Rv1985c, a promising novel antigen for diagnosis of tuberculosis infection from BCG-vaccinated controls

The Biological and Clinical Aspects of a Latent Tuberculosis Infection

Tuberculosis (TB), caused by bacilli from the Mycobacterium tuberculosis complex, remains a serious global public health problem, representing one of the main causes of death from infectious diseases. About one quarter of the world’s population is infected with Mtb and has a latent TB infection (LTBI). According to the World Health Organization (WHO), an LTBI is characterized by a lasting immune response to Mtb antigens without any TB symptoms. Current LTBI diagnoses and treatments are based on this simplified definition, although an LTBI involves a broad range of conditions, including when Mtb remains in the body in a persistent form and the immune response cannot be detected. The study of LTBIs has progressed in recent years; however, many biological and medical aspects of an LTBI are still under discussion. This review focuses on an LTBI as a broad spectrum of states, both of the human body, and of Mtb cells. The problems of phenotypic insusceptibility, diagnoses, chemoprophylaxis, and the necessity of treatment are discussed. We emphasize the complexity of an LTBI diagnosis and its treatment due to its ambiguous nature. We consider alternative ways of differentiating an LTBI from active TB, as well as predicting TB reactivation based on using mycobacterial “latency antigens” for interferon gamma release assay (IGRA) tests and the transcriptomic analysis of human blood cells.

Assessing and screening for T-cell epitopes from Mycobacterium tuberculosis RD2 proteins for the diagnosis of active tuberculosis

The Region of D eletion 2 (RD2) of Mycobacterium tuberculosis encodes reserved antigens that contribute to bacterial virulence. Among these antigens, Rv1983, Rv1986, Rv1987, and Rv1989c have been shown to be immunodominant in infected cattle; however, their diagnostic utility has not been evaluated in humans.

In this study, we screened 87 overlapping synthetic peptides encoded by five RD2 proteins for diagnosing tuberculosis epitopes in 50 active tuberculosis (TB) cases, 31 non-tuberculosis patients and 36 healthy individuals. A pool of promising epitopes was then assessed for their diagnostic value in 233 suspected TB patients using a whole blood IFN-γ release assay.

Only 10 peptides were recognized by more than 10% of active tuberculosis patients. The IFN-γ release responses to Rv1986-P9, P15, P16, Rv1988-P4, P11, and Rv1987-P11 were significantly higher in the active TB group than in the control groups (p < 0.05). The whole blood IFN-γ release assay based on these epitopes yielded a sensitivity of 51% and a specificity of 85% in diagnosing active tuberculosis, and the corresponding results using the T-SPOT.TB assay were 76% and 75%, respectively.

In conclusion, these results suggest that the six epitopes from the RD2 of M. tuberculosis have potential diagnostic value in TB.

Identification of new diagnostic biomarkers for Mycobacterium tuberculosis and the potential application in the serodiagnosis of human tuberculosis

Mycobacterium tuberculosis (M. tuberculosis) regions of difference (RD) encode proteins which are potentially useful as diagnostic reagents for tuberculosis (TB). In this study, 75 genes from M. tuberculosis RD1‐RD16 were successfully cloned from which 68 proteins were expressed and purified. Three serum pools from patients with pulmonary TB (PTB), extra‐pulmonary tuberculosis (EPTB) and healthy controls (HC) were used to preliminarily screen individual RD proteins. The OD₆₃₀ ratio of the PTB or EPTB to the HC group ≥ 2‐fold was positive. As a result, 29 proteins were obtained. The serological response to the identified antigens was further verified using 58 PTB samples with 38 sera from smear‐positive PTB (PTB‐SP) patients and 20 sera from smear‐negative PTB (PTB‐SN) patients, 16 EPTB samples, 42 latent M. tuberculosis infection samples and 40 HCs by indirect ELISA. With respect to the PTB diagnosis, receiver operating characteristic analysis showed that Rv0222 [area under the curve (AUC), 0.8129; 95% confidence interval (CI), 0.7280–0.8979] and Rv3403c (AUC, 0.8537; 95% CI, 0.7779–0.9294) performed better than ESAT6/CFP10 (AUC, 0.7435; 95% CI, 0.6465–0.8406). Rv0222 and Rv3403c demonstrated the highest diagnostic ability in the PTB‐SP group (sensitivity, 86.8%; specificity, 80%), while Rv3403c demonstrated the highest diagnostic ability in the PTB‐SN group (sensitivity, 70%; specificity, 80%). With respect to the EPTB diagnosis, Rv0222 exhibited the highest diagnostic value (AUC, 0.7523; sensitivity, 68.8%; specificity, 87.5%). In addition, the combination of Rv0222 and Rv3403c improved the test for PTB‐SN. These results indicate that Rv0222 and Rv3403c would be potential diagnostic biomarkers for active TB serodiagnosis. Mouse experiments demonstrated that Rv0222 and Rv3403c elicited specific cellular and humoral responses which were characterized by production of IFN‐γ, IgG1, and IgG2a, but a higher level of IgG1 than IgG2a.

