INVITED REVIEW SERIES: TUBERCULOSIS
SERIES EDITORS: WING WAI YEW, GIOVANNI B. MIGLIORI AND CHRISTOPH LANGE
The immunology of tuberculosis: From bench to bedside
KEERTAN DHEDA,1,2,3 STEPHAN K. SCHWANDER,4 BINGDONG ZHU,5 RICHARD N. VAN ZYL-SMIT1 AND
YING ZHANG6
1
Lung Infection and Immunity Unit, Division of Pulmonology and Clinical Immunology & UCT Lung Institute,
Department of Medicine, 2Institute of Infectious Diseases and Molecular Medicine, University of Cape Town,
Cape Town, South Africa, 3Centre for Infectious Diseases and International Health, Department of Infection,
University College London, London, UK, 4UMDNJ-School of Public Health, Centre for Global Public Health,
Department of Environmental and Occupational Health, Piscataway, New Jersey, 6Department of Molecular
Microbiology and Immunology, Bloomberg School of Public Health, Johns Hopkins University, Baltimore,
Maryland, USA, and 5Lanzhou Centre for Tuberculosis Research, Institute of Pathogenic Biology,
School of Basic Medical Sciences, Lanzhou University, Lanzhou, China
ABSTRACT
resp_1739 433..450
Tuberculosis (TB) is an international public health priority and kills almost two million people annually.TB is
out of control in Africa due to increasing poverty and
HIV coinfection, and drug-resistant TB threatens to
destabilize TB control efforts in several regions of the
world. Existing diagnostic tools and therapeutic interventions for TB are suboptimal. Thus, new vaccines,
immunotherapeutic interventions and diagnostic
tools are urgently required to facilitate TB control
efforts. An improved understanding of the immunopathogenesis of TB can facilitate the identification of
correlates of immune protection, the design of effective
vaccines, the rational selection of immunotherapeutic
agents, the evaluation of new drug candidates, and
drive the development of new immunodiagnostic
tools. Here we review the immunology of TB with a
focus on aspects that are clinically and therapeutically
relevant. An immunologically orientated approach to
tackling TB can only succeed with concurrent efforts to
alleviate poverty and reduce the global burden of HIV.
Dr K.D. is an Associate Professor of Respiratory Medicine and
his research interests include mycobacterial host immunity,
development of field-friendly diagnostics and mycobacterial
drug resistance. Dr S.K.S. is an Associate Professor of Public and
Global Public Health. His laboratory-based research focuses on
human antimycobacterial immunity and its environmental confounders. Dr B.Z. is an Associate Professor of Pathogenic Biology
with primary research interests in tuberculosis subunit vaccine
and pathogenesis. Dr R.V.Z.S. is a pulmonologist and research
fellow with research interests in tobacco smoking and tuberculosis immunity. Dr Y.Z. is a Professor of Microbiology
and Immunology and his research interests include tuberculosis
pathogenesis, drug resistance, persistence and host responses to
mycobacterial infections.
Correspondence: Keertan Dheda, Department of Medicine, H
floor (Old Main Building), Groote Schuur Hospital, Observatory,
Cape Town 7925, South Africa. Email: keertan.dheda@uct.ac.za
or keertandheda@yahoo.co.uk
Received 18 November 2009; invited to revise 24 November
2009, revised 1 December 2009; accepted 8 December 2009.
© 2010 The Authors
Journal compilation © 2010 Asian Pacific Society of Respirology
Key words: cytokine, diagnostics, immunology, tuberculosis, vaccine.
INTRODUCTION
Tuberculosis (TB) is one of the commonest serious
infectious disease worldwide, is one of the commonest causes of serious respiratory disability and afflicts
almost 10 million people annually.1 The disease,
fuelled by HIV infection and poverty, is out of control
in Africa. Drug-resistant TB threatens to destabilize
TB control in several other regions of the world.2
Although several factors including HIV infection and
socioeconomic deprivation facilitate the development of TB, contributing factors include suboptimal
diagnostic tools, lack of cheap and effective new
therapeutic interventions and the lack of a widely
available effective vaccine. However, a better understanding of the immunopathogenesis of TB can drive
the development of new immunodiagnostic tools,
accelerate and facilitate the evaluation of new therapeutic interventions and is indispensible to the development of an effective vaccine. There have been
several excellent published reviews on the immunology of TB.3–7 Here, however, we will update the reader
on the latest new developments in the field and, in
particular, focus on clinically relevant and translational aspects of TB immunology.
THE LIFE CYCLE OF MYCOBACTERIUM
TUBERCULOSIS AND CLINICAL
SPECTRUM OF TUBERCULOSIS
INFECTION
There is no gold standard for the diagnosis of latent
TB infection (LTBI; asymptomatic state with no clinical or radiological evidence of active disease but with
Respirology (2010) 15, 433–450
doi: 10.1111/j.1440-1843.2010.01739.x
434
viable M. tuberculosis organisms within tissues).
However, the discovery of new immunodiagnostic
tools such as the interferon-g (IFN-g) release assays
(IGRA) has improved our understanding about
human TB.8–11 Although well documented, it is not
widely appreciated that a substantial proportion
(perhaps up to 50%) of close contacts of microbiologically confirmed index cases, even in many (although
not all) high burden settings, have no immunodiagnostic (positive tuberculin skin test (TST)) evidence of
LTBI.12 The same phenomenon is documented in
repeatedly exposed health-care workers working in
hospital TB wards. Thus, in many exposed individuals
sterilizing innate immunity may presumably prevent
the induction of adaptive immune responses and
explain the persistently negative post-exposure TST
and IGRA test results (Fig. 1). Specific genetic loci may
be associated with the lack of TST reactivity.13 That the
available tools (IGRA or TST) lack adequate sensitivity
to detect LTBI remains a possibility.
Of the remaining exposed individuals who have a
detectable adaptive immune response and presumably develop LTBI, only a small number (~5%) go on
to develop active TB within 2–5 years of becoming
K Dheda et al.
infected (Fig. 1). The remainder (~95%) of those who
become infected remain disease-free for the duration
of their lifetime unless they become immunocompromised by intervening illness, HIV infection or
immunosuppressive drugs, etc. M. tuberculosis
antigen-specific Th1 responses in the lungs,14 and
expanded CD8+ T cells with capacity to control
growth of M. tuberculosis15 in vitro can be found in
household contacts. It is also recognized that some
individuals with presumed LTBI may undergo reversion of the TST or IGRA,16,17 and may convert their
reactions if they become reinfected.16,18 It is reasonable to speculate therefore that some individuals may
clear their infection several weeks or months after
becoming infected; some investigators have termed
this an ‘acute resolving infection’.17,18 Alternatively,
whether this, in a proportion of individuals, merely
represents transition into dormancy or suboptimal
detection (poor test sensitivity), rather than sterilization, is unclear. The various disease and immunodiagnostic phenotypes and their clinical, radiological
and microbiological correlates are outlined in Table 1.
