J e dédie ma thèse à mon épouse, Annick, toi qui a toujours su m’apporter le soutien depuis de nombreuses années. Sans toi, nous n’y serions jamais arrivé. Je dédie ma thèse à mes filles Morgane et Armelle, vous qui m’apportez tant de joie.
D’aussi loin que je me souvienne, la Vie m’a toujours intéressé. Vers l’âge de 4 ans, je me déguisais avec amusement en médecin. Petit, je me promenais avec une loupe pour observer les insectes et la vie aquatique. Plus tard, mes parents m’ont offert un microscope, la panoplie du petit chimiste, etc… En 1990, j’ai participé à l’organisation des « Compa- gnons de la Recherche » grâce à mon professeur de Biologie de l’époque, Mme Poels. A ce moment, j’ai pû dé- couvrir les laboratoires de pharmacie de l’ULB, l’intérêt de chercheurs pour le taxol avec l’espoir de lutter contre le cancer. Peu après, ma future épouse et mes parents me soutenaient dans les débuts des études de Médecine.
En 00, mon stage de Médecine In- terne chez le Prof. Elie Cogan a servi de tremplin pour s’élancer vers le monde, alors inconnu, de la Recherche Médicale.
C’est à la fin de mon premier «grand»
séminaire1 que j’ai fait la connaissance du Prof. Françoise Mascart. Quelques mois plus tard, je menais des travaux de recherche sur la réponse humorale à l’égard d’une adhésine de M. tuberculo- sis au sein de son laboratoire. Ce labo- ratoire de “Vaccinologie et d’Immunité
Hougardy JM. et al. Usual Interstitial pneumonia.
Rev. Med. Brux. 2004;25:78-83.
des Muqueuses” est aussi connu sous le petit nom de «LoVMI».
Au sein du «LoVMI», j’ai fait la connais- sance de plusieurs personnes qui ont définitivement marqué cette période.
Je pense en premier à mes proches col- laborateurs, Stéphane Temmerman et Sammy Place. Ils ont sû me transmet- tre, chacun à leur manière, leur savoir et leurs idées (et bien plus). Je les remercie spécialement et chaleureusement. C’est en pensant à eux, que le mot «équipe»
prend tout son sens.
Mes remerciements vont ensuite vers Virginie Verscheure et Gaëlle Leloux qui ont pu m’aider précieusement dans la réalisation de certaines expériences. Un tout grand merci à Kaat Smits, Françoise Mascart et Camille Locht pour les dernières heures passées à retravailler l’anglais. Merci aussi à: Annick Ocmant, Liliane Schandené, Patrick Stordeur, Jean-Pierre Delville, Martine Ducarme et Zoulika Amraoui, Nour de San, Violette Dirix, Kinda Schepers, Julie Smet, Nadia Assoufi et Magali Anzil. Merci à Annie Drowart, à Jean-Paul Van Vooren. Merci à Kris Huygen et à Alain Lemoine.
Merci à Marc & Yvette, à Louise & Josée, à mes parents.
Enfin, Françoise a notamment su com- prendre et cultiver mon intérêt particul- ier pour la recherche «translationnelle», celle qui est directement transposable lors de la pratique médicale quotidi- enne. L’exploitation des connaissances acquises au sein du laboratoire alors
Préambule
que nous sommes au lit du malade est extrêmement gratifiante. C’est aussi un moyen de s’appliquer à procurer une médecine meilleure. Je remercie donc particulièrement Françoise Mascart et Camille Locht. Grâce à leur dévouement et à leur énergie, la carte d’identité de la HBHA s’est considérablement enrichie depuis sa découverte en 1996.
A l’instar des arômes d’un excellent bor- deau, les résultats de la HBHA sont rich- es et variés. Fruit d’un travail continu de quelques «saisonniers» guidés par ces vignerons acharnés!
Ce travail a pu être réalisé grâce aux
soutiens du F.R.I.A. et de l’ “European
Commission within the 6
thFramework
Program, contract no. LSHP-CT-00-
5067”.
Contents
Préambule 2
Tableofcontents 4
Jurymembers 6
Abbreviationslist 7
Résumé 9
Summary 11
I.Introduction 13
History 14
Natural history of M. tuberculosis infection 18
Primary infection 18
Latent tuberculosis infection
Tuberculosis disease
Primary tuberculosis
Post-primary tuberculosis 4
Epidemiology 5
World-wide burden of tuberculosis 5 Belgian burden of tuberculosis 6
Microbiology 8
Generalities (4, 5) 8
Habitat 8
Structure of the envelope 9
Antigens of major clinical interest and
other main virulence factors 0
Purified protein derivative 0
Virulence factors related to cell envelope function
or secretion 1
Heparin-binding hemagglutinin (HBHA) 1 Early-Secreted-Antigenic-Target-6 kDa
and Culture-Filtrate-Protein-10 kDa 4
Antigen 85 Complex 5
LAM, LM and PIM 5
HSPx or 16-kDa-alpha-crystallin 6
Laboratory diagnosis of tuberculosis 7 R
ecent infection and latent tuberculosis 7Tuberculin-skin test 7
Immunodiagnostic 8
Active tuberculosis 40
Clinical laboratory procedures and
biological samples 40
Microscopy 40
Culture and identification 4
Molecular biology 4
Immunodiagnosis 4
Conventional chest radiography and
computed tomography 4
Immunity and pathogenesis 44
Immunity and intracellular bacteria 44 Immune responses against M. tuberculosis 44
Innate immunity 45
Toll-like receptors 45
Natural killer cells 46
Neutrophils 47
Macrophages and
effector mechanisms 47
Dendritic cells 49
Other cell types: the example of the alveolar epithelial cells 50
Complement 5
Adaptive immunity 5
CD4+ T cells 5
CD8+ T cells 54
Th17 CD4+ T cells 56
Unconventional
T cells 57
Immune regulation and tuberculosis 60
Immunomodulatory cytokines 60
TGF-ß 60
IL-10 61
Regulatory T cells 61
Classification of regulatory CD4+ T cells: naturally occurring or induced 6 Major phenotypic markers of
regulatory T cells 64
Regulatory T cells and infections 70 Subversion of anti-microbial immune responses by the use of regulatory T
cells 7
Antigen-specificity of regulatory T cells during infectious processes 7
A brief global overview… 74
II.Rationales&objectives 76
III.Material&methods 79
IV.Results 82
4.1. HBHA-Induced IFN-γ Release as a Diagnostic
Tool for Latent Tuberculosis 8
4.1. Supplementary data 9
Potential use of a recombinant form of HBHA instead of native HBHA in the HBHA-IGRA 9
4. Regulatory T cells depress immune responses to protective antigens in active tuberculosis 94
4.. Supplementary data 10
4..1 Role of TGF-ß and IL-10 during the in vitro stimulation of PBMC from TB patients with PPD or
HBHA 10
4... CD4+CD5highFOXP+ Treg cells from TB patients also express other Treg cell-associated
markers 104
4... Involvement of local regulatory T cells 106 HBHA-specific IFN-γ secretion during
TB pleurisy 107
Proportions of pleural and BAL CD4+CD5highFOXP+ T cells 107 Depletion of pleural CD4+CD5high T
cells 107
4.. In vitro expansion of CD4
+CD5
highFOXP
+
CD17
low/-regulatory T cells from peripheral blood lymphocytes of healthy Mycobacterium tuberculosis-infected humans
4..1. Supplementary data 119
Role of IL- in the induction/expansion of Treg cells in response to BCG stimulation: 119 Cell-to-cell contact dependent suppressive mechanism mediated by induced/expanded
Treg cells 119
V.DISCUSSION 121
About the release of IFN-γ in response to HBHA 1 HBHA and the natural history of M. tuberculosis
infection 1
HBHA-IGRA and tuberculin skin testing 14 HBHA-IGRA and QuantiFERON TB Gold IT 15 IGRAs and detection of active TB 17 HBHA-IGRA and HBHA-based vaccines 17 Conclusions & perspectives about HBHA-based
IGRA 18
About the involvement of regulatory T cells during M.
