Serologic response to a secreted and a cytosolic antigen of Mycobacterium tuberculosis in childhood tuberculosis

Evaluation of serological assays for the diagnosis of childhood tuberculosis disease: a study protocol

BACKGROUND

Tuberculosis (TB) poses a major public health challenge, particularly in children. A substantial proportion of children with TB disease remain undetected and unconfirmed. Therefore, there is an urgent need for a highly sensitive point-of-care test. This study aims to assess the performance of serological assays based on various antigen targets and antibody properties in distinguishing children (0-18 years) with TB disease (1) from healthy TB-exposed children, (2) children with non-TB lower respiratory tract infections, and (3) from children with TB infection.

METHODS

The study will use biobanked plasma samples collected from three prospective multicentric diagnostic observational studies: the Childhood TB in Switzerland (CITRUS) study, the Pediatric TB Research Network in Spain (pTBred), and the Procalcitonin guidance to reduce antibiotic treatment of lower respiratory tract infections in children and adolescents (ProPAED) study. Included are children diagnosed with TB disease or infection, healthy TB-exposed children, and sick children with non-TB lower respiratory tract infection. Serological multiplex assays will be performed to identify M. tuberculosis antigen-specific antibody features, including isotypes, subclasses, Fc receptor (FcR) binding, and IgG glycosylation.

DISCUSSION

The findings from this study will help to design serological assays for diagnosing TB disease in children. Importantly, those assays could easily be developed as low-cost point-of-care tests, thereby offering a potential solution for resource-constrained settings.

CLINICALTRIALS

GOV IDENTIFIER

NCT03044509.

Antibody responses to mycobacterial antigens in children with tuberculosis: challenges and potential diagnostic value

The identification of easily detectable biomarkers for active tuberculosis (TB) is a global health priority. Such biomarkers would be of particular value in childhood TB, which poses greater diagnostic challenges than adult TB. Serum antibodies can be detected by simple formats that provide extremely rapid results. However, attempts to develop accurate serodiagnostic tests for TB have been unsuccessful. Whereas antibody responses to mycobacterial antigens in adult TB have been studied extensively and reviewed, the same cannot be said for serologic data in pediatric populations. Here we appraise studies on serological responses in childhood TB and discuss findings and limitations in the context of the developing immune system, the age range, and the spectrum of TB manifestations. We found that the antibody responses to mycobacterial antigens in childhood TB can vary widely, with sensitivities and specificities ranging from 14% to 85% and from 86% to 100%, respectively. We conclude that the limitations in serodiagnostic studies of childhood TB are manifold, thereby restricting the interpretation of currently available data. Concerns about the methodology used in published studies suggest that conclusions about the eventual value of serodiagnosis cannot be made at this time. However, the available data suggest a potential adjunctive value for serology in the diagnosis of childhood TB. Despite the difficulties noted in this field, there is optimism that the application of novel antigens and the integration of those factors which contribute to the serological responses in childhood TB can lead to useful future diagnostics.

Humoral immune response against 38- and 16-kDa mycobacterial antigens in childhood tuberculosis

Several enzyme-linked immunosorbent assays (ELISAs) based on mycobacterial antigens have been tried for the rapid diagnosis of tuberculosis (TB). In this study, the value of the 16 and 38-kDa mycobacterial antigens in the diagnosis of TB was investigated in pediatric patients in Izmir, Turkey in whom they were found using clinical and/or bacteriological methods. A commercial ELISA kit was used for measuring IgG against 38 and 16-kDa recombinant antigens. The humoral immune response was analyzed in a group of 32 TB patients (24 pulmonary, 3 lymphadenitis and 2 pleuritis, 2 meningitis and a disseminated TB) and in control groups consisting of 20 healthy children and 20 pulmonary diseases other than TB cases. The sensitivity, specificity, positive predictive value, and the negative predictive value of the test were found to be 25%, 90%, 66.7%, and 60%, respectively, in the TB cases. The ELISA test shows very good specificity, but low level of sensitivity and negative predict value. It was thought that it might be used in combination with other methods to increase diagnostic accuracy, especially for culture-negative TB pediatric cases, which are difficult to diagnose.

