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Review doi: 10.1111/joim.12031
Shortening the ‘short-course’ therapy– insights into host immunity may contribute to new treatment strategies for tuberculosis T. Sch€on1,2, M. Lerm2 & O. Stendahl2 From the 1Department of Infectious Diseases and Department of Clinical Microbiology, Kalmar County Hospital, Kalmar; and 2Division of Microbiology and Molecular Medicine, IKE, Faculty of Health Sciences, Link€ oping University, Link€ oping; Sweden
Abstract. Sch€ on T, Lerm M, Stendahl O (Kalmar County Hospital, Kalmar; and Link€ oping University, Link€ oping; Sweden). Shortening the ‘short-course’ therapy: insights into host immunity may contribute to new treatment strategies for tuberculosis (Review). J Intern Med 2013; 273: 368–382. Achieving global control of tuberculosis (TB) is a great challenge considering the current increase in multidrug resistance and mortality rate. Considerable efforts are therefore being made to develop new effective vaccines, more effective and rapid diagnostic tools as well as new drugs. Shortening the duration of TB treatment with revised regimens and modes of delivery of existing drugs, as well as development of new antimicrobial agents and optimization of the host response with adjuvant immunotherapy could have a profound impact on TB cure rates. Recent
Introduction In the Western world, tuberculosis (TB) was long considered a disease of the past, controlled by effective public health systems that compensated for the relative shortcomings of the available intervention strategies. However, due to the lack of effective vaccines, early biomarkers, rapid diagnostic tools and novel therapeutic agents, the incidence rate is still high, with 1.4 million deaths due to TB in 2010, mostly in resource-poor areas [1]. Of particular concern in sub-Saharan Africa is the high proportion of TB patients co-infected with the human immunodeficiency virus (HIV), further increasing morbidity and mortality. In most individuals, infection with the causative agent Mycobacterium tuberculosis (Mtb) remains latent without symptoms or transmission to others, but in approximately 5%–10% the infection progresses to active disease, killing many of those who are not diagnosed and effectively treated [2]. TB has become the focus of attention in many parts of
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data show that chronic worm infection and deficiencies in micronutrients such as vitamin D and arginine are potential areas of intervention to optimize host immunity. Nutritional supplementation to enhance nitric oxide production and vitamin Dmediated effector functions as well as the treatment of worm infection to reduce immunosuppressive effects of regulatory T (Treg) lymphocytes may be more suitable and accessible strategies for highly endemic areas than adjuvant cytokine therapy. In this review, we focus mainly on immune control of human TB, and discuss how current treatment strategies, including immunotherapy and nutritional supplementation, could be optimized to enhance the host response leading to more effective treatment. Keywords: macrophage, nitric oxide, nutrition, tuberculosis, vitamin D.
the world because of an alarming proportion of cases caused by drug-resistant bacteria. Nearly 50% of culture-positive patients in areas such as Belarus carry multidrug-resistant (MDR) TB, which is defined as resistance to the two most important antimycobacterial drugs isoniazid (INH) and rifampicin [3]. Shortening the duration of TB treatment to increase compliance, development of new antimicrobial agents and treatment strategies to optimize the host response could all have a profound impact on the TB cure rate and the control of the infection, in particular as drug resistance is rapidly increasing. Considerable effort is now being made to develop new vaccines, but how bacteria surviving inside phagocytic cells can delay initiation of an effective immune response and how they can be detected and eliminated more efficiently remain to be determined. Mycobacterium tuberculosis is the most successful human pathogen known, as indicated by its ability to infect up to one-third of the global population.