tuberculosis infection 19
Introduction 19
Broad outlines of publications on Treg cells
& tuberculosis 10
Murine models and regulatory T cells during TB 1
In vitro induction of Treg cells upon stimulation
with M. bovis BCG 14
Antigen specificity of the Treg cells during
active TB 14
Regulation of regulatory T cells: immunotherapy against M. tuberculosis infections? 16
Therapeutic depletion of Treg cells 16 Modification of Treg cells by
TGF-ß modulation 17
VI.Conclusions&perspectives 139
References 141
Thèseannexe 160
Curriculumvitae 161
Jury members
President:
Prof. Gevenois. P.-A. Faculté de Médecine, Hôpital Erasme, Université Libre de Bruxelles, Bruxelles, Belgique.
Members:
• Prof.MascartF.Secretary, Laboratory of Vaccinology and Mucosal Immu- nity, Hôpital Erasme, Université Libre de Bruxelles, Bruxelles, Belgique.
• Prof.MarchantA. Institute for Medical Immunology, Université Libre de Bruxelles, Gosselies, Belgique.
• Prof.SergyselsR. Hôpital St-Pierre, Université Libre de Bruxelles, Bruxelles, Belgique.
• Prof.StruelensM.Hôpital Erasme, Université Libre de Bruxelles, Bruxelles, Belgique.
• Prof.KestensL. Prins Leopold Instituut voor Tropische Geneeskunde, Ant- werpen, België.
• Prof.StengerS. Institut für Klinische Medizinische Mikrobiologie und Hy-
giene, Universitätsklinik, Ulm, Deutschland.
Abbreviations list
A
ADA Adenosine DeAminase Ag85 Antigen-85
AIDS Acquired ImmunoDeficiency Syn- drome
APC Antigen Presenting Cell B
BAL Broncho-alveolar lavage C
cAMP Cyclic Adenosine Monophosphate CCL4 CC Chemokine Ligand 4
CD Cluster of differentiation,
i.e. CD4 : Cluster of differentiation- 4
CFP Culture-Filtrate-Protein CFP-10 Culture-Filtrate-Protein-10 kDa CR Complement Receptor ;
i.e. CR : Complement Receptor- D
DC Dendritic Cells
DNA Desoxyribonucleic Acid
E
ESAT-6 Early-Secreted-Antigenic- Target-6 kDa
F
FOXP Forkhead Box Protein- G
GITR Glucocorticoid-Induced-TNFR- Related protein
H
HBHA Heparin-Binding-Hemagglutinin nHBHA: native HBHA
rHBHA: recombinant HBHA HIV Human Immunodeficiency virus HBV Hepatitis B virus
HCV Hepatitis C virus I
IFN-γ Interferfon-gamma
IGRA Interferon-Gamma-Release-Assay IL Interleukin, i.e. IL-10: Interleukin- 10
IPEX Immune-Dysregulation-
Polyendocrinopathy-Enteropathy- X-linked
L
LAM Lipoarabinomannan
LAMP-1 Lysosome-Associated-Membrane- Protein-1
LTBI Latent tuberculosis infection
LM Lipomannan
M
M. tuberculosis:
Mycobacterium tuberculosis MCP-1 Monocyte Chemoattractant Pro- tein-1
MDR Multidrug resistant
MHC Major Histocompatibility Complex MMP-9 Matrix-Metallo-Proteinase-9 MOTT Mycobacteria Other Than
M. tuberculosis
N:NK cells: Natural killer cells
P
PAMP Pathogen-associated molecular pattern
PBMC Peripheral Blood Mononuclear Cells
PPD Purified Protein Derivative Q
QFT QuantiFERON
R
Rag Recombination-activating gene RD Region of Difference
RNI Reactive Nitrogen Intermediates ROI Reactive Oxygen Intermediates RR Relative Risk
T
TB Tuberculosis TCR T-Cell Receptor
TGF-ß Transforming-Growth-Factor-beta TLR Toll-Like-Receptor,
e.g. TLR- : Toll-Like-Receptor- TNF-a Tumor-Necrosis-Factor-alpha TNFRSF: Tumor-Necrosis-Factor-Receptor-
SuperFamily Treg cells: Regulatory T cells TST Tuberculin skin test TU Tuberculin units
W
WHO World Health Organization X
XDR Extensive drug resistant
Résumé
Globalement, un tiers de la population mondiale est infectée par Mycobacte- rium tuberculosis, l’agent infectieux de la tuberculose (TB). Fort heureusement, seuls 5 à 10 % des individus infectés développent un jour une TB active. Les individus non malades restent cepend- ant infectés à vie, on parle d’infection la- tente. Chaque année, 8-10 millions nou- veaux cas de tuberculose active sont recensés et M. tuberculosis est responsa- ble de 1,5 à millions de décès. Depuis plus d’une décennie, M. tuberculosis s’est étroitement associé à l’infection par le virus de l’immunodéficience hu- maine. Cette alliance néfaste représente une importante menace pour les pays en voie de développement, car ces pathogènes déciment les forces vives de ces populations. Il faut malheureuse- ment rajouter à ce triste tableau une fréquence grandissante de souches multi-résistantes, voire extensivement multi-résistantes. Face à ces souches, les avancées thérapeutiques du siècle dernier sont pratiquement réduites à néant.
Considérant ces données, il est dé- sormais crucial d’améliorer nos out- ils de dépistage de l’infection latente, de diagnostic de la maladie active, de prévention (vaccins) et de traitement.
