccines that reach the stage of large phase 3 field trials fail to provide the desired level of protective efficacy. The application of controlled human challenge models of infection and disease at the appropriate stages of development could accelerate development of candidate vaccines and, in fact, has done so successfully in some limited cases. Human challenge models could potentially be used to gather critical information on pathogenesis, inform strain selection for vaccines, explore cross-protective immunity, identify immune correlates of protection and mechanisms of protection induced by infection or evoked by candidate vaccines, guide decisions on appropriate trial endpoints, and evaluate vaccine efficacy. We prepared this report to motivate fellow scientists to exploit the potential capacity of controlled human challenge experiments to advance vaccine development. In this review, we considered available challenge models for 17 infectious diseases in the context of the public health importance of each disease, the diversity and pathogenesis of the causative organisms, the vaccine candidates under development, and each model's capacity to evaluate them and identify correlates of protective immunity. Our broad assessment indicated that human challenge models have not yet reached their full potential to support the development of vaccines against infectious diseases. On the basis of our review, however, we believe that describing an ideal challenge model is possible, as is further developing existing and future challenge models.

Ferrets as a model for tuberculosis transmission

Even with the COVID-19 pandemic, tuberculosis remains a leading cause of human death due to a single infectious agent. Until successfully treated, infected individuals may continue to transmit Mycobacterium tuberculosis bacilli to contacts. As with other respiratory pathogens, such as SARS-CoV-2, modeling the process of person-to-person transmission will inform efforts to develop vaccines and therapies that specifically impede disease transmission. The ferret (Mustela furo), a relatively inexpensive, small animal has been successfully employed to model transmissibility, pathogenicity, and tropism of influenza and other respiratory disease agents. Ferrets can become naturally infected with Mycobacterium bovis and are closely related to badgers, well known in Great Britain and elsewhere as a natural transmission vehicle for bovine tuberculosis. Herein, we report results of a study demonstrating that within 7 weeks of intratracheal infection with a high dose (>5 x 10³ CFU) of M. tuberculosis bacilli, ferrets develop clinical signs and pathological features similar to acute disease reported in larger animals, and ferrets infected with very-high doses (>5 x 10⁴ CFU) develop severe signs within two to four weeks, with loss of body weight as high as 30%. Natural transmission of this pathogen was also examined. Acutely-infected ferrets transmitted M. tuberculosis bacilli to co-housed naïve sentinels; most of the sentinels tested positive for M. tuberculosis in nasal washes, while several developed variable disease symptomologies similar to those reported for humans exposed to an active tuberculosis patient in a closed setting. Transmission was more efficient when the transmitting animal had a well-established acute infection. The findings support further assessment of this model system for tuberculosis transmission including the testing of prevention measures and vaccine efficacy.

Early alveolar macrophage response and IL-1R-dependent T cell priming determine transmissibility of Mycobacterium tuberculosis strains

Mechanisms underlying variability in transmission of Mycobacterium tuberculosis strains remain undefined. By characterizing high and low transmission strains of M.tuberculosis in mice, we show here that high transmission M.tuberculosis strain induce rapid IL-1R-dependent alveolar macrophage migration from the alveolar space into the interstitium and that this action is key to subsequent temporal events of early dissemination of bacteria to the lymph nodes, Th1 priming, granulomatous response and bacterial control. In contrast, IL-1R-dependent alveolar macrophage migration and early dissemination of bacteria to lymph nodes is significantly impeded in infection with low transmission M.tuberculosis strain; these events promote the development of Th17 immunity, fostering neutrophilic inflammation and increased bacterial replication. Our results suggest that by inducing granulomas with the potential to develop into cavitary lesions that aids bacterial escape into the airways, high transmission M.tuberculosis strain is poised for greater transmissibility. These findings implicate bacterial heterogeneity as an important modifier of TB disease manifestations and transmission.

Occupational Risk of Airborne Mycobacterium tuberculosis Exposure: A Situational Analysis in a Three-Tier Public Healthcare System in South Africa

This study aimed to detect airborne Mycobacterium tuberculosis (MTB) at nine public health facilities in three provinces of South Africa and determine possible risk factors that may contribute to airborne transmission. Personal samples (n = 264) and stationary samples (n = 327) were collected from perceived high-risk areas in district, primary health clinics (PHCs) and TB facilities. Quantitative real-time (RT) polymerase chain reaction (PCR) was used for TB analysis. Walkabout observations and work practices through the infection prevention and control (IPC) questionnaire were documented. Statistical analysis was carried out using Stata version 15.2 software. Airborne MTB was detected in 2.2% of samples (13/572), and 97.8% were negative. District hospitals and Western Cape province had the most TB-positive samples and identified risk areas included medical wards, casualty, and TB wards. MTB-positive samples were not detected in PHCs and during the summer season. All facilities reported training healthcare workers (HCWs) on TB IPC. The risk factors for airborne MTB included province, type of facility, area or section, season, lack of UVGI, and ineffective ventilation. Environmental monitoring, PCR, IPC questionnaire, and walkabout observations can estimate the risk of TB transmission in various settings. These findings can be used to inform management and staff to improve the TB IPC programmes.

Mycobacterium tuberculosis-Specific T Cell Functional, Memory, and Activation Profiles in QuantiFERON-Reverters Are Consistent With Controlled Infection

Reversion of immune sensitization tests for Mycobacterium tuberculosis (M.tb) infection, such as interferon-gamma release assays or tuberculin skin test, has been reported in multiple studies. We hypothesized that QuantiFERON-TB Gold (QFT) reversion is associated with a decline of M.tb-specific functional T cell responses, and a distinct pattern of T cell and innate responses compared to persistent QFT+ and QFT- individuals. We compared groups of healthy adolescents (n=~30 each), defined by four, 6-monthly QFT tests: reverters (QFT+/+/-/-), non-converters (QFT-/-/-/-) and persistent positives (QFT+/+/+/+). We stimulated peripheral blood mononuclear cells with M.tb antigens (M.tb lysate; CFP-10/ESAT-6 and EspC/EspF/Rv2348 peptide pools) and measured M.tb-specific adaptive T cell memory, activation, and functional profiles; as well as functional innate (monocytes, natural killer cells), donor-unrestricted T cells (DURT: γδ T cells, mucosal-associated invariant T and natural killer T-like cells) and B cells by flow cytometry. Projection to latent space discriminant analysis was applied to determine features that best distinguished between QFT reverters, non-converters and persistent positives. No longitudinal changes in immune responses to M.tb were observed upon QFT reversion. M.tb-specific Th1 responses detected in reverters were of intermediate magnitude, higher than responses in QFT non-converters and lower than responses in persistent positives. About one third of reverters had a robust response to CFP-10/ESAT-6. Among those with measurable responses, lower proportions of T_SCM (CD45RA+CCR7+CD27+) and early differentiated (CD45RA-) IFN-γ-TNF+IL-2- M.tb lysate-specific CD4+ cells were observed in reverters compared with non-converters. Conversely, higher proportions of early differentiated and lower proportions of effector (CD45RA-CCR7-) CFP10/ESAT6-specific Th1 cells were observed in reverters compared to persistent-positives. No differences in M.tb-specific innate, DURT or B cell functional responses were observed between the groups. Statistical modelling misclassified the majority of reverters as non-converters more frequently than they were correctly classified as reverters or misclassified as persistent positives. These findings suggest that QFT reversion occurs in a heterogeneous group of individuals with low M.tb-specific T cell responses. In some individuals QFT reversion may result from assay variability, while in others the magnitude and differentiation status of M.tb-specific Th1 cells are consistent with well-controlled M.tb infection.