Collectively, these data support a dynamic model of
TB infection10,11,19 incorporating innate immunity
Figure 1 The spectrum of Mycobacterium tuberculosis infection and the life cycle of M. tuberculosis. Many exposed
individuals (~30–50%) have no immunodiagnostic evidence of M. tuberculosis infection or T-cell priming despite heavy
exposure to M. tuberculosis. These individuals, although unproven, may be innately resistant to M. tuberculosis
infection. Of those who have immunodiagnostic evidence of T-cell priming (+ve TST or IGRA) it is likely that a
substantial proportion have LTBI (clinically asymptomatic M. tuberculosis infection during which organisms are in a
state of non-replicating persistence). Some individuals have transiently positive responses, and may subsequently
‘revert’ their reactions—they may have ‘acute resolving infection’ or clear their infection. These individuals may become
reinfected and progress to active tuberculosis. A proportion of patients with LTBI may also progress to active disease.
Given the lack of a gold diagnostic standard for LTBI some of these postulates are based on circumstantial evidence and
remain unproven. A multitude of clinical conditions (HIV-1 infection, diabetes mellitus, malnutrition, tobacco smoking,
TNF-a inhibitor therapy, helminth coinfection) may permit tipping of the immunological balance and promote transition
from latent infection to active disease. IGRA, interferon-g release assay; LTBI, latent tuberculosis infection; TNF-a,
tumour necrosis factor-a; TST, tuberculin skin test.
Respirology (2010) 15, 433–450
© 2010 The Authors
Journal compilation © 2010 Asian Pacific Society of Respirology
TB immunology
© 2010 The Authors
Journal compilation © 2010 Asian Pacific Society of Respirology
Table 1 Spectrum of TB infection and disease in exposed individuals stratified by symptoms, test results, CXR changes and bacterial burden
Immunological/disease phenotype
1.
2.
3.
4.
5.
6.
7.
Innately immune
T-cell priming but clearance of infection‡
T-cell priming but LTBI
Subclinical TB‡‡ (quiescent disease)
Active TB disease
Treated TB with disease resolution
Old X-ray changes—no history of active disease
Presence of
symptoms
TST status
IGRA status
Presence of
CXR changes
Bacterial
burden†
Persistence of
non-replicating
Mycobacterium
tuberculosis
No
No
No
No
Yes
No
No
-ve
Transiently +ve/then -ve
+ve¶ (may be -ve)
+ve¶ (may be -ve)
+ve¶ (may be -ve)
+ve¶ (may be -ve)
+ve¶ (may be -ve)
-ve
Transiently +ve/then -ve
+ve†† (may be -ve)
+ve†† (may be -ve)
+ve†† (may be -ve)
+ve†† (may be -ve)
+ve†† (may be -ve)
No
No§
No§
Yes
Yes
Yes
Yes
None
None
+
++
++++
+/+/-
No
No
Yes
N/A
N/A
Possible
Possible
435
Respirology (2010) 15, 433–450
†
Bacterial burden refers to the quantitative load of TB bacilli (lowest in LTBI and highest in active TB). In treated TB patients there may be residual foci of
non-replicating but persisting organisms. 3 may progress to 4, and then to 5 and 6 after treatment. In 1 and 2 there may be clearance of infection but these individuals
may in future become reinfected and then progress to 3, and/or 4 and 5.
‡
Clearance of the infection without T-cell priming assumes bacterial sterilisation by innate immune mechanisms, and with no immunodiagnostic evidence of T-cell
priming.
§
No features of active TB but calcified granulomas may be present.
¶††
In these cases the TST or IGRA is often positive but may be negative, for example, even in active TB ~30% of patients have a negative TST/IGRA, TST and IGRA
-ve exposed individuals may progress to develop active TB, and T-cell priming may occur but the infection is cleared and thus there is a poor correlation between
disease phenotype and immunodiagnostic test results.
‡‡
Subclinical TB refers to active disease where M. tuberculosis can often be microbiologically cultured from biological samples, and radiological changes may be
present, but the patient is asymptomatic.
IGRA, interferon-g release assay; LTBI, latent TB infection; N/A, not applicable; TB, tuberculosis; TST, tuberculin skin test.
436
K Dheda et al.
against M. tuberculosis infection, acquisition of LTBI,
conversions (TST or IGRA positivity after exposure
in previously negative individuals), reversions (TST or
IGRA negativity in previously positive and exposed
individuals), immune escape (LTBI undetected by
immune surveillance tools such as IGRA and TST),
acute resolving infection and reinfection. Thus, there
is a poor correlation between immunodiagnostic test
results and disease phenotype, and this may explain
the generally poor positive predictive value of the TST
and IGRA (less than ~5%) for active TB.8 It also suggests that serial testing may be required to delineate
disease phenotypes.20 Consensus is required about
the standardization of terminology (the spectrum of
tuberculosis infection and disease), and further
studies are needed to clarify several aspects of M.
tuberculosis infection. Therefore, the development of
reliable tools, including new imaging modalities (e.g.
antibody-bound isotope markers), to detect LTBI is a
research priority.
by suppressive immune mechanisms, clear correlates
of protection are still lacking for the assessment of
new tuberculous vaccines and adjuvant immune
therapies.
A role for protective immune response also underlies the persistence of LTBI (a state assumed to exist in
a third of humans worldwide) during which M. tuberculosis growth is suppressed. These non-replicating
bacilli are characterized by intracellular lipid bodies55
and foamy lipid-laden macrophages may be associated with TB-related tissue pathology.56
In the parenchymal tissue, M. tuberculosis may
induce the formation of pathognomonic granulomas
(inflammatory reaction characterized by a ball-like
collection of immune cells). Necrotizing granuloma
centres57 provide ideal culture conditions for M. tuberculosis that multiply extracellularly to large numbers,
gain access to the airways and thus eventually disseminate the disease within the lung and to close
contacts.
OVERVIEW OF THE
IMMUNOPATHOGENESIS
OF TUBERCULOSIS
INNATE IMMUNITY TO
MYCOBACTERIUM TUBERCULOSIS
The likelihood and intensity of aerogenic transmission of M. tuberculosis from a TB index case to a
contact person in a shared air space depends on (i)
exposure duration; (ii) intensity of exposure; (iii)
cough and sputum-related host factors; and (iv) M.
tuberculosis strain-related virulence characteristics.
Infectious droplet nuclei are deposited in the alveolar
spaces where M. tuberculosis can be phagocytosed by
alveolar macrophages,21–24 epithelial cells, dendritic
cells (DC)25,26 and neutrophils. Alveolar macrophages
and DC are then believed to transport M. tuberculosis
to local lymph nodes where T cells are primed and
clonally expanded.