Pour atteindre ces objectifs, une des pistes est la caractérisation détaillée des réponses immunitaires. En comparant les réponses immunitaires des sujets in- fectés de manière latente à celles liées à la maladie active, nous pourrons peut- être comprendre certains mécanismes de protection. L’étude des réponses immunitaires induites par la «Heparin-
Binding-Hemagglutinin» (HBHA) s’est faite dans cet objectif. La HBHA est une adhésine exprimée par le complexe M.
tuberculosis. Elle est impliquée dans la dissémination extrapulmonaire du bacille et constitue donc un facteur de virulence. Par ailleurs, une vaccination de souris par seulement doses de 5 µg de HBHA suffit à protéger de l’infection avec une efficacité comparable à celle du vaccin BCG. Chez l’homme, les su- jets sains mais infectés développent d’importantes sécrétions d’interféron- gamma (IFN-γ) en réponse à cet an- tigène, alors que la majorité des patients tuberculeux ne le font pas. Cette dif- férence est importante pour compren- dre une des raisons d’échappement de M. tuberculosis au contrôle immunitaire.
La HBHA est une protéine méthylée et la méthylation s’avère essentielle pour ses propriétés immunoprotectrices.
Nos travaux présentés ici se sont axés sur deux éléments de la réponse im- munitaire à la HBHA chez l’homme : d’une part, l’exploitation de la réponse périphérique d’IFN-γ à la HBHA comme outil de dépistage de l’infection latente et, d’autre part, l’étude des raisons de la faible sécrétion d’IFN-γ spécifique de la HBHA lors de la maladie active.
L’évaluation de la sécrétion périphérique
d’IFN-γ en réponse à la HBHA a per-
mis de démontrer rétrospectivement
que celle-ci permet de détecter plus
de 90 % des sujets réagissant positive-
ment à l’injection intradermique de
tuberculine. De manière intéressante,
l’utilisation d’un test commercial, le
QuantiFERON TB Gold IT (QFT-IT) n’a
permis de détecter que la moitié des
sujets infectés sains. De notre point de
vue, le QFT-IT ne peut être recommandé
seul pour le dépistage systématique de l’infection latente par M. tuberculosis. De manière parallèle, un test de stimula- tion basé uniquement sur la sécrétion d’IFN-γ suite à une stimulation à l’ESAT- 6, composant du QFT-IT, n’a pas permis d’augmenter la sensibilité, ni d’ajouter une plus-value au test basé sur la HBHA.
A l’instar de l’intradermoréaction à la tuberculine, le dépistage de la maladie active reste décevant que ce soit par l’utilisation de la HBHA ou de l’ESAT-6.
La TB active est caractérisée par une basse sécrétion périphérique d’IFN-γ en réponse à la stimulation par la HBHA.
Cette faible sécrétion est cependant réversible, puisque un traitement effi- cace permet d’atteindre des taux d’IFN-γ significativement plus élevés. Ceci nous démontre qu’il s’agit d’une suppression associée à la phase active de l’infection.
Nous avons d’abord évalué l’importance de la modulation de la sécrétion d’IFN-γ en réponse à la HBHA par cytokines immunomodulatrices, l’interleukine-10 (IL-10) et le Transforming-Growth-Fac- tor-Beta (TGF-ß). De manière intéres- sante, alors que ces cytokines sont as- sociées à l’infection par M. tuberculosis, la HBHA n’est inductrice ni d’IL-10, ni de TGF-ß. Les lymphocytes T régulateurs (Treg) expriment marqueurs d’intérêt : le CD5, composant du récepteur à l’IL-, et Foxp, un gène régulateur majeur des cellules Treg. Ces cellules sont décrites comme suppressives de réponses immunitaires déclenchées par des antigènes du Soi et du non-Soi.
Nous avons montré que la proportion de lymphocytes Treg périphériques est augmentée en cas de TB active.
Par ailleurs, nous avons également dé- montré que ces cellules suppriment la sécrétion d’IFN-γ et la prolifération induite par la HBHA après stimulation des cellules mononucléées sanguines périphériques de patients tuberculeux in vitro. Cependant, la réponse anti- HBHA des patients tuberculeux, qui est démasquée par la déplétion des lym- phocytes Treg, n’est pas dirigée contre des épitopes protecteurs. En effet, la méthylation n’influence pas leur sécré- tion d’IFN-γ. De ce point de vue, les lym- phocytes Treg sont impliqués dans la maladie tuberculeuse et influencent né- gativement les réponses dirigées contre un antigène protecteur. Cependant, il semble que la TB active soit également associée à une ignorance d’épitopes protecteurs.
Enfin, nous avons également démontré
qu’il était possible d’induire des lym-
phocytes Treg au départ de cellules san-
guines périphériques de sujets infectés
sains. En effet, la stimulation in vitro
des cellules sanguines périphériques
en présence de BCG et de TGF-ß est un
moyen rapide pour induire l’apparition
de lymphocytes Treg fonctionnels in
vitro. Ceci nous interroge quant aux
rôles des lymphocytes Treg dans la
pathogenèse de la maladie. En effet,
un excès de TGF-ß circulant est observé
dans certaines conditions cliniques à
haut-risque de TB post-primaire. De
ce point de vue, les lymphocytes Treg
pourraient être des acteurs déterminant
dans la perte du contrôle à long terme
de l’infection et, par là, pourraient être
des cibles thérapeutiques d’intérêts lors
de l’infection par M. tuberculosis.
Summary
Mycobacterium tuberculosis is the caus- ative agent of tuberculosis (TB). It is es- timated approximately one third of the World’s population is infected with M.
tuberculosis. Fortunately, only 5 to 10 % of the infected individuals will develop the disease throughout their life. How- ever, the other healthy infected individ- uals remain infected for life: this is the latent TB infection (LTBI). Every year, 8 to 10 million new cases of TB are recorded globally, and about to million of peo- ple die from the disease. During the last several decades the co-infection of M.
tuberculosis and the human immuno- deficiency virus have worsened the pic- ture. This dreadful association currently affects mostly the poorest people of the World. Unfortunately, bad news never stands alone. We now witness increas- ing emergence of multi-drug-resistant and even of extensively-multi-drug-re- sistant M. tuberculosis strains. Against these strains current therapeutics are virtually useless.
The development of new tools for pre- vention (vaccines), diagnostics and treatment is crucial. In order to fulfill these objectives, detailed studies on the immune responses is one of the main tracks to explore. Indeed, the comparison of immune responses in LTBI subjects with those in TB patients may provide some clues to understand immune mechanisms of protection.
Studies of the immune responses that are specific to Heparin-Binding-Hemag- glutinin (HBHA) may be one of these clues. HBHA is an adhesin, which is ex- pressed by the micro-organisms of the M. tuberculosis complex. It largely con-
tributes to the extrapulmonary dissemi- nation of the tubercle bacilli. Hence, HBHA may be qualified as an important virulence factor. Furthermore, vaccina- tion of mice with three doses of only 5 µg HBHA each affords the same level of protection as vaccination with BCG. In humans, peripheral blood mononuclear cells (PBMC) from LTBI subjects secrete significant levels of IFN-γ in response to HBHA, whereas PBMC from TB patients do not. This discrepancy may be a cor- nerstone in the understanding of some of the mechanisms underlying the im- mune escape mediated by M. tubercu- losis. HBHA is a methylated protein, and the methylation is crucial for its immu- no-protective properties.
