Chapter 3 - Experimental Models Used to Study Human Tuberculosis

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Abstract

Mycobacterium tuberculosis causes more deaths in humans than any other bacterial pathogen. The most recent data from the World Health Organization reveal that over 9 million new cases of tuberculosis occur each year and that the incidence appears to be increasing with population growth. Despite the global burden of tuberculosis, we are still reliant on relatively dated measures to prevent, diagnose, and treat the disease. New, more effective tools are needed to diminish the incidence of tuberculosis. M. tuberculosis lacks a natural host beyond humans and, hence, surrogate models have been employed in the study of the pathogen. The discovery and development of new vaccines, diagnostics, or antitubercular drugs are dependent upon the validity of any experimental model used and its relevance to tuberculosis in humans. In this review, a range of experimental models, from in vitro studies with fast-growing low-pathogenic species of mycobacteria to the infection of nonhuman primates with virulent M. tuberculosis, will be discussed.

Introduction

Tuberculosis (Tb) is the leading cause of death from a single infectious organism (National Institute of Allergy and Infectious Diseases, 2006). In 2007 alone, 9.27 million people developed tuberculosis and 1.78 million died of the disease (WHO, 2009). The causative agent, Mycobacterium tuberculosis, is transmitted via aerosols and enters the lung from where it can cause active clinical Tb or persist in latent form over the lifetime of the host (Parrish et al., 1998). Predisposing factors that influence the onset of clinical disease include HIV infection, diabetes, smoking, alcoholism, malnutrition, and overcrowded living conditions (Lonnroth et al., 2009). Clinical tuberculosis invariably commences with the pulmonary form of the disease; however, the pathogen can subsequently disseminate via the circulatory or lymphatic systems and multiply in extrapulmonary host sites such as the skin, lymph nodes, central nervous system, genitourinary tract, and skeleton (Kritski and de Melo, 2007).

The earliest direct evidence of tuberculosis in humans originates from several thousand years ago. Traces of M. tuberculosis DNA, mycolic acid lipids, and paleopathological tubercular lesions have been identified in skeletal remains excavated from the submerged site of Atlit-Yam in the Eastern Mediterranean which dates from 9250 to 8160 BP (Hershkovitz et al., 2008). Indirect evidence is derived from the analysis of phylogenetic markers present in animal and human mycobacterial isolates. This has resulted in an estimation that the most common ancestor of the M. tuberculosis complex emerged approximately 40,000 years ago from its progenitor in East Africa, a time which is believed to coincide with the expansion of “modern” human populations from this area (Wirth et al., 2008).

Despite our long association with M. tuberculosis, we have not yet been able to effectively control or eradicate this pathogen. Previous targets of halving the incidence of tuberculosis between 2006 and 2015 or of eliminating the disease by 2050 will need to be reexamined (Lonnroth and Raviglione, 2008). The World Health Organization has concluded that new preventative, diagnostic, and treatment measures are needed to bring tuberculosis under control (WHO, 2006). The capacity of researchers to deliver new tools to control tuberculosis is acutely dependent on the relevance of the experimental models they use to study the disease and its etiological agent.

Section snippets

Use of Surrogate Models to Study Tuberculosis

M. tuberculosis has a very limited host range with no known natural hosts beyond humans (Brosch et al., 2002). Despite this, the pathogen does not require a zoonotic or environmental reservoir for persistence between episodes of clinical disease. The World Health Organization has estimated that one-third of the world's population is latently infected with the pathogen (Dye et al., 1999). This high-carriage rate provides a reservoir for subsequent disease in susceptible hosts whereby the

Fast-growing mycobacterial species

The value of using fast-growing species of mycobacteria in tuberculosis research was recognized by Selman Waksman in the 1940s. Waksman found that screening for antimycobacterial compounds using the pathogenic M. tuberculosis was a slow process (Sneader, 2005). He decided to screen against the fast-growing nonpathogenic species, M. phlei, and this ultimately led to the discovery of streptomycin, the first antibiotic effective in the treatment of tuberculosis. Since then, other fast-growing

Macrophages and cell cultures

As with in vitro models of tuberculosis, a variety of cell culture models have been used to study the disease. Based on the phagocytosis of M. tuberculosis at an early stage of infection of the host lung, many of the cell culture models have focused on the use of murine or human monocyte-derived macrophages or dendritic cells (de Chastellier, 2009). The use of cell cultures enables researchers to isolate the bacterial processes, which are central to uptake and survival and, in addition, the

The Study of Tuberculosis Pathogenesis in Human Patients

There is little doubt that the more an experimental model recreates the events that occur during the natural infection of humans by M. tuberculosis, the more relevant the data generated will be to the human disease. Improvements can be made to animal models of tuberculosis is to based on our knowledge of the natural history of tuberculosis in humans (Smith et al., 2000). For example, humans acquire tuberculosis through aerosol infection and only a small number of viable bacilli are required to

Conclusions and Future Prospects

It is clear that a large number of in vitro and in vivo models have been developed for the study of human tuberculosis. As well as the biological questions to be addressed, the choice of model will often depend on the financial resources and facilities available to a researcher. It is important that new tuberculosis research is not impeded by local limitations in technology or expertise. The sharing of experimental models to study human tuberculosis will likely hasten progress towards more

Acknowledgments

The support of the Health Research Council of New Zealand (grant number 07/379), the Wellington Medical Research Foundation (grant no. 2006/121), and the University Research Fund, Victoria University of Wellington (grant no. 26211/1496), is gratefully acknowledged.

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