Novel T-cell assays for the discrimination of active and latent tuberculosis infection: the diagnostic value of PPE family

Current approaches for diagnosis of subclinical pulmonary tuberculosis, clinical implications and future perspectives: a scoping review

INTRODUCTION

Subclinical tuberculosis (TB) is the presence of TB disease among people who are either asymptomatic or have minimal symptoms.

AREAS COVERED

Currently, there are no accurate diagnostic tools and clear treatment approaches for subclinical TB. In this study, a comprehensive literature search was conducted across major databases. This review aimed to uncover the latest advancements in diagnostic approaches, explore their clinical implications, and outline potential future perspectives. While innovative technologies are in development to enable sputum-free TB tests, there remains a critical need for precise diagnostic tools tailored to the unique characteristics of subclinical TB. Given the complexity of subclinical TB, a multidisciplinary approach involving clinicians, microbiologists, epidemiologists, and public health experts is essential. Further research is needed to establish standardized diagnostic criteria and treatment guidelines specifically tailored for subclinical TB, acknowledging the unique challenges posed by this elusive stage of the disease.

EXPERT OPINION

Efforts are needed for the detection, diagnosis, and treatment of subclinical TB. In this review, we describe the importance of subclinical TB, both from a clinical and public health perspective and highlight the diagnostic and treatment gaps of this stage.

Diagnostic accuracy of the IFN-γ release assay using RD1 immunodominant T-cell antigens for diagnosis of extrapulmonary tuberculosis

The diagnosis of extrapulmonary tuberculosis (EPTB) poses a significant challenge, with controversies surrounding the accuracy of IFN-γ release assays (IGRAs). This study aimed to assess the diagnostic accuracy of RD1 immunodominant T-cell antigens, including ESAT-6, CFP-10, PE35, and PPE68 proteins, for immunodiagnosis of EPTB. Twenty-nine patients with EPTB were enrolled, and recombinant PE35, PPE68, ESAT-6, and CFP-10 proteins were evaluated in a 3-day Whole Blood Assay. IFN-γ levels were measured using a Human IFN-γ ELISA kit, and the QuantiFERON-TB Gold Plus (QFT-Plus) test was performed. Predominantly, the patients were of Afghan (62%, n = 18) and Iranian (38%, n = 11) nationalities. Eighteen individuals tested positive for QFT-Plus, accounting for 62% of the cases. The positivity rate for IGRA, using each distinct recombinant protein (ESAT-6, PPE68, PE35, and CFP-10), was 72% (n = 21) for every protein tested. Specifically, among Afghan patients, the positivity rates for QFT-Plus and IGRA using ESAT-6, PPE68, PE35, and CFP-10 were 66.7%, 83.3%, 83.3%, 77.8%, and 88.9%, respectively. In contrast, among Iranian patients, the positivity rates for the same antigens were 54.5%, 54.5%, 54.5%, 63.6%, and 45.5%, respectively. In conclusion, our study highlights the potential of IGRA testing utilizing various proteins as a valuable diagnostic tool for EPTB. Further research is needed to elucidate the underlying factors contributing to these disparities and to optimize diagnostic strategies for EPTB in diverse populations.

The PE-PPE Family of Mycobacterium tuberculosis: Proteins in Disguise

Mycobacterium tuberculosis has thrived in parallel with humans for millennia, and despite our efforts, M. tuberculosis continues to plague us, currently infecting a third of the world's population. The success of M. tuberculosis has recently been attributed, in part, to the PE-PPE family; a unique collection of 168 proteins fundamentally involved in the pathogenesis of M. tuberculosis. The PE-PPE family proteins have been at the forefront of intense research efforts since their discovery in 1998 and whilst our knowledge and understanding has significantly advanced over the last two decades, many important questions remain to be elucidated. This review consolidates and examines the vast body of existing literature regarding the PE-PPE family proteins, with respect to the latest developments in elucidating their evolution, structure, subcellular localisation, function, and immunogenicity. This review also highlights significant inconsistencies and contradictions within the field. Additionally, possible explanations for these knowledge gaps are explored. Lastly, this review poses many important questions, which need to be addressed to complete our understanding of the PE-PPE family, as well as highlighting the challenges associated with studying this enigmatic family of proteins. Further research into the PE-PPE family, together with technological advancements in genomics and proteomics, will undoubtedly improve our understanding of the pathogenesis of M. tuberculosis, as well as identify key targets/candidates for the development of novel drugs, diagnostics, and vaccines.

