Mycobacterium tuberculosis (MTB) is a major microbial pathogen that threatens global
health. The WHO has estimated about 9 million new tuberculosis (TB) cases and around
1.5 million deaths due to TB in the year 2013 (World Health Organization, 2014). In
MTB-infected individuals, bacteria may exist in a dormant state for lengthy periods
without causing disease symptoms, or may finally start multiplying and evading immune
control, resulting in active TB in 5–10% of infected cases. At present, factors that
promote progression from latent tuberculosis infection (LTBI) to disease are not totally
understood. Therefore, management of LTBI plays a significant role for TB disease
control given that quiescent bacilli are a big reservoir of potential TB cases.
Tuberculin skin test (TST) has been the classical method used for LTBI diagnosis in
spite of its compromised specificity due to cross-reaction with Mycobacterium bovis
bacillus Calmette–Guérin (BCG) vaccine strain and non-tuberculous mycobacteria. Furthermore,
it has low sensitivity in immune compromised patients, who are in particular those
that mainly need this test because they have a high risk of developing active TB if
they are infected. Interferon (IFN)-γ release assays (IGRAs) appeared more than a
decade ago as an alternative method to TST for diagnosing LTBI. They detect the IFN-γ
released by sensitized T cells after stimulation with specific MTB antigens encoded
in the region of difference (RD) 1 and 11 (ESAT-6, CFP-10 and TB7.7) (Andersen et
al., 2000). Nowadays, IGRAs implementation has improved LTBI diagnosis because of
their higher specificity with respect to TST and good correlation with MTB exposure
degree; however, they do not discriminate between LTBI and active TB (Pai et al.,
2014). As a consequence, there is a need for studying new biomarkers to explore the
biology and the immune response for distinguishing latency from disease.
At present, some new MTB phase-dependent antigens have been studied in LTBI individuals
and active TB patients showing promising results. These studies explore the response
of T cells producing IFN-γ because they are the major players in the protection against
TB. However, variable results regarding these antigens have been obtained since now
due to disparities in methodology and population heterogeneity (Serra-Vidal et al.,
2014; Goletti et al., 2010; Singh et al., 2014). In EBioMedicine, Delfina Peña and
colleagues (Peña et al., 2015) have investigated the efficacy of several DosR regulon-encoded
latency antigens as potential markers for LTBI. The antigens included and studied
in this work were Rv2624c, Rv2626c, and Rv2628; together with ESAT-6 and CFP-10. Interestingly,
Rv2626c was found to produce significant IFN-γ levels in BCG LTBI individuals with
respect to active TB patients or healthy controls. In a first approach, the response
to MTB antigens was investigated stimulating peripheral blood mononuclear cells (PBMCs)
from BCG LTBI individuals (QFT-GIT positive) and healthy controls (QFT-GIT negative),
finding that IFN-γ response was significantly higher in LTBI individuals with respect
to healthy controls upon Rv2626c stimulation. Interestingly, these findings were also
confirmed by flow cytometry. Then, in a second approach, the IFN-γ response to Rv2626c
was also analyzed in a third group of active TB patients stimulating PBMCs and whole
blood with this specific antigen. In contrast to the results obtained with RD1 antigens
(ESAT-6 and CFP-10), stimulation with this latency antigen allowed to discriminate
between active and latent infection since patients with active TB did not secrete
IFN-γ against Rv2626c. Finally, these results were also reinforced by a ROC analysis.
Investigators also identified several specific and immunogenic Rv2626c immunodominant
peptide pools that improved LTBI diagnosis.
Delfina Peña and colleagues study (Peña et al., 2015) investigates an interesting
and promising field; as a consequence, their results are encouraging for several reasons.
First, as currently IGRAs are not designed for distinguishing between LTBI and disease;
investigations based on new antigens linked to latency are required. In this sense,
the novel Rv2626c antigen studied in the present work could be promising and improve
LTBI diagnosis. However, further investigations on other latency antigens different
from those contemplated in this study are still required. Second, the study of new
immune-biomarkers for detecting IFN-γ in vitro may open the door to the development
of next generation assays based on new antigens. Furthermore, potential host biomarkers
are urgently needed to provide risk of LTBI progression as well. In this sense, a
recent published study have assessed the utility of measuring IFN-γ/TNF-α ratio against
16 antigens, finding that Rv2626c and Rv3716c improved QFT-GIT diagnostic performance
and LTBI diagnosis (Prabhavathi et al., 2015). In addition, the use of flow cytometry
is currently being utilized for the study of the immune response and cell surface
marker expression (immunophenotyping) in order to find a signature related with infection
or disease (Harari et al., 2011; Portevin et al., 2014). Together these findings will
help to develop and design novel state-of-the-art techniques for LTBI and active TB
diagnosis in not so near distant future.
Conflict of interest
We declare no competing interests.