This work focused on major issues of the HBHA-specific immune response in humans: the use of the peripheral IFN-γ secretion in response to HBHA as a di- agnostic tool for LTBI and the analysis of the underlying mechanisms to the low IFN-γ secretion during active TB.
In our study, the measurement of HBHA- specific IFN-γ secretion resulted in the detection of more than 90 % of the tu- berculin-skin-test (TST) positive LTBI.
Strikingly, the QuantiFERON TB Gold
IT (QFT-IT), a commercial test, failed to
identify those LTBI subjects in more
than 50 % of the cases. Therefore, we
cannot recommend the use of QFT-IT
alone instead of the TST for the detec-
tion of LTBI. Similarly, a test relying on
the detection of IFN-γ secretion upon
ESAT-6 stimulation, one of the antigens
used in the QFT-IT, was not sufficiently
sensitive for the LTBI detection, nor did
it improve the sensitivity or the specifi-
city of the HBHA-based test. In contrast
to the diagnosis of LTBI, the tests based
on HBHA- or ESAT-6-induced IFN-γ se- cretions displayed poor sensitivity for the diagnosis of active TB.
During active TB, the HBHA-specific IFN-γ secretion in the periphery is low.
However, this weak secretion is revers- ible upon effective treatment, as the IFN-γ response to HBHA is increased af- ter completion of chemotherapy. This is strongly suggestive of an immune sup- pression during active disease. There- fore, we have first evaluated the role of two immunomodulatory cytokines, interleukin-10 (IL-10) and Transform- ing-Growth-Factor-Beta (TGF-ß), in the suppression of the HBHA-specific IFN-γ secretion. We found that neutralization of neither IL-10 nor TGF-ß with specific antibodies induced HBHA-specific IFN-γ secretion by PBMC of TB patients in vit- ro. In contrast, depletion of regulatory T cells (Treg) that express major markers, CD5, a constituent of the IL- receptor, and Foxp, a master regulatory gene, resulted in increased HBHA-specific IFN- γ secretion by the PBMC of TB patients.
These cells are known to be involved in the suppression of immune responses to both Self and non-Self antigens. We fur- ther show that the size of the peripheral Treg cell population increases during active disease. In addition to suppress- ing the HBHA-specific IFN-γ secretion these cells suppress T cell proliferation in response to HBHA in vitro. However, even after depletion of the Treg cells, the uncovered HBHA-specific immune responses are not directed to the meth- ylated epitopes during TB disease.
Finally, we show that Treg cells can be induced (or expanded) from the PBMC of LTBI subjects. Stimulation of those PBMC with BCG in the presence of TGF-ß
resulted in a quick appearance of func- tional Treg cells in vitro. This observation strongly suggests a role of Treg cells in the pathogenesis of TB, in particular in the progression of latency to reactiva- tion. Interestingly, excessive concentra- tion of TGF-ß, associated with various clinical conditions, is high risk factor for post-primary TB. Thus, Treg cells may result in the loss of immune control against latent M. tuberculosis infection.
Therefore, Treg cells may represent po-
tential therapeutic targets during M. tu-
berculosis infection.
I. Introduction
A s an old foe of the mankind, tubercu- losis (TB) has long been known un- der the names of schachepheth, phtisis , consumption and white plague (figure 1). Circa 189, Johann Lukas Schönlein proposed the terminology “tuberculo- sis” (TB) by referring to the potato-like aspect of the lesions observed during active disease (). TB has often been described through graphic representa- tions of individuals with dorsal spine de- formities (Egyptian artworks, circa 4000 B.C) and, through detailed descriptions of phthisis such as those found in the Hippocratic Collection (“Of the Epidem- ics”, around 400-50 B.C), in the Aretae- us and in the Galen’s writings (circa 150 A.D.).
Today, studies on ancient DNA of micro- bial pathogens have shown that TB was widespread in ancient Egypt and Rome and that Mycobacterium tuberculosis, the causal agent, existed in America and in Borneo well before Columbus.
M. tuberculosis DNA was found in North America in a bison metacarpal bone that was about 17,500 years old (). This latter discovery has been interpreted as evidence for possible transmission of TB from cattle (M. bovis) to humans (M.
tuberculosis) during domestication of cattle circa 8,000 to 15,000 years ago.
However, this assumption has recently been questioned, because M. tubercu-
losis presents chromosomal segments that are absent in M. bovis (4), suggest- ing that M. tuberculosis is actually more ancient than M. bovis. In any case, these historical scientific findings underline the exceptional co-evolution between M. tuberculosis and the hominidis. Dur- ing this long period of dynamic co-evo- lution, selective pressures most certain- ly influenced both the host (human) and the pathogen (M. tuberculosis), forcing the adversaries to negotiate compro- mises. Hence, M. tuberculosis has devel- oped particular survival mechanisms and virulence factors. In contrast, the host has elaborated counteracting host defence mechanisms. However, by now, M. tuberculosis has not (yet) completely succeeded in silently infecting all of us, but we have not fully succeeded in con- trolling it either (5).
During centuries, TB has plagued man- kind and affected some of Europe’s most creative individuals, such as Fred- eric Chopin, Albert Camus, Franz Kafka or Edvard Munch. Rational scientific ap- proaches to study and combate TB were developed essentially since the XIX
thcentury. Successive works were per- formed by honourable scientists such as Benjamin Marten, René T.H. Laënnec, Gaspard L. Bayle, Jean Antoine Villemin, Clemens Von Pirquet, etc... (figure 1).
“Mycobacterium tuberculosis is the most pervasive, morbid, and lethal microbial pathogen of humans. From the equator to the polar circles, on all seven continents, males and females, young and old of all races are at risk from this debilitating, disabling, and potentially fatal infec- tion”
M.D. Iseman, 2000 (1).
History
In 188, Robert Koch first identified tu- bercle bacillus “in large numbers, es- pecially at the edge or large, cheesy masses” in all tissues in which “the tu- berculosis process has recently devel- oped and is progressing most rapidly”
(6). The pathogen unmasked, numerous studies were (and are still) conducted in multiple scientific disciplines (i.e.
microbiology, immunology, pharmacol- ogy, etc…). All these efforts are carried on with the most optimistic hopes and believes in the potential control of this dreadful disease.
Next page : figure 1.
+/- 3500 B.C. 2698 B.C.Chinese writings very likely referencing to TB. 1500 B.C. 460-375 B.C. +/- 354-322 B.C.