The double-edged sword of Tregs in M tuberculosis, M avium, and M absessus infection

Immunity against different Mycobacteria species targeting the lung requires distinctly different pulmonary immune responses for bacterial clearance. Many parameters of acquired and regulatory immune responses differ quantitatively and qualitatively from immunity during infection with Mycobacteria species. Nontuberculosis Mycobacteria species (NTM) Mycobacterium avium- (M avium), Mycobacterium abscessus-(M abscessus), and the Mycobacteria species Mycobacterium tuberculosis-(Mtb). Herein, we discuss the potential implications of acquired and regulatory immune responses in the context of animal and human studies, as well as future directions for efforts to treat Mycobacteria diseases.

Does Mycobacterium tuberculosis var. bovis Survival in the Environment Confound Bovine Tuberculosis Control and Eradication? A Literature Review

Bovine tuberculosis (bTB) is one of the globe's most common, multihost zoonoses and results in substantial socioeconomic costs for governments, farming industries, and tax payers. Despite decades of surveillance and research, surprisingly, little is known about the exact mechanisms of transmission. In particular, as a facultative intracellular pathogen, to what extent does survival of the causative agent, Mycobacterium tuberculosis var. bovis (M. bovis), in the environment constitute an epidemiological risk for livestock and wildlife? Due largely to the classical pathology of cattle cases, the received wisdom was that bTB was spread by direct inhalation and exchange of bioaerosols containing droplets laden with bacteria. Other members of the Mycobacterium tuberculosis complex (MTBC) exhibit differing host ranges, an apparent capacity to persist in environmental fomites, and they favour a range of different transmission routes. It is possible, therefore, that infection from environmental sources of M. bovis could be a disease transmission risk. Recent evidence from GPS-collared cattle and badgers in Britain and Ireland suggests that direct transmission by infectious droplets or aerosols may not be the main mechanism for interspecies transmission, raising the possibility of indirect transmission involving a contaminated, shared environment. The possibility that classical pulmonary TB can be simulated and recapitulated in laboratory animal models by ingestion of contaminated feed is a further intriguing indication of potential environmental risk. Livestock and wildlife are known to shed M. bovis onto pasture, soil, feedstuffs, water, and other fomites; field and laboratory studies have indicated that persistence is possible, but variable, under differing environmental conditions. Given the potential infection risk, it is timely to review the available evidence, experimental approaches, and methodologies that could be deployed to address this potential blind spot and control point. Although we focus on evidence from Western Europe, the concepts are widely applicable to other multihost bTB episystems.

Ultra-low Dose Aerosol Infection of Mice with Mycobacterium tuberculosis More Closely Models Human Tuberculosis

Tuberculosis (TB) is a heterogeneous disease manifesting in a subset of individuals infected with aerosolized Mycobacterium tuberculosis (Mtb). Unlike human TB, murine infection results in uniformly high lung bacterial burdens and poorly organized granulomas. To develop a TB model that more closely resembles human disease, we infected mice with an ultra-low dose (ULD) of between 1-3 founding bacteria, reflecting a physiologic inoculum. ULD-infected mice exhibited highly heterogeneous bacterial burdens, well-circumscribed granulomas that shared features with human granulomas, and prolonged Mtb containment with unilateral pulmonary infection in some mice. We identified blood RNA signatures in mice infected with an ULD or a conventional Mtb dose (50-100 CFU) that correlated with lung bacterial burdens and predicted Mtb infection outcomes across species, including risk of progression to active TB in humans. Overall, these findings highlight the potential of the murine TB model and show that ULD infection recapitulates key features of human TB.

'It has become everybody's business and nobody's business': Policy actor perspectives on the implementation of TB infection prevention and control (IPC) policies in South African public sector primary care health facilities

South Africa is increasingly offering screening, diagnosis and treatment of tuberculosis (TB), and especially drug-resistant TB, at the primary care level. Nosocomial transmission of TB within primary health facilities is a growing concern in South Africa, and globally. We explore here how TB infection prevention and control (IPC) policies, historically focused on hospitals, are being implemented within primary care facilities. We spoke to 15 policy actors using in-depth interviews about barriers to effective TB-IPC and opportunities for improving implementation. We identified four drivers of poor policy implementation: fragmentation of institutional responsibility and accountability for TB-IPC; struggles by TB-IPC advocates to frame TB-IPC as an urgent and addressable policy problem; barriers to policy innovation from both a lack of evidence as well as a policy environment dependent on 'new' evidence to justify new policy; and the impact of professional medical cultures on the accurate recognition of and response to TB risks. Participants also identified examples of TB-IPC innovation and described conditions necessary for these successes. TB-IPC is a long-standing, complex health systems challenge. As important as downstream practices like mask-wearing and ventilation are, sustained, effective TB-IPC ultimately requires that we better address the upstream barriers to TB-IPC policy formulation and implementation.

Sensitivity optimisation of tuberculosis bioaerosol sampling

INTRODUCTION

Detection of Mycobacterium tuberculosis (Mtb) in patient-derived bioaerosol is a potential tool to measure source case infectiousness. However, current bioaerosol sampling approaches have reported low detection yields in sputum-positive TB cases. To increase the utility of bioaerosol sampling, we present advances in bioaerosol collection and Mtb identification that improve detection yields.

METHODS

A previously described Respiratory Aerosol Sampling Chamber (RASC) protocol, or "RASC-1", was modified to incorporate liquid collection of bioaerosol using a high-flow wet-walled cyclone (RASC-2). Individuals with GeneXpert-positive pulmonary TB were sampled pre-treatment over 60-minutes. Putative Mtb bacilli were detected in collected fluid by fluorescence microscopy utilising DMN-Trehalose. Exhaled air and bioaerosol volumes were estimated using continuous CO2 monitoring and airborne particle counting, respectively. Mtb capture was calculated per exhaled air volume sampled and bioaerosol volume for RASC-1 (n = 35) and for RASC-2 (n = 21). Empty chamber samples were collected between patients as controls.

RESULTS

The optimised RASC-2 protocol sampled a median of 258.4L (IQR: 226.9-273.6) of exhaled air per patient compared with 27.5L (IQR: 23.6-30.3) for RASC-1 (p<0.0001). Bioaerosol volume collection was estimated at 2.3nL (IQR: 1.1-3.6) for RASC-2 compared with 0.08nL (IQR: 0.05-0.10) for RASC-1 (p<0.0001). The detection yield of viable Mtb improved from 43% (median 2 CFU, range: 1-14) to 95% (median 20.5 DMN-Trehalose positive bacilli, range: 2-155). These improvements represent a lowering of the limit of detection in the RASC-2 platform to 0.9 Mtb bacilli per 100L of exhaled air from 3.3 Mtb bacilli per 100L (RASC-1).