During active TB disease there is an exuberant local
pulmonary immune response characterized by an
alveolitis of activated a/b T-cell receptor-positive
lymphocytes, recently recruited immature macrophages,27,28 and strongly enhanced M. tuberculosis
antigen-specific Th1 responses,29–31 with large
amounts of locally secreted IFN-g. Increased numbers
of alveolar neutrophils can also be found in subgroups of patients.14 M. tuberculosis-induced immune
evasion mechanisms32–34 with production of suppressive cytokines and effector molecules35–38 may counteract protective immune responses and abrogate
bactericidal immune mechanisms.39 The role of regulatory T (Treg) cells in suppressing local immune
responses40,41 and probably permitting mycobacterial
growth is not yet well understood.
Susceptibility to TB is associated with HIV-1 infection (decreased numbers and function of CD4+ and
CD8+ T cells42–46), therapeutic tumour necrosis
factor-a (TNF-a) blockage,47–49 and hereditary IFN-g
and IL-12 receptor abnormalities.50–54 These conditions have provided insights into immune requirements for protection against M. tuberculosis.
However, because the potentially protective Th1
responses (IFN-g and TNF-a) are undermined in vivo
Respirology (2010) 15, 433–450
Upon entry into the host lungs by aerosol inhalation,
M. tuberculosis interacts with various receptors such
as pattern recognition receptors such as toll-like
receptors (TLR),58 complement receptor 3,21 mannose
scavenger
receptor,59
DC-specific
receptor,59
intercellular-adhesion-molecule-3-grabbing
nonintegrin, on the surface of macrophages and DC
(Fig. 2). These receptors recognize components of M.
tuberculosis such as lipoprotein, CpG-containing
DNA, mannose-capped lipoarabinomannan and
phosphatidylinositol mannoside, respectively. Lung
surfactant protein D binds M. tuberculosis surface
lipoarabinomannan and limits the intracellular
growth of M. tuberculosis by increasing phagosome
lysosome fusion.60 In addition, cytosolic nucleotidebinding and oligomerization domain-like receptors
such as NOD2 that recognizes muramyl dipeptide61
and also C-type lectin dectin-1 that interacts with M.
tuberculosis cooperate with TLR-2 to activate NF-kB
and mediate pro-inflammatory cytokine and antimicrobial responses. Toll NF-kB pathway activation
promotes nucleus translocation of NF-kB and activates vitamin D pathway: (i) the activation of NF-kB
results in production and secretion of many proinflammatory mediators including cytokines TNF-a,
IL-1, IL-12, IL-18 and chemokines, which attract neutrophils, natural killer (NK) cells, T cells, and more DC
and macrophages to the infection site,62,63 and nitric
oxide. It is of interest to note that M. tuberculosissecreted protein ESAT-6 could inhibit activation
of NF-kB through preventing interaction between
MyD88 and downstream kinase IRAK4;64 (ii) TLR activation also upregulates expression of the vitamin D
receptor (VDR) and the vitamin D-1-hydroxylase
genes, which converts pro-vitamin D into the active
form 1,25(OH)2D3 and leads to induction of the antimicrobial peptides cathelicidin and b-defensin to
kill intracellular mycobacteria.65–68 NADPH oxidase
2, which mediates phagocytic killing of ingested
© 2010 The Authors
Journal compilation © 2010 Asian Pacific Society of Respirology
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TB immunology
pathogens like M. tuberculosis via reactive oxygen
species, interacts with TLR-2 and affects VDRmediated antimicrobial peptide production.69
Knockdown of NADPH oxidase 2 inhibited 1,25D(3)mediated antimicrobial activity against M. tuberculosis through the modulation of cathelicidin expression
in human macrophages.69 The importance of the
host VDR and TLR in controlling TB is shown in polymorphisms in VDR and TLR-2 that are associated
with increased susceptibility to TB infection.70,71
In contrast, DC-specific intercellular-adhesionmolecule-3-grabbing non-integrin signal pathway
activation leads to production of IL-10 and transforming growth factor-b (TGF-b), which suppress the
immune response.62 Macrophages are heterogeneous
and have different roles during TB infection, that is,
IL-23-producing type 1 macrophages that promote
Th1 immunity to mycobacteria, and IL-10-producing
type 2 macrophages that suppress immunity to mycobacteria.72 The type 2 macrophages have been shown
to induce CD4+ T cells to adopt a Treg CD25+FoxP3+
mTGFb-1+ suppressor phenotype73 (see section below
on Treg).
Interferon-g, secreted from activated T cells and NK
cells have the capability to activate macrophages and
promote bacterial killing by permitting phagosomal
maturation and production of antimicrobial reactive
nitrogen intermediates and reactive oxygen intermediates.74,75 Recent research also found that IFN-g and
TLR signalling pathways induce autophagy in macrophages,76 which enhances the delivery of ubiquitin
conjugates to the lysosome and increases the bactericidal capacity of the lysosomal soluble fraction.77 Th1
cytokine IFN-g facilitates phagosome lysosome fusion
(autophagy) through cell signalling pathway IRGm178
(LRG-47)79 and PI3K,76 whereas Th2 cytokines IL-4 and
IL-13 abrogate autophagy and autophagy-mediated
killing of M. tuberculosis through Akt signalling pathway.80 Besides IFN-g, TNF-a also plays an important
role in killing intracellular M. tuberculosis through
reactive nitrogen intermediates together with IFN-g
and is involved in granuloma formation.81 TNF is
needed for controlling LTBI as anti-TNF antibody
infliximab increases the risk of activating latent TB47
through direct neutralization of TNF and also depletion of granulysin-expressing CD45RA+ subset of
effector memory CD8+ T cells that contributes to the
killing of intracellular M. tuberculosis.82
The CD1d-restricted NKT cells have recently been
shown to mediate protection against M. tuberculosis in
the mouse model.83,84 NK cells also contribute to protective immunity through killing CD4+CD25+FoxP3+
Treg cells.62 In addition, invariant NKT have recently
been found to inhibit development of Th17 cells,85
which may impact the pathology mediated by M.
tuberculosis infection.
The role of neutrophils in host defence against M.
tuberculosis is conflicting. Polymorphonuclear neutrophils (PMN) are the first cells recruited to sites of
microbial entry and express a range of receptors and a
vast arsenal of antimicrobial effector molecules as in
macrophages.62 Most experiments on human PMN
suggest that PMN could be activated in response to M.
tuberculosis, and have the ability to restrict mycobac-
terial growth in vitro.62 For example, PMN produce
human neutrophil peptides 1–3 and cathelicidin
LL-37 and lipocalin 2, which have the ability to kill or
restrict M. tuberculosis growth.86 Furthermore, PMN
could activate macrophages through releasing
granule proteins87 and heat shock protein 72 from
apoptotic neutrophils.88 However, on the other hand,
PMN accumulated in M. tuberculosis-infected sites
seem to have no obvious effect on mycobacterial
growth and play a pathological rather than protective
role in active disease. The results from different
animal models are also conflicting. Some studies
suggest neutrophils have a protective role during
early stage of infection, while others suggest PMN do
not play a role in control of TB.62,86,89 Mast cells seem to
play a role in controlling M. tuberculosis infection as
activation of mast cells via TLR-2 could compensate
the defect in controlling M. tuberculosis infection in
TLR-2 knockout mice.90
V-g-9V-d-2-encoded T cells are the main g-d T cells in
humans, which recognize non-peptidic phosphoantigens without known requirement for MHC molecules.
