Translating basic science insight into public health action for multidrug- and extensively drug-resistant tuberculosis

Abstract

Multidrug (MDR)- and extensively drug-resistant (XDR) tuberculosis (TB) impose a heavy toll of human suffering and social costs. Controlling drug-resistant TB is a complex global public health challenge. Basic science advances including elucidation of the genetic basis of resistance have enabled development of new assays that are transforming the diagnosis of MDR-TB. Molecular epidemiological approaches have provided new insights into the natural history of TB with important implications for drug resistance. In the future, progress in understanding Mycobacterium tuberculosis strain-specific human immune responses, integration of systems biology approaches with traditional epidemiology and insight into the biology of mycobacterial persistence have potential to be translated into new tools for diagnosis and treatment of MDR- and XDR-TB. We review recent basic sciences developments that have contributed or may contribute to improved public health response.

Figure 1

Technology uderlying line probe assays (LPA) and Xpert MTB/RIF. With LPAs (A), DNA is first extracted from M. tuberculosis culture or sputum and the resistance-determining regions of rifampicin, isoniazid, fluoroquinolones and aminoglycosides are amplified via PCR. The labeled PCR products are then exposed to a strip impregnated with oligonucleotide primers specific to both resistance-confering mutations and wild-type sequence. Color change on the strip indicates the presence of either resistance-confering mutations or wild-type sequence for each of the target drugs. Xpert MTB/RIF (B) is an automated nucleic acid amplification test in which the only manual processing required is sputum liquifaction and inactivation. The automated platform purifies, concentrates, and amplifies DNA to identify target sequence indicating the presence of M. tuberculosis and rpoB mutations.

Figure 2

Mechanisms of action and common resistance-confering mutations for key anti-TB drug classes.

Figure 3

The classical paradigm of TB natural history (A) suggested that primary infection occurred due to a single strain (Strain A) and that LTBI due to Strain A was protective against infection with additional strains. If the individual developed active disease, was treated but developed recurrent TB, recurrence would be the result of relapse with the original Strain A. Molecular epidemiology studies indicate a more dynamic paradigm (B) in which reinfection and mixed infections occur. Individuals may be simultaneously or sequentially infected with multiple strains of M. tuberculosis (Strains A, B, C). LTBI may be incompletely or non-protective. Reactivation TB may occur due to a single strain or multiple strains. Individuals who develop recurrent TB following treatment may have relapse due to their original strains (A, B or C) or be reinfected due to exposure to a new strain (Strain D – in this case MDR).

Figure 4

MDR-TB can develop during the course of anti-TB drug therapy via several mechanisms. Resistance may be amplified (A) as a result of sequential drug exposure. A wild-type population of susceptible M. tuberculosis includes a low frequency subpopulation of bacilli with spontaneously occurring INH-resistance (estimated 2.5 mutants per 108 bacilli). Exposure to INH alone selects for survival and replication of the INH-resistant mutants. The resulting INH-resistant population includes a low frequency subpopulation of RIF-resistant bacilli. Now addition of RIF selects for survival of the INH- and RIF-resistant (MDR) bacilli. MDR-TB may also occur due to reinfection. (B) Drug-susceptible is appropriately treated but the patient may have exogenous reinfection with a new MDR-strain either during or following drug treatment. MDR-TB may occur as a result of mixed infection with drug susceptible and MDR-TB strains. (C) MDR-M.tuberculosis may be present as a minority population that is not detected on initial DST. Anti-TB drug therapy will select for survival of resistant subpopulations.

Figure 5

Infection with virulent Beijing strains of M. tuberculosis can result in a distinctive host immunophenotype over time as compared to infection with laboratory strains of M. tuberculosis. During the early stages of an infection, virulent strains may induce a greater T_H1 immune response, resulting in tissue damage and leading to excessive counter-regulatory regulatory T cell (Treg) responses that ultimately impairs host immunity against the tubercle bacillus.