CONCLUSION

This study demonstrates that technical improvements in particle collection together with sensitive detection enable rapid quantitation of viable Mtb in bioaerosols of sputum positive TB cases. Increased sampling sensitivity may allow future TB transmission studies to be extended to sputum-negative and subclinical individuals, and suggests the potential utility of bioaerosol measurement for rapid intervention in other airborne infectious diseases.

Bacterial and host determinants of cough aerosol culture positivity in patients with drug-resistant versus drug-susceptible tuberculosis

A burgeoning epidemic of drug-resistant tuberculosis (TB) threatens to derail global control efforts. Although the mechanisms remain poorly clarified, drug-resistant strains are widely believed to be less infectious than drug-susceptible strains. Consequently, we hypothesized that lower proportions of patients with drug-resistant TB would have culturable Mycobacterium tuberculosis from respirable, cough-generated aerosols compared to patients with drug-susceptible TB, and that multiple factors, including mycobacterial genomic variation, would predict culturable cough aerosol production. We enumerated the colony forming units in aerosols (≤10 µm) from 452 patients with TB (227 with drug resistance), compared clinical characteristics, and performed mycobacterial whole-genome sequencing, dormancy phenotyping and drug-susceptibility analyses on M. tuberculosis from sputum. After considering treatment duration, we found that almost half of the patients with drug-resistant TB were cough aerosol culture-positive. Surprisingly, neither mycobacterial genomic variants, lineage, nor dormancy status predicted cough aerosol culture positivity. However, mycobacterial sputum bacillary load and clinical characteristics, including a lower symptom score and stronger cough, were strongly predictive, thereby supporting targeted transmission-limiting interventions. Effective treatment largely abrogated cough aerosol culture positivity; however, this was not always rapid. These data question current paradigms, inform public health strategies and suggest the need to redirect TB transmission-associated research efforts toward host-pathogen interactions.

Enhanced tenacity of mycobacterial aerosols from necrotic neutrophils

The tuberculosis agent Mycobacterium tuberculosis is primarily transmitted through air, but little is known about the tenacity of mycobacterium-containing aerosols derived from either suspensions or infected neutrophils. Analysis of mycobacterial aerosol particles generated from bacterial suspensions revealed an average aerodynamic diameter and mass density that may allow distant airborne transmission. The volume and mass of mycobacterial aerosol particles increased with elevated relative humidity. To more closely mimic aerosol formation that occurs in active TB patients, aerosols from mycobacterium-infected neutrophils were analysed. Mycobacterium-infected intact neutrophils showed a smaller particle size distribution and lower viability than free mycobacteria. In contrast, mycobacterium-infected necrotic neutrophils, predominant in M. tuberculosis infection, revealed particle sizes and viability rates similar to those found for free mycobacteria, but in addition, larger aggregates of viable mycobacteria were observed. Therefore, mycobacteria are shielded from environmental stresses in multibacillary aggregates generated from necrotic neutrophils, which allows improved tenacity but emphasizes short distance transmission between close contacts.

Preventing Transmission of Mycobacterium Tuberculosis-A Refocused Approach

Traditional tuberculosis (TB) infection control focuses on the known patient with TB, usually on appropriate treatment. A refocused, intensified TB infection control approach is presented. Combined with active case finding and rapid molecular diagnostics, an approach called FAST is described as a convenient way to call attention to the untreated patient. Natural ventilation is the mainstay of air disinfection in much of the world. Germicidal ultraviolet technology is the most sustainable approach to air disinfection under resource-limited conditions. Testing and treatment of latent TB infection works to prevent reactivation but requires greater risk targeting in both low- and high-risk settings.

Evaluation of Peripheral Blood Markers as Early Endpoint Criteria in Guinea Pigs (Cavia porcellus) when Testing Tuberculosis Vaccine Candidates

The guinea pig model of tuberculosis is used extensively to assess the efficacy of novel tuberculosis vaccines. There are established parameters to determine vaccine efficacy in this model, but the science community currently lacks established biomarkers for early detection and monitoring of experimental disease in guinea pigs. To define a set of biomarkers that could be used as benchmarks for disease progression and early endpoint criteria, we assessed serum biochemical and hematology parameters in 2 groups of guinea pigs-one vaccinated with the attenuated Mycobacterium bovis vaccine strain (BCG) and one sham-vaccinated with saline-and then experimentally infected with a virulent strain of Mycobacterium tuberculosis. After infection, WBC showed the strongest differences between saline-inoculated and vaccinated animals, with more subtle changes in other serum biochemical parameters, including ALT and ALP. Therefore, this study provides a starting point for evaluating the utility of blood values as possible early endpoint criteria in the guinea pig model of tuberculosis.

Animal Models of Tuberculosis Vaccine Research: An Important Component in the Fight against Tuberculosis

Tuberculosis (TB), an infectious disease caused by Mycobacterium tuberculosis, is one of the top ten infectious diseases worldwide, and is the leading cause of morbidity from a single infectious agent. M. tuberculosis can cause infection in several species of animals in addition to humans as the natural hosts. Although animal models of TB disease cannot completely simulate the occurrence and development of human TB, they play an important role in studying the pathogenesis, immune responses, and pathological changes as well as for vaccine research. This review summarizes the commonly employed animal models, including mouse, guinea pig, rabbit, rat, goat, cattle, and nonhuman primates, and their characteristics as used in TB vaccine research, and provides a basis for selecting appropriate animal models according to specific research needs. Furthermore, some of the newest animal models used for TB vaccine research (such as humanized animal models, zebrafish, Drosophila, and amoeba) are introduced, and their characteristics and research progress are discussed.

Insights and challenges in tuberculosis vaccine development

Tuberculosis kills more people than any other pathogen and the need for a universally effective vaccine has never been greater. An effective vaccine will be a key tool in achieving the targets set by WHO in the End TB Strategy. Tuberculosis vaccine development is difficult and slow. Substantial progress has been made in research and development of tuberculosis vaccines in the past 20 years, and two clinical trial results from 2018 provide reason for optimism. However, many challenges to the successful licensure and deployment of an effective tuberculosis vaccine remain. The development of new tools for vaccine evaluation might facilitate these processes, and continued collaborative working and sustained funding will be essential.

Potential benefits and harms of the use of UV radiation in transmission of tuberculosis in South African health facilities

The incidence and prevalence of transmitted Mycobacterium tuberculosis have risen very rapidly in modern society. Environmental control measure such as ultraviolet radiation has been introduced in various health care facilities. This preventative measure has been extensively explored in the medical, legislative and public forums. However, the guidelines and manufacturer's claims have created controversies, in terms of prevention of cross-transmission of M. tuberculosis in health care facilities. In this article, the authors reviewed the overall benefits and harms associated with the use of ultraviolet radiation in the prevention of M. tuberculosis transmission. The author concluded that there are still existing gaps in proving beyond any reasonable doubt that ultraviolet radiations absolutely prevent the spread of M. tuberculosis in South African health facilities.

