A brief global overview…
II. Rationales & objectives
T
uberculosis kills about million indi-viduals per year and remains there-fore a major public health challenge (0).The endemic character of this infection may be explained by several factors : 1) the difficulties to establish a rapid and accurate diagnosis of M. tuberculosis in-fection; ) the limited efficacy of the BCG vaccine; and ) the restricted efficiency of the anti-TB chemotherapy.
The detection of M. tuberculosis infec-tion requires either the demonstrainfec-tion of the presence of M. tuberculosis itself within the biological samples, essen-tially for the diagnosis of active disease, or the presence of M. tuberculosis-spe-cific immune responses, essentially for the detection of LTBI. Unfortunately, the sensitivity of the smear test is usually low and the culture generally requires several weeks to become positive (see introduction). In addition, the interpre-tation of the TST is difficult in endemic areas (and/or when performing such test on immigrants coming from high burden countries) due to the high prev-alence of MOTT infections and the sys-tematic BCG vaccination in many coun-tries (86). Moreover, TST does not allow to establish a diagnosis of active disease (87). Therefore, new diagnostic tests are urgently needed for the diagnosis of M. tuberculosis infection and for the dif-ferential diagnosis between latent and active TB.
Although about one third of the world’s population is infected with M. tuberculo-sis, fortunately only 5 to 10 % of these in-dividuals will develop active TB through-out their life. This suggests that immune responses induced by the infection are able to prevent the disease in most cases (88). Therefore, characterization
of the immune responses from M. tuber-culosis-infected individuals, including the differences between LTBI and active TB, should allow us to identify of the parameters correlated with protection against TB disease. These efforts will lead to: 1) the identification of new im-muno-prophylactic strategies which are more effective than BCG vaccination, ) the development of new diagnostic tests for early detection of M. tuberculo-sisinfection and ) the development of immune tests that distinguish LTBI from TB disease. The WHO, underlining their importance, classified these aspects as absolute priorities in the fight against TB.
The identification of new immuno-pro-phylactic strategies and the distinction of LTBI from TB disease require the se-lection of read-outs that faithfully re-flect the protective immune responses against M. tuberculosis. The microbicidal and cytotoxic activities expressed by immune cells against M. tuberculosis are reliable markers of protection (89), but these tests are labour intensive and re-quire equipped laboratory structures that restrict their use at a global level.
Alternatively, IFN-γ secretion has been considered as useful marker associated with protection against M. tuberculosis infection (5, 14, 14). However, the IFN-γ concentrations do not perfectly correlate with protection, and it is now clear that IFN-γ is necessary but not suf-ficient to effectively prevent the Koch’s bacillus from growing (16, 90-9). In contrast to the assessment of the cyto-toxic functions, the measurement of IFN-γ concentrations is easy, affordable and less labour intensive. Taking all this arguments together, the selection of
IFN-γ as a biomarker for the readout in the development of new vaccination strategies was rationally justified. Like-wise, feasibility was a main argument for the development of IFN-γ based im-munodiagnostic assays.
Using M. tuberculosis antigens that in-duce different immune responses dur-ing latency and disease will help to characterize the differences between LTBI subjects and TB patients. As HBHA induces strong IFN-γ responses by the PBMC from LTBI subjects and not by those from TB patients (neither by those from naive subjects), this antigen fulfils these criteria (5). Therefore, the first aim of this work was to evaluate the possible value of the secreted HBHA-specific IFN-γ concentrations as an immuno-diag-nostic test for LTBI and/or TB.
Active TB is characterized by a depres-sion of the peripheral HBHA-specific IFN-γ secretion. This depression is re-versible upon effective treatment, as the HBHA specific IFN-γ secretions from the PBMC of TB patients were increased af-ter completion of anti-TB chemotherapy (5). We therefore made the hypothesis that this depression was the result of a reversible immune modulation. This modulation could be mediated by:
1) Immunomodulatory cytokines such as IL-10 and TGF-ß,
) Treg cells or
) Combined mechanisms.
Monocyte-derived IL-10 and TGF-ß were frequently shown to depress periph-eral immune responses during active TB (179, 186). However, the concept of Treg cells was emerging when we started this work. Treg cells suppress peripheral
im-mune responses even during microbial infection, as it was demonstrated dur-ing infection with Leishmania spp. (7).
In this study, both mechanisms were therefore investigated. We have dem-onstrated a major involvement of Treg cells in the suppression of antigen-specific IFN-γ responses by the PBMC from patients with active TB. An in vitro model for the induction of Treg cells was further developed in order to analyse the possible mechanisms of the induction of these Treg cells during ac-tive TB and therefore their potential role in the pathogenesis of TB. Finally, we compared the role of Treg cells among circulating lymphocytes with their role among lymphocytes collected on the site of the M. tuberculosis infection.