Indo-Aryan Hindu writings very likely referencing to TB. Hippocratic Collection: detailed description of the various presen- tations of phtisis. However, Hippocrates does not propose a common etiology for this disorders. At this time, it is already recommended to cure TB by mountainous retreats. 1483-1553 A.D.Girolamus Frascatorius writesDe Contagioni and reinforces strongly the concept of contagion which is responsible of disease transmission. 1674-1676 A.D.Anthony Van Leeuwenhoek invents the microscope and describes the existence of protozoa and bacteria. 1865 A.D.Jean Antoine Villemin inoculates rabbits with material issued from tuberculous human tissue and demonstrates the transmissible character of the disease but did not isolate the infectious agent. He also characterizes the epidemiologic features of TB. 1821-1902 A.D.Rudolf Virchow describes the presence of lymphocytes and epithe- loid monocytes within the tubercle. 1859 A.D.Hermann Brehmer opens the first sanatorium for TB patients. 1941-1944 A.D.Jorgen Lehmann proposes the para-aminosalicylate (PAS) in order to treat 2 children with osseous and soft tissue TB. This event marks the begining of the TB chemotherapy.
1906 A.D.Clemens Von Pirquet uses the intradermal injection of tuberculin in order to detect the M. tuberculosis infection. 1909-1921 A.D.Albert Léon Charles Calmette and Jean-Marie Camille Guérin: focus on a vaccine made from successive passages of M. bovis. 1944-1945 A.D.Selman Waksman and Albert Schatz: isolation of the streptomycin from cultures of S. griseus and demonstration of its antituberculous efficicency. 1952 A.D.Gerhard Domagk: identification of a singularly potent compound : the isoniazid (isonicotinic acid hydrazide). 1998 A.D.The sequence of the genome of the most widely used strain in research,M. tuberculosis H37Rv, is completely defined. Last decadeThe number of scientific works on TB is growing exponentially in order to increase accuracy in the fields of microbiology, pathogen- esis, diagnosis, treatment and vaccination. 1822 A.D.M. bourru and James Carson introduce the collapse therapy to cure TB before the era of chemotherapy.
1704-1782 A.D.Benjamin Marten publishesA new theory of Consumptions more escpecially of a phtisis or consumtion of the lungs that anticipate in thoughts Pasteur, Villemin and Koch: proposition of “wonderfully minute living creatures” that are responsible of disease.
+/- 1000 B.C.Ancient Israelites do reference to TB (referenced as schachepheth, maqaq or razon) as a wasting disease in the Old Testament. No reliable distinction from other wasting diseases. +/- 150 B.C.Aretaeus, the Capodocian: fine descriptions of the ultime evolution of the consumption (from Latin consumptio meaning “wasting”) +/- 500 A.D.Dark Ages - Clovis the Franc: Royal Touch to heal scrofula.
After observation of the distribution of phtisis, Aristotle believes that something in the air is responsible for disease.
Probable or confirmed TB identified in egyptianmummies from the Nile River Valley. +/- 131-201 A.D.Galen, the Pergamite proposes a rational approach to cure TB: early recognition, rest, a rich diet and gentle cough suppression. 1514-1564 A.D.Andreas Vesalius writes De humani Corporis Fabrica which gave the bases for modern anatomical science.Antonio Benivieni launches the modern discipline of pathology: description of disseminated adenopathies during TB. 1614-1737 A.D.Sylvius and later, Pierre Desult describe the nodular lesions and referred it as “tubercle”. It is also proposed that illness was contagious by sputum.Thomas Willis proposes a more unified conceptualiza- tion of consumption. He had described TB as a systemic illness that is notably responsible of gross lung destruction. Richard Morton publishesPhthisiologia in 1689 where correlation are made between necropsy findings and clinical manifestations. 1882 A.D.Robert Koch bring into focus some sensitive staining methods of rods within tuberculous material. This allows the first identification of the causal agent of TB. Koch also proofs the causal relationship between these stainable objects and the disease TB by several inoculation experiments. These wonderful experiments and observations allow considerable advances in the fied of TB.
1774-1826 A.D.Gaspard Laurent Bayle andRené Théophile Hyacinthe Laënnec perform wonderful autopsy studies. Fine correlations with antemor- tem histories of the illness are established. Laënnec bring together the diverse forms of tubercles in a single disorder and greatly contributed to the antemortem diagnostic of TB notably by his extensive semiological knowledge of TB and by the invention of the sthetoscope.
source : nanobiomed.de HippocratesAristotleGalenA. Vesalius Chest X-ray illustrating sequelae due to collapse therapy source : www.surgical-tutor.org.uk
R.T. Laënnec R. KochC. CalmetteC. Guérin S. WaksmanG. Domagk
J.A. VilleminR. Virchow
+/- 3500 B.C. 2698 B.C.Chinese writings very likely referencing to TB. 1500 B.C. 460-375 B.C. +/- 354-322 B.C.
Indo-Aryan Hindu writings very likely referencing to TB. Hippocratic Collection: detailed description of the various presen- tations of phtisis. However, Hippocrates does not propose a common etiology for this disorders. At this time, it is already recommended to cure TB by mountainous retreats. 1483-1553 A.D.Girolamus Frascatorius writesDe Contagioni and reinforces strongly the concept of contagion which is responsible of disease transmission. 1674-1676 A.D.Anthony Van Leeuwenhoek invents the microscope and describes the existence of protozoa and bacteria. 1865 A.D.Jean Antoine Villemin inoculates rabbits with material issued from tuberculous human tissue and demonstrates the transmissible character of the disease but did not isolate the infectious agent. He also characterizes the epidemiologic features of TB. 1821-1902 A.D.Rudolf Virchow describes the presence of lymphocytes and epithe- loid monocytes within the tubercle. 1859 A.D.Hermann Brehmer opens the first sanatorium for TB patients. 1941-1944 A.D.Jorgen Lehmann proposes the para-aminosalicylate (PAS) in order to treat 2 children with osseous and soft tissue TB. This event marks the begining of the TB chemotherapy.
1906 A.D.Clemens Von Pirquet uses the intradermal injection of tuberculin in order to detect the M. tuberculosis infection. 1909-1921 A.D.Albert Léon Charles Calmette and Jean-Marie Camille Guérin: focus on a vaccine made from successive passages of M. bovis. 1944-1945 A.D.Selman Waksman and Albert Schatz: isolation of the streptomycin from cultures of S. griseus and demonstration of its antituberculous efficicency. 1952 A.D.Gerhard Domagk: identification of a singularly potent compound : the isoniazid (isonicotinic acid hydrazide). 1998 A.D.The sequence of the genome of the most widely used strain in research,M. tuberculosis H37Rv, is completely defined. Last decadeThe number of scientific works on TB is growing exponentially in order to increase accuracy in the fields of microbiology, pathogen- esis, diagnosis, treatment and vaccination.
1822 A.D.M. bourru and James Carson introduce the collapse therapy to cure TB before the era of chemotherapy.
1704-1782 A.D.Benjamin Marten publishesA new theory of Consumptions more escpecially of a phtisis or consumtion of the lungs that anticipate in thoughts Pasteur, Villemin and Koch: proposition of “wonderfully minute living creatures” that are responsible of disease.