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The spread of the disease has not been limited despite the use of the Bacillus Calmette-Gu erin (BCG) vaccine for more than 90 years. Mtb has achieved such success through evolving multiple mechanisms to avoid destruction and adapt to the host. Recent advances in understanding of the delicate balance between the host and Mtb have been reviewed extensively [4–7]. Despite increased knowledge of host defence mechanisms against Mtb, the optimal innate or cell-mediated immune (CMI) response to achieve immunity is unknown [5, 8]. It is evident that human TB is not as homogenous from the host response perspective as it is under strict experimental conditions using laboratory strains of Mtb [9–12]. Animal models are essential for understanding host immunity, especially in the early stages of innate response [13]. However, it is relatively difficult to translate preclinical laboratory findings into novel treatment and diagnostic strategies. From the clinical perspective, the importance of the CMI response is confirmed by several important observations. First, in HIV-infected patients, there is 10% yearly risk of developing TB compared with a 5% lifetime risk in immunocompetent individuals [14]. Secondly, during treatment with tumour necrosis factor (TNF)-a inhibitors or other immunosuppressive drugs such as corticosteroids, there is an increased risk of developing active TB amongst latently infected individuals [14–17]. Moreover, individuals with decreased expression of T helper (Th)1-related cytokines, due to mutations in the genes of the interleukin (IL)-12/IL-23/ interferon (IFN)-c pathway are susceptible to mycobacterial infections [18–20]. Thus, the CMI response against TB plays a fundamental protective role against the development and outcome of active disease. The most important areas to consider to achieve global TB control are: (i) improvement of vaccine efficacy; (ii) development of more effective and rapid diagnostic tools; (iii) establishment of shorter treatment regimens; and (iv) development of strategies for identifying patient groups at risk of developing active disease. Shortening the duration of TB treatment through revised regimens and/or modes of delivery of existing drugs, development of new antimicrobial agents and optimizing the host response via adjuvant immunotherapy could greatly improve TB cure rates. Of note, adjuvant cytokine immunotherapy is unlikely to be accessible for the majority of patients in highly endemic
Review: New treatment strategies for tuberculosis
resource-poor areas. The priorities in such areas should be eliminating poverty, optimizing immunity by nutritional supplementation and treating HIV. In this review, we will mainly focus on the immune control of human TB, and discuss how the current treatment strategies could be optimized through the understanding of innate and adaptive immunity during TB infection. Immunological ‘checkpoints’ of Mtb infection The heterogeneity of human susceptibility to TB can be demonstrated by the L€ ubeck disaster in 1929 [21, 22] when 251 infants were accidentally challenged with an oral dose of virulent Mtb instead of the attenuated strain used in the BCG vaccine; 72 children died within 1 year, 135 developed TB but recovered and 44 became tuberculinpositive but remained well. The heterogeneity of human susceptibility to TB is also evident from studies of close (living in close household contacts) contacts of TB cases. Even after prolonged exposure, only 5%–10% of close contacts of patients with smear-positive TB develop active disease [23]. It has been estimated that up to 50% develop latent TB as indicated by a positive tuberculin skin test [23]. However, a fraction of exposed individuals do not show any signs of induction of systemic immunity to TB, indicating sufficient protection by innate immune mechanisms [13]. This fact has long been disregarded, but knowledge of innate immune functions emerging during the last decade may explain how the host can clear or control TB infection without involvement of the adaptive immune response. Much of the present understanding of host immunity is based on studies using inbred mouse strains. It is clear that there will be little variation in the observed immune response of the infected animals in such studies, with an initial increase in bacterial load in the tissues until the onset of adaptive immunity [24]. However, recent studies in outbred monkeys [25] and rabbits [26] have shown that the heterogeneity of human TB may be better demonstrated using these models. In a study in macaques, it was shown that within the same TBinfected animal, a spectrum of different lesions is present, although with less advanced lesions in tissues from animals with latent TB [27]. On the basis of recent observations, we hypothesize that Mtb has to pass immunity ‘checkpoints’ to establish infection (Fig. 1) [27]. The time it takes for ª 2013 The Association for the Publication of the Journal of Internal Medicine Journal of Internal Medicine, 2013, 273; 368–382
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Review: New treatment strategies for tuberculosis
Checkpoint 1
