Journal Article

Low Cost Tuberculosis Vaccine Antigens in Capsules: Expression in Chloroplasts, Bio-Encapsulation, Stability and Functional Evaluation In Vitro

Lakshmi, Priya Saikumar, Lloyd, Bethany, Yang, Xiangdong, Daniell, Henry, Verma, Dheeraj

2013

Overview

Tuberculosis (TB) caused by Mycobacterium tuberculosis is one of the leading fatal infectious diseases. The development of TB vaccines has been recognized as a major public health priority by the World Health Organization. In this study, three candidate antigens, ESAT-6 (6 kDa early secretory antigenic target) and Mtb72F (a fusion polyprotein from two TB antigens, Mtb32 and Mtb39) fused with cholera toxin B-subunit (CTB) and LipY (a cell wall protein) were expressed in tobacco and/or lettuce chloroplasts to facilitate bioencapsulation/oral delivery. Site-specific transgene integration into the chloroplast genome was confirmed by Southern blot analysis. In transplastomic leaves, CTB fusion proteins existed in soluble monomeric or multimeric forms of expected sizes and their expression levels varied depending upon the developmental stage and time of leaf harvest, with the highest-level of accumulation in mature leaves harvested at 6PM. The CTB-ESAT6 and CTB-Mtb72F expression levels reached up to 7.5% and 1.2% of total soluble protein respectively in mature tobacco leaves. Transplastomic CTB-ESAT6 lettuce plants accumulated up to 0.75% of total leaf protein. Western blot analysis of lyophilized lettuce leaves stored at room temperature for up to six months showed that the CTB-ESAT6 fusion protein was stable and preserved proper folding, disulfide bonds and assembly into pentamers for prolonged periods. Also, antigen concentration per gram of leaf tissue was increased 22 fold after lyophilization. Hemolysis assay with purified CTB-ESAT6 protein showed partial hemolysis of red blood cells and confirmed functionality of the ESAT-6 antigen. GM1-binding assay demonstrated that the CTB-ESAT6 fusion protein formed pentamers to bind with the GM1-ganglioside receptor. The expression of functional Mycobacterium tuberculosis antigens in transplastomic plants should facilitate development of a cost-effective and orally deliverable TB booster vaccine with potential for long-term storage at room temperature. To our knowledge, this is the first report of expression of TB vaccine antigens in chloroplasts.

• Publisher: Public Library of Science,Public Library of Science (PLoS)
• Subject: Administration, Oral / Analysis / Animals / Antigens / Antigens, Bacterial - biosynthesis / Antigens, Bacterial - chemistry / Antigens, Bacterial - genetics / Antigens, Bacterial - immunology / Bacterial infections / Bacterial Proteins - biosynthesis / Bacterial Proteins - chemistry / Bacterial Proteins - genetics / Bacterial Proteins - immunology / Biology / Blood cells / Brassica napus / Cell walls / Chloroplasts / Cholera / Cholera toxin / Cholera toxin B subunit / Development and progression / Developmental stages / Disulfide bonds / Drug delivery systems / E coli / Erythrocytes / ESAT-6 antigen / Escherichia coli / Flowers & plants / Freeze drying / Fusion protein / Gene Expression / Genomes / Genomics / Hemolysis / Infections / Infectious diseases / Lactuca - chemistry / Lactuca - genetics / Lactuca - immunology / Lactuca - metabolism / Leaves / Low cost / Medicine / Mycobacterium tuberculosis / Mycobacterium tuberculosis - genetics / Plant tissues / Plants (botany) / Plants, Genetically Modified - chemistry / Plants, Genetically Modified - genetics / Plants, Genetically Modified - immunology / Plants, Genetically Modified - metabolism / Protein binding / Protein folding / Proteins / Public health / Recombinant Fusion Proteins - biosynthesis / Recombinant Fusion Proteins - chemistry / Recombinant Fusion Proteins - genetics / Recombinant Fusion Proteins - immunology / Sheep / Stability analysis / Temperature / Tobacco / Toxins / Tuberculosis / Tuberculosis vaccines / Tuberculosis Vaccines - chemistry / Tuberculosis Vaccines - genetics / Tuberculosis Vaccines - immunology / Tuberculosis Vaccines - metabolism / Vaccines / Waterborne diseases