+/- 1000 B.C.Ancient Israelites do reference to TB (referenced as schachepheth, maqaq or razon) as a wasting disease in the Old Testament. No reliable distinction from other wasting diseases. +/- 150 B.C.Aretaeus, the Capodocian: fine descriptions of the ultime evolution of the consumption (from Latin consumptio meaning “wasting”) +/- 500 A.D.Dark Ages - Clovis the Franc: Royal Touch to heal scrofula.
After observation of the distribution of phtisis, Aristotle believes that something in the air is responsible for disease.
Probable or confirmed TB identified in egyptianmummies from the Nile River Valley. +/- 131-201 A.D.Galen, the Pergamite proposes a rational approach to cure TB: early recognition, rest, a rich diet and gentle cough suppression. 1514-1564 A.D.Andreas Vesalius writes De humani Corporis Fabrica which gave the bases for modern anatomical science.Antonio Benivieni launches the modern discipline of pathology: description of disseminated adenopathies during TB. 1614-1737 A.D.Sylvius and later, Pierre Desult describe the nodular lesions and referred it as “tubercle”. It is also proposed that illness was contagious by sputum.Thomas Willis proposes a more unified conceptualiza- tion of consumption. He had described TB as a systemic illness that is notably responsible of gross lung destruction. Richard Morton publishesPhthisiologia in 1689 where correlation are made between necropsy findings and clinical manifestations. 1882 A.D.Robert Koch bring into focus some sensitive staining methods of rods within tuberculous material. This allows the first identification of the causal agent of TB. Koch also proofs the causal relationship between these stainable objects and the disease TB by several inoculation experiments. These wonderful experiments and observations allow considerable advances in the fied of TB.
1774-1826 A.D.Gaspard Laurent Bayle andRené Théophile Hyacinthe Laënnec perform wonderful autopsy studies. Fine correlations with antemor- tem histories of the illness are established. Laënnec bring together the diverse forms of tubercles in a single disorder and greatly contributed to the antemortem diagnostic of TB notably by his extensive semiological knowledge of TB and by the invention of the sthetoscope.
source : nanobiomed.de HippocratesAristotleGalenA. Vesalius Chest X-ray illustrating sequelae due to collapse therapy source : www.surgical-tutor.org.uk
R.T. Laënnec R. KochC. CalmetteC. Guérin S. WaksmanG. Domagk
J.A. VilleminR. Virchow
skin test and therefore, are believed to have successfully eliminated the patho- gen (9). Thanks to the major immuno- logical advances from 1954 until now, we may infer that this proportion was probably overestimated because TST presents several drawbacks (see below).
In addition, the number of infected in- dividuals varies with time of exposure, inoculum size, evolution of the disease within the index patient, mycobacterial strain characteristics, including infectiv- ity and virulence, etc… With respect to these observations, it seems reasonable to conclude that initiation of M. tuber- culosis infection is not always successful among humans without knowing exact proportions of effective infections (10).
Within distal airways, M. tuberculosis preferentially infects alveolar macro- phages. Infected macrophages sub- sequently attract additional actors of the natural immunity such as polymor- phonuclear cells (see below). At that time, only a small amount of exudate and few neutrophils surround the in- fectious focus. Within the following few days, M. tuberculosis multiplies, and macrophages collect and ingest the increasing number of tubercle ba- cilli. Progressively, these macrophages tend to aggregate within nodules that modify the alveolar environment. This step is characterized by a modification of the massively infected macrophages, their cytoplasm becoming eosinophilic.
These cells are referred to as “epitheloid cells”. This epitheloid transformation is associated with enhanced bactericidal functions but also contributes to tissue necrosis. Weeks thereafter, central mac- rophages fuse to constitute multinucle- ated giant cells, the “Langhans giant
Natural history of M. tu- berculosis infection
Primary infection
In 1907, Ziesche collected (…) the droplets on Petri dishes placed at vari- able distances in front of phtysic pa- tients. He noticed that 30 to 40 % of the phtysics threw out bacilli up to 80 cm. He even counted the bacilli, which were, in 80 % of cases, less than 400 per half-hour. However, it could in- crease up to 20.000.
Péhu and Dufourt, 1927. (7) Primary infection mainly occurs upon inhalation of aerosolised micro-drop- lets (“droplet nuclei”; ≤ 5 µm in diam- eter) containing the bacillus. Between five and two hundred inhaled bacilli are sufficient to establish an infection (fig- ure , reproduced from reference (8)).
The droplet nuclei are generated in the airways of a TB patient and aerosolised when coughing, sneezing or speaking.
After penetration into the respiratory tree, M. tuberculosis has to avoid the mechanisms of mucociliary clearance from the central airways. Finally, M.
tuberculosis colonizes the sub-pleural
alveolar space. This colonization pref-
erentially affects the middle and lower
lobes of the lung, because they are the
best-ventilated area. This seeding si-
lently overrules natural immunity which
otherwise can eliminate the pathogen
in a significant proportion of cases. Ac-
cording to a publication from Shaw J.B
and Wynn-Williams N. in 1954, 50 to
75 % of exposed cases remain tuberculin
skin test (TST) negative upon tuberculin
Figure 2. Transmission of tuberculosis and progression from latent infection to reactivation tuberculosis.Once infected, most individuals (about 95 %) control their infection by effective immune responses. This stage is called latent tuberculosis, and the only clinical sign is a positive tuberculin skin test. Few infected adult subjects (5 %) further develop active disease during the two first years following the infection (mostly in case of immunodeficiency). Importantly, about 5 % of the latently infected subjects will reactivate their infection and subsequently develop active tuberculosis. At any time of infection, impaired immunological responses are associated with increased risk of disease (i.e. human immunodeficiency virus infection).
Reproduced from: Small PM, Fujiwara PI. Management of tuberculosis in the United States.The New England Journal of Medicine 001;45:189-00.
cells”. Surrounding the core of necrosis, macrophages, epitheloid and Langhans giant cells, a layer of lymphocytes is progressively constituted. This localized concentration of cells constitutes the tu- berculous granuloma (figure ).
During the following weeks, as the an- tigen-specific T cell responses have matured, the granuloma has grown suf- ficiently to become visible to the naked eye as a small, grey nodule. By a proc- ess of necrosis, the centre of this grow- ing tubercle then turns pale yellow. As growth persists, several adjacent tuber- culous granulomas merge together. This area of primary infection is referred to as “Ghon focus” which is a characteris-
tic subpleural lesion ranging from a few millimetres to one or two centimetres in diameter (figure 4).
A few days after seeding in the sub-
pleural alveoli, the drainage of M. tu-
berculosis conveys the bacilli along the
peribronchial lymphatic vessels to the
hilar lymph nodes, where granuloma-
togenous reactions are initiated. Granu-
lomatous reactions within hilar lymph
nodes may be vigorous and tend to be
generally larger than the primary focus
in the lung. The combination of a Ghon
focus with the corresponding, enlarged
regional lymph nodes composes the
primary complex of Ranke (figure 4).