ORFeome-based identification of biomarkers for serodiagnosis of Mycobacterium tuberculosis latent infection

BACKGROUND

The challenges posed by Mycobacterium tuberculosis infection require the gradual removal of the pool of latent tuberculosis infection (LTBI). The current cell-immune-based diagnostic tests used to identify LTBI individuals have several irreversible drawbacks. In the present study, we attempted to identify novel diagnostic antigens for LTBI.

METHODS

A high-throughput glutathione S-transferase (GST)-fusion technology was used to express over 409 TB proteins and sera from LTBI and healthy individuals was used to interrogate these GST-TB fusion proteins.

RESULTS

Of 409 TB proteins, sixty-three reacted seropositive and defined the immuno-ORFeome of latent M. tuberculosis. Within the immuno-ORFeome, the rare targets were predominantly latency-associated proteins and secreted proteins, while the preferentially recognized antigens tended to be transmembrane proteins. Six of novel highly-reactive antigens had the potential to distinguish LTBI from active TB and healthy individuals. A multiple-antigen combination set was selected through analysis of various combinations. A panel of 94 archived serum samples was used to validate the diagnostic performance of the multiple-antigen combination set, which had sensitivity of 66.1% (95% CI 52.9, 77.4) and specificity of 87.5% (95% CI 70.1, 95.1).

CONCLUSION

These results provide experimental evidence of the immunogenicity of novel TB proteins that are suitable for the development of serodiagnostic tools for LTBI.

Antigens Rv0310c and Rv1255c are promising novel biomarkers for the diagnosis of Mycobacterium tuberculosis infection

This study aimed to identify novel immunogenic epitopes from Mycobacterium tuberculosis (MTB) that could be used in tuberculosis (TB) diagnostics. To determine the diagnostic potential of mycobacterial antigens in serodiagnosis of TB, 256 patients were enrolled in a study and divided into two groups: 126 smear-positive pulmonary TB patients (SPPT) and 130 smear-negative pulmonary TB patients (SNPT); 152 bacillus Calmette-Guerin (BCG)-vaccinated healthy people were used as a control. Murine results showed that antigens Rv0310c-E from RD 8 and Rv1255c-E from RD 10 were strongly immunogenic to Th1 cells and induced a great humoral response. Receiver operating characteristic analysis indicated that Rv0310c-E (area under the curve (AUC): 0.800) and Rv1255c-E (AUC: 0.808) performed better than ESAT-6 (AUC: 0.665) and CFP-10 (AUC: 0.623) proteins but were comparable with Rv3425 (AUC: 0.788) protein in a human serum IgG analysis. Rv0310c-E demonstrated the highest diagnostic ability for the SPPT group (Youden index: 0.5602, sensitivity: 69.84%, specificity: 86.18%), while Rv1255c-E demonstrated the highest diagnostic ability for the SNPT group (Youden index: 0.5674, sensitivity: 73.84%, specificity: 82.89%). In addition, combination analysis found that antigen Rv0310c-E, coupled with the Rv3425 protein (Youden index: 0.6098, sensitivity: 87.30%, specificity: 73.68%) had the strongest performance for TB diagnostics of the SPPT group, and the single antigen Rv1255c-E was strongest for the SNPT group. These results suggest that antigens Rv0310c-E and Rv1255c-E are potential antigens for TB serodiagnostic tests, which may facilitate detection of MTB in smear-negative and smear-positive patients.

