Analysis of the shotgun expression library of the Mycobacterium tuberculosis genome for immunodominant polypeptides: potential use in serodiagnosis

A systematic review of potential screening biomarkers for active TB disease

INTRODUCTION

The standard TB Four Symptom Screen does not meet the World Health Organization (WHO) ideal screening criteria for having greater than 90% sensitivity to identify active TB disease, regardless of HIV status. To identify novel screening biomarkers for active TB, we performed a systematic review of any cohort or case-control study reporting associations between screening biomarkers and active TB disease.

METHODS

We searched PubMed and Embase for articles published before October 10, 2021. We included studies from high or medium tuberculosis burden countries. We excluded articles focusing on C-reactive protein and lipoarabinomannan. For all included biomarkers, we calculated sensitivity, specificity and 95% confidence intervals, and assessed study quality using a tool adapted from the QUADAS-2 risk of bias.

RESULTS

From 8,062 abstracts screened, we included 79 articles. The articles described 302 unique biomarkers, including host antibodies, host proteins, TB antigens, microRNAs, whole blood gene PCRs, and combinations of biomarkers. Of these, 23 biomarkers had sensitivity greater than 90% and specificity greater than 70%, meeting WHO criteria for an ideal screening test. Among the eleven biomarkers described in people living with HIV, only one had a sensitivity greater than 90% and specificity greater than 70% for active TB.

CONCLUSION

Further evaluation of biomarkers of active TB should be pursued to accelerate identification of TB disease.

Predicted Structural Variability of Mycobacterium tuberculosis PPE18 Protein With Immunological Implications Among Clinical Strains

Recent advancements in vaccinology have led to the development of the M72/AS01E subunit vaccine, of which the major component is the Mycobacterium tuberculosis (MTB) PPE18 protein. Previous studies have demonstrated the genetic variability of the gene encoding PPE18 protein and the resulting peptide changes in diverse clinical strains of MTB; however, none have modeled the structural changes resulting from these peptide changes and their immunological implications. In this study, we investigated the structural predictions of 29 variant PPE18 proteins previously reported. We found evidence that PPE18 is at least a two-domain protein, with a highly conserved first domain and a largely variable second domain that has different coevolutionary clusters. Further, we investigated putative epitope sites in the clinical variants of PPE18 using prediction software. We found a negative relationship between T-cell epitope number and residue variability, while B-cell epitope likelihood was positively correlated with residue variability. Moreover, we found far more residues in the second domain predicted to be B-cell epitopes compared with the first domain. These results suggest an important functional role of the first domain and a role in immune evasion for the second, which extends our knowledge base of the basic biology of the PPE18 protein and indicates the need for further study into non-traditional immunological responses to TB.

Potential immunogenic polypeptides of Burkholderia pseudomallei identified by shotgun expression library and evaluation of their efficacy for serodiagnosis of melioidosis

The search for novel immunogenic polypeptides to improve the accuracy and reliability of serologic diagnostic methods for Burkholderia pseudomallei infection is ongoing. We employed a rapid and efficient approach to identify such polypeptides with sera from melioidosis patients using a small insert genomic expression library created from clinically confirmed local virulent isolates of B. pseudomallei. After 2 rounds of immunoscreening, 6 sero-positive clones expressing immunogenic peptides were sequenced and their identities were: benzoate 1,2-dioxygenase beta subunit, a putative 200 kDa antigen p200, phosphotransferase enzyme family protein, short chain dehydrogenase and 2 hypothetical proteins. These immunogens were then transferred to an ELISA platform for further large scale screening. By combining shotgun expression library and ELISA assays, we identified 2 polypeptides BPSS1904 (benzoate 1,2-dioxygenase beta subunit) and BPSL3130 (hypothetical protein), which had sensitivities of 78.9% and 79.4% and specificities of 88.1% and 94.8%, respectively in ELISA test, thus suggesting that both are potential candidate antigens for the serodiagnosis of infections caused by B. pseudomallei.