MatureV-g-9V-d-2 T cells are rapidly recruited to the M.
tuberculosis infection site, displaying a potent NK-like
cytotoxic activity, expressing chemokine receptors,
secreting a plethora of cytokines including IFN-g and
TNF-a, and killing M. tuberculosis-infected cells.91 Like
g-d T cells, activated CD1 molecules-restricted T cells
that present mycobacterial glycolipid antigens
produce IFN-g and express cytolytic activity.74 These
antigen-specific g-d T cells and CD1-restricted T cells
have adaptive immune responses, and can have
immune memory and mount rapid, strong recall
expansion after M. tuberculosis reinfection.92 Furthermore, parts of g-d T cells produce IL-17 and express
TLR-1 and TLR-2, as well as dectin-1, and could
directly interact with certain pathogens to orchestrate
an inflammatory response after bacterial invasion.93
Collectively, these data indicate that several pathways and cell types interact to mediate innate immunity against M. tuberculosis, and provide likely
mechanisms for the observation that many individuals fail to display any immunodiagnostic evidence of
TB infection despite significant and prolonged exposure to M. tuberculosis.
© 2010 The Authors
Journal compilation © 2010 Asian Pacific Society of Respirology
Respirology (2010) 15, 433–450
ADAPTIVE IMMUNITY TO
MYCOBACTERIUM TUBERCULOSIS
Mycobacteria-infected macrophages and DC of the
innate immunity present antigens to T cells and B
cells that belong to adaptive immunity. Cytokine
IL-12p40 plays a fundamental role in the pathogeninduced activation of pulmonary DC.94 Macrophage
apoptosis that releases apoptotic vesicles to carry
mycobacterial antigens to uninfected DC can lead to
more effective antigen presentation. Inhibition of
macrophage apoptosis could reduce transfer of antigens to bystander cells and affect activation of CD8 T
cells.95
MHC class CD1-restricted CD4 T cells and MHC
class CD1-restricted CD8 T cells that recognize
peptide antigens and the g-d T lymphocytes as well as
438
K Dheda et al.
the CD1-restricted specific T lymphocytes produce
IFN-g and constitute the protective immunity (Fig. 3).
The CD4+ Th1 cells mount a much stronger IFN-g
response than CD8+ T cells after mycobacterial infection96 and are thought to play a prominent role in
protection.97 The lack of CD4 T cells may result in
delayed distribution of activated CD8 T cells from
draining lymph nodes to distant organs and consequently cause a delayed acquisition of immune protection.98 The CD8 cytolytic T lymphocytes (CTL)
secrete granulysin, granzymes and perforins to kill
mycobacteria-infected cells and are capable of
immune protection against secondary mycobacterial
challenge in the absence of CD4+ T cells.96
The CD4 T helper cells can be differentiated into
Th1, Th2, Th17 and Treg cells. The Th1 cells produce
cytokines, notably IFN-g, TNF-a, IL-2, lymphotoxin
and granulocyte-macrophage colony-stimulating
factor (GM-CSF), which prompt stimulation of
Th1 cells, CTL, and maturation and activation of
Respirology (2010) 15, 433–450
© 2010 The Authors
Journal compilation © 2010 Asian Pacific Society of Respirology
TB immunology
439
Figure 2 Innate immunity to tuberculosis infection. Mycobacterium tuberculosis is phagocytosed by macrophages and
dendritic cells through membrane-bound receptors such as CR3, scavenger receptor, MMR, TLR, NOD2 and DC-SIGN.
These lead to activation of macrophage signalling pathways (NF-kB), causing secretion of pro-inflammatory cytokines,
chemokines, and antimicrobial molecules, and activation of VDR, which induces the expression of the antimicrobial
peptides cathelicidin and b-defensin. In addition, induction of autophagy mediates antimicrobial activity. PMN cells
recognize and engulf M. tuberculosis and secrete antimicrobial peptides to kill bacteria. NK cells, gd T cells and
CD1-restricted T cells are also be activated by specific ligands and cytokines, release cytotoxic factors and secrete IFN-g,
which activates macrophages. CR3, complement receptor 3; DC-SIGN, dendritic cell-specific intercellular-adhesionmolecule-3-grabbing non-integrin; INF, interferon; MMR, macrophage mannose receptor; NK, natural killer; PMN,
polymorphonuclear neutrophils; TLR, toll-like receptors; TNF, tumour necrosis factor; VDR, vitamin D receptor.
䉳
Figure 3 Adaptive immunity to tuberculosis infection. The infected macrophages and dendritic cells secrete cytokines
that include IL-12, IL-23, IL-7, IL-15 and TNF-a, and present antigens to several T-cell populations including CD4+ T cells
(MHC class II), CD8+ T cells (MHC class I), CD1-restricted T cells (glycolipid antigens) and gd T cells (phospholigands).
These T cells produce the effector cytokine IFN-g, which activates macrophages in conjunction with TNF-a to effect
killing of intracellular mycobacteria through reactive oxygen and nitrogen intermediates. In addition, CD8+ cytotoxic T
cells can kill intracellular mycobacteria through granulysin and perforin-mediated pathways. However, CD4+ Th2 cells
produce immunosuppressive cytokines such as IL-4, and CD4+CD25+FoxP3+ regulatory T (Treg) cells produce IL-10 and
TGF-b that may suppress mycobactericidal effector mechanisms.41 A new subset of T helper cells called Th17 cells that
are produced in the presence of IL-23, and are characterized by production of IL-17, is important modulator of
inflammation and recall memory responses. Th17 cells can recruit neutrophils and monocytes, and IFN-g-producing
CD4+ T cells, and stimulate chemokine expression. However, IFN-g in turn can suppress the IL-17 producing Th17 cells.
Thus, there appears to be a more complex cross-regulation of Th1, Th2, Th17 and Treg cell responses, than previously
recognized and the precise role of individual responses in protective immunity remain controversial. GM-CSF,
granulocyte-macrophage colony-stimulating factor; IFN, interferon; TGF, transforming growth factor; TNF, tumour
necrosis factor.