Human Immunology of Tuberculosis

Immunology is a central theme when it comes to tuberculosis (TB). The outcome of human infection with Mycobacterium tuberculosis is dependent on the ability of the immune response to clear or contain the infection. In cases where this fails, the bacterium replicates, disseminates within the host, and elicits a pathologic inflammatory response, and disease ensues. Clinical presentation of TB disease is remarkably heterogeneous, and the disease phenotype is largely dependent on host immune status. Onward transmission of M. tuberculosis to new susceptible hosts is thought to depend on an excessive inflammatory response causing a breakdown of the lung matrix and formation of lung cavities. But this varies in cases of underlying immunological dysfunction: for example, HIV-1 infection is associated with less cavitation, while diabetes mellitus comorbidity is associated with increased cavitation and risk of transmission. In compliance with the central theme of immunology in tuberculosis, we rely on detection of an adaptive immune response, in the form of interferon-gamma release assays or tuberculin skin tests, to diagnose infection with M. tuberculosis. Here we review the immunology of TB in the human host, focusing on cellular and humoral adaptive immunity as well as key features of innate immune responses and the underlying immunological dysfunction which associates with human TB risk factors. Our review is restricted to human immunology, and we highlight distinctions from the immunological dogma originating from animal models of TB, which pervade the field.

Humoral Responses to Rv1733c, Rv0081, Rv1735c, and Rv1737c DosR Regulon-Encoded Proteins of Mycobacterium tuberculosis in Individuals with Latent Tuberculosis Infection

Latent tuberculosis infection (LTBI) is evidence of immunological control of tuberculosis. Dormancy survival regulator (DosR) regulon-encoded proteins may have a role in the maintenance of LTBI. T cell responses to Rv1733c, Rv0081, Rv1735c, and Rv1737c DosR regulon-encoded proteins were found to be most frequent among household contacts of TB cases from Uganda compared to other DosR proteins, but antibody responses were not described. We characterized antibody responses to these proteins in individuals from Uganda. Antibodies to Rv1733c, Rv0081, Rv1735c, and Rv1737c DosR regulon-encoded proteins were measured in 68 uninfected individuals, 62 with LTBI, and 107 with active pulmonary tuberculosis (APTB) cases. There were no differences in the concentrations of antibodies to Rv0081, Rv1735c, and Rv1737c DosR regulon-encoded proteins between individuals with LTBI and APTB and those who were uninfected. LTBI was associated with higher concentrations of antibodies to Rv1733c in female participants [adjusted geometric mean ratio: 1.812, 95% confidence interval (CI): 1.105 2.973, and p = 0.019] but not in males (p value for interaction = 0.060). Antibodies to the four DosR regulon-encoded proteins investigated may not serve as good biomarkers of LTBI in the general population. More of the M.tb proteome needs to be screened to identify proteins that induce strong antibody responses in LTBI.

Genetic diversity of immune-related antigens in Region of Difference 2 of Mycobacterium tuberculosis strains

Region of Difference 2 (RD2) was lost during the ongoing propagation of BCG between 1927 and 1931, a time that coincides with reports of the ongoing attenuation of the vaccine. Some data demonstrate that RD2 plays a role in mycobacterial virulence, and that its deletion from Mycobacterium tuberculosis leads to a decrease in bacterial growth in both a macrophage and a murine model. Human T-cell epitopes of M. tuberculosis are evolutionarily hyperconserved and thus it was deduced that M. tuberculosis lacks antigenic variation and immune evasion. However, two antigens, Rv1986 and MPT64, encoded by RD2 harbored more than one amino acid changes. In this study, we used same set of clinical M. tuberculosis complex (MTBC) isolates from China, amplified the five genes containing T and B cell epitopes other than MPT64 encoded by RD2, and compared the sequences. It turned out that proteins in RD2 region, especially Rv1980c, Rv1985 and Rv1986 may be a special region that undergo antigenic variation in response to host immune pressure and may be involved in ongoing immune evasion. The dN/dS value of all six genes (including MPT64) were 2.33, much higher than 1, which means T cell antigens in RD2 region appeared to be under diversifying selection. Our data support the view that RD2 regions tend to be more variable than we expected to evade host immunity and the immune-related antigens in RD2 were more variable than we expected, especially in T-cell epitope regions.