Gaps in Study Design for Immune Parameter Research for Latent Tuberculosis Infection: A Systematic Review

Background

Immune parameters (IP) have been extensively studied to distinguish between latent tuberculosis (LTBI) and active tuberculosis (TB).

Objective

To determine the IP associated with LTBI, compared to active TB and individuals not infected by M. tuberculosis published in literature.

Methods

We conducted a systematic search using Google Scholar and PubMed databases, combining the MeSH terms latent tuberculosis, Mycobacterium tuberculosis, cytokines, and biological markers, with the free terms, biomarkers and cytokines. Spanish, English, and Portuguese articles comparing the concentration of IP associated with LTBI, either in plasma/serum or in vitro, in adults and nonimmunocompromised versus individuals with TB or without M. tuberculosis infection between 2006 July and 2018 July were included. Two blinded reviewers carried out the searches, read the abstracts, and selected the articles for analysis. Participants' information, diagnostic criteria, IP, detection methods, and biases were collected.

Results

We analyzed 36 articles (of 637 abstracts) with 93 different biomarkers in different samples. We found 24 parameters that were increased only in active TB (TGF-α, CSF3, CSF2, CCL1 [I-309], IL-7, TGF-β1, CCL3 [MIP-1α], sIL-2R, TNF-β, CCL7 [MCP-3], IFN-α, fractalkine, I-TAG, CCL8 [MCP-2], CCL21 [6Ckine], PDGF, IL-22, VEGF-A, LXA4, PGE2, PGF2α, sCD163, sCD14, and 15-Epi-LXA4), five were elevated in LTBI (IL-5, IL-17F, IL-1, CCL20 [MIP-3α], and ICAM-1), and two substances were increased among uninfected individuals (IL-23 and basic FGF). We found high heterogeneity between studies including failure to account for the time/illness of the individuals studied; varied samples and protocols; different clinical classification of TB; different laboratory methods for IP detection, which in turn leads to variable units of measurement and assay sensitivities; and selection bias regarding TST and booster effect. None of the studies adjusted the analysis for the effect of ethnicity.

Conclusions

It is mandatory to harmonize the study of immune parameters for LTBI diagnosis. This systematic review is registered with PROSPERO CRD42017073289.

Infect and Inject: How Mycobacterium tuberculosis Exploits Its Major Virulence-Associated Type VII Secretion System, ESX-1

Mycobacterium tuberculosis is an ancient master of the art of causing human disease. One important weapon within its fully loaded arsenal is the type VII secretion system. M. tuberculosis has five of them: ESAT-6 secretion systems (ESX) 1 to 5. ESX-1 has long been recognized as a major cause of attenuation of the FDA-licensed vaccine Mycobacterium bovis BCG, but its importance in disease progression and transmission has recently been elucidated in more detail. This review summarizes the recent advances in (i) the understanding of the ESX-1 structure and components, (ii) our knowledge of ESX-1's role in hijacking macrophage function to set a path for infection and dissemination, and (iii) the development of interventions that utilize ESX-1 for diagnosis, drug interventions, host-directed therapies, and vaccines.

Diagnostic Accuracy of Monocyte Chemotactic Protein (MCP)-2 as Biomarker in Response to PE35/PPE68 Proteins: A Promising Diagnostic Method for the Discrimination of Active and Latent Tuberculosis

INTRODUCTION

Several studies have been conducted to find new biomarkers for the discrimination of Latent Tuberculosis Infection (LTBI) from active TB (ATB); however, their findings are inconsistent. The aim of the current study was to evaluate the potential of in vitro antigenspecific expression of Monocyte Chemotactic Protein (MCP)-2 for discrimination of ATB and LTBI after stimulation of whole blood with PE35 and PPE68 recombinant proteins.

MATERIALS AND METHODS

The recombinant PE35 and PPE68 proteins were evaluated at a final concentration of 5 µg/ml by a 3-day whole blood assay. Secreted MCP-2 from the culture supernatants were measured by commercially available Human MCP2 ELISA Kit. The diagnostic performance of MCP-2 was ascertained by Receiver Operator Characteristic (ROC) curve and measuring the Area Under the Curve (AUC) and their 95% Confidence Intervals (CI). Cut-offs was estimated at various sensitivities and specificities and at the maximum Youden's index (YI), i.e. sensitivity specificity-1.