Transmission of drug-resistant tuberculosis in HIV-endemic settings

The emergence and expansion of the multidrug-resistant tuberculosis epidemic is a threat to the global control of tuberculosis. Multidrug-resistant tuberculosis is the result of the selection of resistance-conferring mutations during inadequate antituberculosis treatment. However, HIV has a profound effect on the natural history of tuberculosis, manifesting in an increased rate of disease progression, leading to increased transmission and amplification of multidrug-resistant tuberculosis. Interventions specific to HIV-endemic areas are urgently needed to block tuberculosis transmission. These interventions should include a combination of rapid molecular diagnostics and improved chemotherapy to shorten the duration of infectiousness, implementation of infection control measures, and active screening of multidrug-resistant tuberculosis contacts, with prophylactic regimens for individuals without evidence of disease. Development and improvement of the efficacy of interventions will require a greater understanding of the factors affecting the transmission of multidrug-resistant tuberculosis in HIV-endemic settings, including population-based molecular epidemiology studies. In this Series article, we review what we know about the transmission of multidrug-resistant tuberculosis in settings with high burdens of HIV and define the research priorities required to develop more effective interventions, to diminish ongoing transmission and the amplification of drug resistance.

Transmission phenotype of Mycobacterium tuberculosis strains is mechanistically linked to induction of distinct pulmonary pathology

In a study of household contacts (HHC), households were categorized into High (HT) and Low (LT) transmission groups based on the proportion of HHC with a positive tuberculin skin test. The Mycobacterium tuberculosis (Mtb) strains from HT and LT index cases of the households were designated Mtb-HT and Mtb-LT, respectively. We found that C3HeB/FeJ mice infected with Mtb-LT strains exhibited significantly higher bacterial burden compared to Mtb-HT strains and also developed diffused inflammatory lung pathology. In stark contrast, a significant number of mice infected with Mtb-HT strains developed caseating granulomas, a lesion type with high potential to cavitate. None of the Mtb-HT infected animals developed diffused inflammatory lung pathology. A link was observed between increased in vitro replication of Mtb-LT strains and their ability to induce significantly high lipid droplet formation in macrophages. These results support that distinct early interactions of Mtb-HT and Mtb-LT strains with macrophages and subsequent differential trajectories in pathological disease may be the mechanism underlying their transmission potential.

Modeling the Airborne Infection Risk of Tuberculosis for a Research Facility in eMalahleni, South Africa

A detailed mathematical modeling framework for the risk of airborne infectious disease transmission in indoor spaces was developed to enable mathematical analysis of experiments conducted at the Airborne Infections Research (AIR) facility, eMalahleni, South Africa. A model was built using this framework to explore possible causes of why an experiment at the AIR facility did not produce expected results. The experiment was conducted at the AIR facility from August 31, 2015 to December 4, 2015, in which the efficacy of upper room germicidal ultraviolet (GUV) irradiation as an environmental control was tested. However, the experiment did not produce the expected outcome of having fewer infections in the test animal room than the control room. The simulation results indicate that dynamic effects, caused by switching the GUV lights, power outages, or introduction of new patients, did not result in the unexpected outcomes. However, a sensitivity analysis highlights that significant uncertainty exists with risk of transmission predictions based on current measurement practices, due to the reliance on large viable literature ranges for parameters.

Research and development of new tuberculosis vaccines: a review

Tuberculosis kills more people worldwide than any other single infectious disease agent, a threat made more dire by the spread of drug-resistant strains of Mycobacterium tuberculosis (Mtb). Development of new vaccines capable of preventing TB disease and new Mtb infection are an essential component of the strategy to combat the TB epidemic. Accordingly, the WHO considers the development of new TB vaccines a major public health priority. In October 2017, the WHO convened a consultation with global leaders in the TB vaccine development field to emphasize the WHO commitment to this effort and to facilitate creative approaches to the discovery and development of TB vaccine candidates. This review summarizes the presentations at this consultation, updated with scientific literature references, and includes discussions of the public health need for a TB vaccine; the status of efforts to develop vaccines to replace or potentiate BCG in infants and develop new TB vaccines for adolescents and adults; strategies being employed to diversify vaccine platforms; and new animal models being developed to facilitate TB vaccine development. A perspective on the status of these efforts from the major funders and organizational contributors also is included. This presentation highlights the extraordinary progress being made to develop new TB vaccines and provided a clear picture of the exciting development pathways that are being explored.

Research and development of new tuberculosis vaccines: a review

Tuberculosis kills more people worldwide than any other single infectious disease agent, a threat made more dire by the spread of drug-resistant strains of Mycobacterium tuberculosis (Mtb). Development of new vaccines capable of preventing TB disease and new Mtb infection are an essential component of the strategy to combat the TB epidemic. Accordingly, the WHO considers the development of new TB vaccines a major public health priority. In October 2017, the WHO convened a consultation with global leaders in the TB vaccine development field to emphasize the WHO commitment to this effort and to facilitate creative approaches to the discovery and development of TB vaccine candidates. This review summarizes the presentations at this consultation, updated with scientific literature references, and includes discussions of the public health need for a TB vaccine; the status of efforts to develop vaccines to replace or potentiate BCG in infants and develop new TB vaccines for adolescents and adults; strategies being employed to diversify vaccine platforms; and new animal models being developed to facilitate TB vaccine development. A perspective on the status of these efforts from the major funders and organizational contributors also is included. This presentation highlights the extraordinary progress being made to develop new TB vaccines and provided a clear picture of the exciting development pathways that are being explored.

Meeting report: 5th Global Forum on TB Vaccines, 20-23 February 2018, New Delhi India

The 5th Global Forum on TB Vaccines was held in New Delhi, India from 20 to 23 February 2018. This was the largest Global Forum on TB Vaccines to date with nearly 350 participants from more than 30 countries. The program included over 60 speakers in 12 special, plenary and breakout sessions and 72 posters. This Global Forum brought a great sense of momentum and excitement to the field. New vaccines are in clinical trials, new routes of delivery are being tested, novel assays and biomarker signatures are being developed, and the results from the first prevention of infection clinical trial with the H4:IC31 vaccine candidate and BCG revaccination were presented. Speakers and participants acknowledged the significant challenges that the TB vaccine R&D field continues to face - including limited funding, and the need for novel effective vaccine candidates and tools such as improved diagnostics and biomarkers to accurately predict protective efficacy. New solutions and approaches to address these challenges were discussed. The following report presents highlights from talks presented at this Global Forum. A full program, abstract book and presentations (where publicly available) from the Forum may be found at tbvaccinesforum.org.

Prevention of M. tuberculosis Infection with H4:IC31 Vaccine or BCG Revaccination

BACKGROUND

Recent Mycobacterium tuberculosis infection confers a predisposition to the development of tuberculosis disease, the leading killer among global infectious diseases. H4:IC31, a candidate subunit vaccine, has shown protection against tuberculosis disease in preclinical models, and observational studies have indicated that primary bacille Calmette-Guérin (BCG) vaccination may offer partial protection against infection.