Progress in serodiagnosis of Mycobacterium tuberculosis infection

One-third of the world population is estimated to have Mycobacterium tuberculosis infection. Accurate and timely identification of infected individuals is critical for treatment and control. The current diagnostic methods lack the desired sensitivity and specificity, require sophisticated equipment and skilled workforce or take weeks to yield results. Diagnosis of extrapulmonary TB, TB-HIV co-infection, childhood TB and sputum smear-negative pulmonary TB pose serious challenges. Interest in developing serodiagnostic methods is increasing because detection of antibody is rapid, simple and relatively inexpensive, and does not require a living cell for detection. Three types of tests, namely screening tests to overcome diagnostic delay, specific tests for diagnosis of extrapulmonary TB and other bacteriologically negative cases, and tests for vaccine-induced immunity need critical consideration. Several factors must be considered to develop serodiagnostic methods for TB. Antigen recognition by infected individuals is highly heterogeneous due to stage of disease, differences in HLA types, strain of the bacilli, health of the patient and bacillary load. With advances in molecular biological techniques, a number of novel antigens have been identified. Some of these antigens have proven valuable in detecting specific antibodies in some of the most challenging TB patients. The best example is a fusion protein containing several M. tuberculosis proteins (e.g. CFP-10, MTB8, MTB48, MTB81 and the 38-kDa protein) which showed encouraging results in detecting antibodies in sera of patients, including TB-HIV co-infection. This review presents progress made in the serodiagnosis of TB during the last decade.

PPE protein (Rv3425) from DNA segment RD11 of Mycobacterium tuberculosis: a potential B-cell antigen used for serological diagnosis to distinguish vaccinated controls from tuberculosis patients

Proteins encoded by a 9.5-kb DNA segment, termed the region of difference (RD), of Mycobacterium tuberculosis have been demonstrated to be important in bacterial virulence, vaccine development and the design of diagnostic reagents. This study evaluated the immunogenic properties of Rv3425, a member of the PPE family of proteins, encoded by an open reading frame found in RD11 of M. tuberculosis, in comparison with two other well-known antigens, the early secreted antigen target 6 (ESAT-6) and the 10-kDa culture filtrate protein (CFP-10). RT-PCR demonstrated that Rv3425 mRNA is expressed in liquid culture by M. tuberculosis H37Rv. When tested in a conventional ELISA in the form of a His-tagged recombinant protein, Rv3425 revealed a statistically significant antigenic distinction between healthy bacille Calmette-Guérin (BCG)-vaccinated controls and tuberculosis (TB) patients (p <0.0001). The anti-IgG response to recombinant Rv3425 was almost equal to that for CFP-10, and was higher than that for ESAT-6. The results highlight the immunosensitive and immunospecific nature of Rv3425, which shows promise for use in the serodiagnosis of TB.

Cloning, expression, and purification of the 27 kDa (MPT51, Rv3803c) protein of Mycobacterium tuberculosis

A limited number of proteins of Mycobacterium tuberculosis have been characterized so far for their use as potential candidates for diagnosis and vaccine studies. This study was aimed at cloning, expression, and purification of a 27 kDa protein (otherwise known as the MPT51 or Rv3803c protein) of M. tuberculosis. The Rv3803c gene was PCR amplified using primers that contain specific restriction sites. The amplified product was inserted initially into pTOPO and then sub-cloned into pET15b and pET24d vectors, such that the recombinant protein is predicted to contain an N-terminal or a C-terminal histidine tag, respectively. The recombinant plasmids were introduced into Escherichia coli BL21 (DE3) and the recombinant proteins were purified from the cytosolic fractions of the E. coli sonicates by nickel-NTA chromatography. The purity, molecular mass, and the conformation of the proteins were determined by high performance liquid chromatography (HPLC), matrix assisted laser desorption-ionization-time-of-flight (MALDI-TOF), and circular dichroism (CD) studies, respectively. The purified proteins were found to be immunogenic and useful for immunodiagnostic studies of tuberculosis by enzyme linked immunosorbent assay (ELISA), with a sensitivity of 71% and specificity of 95%.

Immunoglobulin M antibody responses to Mycobacterium ulcerans allow discrimination between cases of active Buruli ulcer disease and matched family controls in areas where the disease is endemic

Buruli ulcer disease (BUD) is an emerging disease caused by Mycobacterium ulcerans. In the present study we have characterized the serological reactivities of sera from volunteer case patients with laboratory-confirmed BUD and controls living in three different regions of Ghana where the disease is endemic to determine if serology may be useful for disease confirmation. Our results showed highly reactive immunoglobulin G (IgG) responses among patients with laboratory-confirmed disease, healthy control family members of the case patients, and sera from patients with tuberculosis from areas where BUD is not endemic. These responses were represented by reactivities to multiple protein bands found in the M. ulcerans culture filtrate (CF). In contrast, patient IgM antibody responses to the M. ulcerans CF (MUCF) proteins were more distinct than those of healthy family members living in the same village. A total of 84.8% (56 of 66) of the BUD patients exhibited strong IgM antibody responses against MUCF proteins (30, 43 and 70 to 80 kDa), whereas only 4.5% (3 of 66) of the family controls exhibited such responses. The sensitivity of the total IgM response for the patients was 84.8% (95% confidence interval [CI], 74.3 to 91.6%), and the specificity determined with sera from family controls was 95.5% (95% CI, 87.5 to 98.4%). These studies suggest that the IgM responses of patients with BUD will be helpful in the identification and production of the M. ulcerans recombinant antigens required for the development of a sensitive and specific serological assay for the confirmation of active BUD.