RESULTS

The median MCP-2 response to both PE35 and PPE68 in those with LTBI was significantly higher than patients with ATB. The discrimination performance of MCP-2 response following stimulation of PE35 (assessed by AUC) between LTBI and patients with ATB was 0.98 (95%CI: 0.94-1.00). Maximum discrimination was reached at a cut-off of 86pg/mL with 100% sensitivity and 97% specificity. The highest sensitivity and specificity was obtained using cut off 58 pg/mL following stimulation with PPE68 (100% and 90%, respectively; AUC: 0.94, 95%CI: 0.85- 1.00).

CONCLUSION

MCP-2 induced by PE35 and PPE68 shows good discriminatory power for discrimination of ATB and LTBI. Additional studies with a larger sample size are needed to confirm the advantage of this marker, alone or combined with other markers; however, these findings present a promising method, which can discriminate between ATB and LTBI.

Diagnostic accuracy of interferon (IFN)-γ inducible protein 10 (IP-10) as a biomarker for the discrimination of active and latent tuberculosis

To assess the potency of Interferon (IFN)-γ inducible protein 10 (IP-10) stimulated by recombinant PE35 and PPE68 as a biomarker in differentiating between active and latent tuberculosis. Patients with active pulmonary TB (PTB) (n = 30), latent TB infection (LTBI) (n = 29), and BCG-vaccinated healthy controls (HCs) (n = 30) were enrolled and blood samples were taken from them. The diagnostic performance of IP-10 was evaluated by the Receiver operator characteristic (ROC) curve and the area under the curve (AUC) and their 95% confidence intervals (CI) were calculated. The median IP-10 concentrations following stimulation with recombinant PE35 and PPE68 were significantly higher in TB-infected group (both PTB and LTBI) compared with HCs (P < 0.05). It was also significantly higher in PTB patients compared with individuals with LTBI (P < 0.05). The discriminatory performance of IP-10 following stimulation with recombinant PE35 and PPE68 (assessed by AUC) between TB patients and HCs were similar (AUC: 0.79 [95% CI 0.68-0.89] and 0.79 [95% CI 0.69-0.89], respectively). AUCs of IP-10 following stimulation with recombinant PE35 and PPE68 for distinguishing between PTB and LTBI groups were 0.63 (95% CI 0.47-0.79) and 0.61 (0.45-0.77), respectively. Under the selected cut-off values, the sensitivity and specificity of IP-10 for distinguishing of TB-infected and HCs after stimulation with recombinant PE35 was 74.5% and 73%, respectively and after stimulation with recombinant PPE68 were 76.5% and 63%, respectively. Moreover, the sensitivity and specificity of IP-10 for differentiating of PTB and LTBI following stimulation with recombinant PE35 and PPE68 were 770 pg/ml (sensitivity: 63%; specificity: 62%) and 502 pg/ml (sensitivity: 80%; specificity: 52%), respectively. IP-10 stimulated by recombinant PE35 and PPE68 is a promising biomarker for TB diagnosis. However, it doesn't have desirable sensitivity and specificity in distinguishing between PTB and LTBI.

Screening of healthcare workers for latent tuberculosis infection in the low tuberculosis burden country: QuantiFERON-TB gold in tube test or tuberculin skin test?

Healthcare workers (HCWs) are an important group at increased risk for exposure to various infectious agents including Mycobacterium tuberculosis. Unfortunately, there is very limited data on the prevalence of latent tuberculosis infection (LTBI) among HCWs in Iran. The aim of this study was to analyze the prevalence of LTBI among HCW's of Children Medical Center (CMC), an Iranian referral hospital, and to compare the feasibility of using tuberculin skin test (TST) and QuantiFERON-TB gold in tube test (QFT-GIT) to screen the LTBI among this population. Informed consent was obtained before the study questionnaire was completed. Participants were then evaluated for LTBI using QFT-GIT test followed immediately by TST. The prevalence of latent TB infection in the 101 HCWs, based on QFT-GIT positivity, was 47% (n = 47), while based on TST test a higher prevalence was observed (n = 57, 57%). Forty-five (45%) subjects were positive for both tests, whereas 14 (14%) were positive by either test. Among the TST positive cases, 21% (n = 12) were negative for QFT-GIT. The overall agreement of TST and QFT-GIT was 86% ([45 + 42]/101] and a moderate correlation between these two tests was observed (Kappa value 0.72). The use of QFT-GIT test as a second step in TST-positive cases offers an appropriate tool for LTBI detection in our country where all HCWs are BCG-vaccinated. To enable the effective control and prevention of TB in healthcare settings in Iran, specific and detailed guidelines for dealing with LTBI are highly required. All newly hired HCWs should receive baseline testing for LTBI before commencing employment. In addition, HCWs who work in high-risk departments should receive regular follow-up screening for LTBI.

