Recombinant M. Tuberculosis ESAT-6 Antigen [His, GST]

Tuberculosis (TB) is a chronic infectious disease caused by Mycobacterium tuberculosis (MTB), and the Bacillus Calmette-Guérin (BCG) vaccine, a mutant strain of Mycobacterium bovis, is the only effective and widely used vaccine against TB. In contrast to BCG, the MTB genome has a BCG-deficient region, the region of difference (RD). The RD region is divided into 16 regions, and ESAT-6 is an important virulence protein encoded by the Rv3875 gene in the RD-1 region, which is involved in the regulation of macrophage apoptosis, inflammatory activation, autophagy, and other processes. The ESAT-6 gene, which is 288 bp in length and contains 95 amino acids, serves as a conserved gene for MTB and is present only in pathogenic MTB, and is not expressed in either BCG or non-pathogenic Mycobacteria.

Figure 1. Unraveling protein-mediated pathways in Mycobacterium tuberculosis pathogenesis
(Source: Passos BBS, et al. 2024)

ESAT-6 is a key T-cell antigen with strong immunogenicity. Cell-mediated immune responses are essential for the control of MTB infection, and clearance of MTB is dependent on activation of the macrophage microbicide pathway, antigen presentation, activation of T lymphocytes, and release of Th1-type cytokines. The immune response is dependent on MHC class II molecules to deliver and activate CD4⁺ T cells and promote the secretion of pro-inflammatory cytokines, such as TNF and IL-12, as well as chemokines that recruit immune cells to the site of infection and further activate immune cells, and on the secretion of the cytokine IEN-γ by CD8⁺ T cells and NK cells to activate macrophages. One of the reasons for the current lack of protective efficacy of BCG vaccine is likely to be the insufficient initiation of CD8⁺ T cells, whereas ESAT-6 induces macrophages to produce specific CD8⁺ T lymphocytes that participate in the immune response against MTB infection. Thus ESAT-6 has received much attention in TB vaccine research as a promising vaccine candidate molecule.

Some studies have reported that inoculation of BALB/c mice with ESAT-6 recombinant antigen or DNA vaccine plus an immune adjuvant was effective against attack by the H37Rv strain. Since ESAT-6 antigen is a better vaccine molecule, researchers often use attenuated modified bacteria or viruses as vaccine vectors, and these pathogen vectors mediated ESAT-6 vaccine immunization of the host can produce effective immune response and partially counteract MTB attack. However, none of these vaccines can provide 100% protection, among which the reasons may be: recombinant BCG and Mycobacterium bovis vaccines have low transformation efficiency, the expression of exogenous genes requires special vectors, oral rSt vaccines are susceptible to tolerance, and expression products of recombinant single-amplified Listeria monocytogenes differ significantly from natural proteins.

Mycobacterium tuberculosis ESAT-6
MTB ESAT-6