METHODS

In this phase 2 trial, we randomly assigned 990 adolescents in a high-risk setting who had undergone neonatal BCG vaccination to receive the H4:IC31 vaccine, BCG revaccination, or placebo. All the participants had negative results on testing for M. tuberculosis infection on the QuantiFERON-TB Gold In-tube assay (QFT) and for the human immunodeficiency virus. The primary outcomes were safety and acquisition of M. tuberculosis infection, as defined by initial conversion on QFT that was performed every 6 months during a 2-year period. Secondary outcomes were immunogenicity and sustained QFT conversion to a positive test without reversion to negative status at 3 months and 6 months after conversion. Estimates of vaccine efficacy are based on hazard ratios from Cox regression models and compare each vaccine with placebo.

RESULTS

Both the BCG and H4:IC31 vaccines were immunogenic. QFT conversion occurred in 44 of 308 participants (14.3%) in the H4:IC31 group and in 41 of 312 participants (13.1%) in the BCG group, as compared with 49 of 310 participants (15.8%) in the placebo group; the rate of sustained conversion was 8.1% in the H4:IC31 group and 6.7% in the BCG group, as compared with 11.6% in the placebo group. Neither the H4:IC31 vaccine nor the BCG vaccine prevented initial QFT conversion, with efficacy point estimates of 9.4% (P=0.63) and 20.1% (P=0.29), respectively. However, the BCG vaccine reduced the rate of sustained QFT conversion, with an efficacy of 45.4% (P=0.03); the efficacy of the H4:IC31 vaccine was 30.5% (P=0.16). There were no clinically significant between-group differences in the rates of serious adverse events, although mild-to-moderate injection-site reactions were more common with BCG revaccination.

CONCLUSIONS

In this trial, the rate of sustained QFT conversion, which may reflect sustained M. tuberculosis infection, was reduced by vaccination in a high-transmission setting. This finding may inform clinical development of new vaccine candidates. (Funded by Aeras and others; C-040-404 ClinicalTrials.gov number, NCT02075203 .).

Detection of Mycobacterium tuberculosis bacilli in bio-aerosols from untreated TB patients

Background: Tuberculosis (TB) is predominantly an airborne disease. However, quantitative and qualitative analysis of bio-aerosols containing the aetiological agent, Mycobacterium tuberculosis (Mtb), has proven very challenging. Our objective is to sample bio-aerosols from newly diagnosed TB patients for detection and enumeration of Mtb bacilli. Methods: We monitored each of 35 newly diagnosed, GeneXpert sputum-positive, TB patients during 1 hour confinement in a custom-built Respiratory Aerosol Sampling Chamber (RASC). The RASC (a small clean-room of 1.4m ) incorporates aerodynamic particle size detection, viable and non-viable sampling devices, real-time CO 2 monitoring, and cough sound-recording. Microbiological culture and droplet digital polymerase chain reaction (ddPCR) were used to detect Mtb in each of the bio-aerosol collection devices. Results:  Mtb was detected in 27/35 (77.1%) of aerosol samples; 15/35 (42.8%) samples were positive by mycobacterial culture and 25/27 (92.96%) were positive by ddPCR. Culturability of collected bacilli was not predicted by radiographic evidence of pulmonary cavitation, sputum smear positivity. A correlation was found between cough rate and culturable bioaerosol.  Mtb was detected on all viable cascade impactor stages with a peak at aerosol sizes 2.0-3.5μm. This suggests a median of 0.09 CFU/litre of exhaled air (IQR: 0.07 to 0.3 CFU/l) for the aerosol culture positives and an estimated median concentration of 4.5x10 CFU/ml (IQR: 2.9x10 -5.6x10 ) of exhaled particulate bio-aerosol. Conclusions:  Mtb was identified in bio-aerosols exhaled by the majority of untreated TB patients using the RASC. Molecular detection was more sensitive than mycobacterial culture on solid media, suggesting that further studies are required to determine whether this reflects a significant proportion of differentially detectable bacilli in these samples.

Addressing diversity in tuberculosis using multidimensional approaches

Tuberculosis is a complex disease, which can affect many organs other than the lungs. Initial infection may be cleared without inducing immunological memory, or progress directly to primary disease. Alternatively, the infection may be controlled as latent TB infection, that may progress to active tuberculosis at a later stage. There is now a greater understanding that these infection states are part of a continuum, and studies using PET/CT imaging have shown that individual lung granulomas may respond to infection independently, in an un-synchronized manner. In addition, the Mycobacterium tuberculosis organisms themselves can exist in different states: as nonculturable forms, as 'persisters', as rapidly growing bacteria and a biofilm-forming cording phenotype. The 'omics' approaches of transcriptomics, metabolomics and proteomics can help reveal the mechanisms underlying these different infection states in the host, and identify biosignatures with diagnostic potential, that can predict the development of disease, in 'progressors' as early as 12-18 months before it can be detected clinically, or that can monitor the success of anti-TB therapy. Further insights can be obtained from studies of BCG vaccination and new TB vaccines. For example, epigenetic changes associated with trained immunity and a stronger immune responses following BCG vaccination can be identified. These omics approaches may be particularly valuable when linked to studies of mycobacterial growth inhibition, as a direct read-out of the ability to control mycobacterial growth. The second generation of omics studies is identifying much smaller signatures based on as few as 3 or 4 genes. Thus, narrowing down omics-derived biosignatures to a manageable set of markers now opens the way to field-friendly point of care assays.

Early clearance versus control: what is the meaning of a negative tuberculin skin test or interferon-gamma release assay following exposure to Mycobacterium tuberculosis?

The elimination of tuberculosis (TB) cannot reasonably be achieved by treatment of individual cases and will require an improved vaccine or immunotherapy. A challenge in developing an improved TB vaccine has been the lack of understanding what is needed to generate sterilizing immunity against Mycobacterium tuberculosis (Mtb) infection. Several epidemiological observations support the hypothesis that humans can eradicate Mtb following exposure. This has been termed early clearance and is defined as elimination of Mtb infection prior to the development of an adaptive immune response, as measured by a tuberculin skin test or interferon-gamma release assay. Here, we examine research into the likelihood of and possible mechanisms responsible for early clearance in household contacts of patients with active TB. We explore both innate and adaptive immune responses in the lung. Enhanced understanding of these mechanisms could be harnessed for the development of a preventative vaccine or immunotherapy.

Biology of clinical strains of Mycobacterium tuberculosis with varying levels of transmission

Transmission of Mycobacterium tuberculosis bacilli from one individual to another is the basis of the disease process. While considerable emphasis has been placed on the role of host mechanisms of resistance in establishing or preventing new infection, far less has been expended on understanding possible factors operative at the bacterial level. In this study we established a panel of clinical isolates of M. tuberculosis strains obtained from the Western Cape region of South Africa, each of which had been carefully tracked in terms of their degree of transmission in the community. Each of the panel were used to infect guinea pigs with 15-20 bacilli by aerosol exposure and the course of the infection then determined. Strains with different degrees of transmission could not be distinguished in terms of their capacity to grow in the main target organs of infected animals. However, rather surprisingly, while strains with no evidence of transmission [NOT] in general caused moderate to severe lung damage, this parameter in animals infected with highly transmitted [HT] strains was mostly mild. In terms of TH1 immunity these signals were strongest in these latter animals, as was IL-17 gene expression, whereas minimal signals for regulatory molecules including IL-10 and FoxP3 were seen across the entire panel. In terms of T cell numbers, responses of both CD4 and CD8 were both far faster and far higher in animals infected with the HT strains. At the gene expression level we observed a major three-fold difference [both up and down] between NOT and HT strains, but in terms of proteins of key interest only a few [including PD-L1 and HIF-3] showed major differences between the two groups. Overall, it was apparent that NOT strains were far more inflammatory that HT strains, and appeared to trigger a much larger number of genes, possibly explaining the observed damage to the lungs and progressive pathology. In contrast, the HT strains, while equally virulent, were more immunogenic and developed much stronger T cell responses, while keeping lung damage to a minimum. Hence, in terms of trying to explain the capacity of these strains to cause transmission, these results are clearly paradoxical.