Identification of Novel RD1 Antigens and Their Combinations for Diagnosis of Sputum Smear-/Culture+ TB Patients

Rapid and accurate diagnosis of pulmonary tuberculosis (PTB) is an unresolved problem worldwide, especially for sputum smear− (S−) cases. In this study, five antigen genes including Rv3871, Rv3874, Rv3875, Rv3876, and Rv3879 were cloned from Mycobacterium tuberculosis (Mtb) RD1 and overexpressed to generate antigen fragments. These antigens and their combinations were investigated for PTB serodiagnosis. 298 serum samples were collected from active PTB patients, including 117 sputum smear+ (S+) and sputum culture+ (C+) cases, 101 S−/C+ cases, and 80 S−/C− cases. The serum IgG levels of the five antigens were measured by ELISA. Based on IgG levels, the sensitivity/specificity of Rv3871, Rv3874, Rv3875, Rv3876, and Rv3879 for PTB detection was 81.21%/74.74%, 63.09%/94.78%, 32.21%/87.37%, 62.42%/85.26%, and 83.56%/83.16%, respectively. Furthermore, the optimal result for PTB diagnosis was achieved by combining antigens Rv3871, Rv3876, and Rv3879. In addition, the IgG levels of Rv3871, Rv3876, and Rv3879 were found to be higher in S−/C+ PTB patients than in other PTB populations. More importantly, combination of the three antigens demonstrated superior diagnostic performance for both S−/C+ and S−/C− PTB. In conclusion, the combination of Rv3871, Rv3876, and Rv3879 induced higher IgG response in sputum S−/C+ PTB patients and represents a promising biomarker combination for diagnosing of PTB.

Screening putative antigens as stimulators in the Mycobacterium bovis interferon-gamma release assay for cattle

Bovine tuberculosis (BTB) represents not only a significant economic concern, but also an important public health problem. Currently, interferon-gamma (IFN-γ) release assays (IGRAs) are widely used as an adjunct to the tuberculin test (TST) for the diagnosis of BTB. A great number of international studies have demonstrated that the sensitivity of the IFN-γ assay, which uses purified protein derivatives (PPDs) as diagnostic reagents, is superior to that of the TST. However, there are concerns about its specificity, largely because of the cross reactivity of common antigens shared by pathogenic and non-pathogenic mycobacterial species. The use of pathogen-specific antigens theoretically offers the most effective way to improve the specificity of IGRAs. In this study, we evaluated the potential utility of 13 purified recombinant putative antigens, which are highly specific to the Mycobacterium tuberculosis complex, as diagnostic reagents in IGRAs. A CFP-10-ESAT-6 fusion protein (abbreviated CE) displayed the greatest potential, whereas four region of difference 2 (RD2) antigens, especially Rv1985c were identified as potential candidate antigens, and can be included in an IGRA cocktail, together with CE as stimulators in the IFN-γ release assay for the diagnosis of BTB.

High level of IFN-γ released from whole blood of human tuberculosis infections following stimulation with Rv2073c of Mycobacterium tuberculosis

More efficacious and specific biomarkers are urgently needed for better control of tuberculosis (TB), the second leading infectious cause of mortality worldwide. The region of difference 9 (RD9) presents the genome of the causative pathogen Mycobacterium tuberculosis rather than other species of the genus Mycobacterium, which might be promising targets for specific diagnosis, vaccine development and pathogenesis. In this study, two proteins Rv2073c and Rv2074, encoded by the RD9 were expressed and purified from Escherichia coli system. Following stimulation with both proteins, the levels of IFN-γ secreted by T cells from a total of 49 whole blood samples obtained from clinically diagnosed active TB patients, patients with latent TB infections (LTBIs), and healthy donors, were compared with those of the incubation with recombinant fusion protein of CFP21 and MPT64 (rCM). Our results demonstrated that only Rv2073c could induce a higher level of IFN-γ in TB infections than healthy controls and there was a positive correlation between Rv2073c- and rCM-specific IFN-γ levels in TB infections and healthy donors, respectively. These findings indicate that Rv2073c might be a promising antigen for specific diagnostic reagents and vaccine candidates of TB.