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Time Fig. 1 The ‘checkpoint’ model of host immunity against TB. (1) Eradication of the initial infection by an efficient innate immunity. (2) Innate immunity may not be sufficient to control further dissemination of Mtb to distant foci. (3) Adaptive immunity could control the infection which is maintained in a latent phase. (4) Adaptive immunity may clear the infection in some individuals. (5) Inadequate control of replication, and therefore Mtb may be detected in cultures in the absence of clinical symptoms. (6) Uncontrolled Mtb replication leads to symptomatic TB. (7) TB mortality is high without treatment, in particular amongst patients co-infected with HIV. (8) The bacillary load rapidly decreases during antimycobacterial treatment. (9) In a minority of treated patients, viable bacilli persist and cause relapse. To prevent progression to active TB, there are potential critical time-points for intervention (I1–I4). (I1). Innate immune mechanisms could be enhanced by nutritional supplementation (vitamin D or arginine). (I2) Eradication or control of the infection could be achieved by inhibiting factors that inhibit the Th1 pathway, for example by treating HIV and chronic helminth infections. Strengthening Th1 responses could be achieved by adjuvant cytokine therapy or nutritional intervention. (I3) In the presence of active disease, antimycobacterial chemotherapy is superior to interventions targeting immunity. Antibiotic therapy could be optimized by measuring drug levels and minimal inhibitory concentrations of Mtb according to the concepts of therapeutic drug monitoring. In patients with extensive inflammation and tissue damage, anti-inflammatory drugs could be of benefit. (I4) Bacterial load rapidly decreases during the first weeks of antimycobacterial treatment. At this stage adjuvant cytokine therapy and nutritional interventions could optimize a Th1-directed response and immune control could be regained.
the bacterium to pass the checkpoints, if it does, varies greatly between individuals ranging from a few weeks in patients with an impaired immune response, such as those infected with HIV, to decades in initially resistant hosts. To pass the first checkpoint, Mtb must avoid being killed by the infected phagocytes, including neutrophils, macrophages and dendritic cells (DCs), of which macrophages are the primary target cells. It is likely that the interaction between these cells plays a key function in the early clearance of Mtb. However, Mtb has developed strategies to counteract innate immune mechanisms such as phagosomal maturation [28, 29] and intracellular killing. Apoptosis is also inhibited [30] and partly replaced by host detrimental necrosis [31, 32]. Failure of macrophages to eradicate the initial infection allows progression towards the next checkpoint 370
ª 2013 The Association for the Publication of the Journal of Internal Medicine Journal of Internal Medicine, 2013, 273; 368–382
and the spread of infection to distal sites [33]. Such failure may be due to deficient phagolysosomal fusion or autophagy [34], impaired production of antimycobacterial peptides such as LL-37 or a-defensins [35] and other antimycobacterial compounds such as nitric oxide (NO). Of note, Mtb can be present in apparently normal lung tissue, indicating that the bacterium is able to coexist with its host without causing tissue damage and inflammation [36]. The mechanisms by which macrophages are able to maintain a persister state of Mtb need further investigation, albeit acidification of phagosomes and phagolysosomal protease activity have been determined to play a role [29]; NO, which is produced by human macrophages at sites of Mtb infection, is also thought to be important [37]. There is evidence that Mtb, if exposed to suboptimal doses of NO, can respond to this stressor by switching to an NO-tolerant phenotype [38].
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The second checkpoint is passed when innate immunity fails to control the infection, allowing Mtb to start replicating to an extent that adaptive immunity is utilized. Enhanced inflammation caused by necrosis of infected host cells harbouring replicating bacteria is probably needed for the induction of antigen presentation to induce both cytotoxic T cell activity and an effective Th1 response to enhance macrophage activity via production of IFN-c [39]. Depending on host immunity and bacterial factors, the infection may progress to active disease. However, latent TB is observed in individuals who can control the infection by an effective adaptive immune response. It is also possible at this stage that the infection may be completely cleared. This is clinically important, as traditional tests for latent TB such as interferon IFN-c assays measure the CMI response to Mtb, but do not determine whether the bacteria are still viable [2]. Progression to active disease occurs, due to insufficient immune control, when the third and final checkpoint is reached. Excessive inflammation contributes to the pathology; there is evidence that Mtb takes advantage of immune activation leading to necrosis [31], which is a prerequisite for the bacterium to spread to other individuals. Draining of the bacilli-containing necrotic cores of caseating granulomas or cavities into the bronchial tree is the major route by which Mtb is transmitted. A recent study of antigen variability in Mtb showed that in contrast to many other pathogenic microorganisms, which rely on antigen variability to evade specific immunity, Mtb has highly conserved antigen epitopes. This finding implies that at a certain stage of infection, Mtb manipulates the host immune response to take advantage of T cell-mediated immunity [40]. Necrosis that is thus generated through host immunity is the platform from which Mtb can spread in the human population.