A synthetic gag p24 epitope chemically coupled to BSA through a decaalanine peptide enhances HIV type 1 serodiagnostic ability by several folds

p24 is an immunodominant gag core protein of HIV-1. The synthetic immunodominant epitope of p24 and the recombinant p24 show poor immunoreactivity and specificity, respectively. Their application is, therefore, severely limited in the serodiagnosis of HIV-1, although it is an important marker for early diagnosis. These limitations have been overcome by conjugating the synthetic p24 to BSA through a decaalanine peptide spacer. The engineered p24 shows about 5-fold more efficient immunoreactivity than the synthetic p24, and, at the same time, shows a several fold reduction in nonspecific cross-reactivity as compared to recombinant p24. Our strategy to conjugate the p24 peptide epitope to BSA worked well as a consistent and reliable immunodiagnostic marker. This strategy may also prove useful for the diagnosis of other diseases.

Modern approaches to a rapid diagnosis of tuberculosis: promises and challenges ahead

The limitations of the conventional methods for diagnosing tuberculosis have spurred multi-faceted research activities in this field throughout the world. Chromatographic methods appear promising but may not be widely available in the developing countries. Immuno-diagnostic methods using combinations ("cocktails") of antigens have high sensitivity and specificity and can easily be applied in the peripheral laboratories and in the field settings. Though expensive, molecular methods for diagnosis of tuberculosis have advantages of speed, sensitivity, and specificity. Adequate training of the eligible personnels in molecular methods and prevention of laboratory-dependent contamination may help reduce false positive results. Although, there are no clear guidelines, so far on how to make out the best from the gene amplification methods, yet their use may be encouraged with adequate quality controls, because of the inherent ingenuity and promises of these methods. Phage-based molecular methods provide rapid results in susceptibility tests for anti-tubercular drugs. In future, many sophisticated techniques are expected to hit the market for a rapid diagnosis of tuberculosis. In the developing countries, it is necessary to evaluate availability of suitable infrastructure and trained personnels before adopting modern diagnostic methods.

Translationally controlled tumor protein from Madurella mycetomatis, a marker for tumorous mycetoma progression

About 40 years ago Abs against the fungus Madurella mycetomatis were first demonstrated to be present in eumycetoma patients, a disease characterized by tumorous swellings. To date nothing is known about the individual immunoreactive Ags present in this fungus. In the present study, we identify its first immunogenic Ag, a protein homologous to the translationally controlled tumor protein (TCTP), a well-conserved histamine release factor in a range of eukaryotes. The gene for this Ag was demonstrated to be present in two variants in M. mycetomatis, with 13% aa difference between the two proteins encoded. In vitro, TCTP was secreted into the culture medium. In vivo, it was found to be expressed on hyphae present in developing stages of the eumycetoma-characteristic black grain. Significant IgG and IgM immune responses, against the whole protein and selected M. mycetomatis-specific peptides, were determined. The Ab levels correlated with lesion size and disease duration. Overall, the patients with the largest lesions had the highest Ab level, which lowered with decreasing size of the lesion. After 6-15 years of disease duration the Ab levels were the highest. TCTP is the first well-characterized immunogenic Ag, simultaneously the first monomolecular vaccine candidate, identified for the fungus M. mycetomatis.

Glycolipids of Mycobacterium tuberculosis strain H37Rv are potential serological markers for diagnosis of active tuberculosis

A simple and cost-effective diagnostic tool (TB Screen Test) for the screening of patients with pulmonary and extrapulmonary tuberculosis and for differentiation of those individuals from individuals without tuberculosis, other common infections, and healthy controls has been developed. The serological responses of purified mycobacterial glycolipid antigens were examined by a liposome agglutination assay. The assay was able to detect very low antiglycolipid antibody concentrations in the infected individuals. The sera from the tuberculosis patient group had significantly higher concentrations of antiglycolipid antibody than the sera from uninfected control subjects, with 94% sensitivity and 98.3% specificity. Glycolipids of Mycobacterium tuberculosis H37Rv antigens were isolated, purified, and characterized. After interchelation with liposome particles, these purified antigens specifically bound to the antiglycolipid antibodies present in the sera of patients with tuberculosis, resulting in the formation of a blue agglutination. This protocol clearly differentiates healthy controls and M. bovis BCG-vaccinated subjects from those with active tuberculosis. The resultant diagnostic tool, the TB Screen Test, is more economical and rapid (4 min) than other currently available products and can be used for the mass screening of a heavily afflicted population.