In Vitro Immune Response to Vaccine Candidate Mycobacterium Tuberculosis Antigens in Healthy Adult Ethiopians
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Date
2010-08
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Addis Ababa University
Abstract
Introduction: The global burden of disease due to tuberculosis (TB) is huge and is one of the challenges in the fight against infectious diseases. The widespread use BCG vaccine was for many years believed to be one of the key control measures for TB, but its efficacy has been questioned following the results of trials conducted in developing countries. Therefore, there is an urgent need particularly in developing countries, for a vaccine with acceptable efficacy to combat the TB epidemic. However, protective immunity in TB is incompletely understood. In murine models, cell-subset depletion experiments in vivo showed that CD4 and CD8 άβ as well as γδ T cells all have a role in protective immunity. Studies demonstrated that immunization of mice with live M. tuberculosis induces protection and delayed type hypersensitivity (DTH), whereas heat killed organisms induce DTH only. This experimental observation has sparked interest in antigens released or secreted by mycobacteria in culture filtrates as vaccine candidates and different studies on these antigens showed promising results. Ethiopia being one of the high TB burden countries, this project is designed with the objective to generate background immunological data on immune response to selected candidate TB vaccine antigens of healthy adult Ethiopians in preparation for a TB vaccine clinical trial.
Methods: Tuberculin skin test (TST) and QuantiFERON®-TB-Gold (QFT-G) assay were done. Peripheral blood monocytic cells (PBMCs) were isolated from healthy adults and the immune response following PBMC stimulation with the different TB antigens, ESAT-6, Ag85B and fused Ag85B-ESAT-6, was measured by the in vitro assays, IFN–γELISPOT assay and IFN–γ ELISA.
Results: Our findings showed that 46.7 % and 43.9% of the study participants had positive TST (TST10 mm) and QFT-G assay results respectively. The overall strength of agreement between TST (10mm) and QFT-G assay was very good (Kappa = 0.83), with concordant results in 98/107 (91.6%). Concordance between the two tests was good in participants with BCG scar (85.4%, Kappa = 0.71), but it was found to be excellent (96.6%, Kappa =0.93) in those who had not BCG scar. By using IFN–γ ELISPOT assay, the median (IQR) responses to Ag85B, ESAT-6 and fused Ag85B-ESAT-6 were found to
be 256 (147-408), 305 (133-474) and 429 (267-601) SFC/Million of PBMC respectively. The median (IQR) responses to Ag85B, ESAT-6 and fused Ag85B-ESAT-6 were found to be 0(0.0-189.0), 328.0 (0.0- 2351.0) and 96.0(0.0- 1220.0) pg/ml of IFN-γ respectively as it was measured by IFN–γ ELISA.
Conclusion: An overall strong agreement between TST and QFT-G was observed at both TST10mm and TST>5 mm cutoff values in spite of the routine BCG vaccination at birth. We also have demonstrated the possibility to have false TST positivity in adults as the result of BCG vaccine administered in infancy, at cutoff of TST>5mm, but the possibility of true Latent TB infection (LTBI) is highly likely at larger indurations (TST10mm). QFT-G assay was not able to differentiate recent infection from remote
one. By using ELISPOT assay and ELISA, we were able to demonstrate the existence of effector and central memory T-cells specific to the fused antigens, Ag85B-ESAT-6, and its components, ESAT-6 and Ag85B, in healthy adult populations. The presence of significant ESAT-6 specific memory T-cells in the latent group makes the fused vaccine valuable to be used as post-exposure vaccine. The persistence of Ag85B specific T-cells as the result of childhood BCG vaccination enables the fused vaccine to be used as booster vaccine besides its potential use as post-exposure vaccine.
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Keywords
Vaccine ,Adult ,Mycobacterium tuberculosis