PPE17 (Rv1168c) protein of Mycobacterium tuberculosis detects individuals with latent TB infection

Latent tuberculosis infection (LTBI) is a clinically distinct category of Mycobacterium tuberculosis (Mtb) infection that needs to be diagnosed at the initial stage. We have reported earlier that one of the Mtb proline-proline-glutamic acid (PPE) proteins, PPE17 (Rv1168c) is associated with stronger B-cell and T-cell responses and could be used to diagnose different clinical categories of active TB patients with higher specificity and sensitivity than PPD and ESAT-6. Based on these observations we further tested the potential of PPE17 for the diagnosis of LTBI. We tested 198 sera samples collected from LTBI individuals (n = 61), QFT-negative (n = 58) and active TB patients (n = 79). Individuals were defined as LTBI by QuantiFERON-TB Gold In-Tube test (QFT–GIT) positive results, while active TB patients were confirmed based on the guidelines of the Revised National TB Control Programme of India. The antibody responses against PPE17, ESAT-6:CFP-10 and PPD were compared in these subjects by enzyme-linked immunosorbent assay. We observed that LTBI individuals show a higher sero-reactivity to PPE17 as compared to currently used latent TB diagnostic antigens like ESAT-6, CFP-10 and PPD. The LTBI and active TB patients display almost similar sensitivity. Interestingly, PPE17 could discriminate LTBI positive subjects from the QFT-negative subjects (P < 0.001). Our study hints that PPE17 may be used as a novel serodiagnostic marker to screen the latently infected subjects and may also be used as a complimentary tool to the QFT–GIT.

Introducing the ESAT-6 free IGRA, a companion diagnostic for TB vaccines based on ESAT-6

There is a need for an improved vaccine for tuberculosis. ESAT-6 is a cardinal vaccine antigen with unique properties and is included in several vaccine candidates in development. ESAT-6 is also the core antigen in the IFN-γ release assays (IGRA) used to diagnose latent infection, rendering IGRA tests unspecific after vaccination. This challenge has prompted the development of a companion diagnostic for ESAT-6 based vaccines, an ESAT-6 free IGRA. We screened a panel of seven potential new diagnostic antigens not recognized in BCG vaccinated individuals. Three highly recognized antigens EspC, EspF and Rv2348c were identified and combined with CFP10 in an ESAT-6 free antigen cocktail. The cocktail was prepared in a field-friendly format, lyophilized with heparin in ready-to-use vacutainer tubes. The diagnostic performance of the ESAT-6 free IGRA was determined in a cross-validation study. Compared IGRA, the ESAT-6 free IGRA induced a comparable magnitude of IFN-γ release, and the diagnostic performance was on par with Quantiferon (sensitivity 84% vs 79%; specificity 99% vs 97%). The comparable performance of the ESAT-6 free IGRA to IGRA suggests potential as companion diagnostic for ESAT-6 containing vaccines and as adjunct test for latent infection.

Evaluation of Rv0220, Rv2958c, Rv2994 and Rv3347c of Mycobacterium tuberculosis for serodiagnosis of tuberculosis

Tuberculosis (TB), the leading cause of death among infectious diseases worldwide, is caused by Mycobacterium tuberculosis (M. tuberculosis). Early accurate diagnosis means earlier prevention, treatment and control of TB. To confirm efficient diagnostic antigens for M. tuberculosis, the serodiagnosis value of four recombinant proteins including Rv0220, Rv2958c, Rv2994 and Rv3347c was evaluated in this study. The specificities and sensitivities of four recombinant proteins were determined based on enzyme‐linked immunosorbent assay (ELISA) by screening sera from smear‐positive pulmonary TB patients (n = 92), uninfected individuals (n = 60) and patients with Mycoplasma pneumoniae (n = 32) that potentially cross‐react with M. tuberculosis. The ELISAs showed that Rv0220, Rv2958c, Rv2994 and Rv3347c exhibited high specificities and sensitivities in detecting immunoglobulin G (IgG) antibody, with 98.3/91.3%, 91.7/85.9%, 93.3/89.1% and 93.3/80.4% respectively. According to the receiver‐operating characteristic (ROC) analysis, the area under the ROC of the target proteins was 0.988, 0.969, 0.929 and 0.945 respectively. Western blot was established to evaluate the immunoreactivities of target proteins to mice and human sera. Results demonstrated that Rv0220, Rv2958c, Rv2994 and Rv3347c could specifically recognize TB‐positive sera and the sera of mice immunized with the corresponding protein. Thus, Rv0220, Rv2958c, Rv2994 and Rv3347c were valuable potential diagnostic antigens for M. tuberculosis.