䉳
macrophages as well as granulocytes. The Th2 cells
produce B cell-stimulation factors such as IL-4, IL-5,
IL-10 and IL-13, which promote antibody production
but suppress the Th1 type immune response. The Th17
cells, a distinct subset of helper T cells, produce unique
cytokines of IL-17, IL-17F, IL-21 and IL-22, which
stimulate defensin production and recruit neutrophils
and monocytes to the site of inflammation, and are
involved in the early phase of host defence. The Th1,
Th2 and Th17 subsets may be modulated by Treg
cells, of which there are several types and the list
is growing. The CD4+CD25+FoxP3+ natural Treg cells
are characterized by TGF-b and IL-10 production,99,100
while the Treg cells also co-produce IFN-g.101
In addition to CD4+ Treg cell subsets, CD8+ Treg
cells are also described, which could inhibit T-cell
proliferation and cytokine production.102 The FoxP3expressing Treg cells are expanded during TB infection,103 and inhibit human memory g-d T cells to
produce IFN-g in response to M. tuberculosis
antigens.104 However, phosphoantigen-activated
Vg2Vd2 T cells could antagonize IL-2-induced
CD4+CD25+Foxp3+ Treg cells in mycobacterial
infection.105 CD4(+)CD25(+)FoxP3(+) Treg cells also
produce TGF-b to downregulate CD4+ T-cell
response,39 and suppress the effector-immune
response and induce bacillary dissemination and
disease manifestation.106,107 High per cent of Treg cells
characterized as CD4(+)CD25(high)CD39(+) was also
identified in active TB patients.108 Attenuation of Treg
cells has a positive impact on the protective capacity
of vaccine against M. tuberculosis infection.109,110
Nevertheless, given the extensive immunopathology
and lung damage that characterize human TB, the
precise role of Treg cell (deleterious, beneficial or
bystander) in the immunopathogenesis of TB remains
unclear.
Different cytokines can decide the differentiation
of T-cell sets (Fig. 3). IL-12, IL-18 and IFN-g promote
Th1 cell development, while IL-4, IL-5 and IL-13
induce Th2 cell development. IL-23, Il-6, IL-21 and
low concentration of TGF-b could induce Th17 differentiation, while IL-2 and high concentration of TGF-b
could induce Treg differentiation.99,100 IL-6 inhibits
the generation of Treg cells induced by TGF-b, but
together with TGF-b induces the differentiation of
Th17 cells.111
Traditionally, B cells are not generally thought to
play a significant role in protection against TB.
However, recently it has been demonstrated that B
cells112 are required for optimal protection in M.
tuberculosis-infected mice through interactions with
the cellular immune response and activation of
complement.113 M. bovis Bacille Calmette Guérin
(BCG) directly activates the classical, lectin and alternative pathways, resulting in fixation of C3b onto
macromolecules of the mycobacterial surface,114
which will contribute to mycobacterial killing.
Memory T (TM) cells form after M. tuberculosis exposure or infection. TM cells can be developed into effector (CCR7lo, CD62Llo, CD69hi in humans) and central TM
cells (CCR7hi, CD62Lhi, CD69lo in humans).97 TM cells
proliferate promptly after encounter with antigens,
and produce multiple cytokines such as IFN-g, IL-2,
© 2010 The Authors
Journal compilation © 2010 Asian Pacific Society of Respirology
Respirology (2010) 15, 433–450
440
TNF-a, lymphotoxin and/or GM-CSF.99 Nevertheless,
the identification of specific biomarker signature of
protective immunity remains elusive. Multifunctional
CD4+ T-cell responses, like those raised to the TB
subunit vaccine, Ag85B-ESAT-6/CAF01-induced,115
may be a better marker of protective immunity.
K Dheda et al.
well-conducted prospective clinical trials are required
to clarify their utility for routine use. The emergence of
extensively drug-resistant TB had intensified the
urgency for these studies to be conducted.
IMMUNOTHERAPY FOR
TUBERCULOSIS
MONITORING TUBERCULOSIS
TREATMENT AND PREDICTORS
OF RELAPSE
It has been suggested that the development of TB is
due to failure of immune regulation or inappropriate
immune regulation.116–118 Moreover, much of the lung
damage associated with TB is host-mediated immunopathology rather than due to M. tuberculosisderived virulence factors. The disturbance in immune
regulation may speculatively involve the subversion
of a protective Th1 response, including the generation
of CD8+ CTL, by several mechanisms including Th2like cytokines,118 TGF-b, Treg or other regulatory
cells,116 and hitherto undescribed mechanisms afflicting the downstream protective Th1 pathways. The
observation that those with active TB require
6 months of treatment despite almost 95% of the bacterial sterilization occurring within the first 2 weeks of
therapy has never satisfactorily been explained. Thus,
more effective treatment may require modulation of
the immune system and a switch away from an
immunopathologic phenotype to a protective one.
Restoring this immunoregulatory balance may take
several months.37
Attempts to restore mycobactericidal immunity
with IL-2119 and IFN-g120,121 have been disappointing.
Agents such as steroids,122 thalidomide123 and TNF-a
antagonists have also been studied.124 It is hypothesized that immunomodulatory agents (outlined in
Table 2) may drive an appropriate Th1 response
while concurrently turning on or off the appropriate
regulatory cells. For example, M. vaccae may drive a
Th1 response and CD8+ CTL but at the same turn on
CD25CD45Rblow Treg cells.127 However, clinical trials
of M. vaccae have been disappointing.128–130 It has
been suggested that this failure may be related to the
administration of a single rather than multiple
doses, which is used with success in China.131 A
recent study of multiple-dose M. vaccae in HIVinfected African participants, and published in
abstract form, demonstrated a reduction in mycobacteraemia in the intervention group.132 In a murine
model IVIG was shown to dramatically improve
mycobacterial sterilization but there are no human
data.133 There are several intriguing preliminary
reports, involving small numbers of patients, that a
proprietary extract of several plants from the
Ukraine (Immunoxel) may be associated with
improved outcomes in drug-sensitive and drugresistant TB134–136. Several other potential immunomodulatory agents including Mycobacterium w,137
DNA vaccine-encoding HSP65 of M. leprae,138
HE2000,139 RUTI,140 SCV-07 SciCLone,125 anti-IL-4125
and GM-CSF125 are described in Table 2, and reviewed
in detail in Churchyard et al.125 The value of these
immunomodulatory agents remain unclear and
It is extraordinary that no new TB drugs have been
introduced into clinical management since the discovery of rifampicin in the 1960s. With the advent
of multi-drug-resistant TB and extensively drugresistant TB the concerted global effort to find new
drugs has been re-energized. The development of any
new drug is a prolonged and costly process. The definition of ‘drug efficacy’ includes successful completion of treatment and up to 2 years of follow up to
identify post-treatment relapses. Given that treatment regimens for TB may last for up to 18 months,
and recruitment of large cohorts may span several
years, the time frame for evaluating new drugs in
phase III trials may be as long as 5–8 years.