Review: New treatment strategies for tuberculosis
immunized [23] suggests that several inherent and innate immune mechanisms are active before checkpoint 2. Alveolar macrophages represent the primary niche for replication of the bacillus, and macrophage functions such as phagosomal acidification [29] and autophagy [41] are important mechanisms by which these cells can control Mtb. The innate immune response to Mtb is triggered by several pattern recognition receptors, primarily Toll-like receptors (TLRs) 2 and 9 [7]. Also, cytosolic innate receptor complexes, such as NOD2 and NALP3, recognize Mtb molecules and may affect phagosomal maturation and autophagy [7, 34]. A proinflammatory cytokine and chemokine response is mounted [7], recruiting neutrophils, DCs and macrophages to the site of infection. Recruited neutrophils may have several important functions during the early phases of infection. Initially, they may be beneficial to the host through phagocytosing and killing virulent Mtb via Nadph Oxidase 2 (Nox2)-generated reactive oxygen species (ROS) and other antimicrobial molecules (defensins, LL-37) [35], but also offer a niche that favours bacterial survival and growth. During this process neutrophils become apoptotic. Because apoptotic cells are cleared from the tissue by macrophages, Mtb-containing apoptotic neutrophils can be transferred to the macrophages and mount a proinflammatory response [42, 43]. This can also occur in DCs, where apoptotic neutrophils can facilitate DC maturation and augment subsequent Th cell activation [44]. A well-recognized virulence-associated attribute of Mtb is its ability to impair apoptosis in macrophages [45, 46], as well as in neutrophils, thereby delaying the adaptive immune response [47] and forming an intracellular environment to survive and proliferate.
Details of the immune mechanisms that can be targeted to optimize host immune responses, according to the checkpoint model, will be discussed below. Through interventions targeting the immune system, Mtb could thus be prevented from passing checkpoint 3, allowing more efficient clearance of infection.
Although the early innate immune reaction can be seen as an infectious stage dominated by bacterial proliferation and spreading, it can also lead to a proper adaptive immune response in individuals in whom innate immunity is not limiting the infection (e.g. after checkpoint 1 in the model shown in Fig. 1). It has recently been suggested that neutrophils can form a link between innate and adaptive immune activation during the early phase of TB infection, by enhancing both a proinflammatory macrophage response and DC maturation [42].
Innate immune mechanisms during TB infection
Targeting host immunity for prevention and cure of TB
The fact that many individuals heavily exposed to Mtb do not develop infection despite not being
The rationale for treatment strategies that aim to strengthening the host immune response during ª 2013 The Association for the Publication of the Journal of Internal Medicine Journal of Internal Medicine, 2013, 273; 368–382
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TB is to reduce the risk of disease progression upon exposure and to optimize the treatment of active TB by stimulating the host immune response (Fig. 1). The timing for such interventions is important and should be considered carefully in relation to the immunological life cycle of Mtb. In terms of our model, this means the infection will be stopped at checkpoint 1 and extensive tissue damage prevented (Fig. 1). Interventions based on enhancing host immunity may cover both prevention and active disease such as HIV and chronic worm infection as well as optimizing nutritional status in the case of vitamin D and arginine supplementation. Such strategies can be cost-effective in reducing the risk of developing TB and preventing development of severe active disease in malnourished and HIV-infected individuals. Nutritional supplementation in the treatment of TB Before the availability of chemotherapy, nutritional supplementation was an important part of care in a Sanatorium, although the relative contribution of food supplementation to the cure of TB during this time is controversial [48, 49]. As care, diet and sanitation improved during late 19th and early 20th century in countries that today have a relatively low incidence of TB, the disease burden declined even before the introduction of the BCG vaccination and the development of antibiotics, underlining the importance of social factors and nutrition in reducing the spread of TB [50]. In a large epidemiological study, being underweight (body mass index