Detection of Mycobacterium tuberculosis bacilli in bio-aerosols from untreated TB patients

Background: Tuberculosis (TB) is predominantly an airborne disease. However, quantitative and qualitative analysis of bio-aerosols containing the aetiological agent, Mycobacterium tuberculosis (Mtb), has proven very challenging. Our objective is to sample bio-aerosols from newly diagnosed TB patients for detection and enumeration of Mtb bacilli. Methods: We monitored each of 35 newly diagnosed, GeneXpert sputum-positive, TB patients during 1 hour confinement in a custom-built Respiratory Aerosol Sampling Chamber (RASC). The RASC (a small clean-room of 1.4m ) incorporates aerodynamic particle size detection, viable and non-viable sampling devices, real-time CO 2 monitoring, and cough sound-recording. Microbiological culture and droplet digital polymerase chain reaction (ddPCR) were used to detect Mtb in each of the bio-aerosol collection devices. Results:  Mtb was detected in 27/35 (77.1%) of aerosol samples; 15/35 (42.8%) samples were positive by mycobacterial culture and 25/27 (92.96%) were positive by ddPCR. Culturability of collected bacilli was not predicted by radiographic evidence of pulmonary cavitation, sputum smear positivity, or cough rate. Mtb was detected on all viable cascade impactor stages with a peak at aerosol sizes 2.0-3.5μm. This suggests a median of 0.09 CFU/litre of exhaled air (IQR: 0.07 to 0.3 CFU/l) for the aerosol culture positives and an estimated median concentration of 4.5x10 CFU/ml (IQR: 2.9x10 -5.6x10 ) of exhaled particulate bio-aerosol. Conclusions:  Mtb was identified in bio-aerosols exhaled by the majority of untreated TB patients using the RASC. Molecular detection was more sensitive than mycobacterial culture on solid media, suggesting that further studies are required to determine whether this reflects a significant proportion of differentially detectable  bacilli in these samples.

Tuberculosis Infectiousness and Host Susceptibility

The transmission of tuberculosis is complex. Necessary factors include a source case with respiratory disease that has developed sufficiently for Mycobacterium tuberculosis to be present in the airways. Viable bacilli must then be released as an aerosol via the respiratory tract of the source case. This is presumed to occur predominantly by coughing but may also happen by other means. Airborne bacilli must be capable of surviving in the external environment before inhalation into a new potential host-steps influenced by ambient conditions and crowding and by M. tuberculosis itself. Innate and adaptive host defenses will then influence whether new infection results; a process that is difficult to study owing to a paucity of animal models and an inability to measure infection directly. This review offers an overview of these steps and highlights the many gaps in knowledge that remain.

Clinical Testing of Tuberculosis Vaccine Candidates

It is almost 100 years since the development of bacille Calmette-Guérin (BCG), the only licensed vaccine against tuberculosis (TB). While BCG does confer consistent protection against disseminated disease, there is an urgent need for a more effective vaccine against pulmonary disease. There are several indications for such an improved vaccine, including prevention of infection, prevention of disease, and a therapeutic vaccine to prevent recurrent disease. The two main approaches to TB vaccine development are developing an improved whole mycobacterial priming agent to replace BCG and/or developing a subunit booster vaccine to be administered after a BCG or BCG replacement priming vaccination. In this article we review the status of the current candidate vaccines being evaluated in clinical trials. The critical challenges to successful TB vaccine development are the uncertain predictive value of the preclinical animal models and the lack of a validated immune correlate of protection. While it is relatively simple to evaluate safety and immunogenicity in phase 1/2 studies, the evaluation of efficacy requires complex studies with large numbers of subjects and long periods of follow-up. This article reviews the potential role for human Experimental Medicine studies, in parallel with product development, to help improve the predictive value of the early-stage trials.

Animal Models of Tuberculosis: An Overview

This article provides an overview of the animal models currently used in tuberculosis research, both for understanding the basic science of the disease process and also for practical issues such as testing new vaccine candidates and evaluating the activity of potential new drugs. Animals range in size, from zebrafish to cattle, and in degrees of similarity to the human disease from both an immunological and pathologic perspective. These models have provided a great wealth of information (impossible to obtain simply from observing infected humans), but we emphasize here that one must use care in interpreting or applying this information, and indeed the true art of animal modeling is in deciding what is pertinent information and what might not be. These ideas are discussed in the context of current approaches in vaccine and drug development, including a discussion of certain limitations the field is currently facing in such studies.

Mouse and Guinea Pig Models of Tuberculosis

This article describes the nature of the host response to Mycobacterium tuberculosis in the mouse and guinea pig models of infection. It describes the great wealth of information obtained from the mouse model, reflecting the general availability of immunological reagents, as well as genetic manipulations of the mouse strains themselves. This has led to a good understanding of the nature of the T-cell response to the infection, as well as an appreciation of the complexity of the response involving multiple cytokine- and chemokine-mediated systems. As described here and elsewhere, we have a growing understanding of how multiple CD4-positive T-cell subsets are involved, including regulatory T cells, TH17 cells, as well as the subsequent emergence of effector and central memory T-cell subsets. While, in contrast, our understanding of the host response in the guinea pig model is less advanced, considerable strides have been made in the past decade in terms of defining the basis of the immune response, as well as a better understanding of the immunopathologic process. This model has long been the gold standard for vaccine testing, and more recently is being revisited as a model for testing new drug regimens (bedaquiline being the latest example).

Latently and uninfected healthcare workers exposed to TB make protective antibodies against Mycobacterium tuberculosis

The role of Igs in natural protection against infection by Mycobacterium tuberculosis (Mtb), the causative agent of TB, is controversial. Although passive immunization with mAbs generated against mycobacterial antigens has shown protective efficacy in murine models of infection, studies in B cell-depleted animals only showed modest phenotypes. We do not know if humans make protective antibody responses. Here, we investigated whether healthcare workers in a Beijing TB hospital-who, although exposed to suprainfectious doses of pathogenic Mtb, remain healthy-make antibody responses that are effective in protecting against infection by Mtb. We tested antibodies isolated from 48 healthcare workers and compared these with 12 patients with active TB. We found that antibodies from 7 of 48 healthcare workers but none from active TB patients showed moderate protection against Mtb in an aerosol mouse challenge model. Intriguingly, three of seven healthcare workers who made protective antibody responses had no evidence of prior TB infection by IFN-γ release assay. There was also good correlation between protection observed in vivo and neutralization of Mtb in an in vitro human whole-blood assay. Antibodies mediating protection were directed against the surface of Mtb and depended on both immune complexes and CD4+ T cells for efficacy. Our results indicate that certain individuals make protective antibodies against Mtb and challenge paradigms about the nature of an effective immune response to TB.