Use of early bactericidal activity141,142 and 2-month
culture conversion143 allow for assessment of drug
efficacy but the latter correlates crudely with disease
relapse. Data from several studies reveal that the
median (interquartile range) 2-month sputum conversion rate in patients with pulmonary TB was 81%
(69.5; 87.5) and this was associated with a relapse rate
of 6% (3.8; 12).144 Immunological biomarkers/profiles
hold promise as proxy markers to monitor treatment
responses and predict future relapse.144–146 Although
a multitude of potential markers exist (recently
reviewed144,145) none has been validated in large prospective studies. Cytokines such as IFN-g and the IL4/
IL4d2 ratio have been shown to vary with treatment of
LTBI and active disease;147–150 these changes, however,
are not always consistent151,152 and may be confounded by the natural variation in stimulated cytokine responses over time.16,153 Numbers and ratios of
Treg cells and NK cells differ in TB and non-TB
patients154,155 and surface receptors such as soluble
intercellular adhesion molecule 1 and soluble TNF
receptor 1 show promise in distinguishing active from
inactive TB and evaluating disease severity.156,157
Inflammatory markers such as CRP and neopterin
both decrease with TB treatment but are nonspecific.158,159 Recent studies suggest that a profile
using several cytokines, rather than one biomarker,
may be more useful,146,160–162 and it is likely that the best
approach might entail a comprehensive ‘relapse prediction score’ using clinical variables, for example,
weight, laboratory parameters and immunological
markers. The biggest challenge will be to fund large
cohorts with enough power to identify useful markers
that predict relapse. Immunological biomarkers thus
show promise and in combination with other parameters may be useful indicators of disease response
and efficacy of therapy, thereby accelerating the
development of much needed new anti-tuberculous
treatments.
Respirology (2010) 15, 433–450
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Journal compilation © 2010 Asian Pacific Society of Respirology
441
TB immunology
Table 2 Potential immunotherapeutic agents that have been studied in murine or human models for the treatment
of TB
Agent
Immunoregulatory agents
Mycobacterium vaccae
Mycobacterium w
High-dose IVIG
HE2000
DNA vaccine encoding
HSP65 from M. leprae
Fragmented
lipid-depleted M.
tuberculosis delivered
in liposomes (RUTI)
Immunoxel (Dzherelo)
SCV-07 SciCLone
Anti-IL-4
Supplemental effector
cytokines
Recombinant IFN-g
Recombinant IL-2
IL-12
Recombinant GM-CSF
Immunosuppressive
agents
Thalidomide, newer
thalidomide
analogues and TNF
blockers e.g.
etanercept
High-dose prednisolone
Mechanism
Drives Th1 and CD8+ CTL but downregulates
Th2 through CD4+CD45+Rblow regulatory
cells
Drives a Th1 response in mice but mechanism
of action is unknown. Licensed in India as a
immunomodulator for use in leprosy
Unknown but may involve pathways
implicating sialic acid residues on IgG
Modified form of DHEA (an adrenal steroid).
Mode of action is unknown
Enhances CD8+ CTL activity and
downregulates Th2
Liposomal preparation of the M. tuberculosis
cell wall skeleton. Suggested as an adjunct
to eradicate long-term persisters
Combination of plant extracts used in the
Ukraine. Mechanism of action is unknown
Unknown
Reduced macrophage apoptosis, enhanced
macrophage function, reduced TGF-b, drives
expansion of CD8+ CTL etc.
Theoretically upregulate Th1-mediated
macrophage killing
Promotes T-cell proliferation and granuloma
formation
Drives a Th1 response
Reduced M. tuberculosis replication in
macrophages and DC
Disruption of immunopathological
environment in which M. tuberculosis
resides with replication upon exposure of
bacteria to drugs. Newer analogues are
PDE4 inhibitors
Attenuates TNF-a production
Comment
Single-dose M. vaccae not effective in clinical
trials but full results of multiple-dose studies
are awaited (reduced TB incidence shown in
HIV-positive subjects in the DARDAR study).
Used in China to treat MDR-TB
Preliminary data suggest a beneficial effect in
a murine model and one small human study
Effective in a murine model. No human trials
undertaken
Therapeutic in a murine model of TB. Reduced
coinfection with TB shown in a cohort of
HIV-infected individuals
Effective in a mouse model but there are
conflicting data. Phase 1 studies are planned
Accelerates bacterial clearance in a mouse
model. Phase 1 studies underway
Striking effects in small uncontrolled studies
of M and XDR-TB patients. Larger controlled
studies are warranted
Results from one murine study
Therapeutic effect in mouse models.
Humanized monoclonal IL-4 has been
produced by Glaxo Smith Kline but no
human studies have been performed
Two controlled trials showed minimal effects
and their results have never been published
No beneficial effects found in an RCT (in fact
reduced culture conversion was observed in
the IL-2 arm)
Beneficial in a murine model
Trend to faster sputum conversion in one
human study
Murine and human studies have been
undertaken. Newer thalidomide analogues
show enhanced killing in murine models.
Etanercept showed enhanced culture
conversion in a clinical trial
Large controlled trial showed a dramatic effect
on enhanced sputum culture conversion.
Two other prospective studies showed a
beneficial effect on sputum conversion
These agents, their mechanisms of actions and evidence for their utility have recently been reviewed in detail
elsewhere116,125,126 and this table should be read in conjunction with Churchyard et al.125
CTL, cytolytic T lymphocyte; DARDAR, Dartmouth and Dar es Salaam; DC, dendritic cell; DHEA, Dehydroepiandrosterone; GM-CSF, granulocyte-macrophage colony-stimulating factor; IFN, interferon; MDR-TB, multi-drug-resistant
tuberculosis; RCT, randomized controlled trial; TB, tuberculosis; TGF, transforming growth factor; TNF, tumour necrosis
factor; XDR-TB; extensively drug-resistant tuberculosis.
© 2010 The Authors
Journal compilation © 2010 Asian Pacific Society of Respirology
Respirology (2010) 15, 433–450
442
TUBERCULOSIS IMMUNOLOGY AND
THE DESIGN OF EFFECTIVE VACCINES
Cytokine-specific immune responses to antigens
from M. tuberculosis (such as ESAT-6, CFP-10, Ag85)
have been found in latently infected persons and in
healthy contacts of persons with TB.163–165 The same
antigens have been shown in animal models (murine
and guinea pigs, some non-human primates) to
provide protection against experimental M. tuberculosis infection. The expression of these (such as
ESAT-6, CFP-10, Ag85) and additional M. tuberculosis
antigens either in BCG or through viral vectors is a
unifying characteristic of modern genetically engineered vaccine candidates against TB.