PE and PPE Genes: A Tale of Conservation and Diversity

PE and PPE are two large families of proteins typical of mycobacteria whose structural genes in the Mycobacterium tuberculosis complex (MTBC) occupy about 7% of the total genome. The most ancestral PE and PPE proteins are expressed by genes that belong to the same operon and in most cases are found inserted in the esx clusters, encoding a type VII secretion system. Duplication and expansion of pe and ppe genes, coupled with intragenomic and intergenomic recombination events, led to the emergence of the polymorphic pe_pgrs and ppe_mptr genes in the MTBC genome. The role and function of these proteins, and particularly of the polymorphic subfamilies, remains elusive, although it is widely accepted that PE and PPE proteins may represent a specialized collection used by MTBC to interact with the complex host immune system of mammals. In this chapter, we summarize what has been discovered since the identification of these genes in 1998, focusing on M. tuberculosis genetic variability, host-pathogen interaction and TB pathogenesis.

Early detection of Toxoplasma gondii infection by using a interferon gamma release assay: A review

Antibody-based serological tests are currently the most common diagnostic methods for detection of Toxoplasma gondii; however, these tests bear several limitations. Recently, Interferon-gamma release assay (IGRA), a T-cell-based test, was introduced as an in vitro test for detection of T. gondii infection. Few studies have investigated the potential role of cell immunity in diagnosis of toxoplasmosis. IGRA accurately distinguished infected from uninfected individuals, showing strong lymphocyte activation after in vitro stimulation with T. gondii antigens, even during the first days of life. IGRA is an easy-operation and low-cost method to measure cell mediated immunity against T. gondii. The results of this review underline the importance of evaluating cellular immunity to establish an early diagnosis particularly for congenital toxoplasmosis. Therefore, ELISA-based IGRA holds the potential to become a useful diagnostic tool for early detection of T. gondii infection.

Improving T-cell assays for diagnosis of latent TB infection: Confirmation of the potential role of testing Interleukin-2 release in Iranian patients

BACKGROUND

Since gamma interferon release assays (IGRAs) cannot differentiate between active tuberculosis and latent tuberculosis infection (LTBI), development of rapid and specific diagnosis tools are essential for discriminating between active tuberculosis (TB) from LTBI. Both IGRAs are based on Mycobacterium tuberculosis-specific antigens, namely, early secretory antigenic target 6 (ESAT-6) and 10kDa culture filtrate (CFP-10). The aim of this study was to evaluate the potential value of IL-2 secretion by whole blood cells after stimulation with rESAT-6 and rCFP-10 for discriminating between active and latent tuberculosis.

METHODS

Interleukin-2 and IFN-γ were measured after blood stimulation of 90 cases (30 with active TB, 30 with LTBI and 30 healthy controls) with recombinant ESAT-6 and CFP-10. Receiver operating characteristic (ROC) curve analysis was conducted to determine the best IL-2 and IFN-γ result thresholds in discriminating between cases with active or latent TB, and the corresponding sensitivity and specificity were recorded.

RESULTS

The IFN-γ release assay demonstrated a good sensitivity and specificity (sensitivity 83-84% and specificity 92%) for diagnosis of tuberculosis. The discrimination performance of IL-2 assay (assessed by the area under ROC curve) between LTBI and patients with active TB were 0.75 and 0.8 following stimulation with rESAT-6 and rCFP-10, respectively. Maximum discrimination was reached at a cut-off of 11.6pg/mL for IL-2 after stimulation with recombinant rESAT-6 with 72% sensitivity and 79% specificity and 10.7pg/mL for IL-2 following stimulation with rCFP-10 with 75% sensitivity and 79% specificity, respectively.

CONCLUSION

This study demonstrates that rESAT-6 and rCFP-10 can provide a sensitive and specific diagnosis of TB. In addition, it was shown that IL-2 may be serving as a marker for discriminating LTBI and active TB.