Driving the Way to Tuberculosis Elimination: The Essential Role of Fundamental Research

Tuberculosis has impacted human health for millennia. The World Health Organization estimated that, in 2014, 9.6 million people developed tuberculosis and 1.5 million people died from the disease. In May 2014, the World Health Assembly endorsed the new "End TB Strategy" that presents a pathway to tuberculosis elimination. The strategy outlines 3 areas of emphasis, one of which is intensified research and innovation. In this article we highlight the essential role for fundamental tuberculosis research in the future of tuberculosis diagnostics, treatment, and prevention. To maximize the impact of fundamental research, we must foster collaboration among all stakeholders engaged in tuberculosis research and control to facilitate open dialogue to assure that critical gaps in outcome-oriented science are identified and addressed. We present here a framework for future discussions among scientists, physicians, research and development specialists, and public health managers for the reinforcement of national and international strategies toward tuberculosis elimination.

Rapid impact of effective treatment on transmission of multidrug-resistant tuberculosis

BACKGROUND

Effective treatment for drug-susceptible tuberculosis (TB) rapidly renders patients non-infectious, long before conversion of sputum acid-fast smear or culture to negative. Multidrug-resistant TB (MDR-TB) patients on treatment are currently assumed to remain infectious for months. While the resources required for prolonged hospitalization are a barrier to the scale-up of MDR-TB treatment, the safety of community treatment is clear.

OBJECTIVES

To estimate the impact of treatment on infectiousness among MDR-TB patients.

METHODS

A series of five human-to-guinea pig TB transmission studies was conducted to test various interventions for infection control. Guinea pigs in adjacent chambers were exposed to exhaust air from a hospital ward occupied by mostly sputum smear- and culture-positive MDR-TB patients. The guinea pigs then underwent tuberculin skin testing for infection. Only the control groups of guinea pigs from each study (no interventions used) provide the data for this analysis. The number of guinea pigs infected in each study is reported and correlated with Mycobacterium tuberculosis drug susceptibility relative to treatment.

RESULTS

Despite exposure to presumably infectious MDR-TB patients, infection percentages among guinea pigs ranged from 1% to 77% in the five experiments conducted. In one experiment in which guinea pigs were exposed to 27 MDR-TB patients newly started on effective treatment for 3 months, there was minimal transmission. In four other experiments with greater transmission, guinea pigs had been exposed to patients with unsuspected extensively drug-resistant tuberculosis who were not on effective treatment.

CONCLUSIONS

In this model, effective treatment appears to render MDR-TB patients rapidly non-infectious. Further prospective studies on this subject are needed.

Real-Time Investigation of Tuberculosis Transmission: Developing the Respiratory Aerosol Sampling Chamber (RASC)

Knowledge of the airborne nature of respiratory disease transmission owes much to the pioneering experiments of Wells and Riley over half a century ago. However, the mechanical, physiological, and immunopathological processes which drive the production of infectious aerosols by a diseased host remain poorly understood. Similarly, very little is known about the specific physiological, metabolic and morphological adaptations which enable pathogens such as Mycobacterium tuberculosis (Mtb) to exit the infected host, survive exposure to the external environment during airborne carriage, and adopt a form that is able to enter the respiratory tract of a new host, avoiding innate immune and physical defenses to establish a nascent infection. As a first step towards addressing these fundamental knowledge gaps which are central to any efforts to interrupt disease transmission, we developed and characterized a small personal clean room comprising an array of sampling devices which enable isolation and representative sampling of airborne particles and organic matter from tuberculosis (TB) patients. The complete unit, termed the Respiratory Aerosol Sampling Chamber (RASC), is instrumented to provide real-time information about the particulate output of a single patient, and to capture samples via a suite of particulate impingers, impactors and filters. Applying the RASC in a clinical setting, we demonstrate that a combination of molecular and microbiological assays, as well as imaging by fluorescence and scanning electron microscopy, can be applied to investigate the identity, viability, and morphology of isolated aerosolized particles. Importantly, from a preliminary panel of active TB patients, we observed the real-time production of large numbers of airborne particles including Mtb, as confirmed by microbiological culture and polymerase chain reaction (PCR) genotyping. Moreover, direct imaging of captured samples revealed the presence of multiple rod-like Mtb organisms whose physical dimensions suggested the capacity for travel deep into the alveolar spaces of the human lung.

Tuberculosis vaccines: barriers and prospects on the quest for a transformative tool

The road to a more efficacious vaccine that could be a truly transformative tool for decreasing tuberculosis morbidity and mortality, along with Mycobacterium tuberculosis transmission, is quite daunting. Despite this, there are reasons for optimism. Abetted by better conceptual clarity, clear acknowledgment of the degree of our current immunobiological ignorance, the availability of powerful new tools for dissecting the immunopathogenesis of human tuberculosis, the generation of more creative diversity in tuberculosis vaccine concepts, the development of better fit-for-purpose animal models, and the potential of more pragmatic approaches to the clinical testing of vaccine candidates, the field has promise for delivering novel tools for dealing with this worldwide scourge of poverty.

Transformative tools for tackling tuberculosis

The world is in need of more effective approaches to controlling tuberculosis. The development of improved control strategies has been hampered by deficiencies in the tools available for detecting Mycobacterium tuberculosis and defining the dynamic consequences of the interaction of M. tuberculosis with its human host. Key needs include a highly sensitive, specific nonsputum diagnostic; biomarkers predictive of responses to therapy; correlates of risk for disease development; and host response-independent markers of M. tuberculosis infection. Tools able to sensitively detect and quantify total body M. tuberculosis burden might well be transformative across many needed use cases. Here, we review the current state of the field, paying particular attention to needed changes in experimental paradigms that would facilitate the discovery, validation, and development of such tools.

The Impact of Ventilation and Early Diagnosis on Tuberculosis Transmission in Brazilian Prisons

Prisoners have among the highest incidence of tuberculosis (TB) globally. However, the contribution of the prison environment on transmission is not well understood and structural characteristics have received little attention as effective epidemiological interventions in TB control. We evaluated architectural characteristics and estimated ventilation rates in 141 cells in three prisons in central west Brazil using steady-state exhaled carbon dioxide (CO2) levels. We used a modified Wells-Riley equation to estimate the probability of infection for inmates sharing a cell with an infectious case and projected the impact of interventions, including early diagnosis and improved ventilation. Overall, prison cells were densely populated (mean 2.1 m(2) per occupant) and poorly ventilated, with only three cells meeting World Health Organization (WHO) standards for per-person ventilation (60 L/s) applied in infection control settings. In the absence of interventions, projected mean risk of infection was 78.0% during a 6-month period. Decreasing time-to-diagnosis by 25% reduced transmission risk by 8.3%. Improving ventilation to WHO standards decreased transmission by 38.2%, whereas optimizing cross-ventilation reduced transmission by 64.4%. Prison environments promote high infection risk over short-time intervals. In this context, enhanced diagnostics have a limited impact on reducing transmission. Improving natural ventilation may be required to effectively control TB in prisons.