Macrophages Apoptotic
macrophage Extracellular M. tuberculosis
Giant cells Activated macrophages
/histiocytes
External fibrosis Lymphocytes
Lymphocytic layer
Histiocytes/Giant cells layer
Necrotic core
Figure 3. Schematic representation of the tuberculosis granuloma.From the outer to the central layers : external fibrosis, lymphocytic layer, histiocytes/giant cell layer and necrotic core (see text).
Once infection is settled, M. tubercu- losis rapidly reaches the bloodstream through lymphatic channels and pulmo- nary veins, as demonstrated by the use of radio-labelled tubercle bacilli in experi- mental animal models (11). Interestingly, further extrapulmonary dissemination is mediated by tight interactions of M.
tuberculosis with alveolar epithelial cells (1). Scattered phagocytes throughout the body quickly destroy most of the blood-borne tubercle bacilli. In spite of this elimination, some of the blood- borne bacilli will establish infection in other areas of the body. These areas are most frequently highly oxygenated or- gans such as the apical-posterior areas of the lungs, the central nervous system, the epiphyses and vertebral bodies, the kidneys and the adrenals. Of these areas, the apex of the lungs is one of the most sensitive areas of colonization after hae- matogenous dissemination. This vulner-
ability is thought to be partially linked to its poor ability to produce lymph in synergy with its decreased lymphatic drainage which, together, impede the elimination of bacilli from the summits of the lungs (1). Finally, these metastat- ic foci could progress to cavitary lesions or remain latent for years (see below). Of interest, other cells, such as adipocytes (14) and alveolar epithelial cells (15-17), are usually infected during this process of extra-pulmonary dissemination. This infection of non-professional phago- cytes is not systematically associated with granulomatous reactions and is therefore, often silent. The exact role of this phenomenon is currently unex- plained, but it could constitute a reser- voir that allows persistence of the myco- bacteria for long periods of time (14).
In most cases, prompt repair processes
are initiated by the host, and the pri-
mary infection remains subclinical. For
Alveolar macrophage
M. tuberculosis
Alveolus
Complete destruction No infection
( ? %)
M. tuberculosis proliferates within the AM
Killed AM Released bacilli
Newly recruited phagocytic cells
Hilar adenopathies
Primary lesion
Occult bacillemia and dissemination
II. EARLY PROLIFERATION
& SPREAD I. INITIAL ENCOUNTER
Early spread to apices of lungs
i.e. kidneys, vertebrae
IIIb. ACTIVE DISEASE (TB)
Inefficient control
Liquefaction, cavitation, bacillary proliferation, endobronchial & extrapulmonary spread 8-12 wks
Involution of primary lesions IIIa. LATENT INFECTION
(LTBI)
TST+
Figure4.PathogenesisanddevelopmentofimmuneresponsesduringinfectionwithM.tuberculosis.
(Seetext).
Adapted from : Iseman M. Immunity and pathogenesis. A clinician’s guide to tuberculosis, 1st edition. Ed : Lippincott Williams& Wilkins;000. p6-97.
instance, a fibrous capsule progressively walls off the primary complex. This cap- sule is composed of fibroblasts and col- lagen fibres and prevents spreading of surviving tubercle bacilli.
In parallel, the Ghon focus usually disap-
pears within several weeks or months.
Local eradication occurs sometimes. In
most cases, some of the disseminated
bacilli remain viable in a dormant stage
within the granulomas or other cell
types, including adipocytes, epithelial
cells, endothelial cells and fibroblasts, which are distributed throughout the body. Quiescent bacteria may persist without causing any symptom until a lowering of cellular immune responses.
This stage is referred to as latent TB infec- tion (LTBI). Even if poorly characterized, the loss of immune control will finally lead to reactivation of TB disease, which is referred to as post-primary TB. In some cases, the primary infection continues to progress, and TB disease rapidly occurs;
this is called (progressive) primary TB. In the absence of an appropriate immune responses, the disseminated bacilli will cause the so-called disseminated TB or miliary TB (figure 4).
The classical radiographic sequelae of a primary infection are nodules (eventu- ally calcified), fibrotic bands and scar- ring of the lung apices. On a chest X- ray, the presence of apical fibronodular shadowing is sometimes referred to as Simon foci. Unlike the other lesions, Si- mon foci are associated with significant risk of postprimary TB (0-fold higher risk of reactivation in comparison with normal chest X-ray) (18).
Latent tuberculosis infection
Once the primary infection is initiated, about 90 to 95 % of the TST+ subjects remain healthy throughout their life- time. This control is due to effective immune responses that keep the infec- tion in check. LTBI is clinically defined as
“the presence of a positive TST reaction in someone who is asymptomatic and who has no microbiological evidence of tuberculosis” (19). For immunologists, LTBI may be defined as an equilibrium between the host immune responses
and the growth of the micro-organism that results in containment of the pri- mary infection. Even if not contagious, LTBI subjects constitute the reservoir of the disease and, therefore, represent an important target population that has to be identified in order to fully control the disease.
Tuberculosis disease
Only 5 to 10 % of the M. tuberculosis-in- fected subjects will finally develop ac- tive TB disease after primary infection.
This low risk of active TB at a single-in- dividual level is probably a corner stone in persistence and dissemination of M.
tuberculosis, as about 1/ of the world’s population is infected (0). However, the risk of active disease is greatly increased when immune responses are depressed for any reason. Indeed, in comparison to healthy individuals devoid of risk factors, the relative risk (RR) to develop active TB increases dramatically in the presence of HIV infection (RR: 11) and AIDS (RR: 170), transplantation (RR: 0 to 74), diabetes mellitus (RR: .0 to .6), chronic renal failure (RR: 10.0 to 5.), silicosis (RR:0) or iatrogenic immunosuppression (RR:
11.9) (1). Malnutrition and nicotine ad- diction also represent risk factors with RR between .0 and 4.0.
The most common presentation of TB in adults is pulmonary TB (about 75 %).
However, extra-pulmonary TB, even if often associated with pulmonary le- sions, are more frequent if risk factors are associated (-5).
The outcome of untreated TB is poor,
because 49 % of the TB patients will die
within five years. Among them, 5 % will
0 20 40 60 80 100
Chronic, AFB positive Cured, AFB negative Dead
No treatment Irregular/Inadequate chemotherapy
Standardized chemotherapy
% of tuberculosis patients
18 %
33 %
49 %
30 %
60 %
10 %
2 %
90 %
8 %
Figure 5. Outcome of patients with pulmonary tuberculosis according to the given treatment.
Absence of anti-tuberculosis chemotherapy is associated with mortality in about half of tuberculosis patients. Importantly, about one fifth present chronic forms of disease and constitute the major source of contagion. The risk of contagion is greatly increased upon irregular/inadequate chemotherapy.