Conserved alanine rich protein Rv3878 in Mycobacterium tuberculosis contains sequence polymorphisms

Host immune pressure and associated parasite immune evasion are key features of host-pathogen co-evolution. A previous study showed that human T cell epitopes of Mycobacterium tuberculosis are evolutionarily hyperconserved and thus it was deduced that M. tuberculosis lacks antigenic variation and immune evasion. Here, we selected 162 clinical M. tuberculosis complex (MTBC) isolates from China, amplified gene encoding Rv3878 and compared the sequences. The results showed that Rv3878, a conserved hypothetical alanine rich protein, is not conserved in M. tuberculosis strains and there are polymorphisms existing in the protein. The large number of amino acid changes in its T cell epitopes may reflect ongoing immune evasion.

BCG vaccine in Korea

The anti-tuberculosis Bacille de Calmette et Guérin (BCG) vaccine was developed between 1905 and 1921 at Pasteur Institutes of Lille in France, and was adopted by many countries. BCG strains comprise natural mutants of major virulence factors of Mycobacterium tuberculosis and that BCG sub-strains differ markedly in virulence levels. The tuberculosis became endemic in Korea after the Korean War (1950s). The BCG strain, which was donated by Pasteur Institutes, was brought to Korea in 1955, and the first domestic BCG vaccine was produced by the National Defense Research Institute (NDRI), current Korea Centers for Disease Control and Prevention (KCDC), in 1960. Since 1987, BCG manufacture work was handed over to the Korean Institute of Tuberculosis (KIT), the freeze-dried BCG vaccine was manufactured at a scale required to meet the whole amount of domestic consumption. However, since 2006, the manufacture of BCG vaccine suspended and the whole amount of BCG was imported at this point of time. Now KIT is planning to re-produce the BCG vaccine in Korea under the supervision of KCDC, this will be render great role to National Tuberculosis Control Program (NTP) and provide initiating step for developing new tuberculosis vaccines in Korea.

Mycobacterium tuberculosis region of difference (RD) 2 antigen Rv1985c and RD11 antigen Rv3425 have the promising potential to distinguish patients with active tuberculosis from M. bovis BCG-vaccinated individuals

Antigens encoded in the region of difference (RD) of Mycobacterium tuberculosis constitute a potential source of specific immunodiagnostic antigens for distinguishing tuberculosis (TB) infection from BCG vaccination. We evaluated the diagnostic potential of specific T-cell epitopes selected from two immunodominant antigens, Rv1985c and Rv3425, from RD2 and RD11, respectively, on the basis of epitope mapping, in TB patients and BCG-vaccinated healthy individuals. Using a whole-blood gamma interferon release assay, a wide array of epitopes was recognized on both Rv1985c and Rv3425 in TB patients. Those epitopes that could specifically discriminate TB infection from BCG vaccination were carefully selected, and the most promising peptide pools from Rv1985c showed a sensitivity of 53.9% and a specificity of 95.5%. When the novel specific peptides from Rv1985c joined the diagnostic antigens in the QuantiFERON-TB Gold In-Tube (QFT-IT) assay, the sensitivity was increased from 86.4% to 96.2%, with no drop in specificity. These results indicate that the peptide pools selected from Rv1985c and Rv3425 have the potential to diagnose TB infection by a method that may be routinely used in clinical laboratories.

IP-10 release assays in the diagnosis of tuberculosis infection: current status and future directions

The current state-of-the-art tests for infection with Mycobacterium tuberculosis - the IFN-γ release assays - rely on accurate measurement of the cytokine IFN-γ. Many other potential biomarkers are expressed in concert with IFN-γ, and IP-10 in particular has shown promising results. IP-10 is produced in large amounts, allowing for the development of new and simplified test platforms, such as lateral flow. In this review, we summarize the results of 22 clinical studies exploring the use of IP-10 as an alternative marker to IFN-γ. The studies report that diagnostic accuracy of IP-10 is on par with IFN-γ, but also that IP-10 may be more robust in young children and in HIV-infected individuals with low CD4 cell counts. We conclude the review by presenting limitations of the published works and outline recent developments and future directions.