Bacille Calmette Guérin, derived from M. bovis in
1902, is the only currently approved anti-tuberculous
vaccine and widely used globally.166 It has a very good
safety profile in immunocompetent persons.167,168
Recently identified genetic differences between BCG
strains and M. tuberculosis169 are encoded within the
region of difference 1, which is absent in BCG. The
protective efficacy of BCG as a vaccine has been
evaluated in many clinical studies to assess protection against many forms of TB in various clinical subgroups.168 BCG vaccine-induced protective efficacy
appears to be influenced by multiple factors170–172
including age of the vaccine recipient, background
infection rates with ubiquitous mycobacteria (mycobacteria other than M. tuberculosis, MOTT), virulence
of the infecting clinical M. tuberculosis strains, coinfection with helminths,173–175 acquisition of T-cell
immunity against helminths in utero176 and other
conditions that alter immune responses (e.g. malnutrition). Although protection against adult pulmonary
TB varies from none to some effect in many studies
(summarized in Rieder170), there is good evidence that
BCG protects against disseminated TB (summarized
in Rieder170). Newer studies in household contacts
support the notion that BCG may also protect against
adult M. tuberculosis infection.177,178 Concerns about
potential adverse vaccination events from the live
BCG vaccine (such as disseminated mycobacteriosis
and mycobacterial meningitis) in HIV-1-infected
persons have led to the development of new BCG vaccination guidelines by the World Health Organization.168,179 Similar safety precautions impact on the
efficacy testing and future use of new experimental
recombinant BCG (rBCG) vaccines in populations
with high rates of HIV-1 infection. Despite of these
considerations, current planning for future primeboost vaccination strategies incorporates conventional BCG and/or new rBCG-derived vaccines as the
priming backbone followed by booster vaccines that
are not based on BCG. Some of the rBCG vaccines
improve antimycobacterial T-cell responses through
engagement of an alternative antigen presentation
pathway (endosome escape).180
Thus, current TB vaccine development efforts aim
at preventing primary M. tuberculosis infection
through preinfection vaccination, or at interrupting
the transition from latent infection (LTBI) to active
disease through post-BCG booster vaccination
approaches. The number of promising tuberculous
Respirology (2010) 15, 433–450
K Dheda et al.
vaccine candidates has increased in the last decade
(Table 3). Currently there are eight vaccine candidates
in human phase I and phase IIa trials.188
As vaccines need to be developed for neonates,
adolescents and adults, each of these vaccine recipient groups requires specific considerations related to
the maturity of the immune system, and pre-existing
antimycobacterial immunity (boosting after primary
vaccination), including that conferred by mothers in
the case of neonates. Field trials to assess the protective efficacy of vaccines against M. tuberculosis infection are risky, costly, and require a sophisticated
clinical set-up in areas highly endemic for M. tuberculosis. Alterations of the underlying immune status
affect the vaccination safety, vaccine-induced protection and the possibility to assess vaccine-induced
immunity. As high incidence areas for TB most often
are also areas with high HIV-1 infection rates, HIV-1
infection in vaccine recipients189 must be considered
in the choice of vaccines for future vaccination campaigns. Helminth coinfection that cause a Th2 cytokine bias must also be considered as they can alter TB
immunity and responses to TB vaccines.173,174,190
The urgency to identify and use correlates of protective immunity is particularly obvious in the vaccine
development field in which the study of immunological correlates is a crucial component of vaccine efficacy assessments, prior to embarking on large-scale
efficacy field trials. It is increasingly clear that the
induction of no single cytokine (IFN-g, TNF, IL-2) or
effector molecule (granzyme, nitric oxide, granulysin)
can predict immunological protection. Polyfunctional
immune responses191,192 have recently been considered useful correlates of protection. Attempts to determine functional assays that would allow measurement
of vaccine-induced immunological protection in in
vitro growth inhibition assays are ongoing. In such
assays whole blood or peripheral blood mononuclear
cells (PBMC) from vaccinees (before and after vaccination) will be incubated with isolates of virulent M.
tuberculosis and the vaccine induced inhibitory effect
on M. tuberculosis growth assessed in vitro.
Novel tuberculous vaccination approaches may
also involve the lungs. As human infection with M.
tuberculosis most often occurs via inhalation, and TB
pathology most often affects the lungs, it is logical to
induce pulmonary protective immunity. An aerosolized TB vaccine candidate is currently undergoing
safety and immunogenicity studies in non-human
primates. Preliminary studies have shown strongly
induced local pulmonary antigen-specific T-cell
responses that may have characteristics of protective immunity (Aeras Global TB Vaccine Foundation, Annual Report 2008, http://www.aeras.org./
newscenter/downloads/presskit/Aeras%202008%20
Annual%20Report_Final_9.29.08.pdf).
RAPID IMMUNODIAGNOSIS
OF TUBERCULOSIS
Mycobacterium tuberculosis infection has traditionally
been ascertained using the TST, which measures skin
© 2010 The Authors
Journal compilation © 2010 Asian Pacific Society of Respirology
443
TB immunology
induration caused by a delayed type hypersensitivity
reaction and the sequential cutaneous infiltration of
neutrophils, macrophages and several lymphocyte
subsets10,193 including Treg cells.194 More recently quantitative and relatively specific TB antigen-driven
(ESAT-6, CFP-10, TB 7.7) T-cell responses using PBMC
have been investigated for their utility in diagnosing
TB infection8,195 (IGRA; Fig. 4). These commercially
available assays enumerate the frequency of CD4+
antigen-specific effector T cells in peripheral blood.8,195
Both these assays (IGRA and the TST) are measures of
T-cell priming and in themselves do not necessarily
indicate the presence of potentially viable M. tuberculosis organisms in the tissues of asymptomatic individuals. The IGRA are more specific, and in contrast to
the TST that may be induced by BCG vaccination or
non-tuberculous mycobacterium, correlate well with
the magnitude of exposure to M. tuberculosis.8,9,195
However, exposure per se does not necessarily imply
the tissue persistence of organisms in a nonreplicating state. There is good evidence for the TST,
but not the IGRA, that treating test-positive individuals reduces the subsequent risk of developing active
TB.208,209 Although not precise, the IGRA and TST are
the only available proxy markers of TB infection. The
performance outcomes of these assays have been
reviewed in detail elsewhere.8 Several international
guidelines recommend the use of IGRA, when using
peripheral whole blood or PBMC, for the diagnosis of
LTBI, either alone210 (USA), or in conjunction with the
TST8 (UK, Canada, France, Italy, etc.).
Can the IGRA be used to diagnose active disease?