Adapted from Enarson D, Chiang C, Murray J. Global Epidemiology of tuberculosis. In : Rom W, Garray S, editors. Tuberculosis, second edition ed: Lippincott Williams & Wilkins; 004. p1-1.die within the first two years. In contrast, % of the TB patients will spontane- ously cure their disease. The remaining 18 % will develop chronic infections with positive sputum smears representing thus a source of contamination for other individuals. Fortunately, with the excep- tion of multi-drug-resistant strains (see below), adequate chemotherapy is suc- cessful in more than 90 % of the cases, so that the number of chronic evolution of TB infection is drastically reduced (figure 5) (6, 7).
Primary tuberculosis
Progressive primary TB occurs as a result of weak and/or ineffective cellular im- mune responses. Historically, such pro- gression most often occurred in young children who are considered to have immature cellular immune responses.
Nowadays, this rapid evolution of TB in- fection is more frequently observed in adults as a consequence of HIV infection.
However, in contrast with more classical evolutions, both AIDS and infancy are less frequently associated with the pres-
ence of pulmonary cavities (8, 9).
If the primary infection is inadequately controlled by the host, the tuberculosis granulomas continue to grow within the lung parenchyma or within the re- gional lymph nodes, and their size may reach several centimeters in diameter.
This expansion of the granuloma size may result in an erosion of the nearby anatomical structures like a bronchus or larger blood vessels, leading to the draining of the necrotic focus within the airway and/or through the blood circu- lation. It results in the appearance of a cavity, surrounded by the tuberculosis granulation tissue.
This drainage through the airways may be sudden and massive, resulting some- times in rapid asphyxia of the patient. If less important, it may induce cough and deep inspiration, contributing to the in- ter-individual spread of infection and, for the patient, to the dissemination of the infection to other lung segments.
The erosion of a bronchus by the granu-
lomas may also lead to a bronchus ob-
struction, with a secondary segmental
collapse. A similar situation may be the result of enlarged granulomatous hilar nodes that distort a major bronchus. Fi- nally, the erosion by granulomas of the pleural space may induce the appear- ance of a pleural effusion, a pneumot- horax, a tuberculous empyema or even a pyopneumothorax.
If large amounts of necrotic material are drained into the blood and/or lym- phatic vessels, miliary TB may develop, especially when erosion of the large veins of the mediastinum by enlarged hilar lymph nodes occurs. In this case, the amount of released tubercle ba- cilli is important, and the blood outflow within the blood vessels is high so that the phagocytic system is quickly over- whelmed and unable to eliminate all the bacilli. Miliary TB is characterized by an uncontrolled dissemination that can occur during primary infection or after reactivation of latent foci.
Post-primary tuberculosis
Post-primary TB may be due either to reinfection (exogenous origin), or to reactivation of dormant bacilli within a primary lesion (endogenous origin).
In high incidence areas, many cases of post-primary TB are probably due to ex- ogenous re-infection. In contrast, in low incidence countries, reactivation is the most common cause of post-primary TB and it mostly occurs among old people.
This reactivation generally concerns the subapical regions of the lungs, which are the most vulnerable sites. However, nearly 15 % of the reactivations occur at extra-pulmonary sites without apparent involvement of the lungs (14). This sug- gests that M. tuberculosis may reactivate
from an extra-pulmonary reservoir. As mentioned above, several risk factors of reactivation are now identified and among them, HIV infection, and, with some extensions, severe impairment of the immune system, are the most fre- quently encountered.
Whereas the Ghon focus of primary TB may occur in any lobe, the lesions asso- ciated with post-primary TB are mainly found in the lung apices. During reacti- vation, the tuberculous granulomas are growing with subsequent development of a central necrotic core. Coalescent granulomas finally fuse together and produce large areas of necrosis, that can be expulsed within the airways or the bloodstream. This process, already found in primary TB, is responsible for the appearance of more advanced le- sions, such as cavitations and caseous pneumonia. During long-lasting disease, large cavities may finally replace impor- tant areas of the lung parenchyma, so that the lung becomes unstructured with a diminished respiratory function.
In contrast to primary TB, the involve-
ment of the hilar lymph nodes is gener-
ally more limited in post-primary TB, so
that this form of TB is less frequently as-
sociated with complications secondary
to bronchus compression. In contrast,
massive haemoptysis may rarely occur
as a consequence of the rupture of a
Rasmussen’s aneurysm. These appear in
case of a rapid destruction of the blood
vessels by the caseous reaction, so that
no obliterative endarteritis occurs, and
the muscular and elastic layers of the
vessel are destroyed resulting in the ap-
pearance of an anevrysm.
Epidemiology
World-wide burden of tuberculo- sis
World-wide, every second an individual is infected with M. tuberculosis. Accord- ing to the World Health Organization (WHO), it is estimated that one third of the world’s population is infected with M. tuberculosis (0). During 006, the WHO had registered 1.7 million of deaths due to TB. Most of these deaths were declared in Africa demonstrating that TB mostly concerns the poorest people. In these high-burden regions, HIV is a major trigger allowing for a rapid increase in TB prevalence. Since 004, some stabilization of the general TB incidence was observed but unfortu- nately, this was not the case for African regions where the epidemic continued to grow in parallel with the HIV burden (see table n°1, reproduced from refer- ence (0)).
According to the EuroTB report for 004 (), 414,16 TB cases were notified in the 51 countries of the WHO Europe- an region, with a global incidence of 47/100,000. However, important differ- ences in incidence exist between the European Union/West (1.6/100,000), the Centre (50.7/100,000) and the East (104.7/100,000) regions. In the Central and West European Union, 81 % of the TB cases were defined as new cases (never treated or less than one month for TB).
Seventy-seven percent of reported TB cases in these areas had pulmonary disease and 4 % of them were sputum smear positive for acid-fast bacilli. The great discrepancies in TB incidences underline heterogeneity in effective control across the European Union. For instance, countries belonging to the former Soviet Union showed the high- est incidence among young adults, re- flecting the high transmission rate dur- ing these last years.
Table n°1 – Estimated incidence, prevalence and TB mortality, 2004
Incidencea Prevalencea TB mortality
All forms Smear (+)b
WHO region Number (%)c /100000 Number /100000 Number /100000 Number /100000
Africa 2573 (29) 356 1098 152 3741 518 587 81
South-East Asia 2697 (33) 182 1327 81 4965 304 535 33
Eastern
Mediterranean 645 (7) 122 289 55 1090 206 142 27
Western Pacific 1925 (22) 111 865 50 3765 216 307 18
Europe 445 (5) 50 199 23 575 65 69 7.8
The Americas 363 (4) 41 161 18 466 53 52 5.9
GLOBAL 8918 (100) 140 3939 62 14602 229 1693 27
a Incidence: new cases arising in given period; prevalence: the number of cases which exist in the population at a given point of time; b Smear-positive cases are those confirmed by smear microscopy, and are the most infectious cases; c Numbers are in thousand, % is based on global population and /100000 is referring to population