Transmission and Institutional Infection Control of Tuberculosis

Tuberculosis (TB) transmission control in institutions is evolving with increased awareness of the rapid impact of treatment on transmission, the importance of the unsuspected, untreated case of transmission, and the advent of rapid molecular diagnostics. With active case finding based on cough surveillance and rapid drug susceptibility testing, in theory, it is possible to be reasonably sure that no patient enters a facility with undiagnosed TB or drug resistance. Droplet nuclei transmission of TB is reviewed with an emphasis on risk factors relevant to control. Among environmental controls, natural ventilation and upper-room ultraviolet germicidal ultraviolet air disinfection are the most cost-effective choices, although high-volume mechanical ventilation can also be used. Room air cleaners are generally not recommended. Maintenance is required for all engineering solutions. Finally, personal protection with fit-tested respirators is used in many situations where administrative and engineering methods cannot assure protection.

Institutional Tuberculosis Transmission. Controlled Trial of Upper Room Ultraviolet Air Disinfection: A Basis for New Dosing Guidelines

RATIONALE

Transmission is driving the global tuberculosis epidemic, especially in congregate settings. Worldwide, natural ventilation is the most common means of air disinfection, but it is inherently unreliable and of limited use in cold climates. Upper room germicidal ultraviolet (UV) air disinfection with air mixing has been shown to be highly effective, but improved evidence-based dosing guidelines are needed.

OBJECTIVES

To test the efficacy of upper room germicidal air disinfection with air mixing to reduce tuberculosis transmission under real hospital conditions, and to define the application parameters responsible as a basis for proposed new dosing guidelines.

METHODS

Over an exposure period of 7 months, 90 guinea pigs breathed only untreated exhaust ward air, and another 90 guinea pigs breathed only air from the same six-bed tuberculosis ward on alternate days when upper room germicidal air disinfection was turned on throughout the ward.

MEASUREMENTS AND MAIN RESULTS

The tuberculin skin test conversion rates (>6 mm) of the two chambers were compared. The hazard ratio for guinea pigs in the control chamber converting their skin test to positive was 4.9 (95% confidence interval, 2.8-8.6), with an efficacy of approximately 80%.

CONCLUSIONS

Upper room germicidal UV air disinfection with air mixing was highly effective in reducing tuberculosis transmission under hospital conditions. These data support using either a total fixture output (rather than electrical or UV lamp wattage) of 15-20 mW/m(3) total room volume, or an average whole-room UV irradiance (fluence rate) of 5-7 μW/cm(2), calculated by a lighting computer-assisted design program modified for UV use.

A nonhuman primate toxicology and immunogenicity study evaluating aerosol delivery of AERAS-402/Ad35 vaccine: Evidence for transient t cell responses in peripheral blood and robust sustained responses in the lungs

Bacille Calmette-Guérin (BCG), the only licensed vaccine for the prevention of tuberculosis (TB), provides only limited protection against certain forms of Mycobacterium tuberculosis (Mtb) infection. While infection with Mtb can be treated with antibiotics, the therapy is expensive, toxic, and requires several months for treatment. In addition, the emergence of drug resistant strains limits the impact of antibiotics and underlines the importance of developing a more effective vaccine to control this disease. Given that pulmonary TB is the most common form of the disease, a vaccine capable of inducing lung-resident immunity may be advantageous for combating this infection. New advances in pulmonary delivery make this route of vaccination feasible and affordable. Here, we evaluate the safety and immunogenicity of an aerosolized Ad35-based vaccine, AERAS-402, delivered to the lungs in nonhuman primates as part of a GLP acute and chronic toxicology and safety study. In this study, animals received three high doses (1 x 10(11) vp) of AERAS-402 by inhalation via a nebulizer at 1-week intervals. Aerosol delivery of AERAS-402 resulted in an increase in relative lung weights as well as microscopic findings in the lungs, mediastinal lymph nodes, bronchus-associated lymphatic tissue, and the naso-oropharynx that were consistent with the induction of an immune response during the acute phase. These findings resolved by the chronic phase and were considered to be non-adverse. Furthermore, we observed transient vaccine-specific immune responses in the peripheral blood as well as sustained high-level polyfunctional CD4(+) and CD8(+) T cell responses in the bronchoalveolar lavage fluid of vaccinated nonhuman primates. The data suggest that pulmonary delivery of Ad35-based vaccines can be safe and can induce potent lung-resident immunity.

Tuberculosis vaccines and prevention of infection

SUMMARY

Tuberculosis (TB) is a leading cause of death worldwide despite the availability of effective chemotherapy for over 60 years. Although Mycobacterium bovis bacillus Calmette-Guérin (BCG) vaccination protects against active TB disease in some populations, its efficacy is suboptimal. Development of an effective TB vaccine is a top global priority that has been hampered by an incomplete understanding of protective immunity to TB. Thus far, preventing TB disease, rather than infection, has been the primary target for vaccine development. Several areas of research highlight the importance of including preinfection vaccines in the development pipeline. First, epidemiology and mathematical modeling studies indicate that a preinfection vaccine would have a high population-level impact for control of TB disease. Second, immunology studies support the rationale for targeting prevention of infection, with evidence that host responses may be more effective during acute infection than during chronic infection. Third, natural history studies indicate that resistance to TB infection occurs in a small percentage of the population. Fourth, case-control studies of BCG indicate that it may provide protection from infection. Fifth, prevention-of-infection trials would have smaller sample sizes and a shorter duration than disease prevention trials and would enable opportunities to search for correlates of immunity as well as serve as a criterion for selecting a vaccine product for testing in a larger TB disease prevention trial. Together, these points support expanding the focus of TB vaccine development efforts to include prevention of infection as a primary goal along with vaccines or other interventions that reduce the rate of transmission and reactivation.

Animal models in tuberculosis research - where is the beef?

INTRODUCTION

Tuberculosis (TB) is a major global health problem, and new drugs and vaccines are urgently needed. As clinical trials in humans require tremendous resources, preclinical drug and vaccine development largely relies on valid animal models that recapitulate the pathology of human disease and the immune responses of the host as closely as possible.

AREAS COVERED

This review describes the animal models used in TB research, the most widely used being mice, guinea pigs and nonhuman primates. In addition, rabbits and cattle provide models with a disease pathology resembling that of humans. Invertebrate models, including the fruit fly and the Dictyostelium amoeba, have also been used to study mycobacterial infections. Recently, the zebrafish has emerged as a promising model for studying mycobacterial infections. The zebrafish model also facilitates the large-scale screening of drug and vaccine candidates.

EXPERT OPINION

Animal models are needed for TB research and provide valuable information on the mechanisms of the disease and on ways of preventing it. However, the data obtained in animal studies need to be carefully interpreted and evaluated before making assumptions concerning humans. With an increasing understanding of disease mechanisms, animal models can be further improved to best serve research goals.