Peripheral antigen-specific responses do not distinguish between active TB disease and LTBI. However, it
is reasonable to hypothesize that patients with
non-TB diagnoses, for example, cancer or bacterial
infection, should not have a high frequency of
antigen-specific T cells at the site of disease. Thus, it
should be possible to diagnose active TB if there is a
high frequency of M. tuberculosis-specific T cells
within a specific disease compartment. Indeed, quantitative T-cell responses (T-SPOT.TB (Oxford Immunotec, Oxford, UK)) are accurate and clinically useful
for the diagnosis of pulmonary TB when using BAL
cells in both low-198 and high-burden settings199
(Fig. 4). Our recent work suggests that although IGRA
can be used for the immunodiagnosis of active TB
using cells from induced sputum this approach is not
feasible in clinical practice with the current assay
format (T. Cashmore and K. Dheda, 2010, unpubl.
data). Can the same approach be used in other
compartments? Although preliminary studies were
encouraging we201 and others211,212 have recently
shown that the IGRA are not clinically useful for the
diagnosis of pleural TB at least in high-burden settings (this may be different in low-burden settings213).
Table 3 Promising anti-tuberculous vaccine candidates in human phase I and phase IIa trials
Vaccine
Description
BCG30
rBCG with plasmid over expressing
Mycobacterium tuberculosis Ag85A181,182
rBCG with chromosomal expression of
listeriolysin (for endosome escape)180,183
Recombinant BCG with chromosomal
overexpression of perfringolysin (for
endosome escape) and M. tuberculosis
antigens 85A, 85B, 10.4, Rv3427, rpfA, rpfB,
rpfC40 and DosR-regulated proteins184
Recombinant adenovirus 35 expressing M.
tuberculosis antigens 85A, 85B and 10.4,
boosts BCG or rBCG
Modified vaccinia Ankara 85A vector
expressing M. tuberculosis antigen 85A,
boosts BCG or rBCG185,186
Fusion molecule comprised of a M.
tuberculosis protein from the PPE family
(Rv1196), combined with an inactive serine
protease Rv0125, boosts BCG
Fusion molecule of proteins from the M.
tuberculosis PPE family, inactive serine
protease Rv1025 and AS01 adjuvant
Recombinant M. tuberculosis antigens 85B
and 10.4 combined with adjuvant IC31,
boosts BCG187
VPM-X
rBCG
(AERAS-407)
Crucell Ad35
(AERAS-402)
MVA85A Oxford
GSK M72
Hybrid I
HyVac 4
Clinical
development
stage
Source/producer
Phase I
UCLA/Aeras
Phase I
Max Planck/VPM/TBVI
Phase I
Aeras
Phase I + IIa
Crucell/Aeras
Phase IIa
Oxford/Isis/Aeras/Emergent
Phase I + IIa
GSK/Aeras
Phase I + IIa
SSI/Intercell/TBVI
Phase I
SSI/Intercell/Sanofi Pasteur
Sources: http://www.aeras.org/our-approach/vaccine-development.php?discovery-overview and Barker et al.188
BCG, Bacille Calmette Guérin; rBCG, recombinant Bacille Calmette Guérin.
© 2010 The Authors
Journal compilation © 2010 Asian Pacific Society of Respirology
Respirology (2010) 15, 433–450
444
K Dheda et al.
Figure 4 Rapid immunodiagnosis of TB. An outline of the different antigens, testing platforms, biological markers and
cells from different compartments used for the rapid quantitative immunodiagnosis of active TB and LTBI. *Flow
cytometric platforms have been evaluated (Cosmi et al.).196 $$Cut-points are defined by international guidelines and vary
according to geographical location, BCG vaccination and HIV/immune status. ##Preliminary evidence that HBHA may
help distinguish latent from active disease.197 **Jafari et al.,198 Dheda et al.199 #Cashmore et al.200. +Dheda et al.201
$
Thomas et al.,202 Kosters et al.,203 Kim et al.204 ‡van Zyl-Smit et al.205 ++Kim et al.,206 Tinelli et al.207 BCG, Bacille Calmette
Guérin; CSF, cerebrospinal fluid; LTBI, latent TB infection; HBHA, heparin-binding haemagglutinin; PBMC, peripheral
blood mononuclear cells; PLF, pleural fluid; TB, tuberculosis.
This is likely due to the suboptimal specificity as a
consequence of migration of the M. tuberculosisspecific T cells into the pleural compartment of
patients with non-TB diagnoses, for example, cancer,
bacterial infection, etc.201 Preliminary reports of the
utility of IGRA for the diagnosis of TB meningitis are
encouraging.202 We recently found, in 140 consecutively recruited TB suspects, that T-SPOT.TB is a clinically useful and accurate diagnostic tool for TB
meningitis.214 The utility of a similar approach to diagnose abdominal TB may be feasible.207 Thus, the accuracy of quantitative T-cell responses for the diagnosis
of active TB is compartment-specific.
In summary, IGRA are useful for the diagnosis of
presumed LTBI in low-burden settings.8 Although they
cannot be used to distinguish LTBI from active disease
using peripheral PBMC or whole blood, they are useful
tools for the diagnosis of active TB when using cells
from the site of disease (lung and cerebrospinal fluid).
Thus, an immunologically oriented approach for the
diagnosis of TB is feasible and useful. The main advantage of this approach is the rapid and user-friendly
diagnosis of TB. Further studies are now required to
evaluate alternative testing platforms, alternative
antigens (e.g. heparin-binding haemagglutinin),215
and different biomarker readouts (e.g. IFN-g-induced
protein (IP-10), monocyte chemotactic protein 1
(MCP-1) and IL-1 receptor antagonist (IL-1RA))216 for
the rapid diagnosis of TB (Fig. 4).
Respirology (2010) 15, 433–450
© 2010 The Authors
Journal compilation © 2010 Asian Pacific Society of Respirology
CONCLUSIONS
It is becoming evident that M. tuberculosis infection is
a dynamic state with a wide spectrum of pathology.
Further research using cells from the lung and in individuals who are probably innately immune to TB are
likely to yield important insights into correlates of
immune protection. Although the immunology of TB
is complex and poorly understood an improved
understanding of the immunopathogenesis of TB can
facilitate the design of effective vaccines and evaluation of their efficacy, the rational selection of immunotherapeutic agents, identification of correlates of
445
TB immunology
immune protection, the evaluation of new drug candidates, and drive the development of new immunodiagnostic tools. Nevertheless, these newer tools are
only likely to translate into improved outcomes if
adequate resources are devoted to their evaluation,
and if poverty, political instability and HIV infection
are concurrently tackled on an international scale.
ACKNOWLEDGEMENTS
This work and K.D. was supported by a TBsusgent
grant from the European Commission (EU-FP7), the
EDCTP, an SA MRC Career Development Award, a
Canadian CIHR award and an NRF/SARChI award.
S.K.S. is supported by NIH R21 R21ES016928-02.
R.V.Z.S. is supported by a Discovery Foundation Fellowship and by the Fogarty International Clinical
Research Scholars/Fellows Support Centre NIH grant
R24TW007988. Y.Z. is supported by R01AI44063. B.Z.
is supported by the National Major S & T Projects of
China (2008zx1000301104).
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Respirology (2010) 15, 433–450
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Journal compilation © 2010 Asian Pacific Society of Respirology