Study of the antibody response against Mycobacterium tuberculosis antigens in Warao Amerindian children in Venezuela

This study was aimed at investigating alternate methods for serodiagnosis of tuberculosis (TB), which are needed because bacteriologic diagnosis of childhood TB is difficult. A selection of 80 serum and saliva samples were tested from Warao indigenous children under 15 years of age; 34 high TB suspects (28 positive and 6 negative for the tuberculin skin test, TST) and 46 healthy contact children (32 positive and 14 negative for the TST). Several enzyme-linked immunosorbent assay (ELISA) serological tests were developed to test for Mycobacterium tuberculosis-specific antibodies, including serum IgA, IgG, IgE, and secretory IgA (sIgA) in saliva against 3 specific antigens (PPD, HSP60, 38 kDa). Of these, 2 antigens, PPD and 38 kDa, showed significantly higher reactivity. The sensitivity and specificity of these tests for diagnosis remained limited, between 26.5% and 38.2%, and 77.4% and 97%, respectively. Of all the samples studied and combinations realized between all isotypes and antigens combined with 3 isotypes (anti-PPD IgG, IgE, and anti-38kDa sIgA) managed to detect the largest number of patients, showing an improved sensitivity level of 64.7%, although specificity levels dropped to 81.8%. These results were compared with the Omega diagnostics commercial kit results. The commercial kits showed significantly lower reactivity (sensitivity of 20% and 13.33% to Myco G and Complex Plus, respectively) and a specificity of 100%. This study shows that in indigenous populations of Venezuela, where invasive procedures cannot be used to select samples but evaluation with a chest X-ray for radiological studies is available, the combination of 3 specific isotypes may be a useful tool to increase diagnostic accuracy with pulmonary TB in this population, when used together with clinical and epidemiological criteria.

Potential serological use of a recombinant protein that is a replica of a Mycobacterium tuberculosis protein found in the urine of infected mice

The recent availability of numerous well-characterized Mycobacterium tuberculosis recombinant proteins has revived interest in the serological diagnosis of tuberculosis. Several promising results have been reported, particularly when more than one antigen is used in the test. However, thus far these antigens have not been used in routine diagnostic tests because they lack sufficient sensitivity. In addition, with the exception of one antigen, most recombinant M. tuberculosis proteins do not identify the majority of tuberculosis patients coinfected with human immunodeficiency virus (HIV). Here, we report a newer M. tuberculosis protein that is a promising candidate for increasing the sensitivity of the serological tests, in particular for patients coinfected with HIV. The protein was found in the urine of mice during the early stages of infection with M. tuberculosis (10 to 14 days), thus suggesting that the antigen is abundantly released during the in vivo growth of the mycobacterium. Reverse genetics was used to produce the recombinant protein, which we named U1 (for urine protein 1). Using a conventional enzyme-linked immunosorbent assay (ELISA), antibody to U1 could be detected in 60% of patients with pulmonary tuberculosis with no signs of coinfection with HIV (n = 83). Conversely, anti-U1 antibody was detected in 87% of the sera from tuberculosis patients coinfected with HIV (n = 47). Out of 12 HIV-infected nontuberculosis patients' sera, 9 did not react with U1 and three sera gave borderline ELISA signals (signal/cutoff of < or =1.75). These results suggest that the high efficiency of U1 in identifying tuberculosis patients coinfected with HIV may be related to abundant release of this protein during the initial phase of the HIV coinfection. The immediate availability of the antigen at a time point in which the patient's immune system is still competent would lead to a secondary immune response to U1 that persists for months in the patient's serum.

Recent advances in tuberculosis research in India

Tuberculosis (TB) continues to be the leading killer of mankind among all infectious diseases, especially in the developing countries. Since the discovery of tubercle bacillus more than 100 years ago, TB has been the subject of research in an attempt to develop tools and strategies to combat this disease. Research in Indian laboratories has contributed significantly towards developing the DOTS strategy employed worldwide in tuberculosis control programmes and elucidating the biological properties of its etiologic agent, M. tuberculosis. In recent times, the development of tools for manipulation of mycobacteria has given a boost to researchers working in this field. New strategies are being employed towards understanding the mechanisms of protection and pathogenesis of this disease. Molecular methods are being applied to develop new tools and reagents for prevention, diagnosis and treatment of tuberculosis. With the sequencing of the genome of M. tuberculosis, molecules are being identified for the development of new drugs and vaccines. In this chapter, the advances made in these areas by Indian researchers mainly during the last five years are reviewed.